SF Twin Reverb on the bench

[Lynxtrap]

It's one of these, a '76:

https://schematicheaven.net/fenderamps/twin_reverb_sf_100_schem.pdf

It is in 99% original condition, and the owner wants to keep it as close to stock as possible.
Besides new filter caps it needs a new power switch and likely a new push-pull master volume pot. The pot feels a bit worn out, and the boost has obviously not worked as long as he has owned the amp. Pulling the switch does something to the sound, but definitely no boost. (Yes I know how a functional "pull boost" sounds, but stock it is).

Tube Amp Doctor have a replacement pot in stock, but it does not have the extra tap where the 120pF cap is connected. I've never seen this before, and I'm not quite sure how it's supposed to work.

What do you think would be a good way to deal with the cap without the tap?

 

[Ten Over]

I would probably connect the cap to the wiper instead of to the tap. If you don't like it, leave it off altogether.

 

[Phrygian77]

I personally don't like the bright cap on the master. You could do something like an 8-10pF to the wiper, like a Dumble, or leave it off altogether.

However, before assuming the pot is bad and changing it, fix the other stuff first. Then, hit the pot with some DeoxIT F5 or D5 followed by some F100L.

 

[Paul G.]

I had a pull-boost that was intermittent and made a pop when switched and was noisy when turned. A thorough cleaning was all it needed. Definitely see if you can get the original to work before replacing it.

 

[Lynxtrap]

I would probably connect the cap to the wiper instead of to the tap. If you don't like it, leave it off altogether.

Thanks, and especially for the layout, I wasn't aware that layouts even existed for these.

So the cap is basically a bypass that we usually see connected between input lug and wiper, but with some resistance in series with the cap?

It does seem that the sound gets a bit thin with the master down low, so the effect of the cap certainly seems to be enough as it is.

I have already ordered the pot, TAD was also the only European dealer that seemed to have the original looking power switch in stock.
Postage was 21€ so I thought I'd better throw the pot in the same package. I always wondered who buys stuff from TAD with the prices they have, now I know I checked with the owner of course, he was prepared to pay that kind of money for an original looking switch.

I might try to clean the old pot and see if the boost gets functional.

 

[Bourbon Burst]

I have a 75 TR that I just had serviced. It is hard to hear the difference when pulling the boost switch. I looked up the manual and it says to put all the tone controls to 10 to get the most out of the gain. I haven't tried that yet.

 

[Nicko_Lps]

It does seem that the sound gets a bit thin with the master down low, so the effect of the cap certainly seems to be enough as it is.

If the cap is 120pf as you mentioned, yeah this is the issue.
Up to 47pF will negate this effect you are talking about while still maintaining some of the "lost" high frequencies because the vol is down.

Personally when i installed the MV on mu amp, the 120 was thinning it by alot. Funny because i initially tried a 250pf and i really have no words to explain the monstrosity, the ugliness and the disgust from what i heard.

I removed the 47pf bypass cap from MV and i kept it on my OD channel gain pot.
If the owner wants to keep it as stock as possible... Ask him and take the load off your back

 

[Lynxtrap]

I have a 75 TR that I just had serviced. It is hard to hear the difference when pulling the boost switch. I looked up the manual and it says to put all the tone controls to 10 to get the most out of the gain. I haven't tried that yet.

That seems strange. I haven't heard it in a TR, but for instance in a Deluxe Reverb it certainly has an effect.

Looking at the schematics, the DR does not have the second half of the switch that reduces the signal on V4B in the Twin. The Twin has a 470K resistor after the 1K to ground, while it's 220K in a DR.

Anyone else with experience from these Twins? I don't want to put in the hours trying to fix something that ain't broke...

This one has the 470 ohm cathode resistor on the reverb driver. I'm tempted to change that to something like 1K. That AT7 is driven extremely hard.

 

[Phrygian77]

That seems strange. I haven't heard it in a TR, but for instance in a Deluxe Reverb it certainly has an effect.

Looking at the schematics, the DR does not have the second half of the switch that reduces the signal on V4B in the Twin. The Twin has a 470K resistor after the 1K to ground, while it's 220K in a DR.

Anyone else with experience from these Twins? I don't want to put in the hours trying to fix something that ain't broke...

This one has the 470 ohm cathode resistor on the reverb driver. I'm tempted to change that to something like 1K. That AT7 is driven extremely hard.

I have a '76 that is the same way. The boost is pretty weak. Although, mine has been modified, but it was mostly just blackfacing of the power rail and the PI. The preamp for the vibrato channel is stock except that the reverb driver cathode resistor was changed to 2.2k, so I replaced it with a 680 ohm. It's unbypassed, so the more you increase the resistor, the more negative feedback and less gain it will have.

