nevetslab 11/17/2015 4:36 PM
Sunn 1200S Restoration project
I recently took in a Sunn 1200S Guitar Amp to restore. This one has date codes inside and out back to arly 1969 (4-section Cornell-Dublier Cap Can, still has the Dynaco name stamped into the can). I’m just at the starting gate on this project, and have ordered replacement caps, power resistors grounding jacks and other such items for it.

I did take the time to slowly bring up the AC mains on it, over the course of a day, using a solid-state 5AR4 replacement module to apply the FW rectifier output to the main caps., all tubes removed. Looks like there’s leakage current flowing in at least two sections, as I’m getting around 8mA current flowing thru the resistive string. The last section has about 500uA current flowing into the cap. Not surprised, with a date code of 6912….over 45 yrs old.

But, it was low enough to chance powering up one pair at a time of the Tung Sol 6550 power tubes. Plate current ranged from 14, 15, 18 & 21mA in pairs, with plate voltage at 545V, using one of the 5AR4 rectifier tubes. I haven’t resumed any of the electrical check-out today, taking the time instead to photograph the unit inside and out.

Early version of the 1200S, lacking the switched grounding input jacks, and having only one slide switch, labeled Mid-Boost. All I’ve done so far is replace the AC mains cord & fuse post, eliminating the Ground Reverse cap associated with the 2-wire power cord as it was built back then.

All the grounding throughout this amp is done thru riveted terminal strips, with the HV Secondary C/T getting to chassis thru one rivet, while the negative terminal of the first 20uF/600V cap getting to chassis a few inches away. With all the metal corrosion I see on the outside, and 45 yr old rivet, I don’t’ trust any of that for grounding. Already some signal path to chassis exists between the cap-can ground and the initial charging current of that 600V cap. So, that ought to be fun.


The inside metal surface seems to be free from the corrosion that’s intense on the outside. Looks like a plating on the steel, like maybe Maganese or Zinc Phosphate, though I really don’t know. If time and cost weren’t an object, I’d gut the chassis and send it out to have it sand-blasted, and then treated, before re-building again. I’ll have to wing it as is, so open to suggestions.


I’ve tried a small area of the top corner using a product called ‘sandflex’, available in coarse, medium and fine grit embedded throughout the rubber compound block. I’m sure it will break thru the original coating, but as you can see from the photos, that coating is ancient history. I’m open to suggestions on the outside surface, besides this approach.

Both transformers are quite rusty. I thought of using WD-40 in the process of working thru the rust, but perhaps there’s a better solution? I’m all ears.

I haven’t yet assessed the tube sockets’ contacts. While they are lacking the usual flange-mount, using instead a locking snap-ring to hold them into the keyed hole, the contacts ARE fully circular, rather than the forked type, so that’s encouraging.

The Reverb unit is a Gibbs tank, having a locking mechanism that I restored. Similar in DCR in/out to an Accutronics 4AB3C1B as we find in Fender Twin’s. It mounts to the inside roof of the cabinet, so hum coupling is no doubt an issue.

Leo_Gnardo 11/17/2015 6:09 PM
What a project! Similar to a Sunn PA 120 I fixed up a couple years back.

Good to not trust those riveted grounds. Unless it's verboten, mount some fresh grounding tabs with nuts n bolts, you may need to drill a couple holes. Good to scrape the chassis surface with mini wire brush or 400/600 grit at those points for best contact.

Transformer rust, best solution is the laziest. Leave it be. Wire brushing, sanding, you take a chance on shorting laminations to each other with metal splinters. Soak with oil, no I wouldn't. If you must, paint for looks, so what if it's a little bubbly.

Might be an idea to put in a second bias control, group the low 6550's one side & the higher reading ones on the other. About 35 mA per tube oughta do it.

