Carvin Valvemaster low heater voltage

[m1989jmp]

Hello all,

this is a continuation of my last thread where I've inquired about volume fluctuations in my amp. Replacing the DC heater supply caps and diodes stabilized the voltages, but they ended up too low, only 5.1VDC for V1 and V2 that use DC for heaters.

I ended up removing the DC heater circuit altogether, now all tubes run on AC with the artificial center tap nearest to the V1.

Anyway, my bias tap right from the PT, unloaded or/and disconnected from the board, reads 6VAC. When tubes are inserted, I get around 5.8VAC on the spade terminals on the board where heater tap connects to, 5.6VAC between power tube pins 2 and 7 only a few centimeters away, and 5.1VAC between preamp tube pins 4&5 and 9.

I should mention that this power amp board is poorly designed, with some power tube heater traces around 2mm wide at the narrowest points. Preamp tube heaters are fed from the power to the preamp board via thin ribbon cable.

What are the possible causes for this loss of AC on the same string running in parallel?

The PT is from a Carvin X100B model, it's specs are included below in the schematic; I don't know if this was a factory part blowout so they used up everything they had, or someone replaced a failed PT at some point. X100B is designed with 4xEL34/6L6 and 3x12AX7, so the PT could be under specs.

I'm running it with 2x EL34s at the moment just to be sure, so the heater string is pulling 4.5A at max.

http://carvinimages.com/schematics/v...ster_rev_b.pdf

This schematic below is only relevant because of the PT

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[alexradium]

Hello all,

this is a continuation of my last thread where I've inquired about volume fluctuations in my amp. Replacing the DC heater supply caps and diodes stabilized the voltages, but they ended up too low, only 5.1VDC for V1 and V2 that use DC for heaters.

I ended up removing the DC heater circuit altogether, now all tubes run on AC with the artificial center tap nearest to the V1.

Anyway, my bias tap right from the PT, unloaded or/and disconnected from the board, reads 6VAC. When tubes are inserted, I get around 5.8VAC on the spade terminals on the board where heater tap connects to, 5.6VAC between power tube pins 2 and 7 only a few centimeters away, and 5.1VAC between preamp tube pins 4&5 and 9.

I should mention that this power amp board is poorly designed, with some power tube heater traces around 2mm wide at the narrowest points. Preamp tube heaters are fed from the power to the preamp board via thin ribbon cable.

What are the possible causes for this loss of AC on the same string running in parallel?

The PT is from a Carvin X100B model, it's specs are included below in the schematic; I don't know if this was a factory part blowout so they used up everything they had, or someone replaced a failed PT at some point. X100B is designed with 4xEL34/6L6 and 3x12AX7, so the PT could be under specs.

I'm running it with 2x EL34s at the moment just to be sure, so the heater string is pulling 4.5A at max.

http://carvinimages.com/schematics/v...ster_rev_b.pdf

This schematic below is only relevant because of the PT

http://www.thetubestore.com/lib/thet...-Schematic.pdf

all Carvin transformers have that low value heater voltage,you have to live with that,only thing you can do is mount 6L6s and 12ax7s that work good at those voltages,and maybe run additional twisted wires for the preamp.

[m1989jmp]

all Carvin transformers have that low value heater voltage,you have to live with that,only thing you can do is mount 6L6s and 12ax7s that work good at those voltages,and maybe run additional twisted wires for the preamp.

I presumed the voltages will drop from 6VAC since the PT is tiny for 9 valves it supplies.

However, you're saying thin wires/traces are the reason I'm losing 0.2VAC on power tube heaters and 0.7VAC on preamp tube heaters when compared to spade terminals the heater filament connects to?

Or the power tubes draw so much current it causes a heater voltage drop across preamp tubes? In that case, wiring a new heater wires would not resolve my issue.

One more strange thing... that 3.5VAC relay supply tap measures 10VAC (!) on both sides of the R47 resistor and with the resistor disconnected. I measure 50R between two taps when the amp is powered down...Does that mean the relay supply tap is shorted to the heater tap?

[m1989jmp]

The schematic (and it's logical) shows that heater tap is separate winding than the high voltage tap, yet I have 60R resistance between them.

Edit:

I forgot to disconnect the taps from the board; with the disconnected taps, no continuity between HT/relay winding and heater winding.

Since the 3.5VAC relay is close to the HT winding CT, I was actually measuring heater balance resistors in relation to the ground+HT winding DC resistance.

[g1]

If you break into the heater circuit and do some current measurements, you can calculate the resistances of those thin traces. However, you are already measuring voltage drop from end to end of the traces, so you know there is some resistance there. I would just tack in some wires in parallel (with traces) and see if it helps the heater voltage loss issue.

[m1989jmp]

If you break into the heater circuit and do some current measurements, you can calculate the resistances of those thin traces. However, you are already measuring voltage drop from end to end of the traces, so you know there is some resistance there. I would just tack in some wires in parallel (with traces) and see if it helps the heater voltage loss issue.

Sure thing, but now I'm struggling to understand why the relay tap measures 10VAC. Even weirder is how the relay is still working properly.

Edit: Ok, so with the channel switch S6 switched to CH2, relay supply tap measures 3VAC, so that's fine.

I'll try soldering thicker heater wires tomorrow and switch to 6L6 tubes, maybe the voltages stick around near 6VAC.

First model Valvemasters came without 6L6/EL34 switch, but the bias circuit nonetheless can be adjusted for both.

It makes me wonder if EL34s put too much stress on the filament tap, as well as if these amps (PTs) were designed around 6L6(GC) tubes.

[g1]

Another option in the worst case scenario is to install a separate 6.3VAC transformer just for the heaters.

[m1989jmp]

Another option in the worst case scenario is to install a separate 6.3VAC transformer just for the heaters.

And while I'm at it, install a choke too.

The voltage at power tubes is now at 5.8VAC after putting 2 6L6GCs and 5.5VAC at preamp.

I'll report if anything changes after putting a parallel string for preamp tubes right off the first power tube.

[m1989jmp]

After wiring in additional thicker wires and disconnecting the thin ribbon cable that was supplying preamp heaters, I get 5.8VAC between pins 4&5 and 9.

New wires were pulled from pins 2 and 7 of the first power tube. Now the voltage on the spade connectors right off the heater tap is 5.8VAC, and 5.7VAC on the power tubes.

I'll wire new heater wires for power tubes at some point too, as I get a 0.1VAC voltage drop because of the thin traces.

Thanks all for your help.