Showman AB763

[Elfshimmer]

Got a Showman,initial problem:a sort of "thumb" and endtubes working in full mode...
Began to check some stuff.Found problems with the "vibrato"tube,V3a supposed to have a kathode voltage of 17Volts (found 0V),Anode Volts ok,V3b kathode volts supposed to have 2,1 volts,there also 0Volts mesured,The anode gives me 390Volts and sometimes rising...I see the opto flicker sometimes...a turn on the speed doesn't work...
The mass on the cold side is well connected...The elco's are not shorted,the resistors are ok...
Has anybody any idea?
Thx in advance....

From pretty cold and damp Belgium,

Elfshimmer

[Enzo]

The trem circuit doesn't function until you ground the footswitch jack for it. Have you done that?

[Elfshimmer]

Showman AB763

Thank you for the response Enzo...
You're right the bleedin' obvious is in 50% of the cases the right answer...However this time it was the ol' Mallory 25µf across the 100K that was the badguy!!!!So that solved the vibrato trouble...
I was ready to cry victory...I put a signal from my signalgenerator in,over the dummyload of 4 ohms I looked at my signal out...A mere 14V peak to peak...full blast...
Began to trace from front to back everything goes well to the plates to the anodes of the 6L6's Over 50V peak peak...
There are only two 6L6's (common practice) I biased them at the moment on -49 volts
Is the OT halfdead,anyone any idea...

Thx

Elfshimmer

[Mars Amp Repair]

Well,
this also may be oversimplified, but are you using the speaker jack & not the extension speaker jack? On many of the Fenders, the regular speaker jack will short the output when not in use. Just a thought. Don't know if you'd even get 12vpp with that jack shorted, but could be dirty allowing some resistance.

What is the amplitude of the drive signal on the 6L6's.

Not knowing the plate voltage, I'd have to say that -49v bias seems like it would they would be running rather cold...still wouldn't cause your low output, tho.

Just some thoughts...glen


Just a thought....glen

[Amp Kat]

You could be very well right about that jack Glen and what is the ohms of the OT from CT to each side and across the winding ?

[Elfshimmer]

Recapitulation

You could be very well right about that jack Glen and what is the ohms of the OT from CT to each side and across the winding ?

First off all many thx to you all in an attempt to help me out!
The output transfo gives me 37.4 and 39.6 Ohms primary and 1.1 Ohms secundary.
With 1Volt top top in (functiongenerator) it comes out of the PI at 35Volts peak to peak...
It comes out the speaker out,not the extension speaker out(!),at 4 Volts,adjusting the bias(which works ok) doesn't give me a bigger output.The plate voltage is about 432Volts...
I'm pretty out of ideas I'm afraid...

Another issue does anyone know if the clean channel of a GT Trio is about half the strength of the scream ,mean channel?
In other words you have to put clean on 4 and the other two channels on 2 to have the same level...

Elfshimmer

[Tom Phillips]

Elfshimmer,
A few comments:
1) Verify that you have the proper screen voltage at pin 4 of each power tube. It should be just a few volts less than the plate voltage. If it’s way less or some tubes don’t have screen voltage, then the power will be low.
Could be caused by a bad screen resistor or problem with the screen supply voltage.
2) Have you tried a new set of power tubes? Could just be a very worn out set. Or if they are new, a bad set.
3) It will be good to learn how to measure plate current rather than just bias voltage. There are lots of articles on how to do that. Maybe you already know but have not reported the value.
The grid voltage controls the plate current and is set to achieve the proper operating point. The exact value varies a lot depending on the exact tubes used and other circuit parameters. It is possible that the voltage could appear to be correct but the current isn’t flowing. Another indication of a possible bad tube. You will be looking for 30 to 40ma per tube. I'm not sure what your experience level is. Let's discuss but don't do anything that you are not comfortable with.

Let us know what you find and we will go from there.

Regards,
Tom

[Elfshimmer]

The Amp who says NIH NIH

Thank you for your post...
Well,I did measure on pin 4 where a have a good 432Volts,I didn't measure the plate current...I soldered a 1 Ohm resistor from Kathode to ground...and a millivolt meter across...I did bias them on Idle 45mA,with a signal full blast it's climbing to 120mA...
With the signal reading good after the PI,the bias reading well and swinging with the signal in...The primary OT windings reading well (with a DVM)36 and 37 ohms,the secondary 1 Ohm...
You know what,I have no idea left!

[Tom Phillips]

"The Amp who says NIH NIH" - That's good


OK One more thing checked off.
Have you tried another set of power tubes yet?
(Easy things first)