On the PA120, we wound up with solid state rectifiers (why beat up valuable Bungle Boy GZ34's?), KT88 JJ outputs, and it turned out a clean 120 watts @ clip, maybe not hi fi but good for any instrument amp or PA, 40 - 10KHz at full power. But who's so concerned about 20-20K at full rip? Better bandwidth at lower power, after all the power is Dynaco Mk III with doubled up output tubes. A couple of these, one just running power amp, thru a pair or quad of Altec A7's, there's enough for an old fashioned rock concert. Wear your bell bottoms, nehru jacket, love beads & mood ring, that's a show!
nevetslab 11/17/2015 8:22 PM
I was also thinking about adding a second bias pot for that same reason. Good points made on the transformer laminations. I tried a little WD-40 on a rag on the end bells, and that seemed like it wanted to help. I haven't yet powered it up with all four 6550's in it, and bump the bias up to nominal level....see what the supplies look like then. In this amp, and all the other variants of the 1200, the O/T is powered AFTER the choke (both are in parallel). I'll no doubt end up with a bit more capacitance on the first stage, series-connected caps & ballast across them.
drewl 11/17/2015 9:16 PM
Years ago my bassist found a 2000s in the trash he gave me.
I rebuilt it like you, only needing filter caps and output tubes.

I cleaned up the rusty transformers and painted them.
Mine was a transitional model with sockets for the rectifier and also had the plug in solid state rectifier.

I used it as a practice amp at our studio for years then took it to a friends store to sell it.

Someone from Sweden bought it and a huge matching cabinet someone else was selling. Shipment by boat was as much as the pair!
Leo_Gnardo 11/17/2015 10:33 PM
Quote Originally Posted by nevetslab View Post
In this amp, and all the other variants of the 1200, the O/T is powered AFTER the choke (both are in parallel). I'll no doubt end up with a bit more capacitance on the first stage, series-connected caps & ballast across them.
That "pre filter" is standard issue on Dyna MkIII. Sunn licensed the design & bought a lot of their iron from Dynaco. "Why engineer a power amp when there's a perfectly good one." Worked out OK for both parties. On the PA 120 I did replace can caps with a pair of 50+50uF/500V in series, balancing R's, as I've seen practically all the old Dyna caps fail, why invite a sure fire disaster. Caps further down the chain, replaced with singles, and the one handling preamp grounded locally in the preamp zone. It's a big project, you just chip away one bit at a time, enjoy the improvements as you go.

20 years back I thought I could get a pinch more power out of Dynaco MkIII by skipping the pre filter & running the OT center tap from raw hi voltage as it's typically done. Didn't work so well. Passing a sine wave thru, it developed a "point" as power approached clipping, as if the load was insufficient for the OT tap. And I heard that as distortion. So I put 'em back to normal. Some things, you can't improve.
Tom Phillips 11/17/2015 11:21 PM
Quote Originally Posted by nevetslab View Post
Ö Both transformers are quite rusty. I thought of using WD-40 in the process of working thru the rust, but perhaps thereís a better solution? Iím all earsÖ
The rust itself does not cause a huge problem but it is sure unsightly. A treatment that works well for me is to remove loose rust particles and then apply a rust conversion coating. The attached photos show an amp before and after the transformers received this treatment.
For an even better look, follow with a coat of black paint. There are several rust conversion products on the market. One example is Corroseal. (See Corroseal Rust Converter | Metal Primer | Rust Paint)
trobbins 11/18/2015 12:46 AM
A big amp like this really should have power transformer secondary side fuse protection snuck in somehow. Given the bias tap, any B+ related fuse protection would need to be in the HV winding arms, not the CT.

As well as an extra bias control - it would be best to make the pot wiper fail-safe.

A smattering of grid stoppers in stages that can be overdriven may be noticeable, and the PI may need to be checked for cathode-heater stress at turn-on.
gbono 11/18/2015 4:24 PM
I had a run of Sunn knobs made for me by Rogan the original supplier. Let me know if you need any.
nevetslab 11/19/2015 1:52 AM
Some excellent suggestions made from all of you. Adding the HT fuse protection sounds like a good idea, plenty of room for that.

On the PI cathode-heater stress at turn-on, what are you looking for? Is this specific on the 6AN8 Pentode/Triode tube? The three 6AN8 tubes I have are RCA's, two rescued from de-commissioned old test gear, the other from the 1200S. Of course, that tube socket is underneath the forest of circuit components of those two stages.


I had removed the Sunn knobs, dropped them into my ultra-sonic cleaner, dish-washing soap, ran them for 1/2 hr. Cleaner, though the white ink in some numbers are well faded, and all of the metal dress inserts have corrosion spots, so I would be interested in a fresh set. 'gbono', I'll send you an email on that.....many thanks!


