Vtm120 PT swap on EL34 mod

[Hoggnogg]

I’m in a pickle. I bought a vtm120 for cheap recently that has a fair few mods done, and a bad power transformer. It has been modded to take el34, and fitted with a bias pot. I figure I’d just switch back to 6l6 (which I prefer anyhow) and get the proper PT from peavey and I’d be good to go. Unfortunately Peavey is barely in business and I was unable to get anyone on the phone, let alone a new PT. Further complicating things is that nobody makes an aftermarket PT for a vtm120 either. I’ve done the best I could and obtained a PT for the jcm800, whose voltages and current are mostly spot on, including some extra current to the heaters so the pt won’t blow again, except for the bias secondary. The problem here is the vtm calls for 28v with a centertap for 56v altogether, but the 800 pt bias secondary is 100v, no centertap.
What im figuring on doing is breaking off the single 100v into a pot/resistor voltage divider to provide the leg of 28v that powers the LEDs. What I’m not so sure of is what to do with the bias supply leg. I talked briefly to someone who drew out a quick sketch of a fairly involved circuit he said I should build offboard, but I’m not able to reach him for questions, and I wasn’t aware of the bias mod when I spoke to him. What do I need to do with this bias supply to get this running right with el34s? I have no engineering or theory background if you can’t tell♂️

[Helmholtz]

Schematic?

[Hoggnogg]

https://www.thetubestore.com/lib/the...zdiC9YHXjdaTQI

[Hoggnogg]

I’ll see if I can post the diagram that was drawn for me, you wizards may be able to glance at it and know exactly what it’s doing
*******
turns out I’m not authorized to post attachments

[Enzo]

The amp is 33 years old, Peavey won't have the part anyway..

I doubt the heater current for the EL34s hurt the transformer. Peavey made numerous models set up to switch between 6L6 and EL34 using the same transformers. Transformers do fail, but they are the most reliable part in the amp, and in my experience, Peavey power transformers have been quite robust.

The -56v supply needs about 40VAC to make it. SInce the current demand is tiny, you could use a 100vAC winding and simply drop it down with a couple resistors Center tap that 40vAC to get 20vAC et voila 28vDC.

Correct me if I am wrong, but the only thing I see they use the +28v for is to elevate the heaters and to run the two LEDs. None of that needs more than a handful of milliamps, so again, we can step your 100v down with a couple resistors.

I don't see why the bias supply needs to be complicated. Just reduce the AC going into the existing circuit so it sees around 40vAC like it expects.

[Hoggnogg]

[Hoggnogg]

This guy definitely knows his shit, the problem is that I don’t. If I’m going to put the effort into building this circuit I’ve got to at least understand it somewhat, and if that’s not possible then I at least want someone else to check the work or give a second opinion.

[Hoggnogg]

The amp is 33 years old, Peavey won't have the part anyway..

I doubt the heater current for the EL34s hurt the transformer. Peavey made numerous models set up to switch between 6L6 and EL34 using the same transformers. Transformers do fail, but they are the most reliable part in the amp, and in my experience, Peavey power transformers have been quite robust.

The -56v supply needs about 40VAC to make it. SInce the current demand is tiny, you could use a 100vAC winding and simply drop it down with a couple resistors Center tap that 40vAC to get 20vAC et voila 28vDC.

Correct me if I am wrong, but the only thing I see they use the +28v for is to elevate the heaters and to run the two LEDs. None of that needs more than a handful of milliamps, so again, we can step your 100v down with a couple resistors.

I don't see why the bias supply needs to be complicated. Just reduce the AC going into the existing circuit so it sees around 40vAC like it expects.

Thanks so much for taking a moment on this, first off.
Yeah, that’s what I was thinking-it seems like a pretty simple problem-split another leg and bring the voltage down to what the amp expects to see on the power board. (Note: just so I’m clear, when you say center tap the 40vAC, you just mean splice off the 40v, or is there some process I’m not familiar with?)
In any case, I know enough to know how little I know, so when the advice got complicated I assumed it was something over my head. But as I said, the guy that advised this circuit knows his stuff. Has he just made in complicated for electrical erudition, or is adding this circuit actually advantageous?
im a little worried that I’m going to go for the simpler route and end up blowing something up

[Enzo]

Your guy eft out the rectifier on the +28 branch.

The peavey transformer was a 40vAC winding that is center tapped, so it makes 40vACand 20vAC. The -56v on the drawing mans -56v DC. AC has no polarity, so any time you see + or - it means DC. Now then, to make 56v DC, you need only 40vAC. That is where I got the number. ANd assuming the tap was actually acenter tap, I caalculated what 20vAC becomes, and sure enough, 28vDC.

The Peavey part has the center tapped winding, by tif all you have is a single winding, you can still branch two ways from it making one positive and one negative.

[Hoggnogg]

Your guy eft out the rectifier on the +28 branch.

The peavey transformer was a 40vAC winding that is center tapped, so it makes 40vACand 20vAC. The -56v on the drawing mans -56v DC. AC has no polarity, so any time you see + or - it means DC. Now then, to make 56v DC, you need only 40vAC. That is where I got the number. ANd assuming the tap was actually acenter tap, I caalculated what 20vAC becomes, and sure enough, 28vDC.

The Peavey part has the center tapped winding, by tif all you have is a single winding, you can still branch two ways from it making one positive and one negative.

Thanks so much for talking about some of the principles involved in these specifics. It makes a huge difference to people like me that only know what they have gleaned from conversations and read in articles.
So is the bias circuit he provided meant to bypass the power supply board bias circuit altogether? Is that why it’s so involved, as well as rectified?
I have no plans to bypass the power supply board circuits, and therefore no need to rectify the20vAC (right?), but he threw me for a loop by combining a circuit that’s providing AC to the board with a circuit that’s bypassing the board and providing the -56DC directly.
Would it be advantageous or preferred to use his circuit as opposed to the vtm bias circuit considering it’s utilizing a whole different PT and output tube denomination? Or will the vtm circuit suffice since there’s a bias adjustment mod there that I can use to make up the difference?
This is amazing!—two weeks of mystery and confusion lifted with two comments from someone who’s both knowledgeable and communicative.

[Enzo]

ANything that results in the voltages the amp needs is OK. I myself would just adjust the AC going into the existing board to make it similar to what was there before. His circuit is ssimply pretty much teh same as what is already in the amp, but drawn as a stand alone circuit. Either way would work. I'd pass on building the whle circuit and just drop the 100v with resistors in a "voltage divider".

[Hoggnogg]

Perfect! That was what was nagging me and I feel fully confident in the plan now. A million thanks, really. It’s rare that a problem comes up for me that can’t be figured out with some searches and reading through a ton of threads. Eventually you’ll find someone who had the same issue and how it was solved. But with this I could find nothing. I hope this helpthread finds its way to the next person that has this issue.

[tedmich]

FYI this PT transformer (Peavey 705-18719) is the same as in the Ultra 120, Triumph 120, TripleXXX, Butcher 120, JSX 120 and Rockmaster I believe, possibly others...
They are plentiful on used mkt but heavy to ship! Good luck!