frankeg 6/14/2018 4:18 PM
Silvertone 1333 tube amp
I have a 1956 Silvertone model 1333 to re-cap and I found 2 pots wired as variable resistors between the plates of the 6SC7 & 6SN7GT preamp tubes and their power supply source voltage. The schematic I have shows normal resistors there. Could this be someones mod?
 
galaxiex 6/14/2018 5:14 PM
I would say that's a mod.
Someone trying to tweak something, or didn't have the correct resistors on hand.
 
olddawg 6/14/2018 10:57 PM
Those pots are not from 1956....
 
Enzo 6/15/2018 1:11 AM
Most pots have a date code on them.
 
frankeg 6/15/2018 5:04 AM
Thanks guys. The only thing these pots would tweek is the two preamp tubes plate voltage. I haven't worked with 6SC7 or 6SN7GT tubes but I'm guessing he was playing with their bias for whatever sound he wanted. It still seems a onnce and done thing since you have to take out the chassis to tweek.
 
frankeg 6/15/2018 5:11 AM
Here's the schematic
Here is the schematic. The pots are wired to pin 5 of each preamp tube.
 
g1 6/15/2018 12:58 PM
I'd think someone was tweaking the plate voltage (while disregarding the bias, which seems common ) as you mentioned.
If the pots are rated 1/2W or better, I suppose you could leave them as is.
Out of curiosity, how far off are the set values from the stock values shown on schematic?
 
frankeg 6/15/2018 1:13 PM
You are correct they are CTS pots made in 76 & 77.
 
Chuck H 6/15/2018 1:20 PM
Quote Originally Posted by frankeg View Post
The only thing these pots would tweek is the two preamp tubes plate voltage.
Well, no. They also change the plate load and that changes everything. Gain, current, bias, impedance AND voltage. With a pot as a plate load, and no other circuit parameters considered, it would be very easy to dial in an operational circumstance that is clearly undesirable (at best) or even dangerous (at worst).
 
frankeg 6/15/2018 1:21 PM
I'm taking it back to stock. I don't care for his soldering either. He told my friend that he's had it since the 60's and it's completely stock. That doesn't jive with CTS pots from 76 & 77. I'll let you know the values once I desolder which will be a week or so when the new parts arrive. The more I see of these old amps the more I liken them to a box of chocolates, you never know what you're gonna get...
 
frankeg 6/15/2018 1:58 PM
Quote Originally Posted by Chuck H View Post
Well, no. They also change the plate load and that changes everything. Gain, current, bias, impedance AND voltage. With a pot as a plate load, and no other circuit parameters considered, it would be very easy to dial in an operational circumstance that is clearly undesirable (at best) or even dangerous (at worst).
Hi Chuck! Hope all is well. You always have the right words to expand and clarify what I'm trying to say and yes I totally agree with you! Thanks
 
mozz 6/15/2018 2:05 PM
Those pots are out of a color tv set.
 
frankeg 6/15/2018 7:09 PM
Quote Originally Posted by mozz View Post
Those pots are out of a color tv set.
Hence the blue and red colors right? Makes sense. Thanks for the insight!
 
Chuck H 6/15/2018 7:31 PM
Quote Originally Posted by frankeg View Post
Hence the blue and red colors right? Makes sense. Thanks for the insight!
Oh I gave it a right away because I remember seeing these pots in TV's. I've never serviced them, but I took a few apart It didn't click before mozz mentioned it and then I got a little kick out of it. Fun.
 
g1 6/16/2018 1:10 PM
Quote Originally Posted by frankeg View Post
You are correct they are CTS pots made in 76 & 77.
Well I think Sears stopped branding TV's with the Silvertone name around 73. So I guess my hopes that they might still be official 'Silvertone' parts are dashed.
 