While I did order a fresh 4-section cap-can 30-20-20-20 @ 525V, I'll end up using that for the PI/reverb Driver/Preamp stages, and go with the series-stacked axial lead caps with ballast on the pre/post choke filter stages. Takes all the high charging currents out of the cap-can that does the damage, as well as dropping the supply potential. I also noticed in this early build, the preamp stage and pre-reverb tank drive stage are NOT wired to the final filter stage (10k to 20uf/525V filter cap).

10k grid-stoppers on the power tubes, at the socket pins directly where it should be. Interesting wiring arrangement on the two sets of power tubes. Each pair of tubes is upper/lower, with the second pair fed from the terminal strip containing the plate and screen resistors and lead-wires connecting those from the resistors. V4-V5 & V7-V6, where V4 & V6 are upper pair, V5/V7 are lower pair. Also noticed a 700 ohm load resistor to ground off the 16 ohm tap, and, on the schematics I have, it shows the SPKR jack is 16 ohm, and plugging into the EXT Jack, it/both become 8 ohm load, with the 4 ohm tap not being used. This looks to be wired with the tap not being used (Yel wire), though still connected to the 700 ohm load, and the impedance switching being between the 8 and 4 ohm taps, as seen in the images below (16 ohm YEL, 8 ohm WHT, 4 ohm BRN, Gnd BLK.


Seen along side the 700 ohm load resistor is the 1k/750pF feedback network to the tapped cathode resistor string of the 6AN8 Pentode input stage of the power amp.

I added the 2 Sunn 1200 & 1200S schematics I have, along with a Sunn Dual Rectifier Amp schematic, it being a PA amp (Sun 120?). None show the power supply wiring as it's built in this amp I have on the bench. Only one 600V cap, followed by the chokes and then into the 30-20-20-20 cap-can, with the last section not connected to the preamp stages.
trobbins 11/19/2015 2:58 AM
A hazard that can arise with direct coupled stages is during startup, when V2 is not conducting and hence the grid of the cathodyne is effectively at B+. As well, the cathode of the cathodyne will be at ground. That has been known to cause arcing - and a grid-cathode diode can alleviate that.
drewl 11/19/2015 10:45 AM
I just used Fender numbered knobs, close enough.
gbono 11/19/2015 10:56 AM
Never did understand why Sunholm used the 0-10 numbering. I guess you get "11" spaces while Fender knobs were only 10
g1 11/19/2015 12:03 PM
So that would be one louder?
loudthud 11/19/2015 12:06 PM
The Dynakit preamp that Sunn started with had Rogan knobs but they were tan colored with just a triangle pointer. Sunn used an identical black knob with numbers on the front panel for a couple of years, then changed to numbers on the skirt. Gibson used Rogan on some years except they were grey. Music Man used Rogan knobs with a different font for the numbers.
66tele 12/2/2015 5:40 PM
I sent you a PM yesterday.

nevetslab 12/2/2015 7:53 PM
Quote Originally Posted by trobbins View Post
A hazard that can arise with direct coupled stages is during startup, when V2 is not conducting and hence the grid of the cathodyne is effectively at B+. As well, the cathode of the cathodyne will be at ground. That has been known to cause arcing - and a grid-cathode diode can alleviate that.
I will look into that....many thanks for the explanation and suggestion.

I was just about to start pulling the old supply caps out and installing the new mounting points for the series-coupled supply filters on the first two stages, but stopped to look at what would happen if I just blindly used shorting input jacks in place of the open-circuit jacks Sunn used. While I could do it on the Normal inputs, making one a lower sensitivity input (attenuation) like Fender or Ampeg do, it doesn't work with the Bright input circuit. Their 'Bright' input R/C network if it gets grounded, it screws up the input response and I also loose a lot of sensitivity. Sunn used isolated Break switch contacts on those four input jacks in the later release, which shorted out the volume pot when nothing was plugged in. But, once you're plugged in, the unused input jacks remain open circuit, though effectively loaded by your guitar, so it doesn't really matter. Those jacks (Switchcraft 13A) are around $8 each. This amp doesn't have them. I'll deal with it down stream.

