Marshall VS65R - amp powers on but no sound

[Tone Meister]

Marshall VS65R powers on but no sound - zero. With amp powered on there is no hiss or hum from speaker. Amp belongs to a buddy, amp was working fine until he swapped the single preamp tube, now there is no sound. The tube is fine and the tube socket is not damaged.

No sound from FX SEND to input of another amp. No sound from preamp of another amp into FX RETURN. So neither the preamp nor the power sections are working. I opened the amp last night and noticed the tube wasn't lit. Took a few voltage measurements on the (VS65-60-00) preamp board and on the (B65-62-00) power board.

R10 (power PCB) 43 VDC on one side and 28 VDC on the other
R26 (power PCB) 0.0 VDC both sides of the resistor

Voltages on the tube are as follows:

pin 1 - 387 VDC, 0.002 VAC
pin 2 - 378 VDC, 0.005 VAC
pin 3 - 0.001 VDC, 0.002 VAC
pin 4 - 0.08 VDC, 0.246 VAC
pin 5 - 0.05 VDC, 28.72 VAC
pin 6 - 379 VDC, 0.005 VAC
pin 7 - 0.004 VDC, 0.002 VAC
pin 8 - 0.00 VDC, 0.002 VAC
pin 9 - 0.00 VDC, 0.002 VAC

The heater voltage is obviously out of whack. What could be causing this what would have happened with a simple preamp tube swap? I want to suspect the F3 fuse.

Any help is appreciated.

[J M Fahey]

Marshall VS65R powers on but no sound - zero. With amp powered on there is no hiss or hum from speaker. Amp belongs to a buddy, amp was working fine until he swapped the single preamp tube, now there is no sound. noticed the tube wasn't lit.

R10 (power PCB) 43 VDC on one side and 28 VDC on the other

Looks normal.

R26 (power PCB) 0.0 VDC both sides of the resistor

Well, it's in series with a capacitor and has one end grounded, can't imagine how could it have a voltage different from 0 . Normal.

Voltages on the tube are as follows:

pin 1 - 387 VDC, 0.002 VAC
pin 2 - 378 VDC, 0.005 VAC
pin 3 - 0.001 VDC, 0.002 VAC
pin 4 - 0.08 VDC, 0.246 VAC
pin 5 - 0.05 VDC, 28.72 VAC <-- weird
pin 6 - 379 VDC, 0.005 VAC
pin 7 - 0.004 VDC, 0.002 VAC
pin 8 - 0.00 VDC, 0.002 VAC
pin 9 - 0.00 VDC, 0.002 VAC

That's a 12.6V filament tube.
With 28.7VAC across its ends (Pins 4 and 5) it should be illuminating the whole room.
Even worse, I see 0V of any kind on Pin 9, so 1 filament is getting the full 28.7V (weird) and since the other filament is in series with it, the other one should be also gatting as much. Weird.
And anyway I suspect the filament winding must be 12VAC . Not stated but schematic shows it feeding both filaments in series, with the centertap grounded.
Then, where do your 28.7V come from? Weird.
Ok, now you have a few puzzles to solve

The heater voltage is obviously out of whack.

Looks so.

What could be causing this what

Dunno.Chec that winding, follow tracks, etc.

would have happened with a simple preamp tube swap?

That can't change a transformer winding.

I want to suspect the F3 fuse.

Don't suspect without investigating.
Measure it whether it's open or closed, what voltage you measure across fuse socket ends, etc.

When you find something weird, imagine what you *should* find there, and if not, what might have happened.

Troubleshooting is half measuring, half reasoning.
One half without the other leads you nowhere.

[Tone Meister]

Thanks Juan for the detailed reply. Some of those omissions on the schematic gave me pause, and I was hoping someone here already had a handle on the circuit and could fill on the blanks that are missing from the schematic. The fuse type is one with which I am not familiar, so I posed my questions here before peeling back the board. This is not the actual board but this fuse is like the one I mentioned:

[Jazz P Bass]

That fuse is a typical Marshall fuse.
Parts is Parts has them.
Simple to check with an ohmeter.

The fact that the power amp does not work at all tells me to look at the complete power supply.
High voltage + & - for the power amp.
Low voltages for the opamps.

And yeah, ohm out the fuse.

Note: don't know why you where measuring R26 as that is part of the zobel network.

