Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on January 03, 2014, 09:04:05 pm
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Hi,
I have old chasis with the whole transformer setup and will like to try a dual channel design with cathodine P.I. and cathode bias power section. The original amp run on 500-550vdc on plates. It was EL34 fixed bias, U.L.
So,.. Will it be safe to run it at those voltages in cathode bias mode?
thanks in advance.
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Tube charts say 375 VDC plate volts for EL34 cathode bias. Fixed bias allows for correct bias at higher plate voltages. Cathode bias requires the correct supply voltage -- usually rather low, in order to maintain bias. (EL34's max out at 800VDC, so 375 is rather low. But note that at 800 pate volts, only 400 for the screens.) I suspect that at 500+ plate volts, good cathode bias may be a problem.
It might seem that increasing the value of the cathode resistor will reduce current draw; but it doesn't. As the cathode resistor value increases, it pushes up the plate voltage! The higher plate voltage draws more current through the larger cathode resistor. You just keep dancing around the same high bias point. The only way to get the correct bias is to start with the correct plate voltage for cathode bias. Or, use a higher plate voltage with fixed bias.
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Thanks!
About the G2 limit i was thinking in a VVR so i can drop that voltage to 400-425, but as you explained seems that is not gonna be enought to be in a safe situation. IŽll try the fixed bias mode.
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> How much is too much B+ for a cathode biased EL34 amp?
Depends how high-impedance you can get the output transformer.
> original amp run on 500-550vdc on plates
Assume 30 Watts Pdiss per EL34, 60W per pair.
Let's use "500V" because you will lose some in Rk.
60W/500V= 0.12 Amps maximum cathode current/pair.
500V/0.12A= 4166 ohms equivalent load on B+.
The OT p-p impedance must be roughly twice this, 8K or so.
Power output may be 60W*0.4 or 24W.
A 24W 8KCT OT is unusual but not impossible. In fact it sounds like a Williamson. You should probably use the original OTs. And before a rip-out, try just lightening the NFB to taste.
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Thanks!
About the G2 limit i was thinking in a VVR so i can drop that voltage to 400-425, but as you explained seems that is not gonna be enought to be in a safe situation. IŽll try the fixed bias mode.
In pentodes/beam tetrodes, the screen (g2) dictates the operating condition more than the anode, so you can start with any anode voltage (below the maximum rating of course) and adjust the screen voltage until the desired bias condition is met.
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If someone is interested, i attached the original schematic for a similar model. Mine have 4/8/16 ohm outputs. :icon_biggrin:
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So you have the original UL OPT? If so, why change it to cathode bias as the output power will be lower.
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Look at the schemo, the lowest secondary tap on the OT is 125ohm.
Brad :icon_biggrin:
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So you have the original UL OPT? If so, why change it to cathode bias as the output power will be lower.
The power output is not a problem. I love the simplicity of cathode bias.
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Look at the schemo, the lowest secondary tap on the OT is 125ohm.
Brad :icon_biggrin:
Read my previous post. MINE have 4-8-16 ohm outputs. That schemo is for a previous version.
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Sorry, my mistake.
Brad :w2:
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Thanks! About the G2 limit i was thinking in a VVR so i can drop that voltage to 400-425, but as you explained seems that is not gonna be enough to be in a safe situation. IŽll try the fixed bias mode.
I think VVR is a great idea! 425V is only about 13% higher than 375, which may be close enough tolerance. So you might give cathode bias a try. I'd keep the bias current metered at turn on and be ready to shut down just in case; and/or use a fuse at least for start-up testing.
You could also add a 12V bucking tranny which, together with VVR, should drop down darn near to 375V.
PRR's suggestion to cheat with a large OT primary impedance is also interesting, though the p-p mismatch may rob some frequency response.
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Sorry, my mistake.
Brad :w2:
No worry men. :wink:
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Since maximum output power is not required, then cathode bias could be used even with high Eg2, taking a cue from PRR, running the tubes with higher reflected load impedance may be the ticket, do you know the primary impedance of your OPT?
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I say:
Reduce the NFB (increase the 120K near the OPT in that plan)
Fix the preamp to something more guitar-like.
It's a fine amp as-is, though maybe a bit stiff for guitar.
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Since maximum output power is not required, then cathode bias could be used even with high Eg2, taking a cue from PRR, running the tubes with higher reflected load impedance may be the ticket, do you know the primary impedance of your OPT?
Not yet, but when i get into work iŽll do de measurements and calculations.
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I say:
Reduce the NFB (increase the 120K near the OPT in that plan)
Fix the preamp to something more guitar-like.
It's a fine amp as-is, though maybe a bit stiff for guitar.
I say:
Reduce the NFB (increase the 120K near the OPT in that plan)
Fix the preamp to something more guitar-like.
It's a fine amp as-is, though maybe a bit stiff for guitar.
Mine version in diferent to the schemo one. I have 4-8-16ohm outputs, but the circuit board was totally destroyed and most parts missed, so i donŽt know were NFB was attached in mine.....
My poor english skills and my (even worst) tech skills made hard to me understand some of your answers.
When you talk about "running the tubes with higher reflected load impedance ", means use a higher speaker load than expected? I mean, 16ohm speaker in the 4 ohm output...
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My poor english skills and my (even worst) tech skills made hard to me understand some of your answers.
When you talk about "running the tubes with higher reflected load impedance ", means use a higher speaker load than expected? I mean, 16ohm speaker in the 4 ohm output...
ĄNo pasa nada! Yes, that is the idea, after you find out the specs for the OPT, we can calculate how to do it (or if it is possible).
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Ok. i got it!
Maybe i donŽt start that project in a few months because job, family and other semi-finish amps.... you know what i mean...
iŽll be back!