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Author Topic: fender deluxe reverb reissue bias problem. bias current climbs on both tubes.  (Read 7857 times)

EDWARDEFFECT1

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hi i know this is a solid state site,but i also know there are some good techs here.i have a fender deluxe reverb reissue that blew a fuse. one or the power tubes was shorted.i put new tubes in it and tried to rebias it using a weber bias rite meter.the amp will bias then when you wait about a minute the bias starts going up a tenth at a time.then you keep turning the bias down till there is no more adjustment.there are 2 dcv test points on the print-34vdc(reads -35vdc) and -49vdc (reads 50vdc).the voltages are perfect and they stay rock solid on there readings. the bias probe is good as i just did a couple amps with it with no problem.also to prove it i let the amp run for about 3-4 minutes and finally the tube with the higher bias red plated,so i shut it down right away.the bias is affecting both tubes as the current keeps counting up a tenth at a time on them.all the components on the circuit test good.i also changed the 2 electrolytic s with no difference.the voltages do show no problem with the bias circuit.there are no cathode caps on pin 8 of the 6v6 power tubes.both cathodes are tied to ground.this one is a hair puller.lets see who has the answer to this tough problem?....thanks....ed i will try to attach schematic...thanks...ed! page 5 is the right schematic...
« Last Edit: February 03, 2013, 12:18:50 AM by EDWARDEFFECT1 »

phatt

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Yep likely better minds will answer but I'd try disconnecting the PI Caps to isolate the grids.
something is pulling down the bias and I think leaky inter-stage caps will do that when they die.
Phil.

EDWARDEFFECT1

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hey Phil! thanks for jumping in.i had the same idea.the only other thing might be a leaky 1n4003 diode.it is the first part in the bias circuit. i think we are on the right track as both of these will affect the bias on both tubes.i just fixed an amp just like this and it also had bias trouble.the 22 ohm resistor right after the diode read 22k in circuit and was open. i jumper ed a 22 ohm resistor across it and the bias went back up to normal voltage...anyone with any more ideas welcome.thanks....ed!

Roly

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Yep, a leaky coupling cap would be my first guess also.  If the output stage runs okay with the coupling caps disconnected I'd replace both (and give any others of the same style a very close scrute).  If not I'd suspect a duff valve and check by substitution.

The old trick for testing coupling caps for leakage was to lift the grid end and connect your meter from the free end to ground.  A good cap should charge and settle to zero volts; anything else means leakage and replacement.

If the bias supply voltages are correct and stable as you say then I'd doubt it was the diode; these normally fail short.

Service manual;
ampwares.com/schematics/65_Deluxe_Reverb_RI.pdf

HTH
If you say theory and practice don't agree you haven't applied enough theory.

g1

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 Another thing to try: remove the power tubes and measure the grid voltages (pin5 at each output tube socket).
This  should tell you whether the problem is at one socket or both.  You should get your -35VDC at both sockets and it should stay stable.
  If the problem is at both sockets you may have a bad connection somewhere common to both, such as P18 or P19. 
  If it is a bad coupling cap, then I would expect the problem to be more evident at one socket with the tubes removed.

EDWARDEFFECT1

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i get -43vdc on pin 5 of the power tubes and the voltage stays steady.the only problem i have with the coupling cap thought is that the coupling caps are separate one for each tube,so for the amp to be doing what it is, they both would have to be leaky.what are the odds.i am waiting for new tubes.the tubes are both connected  and if one had a short to the cathode it would affect both tubes (which is what i have)there must be something we are missing as i have tried another set of tubes,but it is possible i have a bad one. i hate electro harmonics tubes from Russia. they seem ok with their preamp tubes but over 400volts the 6v6's seem to go bad. i have had alot of bad ones.the jj tubes seem a lot better.
« Last Edit: February 03, 2013, 06:41:11 PM by EDWARDEFFECT1 »

phatt

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Then are the Valves New,, or more second handy reboxed nos kinda thing?
Phil.

Enzo

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Hi Edward.  yes this is a solid state place, but did you look and see there was a section for tubes here?


