Troubled Twin Reverb

Robert · Post by **Robert** » 31 Oct 2001 1:56 pm

Friends:
I was playing the Dual Professional through the '68 Twin Reverb today - everything sounded great for a couple of hours, then I had a slow decrease in volume, accompanied by a raspy sputter - then nothing but the sound that an electric razor makes. I turned it off and checked the guitar on another amp - it's not the guitar. Nothing like the sounds that a tube makes when they go, either. Too sudden for that, anyway. So what gives? A capacitor, maybe? Thanks in advance for any info. I'll probably get it into the shop quick, but good to hear from all of you, as well.

Rob Yale

Jack Stoner · Post by **Jack Stoner** » 31 Oct 2001 2:47 pm

Could be the rectifier tube.

Rodney Shuffler · Post by **Rodney Shuffler** » 31 Oct 2001 4:35 pm

If it's a Twin Reverb...you can rule out the rectifier tube right away. No way it could be that.

If it was a cap you could probably smell it. Even if there's no smell, I'd have to look at the filter caps to start with.....if it's gone you can see it when you take the chassis outta the cab and look at them.

If that ain't it, holler back and someone here can probably help find out what it is.

Rod

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Rod's Place

Blake Hawkins · Post by **Blake Hawkins** » 31 Oct 2001 6:28 pm

Robert, There is an 85% chance it could be a tube. If the noise you heard was very loud, then it could be one of the smaller tubes
in the pre-amp, reverb or driver circuit.
If you have some spare tubes, try them.
If not, take it to your local amp tech.
I've used tube amps since 1953 and keep a spare set with me when I go to a gig.
There is a 10% chance it could be an electrolytic cap....and a 5% chance a resistor or transformer has failed.
Blake[This message was edited by Blake Hawkins on 31 October 2001 at 06:32 PM.]

Robert · Post by **Robert** » 31 Oct 2001 7:04 pm

Thanks, all:
At under $100., a re-cap sounds about right. I figure it's been about five years on the set that's in the amp now, so . . . please, not a transformer! Seems like that might be a bit pricey. Thanks again, and I'll keep the curious posted once I get to the bottom of it.

Rob Yale

Jay Ganz · Post by **Jay Ganz** » 1 Nov 2001 6:54 am

'68 Twin Reverb amps don't have
a rectifier tube anyhow. They use a
series of diodes.

Rodney Shuffler · Post by **Rodney Shuffler** » 1 Nov 2001 7:05 am

That's why I said there's no way it could be that.

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Rod's Place

Paul Graupp · Post by **Paul Graupp** » 1 Nov 2001 7:38 am

Take a look at those four 6L6GCs or whatever output tubes are used while the amp is on. If they glow Blue it's time for new !!

Regards, Paul

Patrick Smith · Post by **Patrick Smith** » 1 Nov 2001 8:03 am

Knee jerk reaction says a problem tube is afoot....does it behave this way on both channels?....if not then swip-swap the 2 preamp tubes all the way to the right as you look at the back of the amp.....just swap em'....now, has the problem moved from one channel to another?

If it's in both, I would certainly heed the advice on your power tubes and check the PI tube....that may be your problem.

I would bet the farm that it's either a tube problem which is easily fixed, or a solder joint problem, which is easier to fix.

Oh, and when you find that rectifier tube please e-mail me it's approx location, I can't seem to find the rectifier tube in any of my twin reverbs......

PMS
Britney's Louisiana Love Dumplin'[This message was edited by Patrick Smith on 01 November 2001 at 08:04 AM.]

Rodney Shuffler · Post by **Rodney Shuffler** » 1 Nov 2001 8:10 am

Fender amps over 45 watts don't have tube rectos....lol; sorry guys I was just makin' a joke. Rod

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Rod's Place

Patrick Smith · Post by **Patrick Smith** » 1 Nov 2001 9:54 am

Us too Rodney!......LOL Some things are just too fun to leave alone.....

PMS

PS-nice web page!...nice tele!

