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Old 7th July 2010, 11:11 PM   #1
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Default B+ for EL84

The datasheet on the JJ EL34's I'm using to upgrade a Marshall 1974X handwired 18 watt combo to ultralinear says maximum plate voltage of 300V. What is the advantage/disadvantage of using a higher plate voltage. I currently have 375VDC. Also, I have seen some (AX84) use 1K ohm screen resistors. I noted in some threads and designs presented here to use a much smaller value (150-270ohm). What are the design considerations pertaining to this value? Much obliged for your expertise. Christopher
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Old 8th July 2010, 12:18 AM   #2
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34, or 84?
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Old 8th July 2010, 12:36 AM   #3
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Whoops. EL84. I've been looking around and noticed the Dynaco SCA35 uses 360 at the plates. Right now I have this amp biased with 32ma cathode current so right about 12 watts. I'm getting the impression that it's the guitar ampers that like the higher voltage. I'm also figuring that a higher Vp means shorter tube life. What else? I'm mostly a little taken back that the specs say 300 but people still run them with a higher voltage.
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Old 8th July 2010, 12:49 AM   #4
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Quote:
Originally Posted by lindope View Post
Whoops. EL84. I've been looking around and noticed the Dynaco SCA35 uses 360 at the plates. Right now I have this amp biased with 32ma cathode current so right about 12 watts. I'm getting the impression that it's the guitar ampers that like the higher voltage. I'm also figuring that a higher Vp means shorter tube life. What else? I'm mostly a little taken back that the specs say 300 but people still run them with a higher voltage.
The fact that people are doing it nonetheless does not make it a good idea.. I've got a couple of vintage hifi amps that run them at nearly 400V and the modern tubes can't tolerate these voltages at the design currents, particularly with the screens running at the full B+. If you want to run at elevated plate voltages you will need to reduce the screen voltage substantially to prevent meltdowns.. Japanese 6BQ5 and Philips/Telefunken made 6BQ5 seem to be more rugged than most US made types, and the ubiquitous JJ EL84. For reliability you might want to consider the Russian 6P14P-EV or ER as they seem to tolerate the higher voltages well. (They're pretty similar to a 7189A.) There are a lot of nice sounding 6BQ5 amps out there. (I'm thinking Vox particularly)
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Old 8th July 2010, 01:15 AM   #5
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FWIW, I have had no problems with using higher-than-rated plate voltages on EL84 as long as the dissipation isn't exceeded AND the screens are kept well below 300V.
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Old 8th July 2010, 01:17 AM   #6
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Kevin is correct. The Russian 6П14П-ЕВ (6p14p-ev), AKA EL84M, is a genuine 7189 equivalent. The Russky is tough as nails, decent sounding, and attractively priced.

Large valued g2 resistors are necessary to limit g2 current, which is particularly important at "tall" B+ rail voltages. You can't go hog wild, but some voltage over spec. is usually OK, IF you are careful about not exceeding dissipation limits.
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Old 8th July 2010, 01:47 AM   #7
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It is possible to run them at a scary B+

Extract from a post by Roger Modjeski on his RM 10 EL84 amp:

Click the image to open in full size.

Quote:
Let me go through the numbers and tell you how I get more than the specified 35 watts.
First the bias is indeed 30 ma. but for the pair of tubes and measured across 10 ohms, hense the 300 mV set point. Though 25 ma will double the tube life, (under light loading or soft playing) it will increase the distortion. Since the tubes are cheap, I let people decide for themselves.
At the 700V B+ (yes, 700) the idle dissipation is 21 watts/pair or 10.5 per tube, well within the 12 watt rating.
The screens run at 350 V
Load: 13,000 ohms PP, max signal current: 100 mA.
Fixed bias: -25V
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Old 8th July 2010, 03:19 PM   #8
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700V is a bit eye-watering, but I can vouch that the Peavey Classic 50 runs Anodes AND screens at 400V+, and I've seen examples with old-looking Sovtek EL84s [aka ordinary 6П14П] in there and still working fine.

That's how it gets 50W out of 4 x EL84.

The 6П14П has 14W-rated anodes, too.

In a guitar amp, I'd run the anodes at 370V or so (to get that good sound), and use a power MOSFET to set the screens at 300V or so. The FET can keep the hum low when low values of capacitance are used, too (the Brian May special- AC30 uses 22uF in both main positions).

If you have good ventilation, they should last reasonably well even at 12W like this. For a guitar amp, anyway.
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Old 8th July 2010, 05:39 PM   #9
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As for screen resistors, as low as 100 Ohm is OK at 300V. The higher the screen voltage, the higher you need to go. If you're going to let the full 375V onto the screens, best use at least 1K.

Higher values give earlier breakup - might be just what you need!
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Old 8th July 2010, 08:55 PM   #10
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Thanks for all the good info and recommendations.

I have 379V at the plate and with 1K ohm screen resistors there is 360V at the screens. I wonder if I need more resistance.

The transformers are Mercury Magnetics. The friend I'm "upgrading" this for bought all the iron (including a power supply choke to add) before he brought it to me.
I will recommend the Russian tubes.

A a point of humor, I e-mailed the Mercury people and asked about the power transformer they recommend with the higher voltage than stock (316-0-316 as opposed to 275-0-275) and his reply:

Quote:
"That is the correct replacement and part of why it sounds better.
We have sold hundreds of these.
The book may spec that VDC but to achieve tone, it is about safely bathing the tubes in as much B+ and primary impedance as you can."
You can see why I came to you guys for an explanation. I had a pretty good idea you guys wouldn't tell me to "bathe" the tubes.
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