Fender Super Reverb RI ugly low volume distortion

[Marco Pancaldi]

The amp distort at low volume, with both channels.
Tested bias: -51V. Changed inverter and power tubes at first, then substituted one at time all other tubes. Distortion remains.
Voltage DC off tube rectifier 500V on standby, 466V in "play" mode.
Other voltages: x 417V, y 455V, z 468V, power tube plates 466V.
Thanks.
m.p.

[soundguruman]

Once again you have blown speakers?
or a bad output tranny?
Seems like everything is looking normal, by your measurements.

[Dave Curtis, dB AudioTech]

The amp distort at low volume, with both channels.
Tested bias: -51V. Changed inverter and power tubes at first, then substituted one at time all other tubes. Distortion remains.
Voltage DC off tube rectifier 500V on standby, 466V in "play" mode.
Other voltages: x 417V, y 455V, z 468V, power tube plates 466V.
Thanks.
m.p.

It does sound like a light voice coil rub: fuzzy-sounding distortion that is noticeable at low volumes, but seems to disappear at higher volumes. Have you ruled out the speakers? Plug the amp into another cab. At low volume, impedance matching won't be a big issue.

[52 Bill]

Check the plate voltages on the phase inverter tube.

[Marco Pancaldi]

Thanks all.
I will try the speakers but it's not a mechanical distortion.
I have the RI schematics and the voltages are x 261V, y 311V, z 394V - my voltages are absolutely higher.
Mesured resistance for primary coil of output transformer: 45.6 and 48.8 ohm.
I will check the inverter plate voltages.
Checked voltages for B+, plates and screens (output tubes): little or no difference...can be a choke problem?
m.

[pdf64]

'I have the RI schematics and the voltages are x 261V, y 311V, z 394V - my voltages are absolutely higher'

See http://support.fender.com/schematics..._schematic.pdf
The voltages you are refering to may come from the Dexluxe Reverb RI schematic http://support.fender.com/schematics..._schematic.pdf
You really need to eliminate the speakers as being the cause of this; a damaged voice coil can sound like a horrible crossover distortion.
Pete

[Marco Pancaldi]

Pete, you are completely right!! I was reading the WRONG scheme...
I will check the amp with a different load ASAP. Thanks
m.

[g1]

Can't rule out crossover distortion either. -51V bias doesn't say anything about the idle current through the power tubes. Volume has a way of masking distortion, it could still be there at higher volumes, but less noticeable.
What are the voltage readings at TP37 & 38 ?

[Marco Pancaldi]

Checked voltages on inverter plates (thanks 52 Bill...): no voltage on pin 6.
Checked R55 100k plate resistor: completely open.
Installed new (2W) inverter plate resistors (82k + 100k): amp is fine.
Thanks all for your help!!
m.

[52 Bill]

Checked voltages on inverter plates (thanks 52 Bill...): no voltage on pin 6.
Checked R55 100k plate resistor: completely open.
Installed new (2W) inverter plate resistors (82k + 100k): amp is fine.
Thanks all for your help!!
m.

Glad to hear that it's fixed. Remember the sound of that distortion, it's very distinctive, only one half of the signal is being amplified by the power tubes. For some reason I see that failure in a lot of modern Fender amps. It's usually the 82K but sometimes it's the 100K.

[pdf64]

I wonder why? It's not as if they get exposed to unusually high voltages / power dissipations, either on a peak or continuous basis.
Pete

[52 Bill]

I wonder why? It's not as if they get exposed to unusually high voltages / power dissipations, either on a peak or continuous basis.
Pete

I suppose under rated resistors. The thing that I noticed was that in all the years of fixing amps, I can't remember ever replacing these resistors in a black face or silver face amp. I suppose I may have, but I don't specifically remember having them fail. Yet these are a common failure in all the modern Fender amps.

[Enzo]

I don't remember them using 1/4w resistors back when.