Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on April 27, 2011, 10:32:09 am
-
I notice Doug sells 12BH7. Doing a search I found it was suggested to be good as a reverb driver tube. I notice(correct me if I'm wrong please) that the pinout is the same as a 12AT7. Can you drop one in as a substitute or do you need to tweak the circuit. Anyone have schematics? I'd like to try it out.
Anyone use these tubes?
Can I swap one in for a 12AX7 for a gain stage to try or do you need to tweak?
Schematics?
What are these tubes mainly used for?(in guitar amps)
-
You can drop in, since the pinout is the same.
The 12BH7 has a mu much like a 12AU7. So you could simply try a 12AU7 first to see if you like what's happening.
Because of the low gain, you probably won't deliver as much power to the reverb tank as if you used a 12AT7. Another way to say that is that you'd have to have your reverb channel's volume quite a bit higher to deliver the same power to the tank.
You may not mind the change. Unless you really like a lot of reverb, you might have your reverb knob set to 3-4 to just get a little extra meat around your sound. You will therefore notice that you have to turn up the reverb pot quite a bit higher for the same channel volume setting, and the same amount of reverb. That's not a problem for most folks.
You may want to experiement with a larger cathode resistor. I wouldn't be surprised if you needed to raise it to around 8k or so for decent bias. I tried to guess the idle point with curves, and my estimate is around 9mA per triode with a 2.2k cathode resistor. That's a lot of current for this stage, and you might want to tame it down a bit.
-
Can you drop one in as a substitute or do you need to tweak the circuit.
What are these tubes mainly used for?(in guitar amps)
You should know that depending on your amp (and PT heater current rating/availability) this tube uses twice the heater current as the other 12a_7 tubes. Just something to be aware of when subbing various tubes is to know their specs.
-
Thanks guys