Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on September 02, 2010, 11:50:48 am
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I have in my possession an old reverb tank I got with an empty amp cabinet. The input impedance is 7-8 ohms & the output impedance reads @170 ohms. Seems kind of low. Is this tank usable in the one-tube reverb circuit (tubenit's) I'm installing? also has anybody here put any kind of on/off switch on that circuit?
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Did you measure output impedance or did you measure DC resistance with a multimeter - two different things.
To switch off a reverb circuit, ground the input to the tank or the input to the recovery tube.
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8R is a normal input impedance for a transformer driven tank. 170R is a low output impedance compared to 500R or 600R on your typical accutronics tank, but is not devastatingly unusual methinks. AFAICT it is beneficial for bandwidth if the output impedance is at least 5 x lower than the input impedance of the following (recovery) gain stage (which one assumes it will be - even with 600R output impedance), so methinks you ought to be okay. Try plugging it in and see.
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After doing a little more research it almost looks a like low impedance tank is meant to be used with SS circuits, high impedance with tubes. I guess we'll see. I'll try it.
This amp has been such a cool project. The other day when it was still buttoned up, I had it SINGING but, it crapped out pretty badly when cranked to 10. Had to restart it, like a computer. Seems I forgot the grid load resistor before the PI. I can turn it up all the way now but it doesn't sing like that anymore. I'll try a higher value GLR to strike some sort of balance.
In fact, I can see the grids inside the tubes glowing red, maybe overbiased? 48 ma in pentode & 51 ma in triode mode, with 6L6GCs & 5881s, cathode biased. Any thoughts?