Fender Noise !

[Valvehead]

Trying to figure out a Fender Hot Rod Deluxe.
The amp has a real loud nasty hum to it--even with the master on 0.

It still has the hum but at a much quieter level with ALL PREAMP tubes removed.

I tested everything on the separate output board. All looks good.

This points me to the power supply . Any tips on these amps? I havent found any bad components yet .

[stokes]

Are the power tubes matched,or close?Too much of a mismatch in a push-pull amp will cause hum in the output.

[Valvehead]

heres a mp3 of the noise

http://www.fileshost.com/en/file/43700/2hum-mp3.html


ive swapped tubes---its not those.

I notice the bias is only 20mv all the way up. The scem calls for 60mv.

I doubt thats the cause though....

[Enzo]

Get out your scope or your AC voltmeter and check the DC voltages with it. Your bias voltage ought to be smooth DC, as in no AC on it. Is that the case? Pull all the tubes and see. How about the B+, is it clean of ripple and up to voltage? I'd be checking for a filter cap that died or lost its good solder connection.

While you are in there, check the two power resistors for the low voltage supplies. They like to overheat and unsolder themselves.

[mykey]

Trying to figure out a Fender Hot Rod Deluxe.
The amp has a real loud nasty hum to it--even with the master on 0.

It still has the hum but at a much quieter level with ALL PREAMP tubes removed.

I tested everything on the separate output board. All looks good.

This points me to the power supply . Any tips on these amps? I havent found any bad components yet .

seems almost as if one of the diodes in the B+ bridge rectifier has gone bad?
which would explain a drop in B+ voltage and low output tube current?
Hmmmmmm.......

[Valvehead]

dont have a scope

B+ looks perfect

cant find any bad joints on either board

heaters have 6.8 vac

with power tubes in i get 5.7 VAC from cathodes to ground

no tubes in i get 1.5 VAC

power supply diodes check out good

none of the filter caps are shorted

i swapped out the first 2 filter caps--no change

[Enzo]

You get 1.5VAC at pin 8 of the power tubes with them removed???? Does it measure 1 ohm to ground from there with power off? It should measure 1 ohm to ground from there with power on for that matter. Look at the schematic, there is 1 ohm R66 abd a diode in parallel, CR3. If the resistor opened up, CR3 would allow the amp to function, but not right. For that matter, a shorted CR3 would also confuse things.

Those two parts are on the tube socket board in the corner. One end of R66 is in fact the test point for measuring bias. That point is wired directly to pins 8 of the power tubes. Or should be. 5 volts across that resistor would mean 5 AMPS running through the tubes - not likely. Is that where you are meassuring these voltages?


You SHOULD have about 3VAC to ground from either end of the heaters on any of the tubes. Was you 6.8VAC across the heaters? Or to ground. Across them is fine, to ground means something is open. The two 100 ohm resistors from the heaters to ground are R80,R81 and are right behind V2. If one or both of them were open, that would increase hum a lot. of a power tube had shorted internally, it might have burnt those out. New tubes would restore operation, but the loss of those resistors would make the amp real hummy.
OK, B+ is about 430 VDC with less than maybe 10VAC riding on it? Oh wait, with tubes out it will read higher.

How about pin 5 of the power tubes. That is bias, and should read about -50. Look for C43 and CR15, lower left on the main board, looking in the back. The end of that diode closest to the cap is the anode and should have about -53VDC or so, and it should have little AC there. If the AC is substantial, try a new C43.

[Valvehead]

You get 1.5VAC at pin 8 of the power tubes with them removed???? Does it measure 1 ohm to ground from there with power off? It should measure 1 ohm to ground from there with power on for that matter. Look at the schematic, there is 1 ohm R66 abd a diode in parallel, CR3. If the resistor opened up, CR3 would allow the amp to function, but not right. For that matter, a shorted CR3 would also confuse things.

Those two parts are on the tube socket board in the corner. One end of R66 is in fact the test point for measuring bias. That point is wired directly to pins 8 of the power tubes. Or should be. 5 volts across that resistor would mean 5 AMPS running through the tubes - not likely. Is that where you are meassuring these voltages?


You SHOULD have about 3VAC to ground from either end of the heaters on any of the tubes. Was you 6.8VAC across the heaters? Or to ground. Across them is fine, to ground means something is open. The two 100 ohm resistors from the heaters to ground are R80,R81 and are right behind V2. If one or both of them were open, that would increase hum a lot. of a power tube had shorted internally, it might have burnt those out. New tubes would restore operation, but the loss of those resistors would make the amp real hummy.
OK, B+ is about 430 VDC with less than maybe 10VAC riding on it? Oh wait, with tubes out it will read higher.

How about pin 5 of the power tubes. That is bias, and should read about -50. Look for C43 and CR15, lower left on the main board, looking in the back. The end of that diode closest to the cap is the anode and should have about -53VDC or so, and it should have little AC there. If the AC is substantial, try a new C43.

Im getting 1.5mv AC now from 8 to ground. I had a green ground wire disconnected so i can get the board up. Didnt think this wire was that important considering its right next to the Input jack . That was changing the readings--see changes

Pin 8 measures 1 ohm to ground.

CR3 is ok

Measuring at bias test point to ground i get 1 mvAC and 20mv dc

6.8vac was measuring from pin 2 to 7

r80 and r81 are ok

pin 5 shows -44vdc

tried replacing C43 , no change

CR15 has 0.3vac cathode / 40VAC anode
-055vdc cathode / 140mvdc anode

[52 Bill]

Something doesn't sound right with the readings you're getting with the output bias.

Measuring at bias test point to ground i get 1 mvAC and 20mv dc

At 20mV the two output tubes are drawing 10mA each.

pin 5 shows -44vdc

Normally this voltage will be closer to -50 volts.

What are the plate and screen grid voltages on the output tubes?

[Valvehead]

pin 3 349v
pin 4 349v

pin 5 -44mv

[52 Bill]

pin 3 349v
pin 4 349v

pin 5 -44mv

All of these voltages are low. The schematic calls for B+ of 431vdc. Your pin 5 voltage is -44V, not -44mV, I assume?

Pull the output tubes and read the voltages on pins 3-4-5. See if the power supply voltage comes back up. If you really only have -44mV of bias, then your output tubes would be glowing cherry red and the amp would be humming.

[Valvehead]

tubes out =

477v
478v
-44v

[52 Bill]

tubes out =

477v
478v
-44v

OK, so the tubes are pulling down the B+. You didn't answer the earlier question, with the tubes in place, was the bias voltage really -44mV?

In any case, check the bias supply. Do you measure -53 volts at C43? Can you adjust R82 to get more than -44v at C38?

[Valvehead]

OK, so the tubes are pulling down the B+. You didn't answer the earlier question, with the tubes in place, was the bias voltage really -44mV?

In any case, check the bias supply. Do you measure -53 volts at C43? Can you adjust R82 to get more than -44v at C38?

sorry--i meant -44v bias.


now im getting these readings=

at C38 with pot all the way up -55.0v
at C43 -55.7v

[52 Bill]

sorry--i meant -44v bias.


now im getting these readings=

at C38 with pot all the way up -55.0v
at C43 -55.7v

I know that you stated earlier that you had swapped out the tubes, are you sure the output tubes are good? Plug in the tubes and see if you can get it to bias correctly.