Michael Frede · Post by **Michael Frede** » 1 Nov 2001 1:29 pm

Filter caps go bad gradually.A 60Hz hum starts off being barely noticable and then builds up to an obnoxiuos level,but you should still hear your guitar through it.The transformer failures usually involve arcing and/or blown fuses.Loss of bias voltage in the output stage can cause a loud hum and no guitar signal output but not for long as the output tubes will start to melt down and the fuse would blow.BTW,I have seen brand new military spec NOS tubes glow blue or purple from the first time they are used.This by no means indicates that they are bad!Very small amounts of rare,inert gas still remain inside the tube after it is evacuated and are ionized by the high plate voltage.This ionization takes place on the inside edge of the glass envelope and is of no consequence.It is possible a preamp tube has failed(an open heater filament),although rare.From the desciption of the problem,I would suspect failure of a passive componant in the preamp or phase inverter section,or solder connection(s).Of course,anyone who knows what they're doing should find this one pretty quick.

Matt Farrow · Post by **Matt Farrow** » 1 Nov 2001 2:58 pm

No way to tell EXACTLY what's wrong without a good look around. My vote is for a bad solder joint in the phase inverter, or a bad phase inverter tube. I would take the amp to a qualified technician and tell him exactly what you posted here. That should be enough of a description to get him going.

Good Luck!
Matt Farrow
Pharaoh Amplifiers

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Matt Farrow
Marlen 9-string 6+2
Kustom K150

http://www.skybolt6.com/pharaohamps

Robert · Post by **Robert** » 1 Nov 2001 4:13 pm

As usual - lots of good info. I dropped the amp off at the shop this morning. The very trustworthy guy that will perform the work suspects a filter cap. But it didn't go gradually - it went "Pffft!" - PDQ! I hadn't experienced the gradual increase in 60 cycle hum. I'll find out soon enough. BTW - after dropping off the amp I had the car in for $400. in repairs to the emission control system. One of those weeks . . .

thanks,
Rob Yale

Rodney Shuffler · Post by **Rodney Shuffler** » 1 Nov 2001 5:15 pm

I had a filter cap go out one night (thankfully it was during a practice). It went quick and there was no mistaking the smell.....it's kinda like brakes burning, once you smell it you know what it is.

At any rate, I always keep spare caps, tubes etc... I popped some caps in while everyone else smoked and we took off again.

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Rod's Place

Paul Graupp · Post by **Paul Graupp** » 1 Nov 2001 7:12 pm

Michael; You're right about the halo gases but it's the gas that gets between the tube elements that causes the mess. That's why there is usually a gas test when you check tubes. That left over from the getter will usually glow at 455vDC but in the low level tubes you may not see it at all.

Robert, let us know what you find out when they get it all fixed up.

Regards, Paul

Matt Farrow · Post by **Matt Farrow** » 2 Nov 2001 9:00 am

I have a friend in a band in NYC down here in NC playing a gig a year or 2 ago, and at soundcheck, his Fender amp all of a sudden started making some horrible farting noises. Turned out one of the filter caps had broken off the board in the cap pan. I put in a new filter cap the next day and he was back in action - but it goes to show that caps can fail suddenly, too.

Good luck with the amp - it shouldn't cost too much to fix, at least it wouldn't if I were fixing it, can't speak for other techs.

Matt Farrow
Pharaoh Amplifiers

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Matt Farrow
Marlen 9-string 6+2
Kustom K150

http://www.skybolt6.com/pharaohamps

Robert · Post by **Robert** » 8 Nov 2001 7:00 pm

I pick up the Twin reverb tomorrow. Filter caps! Another problem I had was a weak vibrato (the few times I wanted to use it) - I was using a new footswitch with the RCA jacks from Fender. Well - it was wired backwards! No wonder the vibrato wasn't working. So - Terry (Guitar Works, Evanston) is going to re-solder the switch for me, too. I think I'll take the steel to the shop for the test drive. Thanks, all, for your interest and advice.

