Trainwreck type 2 ppimv

[taiwanluthiers]

So for the AB763 build I'm deciding on a ppimv for it, I'm reading about the type 2 trainwreck ppimv which replaces the screen resistor (it's the 1/2 watt 220k one) with a 220k dual gang pot  However the ones it shows are 2 power tubes 50 watt amps. I don't see it show 4 tube types and is it even possible to do this on a 4 tube amp? Or do I need a 4 gang pot (do they even make those?)

[Bieworm]

Can you post a schematic? Normally there are only 2 x 220k grid load resistors on a LTP PI, even in 4 power tube amps. This MV type replaces those resistors. Doesn’t your amp have te same PI? From the junction of the coupling caps/ grid load resistors the grid blockers go to the grids of the power tubes. We’re talking about the 2 sides of the PI on which you can connect 1,2,3… tubes of a push pull pair.

[tubeswell]

What Bieworm said. It’s the same no matter how many tubes in the push pull output stage.

[taiwanluthiers]

It's the same twin reverb AB763 I posted in the other thread. There's 1 resistor connected to each tube on pin 5. It's why I'm asking about it.

https://robrobinette.com/images/Guitar/AB763_Models/AB763_Twin_Reverb_Layout_DIYLC.pdf

Never mind, I think I get it. I was thinking of the resistor on the tube itself and not the pair of 220k on the board..

[Willabe]

It says it on the board, 'power tube grid leak.'

And for most questions we need a schematic, not a layout drawing. Both is best.

You should keep all your questions about the same build in the 1st thread on the Twin. Otherwise you'll end up with several different threads, it will get confusing.

[taiwanluthiers]

Sorry about this, I guess must be lazy eyes... somehow I don't know why I kept thinking of the resistors on the tube itself and not the phase inverter resistors.

Is it possible to merge the threads?

[taiwanluthiers]

I tried to put this mod into the amp and it basically does nothing. Amp sounds the same, tone, volume, or whatever no matter the position I turn the pot. I've even poked around with a meter to ensure I didn't short anything (as I can only find PCB type dual gang pots, and not the large 25mm types) and there's nothing going on. So it seems like the mod doesn't do anything. Right now I've just stuck a type 3 master volume because unlike the type 2, the type 3 actually works even if a little bit of sound will bleed through at high volume.

I am unsure if I need to change some cap value or some such... because I'm all out of ideas.

[SEL49]

The type 2 MV works very well. I suspect an operator wiring error. I think most people don't care for the type 3.

[taiwanluthiers]

Does it require the wires to be crossed? like out of phase or something?

[SEL49]

Does it require the wires to be crossed? like out of phase or something?

No. All it requires is proper wiring IAW the pic you posted in the other thread.

[taiwanluthiers]

I am unsure what I have done wrong then. As far as I know, the pot replaces the grid leak resistors, and somehow that causes the signal from the phase inverter to decrease? I've done that, and it really looks like decreasing the resistance of the grid leak resistor does a whole lot of nothing.

I mean is that how it works? Like if I replace a 220k resistor with say a 10k one, will this cause the volume to decrease?

[tubeswell]

Can you post a schematic of what you did?

[taiwanluthiers]

I took this

Then I had the dual gang pot. Removed the grid leak resistor, connected the part where the phase inverter output goes to the power tubes to pin 3 and 6 of the dual gang pot. Then pin 2 and 5 goes to the power tube (in mine, it's pin 1 on the tube but with a grid stopper, a 1.5k resistor that connects to pin 5). Then pin 1 and 4 are connected together, and then that goes to the bias part of the phase inverter circuit (where the yellow wire starts and goes to the bias pot). A 2.2M resistor is connected to pin 1 and 2 of the dual gang pot, and another 2.2M resistor is connected to pin 4 and 5 of the dual gang pot.

I've double checked and triple checked, and it does nothing.

[AlNewman]

I mean is that how it works? Like if I replace a 220k resistor with say a 10k one, will this cause the volume to decrease?

