Traynors, screen voltage....tube choice?

[GerryJ]

Hello, I have ~ 1968-1972 era YBA-1 bassmaster, YBA-1A Mk II, & YBA-3 Custom Special. The YBA-1 runs ~ 430 volts on the screens & plates, while the Mk II & YBA-3 run ~ 535 volts. All get moderate use (6x/mos), but not approaching power tube distortion.
I've had Tesla EL34s in them, which seem to work....ok. One tube failed after ~5 yrs in the YBA-1 & the Custom Special, & 1 failed in the Mk II by cracking at the base.
( By contrast, the original US RCA 6CA7s in the '68 amp - I'm the original owner- lasted 8 yrs of constant Hard Loud use ! oh well....)
I'm wondering if I need to consider KT-77s for the 535 volt amps?
I have a stash of Tesla NOS El34s. Their spec sheets say 425 v max for screens. Current JJ & Russian EL34s or 6CA7s list ~450-500 volts for screens. Only the JJ KT77s list 600 volts for the screens.
However, I'm wondering how accurate these Slovakian & Russian spec sheets really are? Btw, old US RCA 6CA7s spec'd their screen max at only 425 volts, & they lasted forever.
I also get the impression that NOS Tesla EL34s are considered better than current production.
So should I consider JJ KT77s for the 535 volt bad boys, or are the Teslas in reality just as good (or maybe better)? Thanks!

[Stevem]

If you don't mind shelling out 122 bucks for a pair of KT90s from the Tube store then those would be a life time option.

[didit]

Hello --

So should I consider JJ KT77s for the 535 volt bad boys, or are the Teslas in reality just as good (or maybe better)? Thanks!

Had good success putting JJ KT77s in high voltage amps including the venerable Traynor YBA Mk-II. Cannot comment on your Teslas.

Best .. Ian

[thetragichero]

i like throwing 6550s in em (after ensuring bias feed resistors are 150k or below and that i have enough bias voltage). probably last close to forever

[JD0x0]

First thing I'd do is swap the 47ohm (according to the schematic) screen grid resistors to at least 1k, maybe 1.5k.

I'd probably Zener down the screen voltages below 450VDC and just use regular new production tubes rather than trying to cherry pick rugged enough tubes to handle the duty.

[Roe]

First thing I'd do is swap the 47ohm (according to the schematic) screen grid resistors to at least 1k, maybe 1.5k.

I'd probably Zener down the screen voltages below 450VDC and just use regular new production tubes rather than trying to cherry pick rugged enough tubes to handle the duty.

try 1.8k or even 2.2k

[didit]

Hello --

First thing I'd do is swap the 47ohm (according to the schematic) screen grid resistors to at least 1k, maybe 1.5k.

I'd probably Zener down the screen voltages below 450VDC and just use regular new production tubes rather than trying to cherry pick rugged enough tubes to handle the duty.

try 1.8k or even 2.2k

At the risk of seeming prickly, I'd leave it be and install current-production tubes known to meet the overall specifications -- making any necessary minor adjustments for correct bias. This definitely, rather than taking odd advice of folk who seem unable to read schematics correctly. Pete Traynor has well deserved reputation for robust reliable amp designs, and unless some previous person has been inside one of your amps making "improvements" there's really no necessary changes unless you're after a different sound.

https://www.thetubestore.com/lib/thetub ... ematic.pdf
https://www.thetubestore.com/lib/thetub ... J-KT77.pdf

Another well-regarded current-production replacement option would be --
https://www.thetubestore.com/lib/thetub ... 6ca7eh.pdf

You can find several threads on TAG where these have been recommended as both rugged and providing fine sonic performance. Look particularly for posts from Miles (aka "reeltard").

Best .. Ian

[Roe]

Hello --

First thing I'd do is swap the 47ohm (according to the schematic) screen grid resistors to at least 1k, maybe 1.5k.

I'd probably Zener down the screen voltages below 450VDC and just use regular new production tubes rather than trying to cherry pick rugged enough tubes to handle the duty.

try 1.8k or even 2.2k

At the risk of seeming prickly, I'd leave it be and install current-production tubes known to meet the overall specifications -- making any necessary minor adjustments for correct bias. This definitely, rather than taking odd advice of folk who seem unable to read schematics correctly.

