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Thread: Another mystery amp

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    Another mystery amp

    Hi Guys,
    New to this forum. I have posted this on other forums, searched the Net etc... No luck!! Please can someone try and identify this amp in the hope of obtaining a schematic. 1x12 combo. 2x 12AX7's, 2x 6v6 and 1x EZ81.
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    Lifetime Member Enzo's Avatar
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    I have no idea who made it, but it is a simple amp, four tubes. I don't count the rectifier, we all know what it does. A push pull output, we know what that ought to look like. Two channels. One of the tubes is the phase inverter. SO I bet if you sit down with a pad of paper and a pencil, you could draw up a schematic from the circuits in half an hour tops. If you are familiar with the circuits, and don't have to look up the tube pins, hell, 10-15 minutes.

    I don't know where you are - really ought to include at least your nation on the profile - but since the power switch is labelled "mains" I assume this amp is not from the USA.

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    Quote Originally Posted by Enzo View Post
    I have no idea who made it, but it is a simple amp, four tubes. I don't count the rectifier, we all know what it does. A push pull output, we know what that ought to look like. Two channels. One of the tubes is the phase inverter. SO I bet if you sit down with a pad of paper and a pencil, you could draw up a schematic from the circuits in half an hour tops. If you are familiar with the circuits, and don't have to look up the tube pins, hell, 10-15 minutes.

    I don't know where you are - really ought to include at least your nation on the profile - but since the power switch is labelled "mains" I assume this amp is not from the USA.
    Thanks for the reply,

    1. I will update my profile. Sorry this was my first post. I reside in South Africa. Also I am new to this addictive hobby and have no electronic background.
    2. I will attempt to draw this circuit. Will be a good exercise. Don't feel too confident drawing it,,, but hey.. I have to learn.
    3. I doubt this is a push pull amp. I'm thinking it is a PSE, 5 or 10w (although the OT is small for SE!). Also, this amp has a shitload of gain (hardly any cleans to speak of....even with single coils). So... Surely all this gain can't come from 1/2 a tube? Huge controlled feedback (would make a great rock recording amp).

    Will keep you posted

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    I don't want to be presumptuous, but you COULD have 1/2 AX7 each as a first gain stage per channel, then a second shared gain stage, then the tone stack, then a third shared gain stage right before a simple Cathodyne phase inverter, driving two Push-Pull 6V6s... main point, you could potentially get three gain stages out of it... since you actually have FOUR triodes to play with. But we'll know once you can draw it out.

    Justin

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    Lifetime Member Enzo's Avatar
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    It is conceivable the two 6V6s are in parallel and thus single ended, but far more likely to be a push pull pair.

    Pull up a schematic for an old Fender Deluxe, like the 5E3 or 5D3. I bet your circuit looks very similar.

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    Thanks guys,
    The reason I suspect PSE, is because of the perceived power output. I have a 1961 vintage Epiphone (25w), which is WAY louder!! Will be great if I have a basic 5E3 circuit here. That was going to be my next build. So this would be a good donor chassis. The amp voicing is definitely NOT a Fender type voice, so will redo the tone stack. Actually, once I have checked the voltages, I will determine weather this is worth stripping or just modding.

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    Lifetime Member Enzo's Avatar
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    Loudness isn't a good indicator of power. Doubling or halving power is only a 3db change. Speaker efficiency can have just as large an effect.

    Easy enough to determine. Are the two tube sockets wired in parallel or not? Are there three primary wires or two on the OT?

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    I will try get in there this weekend and revert. Thanks for the input guys.

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    As promised, here is the schematic. There are probably errors.. But you get the idea. I do believe it has a STRANGE tone stack arrangement with the 2 volume controls (instrument and microphone).
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    Last edited by Andresound; 12-19-2016 at 06:33 AM.

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    Have never come across a phase inverter like that before, but I think the 22n feeding the lower 6V6 should come off the PI plate, not the grid.

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    You are correct. My mistake, the 22n is supposed to come off the the plate of the v2b, which is feeding to the "inverted" 6v6. See updated schematic. Yes, the phase inverter is strange, but I wouldn't know...I am new to this game.. Also, weird tonestack (I think).
    Would a bypass cap on the common cathode resistor increase power output?

