Biasing a Fender Twin

[PRR]

> Didn't realize a push-pull-OT has a significant equiv gap as well.

*Fender*, and guitar-amps generally, use E-I lams and often stack 3-3. Inductance is down, but small DC is no-problem.

A 1-1 E-I stack will saturate a bit easier.

Torroids generally have no gap or butt-joints, and can be quite sensitive to DC. These are rare but not unknown in guitar-amp work.

IAC, degausing should clear any ill effect.

 

[CJ]

if the tube lights are in but nobody home then my money is on a cooked or broken screen resistor.

to find out, simple swap tubes and measure cathode current.

Stevie Ray Vaughn used to take a hammer to his OPT, why?

demag? probably not, more likely to damage the grain structure so the OPT sounded like it had barn roof(non grain orient lams) instead of high inductance grain steel,

some folks take out one tube on purpose to lower volume and get a saturated OPT which will result in a warmer compressed sound, this can be hard on a tube as the B+ umps up a bit due to less load on the pwr supply,

got an old Tremolux? some used gapped core OPT's in a push=pull circuit, what happens? less inductance which means less bass,

what kind of 6L6 tubes are you running?

 

[clintrubber]

> Didn't realize a push-pull-OT has a significant equiv gap as well.

*Fender*, and guitar-amps generally, use E-I lams and often stack 3-3. Inductance is down, but small DC is no-problem.

IAC, degausing should clear any ill effect.

Perfect, both interesting and good to hear.

Thanks!

 

[clintrubber]

if the tube lights are in but nobody home then my money is on a cooked or broken screen resistor.

to find out, simple swap tubes and measure cathode current.

I did that when testing, luckily there was current running, so the tube at that position was indeed the bad guy. In addition, it has that 'second getter spot which isnt one'  at the side, so that's some sort of double evidence.

When I have new tubes in, I'll sure have a look at the state of the relevant resistors before inserting these.

some folks take out one tube on purpose to lower volume and get a saturated OPT which will result in a warmer compressed sound, this can be hard on a tube as the B+ umps up a bit due to less load on the pwr supply,

That's interesting to hear!

My SF TR must have been doing this, but inadvertently.

Don't like to keep it that way for next weeks gig... 

what kind of 6L6 tubes are you running?

Old-don't-fix-it-if-it-ain't-broke tubes which I probably should have been replaced already long ago...
they're National Electronics 6L6GC  (in red lettering), don't recall if they're also with label 'made in Russia'.

Like these (not my pic):
http://music-electronics-forum.com/attachments/7151d1259810204-img_0905.jpg

I'll be ordering a quad set of TAD 6L6GC-STR, at least to feel safer for that next weeks gig,
although who knows if swapping for these new ones actually introduces some new-fresh-tube child diseases... 

Bye/thanks

 

[clintrubber]

I'll be ordering a quad set of TAD 6L6GC-STR, at least to feel safer for that next weeks gig,
although who knows if swapping for these new ones actually introduces some new-fresh-tube child diseases... 

And so it went, the first quad set arrived with one leaky tube...

replacement set is in, pluggin that in later today.

TAD advised to bias @ 25 .. 30 mA / each, but with the measured 465V plate voltage (actually high for an early Silverface, not ?) that would mean below 50% of dissipation, which looks a bit low. 
OK, with the balanced bias still in I presently don't have the means for absolute biasing anyway.

Bye

 

[Dr Gris]

If it's not already done you can use the 240V tap on the power trafo. It will lower the plate voltage a bit.
It's been a while since I was inside one but I think the export trafo uses black/yellow for 220V
and black/red for 240V. Better look that up...

//M

 

[clintrubber]

If it's not already done you can use the 240V tap on the power trafo. It will lower the plate voltage a bit.
It's been a while since I was inside one but I think the export trafo uses black/yellow for 220V
and black/red for 240V. Better look that up...

Thans for the suggestion, nice idea.
It's currently set at 230V, and gives 6.2V heater AC.  Using 240V might lower that heater voltage a bit too much then, although it's still within the usually assumed range of +/-7%, or +/-10%.  TAD-datasheet for the 6L6GC-STR states 5.8V - 6.8V though.

The 465V might then be fine enough, it's just that I thought the PowerTX for that early SF would have given less voltage. 

Hmm, I also saw in the doghouse that the (assumed to be original) Mallory caps are still there, might best work there first.

have a good weekend

 

[CJ]

those caps are probably alright, cut open an old blackface 70/350 that was pretty juicy,

Leo knew what he was doing by keeping them in that cool place, also cuts down a bit on radiated stuff coming out of the caps,

bias is not that important if you really crank up the amp,

hook the amp to a dummy load at full power and measure AC volts across the load while turning the bias pot, nothing will happen,

so if the tubes idle cool, you are actually extending their life

 

[Whoops]

Hi Keith,
I have docs on how to perform the Silverface to Blackface conversions, Bias included.
I can send them to you

Tried to contact you by PM but your inbox is full

 

[clintrubber]

those caps are probably alright, cut open an old blackface 70/350 that was pretty juicy,

Leo knew what he was doing by keeping them in that cool place, also cuts down a bit on radiated stuff coming out of the caps,

That's both interesting & confusing  8) , thought it'd be a no brainer to recap a 45 year old amp, but amp is indeed doing fine (with new complete set of power tubes)...

