Hi,
After 30 years using my JTM45 clone (Echolette BS40 from 1968), I've finally decided to apply all the mods I had in mind for so long.
Since a recap was needed, I had to screw up stock german build anyway, let's do it deeply.
I'm mostly cloning a JTM800 preamp, with all stages chaining in series, cold clipper on V1B, warm stage before the cathode follower and attenuators between stages
Preamp seems working ok on scope with no power tubes.
Next mod in my mind is trying to improve the amp's voice, moving master volume from pre to post tone stack. The stock pre is on the cathode resistor of the cathode follower, it tends to scratch a bit event with a new pot, tone setting gets screwed on volume change, and tone shaping is limited.
Problem occurs when I put a 100k resistor on cathode, and a 650k pot between the treble cursor and the grid from the inverter : amp self cuts off at 20/25W (dummy load), after a short period of sort of self oscillating. On Marshall schematics, vol pot is 1M, I don't have any. I doubt it really solve anything looking at the impedances involved in the tone stack.
I reverted back to pre tone stack master and problem disappears, but I'm not satisfied at all, especially because I don't really understand what's going on. I suspect something related to NB, but don't took time trying to disable it before revert back
Any thoughts ?
Thanks
After 30 years using my JTM45 clone (Echolette BS40 from 1968), I've finally decided to apply all the mods I had in mind for so long.
Since a recap was needed, I had to screw up stock german build anyway, let's do it deeply.
I'm mostly cloning a JTM800 preamp, with all stages chaining in series, cold clipper on V1B, warm stage before the cathode follower and attenuators between stages
Preamp seems working ok on scope with no power tubes.
Next mod in my mind is trying to improve the amp's voice, moving master volume from pre to post tone stack. The stock pre is on the cathode resistor of the cathode follower, it tends to scratch a bit event with a new pot, tone setting gets screwed on volume change, and tone shaping is limited.
Problem occurs when I put a 100k resistor on cathode, and a 650k pot between the treble cursor and the grid from the inverter : amp self cuts off at 20/25W (dummy load), after a short period of sort of self oscillating. On Marshall schematics, vol pot is 1M, I don't have any. I doubt it really solve anything looking at the impedances involved in the tone stack.
I reverted back to pre tone stack master and problem disappears, but I'm not satisfied at all, especially because I don't really understand what's going on. I suspect something related to NB, but don't took time trying to disable it before revert back
Any thoughts ?
Thanks
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Did you wire the MV before the 22n coupling cap (looks like C8)?
This is essential as the PI grid sits on a large DC voltage (maybe 100V).
Best post a drawing of your MV wiring.
The level trim-pot at the CF cathode is not a good MV. It will cause scratch noises when turning because there's DC on it.
This is essential as the PI grid sits on a large DC voltage (maybe 100V).
Best post a drawing of your MV wiring.
The level trim-pot at the CF cathode is not a good MV. It will cause scratch noises when turning because there's DC on it.
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Yes of course, the pot was before the 22nF coupling cap.
Isn't the signal pure AC coming from the CF ? When I looked on scope, I got 0V DC from the treble pot cursor, and of course grids from the LTP inverter was 20v biased.
The schematic of this part is the same than marshall design, I'm drawing it on kicad but it takes time as I learn also the software during the process
Isn't the signal pure AC coming from the CF ? When I looked on scope, I got 0V DC from the treble pot cursor, and of course grids from the LTP inverter was 20v biased.
The schematic of this part is the same than marshall design, I'm drawing it on kicad but it takes time as I learn also the software during the process