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Bassman resistor question

Discussion in 'Amps and Cabs [BG]' started by yaryaryar, Oct 23, 2010.

  1. yaryaryar


    Oct 23, 2010
    I've got a '68 Silverface Bassman that burned out a couple resistors last night. I was being an idiot, and running way too much gain into the amp (do you know how awesome that overdrive sounds, though?), and it shut off on me. After determining that it wasn't just a fuse, I opened it up and found the culprits: two resistors connecting each pin of the pilot light to ground.

    Since these resistors are all black and carbony now, I can't determine what value they're supposed to be. Can anyone out there tell me what the values are? Are these resistors going to be a pretty standard value, or will they be very specific to every amp?

    If it matters, the amp is supposedly modded to an AA864 circuit, according to the last owner. I can't say for sure exactly what was done to the circuit.... but I wouldn't guess that my two burned resistors would be all that different circuit to circuit.

    Anyhow, any input is greatly, greatly appreciated. I can also post/send pics if it'll help. Thanks!
  2. On the schematic for my Bassman combo it shows those two resistors as being 100 ohm 1/2 watt. I found my schematic online from a site like Schematic Heaven (?). I'm sure if you do a little surfing you can find your specific model.
  3. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
  4. BillyB_from_LZ


    Sep 7, 2000
    Something else caused those resistors to open...the pilot light circuit is independent of the signal circuit.

    Those resistors are there to lock the voltage of each filament lead to a little over 3 volts above ground...probably for hum control. If those burned open, there must be a reason and one reason would be excessive current caused by excessive voltage...which might suggest a problem with your power transformer...especially if the amp died on you.

    I'd recommend that you check it out (or have it checked out) further before simply replacing those two resistors and flipping the power back on.

    Charlie (Passinwind)...does that make sense?
  5. yaryaryar


    Oct 23, 2010
    Wow, thanks for the quick replies here!

    I should have mentioned - I've done some google digging, and did find some schematics and layouts like those on ampwares.com (fantastic link, by the way). I can't seem to find these particular resistors on either layouts or schematics (looking at both aa864 and ab165).... just arrows saying "to all heaters and pilot light" or "to all 6.3 volt heaters" where those resistors should be.

    I'm no tube amp expert here (obviously), but I've done a few basic amp repairs before, and I'm pretty handy with a soldering iron. But I am admittedly, not great with schematics. Am I missing something on these schematics, or is my amp maybe more "modded" than I bargained for?

    And you're probably right BillyB, I should probably just take it in to a professional. I was just hoping it might be this simple.........

    Another point on the voltage suggestion: I was using an MXR micro gain pedal into the amp.... the micro gain was cranked, causing quite a bit of distortion. Could that have caused a voltage spike? Again, I now realize that may have been pretty stupid.....
  6. craig.p


    Sep 28, 2008
    New Hampshire
    This an interesting problem for several reasons.

    Those resistors may be part of the heater circuit and/or the hum-balance circuit, depending on the amp revision. But they aren't shown on the AA864 prints, which is strange, given the supposed AA864 mod.

    The larger question is, why would those resistors burn out? One possibility is shorted power transformer windings (there are two), which could cause the 300 volts on the plate winding to appear on the 6-volt heater winding, which would take out those two resistors and probably all the tube heaters as well.

    But an even bigger question is why overdriving the inputs would cause a secondary-winding short. Haven't had time to think about that.

    I'd seriously consider taking this amp to a tech. I suspect those burned resistors aren't the problem but rather a symptom of something much nastier going on.
  7. Clark Dark

    Clark Dark

    Mar 3, 2005
    well the "heaters" are also known as tubes
  8. craig.p


    Sep 28, 2008
    New Hampshire
    The heaters are contained within the tubes.
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    There's no reason for resistors to connect each pin of the pilot light to ground. The pilot light is connected directly to the 6.3v tap of the transformer and no resistors should be attached to it at all.
  10. Tusec


    Jan 10, 2010
    There's a reason for those resistors. It has something to do with balancing the heater circuit to reduce hum (I've never sat down and analyzed why they work...maybe someone can explain it...). My '68 Bassman has them.

    I think the guys upthread called it right that the burnt resistors are a symptom of something else going on, probably something fairly expensive. As has been mentioned, it could have been the high voltage winding in the power transformer shorting to the heater winding. It could also have been one of the tubes' plates shorting to its heater filament internally. If that's the case it might only be those two resistors plus a tube that needs replacing.
  11. craig.p


    Sep 28, 2008
    New Hampshire
    Bill, this is a total SWAG on my part and I admit I could be dead wrong, but it could be the pilot-light location for those resistors was the most-convenient "real estate" available. Or, maybe the board had already been laid out without them, and rather than do a re-layout, a "business decision" was made just to nail them onto the pilot light.
  12. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Yup, except I think you meant a cathode (not plate) shorting to the filament (the filament is inside the cathode to heat it up).

    Take a look at the aa371 variant schematic from the Ampwares site. It shows the two resistors. Also, unlike some other Fender amps, the center tap of the filament winding is not grounded. So the resistors are there to provide a ground reference for the filaments. Otherwise, the filament circuit could float and develop a DC offset, or act as an AC feedback path between stages.