Sunn also has a 'brightness' cap across the volume pot's wiper-top of pot, that's not switched. I'll probably replace the volume pot with one having a Pull-switch, so it can be switched in or out.

Chassis is about as clean as I'll get it. Once I'm thru with adding terminal mounting holes and the extra bias pot, I'll apply a coating to the outside surface so it doesn't rust.


As restoration projects go, always interruptions to halt progress.
nevetslab 1/9/2016 6:33 PM
HIGH HUM level on rebuild, now trouble-shooiting and need help
After completing the replacement of all the power supply caps, other aged components and revising the grounding to bypass the 45 yr old aluminum rivets used in the original build, I finally got to powering up the amp.


Having added a second bias pot, now having one each for the top half and bottom half output tubes (6550ís), I began by verifying supply voltages and bias supply adjustment range. All looked favorable there. During the bias adjustment phase, beginning with a solid state plugin rectifier installed in place of the 5AR4 tubes, I ended up adding some 1N5406 rectifiers (two per string) across the rectifier tube sockets, and wired them up to the vacated Ground Reverse Switch, changing it to a Rectifier Mode switch (Tube/Diode)

In Diode mode, I had 535VDC at the O/T C/T winding, set the 6550ís for 35mA/20W per tube, and with the 5AR4ís it dropped to 522VDC & 30mA/18W per tube. Current balance was 3-4mA difference on the extremes..

When I gave a listen to the residual hum/noise for the first time, I was horrified. Massive amount of hum coupling, along wlth veryhigh diode commutation noise, whether in Tube or Diode Rectifier mode.

I was also hearing thump from the Tremolo circuit, which is powered off the bias tap winding. Unplugging the power to that PCB assy, the hum level dropped substantially. Unplugging the preamp tube, thereís still some hum present (16.67mS hum spikesÖ.60Hz), and most of the diode commutation noise much lower. I havenít yet replaced the bias rectifier diode nor added bypass cap across it Thought about changing it a UF4004 (fast recovery diode).

The schematic shows an LDR module in series with the top of the volume control from the input preamp tubeís plate for the Tremolo modulating circuit. That LDR module is now right next to the 1st stage power supply filter stack (pair of 220uF/300V caps in series, with ballast resistors). In fact, in replacing the power supply caps, Iíve added additional capacitanceÖ.1st stage is fed from the Rectifier mode switch, output of which feeds the S/B switch thru a HT fuse (1A S/B), and to a 20uF/600V cap at the input of the parallel inductors. The output of the inductors feeds the second filter stage, it being series-connected 100uF/450V caps, which feed the C/T of the O/T primary. From there, the filters are all in a new cap can, being a 40-20-20-20uF/525V, mounted onto a fiber board, and feed the driver, reverb & preamp stages.,. The C/T winding of the HT supply ties to a ground stud, along with the ground leads of the 1st 3 power supply filter caps. I had the heater C/T lead tied there two, but it is now lifted, in favor of a hum balance pot, installed nearby. A single wire from the cap can ties to this new chassis ground studís solder lugs. All the remaining grounds of the system tie to the cap canís terminals.


So, hearing this excessive hum, and it dropping substantially when I power off the tremolo board, I located where the LDRís Resistor cell was wired in (in series with the tone stack from the 2nd stage tube). I disconnected that, it being wired in via shielded cables, and jumpered the connection so that high impedance circuit was no longer in the power supply field present by the filters and inductors, and closer to the power transformer.

That made no difference, surprisingly. Still had the high hum and buzz. Prior to my making any changes on the amp, I did have it powered up, and listened to it, and it had a lot of hum/noise, though I didnít document it.

The next moves I made, now wondering about the power transformer itself, I disconnected the secondary connections to the circuits, and powered it up, to see where the excitation current was at. 215mA @ 120VAC, 10.7W. That seems a bit high to me, butÖ.I havenít checked to see what it is on my Traynor YBA-1Aís transformer, being about the same VA rating and physical size..

Now, with it disconnected, I moved my HP 712C Power Supply to the bench, and connected up the bias, HT & heater supplies to the chassis. Set the bias to the same -81VDC source level, 550VDC in to the 1st filter stage, 6.3VAC to the heaters, and powered up the system. Minor tweak with the bias, and I had 522VDC at the C/T of the O/T primary, pulling 150mA at idle. The hum/noise level was now very reasonable to my ears.