Rob Yale

Bob Carlson · Post by **Bob Carlson** » 9 Nov 2001 9:00 pm

Robert,
You may already know this, but the reason tube amps have a standby switch Is to keep the tubes warm.

A tube slowly burns Itself up. If you turn one on for only a half hour at a time the deposits will build up and It will start frying bacon. Ask your repair man about how to make tubes last longer.

Bob Carlson

Jack Stoner · Post by **Jack Stoner** » 10 Nov 2001 3:16 am

Bob, that reason is a new on me. And, I had basic electronics training back in 1955 when it was all tubes.

Robert · Post by **Robert** » 10 Nov 2001 7:31 am

Bob:
If I'm going to play for any length of time, I'll use that stand-by switch - many times, though, I've less than an hour to play/practice, so I'm definitely hard on the tubes. They're on for 40 minutes and off again. Playing the steel without the amplifier is not very satisfying, though, so I pay the price. BTW - after getting the amp back yesterday, I played for 2-3 hours (used the stand-by, too) and the old beast sounds great.

Thanks,
Rob Yale

Paul Graupp · Post by **Paul Graupp** » 10 Nov 2001 11:28 am

Jack; Same schooling for me. I always thought the coating on the cathodes to help electron emmision was the failing factor. ie: no emmision, no gain. Since the cathode is structurally right around the filament and is activated by heat, I don't think removing the B+ from the plate via the standby switch would help cathode life all that much.

I always thought of the standby switch as a fast way of coming up to power without the filament warm up period normally required. I changed tubes the same way I change strings.
On a regular basis and not like the old joke; when they break. I still do the same with my Tube-Fex. If I waited until they ran down I know I'd be caught with my drawers down some night in the middle of a song. No need for that when a couple dollars worth of tubes would avoid that risk.

I once had someone write to me that the standby switch was to save "Filter Caps." All I could think was : Huh ??

I used to take old tubes apart and examine them. I've seen hot spot on plates and breaks in the screen or control grids but never any deposits. You can see the errosion of the cathode coating but I wouldn't consider that a deposit. I would attribute the frying sounds so commonly reffered to as intra-elemental gases probably caused by leakage in the tube seal. This would also affect the inter element capacitance values and change the tube parameters a bunch.

Regards, Paul

Blake Hawkins · Post by **Blake Hawkins** » 10 Nov 2001 12:00 pm

Robert, Thanks for the feed back on your amp.
Those of us who take a crack at diagnosing over the internet always like to know what
actually fixed it.
Glad to see you are up and running.

Jack and Paul...I have to agree with you on the standby switch function. My understanding is that it's purpose is "instant on" for the Amp.
The standby switch is usually placed in the High Voltage (B+ line for us Techies) line.
When you activate it, it removes the high voltage from the tubes which stops their function. Although the heaters and cathodes are still hot...there is no current drawn and no emission from the cathode. Also the tube cools down a bit because the plate is no longer dissapating heat. So there may be some less wear on the cathode, however its benefit is probably offset by the metal fatigue caused by the contraction and expansion of cooling the tube down and then heating it up again.
In the Fender Twin the switch is placed after the filter caps and before the tubes...so it is not protecting the filters.
It is, in fact stressing them more because when you open the switch, the load is removed from the power supply and the voltage rises across the caps.
Blake

Bob Carlson · Post by **Bob Carlson** » 10 Nov 2001 9:22 pm

Jack, Paul and Blake.
The information I gave on tubes came from an artical In "Guitar Player" magazine.

They went so far as to say, If you use your amp on and off during the day and evening, to leave It on all the time, but put In standby when not using It.

Bob Carlson.

Jack Stoner · Post by **Jack Stoner** » 11 Nov 2001 4:06 am

Unfortunately a lot of "technical" articles in magazines such as that are not written by someone with a technical background.

I won't say the person that wrote the article you cite is or isn't technical. I've never seen the article.

Just that I've seen a lot of mis-information in many types of print media.