Best .. Ian

Increasing screen resistors on an high-voltage amp with el34s and tube problems is a standard practice recommended by Merlin and others. However, you take it to be "odd advice of folk who seem unable to read schematics correctly"?! In any case,the problem is not just screen voltage typically but also screen current. Kt88s for example may handle 600v at the screens but you still need to limit the current, something that is normally done with a resistor.

[sluckey]

First thing I'd do is swap the 47ohm (according to the schematic) screen grid resistors to at least 1k, maybe 1.5k.

At the risk of seeming prickly, I'd leave it be and install current-production tubes known to meet the overall specifications -- making any necessary minor adjustments for correct bias. This definitely, rather than taking odd advice of folk who seem unable to read schematics correctly.

Did you misread the YBA3 schematic? It clearly shows 47Ω/2W screen resistors.

[GerryJ]

I appreciate everyone's advice! I haven't popped the hood yet to look, but based on the amps bumpers, circuit breaker, etc the 70s schematic is probably the one - looks like the screen grid resistors are 470 ohm, 10 watt.

(I "think" the YBA3 uses the more unusual circuitry that Kevin O'Connor describes as "tying the suppressor grids to the raw bias supply", but that's a different topic).

Fwiw, I'm relatively experienced with biasing (using bias probe socket) & routine cleaning, retentioning, & re-soldering cold joints- but not beyond that yet. No true techs in my area either. So I'm more inclined to tube swaps & bias adjustments that don't require hardware changes, but can do so if required. Thanks again!

[thetragichero]

if you pop the hood there's a good chance the original schematic is still stapled to the top of the lid

[didit]

First thing I'd do is swap the 47ohm (according to the schematic) screen grid resistors to at least 1k, maybe 1.5k.

At the risk of seeming prickly, I'd leave it be and install current-production tubes known to meet the overall specifications -- making any necessary minor adjustments for correct bias. This definitely, rather than taking odd advice of folk who seem unable to read schematics correctly.

Did you misread the YBA3 schematic? It clearly shows 47Ω/2W screen resistors.

Yes. And as part of a fairly unique overall screen supply, yes. Having spent most of my now many years here in Ontario seen a few of those over the many years, and even been inside one. And so can confidently observe, except for the schematic symbol a 6KG6a has near zero resemblance to either an EL34 or KT77 -- cap plate connection and magnoval socket makes for limited interest in tube rolling.

This is way off our initial request in this thread however.

Best .. Ian

[didit]

[...] I'm more inclined to tube swaps & bias adjustments that don't require hardware changes, but can do so if required. Thanks again!

Applauding your wisdom.

Best .. Ian

[sluckey]

And so can confidently observe, except for the schematic symbol a 6KG6a has near zero resemblance to either an EL34 or KT77 -- cap plate connection and magnoval socket makes for limited interest in tube rolling.

Heck, you missed it again. There were three schematics in that linked pdf and the other two were 6CA7 circuits with 47Ω screen resistors too. But the 47Ω resistors were not the point of my comment. I just thought it was unkind to make such a "prickly" comment about someone who can't read a schematic correctly.

[JD0x0]

Yeah, just for further clarification. The YBA-3 schematics have 3 pages. There are 47 ohm screen grid resistors, so I have not misread anything, and as already been mentioned, increasing screen grid resistors for modern tubes, EL34's especially, is standard practice. The only 'issue' you might run into increasing those values, is if you go high enough, you may start to introduce some screen sag at high volumes, but the values suggested in this thread (1k-2.2k) should not be high enough for that to happen and will protect the screens much better than a 47ohm or 470ohm, both of which are too low for modern (EL34) tubes, IMO. The zener will provide additional protection by bringing the screen voltages within current EL34 spec, so common off the shelf EL34's can have a reasonable lifespan.

looks like the screen grid resistors are 470 ohm, 10 watt.

The YBA1 MkII is showing a singled shared 470 ohm screen grid resistor for both tubes. Hence the 10 watt rating. It wouldnt hurt to give each tube it's own screen grid resistor, right at the tube socket. Screens will be less stressed, and the tubes will be slightly more stable, with the resistors right on the tube sockets.