Carvin VTX100 Head Diagnostics Help

[misterc57]

My next project. I picked this up to have something to work on. This is a strange amp (to me). I am still trying to understand how it operates and some of the issues could be my lack of knowing how top operate the amp.

It has (4) EL34 and (1) 12AT7. One of the EL34 was physically broken, I swapped it with one I have sitting around that measures weak. All the other tubes test good.

The 470 ohm 2 W screen grid resistors look cooked, but all measure within 10%. I will be replacing these.

After warming up for a few minutes I start to see smoke rising from the area of the two can filter capacitors. The large black 350 ohm 100 W resistor connected to these cans looks like it has been cooking. It is also hot to the touch. I think it is this resistor that is smoking but it is hard to pin point. Note that it seems to be smoking less on subsequent power ONs, but may still be an issue if I leave the amp on long enough. This resistor reads 359 ohms.

One of the two blue caps on the top left corner of the PS board has physical signs of prior leakage.

Output tube voltage at pins 4 and 5 (I did not measure other pins) are much lower than the schematic indicates. Pin 4 reads 340 VDC (schematic says 470 VDC), pin 5 reads -30 VDC (schem says -44 VDC).

My output looks distorted on the scope. I have a 1K sine wave going in. Picture shows the output with Channel One volume at ZERO, and then at about ONE. My un-trained eye is seeing a lot of noise and clipping. Is that what others see? Is the noise from bad filter caps? Is the clipping normal for this amp or is a clean signal possible?

The amp is drawing about 1 AMP when warmed up. The sustain switch is in the OUT position.

I have attached both the owners manual and the schematic, and some pictures.

Thank you for your help and guidance! MarkO






Carvin VTX-100, STX-125, 250 Bi-Channel Schematics.pdf

carvin_vtx100_stx125_sch.pdf

Volume at ONE


Volume at ZERO

[glebert]

Is the distortion and noise the same on both channels? Have you looked at the waveform at the "channel Insert" (i.e. effects loop) to compare to the output waveform? This will help say if the distortion and noise are in the preamp or power amp. The smoking cap/resistor thing definitely needs to be addressed, but may or may not be the cause of the distortion. I would measure voltages at all pins of the power tubes.

This is a weird amp. I think they sound pretty cool though. Plenty loud too.

edit: how much voltage do you have across that 350ohm resistor? Pin 5 being at -30V instead of -44 will be biasing the tubes pretty hot. The preamp voltages come off that same voltage supply, so they might be low as well.

[misterc57]

Voltages DROP after warm-up.

Figured I should start at the PT secondary and work my way in measuring voltages.

117 VAC on the primary.
700 VAC across the secondary (2 red wires).
49 VAC on the secondary blue wire.

382 VDC at the standby switch.

10 VDC drop across the large 350 ohm resistor. I have 358 VDC and 348 VDC on the two sides of this 350 ohm resistor.

Here is the weird thing. When the amp first powers up I have higher (what would appear more normal) voltages, then after a minute of warm-up the voltages drop. I can see this at the standby switch which starts around 470 VDC and then drops and stabilizes at 382 VDC. I also see this behavior at pin 4 of the output tubes.

VDC falls as the current rises from .5 to about 1.1 Amps.

The VAC at the secondaries does not drop after warm-up.

What could cause this quick voltage drop?

Another observation; pin 4 of the output tubes has about 4 VAC on it. Not sure how much ripple is normal.

Thank you

[Helmholtz]

VDC falls as the current rises from .5 to about 1.1 Amps.

Current draw is way too high and will cause low voltages. A 4xEL34 amp should not draw more than around 150mA at idle and around 0.5A at full output. How/where do you measure current? Are the EL34s redplating?
Bias voltage is way too low (causing high tube currents) and should be adressed first (60µ bias filter caps, resistor values).
What is grid bias in standby?

Leave the amp in standby mode or pull power tubes for further analysis.

[glebert]

Are these measurements with tubes in or out? I would start working backwards from the tubes, disconnecting things until the voltages come up to expected-ish voltages and stay stable. Or if you want to take a bit of a leap look to replace the 40uF/500V caps straight off and see if one of them is leaking after warmup. They are being run pretty close to their rated voltage, and are probably almost 40 years old.

[misterc57]

I am measuring current at my variac. All tubes are inserted. EL34 are not redplating.

Grid bias in standby (measured at pin 5) is -32 VDC.

Current in standby is 0.5A.

I need to explore the (4) 40uF/500V caps.

Appreciate the help! Thank you, MarkO

[Helmholtz]

I am measuring current at my variac. All tubes are inserted. EL34 are not redplating.

Grid bias in standby (measured at pin 5) is -32 VDC.

Current in standby is 0.5A.

I need to explore the (4) 40uF/500V caps.

Appreciate the help! Thank you, MarkO

Now that's a different story as you are measuring AC input current and I was speaking of total DC supply current.
Anyway address the low bias voltage first, leaving the amp in standby.

[misterc57]

Will do. Curious, is it okay to test without tubes inserted at full or partial line voltage?

[glebert]

Will do. Curious, is it okay to test without tubes inserted at full or partial line voltage?

I test without tubes, at least until the voltages are where I want them to be. Theoretically one should watch the voltage ratings on all caps to make sure they won't be exceeded (on preamp tubes the plate voltage may go way up to the B+ voltage). You could use a variac to reduce voltages globally, but if the problem is a leaking cap then you may not see it at lower votlage.

[Helmholtz]

According to schematic, no B+ cap is charged in standby. Bias supply trouble shooting can be done in standby but without variac as we want to see full voltages.

[misterc57]

Progress. Standby ON, I found the bias adjust pot and was able to dial up to -45 VDC. Then I tried with standby OFF and saw pin 4 on the output tube at 432 VDC and 1.5 VAC ripple.

I watched the bias VDC and after a few minutes it began to fluctuate and drop. I know that one of the 60uf caps in the bias circuit had physical sign of leaking and I need to replace both of them. I suspect these are causing the fluctuation in bias VDC???

I also checked the rails going to the preamp and they are around + and -15.7 VDC.

All testing was done on my variac which is also an isolation transformer and dialed up to 117 VAC line output.

Any recommendation on sourcing the 60uf/60V caps or a modern equivalent that is reasonably priced? I see some 68uf/100V radials under $1 each.

47uf/500V okay to replace the 40uf?

Thank you

[glebert]

It is fine to go higher capacitance, especially for filter caps. I would go higher voltage rating than 500V if possible for the 40 or 47uF but it seems like options get fewer above 500v.

[g1]

The 68uF/100V are fine for bias. I typically use 100uF/150V.

[misterc57]

More testing.

I found an 82uf and 100uf cap that I clipped in (after lifting one leg of the original 60uf caps).

Bias is now stable around -44 VDC, pin 4 is about 425 VDC. Is 1.5 VAC okay at pin 4?

Back to my output signal. Still looks bad on the scope. Not seeing a clean signal. I would expect a clean sine wave. Picture is with volume around 30%, channel one.
Channel two looked just as bad, maybe worse.

Where is the "channel Insert" (i.e. effects loop)? No obvious effects loop.

Should I connect a speaker and see how it sounds?

[glebert]

More testing.


Where is the "channel Insert" (i.e. effects loop)? No obvious effects loop.

Actually called Channel Interrupts on mine, which are on the back of the amp, there should be one for each channel. They are TRS jacks so ring is send (or maybe receive) and the tip is the other. At least that is what the way it is on mine. You shouldn't need to break the signal path to probe there.