I have a Hot Rod Deluxe in for repair and it's making me scratch my head. The customer used it for a studio session and it was working fine. He put it in his car, took it home and then it started sounding farty and distorted on every note. He figured it was a tube issue so he replaced the PI. I don't know what he did but when I received the amp the PI was blown. I replaced the PI and now the amp is making sound but it's the same farty, distorted sound he was hearing. The amp buzzes/hums immediately upon taking it off standby with no input and all knobs turned to zero.
I tracked it down to the power amp section. I'm reading a bias of 7mA and the full range of the bias pot only yields somewhere between a couple milliamps and maybe 20mA. The bias supply voltage is solid at -50V. All of the resistors in the power amp section (R59, R60, R61, R62, R64, R65, R66) are good and the 22uF filter cap on the bias supply is good. The plates of the 5881s are reading 350V (B+) which, according to the schematic, should be reading 66.5V(?). I have tried a completely new set of tubes with no change.
I'm not terribly well versed in power amps yet so I appreciate any help anyone can give. Thanks ahead of time.
Schematic: http://support.fender.com/schematics/gu ... ematic.pdf
Hot Rod Deluxe - Very Low Bias
Moderators: pompeiisneaks, Colossal
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Stevem
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Re: Hot Rod Deluxe - Very Low Bias
These amps are quite well known for having one of the PI plate load resistors going open, so check that pin 1and 6 of the PI both have voltage and then post back what you have found.
I always replace them with 1 watt resistors for double the safety from future failure!
I always replace them with 1 watt resistors for double the safety from future failure!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Hot Rod Deluxe - Very Low Bias
I'll try that tonight when I'm in the shop. Thanks!
Re: Hot Rod Deluxe - Very Low Bias
No dice on the plate load resistors; they're fine but I replaced them with 1 watters anyway.
Another odd thing is the B+ is measuring 350VDC when it should be 431V, according to the schematic. All of the PSU filter caps are good and I replaced the diodes in the rectifier. The PT also seems to be within spec, outputting 340VAC.
Any other ideas?
Another odd thing is the B+ is measuring 350VDC when it should be 431V, according to the schematic. All of the PSU filter caps are good and I replaced the diodes in the rectifier. The PT also seems to be within spec, outputting 340VAC.
Any other ideas?
Re: Hot Rod Deluxe - Very Low Bias
How did you confirm the function of the filter caps?
Do you have a sine generator and scope?
Do you have a sine generator and scope?
I build and repair tube amps. http://amps.monkeymatic.com
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Hot Rod Deluxe - Very Low Bias
If you pull the power tubes out if the amp and fire it up do you still have only 350 volts on pin 3 ?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Hot Rod Deluxe - Very Low Bias
I removed one end of each filter cap from the PCB and used an ohm meter to see if each cap charged.xtian wrote:How did you confirm the function of the filter caps?
Do you have a sine generator and scope?
Yes, I have a function generator and a scope.
I was just about to post an update with this information. Yes, if I remove the power tubes, B+ goes to 431V on the dot. I have tried multiple sets of good tubes with the same (bad) results. I also checked all diodes in the PA to make sure none of them were leaking.Stevem wrote:If you pull the power tubes out if the amp and fire it up do you still have only 350 volts on pin 3 ?
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Hot Rod Deluxe - Very Low Bias
Something is not making sense here, your output tubes pulling down the B+ to 350 volts means they are drawing a ton of current yet you are barely reading any idle current out of them and into the OT!
Can you double check that cathode current reading again?
If you pull out one output tube at a time does the idle current that you read basically get cut in half?
Can you double check that cathode current reading again?
If you pull out one output tube at a time does the idle current that you read basically get cut in half?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Hot Rod Deluxe - Very Low Bias
7mA all day long. I know, it doesn't make any sense.
Re: Hot Rod Deluxe - Very Low Bias
What about a high frequency oscillation? Maybe when the PI blew it sent a spike that killed C30?
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Hot Rod Deluxe - Very Low Bias
If you disconnect the red wire on the OT primary does the B+ then go back up to 400?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Hot Rod Deluxe - Very Low Bias
No. C30 doesn't appear to be the problem either.Stevem wrote:If you disconnect the red wire on the OT primary does the B+ then go back up to 400?
Re: Hot Rod Deluxe - Very Low Bias
With no power tubes in the sockets I'm measuring 51.2 ohms between pins 7 and 8. That seems odd, no?
With my scope I measured pin 3 and the noise oscillates at 120Hz. All ground connections appear to be good.
With my scope I measured pin 3 and the noise oscillates at 120Hz. All ground connections appear to be good.
Re: Hot Rod Deluxe - Very Low Bias
No, that sounds normal. Pin 8 is connected to ground, and pin 7 is heater, which is eventually also connected to ground, thru the coil, and then thru a balancing resistor.BungleSim wrote:With no power tubes in the sockets I'm measuring 51.2 ohms between pins 7 and 8. That seems odd, no?
I build and repair tube amps. http://amps.monkeymatic.com
Re: Hot Rod Deluxe - Very Low Bias
If I read your responses correctly, pulling the power tubes causes B+ to reach the specified 431 volts. But you also said no to Steve M's question of whether disconnecting the OT CT from B+ caused the voltage to go back to 431? Did I read that correctly. Finally, through all that, no change in bias current.
Pulling power tubes - voltage returns to spec.
So, indicates powertubes drawing huge current causing big voltage drop.
However, disconnecting the plates from B+ by disconnecting OT CT has no effect. All the while, bias current in minimal.
Would seem to leave only the power tube screens, but rolling tubes has no impact either. Maybe the tube socket is shorted. Try pulling the power tubes and checking for continuity where there should be none on the tube socket. In particular look for a short on the screens.
Best regards,
Mike
Pulling power tubes - voltage returns to spec.
So, indicates powertubes drawing huge current causing big voltage drop.
However, disconnecting the plates from B+ by disconnecting OT CT has no effect. All the while, bias current in minimal.
Would seem to leave only the power tube screens, but rolling tubes has no impact either. Maybe the tube socket is shorted. Try pulling the power tubes and checking for continuity where there should be none on the tube socket. In particular look for a short on the screens.
Best regards,
Mike
Sunnydaze