Totally Baffled now 2475-75

Hoping for some guidance here. One of the 6L6GC glowed cherry red, no output to speakers. Not even a hum. Replaced both tubes and every electrolytic capacitor on the boards. Checked the speakers for any shorts, none, After all this work, the one 6L6GC contines to glow red and now some blue hazing, still no output. I am at a total loss as I have tried replacing most failing parts. Anyone else an offer a suggestion before I trash it? Spending too much time and feel its not worth it anymore.

Don't trash it! Does the same socket location glow red each time?

Yes, and I have replaced the tube sockets and all parts that were attached to them (resistors and a ceramic capacitor). It seems that I replaced every part except for the faulty one. The same tube glows cherry red, if I switch them, then that tube will burn. Could the 1N4003 or 1N5353 diodes be blown? I rarely seen a diode fail.

Obviously one side is having the crap run out of it. When it's running and red plating the one tube what is the voltage read across each of the 3.9 ohm resistors.

My money's on the OT. Have you measured the DC resistances of it?
Sincerely,
Lars Verholt

OK Here are the voltages from the 6L6's:
Pin 1 - 0v
Pin 2 - 0v
Pin 3 - 448(l) 630(h)
Pin 4 - 228(l) 323(h)
Pin 5 - 0v
Pin 6 - 7(l) 8.4(h)
Pin 7 - 0v
Pin 8 - 7(l) 8.4(h)
Regarding additional measurements:
The Orange lead from xmfr is 30vac
After D20 (1N4003) it is 34vdc
After R87 (1.5k, 1w) it is 0v; should be 22v
Could this resistor be bad, or C58 (.1uf) or D15 (1N4748) shorting to ground?

Sounds like you have NO bias control voltage on pin 5 wwhich in your case is positive voltage since the SS driver amps are cathode driven. Check c56, 57, 58 for shorted caps and replace that diode or check it for dead short to ground. Am kind of curious why only ONE tube reds out since you have no control grid voltage-they should both be red.

Only one 6L6 glows red. I will replace the caps & diode (will have to get it on ebay, not sold at radio shack). The one that is glowing is furthest from the audio xmfr. I tried to switch them to see if the one tube is bad, but the other one then glows in that socket! I then tried an old RCA tube in it which I know is weak, and it glows bright blue not red.

I DO think you need to do as Lars suggested and take actual RESISTANCE reading on the OT input and output and see what they are. THEN look at the stuff I suggested. Lars is USUALLY right! (I think he's psycic-or is that physic?)

I replaced the 1.5K resistor, .1uf cap and zenier diode.
OK Here are the voltages from the 6L6's:
Pin 1 - 21(l) 21(h)
Pin 2 - to pin 7 6.30vac (filaments)
Pin 3 - 396(l) 556(h)
Pin 4 - 210(l) 296(h)
Pin 5 - 18(l) 19(h)
Pin 6 - 215(l) 303(h)
Pin 7 - to pin 2 6.30vac (filaments)
Pin 8 - 7(l) 8.4(h)
Still no sound and the one 6L6GC furthest from OT STILL glows blue and Cherry Red. DAMMIT!
I will disconnect the OT now as it HAS to be that! is it possible it is shorted out? how does that happen? are there any replacements or do I have a giant garage door stop now?
HELP!!!

Yes, the OT can go bad. And they do - once in a while. www.mojotone.com has replacements. No big deal.
Cheers,
Lars Verholt

In case that IS what it IS what the problem is.http://www.mojotone.com/amp-parts/transf ormers/Music-Man-5-75-Output-Transformer #.UMZkzOTBFLc

OK, I just removed the OT here are the resistances in ohms:
BRN - BLU 330
BRN - RED 170
RED - BLU 150
YEL - BLK 8
YEL - GRN 4
GRN - BLK 8
Is the OT ok then?

Hi Frank, those measurements don't look too bad. I don't have an amp with a 5-75 transformer for comparison - perhaps another esteemed member here can provide some numbers?
I should mention that I have in the past replaced transformers that measured ok for winding resistance but still didn't work properly (was clearly visible on a 'scope).
Have you tested the cathode driver transistors? The voltage measurements showed that your cathodes were sitting *below* your grids - it should be the other way around. If memory serves the cathodes should be at some 60VDC and you had about 8VDC.
Sincerely,
Lars Verholt

What is a sign that the transistors are dead? Would that cause the RED-OUT condition? Should I just replace them as I have been doing with all the capacitors (which solved nothing from my original problem)?

If they were shorted, you could have uncontrolled current going through them so yes, they could cause it. I would check them first and see what they show. Did you ever measure the VOLTAGE across the 39 ohm resistors AS it was red plating? The driver board components should ALL be tested. Mike

yes, it was solid 21v. I have ordered the driver transistors from Mouser, and will replace them. Do you believe my OT is OK then?

If it's not shorted, PROBABLY OK. Those reading look ballpark but like Lars, I don;t have one here to measure. They ARE balanced pretty well but I don't know what the primary resistances SHOULD be. Mike

Wait a minute-did you say 21V and NOT mV's?

