Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chip on October 23, 2010, 07:46:44 am
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2X6L6GC amp, with 6G16 type tremolo, Long Tailed Pair, and power supply "A"=550, "B"=542, "C"=440, "D"=386
"A" and "B" to 6L6s, "C" to PI and "D" to pre-amp
Is there a maximum voltage that the 12AX7 plates can be subjected to or do you really only need to worry about the voltage drop between the anode and the cathode?
I was going to connect the 12AX7 trem valve to "B" but am I better off connecting it to "C" in the power supply?
The voltage drop across pin6 to pin8 for the trem valve is 283V if I use "B" and 236V if I use "C".
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I was going to connect the 12AX7 trem valve to "B" but am I better off connecting it to "C" in the power supply?
B at 542 V is too high, B at 440 V is exactly what Fender used on their 6G16 Vibroverb.
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If I only use 440V will I get enough wiggle out of the trem circuit for my 6L6 at around -56V bias? Keep in mind that the 6G16 bias is lower and does not need as much wiggle to get the affect.
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Should be OK. If not moving the supply connection one node up is quite easy.
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I love bias vary trem.
FWIW I think the critical issue with modern 12AX7s in these circuits is the heater-to-cathode voltage on the CF stage. Because of the higher voltages involved (in amps with BV trem for 6L6s), I have found problems with modern tubes crapping out with regular monotony (which I have attributed to poor heater/cathode insulation material).
Then after talking it over with one of the local wizz-bang techs (Simcha Delft) for a few months, I hit upon the idea of going with a AC-coupled CF in the trem circuit. This allows really high input impedance, but also means you can experiment with much lower values of Rk in the CF stage to get enough slam. I tested this concept in a 5F6A variant (the attached schematic is the latest version) and the slam is amazing if I say so myself. Not only is it intense, but under different signal conditions and different intensity and speed settings it can get a kind of phasey-warble going on which is quite hypnotic. And since going to this, I haven't any more tube failures (touch wood)
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I have found problems with modern tubes crapping out with regular monotony (which I have attributed to poor heater/cathode insulation material).
Yup, Sovtek 12AX7s and their variants don't accept more than 100 V heater to cathode. Just use elevated heaters or better tubes - BTW, the EH variant from the same factory is much more robust.
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This is good to know. thanks,
If I do fail the valve from over voltage between cathode and heater will it just take out the valve or is this a potential catastrophic failure that will take out all the valves and maybe the transformer?
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If I do fail the valve from over voltage between cathode and heater will it just take out the valve or is this a potential catastrophic failure that will take out all the valves and maybe the transformer?
I've only seen local tube failures, with a loss of continuity and no impact to the rest of the amp. But Murphy's laws can and will apply...
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From my experience with this mode of failure, you will hear the tube starting to crap out when the trem is engaged - you consistently get odd voltage-popping noises that are sort of in-time with the trem. When you replace it with a fresh tube the problem goes away for a wee while, depending on the type of tube. I think good NOS tubes can handle it better, but to-date with the AC coupled CF, I haven't had any problems (The cathode idles at around 170 in the above design, and I also have the heaters elevated to about 40 or 50V).