Traynor YBA-1 Master Volume

[mtlbasslad]

Hello all, here I am back again over my head.
I have a very nice Traynor YBA-1 on the bench, customer wants a master volume installed. Can't quite figure out which of the 3 schematics is right, the one inside has been ripped off (!!) but it does show that the amp was made for 6CA7's.
Looking on robrobinette's site it seems easy to do with a 1 meg pot on the grids. Is it really that simple? Can I just attach the pot to the 2 Pin 5's on the 6CA7's?
Here is a pic of the beast...



I should mention that someone has already done the 3 prong plug update, YAY.
Also that the owner said the power tubes were a 'matched pair' HAHA. One is a very old Sylvania 6CA7 the other a JJ EL34. All caps seem to be original, but the amp 'sounds good' but just too loud. He also questions why I need a cab to test it... seems not to understand the warning on the back panel (in red) not to operate without a speaker...

I'm betting my old friend g1 has seen lots of these in his day...

I actually think I have a handle on this, but would like the collective to reassure me before I mess up this classic.
Thanks in advance,
Lorne

[mtlbasslad]

Can't seem to get a better image, what's changed since I posted last?

[mtlbasslad]

Is this better?

[Chuck H]

You're lucky to have an amp where all the original film caps are still in place. And you mentioned earlier that "ALL" the original caps are in place. Does this include the power supply filters (/decoupling) capacitors??? I had an old Traynor on my bench some fifteen years ago and was surprised to find the original Mallory filter caps still holding up their end (orange cardboard tubes with aluminum cans inside). But at this date I don't think I would trust them. The caps I saw were almost 50 years old but still working and I replaced them anyway because of age and inevitable consequences. Something to consider?

As to master volumes... There are several YBA1's that were all VERY different. Without knowing which one you have it's hard to advise on a master volume. That said, All of the YBA1's did have a decent LTP phase inverter response. So I would say that a post PI MV where a dual pot replaces the grid resistors would be as good as you could hope for. I think it's called the LarMar? It;s been used forever anyhow. Should be a decent choice for that amp..

[mtlbasslad]

Yes, still the original Mallory (made in Canada) filter caps -amazing. I'll take more pics.

[g1]

Hi Lorne. The schematic on pg.7 or 8 of the attached package should be good (the last one on pg.9 already had 3-prong).
Do you mean the 'trainwreck type 3' master from the Robinette site? That is a cross-line type that just cancels out the out-of-phase signals at the power tube grids.

[Mick Bailey]

The Lar-Mar method is also what I'd choose. It's predictably good and in my view sounds much better than other methods for only a few extra parts.

[mtlbasslad]

Hi g1, good to hear from you.
Yes, it' the 'Trainwreck type 3' that I'm looking at. Robinette suggests it over the other methods if it's not a new build.
So is it as simple as I suggested?

[g1]

Yes, very simple. Only 2 lugs of the pot are used. Quick and easy to test without even needing to mount the pot.
It may work well enough for the owner, but as Mick suggested, the LarMar type 2 (on same Robinette page) is preferable for most users.

[Mick Bailey]

Chuck H gets the credit for first mentioning the LarMar control.

[mtlbasslad]

g1 - thanks for the schematics, I had already found them at the 'tube store' but there were so many...
I promised more pics...

[mtlbasslad]

So here we see the original Mallory caps & the mismatched 6CA7/EL34 pair. Also what somebody did with the original schematic - both Traynor & Ampeg used to do this...
I just want to be clear - all I have to do is use pin 5 ? on the 6CA7's ?

[g1]

I just want to be clear - all I have to do is use pin 5 ? on the 6CA7's ?

Yes for the type3, just like the pic on Rob's website, same pin5 as with 6V6. Note the 3rd pot lug is not connected: https://robrobinette.com/images/Guit..._Schematic.jpg

[Chuck H]

I might place it behind the grid stops. I haven't thought it out entirely but something tells me that would be better.

[Greg Robinson]

I might place it behind the grid stops. I haven't thought it out entirely but something tells me that would be better.

Less likely to result in oscillation with the master turned to zero.

But throw my vote in for a Lar Mar style also.

Though I prefer AC coupled rather than DC coupled like Lar Mar, so double value of phase inverter coupling caps, 1M dual gang audio taper pot referenced to ground, AC couple wipers into existing grid leaks - the 2.2Meg "safety" resistors which are the entire rationale of the Lar Mar design is too high a value to prevent thermal/grid current runaway if the wiper loses contact, and AC coupled is less likely to be scratchy from a small amount of grid current which is common in powertubes with grid leaks exceeding datasheet specs as is so often found in guitar amps.