79 super reverb woes strange distortion

[judge74]

I have a 79 super reverb UL amp that I picked up on FB marketplace a few weeks ago. It was super cheap but also needed some work. It did play when I got it. It gets loud but had some hum and a weird distortion on bass notes. Almost like a ghostly trashy fuzz pedal in parallel with the clean signal. It does it at all volumes even 1.

I have done a good bit of work and have gotten rid of most of the hum but not the distortion

So far I have:
Replaced all the original filter caps.
Replaced all the original cathode bypass caps.
Replaced the original bias caps.
Replaced the 47 and 68 k resistors in the bias supply because they had drifted about 50%.
Changed cathode of reverb driver to 2.2k with 25uf.
Removed pull boost circuit.
Replaced scratchy master volume pot.
Added 1 ohm bias resistors.
Replaced 20w 30k resistor to ground (it was open).
Replaced 470 ohm screen grids.
Added 22k resistor with 20k trimmer to back of tube balance pot for bias adjustment.
Resoldered broken connection in reverb tank.
Replaced tremolo roach.
Replaced a couple of bad preamp tubes.
Resoldered some broken grounds by the transformer.
Measured all voltages and they are almost spot on with schematic.

After all that the amp gets loud with no hum. But the strange distortion fuzz is still there. It is on both channels so I am thinking is is between the master volume and the power tubes somewhere. Maybe a coupling cap. I don’t have any test tools besides a multimeter so how do I test that? If I had a oscope I would just trace the signal and look for where it gets ugly.

Any ideas what else it could be? or where I should look next?

Schematic:

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Super-Reverb-Ultralinear-1978-1981-Schematic.pdf

 

[corliss1]

Have you tried a different speaker to make sure that isn't the issue?

 

[judge74]

The only other speaker I have is an old 12 inch Jensen from a 60s Ampeg Gemini. Unfortunately it’s 8 ohm and 30 watts so I can’t do that….

Also I was thinking since it does even at the lowest volume that could make it less likely to be a speaker?

 

[Wally]

+1 on eliminating the speaker as a possible problem. That 8 ohm, 30 watt speaker will suffice for a quick, low volume test. If you are concerned about the impedance match, pull two tubes…one from each side. This will match the impedance and also give a test for a bad power tube operation if you run each pair separately.
Are the phase inverter and power tubes known to be good? Worst case scenario is a bad OT.

 

[darkwaters]

Bought a used speaker cab last year. When I plugged it in, it sounded for all the world like I had a distortion box in my chain. Luckily, I had a spare speaker. Switched speakers. No distortion.

Fiddled with the speaker ( what I perceived to be a very nice Eminence Tone Spotter). Nothing obviously wrong with the speaker except the rubber gaskets were peeling off. Glued and clamped them. Switched it back in. Distortion. Put my old speaker back in. Gave up.

 

[corliss1]

@Wally - super reverb, not twin reverb, can't pull two power tubes and still get sound

@judge74 - hook it up, it'll be fine for a quick test.

 

[judge74]

I may try that in a couple of days. I don’t have much time during the week to tinker with amps…

In the meantime I recorded a quick video on my phone to show what it sounds like. This is with the volume on about 1.5 and you can hear it especially when I hold a note. There is a little hum at the beginning of the video I already fixed that moving some components around with a chopstick.

 

[schmee]

It's a Super with 4 speakers right? Not likely it's one speaker making it too distorted....

You can end down and listen to each speaker separately to see if one is bad.

 

[schmee]

I may try that in a couple of days. I don’t have much time during the week to tinker with amps…

In the meantime I recorded a quick video on my phone to show what it sounds like. This is with the volume on about 1.5 and you can hear it especially when I hold a note. There is a little hum at the beginning of the video I already fixed that moving some components around with a chopstick.

I'm not hearing anything too abnormal........? Not as clean as I might expect but.... You mean the sustaining note that carries on readily, is there a bit of feedback there...?
Where is the master set during this video? Have you tried it on 3 volume? Many Fender amps become a bit distorted at 2 and clean right up at 3. All 3 of my current ones are that way. (BF Deluxe, Pro and SF Princeton rev.)
The place to start is voltages on the tubes pins. make a chart.

 

[Wally]

@Wally - super reverb, not twin reverb, can't pull two power tubes and still get sound

@judge74 - hook it up, it'll be fine for a quick test.

Waking up is for me!! Lol.

 

[corliss1]

I never get anything useful out of sound clips. That sounds fine to me, through my fancy studio monitors.

 

[tbalon]

I have a 79 super reverb UL amp that I picked up on FB marketplace a few weeks ago. It was super cheap but also needed some work. It did play when I got it. It gets loud but had some hum and a weird distortion on bass notes. Almost like a ghostly trashy fuzz pedal in parallel with the clean signal. It does it at all volumes even 1.

