Low plate voltage: adding preamp tubes?

[alfi27]

Hi,

I have a JCM800 from 1981 that has lower plate voltage like the JMP MVs; it measures around 375V if I recall correctly. It has an extra preamp tube already, which the PT hasn't had any issues supplying. I upped the filtering as well, so the bottom end is tight and overall it sounds really good. However, I was thinking that it would have been super awesome to add a clean channel, if that's possible without altering the tone of the gain channel. I have a specific circuit in mind, that is a 3 stage Blackface inspired circuit.

It would require an extra tube (5 in total), a relay, and obviously the circuit itself. The B+ resistors are currently two 4.7k in series, so the preamp voltage could still be increased a bit if required. I would estimate my skills and knowledge to about intermediate, the only thing is that I have never wired up a relay. That shouldn't be too difficult though.

Another alternative, if an additional preamp tube would draw too much current and the relay wouldn't, is to wire up a simple clean channel like the Friedman Runt 20 on the unused triode of the extra tube. That's a great sounding circuit as well, but unfortunately I don't know it and struggle to find schematics. If anyone know it, that might be a better alternative altogether actually.

And finally: I settled on 100k on the 4 ohm tap for NFB, which of course is very little feedback and not the best companion for a clean circuit. Could the relay also switch in another NFB resistor in parallel, or would that cause a massive pop or otherwise be problematic?

Looking forward to hearing from you very knowledgeable people, absolutely love this forum! Cheers

[Stevem]

It can be done , but where the heck are you going to place the blackface tone stack pots?

[alfi27]

There's loads of space on the back, but I'm not too sure if it would be worth all the trouble. I think the Friedman simple clean would be a lot better, but I still need to find the schematic. The closest thing I know about is the Orange Rocker 30 natural channel, which I do have the schematic for, but that's not quite what I'm going for. As it's just one stage it shouldn't be that difficult to tweak and experiment with though, I'm thinking 1.5k/25uf cathode and 100k plate should be a good starting point?

Also, would that stage when bypassed "steal" filtering from the gain channel and notably affect it? And would the tube draw more current when using both triodes as opposed to just one? And finally, again, is the NFB resistor possible to make switchable? If no, no and yes I think I have to do it!

[Stevem]

You really want to stay away from crisscrossing signal carrying wires all around the chassis , especially mounting low level signal ( pot feeds) wires from being mounted near the speakers jacks of the OT and or primary or secondary wires!

Tube wise you will need almost a additional 1 amp of heater current to heat up 3 more preamp tubes so your more then likely going to need to bolt on a small filiment transformer to do that ,also each 12AX7 will pull 8 ma away from your V + current output level you have now.

This will likely drain away some 4 watts of output power that the amp could have had.

[sluckey]

A typical 12AX7 with 1.5K/100K will draw 1mA per triode for a total of 2mA per tube.

[Stevem]

Yup!
I should have posted that all 3 added tubes would pull no more then 8 ma.