VHT Pitbull 45 Cuts Out

[ca7922303]

When playing at gig volume level and on the b/e strings above 12th fret, amp seems to increase distortion and intermittently cuts out. Cleaned all Jack's and noticed that the base of power tubes(EL84s) are running at 450+° F(infared thermometer), with no signal applied. One is 470+°F..

[nosaj]

Check the AC ripple on the filter caps. Check all solder joints. Does it start playing again after it sits for a while? If so that could be indicative of a bad solder joint breaking connection when warmed up then connecting when cold.
nosaj

[Jon Snell]

I would firstly check the bias as they do seem a little on the warm side. 230+°C is hotter than I would like. It could be swamping the output transformer or draining too much power from the main HT supply.

[g1]

It could be swamping the output transformer or draining too much power from the main HT supply.

I was wondering if the amp might be breaking into oscillation as it seems triggered by high notes. If so, the oscillation could also be making the tubes run hot while producing no audio.

[ca7922303]

Check the AC ripple on the filter caps. Check all solder joints. Does it start playing again after it sits for a while? If so that could be indicative of a bad solder joint breaking connection when warmed up then connecting when cold.
nosaj

Would you give me the procedure for checking the ac ripple on the filter caps? Soldered all tube socket pins. It was cutting out momentarily, it was getting a heavily distorted signal only when played on high B &E above the 12th fret at high volume level.

[g1]

To check ripple on the filter caps, you measure the AC volts across the cap terminals. It should be quite low. Also check that the DC volts on the cap are correct.
For a rough number, I would say AC volts ripple at idle should be less than 5% of the DC on the cap. Solid state amps have quite a bit less ripple than tube amps as they use much higher value caps for filtering.

[ca7922303]

To check ripple on the filter caps, you measure the AC volts across the cap terminals. It should be quite low. Also check that the DC volts on the cap are correct.
For a rough number, I would say AC volts ripple at idle should be less than 5% of the DC on the cap. Solid state amps have quite a bit less ripple than tube amps as they use much higher value caps for filtering.

Filter caps are (457vdc) w/no tubes and from (0.06-117vac) measured across, also w/no tubes.

[g1]

(0.06-117vac) measured across, also w/no tubes.

Do you mean 117vac there or is there a decimal point missing? 117vac across a filter cap would not be good.

[ca7922303]

AC across filter caps reading is cycling from (0.06 to 117vac). I'm getting (1.57, 3.06, 0.25, 3.33, 3.19, 3.85, 3.95, 0.06, 117vac). I'm Pushing hold button to get readings.

[ca7922303]

Pushing hold button on & off to get readings.

[g1]

Sometimes the meter may read high while trying to autorange before the reading settles. Or maybe you are taking a reading during power up, or coming out of standby?
Try taking the AC reading without the hold function. Give it a couple seconds to settle.

[nosaj]

What kind of meter are you using?
nosaj

[ca7922303]

Sometimes the meter may read high while trying to autorange before the reading settles. Or maybe you are taking a reading during power up, or coming out of standby?
Try taking the AC reading without the hold function. Give it a couple seconds to settle.

It doesn't settle, it keeps cycling.

[ca7922303]

What kind of meter are you using?
nosaj

Kobalt autorangung

[nosaj]

Take it off autorange an you should get a better measurement.
nosaj