High dissipation on VOX AC30C2

[caesparza]

Hi,

I recently sold a VOX AC30C2, after a week the new owner told me that the amp was making poping noises, so I have to take it back. Measured the tubes on a BK Precision 620 and to my surprise 4 of the 7 tubes tested bad.

I retubed the amp with a fresh set of JJ tubes and made some readings. These amps are known for running hot, but I'm getting readings around 24W which is double of the EL84 12W limit. Am I missing something here?

First I measured resistance from the center tap of the OT to the plates (pin 7) of the power tubes:

T7P7: 17.9
T6P7: 17.2
T5P7: 17.4
T4P7: 17.2

Then I measured the DCV on the same points to determine I.

T7P7: -1.32V
T6P7: -1.32V
T5P7: -1.4V
T4P7: -1.4V

Plate voltages on the power tubes are:

T7: 312.2V
T6: 312.6V
T5: 312.7V
T4: 312.7V

Those measurements gave me the next currents:

T7: 78mA
T6: 76mA
T5: 80mA
T4: 80mA

For a dissipation of:

T7: 24.35W
T6: 23.75W
T5: 25W
T4: 25W

Pin 4 and 5 (heaters) of every EL84 is getting 2.95 VAC.

By the way, there is no red plating on the tubes.

[nevetslab]

Hi,

I recently sold a VOX AC30C2, after a week the new owner told me that the amp was making poping noises, so I have to take it back. Measured the tubes on a BK Precision 620 and to my surprise 4 of the 7 tubes tested bad.

I retubed the amp with a fresh set of JJ tubes and made some readings. These amps are known for running hot, but I'm getting readings around 24W which is double of the EL84 12W limit. Am I missing something here?

First I measured resistance from the center tap of the OT to the plates (pin 7) of the power tubes:

T7P7: 17.9
T6P7: 17.2
T5P7: 17.4
T4P7: 17.2

Then I measured the DCV on the same points to determine I.

T7P7: -1.32V
T6P7: -1.32V
T5P7: -1.4V
T4P7: -1.4V

Plate voltages on the power tubes are:

T7: 312.2V
T6: 312.6V
T5: 312.7V
T4: 312.7V

Those measurements gave me the next currents:

T7: 78mA
T6: 76mA
T5: 80mA
T4: 80mA

For a dissipation of:

T7: 24.35W
T6: 23.75W
T5: 25W
T4: 25W

Pin 4 and 5 (heaters) of every EL84 is getting 2.95 VAC.

By the way, there is no red plating on the tubes.

What is the cathode voltage on your power tubes? Are you set for 50 ohms or 82 ohms on the cathode resistor network? If it's set to 50 ohms, your current calculations suggest that cathode voltage is about 7.7VDC.

Measure the voltage at both settings (50 ohms & 82 ohms), and see if the shared current is similar to what you've calculated. Plate currents as high as you're calculating seem much higher than I've seen in the large collection of AC30-CC2X I maintain here at CenterStaging, LLC in Burbank, CA.

And, something is wrong with the heater voltage you're reading.

I've attached the Service manual for the AC30CC2X

AC30CC Service Manual.pdf

They DO run quite hot, though I've not seen them THAT hot.

Were all of your tubes 'bad' when you sold the amp? While I have a good Hickock 539A Tube Tester, I instead go by how the tubes behave in the equipment and how they measure in-circuit. Most of use here go by that, as that is the tube in the applied circuit.

[caesparza]

Thanks for your answer nevetslab.

This one is diode rectified AC30C2 not the one you got AC30CC2 which has a GZ34 rectifier. I'll take the measurements you mention and come back with the results. The cathode resistor is a 50 ohms.

Here you have a link to the schematic: https://el34world.com/charts/Schemat...Vox_ac30c2.pdf

[Dave H]

I retubed the amp with a fresh set of JJ tubes and made some readings. These amps are known for running hot, but I'm getting readings around 24W which is double of the EL84 12W limit. Am I missing something here?

You need to divide the calculated current by two as the primary current is shared between two tubes.

Also if it hasn't already been done subtract the cathode voltage (about 12V) from the measured plate voltage to get the plate to cathode voltage to use in the power calculation.

80mA/2 * (312-12) = 12W

[caesparza]

Hi Dave,

Thanks for the information, that was the missing part!!.

I got the following readings:

Cathode Resistor Voltage Drop: 8.53 VDC

So the plate to cathode voltage would be:

T7: 303.67V
T6: 304.07V
T5: 304.17V
T4: 304.17V

Dividing the current in pairs I get a pretty good figure for dissipation:

T7: 11.84W
T6: 11.85W
T5: 12.16W
T4: 12.16W

It's the first AC30 that I have to check, I missed completely the fact that the power tubes are connected by pairs. Dissipating a little over 100%, according to the facts I have gathered about the AC30, not bad number at all.

Thanks a lot for the help everybody.

[TimmyP1955]

While you are in there, check that the EL84s' grid stoppers are on the tube sockets, not on the PCB. JJs can have stability (oscillation) issues when the stoppers are on the board. I've seen this with JJ EL84s in (early) Vietnamese handwired AC30s, and with JJ 6L6Gs in a 2001 Fender Tone Master.

[trobbins]

Did you re-measure the heater voltage again to confirm if it was indeed 2.95Vac on the EL84 - that is still a possible problem. You can also measure the DC heater voltage going to the preamp tubes to see if they are nominal 6.3Vdc (if they are about 6.3Vdc, then you are not correctly measuring the EL84 heaters correctly).

Your 17 ohm output transformer primary winding resistances also seem way off the 70 to 72 ohm resistance I measured on my AC30CC2 from B+ to either plate terminal. Do you have another meter for measuring resistance - some meters don't like measuring resistance when it is part of a large inductance.