Music Man 65 Reverb -- 2275-65

[loogie]

My friend picked this up, DOA, actually it makes noise, but not very loud and very distorted.  I did some reading up and I checked the voltage across the 3.9R resistors as well as from emitter to ground and collector to ground (JE1692).  I get 0v across the resistors and from emitter to ground.  I get 50v from collector to ground.  This is with the tubes in, powered up and off standby.  Plate and screen voltages appear sane.  I checked the resistors in situ and they're around 4 ohms.

I wonder if those JE1692s are bad?  Or could it be the op-amp (1458)?  I'm just learning about transistors so I know less than before and maybe I'm reaching.

Bad sounds come from all of the inputs.

[eleventeen]

Without much experience in this sort of hybrid topology, I'll say you've reached an entirely logical conclusion or at least suspicion. It's VERY common for a failure in the output section of an all-transistor amp to suck out the drivers = your JE's. You obviously don't have that, but if there were some sort of tube/ckt failure that either blew up one or both 6L6 (or, the tubes survived, as they often do) the voltage/current excursion could very conceivably kill those driver transistors. Perfectly logical & plausible, in my book.

Does the amp have fresh clean 6L6's installed...indicating that there may have been a big hiccup and the natural reaction was to replace the outputs?

I am not understanding why the apparent outputs of that stage go to the 6L6 *cathodes*....but then I am not looking the whole circuit.

[Willabe]

I am not understanding why the apparent outputs of that stage go to the 6L6 *cathodes*....but then I am not looking the whole circuit.

They drive current through the 6L6's. That's how they get more output from of the tubes.


                 Brad     

[PRR]

> powered up and off standby

Current in the 3r9 resistors is Tube current.

Tube current is probably dead-zero in StandBy. No tube current, no transistor current, so zero across the 3r9 resistors.

What is it with tubes engaged?

If it is melting tubes, we may have to jury-rig to check the transistors.

Also voltage at top of the two series 1N4003, and voltage at wiper of 100r pot.

> outputs of that stage go to the 6L6 *cathodes

Every circuit is a LOOP. The input of a 6L6 is the grid-cathode path and around through the driver. Normally we ground (or ground-bias) the cathode and swing the inputs. Much easier than moving the cathodes. With cheap transistors, MM opted to drive the cathodes (as Brad says). Tube current is *forced* to be the transistor current, which is tightly-wrapped in feedback. It is a transistor amp with a tube booster. Why? It seemed like a good idea at the time. It's not a bad idea. But it sure is "different".

[loogie]

powered up and off standby

Sorry, I should have been clearer.  The standby was switched to 'high' so it was in the play position.  Definitely voltage on the plates and screens.  In the high setting more than I could measure.

Apparently there should be 25mv across one of the 3.9 resistors with the other one not to exceed 55mv.  From what I read this is possibly class B and the idle current is just a trickle, like 6 or 7ma, but can you believe everything you read?

I'll check those other voltages.  If it turns out to be those JE1692s the replacements (I think I would go with the TIP41Cs) look like they would set me back as much as a buck each.  SS does have its benefits. 

[PRR]

Yes, a trickle. That's the only way the tubes would stand such high voltages.

I would not be replacing transistors until I checked their bias, the points I mentioned.

[loogie]

I had the wrong schematic, sorry. 

I've included the voltages at the diodes and the wiper of the pot.  I checked the bottom of the 1K resistor too just to make sure it looked sane.  +16v and -16v are present as well.

[PRR]

The bias diodes D15 D16v are fed through 1.5K from a point you measure as 4.25V.

This is set by the Zener diode bottom-left; however the part value is cropped off.... ah, page 3 of the PDF? Then it is D17 and rated 16V.

And 16V makes more sense than 4V.

Go back to the main low-volts supply. C46 C47 hold the raw LV DC.

C46 must hold more than 26V and less than 47V. 35V may be the happy-spot. If it is closer to 25V, suspect that C46 has failed (suspect C47 also because it is a  similar job and age). C31 C32 C42 C43 may also be past their flavor-peak. Which also throws doubt at C35 C36, because if they cook/age to a short, the transistor bias is very upset.

If the voltage is good, R59 or R53 may have drifted well out of spec. They seem to run hot. Do they look cooked? I'd go ahead and lift one leg to get a good ohms reading. If not within 10%, I'd replace R53 R59 R44 R46 with 5W parts if possible; or very-good 2W parts mounted well up in the cool air.

[loogie]

Thanks very much. I'll start following from LV. 

Sorry about the JPG.

[loogie]

The voltage at R59 (1K) is 40V.  I pulled both R59 and R53 and they check out ok. 
What about D17?  That's pretty critical to the operation of this circuit, right?
Because of the 40V on R59 do you think, for the time being, I can accept C46, C47 and R44, R46 as good and turn my attention to C42 and C43? 

[PRR]

Take out C42 C43. Do you get 16V and ~~1V?

[loogie]

I replaced R59 (I just noticed there are two R59s.  I replaced the 1k) and R53.  Both of the old ones were in spec.  C42 and C43 checked out fine. I calculated the drops ignoring D15, D16 and D17 and it seemed to me the only way there would be a 35V drop across R59 is if there was low resistance to ground on the zener side so I took a chance and replaced the zener.  I might have guessed for the wrong reasons, but that was the culprit.

Its got big clean sound, but the surprising thing to me is how good the preamp sounds when its overdriven.  The tremolo is really nice and it changes its characteristics as the intensity is increased.  It sort of sounds like a rotating speaker.

Have to play it a bit more and replace the power chord, but so far so good.  Thanks for your help.

This was a $50 pawn shop deal.  Cosmetically in great shape.

[PRR]

> that was the culprit.

Glad you found it. Sorry for thinking a Zener failure was less-likely.

[loogie]

If you wouldn't have started me down the resistor/capacitor road I would have replaced the two transistors and then where would I have been?  Better to start looking at the passive stuff first.  It worked out perfectly.

[Willabe]

Great you fixed it!

What did the grid bias voltage end up being after you changed the zener?

I've read (Kevin O'Connor, TUT 4) at idle, the grids sit at about ~+30dcv, cathode sits at ~+60dcv and screen dcv (~+290) is 1/2 of plate dcv (~+585) in a MM.

This was a $50 pawn shop deal.  Cosmetically in great shape.

That's a lot of amp for $50.


              Brad    

[loogie]

I didn't jot down the voltages.  If he doesn't pick it up tonight I'll open it up and get them. 

Ya, nice find, and he flipped the guy for $25.  Guy wanted $75.