Vox AC4 B+ too high?

[powfinger]

Hey,

I'm troubleshooting an old Vox AC4 for a friend that's saying it just isn't as loud as it used to be. It recently had it's filter caps and cathode bypass caps replaced. Tubes seem to be ok as well. In fact, about the only thing that I can see that's different than expected is that the B+ is running at about 300 VDC where as it's marked on the schematic as 270 VDC coming off of the filter caps. This seems to be putting the cathode voltage on the EL84 at 9.4 VDC (as opposed to the 8.5 VDC on the schematic) and the plate current at 60 mA. All this seems much hotter than I would expect. Any thoughts?

Thanks,
Eric

[martin manning]

It's running a bit too hot (too much idle current), but that will usually bring the B+ down. What is your AC line voltage, and what is the DC voltage on the grid of the EL84?

[pdf64]

Schematic http://bmamps.com/Schematics/vox/Vox_ac4.pdf

[Stevem]

What is the cathode voltage on both preamp tubes and have the preamp tubes been tested and or swapped out with known good ones?

[powfinger]

Thanks, guys. I have traded out the EL84 and 12AX7 for known good tubes, but I don't have an EF86 lying around. I've ordered one though, so we'll have an answer later this week. The amp makes sound, it's just kinda muddy and pretty low volume. I know it's only on EL84, but it still seems very low.

The B+ was actually higher than I told you earlier. It's 310 V.

The mains voltage at the wall measures as 123 VAC which would explain part of the problem.

I went ahead and measured the voltage on all the relevant pins of the EL84, EF86, and 12AX7.

Here you go:

EL84
Pin 2 (input grid) 40 mV
Pin 3 (cathode) 9.48 V
Pin 7 (plate) 310 V
Pin 9 (grid) 308 V

EF86
1 (grid) 77.3 V
3 (cathode) 2.57 V
6 (plate) 226 V
8 (connected to cathode)
9 (input grid) 0 V

12AX7
1 (plate 1) 307.7 V
2 (grid 1) 155 V
3 (cathode 1) 158.6 V
6 (plate 2 connected to grid 1) 155 V
7 (grid 2) 0 V
8 (cathode 2) 1.54 V

Thanks again!

[martin manning]

The 40mV on the EL84's pin 2 suggests that the coupling cap from the EF86 is leaking.

[pdf64]

The heater voltage, eg 6.3Vac, and the EL84 cathode resistor value, eg 150 ohms, would also be useful info.

[powfinger]

Ooo, good stuff here. Thanks guys. The cathode resistor on the EL84 reads 156 ohms on the meter. Not too far out.

I'll replace that coupling cap and see what happens.

BUT, the heaters are only getting 3.5 VAC!! Maybe the power transformer has some issues?

Thanks!

[pdf64]

Measure between the heater lines, not heater to chassis ground.
But I guess it will be 7V.

I'm thinking that this is all pointing towards the line being too high for the PT, and so a bucker on it's line supply may be a good idea http://www.geofex.com/Article_Folders/v ... ntvolt.htm

However, none of this is fixing the low volume issue.
That could just be tired electrolytics / drifted resistor.
It may be worth measuring all the resistors, almost all should give an accurate reading whilst still in circuit.

[powfinger]

Right on about the heaters. Rookie mistake. Thanks for the advice and help everyone. Looks like I'm going to be digging a little deeper here.

[powfinger]

After checking all the resistors (and de-soldering a few to double check), several have drifted more than 20 - 50%. I'll be placing an order for these this week and I'll let you know how it all turns out.

[powfinger]

Meant to also say that after replacing the coupling cap off of the EF86 (0.047 uf), the input grid on the EL84 is still sitting at 40 mV.