Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: gitarzysta on March 21, 2021, 01:55:54 pm
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I have one unused chassis and decided to build something higher gain compared to my clone of Princeton Reverb.
Not Soldano high gain, though ;)
I am seriously thinking to get Trainwreck Express preamp and PI and pair with:
1. EL84
2. Low-powered EL34 (think Marshall Mini Jubilee or Origin 20W)
3. 6V6GT
I am not after 100% accurate Trainwreck copy - far from this.
Anyway, what's your suggestions (1, 2 or 3) as far as dynamics go? I basically want my amp to be almost on full and get all the tones (from cleanish to to distorted) using my guitar vol knob. I am not after insane levels of distortion, but the demos of Trainwercks I watched tell me it's pretty capable.
I have many pairs of EL34 so going this route would save me some $$$ - any suggestions about voltages to get into 20W range with 2xEL34? I measured my Origin 20W head in full power mode (20W), and the voltages are:
- anodes: 267V
- screens: 258V
- voltage drop across 150Ohm cathode bias resistor is 16.2V
I was thinking to get these lowered voltages using PVR.
Any feedback will be appreciated!
Gitarzysta
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The Trainwreck Liverpool is essentially an Express with a 4xEL84 poweramp. Could be worth checking out. 2xEL84 should work there too.
The tricky bit with a low powered EL34 amp is to find a suitable power transformer. The Marshall Studio series amps appear to have two separate HT windings, one for preamp and one for poweramp. The powramp seems to have low and high taps to get the 20W and 5W voltages.
You could use powerscaling (or VVR) instead of separate HT windings though.
Good luck!
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Yes. I was thinking about PVR.
Marshall Origin power scaling uses an interesting concept: the lower output power the higher plate voltage but lower screen voltage. I believe Studio amps use similar approach. Origin 20 voltage table for its 3 power scaling settings is attached below if anyone is interested.
But as I do not want power scaling per se, just permanent lower wattage, I guess a simple PVR would work.
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Hammond 369KX with SS rectifiers should deliver ≈300V B+ with EL34 coupled to a Hammond 1750NA OT - 6KΩa-a - also, should be near to pure class A P-P operation. DC bias would be ≈90mA per tube with ≈15V Vk so start with ≈150-180Ω under each EL34 cathode, or ≈75-91Ω common under EL34 cathodes. 1KΩ screen resistors for each EL34 would be recommended. should push out ≈20-22W to appropriate load.
--pete
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Hammond 369KX with SS rectifiers should deliver ≈300V B+ with EL34 coupled to a Hammond 1750NA OT - 6KΩa-a - also, should be near to pure class A P-P operation. DC bias would be ≈90mA per tube with ≈15V Vk so start with ≈150-180Ω under each EL34 cathode, or ≈75-91Ω common under EL34 cathodes. 1KΩ screen resistors for each EL34 would be recommended. should push out ≈20-22W to appropriate load.
--pete
Thank you. Please coach me, though. 6k Raa is EL34 anode to anode resistance at this low voltage and needed to keep it in class A operation? (so 8 and 16 Ohm taps of this particular transformer will still result in this reflected total load of 6k? Typical class AB PP operation conditions for EL34 show Raa=3400)