Low Voltages on 78 Twin Reverb 133 (UL)

[caesparza]

Hi everybody.

I recently got a 135W Twin Reverb for Recap and Maintenance. After the process I measured the voltages and they seem to be low compared to the schematic. One thing that I found strange is that with the new set of tubes, the amplifier sounds distorted while with the old ones (that tested ok on a BK precision 620) it sounds a lot better. I haven't got to the BIAS point yet, cause I want to address the voltage thing first.

I'm going to give some voltages for the filtering section, just to give you an idea:

Heater Voltage (AC): 5.52

Voltage on Stand By Switch: ON 350 (Schematic 500) OFF 444 (Schematic 520).

Plate Voltage on power tubes right now is around 395, isn't that like 100V less than the normal reading?

Voltages on both sides of the 2K7 10W resistor are: 294 and 347 they should be 410 and 520.

Could the amp have a problem with the power transformer or maybe the caps and diodes on the rectifier section (I read 346v just after the rectification section)?

Thanks a lot.
Carlos

[nickb]

If you are getting (all reasonable) voltages and not blowing fuses then the transformer is fine. Have you checked your wall voltage and how does that compare to the setting of the voltage selector on the back? Lastly, a rather remote possibility, does it look as if anyone might have worked on the voltage selector wiring and messed it up?

[caesparza]

115.5 AC just 4.5 under the 120.

[mozz]

"If you are getting voltages and not blowing fuses then the transformer is fine. "

Not always 100% true.

Reasonable voltages, yes. I think there is a problem if filament is only 5.5vac. Can the secondary winding short to the filament winding?

No schematic but plate voltage is higher than the standby voltage? Filament voltage with all tube removed? You can't bypass the bias check till you get the voltage fixed, bias may be too high causing the voltage to really sag. Pull your tubes and tell us pin voltages.

[nickb]

115.5 AC just 4.5 under the 120.

Did you check the voltage selector and wiring as asked?

Now measure voltage at the primary wires, not before in case there is a bad connection in there somewhere. The very low heater voltage makes me think that it's wrong going in.

[g1]

And if you haven't double checked your meter today, do so.

[caesparza]

Hi mozz,

Thanks for your answer. Can you explain to me this "Can the secondary winding short to the filament winding?"? I'll have to read more about power supplies I guess.

I started to trace the voltages right from the wall, this is what I'm getting:

Wall V: 115.5 (AC).
PT Primary voltage (measuring between the two red cables): 325.5 AC, correct me if I'm wrong but that should translate to 458.95 VDC (when rectified), isn't it?. At the output of the Bridge rectification circuit (diodes-capacitors) I'm getting 347 VDC. The voltages are in general low, even in the preamp tubes, that ones are biased with the voltages that the filter section provides.

Should I remove the power tubes or the preamp tubes as well to make the measurements you asked?

Here is a link to the schematic: http://acruhl.freeshell.org/mga/sche...ate70s135w.jpg.

Thanks in advance.

[caesparza]

Hi Nickb,

No voltage selector on this one. You mean like the import models?

Primary voltage (red cables of PT) read 325.5 VAC..

[g1]

Those (red) are the secondary wires of the PT.
I think he wants you to check the primary where the 115V from the wall meets the transformer, to make sure it is all getting there. (after the power switch, fuse, etc.)

115V from the wall seems a little low. Where are you? Is it always like that?

[Enzo]

Is that 325v red to red, or each red to ground?

[caesparza]

Yes my mistake.

Primary (black-white) reads 106.5 VAC. I'm located in Colombia South America.

[caesparza]

Yes red to red Enzo.

[nickb]

Yes my mistake.

Primary (black-white) reads 106.5 VAC. I'm located in Colombia South America.

Well there's your problem. That's 10V less than at the wall. It's your mission to figure out why, should you choose to accept it....

[pdf64]

Yes my mistake.

Primary (black-white) reads 106.5 VAC. I'm located in Colombia South America.

Is that normal for your region?
The actual mains voltage will affect all others in an amp; it should be the first thing checked at each bench session.

[caesparza]

Hi guys,

I guess I was doing something really stupid with the measurements, just bear with me, I'm not an expert on the subject.

I was running the amp through a current limiter for safety. I plugged the amp straight to the wall and now I get 118.5 VAC on the primary and the rest of the voltages are right.

Here you have some of the voltages:

Heater Voltage: 6.23 VAC
Stand By Voltage: 501 off 499 on VAC
At both sides of the 2K7 10W resistor: 400 - 487 VDC

The plates of the power tubes are reading 485 VDC
The grids read: (V7)-52.8 (V8)-52.8 (V9)-56.3 (V10) -56.3
Resistance between secondary main and plates: (V7)36.9 (V8)36.9 (V9)34.9 (V10)34.7
Voltage drop between secondary main and plates: (V7)1.54 (V8)1.54 (V9)1.41 (V10)1.41
Plate Current: (V7)41.73mA (V8)41.73mA (V9)40.40mA (V10)40.63mA
Dissipation: (V7)20.24w (V8)20.19w (V9)19.59w (V10)19.70w

These values are read with the old tubes on the amp.

The BIAS numbers seem to be around 65%-67% of 30W Max dissipation, looks good to me, I just need to balance both sides. Client wants to BIAS this amp to 80%, I think that is not a good idea, what do you think?

I'll add a pot and a resistance to make the BIAS adjustable, preserving the Balance control.

I'm hearing a popping noise from time to time. Is that normal after a recap job?