SF Twin Powering On Distortion - HELP!

[DrFramus]

I recently pickued up a 1970 SF Twin Reverb. The amp sounds great when it is powered on for a minute or two. Yet, during the power up cycle (power on for tubes to warm, standby on for playing) I am getting some nasty distortion out of the amp. It happens regardless of how long I let the tubes warm before swithing the standby on. I have experimented with normal 2 minute warm up all the way up to 1/2 hour warm up time and it still occurs. The distortion happens immediately once the standby is switched on for playing. The distortion lasts between 5 - 10 seconds and then immediately goes away with a single little pop sound. The pop sounds like a short circuit that was instantly corrected. After that the amp plays fine and has no issues.

At first I thought the issue could be the input jacks not grounding properly because I first noticed it when the amp was powered on and I was plugging into it. But I have found that the distortion happens regardless of whether there is a guitar plugged in or not.

Any help in diagnosing the problem would be greatly appreciated.

Thanks!
Dr. Framus

[Kap'n]

How old are the filter caps?

[DrFramus]

Filter caps are original. Yet they show no leakage, dimpling, or other physical signs of being trashed. I know that doesn't mean they are not bad, but that was one of the things I looked for.

Dr. Framus

[Kap'n]

The first thing I'd do is replace them, then I'd troubleshoot. I'd bet money it's the caps.

Oh, BTW,. welcome to the board!

[DrFramus]

Thanks for the welcome.

I was contemplating a cap job anyway just based upon their age. I will give it a shot and see how it works out.

Thanks!
Dr. Framus

[DrFramus]

Do you know if anyone sells capacitor kits for cap jobs, or do I need to just go through the schematic for my amp and create a list of what is needed?

[Kap'n]

Somebody must make kits, but it's fairly easy to figure out what you need.

You'll want to replace every capacitor under the "doghouse" on the bottom of the chassis. There's three 20uF/525V ones, and two 70uF/350V ones. Observe the polarity of the originals - one is physically inverted compared to the others!

Additionally, you'll want to replace all of the 25uF/25V capactors on the main board. There's something like 7 of them. They can be covered in cardboard, but they might be small white cylinders with purple print.

Lastly, there's a 50uF/70V one on the bias board. On this one, the + side goes to ground.

To retain the tone, don't go significantly (+/- 20%) over or under the uF values. You can go over on the voltage ratings, but don't go under.

Observe all safety guidelines.

[DrFramus]

You da' man, Kap'n! Thanks for your help.

[DrFramus]

Lastly, there's a 50uF/70V one on the bias board. On this one, the + side goes to ground.

To retain the tone, don't go significantly (+/- 20%) over or under the uF values. You can go over on the voltage ratings, but don't go under.

What value cap are people using these days for the 50uF/70V on the bias board?

I have found suitable replacements for everythin but that. Right now the best I can come up with is a 50uF/50V. It seems like that might work, but the voltage rating is a little hairy since the nominal voltage in that leg of the circuit is 50V. There is just not much margin in using a 50uF/50V cap.

[JAM]

Dont go down in voltage rating. You can use a 100/uf 100v for the bias cap(s)

[DrFramus]

You'll want to replace every capacitor under the "doghouse" on the bottom of the chassis. There's three 20uF/525V ones, and two 70uF/350V ones. Observe the polarity of the originals - one is physically inverted compared to the others!

Additionally, you'll want to replace all of the 25uF/25V capactors on the main board. There's something like 7 of them. They can be covered in cardboard, but they might be small white cylinders with purple print.

Lastly, there's a 50uF/70V one on the bias board. On this one, the + side goes to ground.

I replaced all of the capacitors listed above with Sprague Atoms. The amp sounds much tighter and i have much better response out of the tone controls. Yet, I am still experiancing some nasty distortion when I power up the amp. One more observation - during the distortion I noticed that a pair of the 6L6s (V7 and V8) are pulsating blue in synche with the distortion until the distortion stops. When the distortion stops and I hear the "pop", the blue goes away immediatly and all of the tubes glow normally.

Has anyone ever heard of this before, or have any ideas about what I should check next?

Thanks!
Dr. Framus

[Kap'n]

Has anyone ever heard of this before, or have any ideas about what I should check next?

To me it sounds as though there's something screwy between the phase inverter and the grids of the power tubes - probably in the bias supply. Make sure all the connections are where they should be, and the voltages are within spec.

Does it have a "bias balance" adjustment, or has it been blackfaced?

[DrFramus]

To me it sounds as though there's something screwy between the phase inverter and the grids of the power tubes - probably in the bias supply. Make sure all the connections are where they should be, and the voltages are within spec.

Will do. Is the phase inverter tube V6?

Does it have a "bias balance" adjustment, or has it been blackfaced?

It is "bias balance" adjustment. The wiring is consistent with the AA270 Schematic.

