Twin Reverb UL hums even when all controls are set to 0

[mogazi!]

Got this amp last week, and but I can't remember if the problem was present when i went to try it out. The amp works wonderfully apart from this "soft" hum that is constantly present even when all the controls are set to 0. There seems to be a slight hum/buzz even when the amp is on standby, although it's not from the speakers. I am using a set of 4 balanced semi-new Svetlana 6L6 all matched by Watford valves less than a year ago. I haven't tried tweaking the Hum Balance or Output Tubes Matching controls as I'm not really sure of how to use them...

Could something (ground connection?) have come undone during transport? Should I try the hum balance control? Could it be something more serious?

Thanks!

[pdf64]

With controls on '0', adjust output tube matching to minimise hum.
Then turn controls up full and adjust hum balance to minimise hum.
To get best tone, it would be a good idea to get a tech to check the bias.
Peter.

[Chuck H]

The amp could probably use new filter caps. You should do it before a cap blows and covers it's surrounding components with smelly, caustic electrolyte goo. Without knowing the amps history it could be that it sat around unused for a loooong time. That's the worst thing for old filters and if they've never been changed they're over due anyhow. This is almost certainly your hum issue.

Chuck

[mogazi!]

Heya, thanks for the quick replies.

Not sure if this fixed the problem or made it worse but, i decided to have a go with the two trims before spending more money on a new set of caps.
The Output Tubes Matching seemed to do the trick. There is a precise point where the hum is almost completely gone. Past that point (or before it) the hum comes back (increasing the further I moved away from the "sweet spot").

I've left the amp on for an hour and so far no glowing tubes or burning smells of any sort although the chassis over the power tubes gets pretty hot but i guess that's common right?. A slight hum seems to come back when i turn the amp back on but goes away after about 5 minutes (maybe after the valves properly warm up?). Also every now and then, I hear a very very quiet helicopter kind of sound it starts randomly and last for a few second and then goes away but even that is inconsistent and just seems to happen occasionally...

help!! Do you think is just the capacitors?

Also, with both Normal and Vibrato channels at 0 if I turn up the Master volume i get a frying kind of sound, kind of like when you have a cable plugged in the amp and unplugged on the other end... Any idea?

[pila]

I think the "frying" kind of sound you hear is just amp "hiss" At high volume the sensitivity goes up, and the pre-amp "tube noise" becomes apparent.

[pontiacpete]

I, also, would change the filter caps.

[Chuck H]

That hissing and popping could be small arcs and cold connections from oxidized tube pins and sockets or switching jack contacts. I'd bet the amp sat unused for a while. You should replace the filters and clean all the contacts. It could save you a much more costly repair.

Chuck

[mogazi!]

Yes, the amp has been sitting around for about a year completely unused (or at least that's what the guy told me) after the transformer was replaced.
I'll change the caps. Is the chassis layout for the 100W_Master_Vol the same as for the 135 Ultralinear?

Are these capacitor values correct? From left to right:

100uF / 350V DC
100uF / 350V DC
20uF / 500V DC
20uF / 500V DC
20uF / 500V DC

I assume Sprague Atom Electrolytic are the way to go. I noticed a popping sound synchronized with the speed of the vibrato. Filter capacitor again? There also seem to be a popping sound when i switch to standby... I saw on the Watford Valves website that there is a Fender Bypass Cap does that have anything to do with it? Is that a different capacitor than the ones in the doghouse? ???

About the contacts: would a contact cleaner like this be ok? Ah, and just to make sure: to clean tube sockets and pins i just put a drop of cleaner in every hole and put the tube in and out several times right? What about the rest of the contacts? should i use like a toothbrush with the contact cleaner? Any other suggestions?

Sorry about the many questions... Just wanna make sure I don't fuck up things more than they already are...
Thank you for your help so far!

[mogazi!]

Sorry for the double post but I had a look the inside of the amp since the last post. The caps don't look as if they are 20 years old plus the amp seem to have been serviced quite recently (probably when the transformer was replaced a year ago or so...)

The only strange thing I found was on top of one of the Power Tubes (V8). It seemed like something went on fire there...
I also noticed that inside the doghouse some of the of the cap leads seem to touch the resistor ends before they reach the actual connection. Don't know if that makes any difference electrically but since we talked about arching i was just wondering if that might have anything to do with it.

Here are some pictures (High resolution so please be patient):

The Chassis:


The Caps:


The "Touching" resistor (there is another one that is very close to touching)


The "Burned" area above V8 (power tube)


I was looking again at the amp and I noticed a couple of things about the V8 Socket. First of all the contact #7 has dropped inside the pin socket (you can clearly see it in the picture) and is touching with contact #8. Contact number 6 has completely drowned into the socket although the resistor was still connected to V7 "on air" probably by the guy who did the servicing. So I'm guessing I need a new valve socket as a start...


-------
Just a small Addition:
I've looked at the values written on the caps and found these:



Does anyone know if they're correct?

[pdf64]

That's why should fit flameproof resistors on the screen grids, there's a lot of collateral damage around the tube socket. Get it, and all the resistors (470 and 1k5 on other sockets also), replaced, 1 watt metal flameproof for screen grid 470. Don't put higher wattage ones in, they need to fuse open circuit when there's a problem (but not set fire doing so).
The caps look about 12 years old, the dark blue ones have a 9742 code on them. Those grey IC ones aren't the best for long life. Make an assessment from the ripple voltage and condition.
The servisol is not the right stuff, it's got a lube in it, bad for high DCV. Best just to use take head cleaner (iso propyl alcohol IPA from chemist). But servisol ok for jack socket contact and switches.
Someone has had fun fitting monster screened cable in there. Are the joints properly sleeved/insulated to prevent shorts?
I can't get that Watford link to work.
Nice amp with a little TLC. Peter.

[Chuck H]

+1 on what Peter said. But not knowing the full history still leaves a few things suspect. For one, I wouldn't assume the age of the caps (except the blue ones). As noted, those grey ones aren't the best for long life. And they have looked the same for about 25 years so it's hard to know the actual age. I just had a set of those blow about four days ago. The amp was 20 years old and still in the original box. When the owner fired it up the main filters just smoked after about five minutes. Point is those caps don't need to be used to go bad. In fact it's worse if they're not used. Even the blue ones at 12 years old are due for replacement and I would guess that the IC caps are older than that.

The work under the doghouse is funky. Just replace the whole mess...Really.

Chuck

[pila]

Appears to be a nice amp. Like Chuck says, re-do all that stuff. I have the attitude " when in doubt, throw it out", when it comes to caps of unknown age. A nice amp shouldn't be living on borrowed time. You will also protect your investment

[lowell]

just my 2 cents but looks as though your amp lost a pin on the power tube that had a fried screen resistor. i'd change that socket too while you're in there doing preventative maintenance. Pin 6 is not necessary for 6L6 but it's nice to keep that screen resistor mounted on something stable.

[lowell]

oh and i forgot whenever i do a cap job like you're doing I always replace the bias supply and bias feed resistors. The little board behind the jewel lamp w/ the 3 components on it. Cap, resistor and diode. Then the 220k resistors between the 2 large coupling caps at the end of the main board closest to the power tubes are the feed resistors. I assume you're already replacing the 1k5 resistors on the power tube pin 5... if not I'd do it. You don't want to have to deal w/ the possibility of your bias supply failing. It is WAY worth it to be sure all those parts are new and solid.

[Sweetfinger]

Someone has had fun fitting monster screened cable in there.

Whatever it takes to pass the hours by in the padded cell!

Tha "bacon frying in a pan" sound is likely one or more noisy plate resistors.