Fender Deluxe Reverb Reissue (2002)

[PLF]

I have a Fender Deluxe Reverb Reissue (2002) that the 6V6 power tubes suddenly started to glow RED and subsequently shorted out. I checked all the tubes and decided to replace all of the tubes. I also found the 47uf 500V located at C32 was dead. I replaced the bad capacitor and when I turned the amp on, the power tubes again glowed RED, so I quickly shut it off. I've checked the other caps and they are pretty much within tolerance, (20%). I've tested the transformers and their outputs are good. What am I missing that would cause the tubes to be overheated like this?

[Leo_Gnardo]

What am I missing that would cause the tubes to be overheated like this?

Bias voltage. Either there's none or what's there is insufficient. With output tubes removed, what voltage do you read on pin 5 of output tube sockets?

[52 Bill]

Welcome to the place.

I have a Fender Deluxe Reverb Reissue (2002) that the 6V6 power tubes suddenly started to glow RED and subsequently shorted out. I checked all the tubes and decided to replace all of the tubes. I also found the 47uf 500V located at C32 was dead. I replaced the bad capacitor and when I turned the amp on, the power tubes again glowed RED, so I quickly shut it off. I've checked the other caps and they are pretty much within tolerance, (20%). I've tested the transformers and their outputs are good. What am I missing that would cause the tubes to be overheated like this?

The tubes are doing what is commonly called red-plating. The tubes are drawing so much current that the plates are heating up to a cherry red color.

The reason for this is a lack of bias on the input grids. Remove the two 6V6's and carefully measure the dc voltage at pin 5 of the two empty tube sockets. you should have somewhere around -40 to -50 (negative) volts there.

If the bias voltage in not there, then you need to figure out why. Broken wire, bad bias pot, bad bias supply cap, bad bias supply diode, a bad coupling cap coming from the phase inverter, etc. A bad power tube can also do this, but having replaced the tubes, it would be unusual for the new tubes to be bad as well.

Measure the voltages and let us know what you find out.

[PLF]

Bias voltage. Either there's none or what's there is insufficient. With output tubes removed, what voltage do you read on pin 5 of output tube sockets?

Thanks for getting back to me so quickly! I get readings of -149.5 and -149.1 respectively.

[PLF]

Welcome to the place.
The tubes are doing what is commonly called red-plating. The tubes are drawing so much current that the plates are heating up to a cherry red color.

The reason for this is a lack of bias on the input grids. Remove the two 6V6's and carefully measure the dc voltage at pin 5 of the two empty tube sockets. you should have somewhere around -40 to -50 (negative) volts there.

If the bias voltage in not there, then you need to figure out why. Broken wire, bad bias pot, bad bias supply cap, bad bias supply diode, a bad coupling cap coming from the phase inverter, etc. A bad power tube can also do this, but having replaced the tubes, it would be unusual for the new tubes to be bad as well.

Measure the voltages and let us know what you find out.

Thanks for getting back to me so quickly! I get readings of -149.5 and -149.1 respectively.

[52 Bill]

Thanks for getting back to me so quickly! I get readings of -149.5 and -149.1 respectively.

While not impossible I guess, I doubt that your reading is -149 dc volts, so maybe -149 mV (millivolts)?

If this guess is correct, then the bias voltage is either disconnected or the supply has died.

Look at the inside of the chassis, somewhere near the power transformer. There should be a screwdriver adjustable pot mounted there. That is the bias adjustment pot. There should be three wires connecting it to the circuit board. Read the voltage to ground from each of the three pot terminals. There should be maybe -60 volts dc on at least one terminal. If not then you will have to read the voltage at the circuit board.

Let us know what you find out.

[PLF]

My bad. Yes, -149 mV. I also tested the voltage using the bias pot and it would go down to -105mV. The gray wire terminal on the bias pot reads -168 mV, the black wire terminal (center) reads -120 and the white wire terminal reads -118 mV with the pot all the way counter clockwise from the adjustment side (external chassis).

