Organ - Allen model 20a amp

[WindyJohn]

If the goal is guitar amp, you probably want a full on preamp ahead of it with tone controls and all that stuff. This is just a power amp.

I'm on the fence as far as what I would like to do. I could go the route of a total rebuild using the existing iron, or design a preamp, like you said.

 

[gadget73]

even with a total rebuild you'd need to add the preamp bits to the mix. Just depends if you want it all on a common chassis or not I suppose. There should be enough power supply onboard to run an external preamp through an umbilical cable. Likely just feeding a couple of 12ax7.

 

[BmWr75]

Haha, I see now. I never noticed who posted that. It was me!! I'm guessing it needs a preamp level input to drive the gain stage before the phase splitter. Did the person who posted the gain reduction modification specify how the input was being driven?

I use a preamp to drive the Allen 20 amps with the gain reduction mod.

 

[nerdorama]

For a straight in guitar amp you would probably want to convert the input stage from 12AYU7 to 12AX7 to get the gain you need. But unless you use pedals or some other circuitry you will only get a clean channel.

 

[trobbins]

WindyJohn, many guitar amps don't use global feedback, or just a low level of feedback. Removing the global feedback would allow the feedback stage to have its cathode capacitor bypassed - in, addition, the input stage can be cathode capacitor bypassed - and that would raise the gain a lot when using a 12AX7. That may be all that is needed to provide some output stage over-drive with a guitar pup. I'd also relocate the vol pot to between the input stage and feedback stage, to better utilise the pot as a gain pot. If every you got adventurous latter on, adding a PPIMV in lieu of the 220k grid-stoppers should provide more stage gain/overdrive flexibility.

Then there is just basic housekeeping restorations, including some ss diode protection of the 5V4. Perhaps use an elevated heater from a filtered B+ divider rather than an output stage cathode with larger signal swings - although that may not be noticeable.

 

[WindyJohn]

WindyJohn, many guitar amps don't use global feedback, or just a low level of feedback. Removing the global feedback would allow the feedback stage to have its cathode capacitor bypassed - in, addition, the input stage can be cathode capacitor bypassed - and that would raise the gain a lot when using a 12AX7. That may be all that is needed to provide some output stage over-drive with a guitar pup. I'd also relocate the vol pot to between the input stage and feedback stage, to better utilise the pot as a gain pot. If every you got adventurous latter on, adding a PPIMV in lieu of the 220k grid-stoppers should provide more stage gain/overdrive flexibility.

Then there is just basic housekeeping restorations, including some ss diode protection of the 5V4. Perhaps use an elevated heater from a filtered B+ divider rather than an output stage cathode with larger signal swings - although that may not be noticeable.

All great ways to go. The possibilities are endless.