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Summing the output of two identical tube amps into one cabinet

Discussion in 'Amps and Cabs [BG]' started by Colonel Monk, Dec 17, 2011.

  1. OK, hear me out.....

    I've got a power amp, Peavey Classic 120 / 120 that is a dual mono tube amp which is bridgeable to 240W.

    It seems that the mono/dual switch is simply connecting the outputs of CH1 and CH2 together. When bridged you use the outputs for CH1 but at 240W instead of 120W.

    I ask, because I also have two identical Peavey Classic 120s which are the single channel 120W version, and began to wonder if a parallel speaker cable was made why wouldn't one be able to use them together in a single large cabinet like an 810.

    Fully aware this would be a bad idea with SS amps, but seems that tube amps play by different rules in some ways I don't quite understand due to transformers on the outputs.

    Looking for advice from people who know their tube amps, not so much opinions from laymen why you'd never try it. Relax, without some scientific advice saying it will work I don't intend to try it.


  2. You just sent me on a search for the schematic. No tube amps don't play by rules that are THAT different. I didn't find the schematic but the OT's would have to be closely matched, the left channel would be positive swing and right channel negative swing (in bridged mode only the channel one controls are active). How they interconnect the OTs is what interested me (unless they share the same core).
  3. Yeah, I should have a long hard look for both schematics. I will need to have them looked at sooner or later....

    The 120 / 120 has two separate sets of power and output transformers, but the two quads of 6L6GCs share the same board. The output board is a daughter board with all the the speaker outputs and the mono/dual switch and impedance selectors mounted to it.

    The output transformers are both connected to the daughterboard directly with molex connectors.

    Daughterboard has no components but jacks and switches on it.

    So it does look like they are just paralleling the outputs with the switch without any new devilry.

    My application:

    I have 4 different practice locations if you include home, and so modularity in my gear is a new kick I'm on, to have what I need in all locations but also be able to bring them together for a gig.

    So the 120/120 is at practice space in San Francisco and can handle gigs alone. One mono 120 is at guitar players house, the other at home or at the other practice place with the 810

    So if this works, I can bring the two 120s together with an 810 I use or can also use them seperately with the Scout cabs I have.

    I've gotten to the point (and with a bad knee, having surgery in the new year) that playing 2-3 times a week I just want to power down and go home and not break gear down and move it around constantly. I'm sure we can all agree on that. It's worth it to me to invest a little more in my gear for that luxury.

    C. Monk
  4. I hear you on this, but I don't quite understand the "swing" thing.

    Here I would appreciate a layman's explanation.

    If I was going to gang the 120s together I'd likely replace the tubes together first but it would be tough to think that the output of the two would be that close together.

    But does it matter THAT much?

  5. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    bridging flips the polarity of one amp and sets it in an opposite, "push-pull" mode with the other amp. i think they have to have a common negative to reference their voltage swings off of, so i don't see two individual amps being somehow bridged together.
    you'd have much better luck wiring that cab to be two separate 4x10s, then hooking up one amp per 4x10 section.
  6. Hi Walter.

    Thanks. That sounds plausible.

    I may be able to rewire the 810, it's not mine.... but the new 810s are wired like this in a more flexible way, I believe you just use a small cable to combine them into a single 4 ohm load.

    I'll look into that.

  7. With my very limited knowledge of amps I'd say you're barking up wrong tree to think it's even feasible. I don't see how a tube amp is so different from a ss amp that loading the outputs against one another can come out with anything other than fried amps.

    What's the input impedance of a tube amp output? Bet it's less than a speaker cab by a long way, instant fried amp.
  8. teemuk


    Mar 1, 2011
    I'm not quite sure what the OP is even trying to do but generally bridging amps requires that the two amps operate almost identically (aside from one of them amplifying a phase-inverted signal) and ensuring that identical operation is way more difficult with tube amps than with solid-state amps. Unless the manual specifically says you can safely bridge the amplifier to another of similar model I wouldn't even dream of doing it. ...Unless I want to see a brief and somewhat unspectacular lightshow enhanced with puff of smoke and nasty smell.
  9. Right, with your "very limited knowledge" of amps....

    Did you read the other posts sir?
  10. Teemuk,

    Clearly you didn't read the first post all the way thru OR the other posts in the thread.

    I don't care what you would dream to do, some of us think outside the box.

    FYI, the first amp mentioned was designed for bridging and is, in fact, a tube amp.

    A little further in the thread I got some scientific info why that works and why it might not work for my pair of 120s, but I guess you didn't read that.
  11. teemuk


    Mar 1, 2011
    But it’s not just that. The designer of that amp also made an attempt to equalize the gain by driving both output sections from a single input stage, that including also the phase inverter section (see how the switch toggles a relay for that function). This gain equalisation is very important because parallel amps are just as delicate to close matching as bridged amplifiers. In fact, they may be even more troublesome. As I said, you need to be careful in ensuring both amps operate about identically. This is Peavey’s method to do so.

    Yes, both paralleling and bridging can work with tube amps. Never said it wouldn’t, but the issues from mismatches are far greater than in solid-state systems due to much wider tolerance spread of components.

    So, matched in the way of 120/120 scheme the output paralleling works at least in the particular model, but you will run into issues if you simply try to parallel two individual amps by connecting their OT secondaries together. That of course unless you figure out ways to ensure the voltage gains are about the same, as well as parameters like output impedance. That is quite difficult in solid-state systems, even more difficult in tube systems. With the wide spread of component values in the input stages and PI’s, and wide spread in tube gains overall the thing is not as simple as just paralleling two Classic 120’s.