Hmmmm. What if I disconnect the heater winding from the Sunnís transformer, and run it from the HP supply? Iíve had excessive field radiation problems on Vox AC30ís improve by using an added heater transformer in the chassis before. So, wired that up. Didnít solve the problemÖI again had high hum & high diode commutation noise again.

Thatís where Iíve stopped for the moment. Not sure if I have a major problem with the power transformer. My AR 4040AT Hypot is in storage, so I havenít checked it for leakage. My next attempts are to try and power this up with the transformer from the Traynor YBA-1A, though by way of extended leads, not yet wanting to remove the transformers and swap it out.

Iím still pondering the changes made in the grounding, and the added filters installed, with their orientation. Also, since Sunn has two inductors wired in parallel, is there a phasing issue there? Will it make a difference with one reversed?
trobbins 1/9/2016 7:12 PM
Given you have external power supplies available, perhaps try and swap in/out some of the on board power supplies to see if any particular supply (B+, bias, heater) is a major point of hum entry. Similarly, try and work back from output stage to see where main ingress occurs (that may mean disconnecting feedback if used, and temporary disconnection of coupling caps between stages).

You may be lucky and find a silver bullet, or need to methodically wade through a few main entry mechanisms.

Any change to original has consequences - such as replacing a valve diode with ss. Perhaps if you review how well the different stages are locally decoupled to their local 0V star nodes (or not!), and then connected together. SS diode related transients on the B+ winding can glitch over to bias and heater windings, so perhaps review the smallest current rated UF diode you can use via PSUD2, and check how well the diodes and first filter cap wiring and layout are done.
Justin Thomas 1/10/2016 12:18 AM
Quote Originally Posted by g1 View Post
So that would be one louder?
No, it's one quieter. It's negative volume for obnoxious singers and bassists.

MarkusBass 1/10/2016 1:53 AM
Quote Originally Posted by gbono View Post
Never did understand why Sunholm used the 0-10 numbering. I guess you get "11" spaces while Fender knobs were only 10
This is actually very simple to understand: with numbers from 0 to 10, 5 is exactly in the middle (it makes easier to manufacture the front panel and to remember settings of the amp). With numbers from 1 to 10, half is somewhere between 5 and 6. With the first case it is also easier to understand setting e.g. gain to 0. Setting the knob to 0 means you get no signal on the output (which is true) while setting the knob to 1 suggest that there is just a little bit signal on the output - more than 0 (which is not true). I would say that scales from 0 to 10 are much more intuitive than scales from 1 to 10 and I wonder why Fender did not think about it.

Enzo 1/10/2016 2:56 AM
I have to admit I never thought about it. I just turn on an amp and spin the controls to where it sounds good. Numbers be damned. I leave this matter to the amp fans.
Justin Thomas 1/10/2016 6:00 AM
Quote Originally Posted by MarkusBass View Post
I wonder why Fender did not think about it.
Probably because the knobs they used had the best bulk discount. And they started using them BEFORE the "bean counters" @ CBS took over.

Mick Bailey 1/10/2016 6:08 AM
Quite recently I'd finished work on a Fender Supersonic and gave the owner a quick demo. I thought it was one of the best sounding amps I'd ever heard when it was finished, and he agreed.

But then he said, "No...hang on - that's all wrong" and adjusted the controls to give a fairly average sound. He asked me why I'd set it the way it was, my reply was that it sounded good that way. He then informed me it was set incorrectly, and there is a mathematical 'rule' that he'd read, and that all Fenders had to be set a certain way. It's a numbers game. Anyhow, that was that.

The way I had it set was described by Matt Schofield;

“You’ll find that every pot has an obvious cusp point in its rotation – every single one – where it goes from not doing much, to where it really kicks in. So I sweep through the pots one by one and find those spots. All amps will do this – bass is especially easy to hear... nothing, nothing, nothing, then bang, there it is! If you do that, the amp is set as evenly as it can be; not too trebly, not too dark. With everything set right on the cusp, you can push it over by digging in, or pull it back by playing softer. It makes everything really responsive and sensitive to what you do. The rest is up to you!”
Justin Thomas 1/10/2016 6:13 AM
Quote Originally Posted by Mick Bailey View Post
and there is a mathematical 'rule' that he'd read, and that all Fenders had to be set a certain way. It's a numbers game. Anyhow, that was that.
I've read 1,000 posts that say treble on 10, mids on 6, Bass on 0. That's the most-often cited "rule" I've heard about Fenders: don't turn the Bass above 2. Tried it once on my Bassman. Friggin' OUCH.

