Okay. So. I still haven't finished my "top caps" project, and I have a 6DY7-based hifi project waiting behind that. But I spent the last week out of town and away from my stuff, so I started noodling on the next guitar amp project.
Donor is an early 70s Bogen CHB-35A. Original tube complement is 6EU7, 12AX7, 6C4 concertina PI, and a pair of 7868s in AB1. So let's see ... five triodes and a push-pull output section ... sounds like a one-channel AB763, if I do the reverb in solid state and eschew the tremolo. I've even got a pair of matching 1x12 cabinets that came as part of the complete PA.
The basic plan is to reuse the iron, chassis, and sockets. I recently bought some 6P41S tubes, so those can go in the power amp. I've also got a bunch of 6N2Ps, which make fine 12AX7 substitutes. Add a 6AV6 to go in the 7-pin socket and we've got everything we need.
I adapted Robrob's one-channel AB763. Notable changes:
Other than those things, depending on your perspective, it's either a Super Reverb with diode rectification, or a Bandmaster with reverb added. The preamp is pure AB763.
About the reverb ... I found a lot of conflicting information about how best to make use of the BTDR built-in to a circuit. I took the simplest approach I could: A voltage divider to attenuate down to its 1.5V limit, an LND150 source follower to handle the low input impedance, and a voltage divider network to get the signal back to the mixer tube. HOWEVER, I know this circuit isn't right. I would appreciate any thoughts y'all might have.
I'm copping out on the 5V power supply for the reverb and using a USB wall-wart, with some additional filtering.
Donor is an early 70s Bogen CHB-35A. Original tube complement is 6EU7, 12AX7, 6C4 concertina PI, and a pair of 7868s in AB1. So let's see ... five triodes and a push-pull output section ... sounds like a one-channel AB763, if I do the reverb in solid state and eschew the tremolo. I've even got a pair of matching 1x12 cabinets that came as part of the complete PA.
The basic plan is to reuse the iron, chassis, and sockets. I recently bought some 6P41S tubes, so those can go in the power amp. I've also got a bunch of 6N2Ps, which make fine 12AX7 substitutes. Add a 6AV6 to go in the 7-pin socket and we've got everything we need.
I adapted Robrob's one-channel AB763. Notable changes:
- Reverb has been replaced with a simple BTDR-2 circuit. More on that.
- Power supply is very different. The donor uses a voltage doubler and a capacitor-coupled bias supply, so I pretty much had to stick with those. The 6P41S has very low screen voltage specs, so I'm pulling them from the 1/2 B+ point on the doubler. It also needs -65V bias, dang. And I'm omitting the choke in favor of an extra RC filter.
- No tremolo.
- 6N2P for phase inversion (as opposed to 12AT7). I like the hotter sound of a 12AX7 in the PI in BF Fenders.
Other than those things, depending on your perspective, it's either a Super Reverb with diode rectification, or a Bandmaster with reverb added. The preamp is pure AB763.
About the reverb ... I found a lot of conflicting information about how best to make use of the BTDR built-in to a circuit. I took the simplest approach I could: A voltage divider to attenuate down to its 1.5V limit, an LND150 source follower to handle the low input impedance, and a voltage divider network to get the signal back to the mixer tube. HOWEVER, I know this circuit isn't right. I would appreciate any thoughts y'all might have.
I'm copping out on the 5V power supply for the reverb and using a USB wall-wart, with some additional filtering.