You had tube failures, and replaced the tubes, not one of them red plates.  My first suspect is those new tubes.  Just because they are new doesn;t mean they can;t be bad.   The clue is that your bias voltage apparently stays steady at the tube grid.   If the bias voltage at the grid was diving, then I might suspect coupling caps, but leaky coupling caps by their nature will bring down teh bias volotage, otherwise they woulde have no symptom.

Bottom line, if the bias voltage is proper and remains steady, then all the bias stuff is working.

You are not losing your bias supply here, by your own measurments, so don't confuse yourself by bringing into it some OTHER amp repair that has an open resistor in the bias circuit.

SInce you had a tube failure, have you checked any screen resistors for value shift?

EDWARDEFFECT1

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thanks to all the solid state guys on this site that helped me out. i found the problem! bad electro harmonics tubes.as i said earlier the electro harmonics 6v6's are junk.i have 427vdc at the plate and the bias is rock steady with a new set of jj 6v6's.wish there was a way to test tubes accurately.my buddy tested one of the old power tubes that blew the fuse on a b&k 707 tester and one was shorted. i brought the same tube home and tested it on my b&k tester(600 series) and my sencore mighty mite tester and couldn't find the short.by the way the 2 electro harmonics that made the bias climb were tested on his tester and they tested good.they were brand new tubes.guess the tube testers are very unreliable.thanks again....you guys rock when it comes to repairs,building and time spent to help others.....thanks to everyone that help solve this probleml!!!!.....ed!

saying for today.wisdom comes from many at solid state forum!!!!    thanks!!

J M Fahey

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Tube testers are reliable ... within their limitations.
Most apply a relatively low voltage, enough to test emission and such, or gross shorts, bhut some defects are only apparent under full voltage stress, say from 300V to 500V.
Remember most guitar amps *abuse* their tubes way beyond what datasheets allow.
Personally, I have a couple homebuilt dedicated tube testers, with a couple sockets, using 300 to 500V "borrowed" fom any amp that happens to be sitting on the bench.
Nothing beats testing under real life conditions.

Enzo

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Just my opinions...

If I had to characterize a big box of random tubes, a tube tester might have some value.  But for troubleshooting, I find them about useless.  I maintain that a tube tester  - a TT for short - can verify a tube is bad for you, most times, but it rarely can tell you a tube is good. 

It is a rare tube tester that can tell you a tube is microphonic, or noisy.  And it won't tell you if a tube just sounds bad for some reason.  And as JM indicates, most do not put real world conditions on a tube.  COnsider testing your car while it idles in the driveway.  You will never find out that it shimmies on the freeway, or gasps for breath on hills.

A TT may not show a short, but we just heard that that is no guarantee.   It will show emissions, yes.  And if I am working on an amp, I probably already knew the tube lacked emissions.  The TT just verifies it for me.   And rare as open heaters are, I can find those faster with my ohm meter than I can set up the TT and run the tube through it.

But in a repair situation, it is SOOOO much faster and more definitive if I suspect a tube, to just stick a new one in its place and find out.

There ARE some very serious TTs out there, and you can certainly invent one.  But I refer to commercial TTs like one would have around an electronics shop.

Roly

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Yup.  :tu: I think TT's are just as likely to mislead as inform.  The ultimate test is how the bottle works in the amp on the bench.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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And an amp on the bench offers a much faster and far more comprehensive set of tests than a TT.


At times in the past, I would think one day I'd make a little amp with the sockets conveniently placed so I could sub tubes into it for a test bed.   Then right after that I'd think, yeah yeah, but I got enough other stuff to do first.

J M Fahey

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Well, you needn't *make* an amp ;)
An old Bandmaster or similar chassis is a good testbed and naybe you have one in your junk pile.
Sorry, "to repair stuff room"

Enzo

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Well, if I need a tube, chances are the unit I am working on is available as a test bed, for that matter.


I have a vision of a project, a self-contained little Champish thing with sockets on top.  Easily set out of the way, and no setup to use.


And little likelihood of me ever doing it.