Enzo 1/10/2016 7:06 AM
All this, and considering that the numbers on the knobs have zero relationship to the circuit. They are merely there so the user has a reference point. They could just as easily have labeled the knobs a-b-c-d-e-f-g...
nevetslab 1/11/2016 1:45 PM
Additional details on Power Supply Revisions, Hi Hum level
The multiple postings on the merits of 0-10 vs 1-10 marked control knob legends is interesting, but at the moment, down-stream to the present state of the high hum level I now have, having installed fresh power supply caps and revisions in grounding to this 45 yr old Sunn 1200S Guitar Amp.

I've included some labeled original images of how it was wired up, and the present approach taken to add new filter caps inside the chassis, using the available space around the Tremolo PCB Assembly.


The original wiring had the 20uF/600V Axial Lead cap grounded to a chassis solder lug, mounted to one of the power transformer mounting screws, along with the C/T of the Heater winding. The C/T of the HT winding is connected to the Ground terminal of a terminal strip (aluminum rivet), along with the ground lead of the 1st bias stage filter. The output of the dual 5AR4 Rectifier tubes feeds the S/B switch (routed close to chassis floor), and output of it ties to the 20uF cap, and input to the pair of parallel inductors. In the second image, we see the output of those inducors (brown wires) connect to the 40uF filter section of the 40-20-20-20/525V Cap Can, which is the main filter section of the amp. It's also the ground collection tie points from the Driver Stage, Cathodes of the power tubes, ground from the Tremolo circuit and ground from the bottom of the tone stack. The rest of the circuit grounds of the preamp & reverb driver stages get to chassis locally on their respective terminal strips underneath the output transformer. Secondary of the O/T ground wired to grounded phone jacks, input grounds for the 1st preamp stage made at the 4our input jacks.

I've presently lifted the local grounds of the preamp stage circuits, and brought them to the new Cap Can ground, along with the bias filter caps. The 1st three filter supplies, along with the HT C/T lead connect to the new Ground Stud adjacent to the new Cap Can.


The ripple levels at 120V/60Hz mains are shown below.

Stage -----Tube Rect---Diode Rect

1st---------8V P-P------7V P-P
2nd--------700mV P-P---500mV P-P
Bias 1st----2V P-P
Bias splits--1.5V P-P
Bias wiper--25mV P-P

The rest of the details are in the post I made on 1/9/16, which seems to have been lost in all the dialog around control knobs. I'm not certain that I've made some bad decisions in the placing the new caps surrounding the tremolo board, instead of mounting them to a plate above the floor (underneath the power transformer) or inside an enclosure on the top of the chassis (like Fender does underneath their chassis) and keep the loop area of the filter wiring well contained).

When I had first begun, after slowly charging the original filters over the course of a day, and getting basic measurements of the system, I did give a listen to the amp, and it had high hum level, both 60Hz & ripple, with dried up 45 yr old filter caps. I didn't make detailed measurements then.

At any rate, in it's present state, the amount of hum coupling, coupled ripple and diode commutation noise is overwhelming, so I"m now looking for solutions to that. Amongst these to try is substituting the power transformer with that from my Traynor YBA-1A, being nearly the same VA rating and size, though it lacks a bias tap.

The excitation current on this Sunn 1200S transformer is 215mA @ 120VAC/60Hz, 10.7W. That seems high to me, but, I don't yet have numbers from my Traynor's transformer to compare it to.
trobbins 1/11/2016 2:10 PM
Quote Originally Posted by nevetslab View Post
The C/T of the HT winding is connected to the Ground terminal of a terminal strip (aluminum rivet), along with the ground lead of the 1st bias stage filter.
Does the C/T of the HT winding go directly to the first filter 20uF cap negative terminal, as close to the cap body as possible? Given that you then go through a choke before powering any circuitry, and the choke goes to your can cap, then the can cap negative tab should be your star point for all major current loops - including a negative wire coming from the first filter 20uF cap negative terminal; and the output stage cathode returns. But I'm a little lost with the photos and descriptions of all the various can and axial capacitor connections and multiple chokes etc - are you able to sketch up a simple power supply diagram and photograph it?
nevetslab 1/11/2016 4:24 PM
I'll have to draw up the circuit, but for the moment, I've marked up a Mesa Dual Rectifier schematic to show the HT Power Supply as it's currently wired up.