frankeg 7/25/2018 6:33 AM
Quote Originally Posted by g1 View Post
I'd think someone was tweaking the plate voltage (while disregarding the bias, which seems common ) as you mentioned.
If the pots are rated 1/2W or better, I suppose you could leave them as is.
Out of curiosity, how far off are the set values from the stock values shown on schematic?
Sorry this took so long (Summer activities). The blue pot was wired between pin 5 (plate) of the 6SC7 and hot rail and measured 13k ohms out of circuit. Pin 2 (2nd plate) of the 6SC7 had a 560k fixed resistor. The stock 390k resistors (R7 & R8) were removed. The red pot was wired between pin 2 of the 6SN7GT (plate of 2nd pre) and the hot rail and measured 7.31k ohms. The stock 68k resistor (R19) was removed. Keep in mind that I have no idea how it sounded because it had bad caps in it when I got it.
 
frankeg 7/25/2018 6:38 AM
Quote Originally Posted by frankeg View Post
Here is the schematic. The pots are wired to pin 5 of each preamp tube.
Could someone tell me the purpose of C20 tied to the primary winding & ground? Is this for the 2-prong pwr polarity like Fender early Fenders had?
 
Chuck H 7/25/2018 7:22 AM
Quote Originally Posted by frankeg View Post
Could someone tell me the purpose of C20 tied to the primary winding & ground? Is this for the 2-prong pwr polarity like Fender early Fenders had?
It's called a "death cap" in modern parlance. That's because if it shorts you have a hot chassis. It's purpose is to reduce noise in the power supply. If you're doing things right the amp should get a proper, chassis grounded three prong AC mains cable and that cap should be removed.
 
frankeg 7/25/2018 7:26 AM
Thanks Chuck! I thought so but it's arrangement looked different than what I've seen in Fenders. It will be removed and a proper 3-prong cord installed.
 
Justin Thomas 7/25/2018 7:29 AM
Off-Topic: glad I can always provide a sig line or two, Chuck!

Justin
 
Chuck H 7/25/2018 7:34 AM
Quote Originally Posted by Justin Thomas View Post
Off-Topic: glad I can always provide a sig line or two, Chuck!

Justin
It was "computers masquerading as amplifiers" that got me For me that's like putting a ball hitch on your smart car and calling it a tow truck
 
frankeg 7/27/2018 9:34 AM
Well it's all back together and working except that the tremolo is weak. I was hoping on getting away without messing with those caps since they're really hard to get at. Do you think anyone would notice if I used .015 uf caps instead of the stock .02 uf? I have a few .015's handy.
 
Enzo 7/27/2018 9:44 AM
There aer two parts to any trem: the oscillator (LFO) and the interface.

The LFO is the part with the three 0.02uf feedback caps, and it makes the pulsing we use.

The interface is whatever circuit we use to couple that pulsing to the signal path in order to modulate the signal.

If your trem is working, as in making trem, then the oscillator is working. Changing the three caps won't change that, it already works. Changing them from 0.02 to 0.015 will in fact not make the trem stronger, but it will peed the trem up. In my experience, many if not most amps have a trem that is too fast already. I want to be able to turn the speed down to a sexy throb. Many of them have a slowest setting that is still too fast. SO I swap in larger feedback caps, not smaller.

But a weak trem is usually in th interface. or the design is for a weak trem effect in the first place.
 
g1 7/27/2018 11:47 AM
Agree with Enzo above, look at the second half of the trem tube, not the oscillator side.
Have you tried another 6SJ7 tube there? R30 & R31 are good?
C16 and C17 caps ?
 
frankeg 7/27/2018 1:42 PM
Thanks Enzo/g1. I listened to another 1333 on youtube and the tremolo was strong. I'll check the output side. I don't have another 6SJ7 to try. I did change out C16 because it was in bad physical shape. Keep in mind that with my old guitar ears, I may be imagining the trem effect, it's that subtle! So I may still change some caps on that side. I found two .22uf and one .33uf that I can use.
 
Enzo 7/27/2018 1:49 PM
C17 is the one that delivers the trem pulses to the intensity pot. I'd sub something in there if you have not already. Is there any DC sitting on the top end of R32?

Turn intensity pot all the way up. Now measure resistance to ground from the grid of either powr tube. You SHOULD sdee about 1.3M. If you see only 300k, then the trem switch is either ON or shorted.