'76 Twin Reverb Head Project

I saw this chassis on Reverb for $500. No cabinet, no glass, and no reverb tank. Too much? For what it is, probably, but this is exactly what I was looking for ... and, these things aren't getting cheaper by the minute either. These guys had a little too much to drink last night...

www.tdpri.com

 

[Dacious]

I had a 70 watt Pro Reverb which sounded and weighed like a Twin Reverb - pulling the boost created a slight volume hike and a transistor-y trebly unpleasant sound lacking mids and bass. After I tried it once, I never did it again. I never played with the tone controls as I didn't ever conceive using it.

The 78 Vibrolux Reverb had a slightly thicker sound on pull boost but still very buzzy and thin, I'd still never use it. Maybe for some jazz fusion.

Whoever signed off on these at Fender in the seventies obviously had never listened to a Marshall but been briefed to add distortion.

 

[Lynxtrap]

Well, I did not need to swap the MV pot out, in fact I didn't have to do anything to it. I lifted it out of the chassis and the boost worked. It seems as if half of it grounded to the bottom of the chassis when mounted to the panel. I put some volcanic tape around it and put it back, reattached the reverb, and it's still working.

To have any real effect the channel volume has to be set high (and the MV low, because...it's a Twin). Then you get some surprisingly decent overdrive out of it, definitely the best "pull boost" I've heard.

The bias was extremely cold, and for some reason the resistor to ground from the balance pot had been changed to an even larger value than on the schematic = even colder. The output tubes ran at 8-9W plate dissipation.

I checked with the owner and installed a bias adjustment trimpot on top of the balance pot, and put the tubes at about 20W.

The tubes differed a bit, and it seemed to follow the sockets even if the 1.5K grid resistors measured fine. There was no hum or anything, so I left the resistors as they where.

I measured some DC after some of the coupling caps in the preamp, about 0.3-0.4V. Swapped the blue blobs out for Mallorys, but still get the same DC readings. Conductive circuit board, or just normal?

This amp sounds glorious by the way!

 

[Lowerleftcoast]

Swapped the blue blobs out for Mallorys, but still get the same DC readings. Conductive circuit board, or just normal?

A little DC from the tube control grid is handled by the grid leak resistor so, yes there is a little DC present.

The conductive boards I have run across do not give a consistent measurement of DC.

 

[Lynxtrap]

I have noticed that the output tubes start drawing more current as they get hot, measured after about 45 minutes with the amp on. I tested it because a faint crackling/frying pan noise started to appear, which I seem to remember tubes can do when biased too hot.

Any thoughts what the reason might be? The grid or screen resistors getting heated up by the tubes? It still has the original carbon comps in those positions.

 

[Ten Over]

I have noticed that the output tubes start drawing more current as they get hot, measured after about 45 minutes with the amp on.

How much more current?
Are the voltages on Pins 5 of the 6L6's changing during that 45 minutes?

 

[Lynxtrap]

How much more current?
Are the voltages on Pins 5 of the 6L6's changing during that 45 minutes?

It's not a lot, about 4-5 mA. I could basically bias it a bit colder to begin with, but I don't like this behaviour.

I have not yet investigated this issue with the amp open, I only ran it with a bias meter connected to all input tubes, so milliamps is all I've got for now. I figure plate voltages are bound to go down as the tubes draw more current.

I just thought I'd throw the question out here in the meantime, in case this is a known issue for you more experienced guys.

 

[Phrygian77]

You are checking the bias with no signal, right?

Tubes when they got hot will naturally run hotter, and bias themselves hotter to the point that without a sufficient grid leak path, they will go into thermal runaway. That's an issue with fixed bias.

 

[Lynxtrap]

You are checking the bias with no signal, right?

Tubes when they got hot will naturally run hotter, and bias themselves hotter to the point that without a sufficient grid leak path, they will go into thermal runaway. That's an issue with fixed bias.

Thanks! How do we handle this issue?
Yes, no signal.

 

[peteb]

It's not a lot, about 4-5 mA. I could basically bias it a bit colder to begin with, but I don't like this behaviour.

the bias current is not supposed to go up.

it is more likely that it will drop. Heated components have higher resistance causing the current to drop.


you are supposed to be able to leave these amps on 24-7, although I always shut mine off within a couple or few hours.


I believe I recently had a bad connection on pin 5 both disrupt the signal and most likely lose some or all of the bias and had the amp’s current draw rise up. I caught it by hearing the noise and then looking at the power monitor. I cleaned the tube pins and it stabilized.

 

[Phrygian77]

Thanks! How do we handle this issue?
Yes, no signal.

That's what the grid leak resistors are supposed to prevent. Fender in the AB763 twins used 220k grid leaks, which was not really enough, but shouldn't an issue an unless the tubes are biased too hot to begin with. The later amps use 68k grids leaks I believe.

 

[Phrygian77]

it is more likely that it will drop. Heated components have higher resistance causing the current to drop.

Have you read anything about thermal runaway?