Bandmaster blackface low power

[binito]

I 've a fender bandmaster blackface with low power.
I changed the power tubes , resistors of screen by Xicon 470 ohm 5w, and grids for 1k5 1w carbón resistor.
I adjusted the bias and all voltages are correct.
I inserted signal and measures with oscilloscope:
on pin5 of power tubes 120V p-p,
in the pin 3 400Vp-p Aprox.
and the speaker output only 20 V p-p with volume to 10
The tone is good but the volume no is enough.
Is bad the OT?

[pdf64]

20Vp-p = 10Vp = 7.1Vrms = ~13 watts across 4 ohms; I see your concern, that seems a bit low.

Do the idle voltages line up with those noted on the schematic http://bmamps.com/Schematics/fender/...b763_schem.pdf

What do the HT voltages at the standby switch, and after the choke, fall to at full power output?

Do the transformers fitted to the amp have the correct part numbers (see schematic)? Maybe they are incorrect replacement types?

[Jazz P Bass]

With a nice healthy signal at pin 5 (as you have) & proper plate voltages (as you have) & still the output is low, I would suspect the OT.

Are you using the proper load for the OT?
The factory load is (2) 8 ohm speakers in parallel (4 ohms).

[Mick Bailey]

The main OT failure condition is a shorted turn. This saps the energy and severely reduces output, but it usually gives a distorted output, too. A near-short on the output leads will reduce volume but give some signal. I had a Vox AC30 where it was producing just a couple of watts and there was a shorted speaker lead.

You could have a high resistance on the secondary side of the OT. Make certain there isn't a bad solder joint or corroded connector. Are you using a dummy load or the actual speaker to test?

Maybe check the transformer using the neon trick to eliminate a shorted turn possibility.

[J M Fahey]

Hola Binito

The Super-Secret Transformer Tester

[Richard]

Is a bad speaker shorting jack a possibility?

[binito]

The main OT failure condition is a shorted turn. This saps the energy and severely reduces output, but it usually gives a distorted output, too. A near-short on the output leads will reduce volume but give some signal. I had a Vox AC30 where it was producing just a couple of watts and there was a shorted speaker lead.

You could have a high resistance on the secondary side of the OT. Make certain there isn't a bad solder joint or corroded connector. Are you using a dummy load or the actual speaker to test?

Maybe check the transformer using the neon trick to eliminate a shorted turn possibility.

I try with other Bandmaster Blackface and with 8ohm dummy load (I know it should be 4 ohm)
Other Bandmaster with power ok:
Pin 5 power Tube - 200Vp-p
Pin 3 power Tube - 700Vp-p
Output transformer secondary - 45Vp-p

The Bandmaster with low power:
Pin 5 power Tube - 200Vp-p
Pin 3 Power tube - 300Vp-p
Output transformer secondary - 20Vp-p

i tried direct connect the OT secondaty to 8ohm Dummy load. i will try the OT with Neon test.
But i think this OT is shorted.
The measures in DC resistance
primary: 75 ohm - 85 ohm
secondary : 0,5 ohm

Thanks.

[Mick Bailey]

What are the DC voltages on your output tubes with no input signal?

The transformer resistance looks about right - the difference between the two primary halves is due to the mechanics of the winding - the turn count is the same for each side, even though the resistance isn't. You still need the neon to tell you if a turn is shorted, though.

[binito]

What are the DC voltages on your output tubes with no input signal?

The transformer resistance looks about right - the difference between the two primary halves is due to the mechanics of the winding - the turn count is the same for each side, even though the resistance isn't. You still need the neon to tell you if a turn is shorted, though.

Plate - 485Vdc
screen - 483Vdc
Grid - -54Vdc

[The Dude]

Is a bad speaker shorting jack a possibility?

Anything is possible. I would first unhook the secondary and wire it directly to a load bypassing all jacks before spending money on a new OT.

[Randall]

Cool! I have never seen this tester before. Question, where it says connect to any winding... is the neon testing for the winding it is connected to? Would one move the neon to winding two in the illustration to test for it? And then winding one?

[Enzo]

Just one winding. They are all linked together magnetically. If any one winding is shorted, even just two turns side by side, it will swamp the thing. Imagine if you put a dead short across the heater winding of a fender transformer - it would have an effect on the high voltage winding too. And it would vastly increase the current on th primary side.

So a free running transformer will "ring" and flash the light, while one with a compromised turn somewhere will be damped out and not ring.

[binito]

Finally . The OT is shorted.
Thanks

[The Dude]

Finally . The OT is shorted.
Thanks

I liked your post purely for the information sharing- not because you have a shorted OT. That sux.