Fender Twin Reverb Silverface Repair Issue

[Shannon]

Hi,

I have been trying to repair my Twin Reverb for some time now and have failed to find the exact cause of the problem.

The problem is as follows: On switchon, from standby, the amp starts making a low frequency bellow through the speakers that builds up from a small soft sound to a really loud bellow that seems to threaten to blow the speakers! The sound has an element of ringing contained within, as though there is an oscillating component that increases in amplitude.

I posted about this some time ago
Fender Twin Reverb massive hum!
and using the advice I got there, the sound disappears when I remove V4 from the circuit. Further more, you can plug into channel 2 and still get clear volume from the guitar out of the amp but at a much reduced volume despite cranking up the volume knob.

Now this is a strange circuit as it has had numerous modifications including an odd 12.5+12.5uF 500V capacitor that is tied to one of the chokes. Also, if I compare the layout diagrams, it does not match the existing ones as there is a choke between V3 and V4 (with the aforementioned cap) that is not evident on existing layout diagrams.

So, since most of the posts suggest capacitor issues, I have replaced the 22nF, 47nF and 100nF caps with polyester 630V equivalents as the old ones looked really swollen and distorted. This has not made any difference.

I am considering replacing the 25uF 25V and 5uF 25V caps (all of them) with 22uF values (can't source 5uF or 20uF locally).

Kinda at my wits end now....any help would be appreciated. Specifically on what V4 does and how I can debug this problem further...

[52 Bill]

Originally Posted by Shannon

Now this is a strange circuit as it has had numerous modifications including an odd 12.5+12.5uF 500V capacitor that is tied to one of the chokes. Also, if I compare the layout diagrams, it does not match the existing ones as there is a choke between V3 and V4 (with the aforementioned cap) that is not evident on existing layout diagrams.

I'm not sure, but I think what you are calling a "choke" between V3 and V4 is actually the reverb driver transformer. Where are these 2-12.5 uF caps? Inside the chassis or under the cap cover? What do they connect to?

Originally Posted by Shannon

So, since most of the posts suggest capacitor issues, I have replaced the 22nF, 47nF and 100nF caps with polyester 630V equivalents as the old ones looked really swollen and distorted. This has not made any difference.

The caps you have replaced are the coupling caps or signal caps. After reading the earlier thread, most suggestions were to replace the filter caps.

Originally Posted by Shannon

I am considering replacing the 25uF 25V and 5uF 25V caps (all of them) with 22uF values (can't source 5uF or 20uF locally).

That value will work as a replacement for those caps.

Originally Posted by Shannon

Kinda at my wits end now....any help would be appreciated. Specifically on what V4 does and how I can debug this problem further...

V4 (the fourth tube from the right side of the chassis) does two things, one side of the tube amplifies the reverb signal that comes from the reverb springs, and the second half of the tube amplifies the mixed reverb and vibrato channel signal before it goes into the power amp.

When you pull V4 out can you play the amp through the normal channel?

[Shannon]

Bill,

I'm not sure, but I think what you are calling a "choke" between V3 and V4 is actually the reverb driver transformer. Where are these 2-12.5 uF caps? Inside the chassis or under the cap cover? What do they connect to?

The cap is one physical metal can, screw fitting, with 3 lugs. Two are wired together and connect to the red wire of the reverb driver transformer. The remaining lug, looks like the common pt, connects back to the circuit via a resistor. I'll post the full details once i reopen the amp tonight. The cap is labelled, 12.5 + 12.5 uF 500V. Reminds of the caps one might find in old car distributors The cap is physically in the chassis next to the circuit, not mounted down. Definitely looks like an aftermarket fit.

The caps you have replaced are the coupling caps or signal caps. After reading the earlier thread, most suggestions were to replace the filter caps.

Correct. I can't source these in South Africa. Checked all agencies and large suppliers. Even phoned an valve amplifier manufacturer - they import as well.

That value will work as a replacement for those caps.

Thanks. Will start working on these tonight.

V4 (the fourth tube from the right side of the chassis) does two things, one side of the tube amplifies the reverb signal that comes from the reverb springs, and the second half of the tube amplifies the mixed reverb and vibrato channel signal before it goes into the power amp.

When you pull V4 out can you play the amp through the normal channel?

No. The normal channel is silent despite cranking up this channel volume. Only the vibrato channel works - barely.

[Shannon]

Some more info:

That odd cap (12.5+12.5uF 500V), is connected to the red wire of the reverb transformer, but goes through an 11K resistor (11.7K measured) to the board at label point D on a AA270 layout diagram. The standard circuit shows that the red wire connects directly to the board between 2 100k resistors and physically between V3 and V4 in terms of alignment. The other side of the cap connects to the vibrato pedal jack.

[52 Bill]

Originally Posted by Shannon

That odd cap (12.5+12.5uF 500V), is connected to the red wire of the reverb transformer, but goes through an 11K resistor (11.7K measured) to the board at label point D on a AA270 layout diagram.

