'67 Bassman problem......ideas???

[StratTone]

Ok, just wanted to get some ideas before I go to test my Bassman later. Was playing on it the other day and got some smoke. Burnt out one of the 1 Watt 470 ohm plate load resisters. I only had a 5 watt ceramic laying around to replace it with but when I got it going again no matter what I do the current is 80ma or so. One tube acts just fine but the one that had the PL resister go bad is way high no matter. It is new matched tubes in there now and not a balancer issue. I am thinking a bad cap in the power supply. Any other ideas?

[Blue Strat]

New tubes CAN be bad. Try switching sockets to confirm.

If it's not the tube, the bias supply isn't getting to pin 5 of that socket or the socket is not making contact with pin 5 of the tube.

[Swarty]

Those amps have a bias balance scheme, so unless you've had it changed one tube will not change when you adjust the bias pot. Also, not sure what the PL resistor is you are referring to. But if it is taking out screen resistors you are dealing with a bum tube. I also think if your tube was not getting bias voltage (pin 5) it would go much higher than 80mA.

[stratman_el84]

+1 on the bias-balancing circuit. I always rewire those to a standard bias-adjustment type setup unless the customer wants it left as-is. You may find that the bias voltage is being blocked or dropped significantly to one side due to a bad bias balance pot.

After testing for bias and tube-pin connection to the socket along with the other checks mentioned, check for DC leakage from the blocking caps going from the plates of the PI to the power tube grids, especially on the side that blew out the resistor (which I agree with Swarty on that it's probably a screen resistor, not a plate resistor).

Also check *both* screen resistors, as the other may be out of tolerance from stress and age as well as for any grid resistors that may have opened or gone out of tolerance. I'd check the PI blocking caps last if you haven't found any other causes for the problem, as usually it's a bad resistor somewhere or a bad bias pot. Good luck!

Cheers!

Strat

[StratTone]

Ok, I did some probing and got -44 one pin 5 and -37 on the socket with the problem. The difference I'm sure is the balance pot, right? Anyway you guys got any other ideas?

[Blue Strat]

Yes, turn the bias balance pot and see which pin 5 moves. Hopefully the -37 will adjust more negative which will result in less idle current in that tube.

[StratTone]

Yes, turn the bias balance pot and see which pin 5 moves. Hopefully the -37 will adjust more negative which will result in less idle current in that tube.

No matter how far I turn it I can only get one tube down to 57ma and the other to 80 or 90 or so. Both the screen grid and plate load resisters are in tolerance. I tried another set of tubes since these were GT grade 7s and it is still the same. Now I'm confused, plus I still haven't found a nice clear schematic of the AB165. Do you know of a place where there is a revised clean schematic?

[Blue Strat]

www.schematicheaven.com (http://www.schematicheaven.com)

Are you able to get the same voltage reading on pin5 of both tubes by adjusting the pot? If not, there's a problem in that circuit. It really needs to be converted to "true bias" as opposed to "bias balance" anyway.

[Swarty]

I too am confused, what are you referring to as "plate load resistors"?

[Blue Strat]

I too am confused, what are you referring to as "plate load resistors"?

He must mean screen (pin 4) and grid (pin 5) resistors.

[StratTone]

He must mean screen (pin 4) and grid (pin 5) resistors.

Sorry your right. I've been known to screw up my words. haha Well I guess it really just looks like I just don't have any tubes that will draw correct with the curcuit. I took the tubes out like you said and I could get a good -37 on each. I'm going to make it a true bias tonight. I have a schematic for the 864 (I think thats it) and use it to change my 165. Looks like it is just disconnecting the current bias circuit and moving one 220k resistor.

[booj]

Make sure you don't have leaky coupling capacitors.

[jawjatek]

I just got a "67 AB165 Bassman myself I am fixing up. Here is a link to the bias conversion info: http://www.fenderholic.com/bassman_bias%20mod.htm. I replaced the 15k on the bias pot with a 27k. HTH