Fender Hot Rod Deluxe breaking up early on clean channel

[Delta362]

I have a Fender Hot Rod Deluxe in with a complaint about the amp distorting on the clean channel. Board has 1996 date printed on it. During troubleshooting, I found a couple of the power supply filter caps were leaking fluid so I replaced the four 500V filter caps. I also replaced the bias cap. The 470 ohm 5W cement resistors had some board discoloration under them so I replaced those and elevated them. I also resoldered the two 16V zeners.

Next up I found the phase inverter 100k plate resistor R58 was open. I replaced that as well as the 82k other plate resistor. Power tubes are new and biased to factory spec of 60mV at TP30. I tightened the speaker mounting screws.

It's still distorting around 3 (of 12) on the clean volume. Using the preamp out into another amp's effects loop return sounds clean well past 8 of 12 so the preamp section seems to be working correctly and leads me to believe the issue is with the power amp section.

Screen grid resistors measure well within tolerance as do the grid stoppers. I've reflowed the power and phase inverter tube sockets. Voltages at pin 3 are around 440 and pin 4 is within 1 volt of pin 3. Now I'm only able to get about 4 or 5 before I have breakup.

Any suggestions for other places to look at? Could C26 or C27 be leaky causing the issue? Maybe C30? What am I overlooking?

Thanks!

[Enzo]

You are overlooking the troubleshooting and moving direct to guessing at bad parts. SO the preamp out is clean at higher levels and the power amp seems affected? OK, what does the signal look like at the phase inverter. Is that clipping? What does a test signal look like at the plates of the PI? Hell, what does the distortion look like at the speaker? Do the signals look clean at the power tube grids? DO they two sides look the same as each other?

Are we plugging the speaker into the MAIN output jack, not the AUX SPKR jack?

Did we go in through the power amp jack yet?

And my personal peeve with these things: you did resolder all the power tube socket pins. I almost always find the solder cracked there.

[glebert]

I'd measure the DC voltage at TP26 to make sure both side of the PI are conducting properly after you replaced those plate resistors.

[Delta362]

Running a signal into the power amp in jack, the phase inverter pins 2 and 7 don't clip until they see about 5.5Vp-p (1.95vrms). The phase inverter output plates start to clip when its outputs reach 80Vp-p, which appears to occur slightly before the input begins to clip. Pin 1 has 255VDC, Pin 6 has 245VDC and pins 3/8 have 38VDC. Waveforms for both in and out of phase look the same.

For the load, I'm using the far left jack with only the one switch contact, not the jack with the extra switching contacts that's shown for the extension speaker jack. My load is a 6.8ohm 100 watt resistor since I don't have the 8 ohm version handy (I'll get some on order). At the speaker jack output, I'm getting a Vrms reading of 15.1V when that waveform starts to clip (coinciding with the phase inverter output clipping), which is at around 33.5W. So it looks like the phase inverter is clipping at that point and the power tubes are just passing that clipped signal.

Is the phase inverter clipping too soon?

[Helmholtz]

I'd say the measurements look ok.
Specified output at 5%THD (means at a little clipping) is 16.3Vrms into 8R, corresponding to 33.2W.

[g1]

You should have around -50V at the power tube grids. That implies to me that you should be able to get more than 80V p-p out of the PI tube.
When you measured that 80V p-p, did you have the power tubes removed? If not, try with the power tubes removed.
If not, you may only have -40VDC at power tube grids, and you are seeing grid conduction rather than PI clipping.
Did you check the bias sense resistor and it's protection diode? (R66, CR3)
What is the VDC at power tube grids?

[Delta362]

My power calcs seem to correlate fairly well with the specs, Helmholtz, so perhaps I'm chasing a ghost with too hot of a signal from my guitar.
Edit: ...as I found and replaced the open phase inverter plate resistors which would have accounted for the original dirty clean signals. /Edit

g1, grids have -48.6VDC for both power tube sockets with tubes out, and -47.1VDC with tubes in.. My initial measurements above were with the power tubes in. I removed them and tested the PI outputs until they clipped. Seemed to get a little higher than 80Vpp, reaching about 110Vpp or so before I noticed the waveform starting to flatten at the peaks.

Bias sense resistor and diode measure 1.0 ohm in circuit, de-energized. I didn't pull the diode to check it since if it was shorted, I should have seen less than 1 ohm.

[Helmholtz]

Many if not most Fender amps start distorting at around 3 with humbuckers or powerful singlecoils.

[Delta362]

Many if not most Fender amps start distorting at around 3 with humbuckers or powerful singlecoils.

That is excellent to know! I had not noticed it before when testing my gear, but perhaps I wasn't testing the clean channel fully.

[g1]

Agree with Helmholtz, if it is reaching full (clean) power of 33W when you get it to clip at "3" on volume, then you just have a hot input signal.
Lots of guys chase around what they believe should be a standard volume level setting that the amp should stay clean up to. And they will ask you to chase it down for them.
There is no such standard, and it is very dependent on the input signal strength. Other controls like tone etc. will also have a big impact.