Tweed Filter Caps

[Jammin'John]

My Tweed Pro,5e5a makes a nasty distortion while I'm playin',when I first fire her up !
My friend thinks it's one of the filter caps.
It goes away after I play for 15 min.
I notice that a lot of the tweeds,mine included,have 16uf-450v,8uf-450v caps. These are the filter caps ?
I also noticed that in other sections 25uf-25v caps are used.
If one of the filter caps is bad should I use the same value cap or larger ?
Is there a brand I should get ?


JJ

[Firestorm]

That sounds like it could a filter cap problem for sure. If the caps are original, you can assume that they are bad -- all of them -- they weren't built to last that long. The 25/25 bypass caps, too. Theoretically, they wear out faster. You can safely go to higher voltage ratings -- 475V, 500V, but be careful about changing the uF rating. These amps are sensitive to changes there (especially the 8uF). These are available.

I'd keep the filter on the PI at 16uF; the other two you could kick up a bit if you want to try to clean it up. Especially since you might have it tubed with modern 6L6s, which are different animals than the original 6L6GBs. Maybe 22uF. Some people might say 30uF, but part of the "character" of these amps is the out of tune quality they get at higher volumes. Pretty much a function of being undercapped. Depends on whether you like that.

I used to use nothing but Sprague ATOMs (and still haven't had a problem with one), BUT a lot of folks have had some issues -- and they have gotten pricey -- so a good bet seems to be the F&T caps from Germany. On the 25/25 caps you're pretty much good with anything that has reasonable tolerances.

[Jammin'John]

"That sounds like it could a filter cap problem for sure. If the caps are original, you can assume that they are bad -- all of them -- they weren't built to last that long. The 25/25 bypass caps, too. Theoretically, they wear out faster."

It's a clone.They are not very old.

"You can safely go to higher voltage ratings -- 475V, 500V, but be careful about changing the uF rating. These amps are sensitive to changes there (especially the 8uF)."

I don't want the tone to change .

I'd keep the filter on the PI at 16uF;

Is that the first one ?

http://www.ampwares.com/schematics/pro_5e5a_schem.pdf

"the the other two you could kick up a bit if you want to try to clean it up. Especially since you might have it tubed with modern 6L6s, which are different animals than the original 6L6GBs."

I'm runnin' KT66's.

Maybe 22uF. Some people might say 30uF, but part of the "character" of these amps is the out of tune quality they get at higher volumes. Pretty much a function of being undercapped. Depends on whether you like that.

I wouldn't want to change much of the tone/feel.

"I used to use nothing but Sprague ATOMs (and still haven't had a problem with one), BUT a lot of folks have had some issues -- and they have gotten pricey -- so a good bet seems to be the F&T caps from Germany. On the 25/25 caps you're pretty much good with anything that has reasonable tolerances."

Probably one of the 16uf ones is the culprit.

Who sells F&T ?
Will I HEAR a difference ?
Are they expensive ?

Thanks,
JJ

[Firestorm]

Ah, a clone. If the caps aren't that old, I'd look elsewhere first: bad tube, bad resistor, bad solder joint, dirt in a pot. Sometimes a cold solder joint will sound bad until it heats up and makes better physical contact because of the expansion. You don't want to replace caps to discover they weren't the problem. And, based on your question of which cap went to the PI, I wonder if you want to be messing with them. Caps can store enough juice to burn painful holes in your flesh, and if you touch everything just right (just wrong, actually) kill you dead. Know a good tech nearby?

[Jammin'John]

I know how to drain the caps.
I just replaced the 1 meg normal volume pot.
I know about the lethal voltages.
I have a good VOM.
How would I check a bad solder joint ?
Just because the caps are almost new doesn't mean that they are all good.
The 1 meg pot was almost new & it crapped out.
What if I hooked my voltmeter to each cap and tried playin guitar while observing the meter.
If a cap was bad wouldn't the voltage drop when I'm playing ?

JJ

[Firestorm]

Okay. A couple of things: I agreed that the caps could be an issue 'cause I assumed it was an old amp. In a newer amp, the caps wouldn't be the first place I looked. You can't really monitor voltages as you're playing unless you did the same thing back when the amp was running right. The voltages will change as you play because the power supply will sag under load. There are a lot of things besides filter caps that could be odd:

The fact that you replaced a 1M pot -- why? what was it doing? which pot? are you certain you got it installed right?

It could be a coupling cap somewhere -- you'd need to check for incorrect voltages in the stage feeding the cap or the presence of DC on the grid the cap feeds.

It could be related to running KT66s in an amp designed for 6L6s -- what amp is it? Can the power supply comfortably deliver the additional heater current those tubes suck up? Can the bias supply provide the right voltage to bias those tubes correctly (the stock circuit is non-adjustable -- did you change it?) In some circuits, the KT66's need bigger grid stopper resistors to run stably. That circuit has the screens at a higher potential than the plate -- not sure if KT66s like that. Specs for the KT66 insist that each tube have it's own bias supply or it's own cathode resistor (if cathode-biased). Not always an issue, but sometimes.

I think the amp should be diagnosed systematically, rather than by parts replacement. The exception: the tubes. Try replacing tubes with known good ones to rule out a tube problem.