I have done a good bit of work and have gotten rid of most of the hum but not the distortion

So far I have:
Replaced all the original filter caps.
Replaced all the original cathode bypass caps.
Replaced the original bias caps.
Replaced the 47 and 68 k resistors in the bias supply because they had drifted about 50%.
Changed cathode of reverb driver to 2.2k with 25uf.
Removed pull boost circuit.
Replaced scratchy master volume pot.
Added 1 ohm bias resistors.
Replaced 20w 30k resistor to ground (it was open).
Replaced 470 ohm screen grids.
Added 22k resistor with 20k trimmer to back of tube balance pot for bias adjustment.
Resoldered broken connection in reverb tank.
Replaced tremolo roach.
Replaced a couple of bad preamp tubes.
Resoldered some broken grounds by the transformer.
Measured all voltages and they are almost spot on with schematic.

After all that the amp gets loud with no hum. But the strange distortion fuzz is still there. It is on both channels so I am thinking is is between the master volume and the power tubes somewhere. Maybe a coupling cap. I don’t have any test tools besides a multimeter so how do I test that? If I had a oscope I would just trace the signal and look for where it gets ugly.

Any ideas what else it could be? or where I should look next?

Schematic:

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Super-Reverb-Ultralinear-1978-1981-Schematic.pdf

 

[Lowerleftcoast]

WAG:
Is the bias set too cold?

Try placing a 100pF cap across the Phase inverter. (Possibly a ultra high frequency oscillation making the power tubes work too hard.)
The 5F6A below uses a 47pF but 100pF is what I usually use.

 

[judge74]

WAG:
Is the bias set too cold?

Try placing a 100pF cap across the Phase inverter. (Possibly a ultra high frequency oscillation making the power tubes work too hard.)
The 5F6A below uses a 47pF but 100pF is what I usually use.

View attachment 1078851

Thanks for that! It makes sense I’ll give it a try.

As far as the bais goes, I added a trimmer to the balance pot so I could set the overall and balance too. I have it written down somewhere but it’s around 32mA at 500 volts with bot tubes really close it was around 50 to 55%

 

[Jasonpatrick]

Thanks for that! It makes sense I’ll give it a try.

As far as the bais goes, I added a trimmer to the balance pot so I could set the overall and balance too. I have it written down somewhere but it’s around 32mA at 500 volts with bot tubes really close it was around 50 to 55%

Is that how hot these run? 500v? Jfc!

 

[Lowerleftcoast]

Try it up about 38mA. That would be about 60%MPD. Remember, the 1 Ohm resistor measurement is measuring plate and screen.

 

[Bluzzi]

I'm not too familiar with the SR model. There is a Master volume I think right? If so I believe also it is push-pull for switching it on and off. Try fooling around with it. Maybe it's my computer speaker that is nto great but I couldn't hear anything extreme in the sound.

If you can try and maybe rent a speaker cabinet to test out. Even at 8ohms you should be OK plugged into the 4ohm output (there should be one for 2ohms and another for 4ohms). Higher impedance is usually safe (but get confirmation from others here). It just drives the speaker at lower volume.

 

[corliss1]

@Bluzzi - none of that is right about a Super Reverb. Master Volume had a pull boost on later amps, and there is only one speaker out, with an extension speaker in parallel.

@Jasonpatrick - schematic shows 500V on the CT - https://www.thetubestore.com/lib/th...er-Reverb-Ultralinear-1978-1981-Schematic.pdf - gotta get that extra wattage somewhere!

 

[judge74]

Yeah the ultralinear fenders have ultra high voltage. Power and headroom! I really like the extra percussive clean that it has. I also appreciate the extra mid control on the EQ.

It actually is 495 at the plate.

I got rid of a lot of the noise by moving to the other room. The output I was plugged into had dirty power and a bad ground. It is quiet enough now plugged into a better outlet that I can live with it for awhile. But for an amp that just had the power supply rebuilt it is still noisier than I would want. Even though it is still not dead quiet the aggravating buzzy fuzz is gone.The cell phone really didn’t capture it, I swear it sounded worse in person. I have had a lot of vintage clean amps over the years and it just wasnt right.

There is still a hum when I turn the master volume past 8 (it is a new pot and I got rid of the pull boost) it is kind of a shame because that is how I would normally play it. Pre on 2 or 3 master on 10.

I will dig back into it in a few days. I have a few things I need to finish up and hopefully I can find the master volume hum.

I replaced all the caps around the phase inverter with some yellow Mallory’s I had laying around from another project. I just wanted to verify if one of the original ones could have been causing the problem but that wasn’t it. So I am going to open it back up and put the original caps back.

 

[cosmicrepairdude]

I have had similar sounding hums after low notes from loose screws, components, etc. Yes, listen to each speaker separately as suggested, but also tighten every screw, nut, and bolt you can find, including corners, feet, handle, everything. Listen around the cabinet as the notes linger and see if you can locate where it's coming from - unless it's coming equally from all 4 speakers. I actually had stiff grill cloth vibrating against the baffle on one amp.