Hi all!
I will start building this project now and have some questions.
Has anyone here on the forum built this? How does it sound?
I have built four PP EL34 and KT88 amps. But all have had fixed-bias.
Is there a big difference sonically between cathode and fixed bias?
Thanks //Daniel
I will start building this project now and have some questions.
An externally hosted image should be here but it was not working when we last tested it.
Has anyone here on the forum built this? How does it sound?
I have built four PP EL34 and KT88 amps. But all have had fixed-bias.
Is there a big difference sonically between cathode and fixed bias?
Thanks //Daniel
I tried to mod in paint to show what I've done on an amp.
A different connect on the input tube, G2 resister 390 ohm to triode connect it, screen to cathode.
Constant current sink on the LTP, the trimmer pot is not necessary for 5% resistors that you measure and match. I've used the Russian ECC99 here, great driver tube. This is worth the effort to do! Mosfet configurations also will work.
I like a solid bias on the output tubes so I use adjustable fixed bias. I like the low end of the music where you can feel the speakers breathe.
For some power transformers missing the 70 volt tap, I use a small 6.3 to 110 100ma transformer backwards connected to a 5vac tap. That will give be about -90 volts DC. I use a voltage divider with pots to derive an adjustable neg bias range from -20 to -50VDC to tame the output tubes to 45 -50ma. If the B+ is around 500V, I increase the neg range, so -30 to -60VDC.
I did not add this to the markup, you should be able to do this.
A different connect on the input tube, G2 resister 390 ohm to triode connect it, screen to cathode.
Constant current sink on the LTP, the trimmer pot is not necessary for 5% resistors that you measure and match. I've used the Russian ECC99 here, great driver tube. This is worth the effort to do! Mosfet configurations also will work.
I like a solid bias on the output tubes so I use adjustable fixed bias. I like the low end of the music where you can feel the speakers breathe.
For some power transformers missing the 70 volt tap, I use a small 6.3 to 110 100ma transformer backwards connected to a 5vac tap. That will give be about -90 volts DC. I use a voltage divider with pots to derive an adjustable neg bias range from -20 to -50VDC to tame the output tubes to 45 -50ma. If the B+ is around 500V, I increase the neg range, so -30 to -60VDC.
I did not add this to the markup, you should be able to do this.
Attachments
I redrew one of my amps with a EF86 on the input and tried to match your circuit. It has Schade feedback on the outputs that can be left off. I also use a damper tube as a B+ delay device in the power supply, that can be converted to tube instead of solid state or just leave it out, no delay. I put blue leds under some of the tubes, power on indicator etc... that also can be left off.
I simplified the bias on the LTP, and it needs the heaters to be elevated in voltage due to the LTP cathode to heater can't exceed 100V....
I use larger coupling caps, PIO russian caps are nice, I prefer .3 to 1u. I like large caps in the B+ feeding the output transformers for the low end...
It was drawn for the iron that I had on hand at the time...
Food for thought...
I simplified the bias on the LTP, and it needs the heaters to be elevated in voltage due to the LTP cathode to heater can't exceed 100V....
I use larger coupling caps, PIO russian caps are nice, I prefer .3 to 1u. I like large caps in the B+ feeding the output transformers for the low end...
It was drawn for the iron that I had on hand at the time...
Food for thought...
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