The Original wiring had the HT C/T tied to chassis at a riveted terminal strip, while the negative lead of the 20uF/600V axial cap was a few inches away. The inductor output went to the cap can, which was the collection point of the grounds of the power amp stage. it is no longer wired like that.

The present 1st stage filter, being a pair of 220uF/300V caps in series, the C/T joins that ground at the chassis stud between the pair of inductors, so no, it's not as close as I could get it to that cap. Those are the Gray caps near the middle of the chassis, along the bottom edge of the Tremolo PCB assy.

I'll sketch out a better drawing the middle of another repair at the moment.
trobbins 1/11/2016 6:30 PM
It may not be a silver bullet, but I can suggest that the key current loops are the PT secondary HT taps and CT going through the diodes to the 2x 220uF series cap. The interconnect wiring should be as short and twisted as practical, and end up connecting as directly to the cap terminals as possible. This is of most importance for the ss diode pathways, as they have much higher peak current pulses. The 2x 220uF cap terminals then would normally become the take off points for the rest of the powering circuitry. It may appear pedantic, but without that commonly applied method to minimise mains rectifier related noise, then you may spend time chasing other red-herrings.

As the circuit uses a standby switch with caps on both sides of that switch, then I suggest that is pretty ugly for the switch, and some series resistance should be with the switch contact to minimise the peak current. If you add 100 ohm then you limit the peak current to 5A, which is still fairly high, but possibly ok. As that 20uF cap is also going to take diode current pulses, it too should be separately wired to the 2x 220uF cap terminals by short twisted cabling.

The original ss diodes were 1N4007. The replacement 1N5406 have a lower PIV, and will let through more reverse recovery current and more primary winding voltage spike, as they are 3A rated. Perhaps series UF4007 would be a better option.
nevetslab 1/11/2016 7:59 PM
Sounds like I'm back to creating a power supply module to mount the caps and rectifiers, getting the input and gnd terminals next to each other (distance just over 1 diameter of the 220's and 100's. I had considered that before....should have gone that route it appears. No worries. Easy salvage job, though mounting the plate may be interesting, since the P/T mtg hardware & P/T has precedence over chassis floor in that area. Love to get the inductors onto that plate, though I doubt I'd have the space. I'll give this some thought.

Perhaps giving up the 20uF/600V cap at the input of the inductor would be wise. Use instead a high voltage film cap, around 47nF? Looks like that was what Mesa used in that circuit diagram I marked up. I'll check to see what the DCR of the inductors is.

I hadn't considered the slower speed of the 1N5406 3A rating letting higher primary voltage spikes thru. Using the UF4007's sounds like a better solution there too. I was seeing a lot of commutation noise in the Tube Rectifier mode.
nevetslab 11/14/2017 5:29 PM
I had set this project aside for a couple months back in Jan 2016, so I could earn funds to keep me going. When I got back to the project in mid-March, 2016, I reviewed where I left off.

After substituting a different power transformer as well as the entire power supply (from my HP 712C Tube Amp power supply), nothing was showing a solution to the hum and buzz I had. I swapped out the slower 1N5406 3A rectifier diodes with UF5407 3A Ultra-Fast diodes, removed the 20uF/600V 2nd stage filter, vastly shortened the supply wiring and installed a rear-panel mounted Diode/Tube rectifier mode switch. Marginal improvement, but no cigar.

I decided to restore the original grounding, which was still easy to do, where all the grounds of the amp got to chassis by way of solder terminals via 45 yr old rivets. When I had powered it back up, the bloody amp was VERY quiet…Well, I’ll be buggered!! The original ‘RF- grounding method used in so many amps over the years, sometimes successful, other times with lots of hum, was in this case the best solution.


