Tubes that can handle 500VDC on screens, or tame the power supply??

[Gtr0]

Hi All.

I have a high gain amp that puts out 516 VDC on the plates and 511 at the screens. It is an EL34 based amp so screen resistors are 1K 5W. I have popped a few tubes in the last year, never the same position (tubes are losing vacuum, turning white) and someone mentioned that running the bias at 70%, with current producton tubes and with these voltages, very well may be too much. I play a lot - probably some 30+ hours per week.

So let us assume this is true for the moment, the only tubes I can even find that are close to accepting 510VDC on the screens are the TAD EL34B-STR (claiming 500 VDC on screens max). Shuguang I guess? I have a set of these in a plexi and I find that they sound fine.

The transformer is custom made and similar to PT for a SLO100, so it's a 360-0-360.

The question is should I have a new tranny made, something that will yield something a bit less like 340-0-340??? Or should I put a dropping resistor after the rectifier circuit? Or is there a set of tubes out there that will accept a bit of abuse in this area (by your experience)? Or even perhaps raising the screen resistors??

I have used JJ E34L for the last decade and a half... But after popping two of them some months back, I bought a set of Tung-Sol EL34s... it was about a month going before I lost one about a week ago. Again I assume they are losing vacuum because they are turning white above the getters.

My temperature probe says the tube's surface runs at about 280 degrees F. But damn does it get HOT in the room.

Thanks for any input you may be able to offer.

Thanks.

[Zozobra]

I know folks with vintage amps that run high plate/screen voltages that have been using the Shuguang's. You can buy them cheap from RS :
https://uk.rs-online.com/web/c/semic...triode-valves/

I've used them in the past in a Trace Elliot Speedtwin and that runs about 500-510. It also has a standard Marshall 100W replacement OT as the tech who repaired it is a hack. It really should have a 2k-2.2k primary for that kind voltage.
Screen voltage rating do get vastly abused in most amps. Said speedtwin also runs 6L6's and they are only rated for 450V on the screens and they see 500! Same is true of the SLO.

[pdf64]

What (loaded) Vac is the PT putting on the heaters? If there's some leaway there, a bucker may be a good option.
Are there any higher voltage taps on the PT primary?

[alexradium]

for the best reliability,i would regulate the screen voltage to 400V max,with a mosfet and zener diodes.
EL34 tubes are very unpredictable as far as lifespan,Winged C were good but expensive,today the most reliable seems to be the EHx,but over 500V?

[Pedro Vecino]

I have popped a few tubes in the last year, never the same position (tubes are losing vacuum, turning white)
...before I lost one about a week ago. Again I assume they are losing vacuum because they are turning white above the getters.

So much repetition is suspicious. Do you use any type of spring retainer on them? Sometimes they do more damage than other things. When a tube turns white it means there is a crack

[Chuck H]

I built a custom order a few years ago that was high-ish voltage, but not as high as your rig. I blew up a bunch of tubes of different brands. Shuggies were the only ones to consistently survive and that's what the amp is using now. Unfortunately it's a combo amp and the Shuguang EL34's tend to be microphonic. I ended up buying three pair to get one that was good enough. If I had known I would have bought a sextet and selected the best two. I have the other four tubes in my stash marked "head only". They sound as good as any EL34 IMO.

[nsubulysses]

JJ 6CA7 is rugged on even higher voltages than 510V plate and 500V screen.

maybe the choke needs to be replaced with a 300-1000 ohm resistor ?? (tame the power supply idea)

Also though, it seems like the tubes are not dying because they would short and blow a fuse, or just turn on but not work at all . if the tube is losing vacuum the tube is not failing because of high voltage IMO. It would be failing from........being banged around???

[J M Fahey]

Rather than looking for Terminator approved tubes , which to boot are used beyond any manufacturer approved rating (they *should* know ) I´d lower screen voltages to sensible levels.

The simplest and cheapest solution is to downgrade the screen supply: drop the choke and use a plain resistor instead.
Anything from 1k 5W or 10W and up, whatever´s needed so at idle they sit at no more than 450V, dropping more when amp is driven full blast ... which is good.

Or you can go Hi Tech and use a MosFet regulator set to fixed 400V.

[nsubulysses]

It is good to drop the screen supply as juan says but it will make it sound more mushy and undefined in pick attack. Which I would guess this player wants since he is playing an SLO clone.

What would you do to re-tighten the tone? Adjust NFB resistor?

I usually use a 20W 1K resistor for 140W amp.
This is a nice one -- https://www.mouser.com/ProductDetail...NE5tU87GTlE%3d

I thought I could be a kinda lazy and use 500 ohm 10W in series with 470 ohm 6.5W in series (no 20w resistors around to use). By 3rd practice the 6.5W almost fell out because it was getting so hot the solder was melting and the resistor almost becaame loose in chassis! This is with amp absolutely cranked so it's an extreme scenario but I like 20W just to be safe.

if you look at 5150 and a bunch of other amps of similar 100W power rating that don't want to buy chokes, they usually use 400 or 500 ohm 10W resistor. Ampeg V4 is about 540V plate and 530V screen and they use 470 ohm 7W, which occasionally falls out of circuit from getting too hot if you crank the shit out of it.

[Jazz P Bass]

The failed tubes turning 'white' is not indicative of overvoltage or a screen failure.

'White' means loss of vacuum, plain & simple.

You may very well have a mechanical/ shock issue.

30 hours a week play time, yes.
But is the amplifier being transported?

[Chuck H]

But is the amplifier being transported?

As in loaded onto a rolling cart while still piping hot and wheeled out into the cold night air through the back door of a bar, grating along over bumpy, busted up asphalt past sleeping bums and broken Thunderbird bottles then heaved into the back of a van sitting on the metal floor for the drive home on surface roads.?. I did that all the time when I was gigging. I never cracked a tube but I did get circuit faults on the boards of my Marshalls three times.

[dstrat]

biasing a high gain type amp 70% might be a problem? Why everyone does this is beyond me.

many a burn , melted 6550 in my marshall over the years.

[g1]

Agree with dstrat, 70% at higher plate voltages is often a problem. Try lowering it until you have too much degradation of the tone. Bias as low (%) as you can get away with. The voltages will increase somewhat, but that is not as much a problem as higher dissipation.
Now it seems your problem is a mechanical one (cracked glass/vacuum leak), but the excess heat may be a contributor.
Nevetslab has posted here about needing to insulate the metal type 'top hat' retainers which were causing cracks in their SVT rental inventory. Could be a similar issue if you are using those type of retainers, like Pedro mentioned above.

[Gregg]

The easiest way to lower the screen voltage is to add 5W zeners in series with each tube screen.

[J M Fahey]

broken Thunderbird bottles

Had to Google that.
Wow!!!!