(Pics below)

I saw one of the original 20uF caps had been replaced  by two stacked 47uF caps; at least I assume they didn't suddenly ran out of  20uMallory-caps at the factory. It's an export model though, so not too likely that Mexico-labeled caps were used when replacing. But OK.

bias is not that important if you really crank up the amp,

hook the amp to a dummy load at full power and measure AC volts across the load while turning the bias pot, nothing will happen,

so if the tubes idle cool, you are actually extending their life

Cool, the original set wasn't sweating too much either w.r.t. the mA's and it sounded mighty fine.
With the new quad set everything seems to run fine @ roughly 29 mA per tube, without any bias-adjusting (note this amp has the stock balance pot).  I'll measure again after some evenings of use.

Now those flubby Oxford speakers... but that's for another time ...

Bye

 

[Whoops]

those caps are probably alright, cut open an old blackface 70/350 that was pretty juicy,

Leo knew what he was doing by keeping them in that cool place, also cuts down a bit on radiated stuff coming out of the caps,

That's both interesting & confusing  8) , thought it'd be a no brainer to recap a 45 year old amp, but amp is indeed doing fine (with new complete set of power tubes)...

(Pics below)

I saw one of the original 20uF caps had been replaced  by two stacked 47uF caps; at least I assume they didn't suddenly ran out of  20uMallory-caps at the factory. It's an export model though, so not too likely that Mexico-labeled caps were used when replacing. But OK.

bias is not that important if you really crank up the amp,

hook the amp to a dummy load at full power and measure AC volts across the load while turning the bias pot, nothing will happen,

so if the tubes idle cool, you are actually extending their life

Cool, the original set wasn't sweating too much either w.r.t. the mA's and it sounded mighty fine.
With the new quad set everything seems to run fine @ roughly 29 mA per tube, without any bias-adjusting (note this amp has the stock balance pot).  I'll measure again after some evenings of use.

Now those flubby Oxford speakers... but that's for another time ...

Bye

Those Mallory paper caps are way over their timelife, I would replace them in a pinch.

 

[CJ]

here is an interesting viewpoint>

"The Truth About Vintage Amps
Skip Simmons Amp Repair | Loma Rica, CA, USA
Repairs/Info: [email protected] | 530.771.7345

I would like to dispel some of the myths, misinformation, rumors, and guesses out there about using and restoring vintage amplifiers. Any of my customers will tell you that I tend to be blunt, but I’m just trying to save us all some time. If everything you know about tube amps came from the internet, you might want to prepare for a shock.

You probably don’t need new tubes.

You don’t need new transformers.

You might not need new capacitors. I know you read a book that said otherwise, but the book is wrong.

You do not always need a three-prong plug. Putting one on your mint tweed Champ is just plain wrong.

"Recapping” a vintage amp can alter the tone significantly. If you were lucky enough to own a great old amp, why in the world would you want to alter the tone significantly? Read "Recapping . . . the Saga Continues"

Most “techs” make extra money by selling current-production parts and accessories that you probably don’t need.

Restoring a vintage amp does not mean replacing all the parts. “Techs” who routinely advise wholesale parts replacement often do so because they don’t have the knowledge and experience to diagnose problems. They are merely hoping that changing all the parts will fix the problem.

I often have amps sent to me AFTER they have supposedly been repaired elsewhere. Can you believe that a very well known shop replaced every single resistor and capacitor in an original ’60 4-10 Bassman, but failed to spot the actual problem – a bad choke? Hmm. I wonder if anyone in that shop had ever heard a real tweed Bassman... they thought the amp sounded great.

I’m sorry, but “any solder junkie with two beers worth of parts” (March ’08 VG) cannot make an old amp better.

Unnecessary parts replacement and poor quality repairs will ultimately decrease the value of your vintage amp. For some reason, techs who wouldn’t dream of replacing the original pots in a ‘57 Strat have no problem gutting a ‘57 Bandmaster.

All the new boutique amps out there are just fine. Most of them are very well made and should have no real servicing problems for many years. There is just one problem. An original old amp just demolishes a “clone” of the same amp, even if it is “hard-wired” and uses “custom-made capacitors.” Why? Because there is a LOT more to good amp tone than just copying a schematic!
 

Skip Simmons Amp Repair • 4824 Bevan Road • Loma Rica, CA 95901

 

[clintrubber]

You might not need new capacitors.

"Recapping” a vintage amp can alter the tone significantly. If you were lucky enough to own a great old amp, why in the world would you want to alter the tone significantly? Read "Recapping . . . the Saga Continues"

Thanks for the info & quotes. Triggered by that, I read some more there:

http://www.skipsimmonsamps.com/amp-repair-articles.html

Filter caps certainly can go bad, and I replace lots of them, but I don't consider it a "must," especially on amps from the eighties or newer. Replacing them will not cause a loss of tone, or make the amp sound less "vintage." Coupling caps and caps used in tone circuits are a different story.

So trying to come to a conclusion, it sounds like it'd be OK to replace the electrolytics without fearing a noticable change in sound (I don't intend to perform HiFi-ear-nonsense on this). 
But obviously not replacing the other caps, no reason for those, amp sounds fine. 

Given though that the amp sounds fine, I'm wondering though what the most likely error-scenario is: say these electrolytics go bad (lose their capacity) --> hum will develop, but nothing will get damaged, not even the gig, since then switching to a backup amp. 

In other words: as long there's nothing wrong so far, how likely is it that cap-damage takes out transformers, tubes & whatnot ?

Thanks/bye
 

 

[clintrubber]

I'll be ordering a quad set of TAD 6L6GC-STR,

FWIW, I also ran into a new quad set of Groove Tubes of 6L6S, Slovakian-made as it seems, discounted (probably old stock).
Let's see how these compare to the 6L6GC-STR.