EL 84 Plate voltage

[Wayne]

How much voltage is too much for a run-of-the-mill EL 84? Yes, I know, the data sheet says 300V, but I also know that this is routinely "pushed" to around 400.

I have an amp (a Traynor YGM-3, FWIW) that I believe was built in the 110V wall voltage era. With 117V, which is usual for my neck of the woods, heater voltage measures 6.6, and the B+ is 430V. Because of the Traynor non-adjustable fixed bias the tubes are running way too hot now, about 11 W each. I can make the bias adjustable and cool down the outputs, but will I still have trouble with that much plate voltage? I've already had a small lightning storm inside one of the bottles, but the tube seems fine.

Any thoughts/opinions?

Tks,

W

[rsi]

I would put some zeners on the B+ line to drop the plate and screen voltages. Either that or start using 7189s, which have a higher voltage rating. Modern production EL84s won't last very long running at 400V+. NOS ones would probably last longer, but then you risk blowing up expensive tubes.

The other approach is to get the line voltage down to something around 110-117. You can do this with an additional transformer (details at http://www.geofex.com/Article_Folders/v ... ntvolt.htm) or you can put a large power resistor on the AC line before the PT. You would have to try different values to get the voltage drop you want. I did some experimenting and found that a 27R 50 watt resistor dropped my AC line voltage from 120 to 108 (this will depend on the current draw of your PT). The resistor gets very hot though. One side effect of this approach is that it will drop your heater voltages also (and every other voltage coming from the PT). So, if you take this approach make sure your heaters don't drop more than 10 percent from the standard 6.3 volts.

[Alexo]

I think a couple zeners might be your best bet. I've seen them run up to 450 volts though, but your plate voltage will likely rise as you cool down the bias.

[Wayne]

Thanks for your replies. I have since dug up the schematic (this is one of the few Traynors I've seen that didn't have it glued to the top plate) and the spec'd voltage is 398. From what I've read since my first post, even that might be a bit much.

I had seen the "vintage voltage" article before and will probably implement that in the end but I don't have a suitable transformer at hand, so for now I think I'll series R either the primary or the B+ to get down below 400V. It's tough sourcing parts in a small town - most of the electronic repair shops hide now when they see me coming

I think with a speaker upgrade, this could be a great little amp!

W

[El_Martin]

Hi!

There are some EL84 amps w. hi plate voltage. The reliable ones take care of the fragile screens. 2k2/5W in series will be some kind of insurance for them.

Zeners to cut B+ won't hurt either.

The russian EL84M military version are known to withstand some punishment. They're also known to be a bit on the harsh side, tonewise.

HTH

Ciao
Martin

[Phil_S]

430v seems too high to me. Running a pair of old USA made RCA, GE, Sylvania, etc, into the low or mid 300's is probably OK, but 430 seems way out of whack. You might want to get JJ's or Sovtek EL84M (poo tone but durable).

There are suggestions on how to drop B+, so I don't need to go there.

[UR12]

I'm not a hugh fan of Mesa but they run their Rocket 44 El84s at 400v. If I had to make a guess though I would say that they are Russian rebranded. Probably closer to the 7189

[Structo]

I just used a zener diode to drop B+ in an amp.

This is on a 6V6 amp and the plate voltage was at 465v unloaded.
I installed a 130v 50w reverse zener diode on the center tap of the HT winding. This is a cathode to case zener.

This brought the voltage down to 322v with the tubes installed.
Still a bit high for 6V6's but 130v was the biggest zener my local guy had.

I installed the diode near the power supply ground and drilled a hole there, put heat sink on the zener and mounted it tightly.

I also put a .01uf ceramic cap from the anode to cathode on the diode to help quiet any switching noises.

[swarty]

If you've got arcing inside the bottle I'd try cooling off the screens per the suggestion above. You may also want to consider changing to cathode bias as this may also be a bit easier on the tubes as in my Marshall 20s that run just over 400V and seem to treat the tubes fine; in this case it is JJs in one and Tungsram in the other.

[Wayne]

I had considered cathode bias as many earlier and later versions of this amp were so equipped. Not going there just yet, though.

I didn't have much time last night after work, but I did sneak down to the workshop and experiment with a series resistor in the B+. Without changing anything else (like bias), 500R puts the idle plate voltage at 400V. At full output into a load, B+ drops to 300V. Didn't try any sonic tests as the gang was asleep, but I don't know how that much sag will sound. Couldn't find my stash of zeners, but I think that might be next.

Also going to drop about 5 more volts off the screens. They currently run about 5V lower than the plates - I'm gonna shoot for about 10.

W

[krash]

2.2K 3W-5W resistor for the screens is a very, very good idea. Zero tone penalty. You lose about 1/2 to 1 watt of total power max.

[El_Martin]

Btw: the 2k2/5 W info was given to me by Larry/novosibir...

He didn't invent screen resistors, but one has to give credit, where credit is due. TNX, Larry!

Ciao
Martin

[Wayne]

Poked around in my "electronic inventory" earlier tonight. Because my heater voltage was running a little high (6.8V) as well as the B+, I opted for a series R in the primary of the PT. The ideal resistance seems to be about 15 ohms (based on heater voltage), but I didn't have that w/enough power rating. I settled on 11R (2x22R) and so far I'm pretty happy with the results. The heaters still run a bit high on standby (of course), but drop right in the pocket once the B+ starts flowing. B+ at idle is still 420, but I'm not worried - it quickly drops to about 360 once I start to play. Bias current is about 42mA for the pair (about 8W per tube). Increased the screen grid resistor from 470 to 1k shared. Sounds zillions better, no lightning storms, and I don't think I'll be ordering output tubes every other week!

The only concern I have now is that the heater voltage "bounces" a bit with the signal. I'm sure this happens to a certain extent with most PTs, but by adding resistance to the primary I've exaggerated that effect. Any opinions on whether or not that will be a problem?

Thanks again for helping me learn...

W