kka 7/27/2018 6:45 PM
2 Tube Guitar Amps into 1 speaker ?
Hello folks, I had a discussion with a friend who runs 2 tube amps simultaneously into one 1x12 cab. He built a device that allows the 2 amps' speaker jacks to be connected to that same cab and he connected his guitar with a Y splitter to both input jacks. So he blends the 2 amps into the same speaker. I had strong reservations if this would cause damage to the amps, but he said he's been playing them like that for a while without issues.

I'd be curious to know if someone could technically explain why this setup either can work, or why it's a bad idea and could cause damage.

Thank you kindly for your time and input !
 
g1 7/27/2018 6:58 PM
Both amps are definitely running into the speaker at the same time? Or he built a switch box and only one amp is connected to the speaker at a time?
Both amps driving the speaker at once should make trouble. If not now, at some point. They will at times be fighting each other.

Ask him how the circuit he designed works. If he can't give a reasonable explanation, then he's not qualified to comment on the reliability either.
 
Enzo 7/27/2018 7:03 PM
I knew a guy who got totally drunk every night at the bar, then he drove home. He did this every night for years. So does that mean it is an OK idea, because he got away with it?

Connecting the outputs of two separate amps to the same speaker is like hooking them to each other. And it is a VERY bad idea.


At lower levels, he can get away with it because the amps are not pushing hard. Also if he Y's the guitar into both amps and they happen to have the same phase relationship, both outputs tend to be similar. What can happen is when any DIFFERENCE exists between the outputs, those voltages oppose one another. Imagine one output trying to go to +20v connected to another whose output is trying to go to -20v.
 
kka 7/27/2018 7:24 PM
Yes he basically has both amps' OT secondaries connected and then both amps go into 1 speaker. He said that he makes sure that the amps are in phase.

So if this is indeed a bad idea I would like to technically understand why it is and what exactly will happen to the amps in terms of damage. I'd like to relate that to him. Thank you
 
Lauri 7/27/2018 9:37 PM
There are safe ways to do it but connecting speaker outputs of two amps in parallel is a bad idea. Maybe he has the OT secondaries connected in series or his mixing device has dummy loads and he mixes the signals together with power resistors.
 
J M Fahey 7/27/2018 10:38 PM
Waste of time speaking to stones, let the amps eventually explode, that will *technically* explain him why.
 
kka 7/27/2018 11:50 PM
Quote Originally Posted by J M Fahey View Post
Waste of time speaking to stones, let the amps eventually explode, that will *technically* explain him why.
I would like to understand this. Could you please explain? Thank you.
 
DrGonz78 7/28/2018 1:04 AM
My cousin is one of the smartest people I know... However... He once told me a story about working in a hi-fi audio store and he decided to parallel two Harman Kardon power amps together with the same speaker system. Now this is a bit different scenario than tube amps and it did not even last a second. He blew out both power amps, probably just turning them on really. Anyhow he felt like a jackass and had to explain it all to his boss. I don't think he worked there very long. Still he is one of the smartest people I know, but even the smartest are still capable of doing the stupidest things.
 
Enzo 7/28/2018 1:14 AM
Smartest people are not always the wisest.
 
Helmholtz 7/28/2018 7:35 AM
Quote Originally Posted by kka View Post
I would like to understand this. Could you please explain? Thank you.
Maybe it helps to realize that OTs are not one way devices, rather they work in both directions. This means that the output voltage produced across the load appears stepped up by a factor of say 25 at the primaries of both amplifiers' OTs (resulting in up to several hundreds of volts), even if only one of the amps is driven and may easily exceed transformer and tube ratings with both amps delivering power. The risks of this misuse are hardly predictable.
 
olddawg 7/28/2018 7:12 PM
I used to work for a LARGE stereo/electronics store. One of the most difficult things was maintaining the switching matrix for speakers and amplifiers in the demo rooms for both home and auto systems. The salesmen could destroy anything. As far as your friend goes.. many players use a “split” stereo cab and 2 heads nowadays. Usually it’s a 2x12” or a 4x12” wired stereo and they “blend the tones” that way. Hooking two amps to a single load is asking for disaster. The exception is a “bridging amplifier” setup... but that isn’t what you are describing.
 
glebert 7/28/2018 10:36 PM
I guess I can't figure out why anyone would do this. Then again, I think if you need to blend two amps to get your magic sound, maybe your magic sound is not reasonable.
 
Randall 7/29/2018 12:40 PM
For the same reason you don't stick your tongue in a wall outlet or piss on an electric fence.
 
nosaj 7/29/2018 12:44 PM
Quote Originally Posted by glebert View Post
I guess I can't figure out why anyone would do this. Then again, I think if you need to blend two amps to get your magic sound, maybe your magic sound is not reasonable.
Doesn't magic smoke make magic sound?
nosaj
 
GainFreak 7/29/2018 12:51 PM
From what I can remember Peavey Classic 120/120 in mono mode is running both channels in parallel at half the impedance so maybe yes, you can run two amps in parallel but under certain circumstances.
 
Chuck H 7/29/2018 12:57 PM
Quote Originally Posted by GainFreak View Post
From what I can remember Peavey Classic 120/120 in mono mode is running both channels in parallel at half the impedance so maybe yes, you can run two amps in parallel but under certain circumstances.
I don't have that schematic, but it's likely that since both OT's are in the same chassis Peavey opted to parallel the OT's at their primary windings (good) rather than at their secondary windings (not good).
 
GainFreak 7/29/2018 1:19 PM
Actually they paralleled the secondaries but they are running the power tubes from the same preamp/PI. Below is the schematic.

[ATTACH]49906[/ATTACH]
 
J M Fahey 7/29/2018 1:54 PM
Yes, power tube grids are in parallel.

So NOW it is absolutely guaranteed that current through tubes is in perfect phase from DC to a couple MHz ... try that with separate amplifiers fed a guitar signal at the preamp input .
 
christarak 7/30/2018 2:37 AM
Quote Originally Posted by J M Fahey View Post
Waste of time speaking to stones, let the amps eventually explode, that will *technically* explain him why.
Oh Juan! You made me laugh!! "speaking to stones"...love it
 
drewl 7/31/2018 1:52 PM
Bogen tube power amps were designed to be paralleled together for more power.

They call it Cathode paralleling. But they are identical amps being run together.
 
loudthud 7/31/2018 2:47 PM
I just Googled it. Turns out there is a Metal band called "Speaking to Stones" and they have a CD available on Amazon
 
Jazz P Bass 7/31/2018 4:05 PM
Quote Originally Posted by drewl View Post
Bogen tube power amps were designed to be paralleled together for more power.

They call it Cathode paralleling. But they are identical amps being run together.
I like that way the they 'jumper out' the 10 ohm bias read resistors.
 
g1 7/31/2018 7:23 PM
The 2 examples given (PV Classic 120 and Bogen MO100) show the great lengths they go to in order to ensure that both matched power amps and OT's get the exact same input signal from a single preamp.
Like JM explained,
Quote Originally Posted by J M Fahey View Post
it is absolutely guaranteed that current through tubes is in perfect phase from DC to a couple MHz ... try that with separate amplifiers fed a guitar signal at the preamp input .
This is pretty much the opposite of what was presented in the first post, where the owner wants to blend 2 different signals from different preamps and poweramps.
It is the blending of dissimilar signals that is the problem, which causes the amps to work against each other, and will likely result in failure of one or both amps.