    Did you have the head on top of the speaker? If so, I'd guess that the vibration either damaged a tube (wore out the insulating coating on the filament), or loosened a power tube in its socket (leaving the cathode ungrounded). This sort of mechanical damage seems more plausible than an internal short in the power transformer between two windings that are probably pretty well separated.

    Some ideas for diagnosis: first, unplug the power cord, and make sure the power supply caps are discharged! The schematic shows bleeder resistors, but don't bet your life on them. Pull all the tubes, remove the burned resistors, and test for shorts between the filament winding, other power transformer windings, and ground. Check all the tubes for filament to cathode isolation. Eyeball the wiring, especially the ground connections of the output tube cathodes. Retension the tube socket pins.
  13. yaryaryar


    Oct 23, 2010
    Hey everyone,

    Thanks so much for all the advice here. It seems the smartest option would be to take this to a pro... but I'm stubborn. And cheap. So I went to my local Radioshack as soon as they opened today, and picked up a handful of 100 ohm, 1/2 watt resistors.

    The first thing I did was to just replace the resistors, and carefully turn it on (fire extinguisher nearby). The resistors immediately burned up, along with a loud sort of high-pitched hum from the speaker. 2nd attempt: I replaced the resistors again, and also replaced the power tubes with a couple old 6L6s I had laying around. Turned it on and..... no fires! It makes sound!

    So it seems the problem was within the tubes, either the heating element or the cathode shorting to the filament, or whatever. So it looks like my next step will be buying some new power tubes, since I don't want to leave the old ones in there. And probably some praying or finger-crossing that I don't start any more fires.

    And yes, the head was on top of the cabinet, as I always have it.... so that looks like one habit that will have to change.

    Thanks again, everyone!
  14. The resistors are there to ground the heater supply. Each resistor balances the voltages and the centre point (0V) is the ground. Some amps like the B15 actually have a pot that can be adjusted for minimum hum. This is wired to each heater rail and the slider is grounded.

    If you have a tube with a heater short the resistors will burn out. This was the OP's problem.
  15. rickdog

    rickdog Gold Supporting Member

    Mar 27, 2010
    Now THAT is what I call a rock & roll approach to amp troubleshooting! :D

    And that's why I like tubes... you can do stuff like this and (probably) not blow up the whole thing! I've tried (and failed) to repair transistor amps where one failed component took out half the semiconductors on the PCB.

    I have some pieces of firm plastic foam I put between the head and cab, and pretend that helps with this problem :)

    When was the last time you changed output tubes? Maybe their time had come....
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Must have been a CBS 'improvement'. My pre-CBS Fenders didn't have them. If they were trying to make it better it's odd they didn't go to a DC heater supply.
  17. yaryaryar


    Oct 23, 2010
    Hi all,

    I was hoping to find some amp advice here again.... my Bassman was working fine for a couple weeks, but last night, in the middle of a show, it died again, smoke and all. I got it home, opened it up, and sure enough, the same two resistors had burned up again.

    To recap: Last time someone suggested there might be a short in the power tubes, sending far too much current across those resistors. I replaced both power tubes with brand new Tung Sol 5881s (same that were in there previously), I biased the amp, on the low side, even. And I got a couple hours of playing on it before the show, during which I saw no problems. Then last night: poof.

    Anyway, if anyone has any awesome ideas, I'm all ears. I realize it may be time for a professional to have a whack at it... but I'd love to do it myself at all possible. It's a learning process, you know?
  18. The amp should have 6L6GCs not 5881s. The 5881 has lower plate dissipation than the GC that's why guitar players like them, they breakup earlier. My suggestion is to take the amp to a tech and get it sorted out. You'll just keep blowing tubes until you do.
  19. Tusec


    Jan 10, 2010
    Hmm, this is an interesting problem.

    My stab is that it's still a tube shorting to the heater circuit, but it's unlikely that this would happen twice in a row spontaneously. So I'm thinking the short must be a symptom of something else, not the root cause. Something else is making the tube go nuts and short itself internally.

    One thing I'll mention is that 5881s have a lower dissipation and plate voltage rating than 6L6s. This may be significant given the brutal plate voltages that the Silverface Bassmans used (it's been years but IIRC mine was about 450V on the output tubes). Maybe they're just not up to it. On the other hand, I don't think most modern "5881" tubes are really 5881s. They might be spec'd higher than the originals.

    You might pull your output tubes and ohm out the pins to see if some of the elements really are shorted together. You might also look for faulty wiring or carbon bridges around the tube sockets, though I think this low probability.

    This could turn out to be a real headscratcher with a very subtle chain of causes.

    Some other thoughts:

    If you have the AB768 circuit you might have cathode bypass caps on the output tubes that could get squirrely and have an intermittent short. If one shorted, the bias could go haywire and the tube could runaway.

    The culprit could also be in the bias circuit itself. There's an electrolytic cap there that could go bad. It's SOP to replace that cap on older amps and yours might not have been done. A faulty bias circuit could cause a runaway tube.

    It seems like the possibility of some kind of short in the PT windings might be back in play too.
  20. whys that odd? a decent dc heater supply is expensive to design and can prove to be just as noisey as ac unless you're careful

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