Dear All!
I try to mod high gain guitar amp, it had a lot troubles with frequency range balance.
After all anode bypass capacitors and plate-to-grid caps were remowed, I found that was the reason of specific sound which I don't like . ( That filters was used on each amplification stage!)
But with tonal improvement, dynamic range dropped, so it is now around -38db ( seems designers decided to simply put filters to solve this problem. Noise appear when guitar or re-amp box connected).
Hiss 100% comes from first two stages. I tried to filter out hiss by placing 470pf ceramic cap between plate and grid , and hiss dissapeared (but filter damaged guitar tone, so this is not a solution).
This is how first two stages look like:
Imgur: The magic of the Internet
What can I do to reduce hiss? I think the problem might be with voltage divider (R1, R2 - 180k), or maybe there is too much amplification? May gain reduction help (with removig that divider from second stage) ?
Unfortunately I cannot easily bypass second staege - amp uses that optocouplers for routing ( Maybe hiss comes from there?)
Original schematic:
Imgur: The magic of the Internet
I try to mod high gain guitar amp, it had a lot troubles with frequency range balance.
After all anode bypass capacitors and plate-to-grid caps were remowed, I found that was the reason of specific sound which I don't like . ( That filters was used on each amplification stage!)
But with tonal improvement, dynamic range dropped, so it is now around -38db ( seems designers decided to simply put filters to solve this problem. Noise appear when guitar or re-amp box connected).
Hiss 100% comes from first two stages. I tried to filter out hiss by placing 470pf ceramic cap between plate and grid , and hiss dissapeared (but filter damaged guitar tone, so this is not a solution).
This is how first two stages look like:
Imgur: The magic of the Internet
What can I do to reduce hiss? I think the problem might be with voltage divider (R1, R2 - 180k), or maybe there is too much amplification? May gain reduction help (with removig that divider from second stage) ?
Unfortunately I cannot easily bypass second staege - amp uses that optocouplers for routing ( Maybe hiss comes from there?)
Original schematic:
Imgur: The magic of the Internet
Guitar Amplifier questions are welcome on the Tubes / Valves portion of DiyAudio (Vacuum Tube Hi Fi / Stereo threads).
However . . . I will remind people that there is Also an Instruments & Amps portion of DiyAudio (Guitar Amp threads).
Guitar Amp fans regularly read that portion of this forum, and are more experienced than I ever will be about Guitar Amps.
Many persons read Both the Tubes / Valves and Instruments & Amps portions of DiyAudio.
Just Sayin'
However . . . I will remind people that there is Also an Instruments & Amps portion of DiyAudio (Guitar Amp threads).
Guitar Amp fans regularly read that portion of this forum, and are more experienced than I ever will be about Guitar Amps.
Many persons read Both the Tubes / Valves and Instruments & Amps portions of DiyAudio.
Just Sayin'
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I simulated the hum level in each stage the snapshot attached each one with and without filter. Although the total gain of 2 stages is about 1000, the hum (from PSU) is mostly amplified by 2nd stage. If the hum come from heater supply you can connect and disconnect heater ground to see any difference, or use hum balance pot.
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Thanks Koonw! Unfortunately this is not the reason of high frequency hiss.
I disconnected one pole from PSU to heaters, no difference. But I think the problem is in topology, it was all the time in front of my eyes 🙂
Right after the guitar input comes controller with diodes & crystal oscilator.
Person who made this descision was so wrong... Yesterday I placed 10k grid stopper (C2 on pcb- after my modifications, and 1 megohm resistor marked as C1. Distance from tube is around 5 centimeters), and noticed that part of pcb became microphonic (due to bad EMI filtering), I will bring back previous value 67kOhm, and solder directly on tube socket.
It should help I hope. If not, then I should find the way how to make shield around input jack and grid resistors.
Also I updated asc file with proper capacitor values - they specified 682uF/630V in schematic, but in fact here amp has 0.22uf, anyway that values doesn't matter anymore...
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