[Jammin'John]

Okay. A couple of things: I agreed that the caps could be an issue 'cause I assumed it was an old amp. In a newer amp, the caps wouldn't be the first place I looked. You can't really monitor voltages as you're playing unless you did the same thing back when the amp was running right. The voltages will change as you play because the power supply will sag under load. There are a lot of things besides filter caps that could be odd:

How can you check a filter cap to see if it's ok ?

The fact that you replaced a 1M pot -- why? what was it doing? which pot? are you certain you got it installed right?

The normal channel vol. pot would crackle badly and if you struck it lightly with a pen it would make loud crackling sounds almost like a bad preamp tube,

It could be a coupling cap somewhere -- you'd need to check for incorrect voltages in the stage feeding the cap or the presence of DC on the grid the cap feeds.

It could be related to running KT66s in an amp designed for 6L6s -- what amp is it?

5e5a http://www.ampwares.com/schematics/pro_5e5a_schem.pdf

Can the power supply comfortably deliver the additional heater current those tubes suck up?

It is designed to run a 5u4 so instead I have a 5r4 in it.

Can the bias supply provide the right voltage to bias those tubes correctly (the stock circuit is non-adjustable -- did you change it?)

It is cathode bias now.

In some circuits, the KT66's need bigger grid stopper resistors to run stably. That circuit has the screens at a higher potential than the plate -- not sure if KT66s like that. Specs for the KT66 insist that each tube have it's own bias supply or it's own cathode resistor (if cathode-biased). Not always an issue, but sometimes.

I have 1 cathode bias resistor.

I think the amp should be diagnosed systematically, rather than by parts replacement. The exception: the tubes. Try replacing tubes with known good ones to rule out a tube problem.

I did.

JJ

[Jammin'John]

I'm runnin' KT66's in this amp with no issues.
http://www.ampwares.com/schematics/twin_5e8a_schem.pdf

JJ

[Firestorm]

A leaky filter cap will draw too much current through the resistor that feeds it. If the voltage drop is out of whack, that can pinpoint the problem. Some guys suggest you can test filter caps by discharging then disconnecting them -- then connect a 9V battery across the leads to charge it to 9 volts -- then disconnect the battery and see how long the charge takes to dissipate. If one cap dumps its charge way faster than another, it may be suspect.

I know what schematic you referred to, but who built the amp? What are the specs of the trannies? They have to be able to cope with KT66s.

Changing from 5U4 to GZ34 won't really solve the heater current issue. The rectifiers are on the 5V winding, not the 6.3V winding.

What cathode resistor are you using (ohms/watts)? Bypassed or not?

How well matched are the tubes, esecially for current? Sharing a cathode resistor (technically a KT66 no-no) means that if one tube goes haywire, it can screw up the other.

[Jammin'John]

Maybe these will help.

JJ

[Firestorm]

Nobody gets any awards for lead dress on this one. Jeepers, there's a lot of extraneous wire.

If you suspect a filter, start to the right of the blue 30uF Sprague. Those look like small Asian numbers. I never liked them for guitar amps. The Sprague might be crap, too, but I'd look at the others.

You've got cathode bias wired (I can't read the values), but there is still a bias supply cap in there from the original (?) fixed bias arrangement! Is that totally disconnected?

I think you need to clean up the leads, verify that the fixed bias is not in circuit, MAYBE replace some of the 16uF and the 8uF caps with F&Ts. And make sure the cathode resistor is the right value. Actually, the tube books say you should split it in two, but you might not have to.

I thought you said 5AR4 before, but I see it's a potato masher 5R4. That might be okay , but I never found specs on them I totally trusted. I'm a Mullard 5AR4 man, myself.

[Structo]

Yes, I can almost guarantee that if you neaten up that wiring you will cure some of the hash. If not, it will sure look better.

[Jammin'John]

Hey,
Don't beat me up too bad. I'm a player,NOT a tech.
I got the chassis at an estate auction for $50.
The man who died had a little music store.
It's a Weber kit.
I had my tech finish the build for cheap and had a nice TV cab & speaker made.
I wanted cathode bias 'cause I play blues & like the compression.
There is a trimpot so I can dial in the bias a little.
I got a screamin' deal on a bunch of KT66's.
I'm runnin' them in a Low Power Tweed Twin with no issues.
I have the 5R4 in there because I wanted to drop the B+ because the KT's are not as robust as modern 6l6's and because they draw a lot of heater current I was tryin' to give the PT a break..
Those are asian caps ?
I just replaced the normal vol. pot with a Bourne.
What do you mean,sloppy lead dress ? he he

JJ

[billyz]

Lead Dress for sure. Shorten those Grid and plate wires , as well as the wiring to and from inputs and controls. As for caps, F&T has some 16uf @ 475 VDC rated ones. I have heard Sprague is better now, but they did have a bad run a while back after the big move offshore. Personally, I have used 20uf in place of 16uf and Liked it. I would keep the 8 uf to no more than 10uf. More important is to keep the Voltage rating as high or even higher, especially with newer made components. They seem to fail the most when run close to their design limit.

I have had tubes behave this way, usually preamp tubes, but even power tubes.

Is that a bias balance pot between the power tubes? with cathode biased I would rather have seperated cathode resistors/bypassed.

[billyz]

Since you had a tech finish it, I would say you need a new tech. He is very Sloppy.