    Thinking outside the box is great, but realizing the limitations and troubles of different types of “experiments” (to which manufactures often advise against of) can save a bunch of expensive equipment. You asked for a question and got some answers to it, why the smug attitude?
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Use a dual driver section cab, each with it's own jack. Or use two cabs. True bridging can be done, but it requires a skill set that you do not possess, and isn't worth the bother anyway,
  13. My intent was not to be a jerk but I deliberately tried to isolate the thread of this sort of alarmist "your idea is dumb and you don't know what you're doing" "I don't know why you'd want to do that" "you'll fry your amps" kind of replies.

    This seems to be a recurring theme in the amps forum, Where people jump into a thread after they've read the title, tell the OP they don't know what they're doing (in so many words) without reading the thread or offering any real help or solution the OP was searching for. Whereas in the effects/bass forum people are more accepting of strange ideas that creative musicians often come up with.

    I asked for input from people who really KNOW tube amps not from people who have an opinion about how they work.

    Further, I stated I had no intent of trying anything without clear knowledge from an expert that it would work.

    I got a few decent replies at the beginning of the thread, both explaining that "bridging flips the polarity of one amp and sets it in an opposite, "push-pull" mode with the other amp. i think they have to have a common negative to reference their voltage swings" and this indicates it's not as simple as connecting the outputs.

    This is the kind of information I was looking for.

    I'm not trying to chase people away from TB, but sometimes it's not the friendliest place because people don't actually read an OPs post or the thread before offering judgement.

  14. Actually they will have a common zero point (0.0v), one "side" will handle the positive swing from 0.0v and the other side will handle the negative swing from 0.0v . Music or instruments {edit}produce AC voltages which have a positive and negative voltage voltage "swing" from a zero reference point be it a sine, square, triangle wave or any combination of those. :)
  15. +1 Yet again I agree with Bill. While it can be done you have to know precisely what you are doing.

    If you remember Bill, Norm Thagard slaved a few tube stereo amps together in an article in Ax or AA magazines. I hope my memory has got the right name! :)
  16. Further....

    Right, it was explained that it's not that simple as making a cable early in the thread. I don't have reading comprehension problems, I got that that from the posts from walterw and B-string...

    I have to say, I'm always surprised by assumptions people make about how "delicate" tube amps are and how there's all this magic happening under the hood. Really? It's the oldest amplification technology there is other than the phonograph.

    It's been around more than 2x my lifetime. It worked well wayyyyy back when there probably wasn't any kind of decent component tolerance. 3 of 4 of these peavey tube power amps I recently acquired have the original tubes and are 20 years old and they still work.

    They aren't hand-wired, they were made for production without alot of human interaction, and there's no adjustments in the circuitry that one can see.

    But you made a point of saying multiple times how delicate and necessary it is for everything to be so closely matched, and that it's quite difficult to do bridging...

    Well either:

    1) it's not that important


    2) it's not that difficult

    I'm going to go with number 2, because there are thousands of SS amps on the market that support bridging and do it flawlessly, and because manufacturing of electronic components such as are in modern amps is pretty much old hat. Something I just grabbed from the internet:

    "thin film resistor tolerances of 0.1, 0.2, 0.5, or 1%"

    Does that sound like there's that much variance in the components? Not really.

    Sorry, it just felt like I was being lectured once again how I take for granted how delicate my amps are. But I own a bunch of old amps that just keep on taking abuse and they run just as designed and ask for more. And many people here have tube amps that are 40 years old and really didn't require much maintenance in those 40 years.

    I'm a mechanical engineer, and I only wish that parts I design had the low low tolerances and low cost of mass-produced electrical components. I should have been an electrical engineer, you get to create your bomber designs using off the shelf components, and in my experience in capital equipment, the EE generally goes home after 8 hours and the ME has to work 10 to even come close to staying on schedule.

    Anyway, apologies for the rant but the lecture about how complicated amps are is just what I was trying to avoid.

    There's alot of good info here but it can be frustrating to sift thru the opinion.
  17. Thanks, clear and concise = :cool:

    I'll continue to learn, if you'll continue to teach.
  18. Right, +1 to Bill as he *does* know what he's talking about.

    Let's not forget, just because we talk about doing things doesn't mean we WILL do them.

    In his opinion it might not be worth doing, but that doesn't mean it's not worth talking about. After all, this is TALKbass.

    So someone did it before, and that means it's *possible*. Whether or I not I possess the skillset is irrelevant, if I already had the skills I wouldn't be asking you guys.

    So anyway, I continue to be open to discussions about how it could be done or in theory what would need to be done, regardless of the practicality... It wasn't that practical to go to the moon either but we did it anyway....
  19. seamonkey


    Aug 6, 2004
    Depending on the schematic
    You can sum the output of two tube amps by wiring them in series - which doubles the voltage to the load.
    They need isolated grounds, and internal feedback should be on the primary of the OT.
    Two 8 ohm taps wired in series needs a 16 ohm load. You'll need to rewire the cab. Go back to what Bill says and rewire the cab for two amps.
  20. to bridge two amps you feed one with a signal 180 degrees out of phase from the other. This will drive the output of one amp positive while the other is driven negative. This is true for SS too. The load is connected across the output terminals same a SS. Ground is the common for both amps. If you are driving an 8Ω load each of the amps should be set to 4Ω as the amps are in series.

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