1969 Dual Showman Reverb low volume problem

[Kbates]

My son recently purchased a Dual Showman Reverb at an estate sale, was very noisey and low volume when we tried it out. We did a full cap job, replaced power switch, checked all tubes, cleaned all pots. Now the amp sounds okay but is maybe a third as loud as it should be. I am only getting 3.6 volts at the heaters even with the tubes out. Disconnected the two wires going to the heaters from the PT and they also read 3.6 volts.

I'm at a loss, even tried another PT I had and it also reads 3.6.

Any help would be appricated greatly.

Thanks
Kevin

[sluckey]

I'm at a loss, even tried another PT I had and it also reads 3.6.

Don't read filament voltage in respect to chassis. Read across the filament, ie, for a 6L6 measure AC filament voltage between pin 2 and 7.

[Kbates]

Thanks, I'll try that tonight, I tried another black face twin and was getting 6+ volts to ground, that's why I thought I had a heater problem

Kevin

[jim]

The power tubes "check-out" OK but they are likely beat.  Replace with a fresh set and bias. It may very well come alive.   Jim

[Ed_Chambley]

Read across your your heater leads as aforementioned.  Probably fine since you tried 2 PT's.  Get a voltage reading of all the tubes and post them.  Probably something simple like a bad solder joint or a mixed up mod causing a wiring problem.  Most of the low output I have had resulted in wiring/solder except one.

[Kbates]

Thanks, I'll check it out tonight

[stratele52]

Low power could come from Phase Inverter circuit . Check resistors value in this circuit . PI is the tube closer to the output tubes.

Heater voltage is always 6.3 volts , work ok at 6 volts too . AC voltmeter probe directly ON tube's pins.

[phsyconoodler]

-bias
-speaker jack ground
-tubes
-check all voltages at all tubes
-pot grounding
-bypass caps replaced?One bad one can suck all the volume
-both channels low volume?
-incorrect screen resistor values  470k instead of 470 ohm?

[Kbates]

Bias balance set @ -45
Speaker jack grounded
Tubes all good
All pots appear to be grounded
All bypass caps were replaced
Both channels low volume
Screen resistors 470ohm

Link to voltages:
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=14114

[HotBluePlates]

Enter your voltages again, but note that pins 2 and 7 of 12AX7's are the grids, not the heaters. They should be 0v for most positions except the phase inverter.

[Kbates]

Values fixed:
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=14114

[Danskman]

Mmmh... Strange voltage on 6L6 pin8, should be 0VDC... V2 voltages no good, too, should be close to V1 voltages.
This amp could have been modded. Could you shot some detailed pics and post them somewhere, this can help us to help you.
Best regards and good luck,
Danskman

[PRR]

Cathodes pin 8 of 6L6 should be dead-zero, not 5V, unless somebody has done a significant modification. Do cathodes go right to ground?

As said, both V1 and V2 should have very-similar voltages, noted on service plan as plate=210V cathode=1.6V. The "2.5V" reading could be a wrong tube (12AT7 in place of 12AX7). However that would be a mild loss of gain, you should still get 80 Watts if you strum hard.

V5 cathode 343V is wrong, but that's apparently just Tremolo so if Intensity knob is full-down it will not cause "a third as loud as it should be". Deal with that another day.

[stratele52]

Low power could come from Phase Inverter circuit . Check resistors value in this circuit . PI is the tube closer to the output tubes.

Don't forget this quick check.

Should I say I have same problem on a Fender amp. Was a resistor in PI

[sluckey]

A couple of the Showmans had resistors under the cathodes. Bet his is one of the oddballs.

http://www.el34world.com/charts/Schematics/files/fender/showmanreverb_aa768_schem.pdf
http://www.el34world.com/charts/Schematics/files/fender/showman_ac568_schem.pdf

[stratele52]

These Showmans are not the "good" one. Easy to modify

[Kbates]

PI resistor values are all good…photos below (let me know if you need to larger/different photos).

[stratele52]

Pictures are good, lots of work are done in this amp .

