Late 70's Vibrosonic Reverb issues

[Axtman]

"Get out your tissues, we've got issues." - Dave from Dave's World of Fun Stuff

1) The amp sounds fine on Normal channel but is distorted on Vibrato channel. The distortion is especially pronounced on bass notes.

2) The tremolo (vibrato) ticks but only when a note is played. It did tick without input but became silent when I installed a 0.01 uf cap per Fender Service Bulletin #9.

BTW This amp is basically a late 70's (135 watt UL) Fender Twin Reverb. Fender made several amps off the Twin Reverb chassis. They just had a different name and speaker configuration.

Any help would be appreciated.

[Enzo]

So one channel distorts. Try swapping preamp tubes first. The get out a meter and look for DC voltage issues, like a cathode without any or plate with really high or low. And is ther DC on any grid wher none ought to be? Thinking leaky coupling cap.

[bob p]

Vibrasonic. Great amp. Hopefully you got one with the JBL or EV speaker. I always thought it odd that Fender would reissue the 15" version of the Twin Reverb as a Twin Reverb (and give it a black faceplate) instead of calling it a Vibrasonic.

So many times we get zero history on an amp. Like if it's been serviced before, or if it's still got original caps. That's relevant information.

It's also typical that we get requests for help without any useful information to guide us... it's as if people expect that Enzo the Oracle can just psychically determine what's wrong with their amp.

I'm not that good. I need help. I need basic information, like a voltage table. A table of pinout voltages on the tubes would give us a clue what the problem is, so we wouldn't have to take generalized stabs in the dark. With a useful starting point like that we can provide guided advice and we're not forced to guess at answers by pulling recommendations out of thin air.

If you could post a voltage table, you might get an immediate answer. Often times the nature of the problem just jumps out at you once you organize basic data.

[bob p]

I find that thinking about what's happening with the load line often helps to reveal why a tube is distorting. Basic load line examination tells us what operating conditions the tube needs to operate properly and why it will distort if those conditions aren't met.

1) Ck is the first place that I'd look if the amp hasn't been recapped recently. If that's the problem then the voltage table should be a dead giveaway. Like Enzo said, checking for abnormal plate voltage could be the problem. Or DC leaking to grids. In any of thsoe cases, a voltage table should readily identify the problem. Checking idle voltages and verifying that they are all in spec is a cheap high yield test.

2) I haven't seen those Fender TSB. Can you post them?

[Enzo]

And check that the reverb boost pull switch is not engaged.


Any of these?

[Axtman]

I replaced all the electrolytic capacitors in the amp.

Yes the amp has the orange JBL speaker.

Here is the voltage chart.

Tube Location Type
Pin (voltage on the schematic) voltage measured

V1 Normal channel preamp 7025
1: (250) 250
3: (1.8) 1.7
6: (250) 269
8: (1.8) 2.0

V2 Vibrato channel preamp 7025
1: (250) 223
3: (1.8) 1.9
6: (250) 196
8: (1.8) 1.9

V3 Reverb driver 12AT7
1 & 6: (395) 378
3 & 8: (6) 6.4

V4 Reverb 7025
1: (250) 254
3: (1.8) 1.6
6: (250) 252
8: (1.8) 1.6

V5 Vibrato 12AX7
1: (250) approx 260
3: (1.8) 1.6
6: (330) 252
8: (12) 11

V6 Phase splitter 12AT7
1: (280) 265
3: (110) 106
6: (280) 267
8: (110) 106

[Axtman]

Here is the schematic diagram.

[bob p]

Any of these?

It looks like TSB9.pdf is the one that deals with vibrato ticking. I've never seen that TSB, though I do remember the subject coming up now and again. If anyone has TSB9 I'd like to see it.

[Enzo]

SB9:

[Jazz P Bass]

You will have to 'zip' that file.

[bob p]

Knowing that the amp has been recapped is useful information, thanks.

When you say that the Vibrato channel distorts, does it matter if the reverb or trem circuits are active or does it happen in spite of switching them in/out?

V2 Vibrato channel preamp 7025
1: (250) 223
3: (1.8) 1.9
6: (250) 196
8: (1.8) 1.9

Thank you for putting the reference voltages in your table.

In general the Fender schematics require voltages to be +/- 20% of the stated value. I notice that all of them seem to be in-spec except for the plate voltage on V2b, which is right at the 20% lower limit. That number seems suspicious, though it may not be the root of the problem.

Take a look at a quick and dirty loadline comparison:



The Green curve is what you're supposed to have according to the schematic. The Red curve is your measured results. I've assumed that the value of the plate load resistor has not drifted and remains at 100k, though that may not be a valid assumption and the Y-intercept for the red line is a guess at best.

Notice that when you drop 20% of the tube's plate voltage the load line is shifted to the left and your bias point has shifted downward on the -1.9V bias curve. The tube is now operating a relatively cooler bias point and it's encroaching into the non-linear areas of the plate characteristics at the bottom of the graph. The tube is now operating closer to cut-off than it should be. That could cause distortion on large signals. It'd be interesting to check to see if the plate load is still 100k.

I suppose that it's also worth checking the coupling caps to assure that none of them are leaking DC. The usual suspects would be the 3 caps in the tonestack and the coupling caps that follow V2b.

Do you have a scope? If so I'd trace a LF signal and see where the distortion occurs in the circuit.

[bob p]

You will have to 'zip' that file.

what's up with that? it seems that The Oracle's PDF isn't working.

[Enzo]

It means the same file had been posted earlier before the big server crash.

Try it now.

[bob p]

I tried a couple different PDF readers and I'm getting a white page with the letters "NZO" in the upper right corner.

[Jazz P Bass]

Zip it.