Yes, that's pretty much how it works.
You're working through the bias supply, but if you look at the resistance to ground for AC signal, it's max 240k.
It's complicated by the potentiometer, as it creates a voltage divider between signal/ground/output.

Fortunately, as we all know, it isn't rocket surgery.

[taiwanluthiers]

I removed it completely and put the 220k resistor pair back into the board, and I'm using type 3 PPIMV. I do not know if it's the pot or something else, but this too much checking and rechecking and not knowing what it's doing to the bias as well.

What I've learned is that RG176 coax cable provides MUCH better shielding compared to twisted wires... I do not know why. But I do know that both the volume and input controls, as well as tremolo, MUST use coax cables or else I get feedback. Once the coax cable is used the feedback and oscillation goes away almost as if by magic.

[tubeswell]

I took this

Then I had the dual gang pot. Removed the grid leak resistor, connected the part where the phase inverter output goes ...

Too tired to focus on explanations today. Here's a free easy to use schematic drawing application for next time. http://jschem.bplaced.net/

[taiwanluthiers]

I just looked over the modification, and if the goal is to replace the grid leak resistor with a pot, then why are we diverting the phase inverter signals at all?

From what I can see, the phase inverter signal just goes through the grid leak resistor (normally) then to the bias circuits. If decreasing the resistance is what causes the volume to go down, then why aren't the bias wires connected to the middle pin of both pots instead of the phase inverter wires?

It seems to me, if the bias wires are connected to pins 1 and 4 on the dual gang pot, then it's going to read 250k no matter what you turn the pot, and so no change in output volume.

Correct me if I'm wrong, but it would seem like the bias wire should go on the middle (pin 2 and 5) rather than the first (pin 1 and 4).

No wonder it's doing nothing.

[pdf64]

...
Correct me if I'm wrong, but it would seem like the bias wire should go on the middle (pin 2 and 5) rather than the first (pin 1 and 4).

You're wrong.
Do you understand the theory of operation of a pot?

[taiwanluthiers]

I know how a variable resistor works. The slider is the center part.

So my question is, how is varying the resistance on the phase inverter path affecting the volume? I know the type 3 mv does that but that uses a 1 meg pot, and it works by mixing the signals to cancel it out.

But the schematic and layout tells me it's only varying the phase inverter signal strength, not the bias path.

What am I missing?

[pdf64]

I know how a variable resistor works. The slider is the center part.

A potential divider is 2 resistors. Yes it can be used as a single variable resistor, but a potential divider is a different thing.
Do you understand the theory of operation of a potential divider?
Because  "my question is, how is varying the resistance on the phase inverter path affecting the volume?" raises doubt that you do.

 

the schematic and layout tells me it's only varying the phase inverter signal strength, not the bias path.

That's how it works, it would be extremely undesirable for the bias voltage to vary as the volume is adjusted.

[taiwanluthiers]

I don't know then. I've installed the mod as the schematic shows and turning the pot does nothing. Maybe the dual gang pot is not actually dual gang? I don't know. Maybe it's acting as a single pot rather than dual gang.

Honestly I've just seen a lot of condescending attitudes here, lots of "lead dress" or whatever (and it's something I'm improving as much as I can). I don't understand how it works, I see the schematic/layouts and I follow it to the best of my ability. Not a lot is explained as to how it works. In fact the site that talks about it has like 2 paragraphs on how it's better than just a type 3 MV without really explaining how it even works. Just saying "back to the drawing board" is just unhelpful.

What books or huge mountains of books do I need to read to understand it? What could cause it to basically do nothing? I took it out because I'm scared that all it's doing is changing the bias, and I definitely don't want to do that. Without a real oscilloscope I can't see what's happening. I got those little cheap ones off Aliexpress and I don't want to risk destroying it to try and read the phase inverter signal.