Do you mean the 3.9 ohm 1W resistors R73 & R81? There are no 39 ohm resistors near the tubes (?)

Yes-I meant the 3.9 ohm resistors on the driver board.

I type as bad as I write!

Hi Mike! OK I measured the 3.9 Ohm resistors. On the transistor sides they are:
66mv (H) 55mv (L) and 1.4v (H) 1.05v (L)
the other side of the resistors are:
8.0mv (H) 5.9mv (L) and 8.9mv (H) 6.9mv (L)
are the transistors the issue?

Measure the voltages ACROSS the resistors-from one leg to the other.

OK I assume you mean in mv current then?
on one resistor, it is:
.46ma (l) .55ma (h)
The other one:
189ma (l) 264ma (h)
I am expecting the new transistors sometime this week.

Hi Frank,
how did you calculate those numbers?
Sincerely,
Lars Verholt

I am using a VTVM on the "ma" setting across the 3.9 ohm resistor measured on the 0-200ma scale.
NEWS FLASH! I replaced the two power transistors with 2N6488's and the amp came alive, the HOT CHERRY RED is GONE! However, there is a slight AC hum still from the speakers, or is that normal? This is in reverb mode, else pretty quiet, not even a hiss. The new readings across the 3.9 ohm resistors are (in ma):
.367 ~ .410 (standby)
1.5(l) 1.7(h) Resistor #1
1.6(l) 1.8(h) Resistor #2
Are these numbers OK? Seems the amp is working but with all these parts replaced (every electrolytic, the tube sockets, the two 6L6GC's matched, and some .1uf caps that looked "greasy") I am sure it needs resetting to spec.
How to I set the bias correctly? I see two trimpots, one large one with a blue plastic wheel next to the transistors and 3.9 ohm resistor and one small white wheel one on the big PC board.
I believe there is light at the end of the tunnel now for me, and it isn't a freight train heading the other way!
Please sdvise how to set the bias.

Hi Frank,
your readings on the mA scale are not all that useful. You have to measure the voltage across each resistor in mV. The bias should be adjusted so you get 25-30mV.
Sincerely,
Lars Verholt

Hi Lars, my mistake I meant the mV scale 0-200mv.
OK here are the latest measurements:
bias across the 3.9 reststors: 25.2 - 26.1 stable!
6L6GC voltages at each pin (high setting only):
1 - 23v
2 - 6.3vac
3 - 730v
4 - 365v
5 - 23v
6 - 365v
7 - 6.3vac
8 - 62v
It worked for about 15 mins beautifully, now is dead in the water, not even a hiss. Has the OT failed? I thought I had it licked but this amp is definetly cursed!
HELP!

Ok, those voltage readings are right on the money. You say you have no sound - but are those voltages all still there?
As with any repair job - once you start poking around, you _always_ run the risk of upsetting B while repairing A. I'm sure car mechanics, HVAC guys and doctors would agree =)
If your OT was being pounded by a pair of power tubes drawing waaay too much idle current, who knows...
We'll get you through this.
Cheers,
Lars

Yes, voltages quite stable, both tubes nice filament glow, I rechecked them again in my tube checker, 100%. I used the 9v battery test of the speakers themselves, and each get the faint popping sound, so they are OK. Should I again recheck the OT resistances? What else could be the problem now? I hate to order a $100 OT and find it isn't the problem. Strange it worked great for 15 mins after replacing the transistors, then I unplugged it for the night. Next morning I went to adjust the bias down to 25mv (it was about 90mv before) and she's dead in the water, no sound or hum or anything.

I hate to bring this up but have you checked the CORD going to the speakers? I know it sounds obvious but you would be surprised how many people overlook THAT. Do you have another amp to use as a sound source? If so, you could whip up one of these: http://www.tedsforums.com/forum/index.ph p?topic=8797.0
and track your signal through. I use an actual signal tracer from the previous century or my oscilloscope but the theory's the same. It would at least let you isolate where your signal is getting lost. Mike.

OK, you guys are NOT going to believe what the problem is! For some reason the jacks have lost contact to ground! the small springy leaf was not contacting the larger curved spring which would make contact with the plugged in instrument. Pardon the rude drawing:
__________
_______.
0
as shown above this smaller leaf was not touching the larger one. I bent the larger one down and used a flat-blade screwdriver to push the lower one up. It looks like this now without any instrument plugged in:
__________
______|
0
I did this to all of them, and the amp came alive so sweet! GO FIGURE! I must have bent the jacks back when replacing the wire wraps after removing them all during board & parts replacement.
I need a good cold one now, with my Strato on my knee! rock on!

Glad you got it together. Two problems at once is sometimes a bitch. Congrats. Mike.

Good job!
Happy holidays!
/Lars Verholt

Man - I read this thread not knowing that much really about electronics. An intriguing read -It turned out to be a great result.
Nice one guys.