Bogen MX60a

[blindbrett]

i have recently aquired a bogen mx60a pa tube amp. I found some old threads regarding converting these to guitar amps by cascading the inputs etc. I would like to do the same with this amp and was wondering if anyone was familiar with these or had a schematic? I am fairly competant with tube electronics although I am still learning and just require some assistace with rewiring the amp to make it more guitar friendly. thanks for any help you can offer, it is appreciated.

Brett

[m6tt]

I am actually in a similar situation. I acquired a 50-watt McGohan PA for cheap, dual 6ca7 (sylvania), 12ax7 dual inputs, 12ax7 for the tone controls and phono in, 12av6? post tone control amp?, 12at7 phase inverter/driver. It's thankfully not at _all_ frequency response limited to vocal range or anything, but it sounds very, very clean. Too clean for guitar and way too clean for harmonica. The 6ca7s are ultralinear connections, the some of the inputs have transformers on them for impedance matching, alltogether it was designed to be fairly good at not changing sounds but making them very, very loud. I've had some success changing the input tube to a 12ay7, the tone tube to an 12at7, and leaving the 12at7 in the phase inverter. I swapped the 12av6 (why would this tiny radio tube be needed?) with a mullard one I had...it was still too clean but a little more aggressive. Finally the guitarist just stuck a shitty pedal in front of it (a Cool Cat I think) and used it to overdrive the inputs.
I am considering changing the phono control out (it was a PA) and using it to overdrive something like a 12sn7.

[blindbrett]

I was wondering if anybody could explain to me what the nfb winding is. I keep reading that it is a good idea to disconnect the nfb winding when converting to a guitar amp but I have no idea where to find it. I believe its out of one of the transformers but I dont know how to find it. Any suggestions....and what exactly does disconnecting it do to benefit the amp with regards to guitar? thanks

[Ty_Bower]

Negative feedback reduces distortion at the expense of gain. Many guitar players seem to want all the gain they can get, so they generally don't want NFB. It also seems that a certain amount of distortion in a guitar amp contributes to the "tone".

Here's a thread where I found a transformer with a feedback winding. It's fairly obvious on the schematic mentioned in the thread.
Mystery transformer

[Tom Bavis]

Generally you can find a resistor that goes from one of the speaker taps back to an earlier stage in the amp - sometimes with a capacitor in parallel. Occasionally it's a separate winding - Stromberg liked to do that. Remove the resistor (and cap, if present) and feedback will be gone.

With feedback, you have a clean output with an abrupt transition to clipping - can sound nasty, depending on how much feedback is used. Without feedback, there will be more distortion, but a more gradual onset of clipping. Expect a large increase in gain, more hum and noise, and more effect from the speaker on overall sound. You might want to reduce gain elsewhere.