Traynor YBA-1A High Screen Voltage

[rockinrob]

I'm a bit over my head working on a Traynor YBA-1A. The B+ is running around 570V, so my screens are seeing around the 560 range.

I have popped several EL34s (most of my spares stash) and I can't really afford to keep "trying" out fixes.


The amp will work, and sounds great, but when it is pushed, either cranked up or with a pedal while it is cranked up, the fireworks begin...

As of now, it has 1K screen resistors on each tube, along with 5K swamp resistors.


I'm about to replace the tube sockets in case there is an arc I can't see, and I have been researching and going over possible solutions to knock off 100v or so, to allow for 500v rated screen tubes.


I have no experience with Mosfets, but have come across RG Keen's mosfet follies article, and was thinking of implementing the lasted one after the B+, before the Screen supply

I don't understand the resistor value selection and how/why I would need to change those. Also, I see similar but different circuits for VVR, and am wondering if this would be a better solution as the implementation is different, but according to my understanding of them both (based on googling) it appears they are a similar idea.

I've ordered several STP3NK90ZFP mosfets and also several 1N5378BRLG zeners, but realized I did not include the smaller protection zener, so I will be ordering that shortly and thought I would ask for some advice here - am I on the right track?


One last question - I repaired a different one of these amps for a buddy, fired it up and he has been gigging with it for 6 months in a loud band that uses pedals all the time, with no issue and running 425v screen rated tubes! I really would like to understand what is going on here - the voltages are similar.

[xtian]

I have had excellent luck with RG Keen's MOSFET B+ reducer, dropping up to 100 volts. Here is my build thread:

http://ampgarage.com/forum/viewtopic.php?t=27035

(Ignore Google's malware warning--there's no malware on ampgarage.com)

[Enzo]

I don't see that you gave us any information about how the tubes are biased in your amp. Voltage is one thing, but it is current that melts tubes.

[rockinrob]

ok, good call. I'll look at my notes when I get home

[rockinrob]

According to my calculations, 28ma and 29ma

The OT resistance is 57ohms and 52ohms, Voltage is 569vdc, and plate to CT voltage is 1.61vdc and 1.51vdc

[Jazz P Bass]

Those 1K screen resistors are not going to cut the mustard if you are coming off of the 570V supply.
(Why That is so high, I don't quite get. The original schematic shows 440Vdc.)

The stock circuit has the screens coming off of a separate 434 V supply from the 440 and then through another, honkin 470 ohm 10 watt resistor.

[rockinrob]

That is the yba-1, this is a different amp (it's confusing!)

[g1]

So you added 1K screen resistors at each power tube? Did you keep the 470R10W (R25) shared screen resistor?
And R38 is still 470R ?

[rockinrob]

Correct - power supply is stock (new electrolytics though) but I added 1k 5w screen resistors on each tube.

[mozz]

What are the filaments at? If much higher than 6.3vac, it would be best to drop the voltage on everything, a bucking transformer on the primary.

[rockinrob]

I'm running 123VAC at my house, and I want to say the heaters were at 6.6 - however, the guy that owns the amp is running 118VAC, and the schematic is at 117 I believe

[g1]

When idling I don't expect the screens to be much lower voltage compared to the plates (than what you have).
Any idea what kind of load is being used?
Also, the other amp that is working fine, any idea what it's idle current was like and what load is being used?

[Chuck H]

I had some trouble finding el34's that would perform for overdrive without popping at only 470Vp. They're much more delicate than they use to be I guess.?. That must be the Bassmaster that's known for getting 96 watts or something (I still say NOPE!). They're really not supposed to be clipped I think. The idea being that they're so damn loud it would be unpleasant (also NOPE!). I think that to get tubes to survive in that amp under heavy clipping you may want to consider a different tube type or a drastic voltage reduction.

[Leo_Gnardo]

If I'm not mistook, the Traynor YBA-1A was the first bass amp I bought new in 1971. Ran for years with its original - 6CA7's. Again I may disremember but I recall they were marked Sylvania. Whatever, they were NOT skinny-glass EL34's, for sure. Although marketers tried to promote the notion that EL34 and 6CA7 were equivalents, they clearly were not identical.

In more recent times I retubed an old Orange 120 with 6CA7, also increased the screen grid R's to 1K5 or 2K2. The amp had a habit, as they do, of eating EL34s. Owner mentioned it didn't sound the same as when it was running EL34 BUT he's had the amp in use, playing loud shows over a year now and I haven't heard of the output tubes failing. Sometimes a little bit of a sacrifice has to be made, tone vs reliability.

It's been claimed the EL34's of yore, real Mullards, perhaps also Telefunken, RTF, Siemens, were a good deal tougher than current (read last 30+ years) offerings and would put up with extreme circuit conditions. Anybody who has enough money to try hi B+/hi screen grid voltage amps with NOS tubes of these types is welcome to try, and please do report your results.

[Chuck H]

Being as the max spec for el34's is 800V plate and 500V screens one would think they could survive some abuse at 570Vp even with high-ish screens. But I think the specs, which remain consistent for new tubes as old, are just regurgitations of old data sheets. No way you could put 800Vp on a new JJ el34. No way. So what do you do with tubes that DON'T have any actual spec sheet? I guess you just try $h!t until something works?