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    Last edited by Andresound; 12-19-2016 at 07:52 AM.

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    A bypass cap for the output tube cathodes won't increase power output, just gain.
    So it will get to full power at a lower setting on the volume control.
    Power output is pretty much set by the capability of the power supply (power transformer).

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    That phase inverter is called a "paraphase". It takes advantage of the fact that a standard common cathode stage inverts the signal, and uses the signal driving one of the power tubes as the input to another triode which creates the inverted signal for the other power tube. It looks like it's using negative feedback to control the gain so that both sides see roughly equal drive, but I bet it's not actually all that equal which is probably a factor in why, as you say, the amp sounds like it has a shitload of gain.

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    It doesn't look like the "normal" way I've seen a Paraphase drawn, but I know that in either RDH3 or RDH4, there are a good 6 or 7 ways of phase splitting or inverting that are not seen commonly in guitar amps. I guess these days, there are only 2 in heavy use - Cathodyne & Schmidtt (LTPI)? Given the pros and cons listed for each, some of those unused ones sound like they might be worth considering in guitar amps. Fidelity? We don't need no stinkin' fidelity! So I shouldn't be surprised that a Mystery Amp has a Mystery PI.

    Justin

    To clarify: why it didn't look like a Paraphase to me is because it looks as though the full amplified signal of the first stage is passed straight to the grid of the inverting tube, without the usual voltage divider to knock the incoming inverting signal to the same level as the incoming non-inverting signal... weird, for sure!

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    I believe the feedback should go to the cathode of the first triode that forms the driver stage pair.

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    Quote Originally Posted by Justin Thomas View Post
    It doesn't look like the "normal" way I've seen a Paraphase drawn, but I know that in either RDH3 or RDH4, there are a good 6 or 7 ways of phase splitting or inverting that are not seen commonly in guitar amps. I guess these days, there are only 2 in heavy use - Cathodyne & Schmidtt (LTPI)? Given the pros and cons listed for each, some of those unused ones sound like they might be worth considering in guitar amps. Fidelity? We don't need no stinkin' fidelity! So I shouldn't be surprised that a Mystery Amp has a Mystery PI.

    Justin

    To clarify: why it didn't look like a Paraphase to me is because it looks as though the full amplified signal of the first stage is passed straight to the grid of the inverting tube, without the usual voltage divider to knock the incoming inverting signal to the same level as the incoming non-inverting signal... weird, for sure!
    There is a possibility I have drawn it wrong. I will check

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    Lifetime Member Enzo's Avatar
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    I wouldn't worry so much whether it looks "correct" for a certain type PI, but rather will the circuit work. Ther may be better ways or a "proper" paraphase might be different, but they could have purposely or ignorantly have designed it different. But it needs to drive one power tube with a signal and the other with its invert. SO we need to see if it will.

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    Quote Originally Posted by Enzo View Post
    I wouldn't worry so much whether it looks "correct" for a certain type PI, but rather will the circuit work. Ther may be better ways or a "proper" paraphase might be different, but they could have purposely or ignorantly have designed it different. But it needs to drive one power tube with a signal and the other with its invert. SO we need to see if it will.
    Hi Enzo,
    This circuit actually was a working amp. The cabinet was cheaply made, so I didn't expect much. Very distorted and no 'balls" with no sparkle/chime. Could very well have been "ignorantly designed". The HT is a low 290v. What do you guys suggest I change to temporarily improve matters. I will eventually use this chassis to build a 5e3 design. I already have OT and MULLARD OS 5y3 recto lying in waiting. Need to get a PT (325/325) in the new year.

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    OK.. Have finally got a PT (unfortunately it's a toroid), but it will do. Stripped chassis and done a temporary board for 5E3.
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    How did the amp repair come along? All good and grafting?

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    Old Timer olddawg's Avatar
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    From the tube complement he listed earlier it looked like an 18 watt Marshall setup except with 6V6s instead of EL84s and kind of a low B+. I would have put in an SS rectifier and made a Spitfire or something similar out of it.

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    Hi Guys,
    Been a while. I eventually built a 5E3 type amp into the cab. Was a quick rebuild on existing board. Amp worked, but was noisy. I have since built up a new turret board, so will revisit this project and post. I am thinking of a Tweddle Dee type 5E3. Thank you for all the help.

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