Traynor YB1 MKII Mod

[shaun]

Heyo.
I'm working on a Traynor YB1 1A MkII. Someone modded it some time ago to have a switchable Pentode/Triode output. The EL34s are burnt, so I'm guessing they didn't get the bias quite right. Not having looked inside one of these before, there is an item I'm not familiar with, and I was hoping someone could enlighten me.

There are two things that look at first glance like old style paper capacitors of some sort, but the schematic says otherwise. They seem to connect the plates of the EL34s, and the symbol looks like diodes back to back maybe.

Can anyone tell me what they are? I would like to restore the original circuit and am wondering if these components go bad over time. There's is high voltage on the plates - 540! - and the PT and OT are enormous. So I'd like to keep it simple. The bias pot is about as flimsy as it gets - a bit tricky to see in the pic, but it is sort of goo-ed upright to the board just above the right 5W ceramic resistor. So that could do with a decent replacement. I'm assuming that IC 47uF 500v e-cap is an addition - it had a bad solder joint - just sorta hanging off the ground lug.

A pretty interesting amp, although I'd need to be 20 again to carry it to a gig.
Pic and schematic attached.
Thanks.

[rake]

Found this:
Traynor Bass Master YBA-1A MKII schematic. So as you can see, this YBA-1A MKII has 537V B+ for the power tubes. With this B+ it will do about 65W at clipping. Original Traynor YBA-1 from the previous post only has a 430V supply and will do about 40W at clipping. Check out the flyback diodes they have going from plate leads of the power tubes to ground. They are listed as what seems to be 6RS20SP20820 and 6RS20SP787 and are actually called thyrectors (6RS20SP series). They are selenium and should be probably just be removed or replaced. They will eventually fail because selenium rectifiers increase in resistance over time. So as the decades go on they run slightly warmer and warmer, until they usually short. Selenium is toxic and burnt/eploded/releasing the smoke selenium rectifiers should be avoided. You can just replace them with those R3000 diodes or three 1N4007 in series.

Look here: https://irationaudio.com/2017/08/21/traynor-bass-master-yba-1a-mkii/

[shaun]

Wow. Fantastic info, rake. Many thanks!  A thyrector...sounds like a medical procedure :).

They don't seem to go to ground, just between the plates. I assume (like the ass I usually make out of me) they are there to stabilize the voltage between the power tubes. I like the idea of popping in a couple of R3000s, but I wonder what would happen if I simply removed them altogether. They're probably there as a safety device, so best to keep them I suppose, especially with that crazy plate voltage.

[PRR]

> They don't seem to go to ground, just between the plates.

It seemed like a good idea at the time.

It may have lasted all of Pete's life, which is better than a lot of electronics.

In recent decades many makers have put reverse diodes to ground, relying on transformer action so when one plate is clamped to ground the other won't rise above twice B+, instead of 10* B+ as when unclamped.

[rake]

Diodes are cheap. They're like deer nuts. They're under a buck! 
I'd just take two pairs + to + and mimic the thigh-rectums! 
Do you want a noisy amp to save maybe $4.00?????
In most cases, it was done for a reason.

[shaun]

> In recent decades many makers have put reverse diodes to ground, relying on transformer action so when one plate is clamped to ground the other won't rise above twice B+, instead of 10* B+ as when unclamped.

Thank you PRR. Makes good sense. I like your graphics too :).