[Jazz P Bass]

Unfortunately, the Deluxe RI does not provide a bias adjust pot.

The bias voltage circuit consists of diode CR1 (1N4003), resistor R69 (Flame proof, 22 ohm) and capacitor C36 (100uf/ 100V).
The schematic indicates that there should be -49Vdc at the junction of R69 & C36. (TP5)

I have included a partial schematic & a partial layout, along with the complete service manual.

65_Deluxe_Reverb_RI Bias Circuit.pdf

65_Deluxe_Reverb_RI TP5.pdf

65_Deluxe_Reverb_RI.zip

[52 Bill]

Unfortunately, the Deluxe RI does not provide a bias adjust pot.

Jazz, there is a bias pot, look at the schematic for the power amp.

PLF, read the voltage at TP5 to see if there is -50 volts dc there. If not, you need to see if there is ac coming from the transformer to the bias supply diode CR1. You need to test CR1 to see if it is still conducting, the 22 ohm resistor to see that it is okay and test the filter cap C36 to see that it is not shorted.

[gnappi]

I have a Fender Deluxe Reverb Reissue (2002) that the 6V6 power tubes suddenly started to glow RED and subsequently shorted out. I checked all the tubes and decided to replace all of the tubes. I also found the 47uf 500V located at C32 was dead. I replaced the bad capacitor and when I turned the amp on, the power tubes again glowed RED, so I quickly shut it off. I've checked the other caps and they are pretty much within tolerance, (20%). I've tested the transformers and their outputs are good. What am I missing that would cause the tubes to be overheated like this?

Did you check the coupling caps for shorts?

[52 Bill]

Did you check the coupling caps for shorts?

That is a possible cause but in this case, where all of the bias voltage is missing, a bad coupling cap is an unlikely cause. First off, there is enough isolation between the power tube input grids that both coupling caps would need to be leaky in order for both grids to go to zero.

When a coupling cap gets leaky, the odds that it would leak just enough positive voltage to cancel out the negative bias voltage is really unlikely. The shorted cap would more likely cause a positive voltage to appear on the power tube input grid.

[gui_tarzan]

I had a SF DR recently that had the same issue. It turned out the bias voltage was missing. Gone. Kaput. I re-did the bias circuit and it was happy again without any red-plating.

[PLF]

I had a SF DR recently that had the same issue. It turned out the bias voltage was missing. Gone. Kaput. I re-did the bias circuit and it was happy again without any red-plating.

Can you tell me what parts you replaced? Thanks!

[nickb]

Can you tell me what parts you replaced? Thanks!

Not wanting to jump in on anyone's toes, but there are only three components in the bias supply: CR1, R69 and C36. Ah but which one failed and why?

Remove the 6V6's and get busy with your meter. Measure TP5, the junction of R69/CR1 and using the ACV range check the cathode of CR1.

Dang: Missed 52 Bills's post saying much the same thing.

[PLF]

Jazz, there is a bias pot, look at the schematic for the power amp.

PLF, read the voltage at TP5 to see if there is -50 volts dc there. If not, you need to see if there is ac coming from the transformer to the bias supply diode CR1. You need to test CR1 to see if it is still conducting, the 22 ohm resistor to see that it is okay and test the filter cap C36 to see that it is not shorted.

Hi 52 Bill. Sorry to let this get so stale. My Father passed away and with the holidays, I wasn't working on much. At TP5 I get -155 mv. On the bias pot I get the following readings: Gray Wire = -168mv, Black Wire = -120mv, White Wire = -118mv. I think I had the pot turned all the way down for those readings. The CR1 diode tests AOK. C36 tests AOK. R69 has me rather bamboozled. The resistor that is in the R69 spot is a Metal Film type with a rating code of 0445 or 4.4M uf. It looks original and it tests 5.7 M uf. The specs are for a 22 uf Metal Film and they indicate that spec is required. Bias supply to CR1 reads 19.6Vac. Thank you for all of your help. I hope your holidays have been great!