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Author
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Topic: What the **** did I do to my amp?
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Mark Herrick
Member From: Los Angeles, CA
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posted 02 May 2005 03:14 PM
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I was attempting to try the reverb-on-both-channels Fender mod again, but decided to check the bias settings I made the other day.
I was measuring the plate voltage on the power tubes (pin 3) and I think I shorted pin 3 and pin 2 on the V8 tube.
The weird thing was that when the sparks flew on V8, I also saw sparks over on the other end of the chassis near V1.??? Now my ’74 SFTR has a nice, loud buzz.
If I pull V1 or V2, I still have the buzz. If I pull both V1 and V2, no buzz; probably because there’s no signal at all, right? What did I do to my amp?
I can’t afford to take this to an amp tech right now, so if I can’t fix it myself it’s going to be sitting for a while. |
Donny Hinson
Member From: Balto., Md. U.S.A.
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posted 02 May 2005 04:16 PM
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Shorting pin 2 & 3 of a 6L6 will throw full plate voltage onto the heater string, which might have caused an inter-electrode short in one of the preamp, or reverb tubes. Try swapping them out if you have any extra ones. The only other thing I would think might cause a continuous hum would be a bad filter cap. Voltage surges (dead shorts with power applied) have been known to ruin them in an instant!Next time...don't fix it if it ain't broke!  |
Mark Herrick
Member From: Los Angeles, CA
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posted 02 May 2005 07:11 PM
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And how, specifically, would I go about checking the filter caps? |
Mike Neer
Member From: NJ
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posted 02 May 2005 08:20 PM
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Very carefully! http://studentweb.eku.edu/justin_holton/caps.html [This message was edited by Mike Neer on 02 May 2005 at 08:22 PM.] |
Ray Minich
Member From: Limestone, New York, USA
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posted 02 May 2005 08:31 PM
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Neat website Mike. So this is the "mother" page to "The Sound of Capacitors". Good stuff!Mark, to check the final's bias currents you really have to break the plate feed and use a milliameter to do it with the least amount of inference (otherwise your "Infering" the plate current from some other reading).[This message was edited by Ray Minich on 02 May 2005 at 08:34 PM.] |
Larry Behm
Member From: Oregon City, Oregon
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posted 03 May 2005 04:52 AM
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Box the whole mess up and send it to Ken Fox, he will fix it.Larry Behm |
David L. Donald
Member From: Koh Samui Island, Thailand
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posted 03 May 2005 05:24 AM
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Sounds like a fried cap not taking it's low frequency to ground like it should.Sending full plate voltage to cap wil do that. It is often cheaper to fix something fried, if the unit runs, than have to look for something cutting off all function.
Consider that lucky. You shold be able to see which stage on a scope no problem[This message was edited by David L. Donald on 03 May 2005 at 05:25 AM.] |
Kevin Mincke
Member From: Farmington, MN (Twin Cities-South Metro) USA
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posted 03 May 2005 08:13 PM
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What Larry said............... |
Dave Grafe
Member From: Portland, Oregon, USA
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posted 03 May 2005 10:33 PM
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glad you didn't get bit, them things can put you on yer butt if the sparks fly the wrong direction[This message was edited by Dave Grafe on 03 May 2005 at 10:37 PM.] |
Mark Herrick
Member From: Los Angeles, CA
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posted 04 May 2005 05:15 PM
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OK, bear with me on this. Someone on another forum suggested that maybe I fried my hum balance pot by shorting pins 3 and 2 on the power tube. So I read this on the Hoffman Amps site regarding the heater wires and eliminating the hum balance pot and had a few questions: quote:
Make sure that the heater wires on the power tubes stay in phase. Pin two should go to pin two and pin seven should go to pin seven. If you see that they are crossing over from pin 2 on one power tube to pin 7 on the next power tube, you need to unsolder them at one of the power tubes and reverse them. This has the effect of canceling hum in the power amp. You will find them wired wrong all the time.
My heater wires are all crossed up; from V10 to V7 they alternate between pin 3 and pin 7 on each adjacent tube.
quote:
The 6.3 vac windings then go to the pre-amp tubes in the same way except that the pin numbers change. On the 12AX7 and 12AT7 tubes, pins four and five are soldered together as one connection and pin nine is the other connection.Pin two on the last power tube goes to pins four and five on the pre amp tube that is closest to the power tubes. Pin seven on the last power tube goes to pin nine on the pre amp tube. The 6.3 vac winding continues down the line in a twisted pair fashion to the rest of the pre-amp tubes.
