Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on February 19, 2022, 06:26:04 pm
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Question, am I going to have a problem with using 470 ohm 3 watt screen grid resistors with EL-34 tubes?
I'm in the process of converting a Bogan CHB-100 to EL-34 power tubea for a friend. The schematic shows the plate voltage at 465 volts and the screen grid voltage at 450 volts.
All the Marshall schematics that I have in my collection seem to show 1Kohm screen grid resistors, and most of them show 5.6Kohm control grid resistors.
I haven't worked with EL-34s in the past, so I'm not sure how to proceed on this.
Also, the Marshall schematics seem to show 150Kohm negative bias resistors for some amps and 220Kohm resistors for a few others, not sure how much of a difference that makes.
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But what’s feeding those resistors? The rest of the circuit is required really.
EL34 draw 1.5A heater current, are you sure that the PT heater winding can cope?
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the 7868 draws .8 of an amp heater current, so 4 of them draw 3.2 amps of current. 4 of the EL-34s would draw 6 amps, so that's quite a difference. If I were to just use 2 of the EL-34s, I'd be safe for heater current, and I can set up the output transformer output side to match the impedance requirements of the EL-34.
The other thing I could do is to use a quad of 7591As. I've converted a CHB-35 to 7591As, basically the same tube in a different bottle.
Still, the question about screen grid and control grid resistors is one that I don't know the answer to...
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Also, the Marshall schematics seem to show 150Kohm negative bias resistors for some amps and 220Kohm resistors for a few others, not sure how much of a difference that makes.
look at the marshall schematics you're comparing carefully: what power tubes are they using?
(willing to bet they're using 150k with 6550s for the export market and 220k for el34s for the uk)
i agree that el34s might be a bit too much for this. why not 6l6gc?
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i agree that el34s might be a bit too much for this. why not 6l6gc?
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The output transformer doesn't have the optimal loading for a 6L6 quad. I'm sure I could still use the 6L6, but if I'm going to go from the 7868 to an octal tube, and not the EL-34, I might as well use the 7591A, it has the same specs as the 7868, just with an octal base.
The reason I was thinking about the EL-34 was because it uses a similar loading to the 7868. I wasn't thinking about the heater current...
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Still, the question about screen grid and control grid resistors is one that I don't know the answer to...
EL34 screen current tends to run higher than 6L6 screen current. Use the "EL34-typical 1kΩ" screen resistor rather than the "6L6-typical 470Ω" screen resistor.
Higher Resistance ---> keep the Screen from melting
Lower Resistance ----> more watts cause Screen Volts don't sag as much
If you're prioritizing tube-protection over every last milliwatt, err on the side of too-high screen resistance.
As for G1 resistors: you're trying to see EL34 from singing & squealing on its own. EL34 has even more Gm than hot-rod 7591 & 7868. More G1 resistance might avoid a penalty for imperfect wiring.