JCM800 Micro EF80 Build

2L man

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Here's another idea. I have an AS-05T240 in my junk drawer. It has both a 240V and 220V HV tap. I can run the 240V tap in the amp and beat the crap out of the EF80 plates at ~300V. I think Rob has stated before that he's run them at 300V. This will also bring up the rest of the voltages at the B+ nodes. I could also use the 220V tap and raise voltages marginally.
I can't see any reason why using parallel feeds (rails) for different stages can not be used? Perhaps there comes something positive when typical series feed is used and stages interact? But if there comes interaction I think it is mostly between B+1 Output Transformer/Anode(s) and B+2 Screen(s). Because for the Phase inverter B+3 filter feed there is usually 10k dropper and the capacitance is quite high for much smaller current. And following stages B+ usually at least 10k.

So after first additional filter capacitor (which now becomes a B+0) divide feeds to two "rails" one for B+1/B+2 and another for B+3, B+4... Then B+3 voltage comes higher and then B+1 can be dropped lower using a Sag-resistor.

Now the Sag-resistor and B+1 filter capacitors can be "tuned" how you like. Perhaps a potentiometer can be good for testing if powerful enough can be found? Or just a SPDT switch which has an Off in the middle and three different value resistors?

Perhaps instaling one more tube socket for 12AU7 does not make EF80 amp too complicated.

Now I have been thinking more... if you have a CT power transformer it is possible to get solid bias circuit and using fixed bias for 12AU7 make it sound even better. And if you wan't both require only one SPDT switch to change between Cathode and Grid bias.
 

owlexifry

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I can't see any reason why using parallel feeds (rails) for different stages can not be used? Perhaps there comes something positive when typical series feed is used and stages interact? But if there comes interaction I think it is mostly between B+1 Output Transformer/Anode(s) and B+2 Screen(s). Because for the Phase inverter B+3 filter feed there is usually 10k dropper and the capacitance is quite high for much smaller current. And following stages B+ usually at least 10k.

So after first additional filter capacitor (which now becomes a B+0) divide feeds to two "rails" one for B+1/B+2 and another for B+3, B+4... Then B+3 voltage comes higher and then B+1 can be dropped lower using a Sag-resistor.
it almost sounds like you're describing my earlier suggestion... apparently too much sag for AB/push-pull (cos that first 'sag-resistor' will need to drop about 80V)..

but then again, others have said this sag should be minimal because it won't be pulling that much current...

Now I have been thinking more... if you have a CT power transformer it is possible to get solid bias circuit and using fixed bias for 12AU7 make it sound even better. And if you wan't both require only one SPDT switch to change between Cathode and Grid bias.
one of the main challenges ive seen discussed around fixed bias, is that it needs to be quite low (<10V) for 12AU7/EF80. Looks like some folks have attempted, but how well this really works appears yet to be determined..
https://www.tdpri.com/threads/ef80-pp-amplifier-with-adjustable-fixed-bias.1065339/page-2


in the absence of locating/sourcing a PT that actually has x2 separate secondary windings at the correct voltages...i'm starting to think x2 separate PTs might just be the easiest way to go...

363AX -> <300V B+ (EF80 power amp)
369BX -> ~380V B+ (preamp + heater circuit)
1654741628551.png

expensive solution, but if it gets the job done..
 

pelayostyle

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On whether you think it's best to just leave the circuit alone and keep the lower voltages on V1-V3 or somehow try and figure out a way to raise the voltages closer to original specs.
 

Paul-T

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If Robrob's too busy to answer - and goodness knows how much we appreciate all the free advice he's always doling out - I wonder if @dougsta has any thoughts? You were one of the first folks to champion the EF80, do you think that involved any compromise on settling for lower voltages for the preamp?
 

pelayostyle

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I installed the AS-05T240 and used the 220v rail. Voltages did go up across the board by ~18V. EF80’s are getting 290V at the plates. I’m afraid using the 240V rail will send them over 300V. I really don’t want to kill these NOS Amperex’s.

I did notice that the clean headroom is higher. This is probably a result of raising the voltages on the preamp tubes. Makes sense. Other then that, I can’t tell much of a difference.
 




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