Again, my wires at this point swap and go from V7 pins 2 and 7 to V6 pins 9 and 4/5 respectively. After that, from V6 to V4 they are consistent, then they switch pins at V3, then stay consistent again to V1 and then to V2. Do you think I should re-wire the heater string to be consistent, as Hoffman suggests? And finally: quote:
If your amp has a hum balance pot I would consider getting rid of it and replacing it with two 100 ohm resistors across the back of the lamp assembly. Replacing the hum balance pot with two 100 ohm resistors is just something I do because I don’t like the hum balance pot system and I don’t trust the pot. I have seen too many of the pots fail.If your power transformer has a green wire with a yellow stripe and it is soldered to the chassis ground you already have a heater supply center tap and you do not have to install the 100 ohm resistors.
I actually prefer the two 100 ohm resistor method because they act like fuses if a power tube plate wire ever shorts out from pin 3 over to pin 2. Pin 2 is one of the heater wires and is a direct short to ground for the high voltage that is found on pin 3. This happens all the time and you can save your power transformer by having the pair of 100 ohm resistors instead of a transformer center tap. The 100 ohm resistors will melt in half if the plate wires ever short out to pin 2. This is way cheaper than replacing a power transformer.
My amp does not have the heater supply center tap, just the two green wires to the pilot lamp then to V10 pins 2 and 7. Should I disconnect the hum balance pot and install the 100 Ohm resistors? And do you think I may have damaged the power transformer?[This message was edited by Mark Herrick on 04 May 2005 at 05:18 PM.] |
Donny Hinson
Member From: Balto., Md. U.S.A.
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posted 04 May 2005 07:58 PM
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Mark, get it working first!  I've never experienced problems with the hum-balnce pots, so unless you do, I see no reason to rewire the amp's tube filaments or change the hum balance circuit. I don't think you've damaged the transformer. If you did, the amp wouldn't be humming! Uhhh...did you check the tubes? |
Mark Herrick
Member From: Los Angeles, CA
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posted 05 May 2005 12:08 AM
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quote:
Mark, get it working first!
It does work...but it has a buzzzzz... quote:
Uhhh...did you check the tubes?
Uhhh...yeah...swapped all tubes with known good tubes...buzzzzz... |
David L. Donald
Member From: Koh Samui Island, Thailand
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posted 05 May 2005 01:02 AM
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Disconnect one side of the hum balance pot,
and check to see it it is ok with an ohm meter.Do you have a scope and a signal generator? If you put a sine wave at low level into the amp, and follow the stages till you suddenly see a 60 hz signal on the other waveform,
then you will know at what stage the huim is being generated. Yes if the power transformer were damaged nothing would work at normal levels.
Or nothing at all. Right now you seem to have normal levels,
but an abnormally raised hum level added too it. |
Dave Potter
Member From: Republic of Texas (near San Antonio)
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posted 05 May 2005 08:10 AM
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quote:
Make sure that the heater wires on the power tubes stay in phase. Pin two should go to pin two and pin seven should go to pin seven.... You will find them wired wrong all the time.
Might be based in fact, but if pin shorting is what started this, I wouldn't waste time tracing wires at this point. |
John Daugherty
Member From: Rolla, Missouri, USA
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posted 05 May 2005 09:12 AM
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Do you hear the hum with the gain controls at minimum? If so, the problem is is most likely the power supply caps or one of the output tubes isn't working. I don't know where the gain control is,circuitwise, in your amp.
I have seen hum caused by one side of the hum balance circuit opening. Some amps use 2, 100 ohm resistors instead of a pot. |
Mark Herrick
Member From: Los Angeles, CA
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posted 05 May 2005 02:00 PM
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quote:
Disconnect one side of the hum balance pot,
and check to see it it is ok with an ohm meter.
It is a 100 Ohm pot. Should I be looking for a range of 0 - 100 Ohms as the pot is rotated? quote:
Do you have a scope and a signal generator?
Nope. I'm not a REAL doctor... quote:
Might be based in fact, but if pin shorting is what started this, I wouldn't waste time tracing wires at this point.
My thoughts as well. quote:
Do you hear the hum with the gain controls at minimum? ... I don't know where the gain control is, circuitwise, in your amp.
Yes. This amp has a master volume control that I always leave full on. Here is a link to what I believe is the closest schematic (but it doesn't have the hum balance pot): 100W CBS quote:
...or one of the output tubes isn't working.
I swapped all the preamp tubes and all the power tubes for known good ones and still have the buzz. quote:
Some amps use 2, 100 ohm resistors instead of a pot.
Just got some 100 Ohm resistors at the electronics store. Thought I would check the hum balance pot first. |
Mark Herrick
Member From: Los Angeles, CA
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posted 05 May 2005 04:46 PM
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It seems to have been the hum balance pot that got zapped. I replaced it with the two 100 Ohm resistors at the pilot lamp. Now the buzz is all but gone; only the noise from the single coil pickup when the pedal gets near full volume.I may try rewiring the heater string anyway to see if that makes any difference at all. One opinion seems to be that it will make little or no difference. ------------------
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