I cleaned up the wiring a bit, now moving forward to deal with the Tremolo circuit, which had it’s own high hum problems. The LDR-opto-cell was glued upside down to the PCB in the middle of the chassis, surrounded by the hum field of the HT power supply, was not quite like that shown in the two schematics I had found on this amp. When I had replaced it with a jumper, the preamp residual noise dropped down considerably. So, I ended up moving the opto-cell to sit between the Volume pot and the Treble pot. It ended up with a mu-metal shield folded up and stuffed inside a brass shield that I soldered together, with openings for the connection to the LDR and lamp terminals. Re-installing it into the circuit did raise the noise level a little bit, but it was tolerable.


I increased the filtering of the power supply that ran the 3-transistor oscillator circuit, which got that quiet. Next was the Reverb return circuit. The original single-transistor circuit was horrible, so I replaced it with a an op amp stage, and mounted it atop a standoff on the Tremolo PCB. The original Gibbs reverb tank worked, did have the locking mechanism for transport, but….it was dark and murky sounding. After borrowing an Accutronics tank from a Fender Twin, that sounded much better, and ordered one that was suited for face-up mounting to the inside roof of the cabinet (4AB3C1A). I ended up tweaking the voicing caps on both the drive and return circuit, as well as lowering the gain of the return circuit.

That left the input circuit. The original input jacks were not grounding-type, so with nothing plugged in and the volume turned up, it was noisy. I finally came up with configuration that kept the original Bright Filters as well as the Normal inputs, together with an attenuated set of jacks, all that ground when nothing is plugged in so the amp was quiet. That required using a pair of Switchcraft type 13A Transfer jacks added switching contact) along with the grounding-type12 jacks. I will load the entire amp circuit into my schematic capture program, so there will finally be a complete revised schematic coming, and will post that when done. I still haven’t found a schematic that shows the 3-deck pot Contour circuit. For some reason, the pdf sketch of the input jack wiring isn't coming up properly, so I'll have to fix this later


I finally got this amp completed in late May 2016. It came out really nice, both in performance, sound and appearance. Apart from increasing the power supply capacitance, and minor mods to the Reverb circuit, the tone circuits, gain stages and output stage remained stock. Was it worth the 6 months of agony and labor I invested in it? Not financially, to be certain, but was worth the effort to see one of these nicely restored
nevetslab 11/21/2017 1:20 PM
Sunn 1200S Revised Guitar Amp Schematic 2016
I finally got the revised 1200S Schematic composed on National Instruments Multisim 11.0, the schematic capture program I use the most. There are a number schematic symbols that are clumsy..the T/S + Normal jacks don't show the Normal contact shorting to the input leave spring, and they didn't have any T/S Transfer jacks (Switchcraft # 13) which I used on the input jacks together with the # 12 T/S+N jacks to solve the automatic grounding of the inputs when nothing is plugged in. The 4 input jacks now have 0dB Normal & Bright, along with -6dB Normal & Bright inputs, with the -6dB Bright curve being a bit different than the 0dB Bright, just for added voicing. There was originally a 470pF Brightness cap across the Volume pot, which is now on a Pull Switch of the Volume Control.

I added a second Bias Adjust pot so the upper and lower output tubes have their own bias adjustment. I Also added a rear panel Rectifier Mode switch. With 5AR4 rectifier tubes, the difference isn't that different, but with 5U4GB's, there's a greater voltage drop in the rectifier tubes. I never got arund to trying it with the 5U4GB's, which is what you find on the Mesa amps having that feature.

Somehow, by the time I finished the project in late May 2016, I never recorded all of the supply voltages thru-out the amp. The Voltage taps thru the power supply decoupling stages were calculated from the plate current values I got from an earlier schematic. In looking at my before and after photos on the original 4-section power supply cap can, it looks like there was a 100k decoupling resistor to the preamp tube section, instead of a 10k, which was shown on that earlier published schematic. Though some other photos I took of that original section, it does look like a 10k 2W Carbon Comp resistor WAS installed.

The schematic program didn't have power and output transformer symbols that were complex as is used on this amp, so I had to cobble symbols together to best-approximate them. I also saw in my notes that I never measured the inductance of the two chokes in the power supply.