SF Twin Reverb on the bench

dsutton24

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Have you read anything about thermal runaway?

Yeah, different materials react differently to heat.

The biggest worry in an old amp is something failing in an unexpected way. That's what causes fires, not theory learned from the internet.
 

Lynxtrap

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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.

This has the 68K grid leaks. Would it be better to up them to 220K?

The amp's preset bias was extremely cold as it came in. I installed adjustable bias in addition to the balance pot. I set it to about 17-19W plate dissipation (the tubes differ from each other within that range).

That is just a bit over 60%, and should not normally send the tubes off into runaway in a well functioning circuit, should it?
 

Lynxtrap

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@Lynxtrap, I skimmed through the thread a second time, but I did not see that this am0 has fresh electrolytics. Did I miss that report??

I might have forgotten to mention it, but yes, I put brand new F&T's in it.
All resistors measured within specs, and he wants the amps as close to stock as possible, so I did not replace any resistors.

This is the output stage and PS as it is now, changes and voltages notated.

Twin PS.jpg
 
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Wally

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If I understand things, you are monitoring the current draw with a bias meter on each tube. I have a bias probe, which I bought for a reason other than Biasing amps. I bought it to do crude matching of tubes.
I have never seen current draw drift. In your situation, I would remove the bias meters and monitor the current draw in a different manner.…simply because I do not like ‘foreign’ objects in a circuit. Others may have experience with letting bias meters sit in a circuit and would say that I am being a bit off track, but I would have to do the experiment to see if the amp is stable with the meters removed.
At the very least, the amp needs to be open on a bench so one can get a handle on what is causing the current draw drift….
 

Lynxtrap

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If I understand things, you are monitoring the current draw with a bias meter on each tube. I have a bias probe, which I bought for a reason other than Biasing amps. I bought it to do crude matching of tubes.
I have never seen current draw drift. In your situation, I would remove the bias meters and monitor the current draw in a different manner.…simply because I do not like ‘foreign’ objects in a circuit. Others may have experience with letting bias meters sit in a circuit and would say that I am being a bit off track, but I would have to do the experiment to see if the amp is stable with the meters removed.
At the very least, the amp needs to be open on a bench so one can get a handle on what is causing the current draw drift….

I have a TAD bias meter with four probing sockets that allows you to monitor the current on all four tubes simultaneously. I bought it just the other week, mostly to do fast and rough checks, so it is new to me.

I will open the amp and check the situation without the bias meter. However, I don't think the meter is to blame.

I had already wrapped up this job and brought the amp along for band practice just to make sure it behaves while in use for a couple of hours.

I gradually started to hear the frying pan sound of hot tubes when I wasn't playing and figured that the bias must be off.

When I got home I did the test with the meter. From the start, current draw was as I had set it. About 45 minutes later the noise started appearing, and the current had increased by about 5-6 mA on all tubes.

A previous tech had installed a larger resistor to ground in the bias supply, which puzzled me as it ran extremely cold. I wonder if this might be the explanation.

Output tubes are TAD 6L6GC-STR.
 
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Lynxtrap

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By the way, I guess it is possible that some kind of oscillation could cause this?
 

peteb

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Have you read anything about thermal runaway?

hi Phrygian,

yes. I believe you introduced that term to me and we have discussed it.

my interpretation of thermal runaway is the strictest interpretation, that heat causes more heat. I think this is commonly associated with chemical reactions.

in the less strict interpretation, thermal runaway means any increase in current beyond beyond what is desired. And this will cause heat, but the heat itself is not the cause of the extra current.
I have never seen current draw drift.

there you have it. Wally has not seen it.



it is my observation that when we see extra current in amps, here on the TDPRI or in person, it always seems to be a deficiency in bias voltage that is the cause.

The biggest worry in an old amp is something failing in an unexpected way.

right.


going back to the concept that tube amps are meant to run 24-7.

I like to keep the heat down for maintenance reasons.


i don’t leave my amps on for long periods of time.


there is one condition however where I think I would feel comfortable leaving an amp on for 24 hours.

1. It is somebody else’s amp.
2. use a fan to cool the PT.
 

Lynxtrap

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I hope the problem is solved now. I put the amp on the bench tonight and started looking over everything again.

I then discovered that the 1K resistor between the PT bias supply and the diode was not properly soldered at the diode end. It basically rested in a hole in the blob of (factory) solder and obviously it has made contact, but I could lift it with my fingers.

While I was at it I changed it to a metal film 1K. I hope that took care of the current draw issue.

Also, poking the mess of wires between the MV pot and the board with a chopstick made some noise. I lifted the MV out of the amp, and most of the frying pan noises and some additional hiss disappeared.

I changed the crazy factory grounding for the MV (it is running back and forth through the shields of two wires) and just grounded it all together with the intensity pot to the copper strip. That took care of the noise.

Instead, the pull-boost switch started acting up again, I think I might swap the entire pot out tomorrow.