OK, someone has made some changes to the power supply. The 2 X 12.5 cap gives the equivalent of 25uF as a filter cap. If caps are as hard to get as you say, they may have done this to make it work, but it seems that they have mis-wired the power supply in doing so.

The red wire from the reverb transformer should go to the "B" node of the power supply. The "D" node should be connected to the "C" node via the 10K (not 11K) resistor. What are the changes to the circuitry under the cap can?

I would suggest removing all of the added in "mods" and rebuild back to correct specs. Is there someplace that you can import new filter caps from?

[Shannon]

I would suggest removing all of the added in "mods" and rebuild back to correct specs. Is there someplace that you can import new filter caps from?

Good idea. The power supply pics are attached. I wonder if the 12.5+12.5 was to compensate for the lack of space in the filter cap tray - you've already indicated this as a possible adjustment to the PSU circuit.

The values seem to confirm this as the arrangement of 10uF caps only make an effective 10uF @ 700V....

The closest values I have are 68uF 400V radial as a replacement for the 70uF 350V existing caps and 47uF 350V radials as a replacement for the 20uF 550V caps.

I could combine the 47uF in series for each 20uF leg? The caps are sufficiently smaller that the originals to work.

[52 Bill]

Originally Posted by Shannon

...I wonder if the 12.5+12.5 was to compensate for the lack of space in the filter cap tray - you've already indicated this as a possible adjustment to the PSU circuit.

Yes and no. The 4-10uF caps are replacements for the "B" node 20uF cap. The 2 x 12.5 cap and 10K resistor is added circuitry that doesn't make any sense to me.

Originally Posted by Shannon

The closest values I have are 68uF 400V radial as a replacement for the 70uF 350V existing caps and 47uF 350V radials as a replacement for the 20uF 550V caps.

I could combine the 47uF in series for each 20uF leg? The caps are sufficiently smaller that the originals to work.

Yes that will work. I'd start by removing the cap and resistor inside the chassis as well as the 4-10uF caps on the power supply board. Then rewire the power supply back to original. I would just start by replacing the missing "B" node 20uF cap and see what happens when you power it up.

[Enzo]

I think we are all way off base. I think he has a feedback in his reverb pan. A low note that builds to a bellow is not consistent with bad caps, but it is exactly what a reverb feedback sounds like. Pulling the reverb tube stops it, also consistent.

Turn your reverb off, does the noise stop?

If so, then we have the problem solved.

There4 are various thijngs to do for a feeding back reverb, and we can discuss them if you verify that is the issue here.

[52 Bill]

Originally Posted by Enzo

I think we are all way off base. I think he has a feedback in his reverb pan. A low note that builds to a bellow is not consistent with bad caps, but it is exactly what a reverb feedback sounds like. Pulling the reverb tube stops it, also consistent.

Enzo, I agree with your diagnosis here, but in reading the earlier thread I thought that it still made this sound with all controls turned down.

In any case, I hate the hack job that was done to the amp by the previous "Tech".

[Gtr_tech]

And its time for all those old brown Mallory's to go....

[Shannon]

I think we are all way off base. I think he has a feedback in his reverb pan. A low note that builds to a bellow is not consistent with bad caps, but it is exactly what a reverb feedback sounds like. Pulling the reverb tube stops it, also consistent.

Turn your reverb off, does the noise stop?

It did! Can't believe I missed such an obvious check! Anyway, played through the amp for the first time in months!
Feedback starts only when it gets to around 7 on the Reverb dial. Fine in all other combinations. Thanks everyone!

Enzo, I agree with your diagnosis here, but in reading the earlier thread I thought that it still made this sound with all controls turned down.

Apologies 52 Bill. Should have tried this earlier.

Amazing the things I've noticed now. There is a fair bit of what I can best describe as audible hiss/buzz (more like an electric fence - but softer and steadier - not impulse) with the occasional crackle especially when I touch the strings after picking the guitar up. I assume a replacement of the filter caps may improve the overall hiss issues.

How do I go about resolving the Reverb issue?

BTW: I tested all the caps I replaced thus far (100nF, 47nF, 22nF), and despite being really old, none are really way out of spec.

[Enzo]

Your reverb is feeding back acoustically. Several things to do.

First, do not screw the reverb pan down tight to the cab floor, the screws are only there to keep it from going somewhere.

Your reverb should be inside a vinyl bag, if not, get one.

If there is a bag, pull the reverb pan from it and inspect. the inner long spring assembly is suspended in mid-air by four short springs at its corners. Make sure all four are intact, we don;t want the innards lying on the floor. Make sure the long springs are intact and mounted at both ends.

If there is not one already, cut a piece of cardboard the size of the pan and cover the pan's open side with it. Tape it in place.

Sometimes the wide flat top surface of the pan can resonate and ring. Dampen that by taking a strip of self-adhesive foam rubber weatherstripping, and stick it on that top surface running down the length of it. Auto makers do this trick inside large body panel on their cars to prevent "sheet metal sounds."

SOmetimes turning it uopside down in the bag works.

And sometimes you simply need to replace the pan.

[Shannon]

Enzo,

I'll complete the suggested actions and report back. Thank you.