[Rontone]

Have you checked https://robrobinette.com/The_Trainwreck_Pages.htm

Almost halfway down the page is the Master Volume types, with RobRobs extra notes on each type

Can you post a few pics of the master volume pot and wiring

[SEL49]

I don't understand how it works

This is not complicated. If you understand how the volume control works in this amp, you should understand the type 2 MV because the MV is exactly the same as the volume control. It's wired the same and it works the same.

The MV pot makes a voltage divider for the signal coming from the PI. The wiper picks off a portion of the PI signal and passes that on to the 6L6 grids. That portion can vary from all the PI signal down to none of the PI signal.

If the junction of the two pots were connected to ground (as would be the case for a cathode biased power amp) maybe the circuit would be easier to understand. But the junction of the pots is connected to the negative bias supply. Guess what. The bias supply IS AC GROUND due to the bias filter cap, as far as the signal is concerned.

Compare the attached drawings and you should see just how simple this mod really is.

[mresistor]

Anyone..  considering the type 2 diagram above   why not connect the bias after the pot wiper?

[Rontone]

Anyone..  considering the type 2 diagram above   why not connect the bias after the pot wiper?

The 220k resistance across the pot keeps the bias voltage constant,

I believe if you add the bias connection after then the wiper would alter the bias voltage when moved

[AlNewman]

Anyone..  considering the type 2 diagram above   why not connect the bias after the pot wiper?

Then it would be basically a dead short to ground for the AC signal.  So the volume would be off all the time.

SEL's diagram doesn't include the 1.8M safety resistors.  You theoretically you could plug the bias in at the bottom of those, but then you'd need a capacitor at the bottom of the pots to keep your DC bias, while maintaining grid leaks for the signal.

[mresistor]

guys  see graphic in my post

[SEL49]

Anyone..  considering the type 2 diagram above   why not connect the bias after the pot wiper?

Your pic is for a type 1 MV, not the type 2.

[taiwanluthiers]

Just one thing... it's a pain to post pictures here because my phone's picture size is WAY too large to allow me to simply post it directly from the phone, as the site's software is designed for desktop and not PC (not to mention it fills my entire phone screen with ads before letting me do anything else). It's one reason why I don't like posting pics. I have no idea how to shrink/resize pictures from the phone (I can do it on the desktop). I do not know if there's a way for the forum software to do so automatically.

That said, here is the wiring I did. I kept the color the same so I don't get the phases crossed as that would end up getting positive feedback oscillations. Pin 1 and 4 are crossed and the yellow wire goes to the bias point (below the grid leak resistors). The middle pins with the resistor ( did not have any 1.8M resistors on hand so I paralleled 2 3.2M resistor to get it down to the right value, but before I used a 2.2M resistor) goes to the power tube grids. The last 2 set of pins (3 and 6) goes to where the phase inverter is. The 220k grid leak resistor is taken out of course.

Like I said it does nothing. Amp acts like the mod hasn't been installed. So I gave up on this because it's too much BS. Perhaps it's the wrong kind of pot even though measuring with an ohm meter tells me it has the right resistance values (and it is indeed a dual gang pot). Edit: It does dampen the volume by a TINY bit if the pot is turned to the max... Maybe it needs a higher value pot?

[mresistor]

Anyone..  considering the type 2 diagram above   why not connect the bias after the pot wiper?

Your pic is for a type 1 MV, not the type 2.

Oh ok  thanks

[mresistor]

This configuration seems good.

[tubeswell]

I believe if you add the bias connection after then the wiper would alter the bias voltage when moved

Not if there is a coupling cap between each pot wiper and the Grid leak resistor

[pdf64]

...
Like I said it does nothing. Amp acts like the mod hasn't been installed. So I gave up on this because it's too much BS. Perhaps it's the wrong kind of pot even though measuring with an ohm meter tells me it has the right resistance values (and it is indeed a dual gang pot). Edit: It does dampen the volume by a TINY bit if the pot is turned to the max... Maybe it needs a higher value pot?

I suspect you’ve made a wiring error.
A potential divider is duty bound to rdduce potential, it's not like it can decide not to