Power transformer look to be non original ; all wires are cut . Is there a problem there ?

Are you sure all tubes are good ? How do you test them ? Tube tester are not good to be sure unless you have very very high quality.

You need a signal generator and scope to follow signal path to see where it drop. Or follow schematic to s be sure evrything is original , no signal capacitor sending singnal to ground  or not new resistor to drop signal.
A signal generator will help to check output transformer too . How is it ; original or "replacement" like the PT ?
I don't know how you are qualified , sorry but not sure if you know enough amp / electronic to trust your tests.

[HotBluePlates]

Yeah, someone has replaced every cap, all preamp plate resistors, and just about all the wire in the amp.

The cutting on the power transformer is a concern, because the original leads were cloth-covered meaning it could be a Fender transformer. Can you post the numbers on that transformer?

With as many replaced parts and rewiring as there is, there are many places for errors to occur. You're not just troubleshooting an amp, but someone else's previous workmanship.

[stratele52]

With as many replaced parts and rewiring as there is, there are many places for errors to occur. You're not just troubleshooting an amp, but someone else's previous workmanship.

+10000

[Bub]

http://www.el34world.com/charts/pt_conversions.htm
http://www.el34world.com/charts/bias_conversions.htm

[HotBluePlates]

If you clip off those 150Ω resistors, you will need to have a method to attach the tube cathodes to ground (the chassis).

Only a 100w solder gun is gonna be close to hot enough to actually solder to that chassis (maybe not even then), unless you have some of the massive irons used to solder copper pipe. So instead, run a heavy wire to each cathode, then have a length of it to which you crimp and solder a ring terminal. Bolt that terminal to the chassis where you have the first filter cap ground and PT center-tap grounded (probably a PT mounting bolt).

If you want to be able to easily and safely measure idle current (without something like a bias probe), you might solder one leg of a 1Ω to each socket's pin 8, then use your wire to connect the opposite end of all the 1Ω resistors together before your ground pigtail.

Make sure the output tubes are not in the sockets while you're doing all this.

Be careful about biasing after you do this. Convert to bias circuit over to be a bias adjust, and set it for maximum negative voltage at pin 5 of the output tubes. The schematic indicates 470vdc in the B+, and you're showing 461vdc on the output tube plates. If you have true 6L6GC's with a 30w plate dissipation rating, you can idle anywhere from about 35-45mA per tube.

[stratele52]

+1 HotBluePlates

Or what you can do is simply solder  an 1 ohms resistor on your 150 ohms with regular solder iron . Those two resistors in parallel will give you a total of ;

0.9933774834437087  ohms . Almost 1 ohm.   Good for bias mesurement  . And youre 150 ohms will be out of circuit.

[Kbates]

In regards to the many appriciated responses the transformer was changed with another known good transformer to try and remedy the problem. Same probelm with both transformers, I intend on putting the original back in.

regarding tube testing, all the tubes from this amp were interchanged on a twin reverb and all worked fine. Power tubes and V6 and pulled and ran at the same time. Preamp tubes interchanged one at a time with coresponding tube in working twin.

[stratele52]

In regards to the many appriciated responses the transformer was changed with another known good transformer to try and remedy the problem. Same probelm with both transformers, I intend on putting the original back in.

Don't need to try another transformer. The voltages you show us it is a proof that it is not the transformer. Or you make some mistake in reading voltage .
Better to follow schematic and see if all capacitor and resitors are ;

1- The good one
2- At the right place
3- No more resistor or capacitor  than schematic show/ need

Also look for:

1-Check B+ voltage when playing guitar on the amp to see if B+ voltage drop. If it drop you'll loosing sound.
   If drop = defective B+ resistor  or bad solders.
2-You could have a "light short"  to ground sending a part of the signal ( sound ) to ground =loosing volume.
 This could be a mod with one more resitor or a solder touching chassis metal.

3-Also a mod with more resistor in serie with any tube's grid will stop signal / sound.

You'll need a lots of time , good luck