Vox AC15C1 issues, volume drops and weird heater voltages

[sgmezei16]

Hi Folks,

I have a Vox AC15C1 Chinese made combo on my bench that I need some help with. Here is the Voltage Chart:
http://el34world.com/charts/valve/ValveData.php?e=view&f=17889

I am concerned about a few things going on:
Low Plate voltage on the EL84's, I have substituted other good pairs with minimal difference.

Kathode voltage (pin3) on the EL84's, other voltage charts I looked at had 11.4-15V and mine is holding steady at 4V

Low Plate Voltage on V2, when I swapped in a Tung-sol the voltages jumped up to 150V on pin1 and 190V on pin6

The Heater voltages!!! When measuring from pins 4/5 to pin9 I am getting some weird readings
V1,2,3 when I measure on AC I am getting 0.08V on pins 4 and 5 to 9, however when the amp was first turned on it was jumping around from 0.03 to 4.5V
V4 EL84 is measuring 0.4V AC between pins 4 and 5
V5 EL84 is measuring 5.4V AC between pins 4 and 5

Am I just crazy or should I be measuring these heater voltages differently? The power tubes heaters light up but it seems the preamps are not. They are sometimes warm to the touch but sometimes not.

I am not sure if I quite understand how this bridge rectifier is working. Am I right to think that the EL84 heaters are running on AC and the preamp tubes are running on DC?

Not sure where to start tackling this so any advice would be appreciated!!

[PRR]

> the preamp tubes are running on DC?

Yes. Put your meter on DC. Read pin-to-pin; pin-to-Ground will give meaningless numbers.

The big bottles are heated with AC so put your meter on AC for them.

What is it doing? Not doing?

[sgmezei16]

The output volume is not near what it should be.
The heaters on the preamp tubes aren't visibly glowing.

The preamp tubes all measure 3.2V from pin 4 or 5 to pin 9 on DC
The power tubes measure 4.2V from pin 4 to 5 on AC

Whats happening is the amp works for a minute or two then volume decreases to nothing. No output.

[shooter]

Pull ALL the tubes.  put your meter on AC volts, measure pins 4 and 5 on either EL84 socket (One lead on 4, one lead on 5).   switch to DC volts, measure pin 7 (red lead) Blk lead on ground.  Power down - discharge.  Install the preamp tubes.  Power-up and make the same measurements.  Leave the amp on for a minute or so and verify nothing changed.

You should have around 6.3VAC p4-p5 (filaments) and guessing around 250 - 350VDC on pin 7 (plate)

[sgmezei16]

Alrighty here is some more info. With the tubes removed, these are the voltages I am getting

V1
1. 440
6. 440

V2
1. 441
6. 445

V3
1. 441
6. 442

Heaters on the preamp tubes are each showing 8.58V (DC)

V4 and V5
7. 449
Heaters are showing 7.1V (AC)

This is obviously WAY different than with the tubes in. I am having a hard time understanding whats going on here....

[TubeGeek]

The output volume is not near what it should be.
The heaters on the preamp tubes aren't visibly glowing.

The preamp tubes all measure 3.2V from pin 4 or 5 to pin 9 on DC
The power tubes measure 4.2V from pin 4 to 5 on AC

Whats happening is the amp works for a minute or two then volume decreases to nothing. No output.

Does the signal gradually disappear or does it turn off like a switch is flipped?


Is there a complete schematic available? found a link to them: https://app.box.com/s/n4rogzmzmt2i7dsubfno.  (Not sure if this is correct info)


Things I'd start checking:


Verify the el84 cathode bypass cap and resistor.

Why is your plate voltage low…start where it begins.

What is your vdc at B+1? (at + of c62)


Verify the 180 ohm 5W resistors (r87 & r88) right off the PT secondary HVT...

[Willabe]

With the tubes removed, these are the voltages I am getting

This is obviously WAY different than with the tubes in. I am having a hard time understanding whats going on here....

If your asking why the voltage is so high with the tubes removed it's because now there's no load on the power supply (PSU). The more the current you pull through a resistor (R) the more the voltage the R will drop. No tubes, no current pulled through the B+ dropping R's, no voltage dropped.


               Brad                                  

[shooter]

V1
1. 440
6. 440

The voltage chart you linked is more like 140.  I suspect you have a bad ground, so do like Willabe suggests, start at the source, your power rail.  DC volts across each of the big caps.  Gator clip your neg lead to a ground stud in the chassis and record the Pos side of each filter cap.  I don't have a ac15c1 schematic, if you have a copy or link that would help. with power off/discharged, I would spend a lot of time doing a good visual inspection, wiggle wires and components, do they feel like your first teeth did as they began coming loose?

[sluckey]

All plate voltages (even on V1) will pretty much be whatever max B+ is whenever there are no tubes in the amp.

[shooter]

  My bad!, missed the all tubes out,  even though I did ask for filaments readings

[TubeGeek]

Update:


This amp is now on my bench.


So far it is not acting up.  I have had it on for about an hour now, all voltages are measuring within reason.


 


I'll leave it on while i work on other amps today and maybe the volume loss will happen.