Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: daveyajd on March 16, 2012, 11:36:44 pm
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I have a Fender Bantam Bass that I have posted many questions about in this forum. So, first of all, I would like to thank all of you for all of your information. With your help I have learned a lot about circuits and amp repair/modification. And my amp sounds better than it ever has. Although as the other guitar player in my band says, "It's still a Fender." :wink: I won't bore you all with the list of things done but basically I rewired to AB763 specs. using Doug's vintage cloth wire and parts as I went.(Very nice and always super fast delivery.) Tonight I re-voiced the normal channel a little bit. 220k plate resistors, and 2.2k cathode resistors. I previously separated the shared cathodes so I left all cathode bypass caps at 25/25. Except V2b which is 22/35 because that's all I had on hand.
Here are the voltages I am getting:
Pv = 437
PI 1 = 262
PI 2 = 108
V2 1 = 270
V2 6 = 270
V1 1 = 235
V1 6 = 388
Are my V1 voltages what I should expect? Channel 1 seems to have a little more gain, less headroom. As expected. And Channel 2 seems to just flat out rock now. I wasn't expecting the change in V2 because I had separated the two stages. Should I change any other values in the circuit for V1? I believe these mods are the Torres mods but I haven't done the .68 cathode cap or 56k slope resistor yet. What will that do?
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V1 6 voltage seems very high too me did you put a plate resistor ?
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I just checked the schematics and the voltages you're reading are definitely wrong: on V1 6 you should get 240V, I did not check the others. Here's the schem.
Colas
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Yes that's true. Actually it states 240v on the schematic and 270v on the layout. If you look closely the V2 is wired incorrectly on that layout also. I chalk it up to the period of time for CBS Fender. Anyway, since I have changed most values to match those of a BFSR AB763 I compare with those voltages which are 270v on preamps and 230v on PI. I was thinking voltages were weird on V1 as well. What would be the cause of this? Prior to changing plate resistors and cathode resistors all voltages were very close to 270v. Except the PI which has always been about 270v on pin 1 and 108 on pin 6.
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Except the PI which has always been about 270v on pin 1 and 108 on pin 6.
There's something wrong there, those 2 plate voltages should be very close to each other.
It might be a wrong value R or something's miswired. It happens to the best of them. I'd re-check all of the PI R's with a meter and then triple check all the wiring hook ups for the PI.
Same thing with V1a/b, they should both be very close in voltage to each other. V1a, pin 6 the higher voltage might be that it's not drawing any or very little current. Check the R values and the cathodes ground. A bad solder conection on the cathode/R would cause no current to flow in that triode stage.
Brad :icon_biggrin:
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So I checked a couple of my resistors. The 1M on pin 7 of my PI is not measuring out. Same with the 220 Pr on V1. I will try to replace those tonight and recheck my voltages.
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I think you found it, might be more bugs, but as the guy's say, divide and conquer.
Brad :icon_biggrin:
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It was the PI tube. After a couple swaps I have one that sits at right about 240v. Still not sure about the V1. More investigating tomorrow.
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So I discovered an incorrect cathode resistor that was throwing V1 voltages off. Now they sit at 233v. So here is the link to the series of value changes I am considering:
http://www.blueguitar.org/new/schem/misc_mod/bfhotrod.pdf (http://www.blueguitar.org/new/schem/misc_mod/bfhotrod.pdf)
I have separated and changed the cathodes(did not use the .68uf but a 22uf/35v instead), and changed to the 220k plates. My next changes will be the 56k slope resistor and the 2.2 M to ground but I am curious to know how these will affect the sound. Not just tonally, although that too, but how the values are interacting with plate voltages and such to raise/lower gain. That way I can fine tune a little bit. Thanks.