During some of this I had the amp on for like an hour and all voltages were stable, but I had turned bias way down to like 12W.

I monitored one of the output tube with the probe, set the bias at around 44 mA (18W) and it was stable for about half an hour. I then ran out of time, I'll do some extended monitoring tomorrow.
 

Wildeman

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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.
Don't bother man, it's really not good😉 I tried it once...
 

Wally

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Regarding turning all tone controls to 10, +1 with @Wildeman. You can try it, but the signal will go to pieces on the h8gh and the low end. Try this instead…..turn the Bass and Treble to zero and dime the Middle And the Volume. Set the MV to please. That would be as old as it gets…unless you want o do this….
Apply the reverb to both channels. This adds the third gain stage to the Normal channel. Then, revoice that preamp for more midrange response so that you have the BF/SF in the Vib channel and a much hotter channel with more midrange gain in the Normal channel. I also like to install an adjustment pot in series with the shunt resistor in the negative feedback loop. All of this is reversible.…but ime no one wants it reversed once they experience the results.
 

dsutton24

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I've got a pull boost / master volume Silverface Twin. When I got it that pull boost master volume pot had disintegrated, and a replacement wasn't easily available. Funny thing, the front panel on my amp doesn't have the 'pull boost' legend on it. The pull boost wiring is easy to eliminate, so I just stuck an appropriate pot in the master volume spot. Works like a charm.
 

Wally

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there you have it. Wally has not seen it.

lol….That does not mean that it does not happen but rather that it has not been a problem that I have encountered. I surely have not seen it all.
Kudos to @Lynxtrap for going inside and finding a problem in that bias circuit.
 

Bourbon Burst

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I've got a pull boost / master volume Silverface Twin. When I got it that pull boost master volume pot had disintegrated, and a replacement wasn't easily available. Funny thing, the front panel on my amp doesn't have the 'pull boost' legend on it. The pull boost wiring is easy to eliminate, so I just stuck an appropriate pot in the master volume spot. Works like a charm.
Mine doesn't have a pull boost legend either. I only knew about it from the guy I bought it from.
 

JohnFender

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It's one of these, a '76:

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?
Can I see a picture of the amp?
I have a 76 too if you need pictures of the inside for any reference
I think mine is all original on the inside
 

JohnFender

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I have a pull boost/master volume on mine and it still works good, I only it used a dozen times still fun to wonder how music was at in 76, when I first got mine I had some trouble finding out which schematic to look at because if I remember correctly that year they have some with no master volume and then some had a master volume and another one with the pull boost added.
Maybe it's due to having jbls in my master volume 76 compared to my non master volume 70 with Oxfords but i would argue that you would have to have your master volume around 3/4 of the way for them to be about the same, I always have my master volume below 5, I would keep the master and skip the boost if parts are unavailable
 

Bourbon Burst

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I have a pull boost/master volume on mine and it still works good, I only it used a dozen times still fun to wonder how music was at in 76, when I first got mine I had some trouble finding out which schematic to look at because if I remember correctly that year they have some with no master volume and then some had a master volume and another one with the pull boost added.
Maybe it's due to having jbls in my master volume 76 compared to my non master volume 70 with Oxfords but i would argue that you would have to have your master volume around 3/4 of the way for them to be about the same, I always have my master volume below 5, I would keep the master and skip the boost if parts are unavailable
I have a 75 master volume with boost. Just curious, why do you keep your MV low?
 

JohnFender

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I just like the way it sounds there, I think I'm compensating for the way jbl d120f speaker output the sound, I usually keep the volume and master close to each other and just tweek them a little from there, I also keep all my eqs at 5, easier to set your sounds when using multiple pedals, there's no right or wrong way, find what sounds better to your ear and that will make you play better
 

Ten Over

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I have a pull boost/master volume on mine and it still works good, I only it used a dozen times still fun to wonder how music was at in 76, when I first got mine I had some trouble finding out which schematic to look at because if I remember correctly that year they have some with no master volume and then some had a master volume and another one with the pull boost added.
Maybe it's due to having jbls in my master volume 76 compared to my non master volume 70 with Oxfords but i would argue that you would have to have your master volume around 3/4 of the way for them to be about the same, I always have my master volume below 5, I would keep the master and skip the boost if parts are unavailable
MV appeared in '72. MV w/switch appeared part way thru '73. UL 135W appeared part way thru '77.
 

Bourbon Burst

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I just like the way it sounds there, I think I'm compensating for the way jbl d120f speaker output the sound, I usually keep the volume and master close to each other and just tweek them a little from there, I also keep all my eqs at 5, easier to set your sounds when using multiple pedals, there's no right or wrong way, find what sounds better to your ear and that will make you play better
Okay thanks. Mine is still new to me so I have yet to really try different settings. Most people have said to turn it to 10 and just use the channel volume to sound best.
 




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