[Kap'n]

Will do. Is the phase inverter tube V6?


It is "bias balance" adjustment. The wiring is consistent with the AA270 Schematic.

Yes, I think. It's the small tube closest to the power tubes. I can never remember which direction they number them from. After digesting this for a few minutes, I'm thinking a cold solder joint somewhere in the bias supply.



I'm looking at the AA270 layout here.

Did you replace the capacitor hanging off of the bias balance pot too? If that doesn't do it, I'd probably shotgun replace the rest of bias circuit (except the pot), and convert it to a blackface style bias while I was at it.

[TT100]

The 100K plate load resistors on the preamp tubes can cause nasty splattering sizzle on power up too. They're carbon comp and can absorb moisture over time. My '68 Pro R had this issue a few months ago. I don't think this would affect your output tubes in the way you describe though.

Worst case is going to be the eyelet card has absorbed moisture and is shorting between eyelets or the insulator card under the eyelet board has done the same and is not insulating. Sliding an insulator under the eyelet card might take care of the latter.

As you've changed the filters and it's still noisy I'd do the plate load resistors next. If you replaced the filters yourself you can do this very easily.

Just be careful and hang in there. You'll end up with a great, reliable twin.

BTW, whats the climate like where you are, or more importantly, where the amps been?

TT

[DrFramus]

The 100K plate load resistors on the preamp tubes can cause nasty splattering sizzle on power up too. They're carbon comp and can absorb moisture over time. My '68 Pro R had this issue a few months ago. I don't think this would affect your output tubes in the way you describe though.

Is there a way to test for this or do I have to replace them to find out if that was the problem?

BTW, whats the climate like where you are, or more importantly, where the amps been?

Thanks!
Dr. Framus

The amp came from Nashville, TN and now resides in tropical Buffalo, NY. The amp is stored in my house heated with a forced air furnace which acts as the worlds best dehumidifier. So for the past 7 weeks the amp has been in desert dry conditions. I am not sure how long the Carbon comps will retain moisture but the fiber board should be pretty well dried out by now.

[TT100]

Is there a way to test for this or do I have to replace them to find out if that was the problem?

Any meaningful testing would have to be under working conditions. Sure you could put a scope across them and "see" what noise coincident transients you might find but we're just talking about very inexpensive, easily replaced resistors. I'd just change them.

You have cleaned and tensioned the tube socket contacts too, right?

Moving on to the realm of the theoretical; If the resistors don't fix it and the tube sockets are good it could still be one of the fiberboards. If any arcing or moisture-induced shorting has taken place there will likely be carbon traces along the path. These would still be conductive after moisture is gone. As the board warms up and expands the paths could become an open circuit and the noise stops.

Let me stop here and make clear I'm no amp "know-it-all." A lot of what I know comes form working for many years in broadcast engineering, the rest, as applies to this kind of stuff, comes from a couple of years on forums, the web and trial and error.

Here's a link to Hoffman's site regarding amp noise.

Good source for help

Let us know what happens next!

;)

TT

[DrFramus]

After digesting this for a few minutes, I'm thinking a cold solder joint somewhere in the bias supply.

I will go though the circuit with a hot iron and check it out. Is the bias supply considered everything going back to the phase invertor tube?

Did you replace the capacitor hanging off of the bias balance pot too?

Definately - both 50mf/70V caps were replaced and wired with positive ground per the schematic.

If that doesn't do it, I'd probably shotgun replace the rest of bias circuit (except the pot), and convert it to a blackface style bias while I was at it.

What does this include? There are the obvious changes to the resistors on the bias pot and removing one of the caps on the bias pot. Yet, following the circuit back to the phase invertor tube there are some differences in the resistor values that I am not sure I should change.

[DrFramus]

What does this include? There are the obvious changes to the resistors on the bias pot and removing one of the caps on the bias pot. Yet, following the circuit back to the phase invertor tube there are some differences in the resistor values that I am not sure I should change.

Never mind - I found details on the Hoffman site after wandering around using TT's link. Thanks!

[Kap'n]

I will go though the circuit with a hot iron and check it out. Is the bias supply considered everything going back to the phase invertor tube?

The bias supply is considered everything from the (usually 220K, in your case 68K) resistors back through the mini "bias board" to the transformer. Obviously the series rectifiers on the bias board are part of the main power supply.

[hawkeye2u]

You could always try some freezer spray and spray various sections, components, since the fault only occurs when u apply the full voltages, not the heaters, if U can determine what components are playing up heat them with a hair dryer, freeze them again until you nail it
Trust me it works
OH DONT SPRAY THE GLASS ON THE TUBES!!!!!!!!!!!!!!!!!! as the freezer is
-60 deg C (CRACK POP GOES the glass)

I forgot to ask what sorta distortion are you hearing? a squeel out of the amp or dist when you playing it? vol up or down, reverb on or off etc

[TT100]

Be careful with freeze spray around high voltages. The condensation on the components and board may cause and arc.

