fender hot rod deluxe low volume & distorted output

[methodofcontrol]

fender hot rod deluxe from '98.
it kind of works... it passes a signal and the tone controls do their thing but the output is VERY low volume
and it's quite distorted.
i ran a cable from the pre-amp out jack and it sounded fine. i then tried the power amp in jack and still had
the bad sound. so, i figure the problem is in the power amp.
voltages on power tube sockets w/ tubes pulled are Pin 3: 475v, pin 4: 475v, pin 5: -48v
according to the schematic, pin 4 is a little high. pin 3 seems to be VERY high and pin 5 seems normal.
i lifted the grounded end of one of the R3000 diodes off of pin 3 and measured using the diode function of my multimeter.
i would've thought they should measure at about .5v each, but the one that i lifted measures @ 1.5v, while the one still in place
measures @ 1v. i get 0v when i try to measure CR3 (1n5062).
what else could be causing pin 3 to measure so high? did the OT go bad?

[52 Bill]

The plate voltages are probably high because there are no tubes installed. Check the tube socket board for cracked solder joints and check the driver tube plate resistors, especially the 82K.

[Enzo]

Oh foo, before making it complicated, check to see if the speaker is plugged into the extension jack instead of the main speaker jack next to it.


If the B+ was 10% high, how on earth would that result in "VERY low" output? Put the tubes in, voltage readings without tubes are meaningless.


SOmewhat more complicated? OK, look at the solder on the power tube socket pins. ANy cracks? I resolder them every time I go in there on that series amp. Then go next door to the phase inverter tube. Got plate voltage on BOTH pins 1 and 6?

[52 Bill]

Oh yeah, good one! Wrong speaker jack.

[methodofcontrol]

correct speaker jack: check!

voltage reading on power tubes: pin 3 AND pin 4 are reading 435v

solder joints: looking good, but re-soldered anyway!

phase inverter plate voltages: pin 1: 0v, pin 6: 213v

voltage at point "Y" on schematic: 404v

nothing looks obviously burnt out or anything on the board.
it makes sense to me why if R57 (82k on pin 1 of PI) was open there would be no voltage on that plate.
but i don't understand how that would effect the voltage on pin 6 of the PI? R58 (100k on pin 6) is fine.

[methodofcontrol]

i just replaced the 82k, and now everything sounds good.
the voltages @ pin 1 and 6 of the PI are both reading about 253v.
schematic says they should be @ 11.5v, i guess the schematic is just wrong.
that seems pretty low.

[52 Bill]

i just replaced the 82k, and now everything sounds good.
the voltages @ pin 1 and 6 of the PI are both reading about 253v.
schematic says they should be @ 11.5v, i guess the schematic is just wrong.
that seems pretty low.

I've seen that 82K resistor fail a lot. A few weeks ago I think I had 3 of them in one week, that's why I suggested it.

I haven't looked up the schematic, but are you sure you're not reading an ac voltage figure?

[methodofcontrol]

yup...253vdc. but it does measure about 10vac.
most test points and their voltages are drawn in circles on the schematic, but a few are drawn in rectangles.
circle means AC? rectangle means DC? i'd buy that. if that's S.O.P. on a schematic, it's the first i've seen.

can someone explain to me how one would know to look at the 82k resistor for the issue i was having?
(btw, i've played the amp for about 40 minutes at half volume w/ no other repairs attempted... seems to be strong again.)

[g1]

It's S.O.P for Fender. Notice that the first circled voltage at the input jack is specified as 1khz sinewave, that is AC.
As far as knowing to look at the 82k, it's because there was no voltage at pin1 of that tube. Open plate resistor most likely cause.
52Bill knew right off the bat it was suspect due to experience with this model.

[52 Bill]

Think about what the phase inverter does. With the open 82K resistor there is no drive directly to one of the power tubes and there is also reduced signal to the cathode driven second half of the PI, resulting in low power and distorted output.

If you work on enough of these amps you sometimes start to see the same problems again and again. You also start to recognize the symptoms. Low power and distortion can be many things, like using the wrong speaker jack or an open screen grid resistor on one of the power tubes or as in this case an open plate resistor in the driver stage. Each problem has a slightly different sound and you start to be able to hear where to look for the problem.

There is one other thing that I always check on these amps, the ext. speaker jack. The output impedance is selected by a switch on this jack. If the contact is dirty or oxidized it will cause the speaker circuit to go open, causing signal cutoff and a no load condition. Be sure to check yours.

[g1]

There is one other thing that I always check on these amps, the ext. speaker jack. The output impedance is selected by a switch on this jack. If the contact is dirty or oxidized it will cause the speaker circuit to go open, causing signal cutoff and a no load condition. Be sure to check yours.

Blues Deville? Is OT failure common with these?

[Enzo]

OT failure is not common in ANY amp. Transformer failure of any kind is not common.



Far more likely would be the jack trouble 52Bill refers to. Dirty contacts means no sound.

[g1]

Sorry, I was not clear on that. Bill mentioned the ext. spkr. jack switch getting dirty/oxidized leading to a no load condition. I was asking if this (no load condition) lead to OT failure more often than in other Fender models (that don't have the series switch in the ext.spkr. jack). To use the word "common" was probably stretching it.

[52 Bill]

I was asking if this (no load condition) lead to OT failure more often than in other Fender models (that don't have the series switch in the ext.spkr. jack).

No, I can only remember replacing one output transformer in any of these amps. It's not like the ext. speaker jack is always going open circuit.

Whenever I have one open, I will send a signal through the amp and while listening I will lightly press on the hot contact of the ext. speaker jack. In most cases the signal will start to break up. I don't see the break jacks on these amps causing the signal to drop out very often, but I do see a lot of problems with these ext. speaker jacks.

I was just trying to pass along another of the many common things that I see on these amps. I'm not saying that all of these amps will have this problem, I'm just saying that I do see it happen a lot.

[Enzo]

Once you are already inside the amp, it is the sort of thing you can check in 10 seconds, so why not make it a regular check.