[Tone Meister]

That fuse is a typical Marshall fuse.
Parts is Parts has them.
Simple to check with an ohmeter.

The fact that the power amp does not work at all tells me to look at the complete power supply.
High voltage + & - for the power amp.
Low voltages for the opamps.

And yeah, ohm out the fuse.

Note: don't know why you where measuring R26 as that is part of the zobel network.

I measured that R before I got my hands on a schematic. You know that at best I am a hack. I will get back around to the amp later today and will check out those voltages.

[J M Fahey]

And momentarily, so you are not paralyzed until the proper one arrives, you can solder 2 wires to *any* fuse holder you have available, fit any (250mA ??? ) fuse and go on testing.

[Enzo]

To measure the fuse while in circuit, you MUST remove the tube first. otherwise low resistance parallel circuits will fool you.

Pin 9 is grounded, and pins 4 and 5 go to either end of the 12v heater winding. The winding itself is not center tapped. Don't measure heater volts to ground and assume the difference is the voltage on the tube. measure from pin 4 directly to pin 5. Got 12vAC? The heater winding is not referenced to ground so voltage readings with respect to ground do not mean a lot.

[Tone Meister]

Missed your last reply Juan and yours also Enzo. In the meantime, F1 fuse was open and I did exactly what Juan had suggested and subbed in one for testing purposes. Amp runs fine that way and I have the proper fuse on the way. My buddy had swapped in a 12BZ7 and I reckon the extra current draw was enough to smoke that fuse.

Any way, problem solved. No idea what made me suspect F3 when the problem was F1.

[Tone Meister]

Yep, the 12AX7 heater draws 150mA in series while the 12BZ7 heater draws 300mA in series.

[J M Fahey]

At least your friend was honest (maybe because he was a "buddy" and not "a customer") about the 12BZy .

Regular ones swear over a 10 ft stack of Bibles that "it was working and it turned off on its own" , that "they touched nothing at all" and IF somebody did, it probably was:

[Tone Meister]

Well, the musicians in my crowd like to experiment and he's no different, he just had no clue the BZ7 would draw twice the current of an AX7. We've used a 12BZ7 in phase inverter in some Marshalls and Fenders to push the power section harder but those circuits can handle the extra current draw. I reckon he thought he was gonna increase the balls in that Valvestate, but what he ended up with was a gelding.

[Tone Meister]

The 12BZ7 will "thicken" the tone in many preamp circuits as compared to 12AX7 types, and some players find this favorable. Buddy plays heavier music and their band uses these hybrid amps for rehearsals.

Buddy still dead set on trying the 12BZ7 in this amp, but the BZ7 draws too much heater current for the stock 250 mA fuse. Does anyone see a problem with increasing the F1 fuse to 315 mA to accommodate the extra current requirements of the BZ7? If not, what could be done to the circuit to make a 12BZ7 work? Worth doing or abandon the idea altogether?

I have both 250 mA and 315 mA fuses coming in today.

[g1]

You can adjust the fuse value but the question is whether the heater winding on the PT will support the extra current. You are going to be asking for 100% more heater current so it may overheat.
An extra add on transformer to run the heaters would avoid damaging the main PT.

[J M Fahey]

And getting thicker sound can be easily be achieved by proper choice of a 20 cents cap and a 10 cents resistor and in a way much more consistent and predictable than changing a tube, which can have a very small effect at best.

[Tone Meister]

You can adjust the fuse value but the question is whether the heater winding on the PT will support the extra current. You are going to be asking for 100% more heater current so it may overheat.
An extra add on transformer to run the heaters would avoid damaging the main PT.

Exactly, and I can't find any specs on the power transformer, so without some assurance the heater winding could supply the extra required current I will leave well enough alone. After all, the heater is only responsible for one preamp tube so one can only guess how much current it can actually provide. I was hoping someone here might know the specs off hand.

And getting thicker sound can be easily be achieved by proper choice of a 20 cents cap and a 10 cents resistor and in a way much more consistent and predictable than changing a tube, which can have a very small effect at best.

Juan, us guitarists are tweakers, and sometimes that just gets us into trouble like it did my buddy with this amp. Since I can't be sure I can safely increase the fuse value and use the 12BZ7 then I am gonna just replace the stock fuse and button it up. Don't you agree?

The stock 12AX7 is pretty much worn out so a new tube won't hurt anything. At least we know it won't draw more current than the 250 mA fuse will pass.