TT

[DrFramus]

After digesting this for a few minutes, I'm thinking a cold solder joint somewhere in the bias supply.

A quick update -

Kap'n, I think you may be on to something. I did not have time to do any soldering last night, but a closer inspection of the bias board this morning revealed something that could very well be the root cause. There is a cold solder joint between the 1K resitor and the diode in the middle of the bias board that run next to the 50mf/70V bias cap. It is pretty bad. I was able to slide the resistor lead freely right through the solder. I am going to fix the joint this evening and I will let you know the results.

Thanks,
Dr. Framus

[Kap'n]

There is a cold solder joint between the 1K resitor and the diode in the middle of the bias board that run next to the 50mf/70V bias cap.

I bet that's it! Congrats.

Don't forget to heat sink the diode lead with an alligator clip, or a pair of needlenose, so it doesn't fry.

[TT100]

I was able to slide the resistor lead freely right through the solder.

Good eye. I made a similar joint while working in a hurry on a filament lead. The amp hummed like mad. Voltage was present at the pin but the poor joint couldn't pass the required current. Touched it with the iron and it was fixed.

Intermittent bias would surely have your output tubes glowing erratically.

TT

[DrFramus]

I thought I found the promised land with the cold joint. I fixed the joint and I have the same condition as before. 10 seconds of BUZZZZZZ when I power up the amp.

Should I have replaced the "chocolate drop" caps when I replaced the filter caps? I was wondering if the .1 caps between the phase inverter tube and the bias circuit were getting flakey. Maybe I am streching here....

[Kap'n]

Should I have replaced the "chocolate drop" caps when I replaced the filter caps? I was wondering if the .1 caps between the phase inverter tube and the bias circuit were getting flakey. Maybe I am streching here....

The chocolate drop(pings) with the yellow writing are some pretty bad sounding capacitors. It's also possible to determine if they'e passing DC. You might just be better off replacing then with Mallory 150's, or Ted Weber caps.

If those caps are leaking, they could also be causing problems. Even if they aren't the culprit, replacing them with good caps will make your amp sound better.

[DrFramus]

You might just be better off replacing then with Mallory 150's, or Ted Weber caps.

I noticed that you did not mention the Sprague Orange Drops. Any Reason? How do they compare to the Mallory 150's?

Dr. Framus

[Kap'n]

I noticed that you did not mention the Sprague Orange Drops. Any Reason? How do they compare to the Mallory 150's?

I don't like them. They sound better than the "turd drops" but, they give an artificial cold sound, IMHO.

Note, there are a few different types of Orange Drops. Some may sound better than others. YMMV, etc.

[DrFramus]

The envelope please.... And the winner is....

Bad power tube! :blbros

After changing out the filter caps and tone caps, the buzzing persisted. I fixed a couple cold solder joints and the buzzing persisted. I changed the biasing scheme to adjustable bias from the balanced method and the buzzing persisted. So on a wing and a prayer I decided to swap the power tubes into different sockets. Low and behold, the pulsating blue glow that was on V7 and V8 during the distortion period moved to V9 and V10. So I installed a new quad and the distortion is gone. The amp came with a quad of old Sylvania 6L6GCs that I am sad to see go because I can not afford to replace them. It appears that there was a short in one of the tubes that corrected itself after running some current through it. When they worked they just sounded glorious. I am not willing to sacrifice reliabilty for that little bit of tone though. So I installed a quad of SED 6L6GCs.

Right now I have the amp biased to 35 mA. I am planning on experimenting with higher current though. Does anyone have experiance with the current limits on silverface output transformers before they get cooked? The highest I was thinking of going is 50 mA, but I am a bit hesitent with no prior experiance on the amp.

I am glad that I have done all this work even though I could likely have gotten away with a new set of tubes. I learned a lot about my amp and I did a bunch of work I would have done eventually anyway. Plus, now I have a much higher regard for the reliabiliity of the amp since just about anything that frequently goes bad on these amps has been replaced.

Thanks for working through it with me Kap'n and TT.

Dr. Framus

[Kap'n]

Good detective work! :yay

Not sure what the current ratings are on Twins. I know people run as high as 40mA or so without problems, or so I've heard. 50 might be pushing it.

There's always the unscientific "heat test." Run the amp for an hour or so. Transformers can get hot enough to touch, but not to leave your hand on. If they get too hot to touch, they're too hot.

If your flesh sticks, turn it down....:lol

[TT100]

If your flesh sticks, turn it down....:lol

:bwa

Good going Dr. :yay

You took care of some other things that you won't have to worry about for a while now, learned a bit about your amp and ended up finding the problem was as easy as a tube swap. You know what to try first next time now, don't you!

;)

Time to plug and play!

:blbros

TT