Low bias current - Carvin TS100

[SacredGroove]

Hello,

I've recently acquired a Carvin TS100 power amp. I'd purchased new power tubes for it and when I went to bias them, I get a very low current measurement of 10mA across both tubes on channel 1. This measurement happens when the bias pot is fully clockwise. The new tubes are JJ E34L. The tubes that came with it are Groove Tube GT6L6-R. After I reinstalled the GT's, the bias current is higher, but still quite low at 20mA. All current measurements are taken from the transformer center tap, per the amps instructions.

I've attached the manual and schematic. Can someone here lead me in the right direction to get the amp biased correctly, or repaired?

Thanks!

[The Dude]

What is your bias voltage (voltage at the output tube grids)? How does it compare to channel 2? Does channel 2 bias properly? Both channels use the same bias supply, so if one channel won't bias and the other will, the problem is likely in the output circuit of the channel. If neither channel will bias, it is more likely a bias supply problem.

[Enzo]

Clockwise is the coldest setting on the bias pot, have your tried going the other direction? Also you appear to have a EL34/6L6 switch, are you setting that correctly for the tubes in use?

[J M Fahey]

6L6 and EL34 have very different bias requirements, amp does have a switch for that but it´s internal, DID you set it for EL34?

What voltage do you measure at C9 and C44 negative pins respectively?

[SacredGroove]

What is your bias voltage (voltage at the output tube grids)? How does it compare to channel 2? Does channel 2 bias properly? Both channels use the same bias supply, so if one channel won't bias and the other will, the problem is likely in the output circuit of the channel. If neither channel will bias, it is more likely a bias supply problem.

Bias voltage is -48V. Channel 2 doesn't bias properly either.

[SacredGroove]

Clockwise is the coldest setting on the bias pot, have your tried going the other direction? Also you appear to have a EL34/6L6 switch, are you setting that correctly for the tubes in use?

Good catch on the bias pot. However, I had tried the other direction. Also, I toggle the switch, depending on the tubes I have installed.

[SacredGroove]

6L6 and EL34 have very different bias requirements, amp does have a switch for that but it´s internal, DID you set it for EL34?

What voltage do you measure at C9 and C44 negative pins respectively?

Both C9 and C44 measure -49.5V. Switch was set for el34.

[g1]

Looks to me like an error in the user manual where they say to disconnect QC2 and QC16, but according to the schematic it should be QC2 and QC32.
Are you getting good screen voltage at pin4 of the power tubes?

I would also try the method they mention with the 1ohm resistors, just in case it is related to the current range on your meter.

[SacredGroove]

UPDATE:
Last night and this morning, I was able to confirm that the original GT6l6 tubes can be biased, according to the manual and I am now reading 50-51mA per channel. This is only ~25mA per tube, but I think I can get them a bit hotter. The 2 channels are interactive with each other and it seems that if I get channel 2 set then I can dial in channel 1. However, at this point, I would much prefer the E34L/EL34 type, so I will try them again tonight.

[Jon Snell]

With 465volts HT, 25mA per valve, is plenty hot enough as that equates to about 25W dissipation. This amp is not designed to be run in Class A.

[Helmholtz]

From 465V B+ and 25mA I get a plate dissipation of roughly 12W per tube.

25mA per tube is fine and biasing hotter would just increase internal amp temperatures.

[SacredGroove]

From 465V B+ and 25mA I get a plate dissipation of roughly 12W per tube.

25mA per tube is fine and biasing hotter would just increase internal amp temperatures.

The GT6L6R is ~25W tube. There is ~457V at pin 3. I calculate 38mA for 70% plate dissipation. (25 / 457) 70%. Is that correct?

[Helmholtz]

I've never found a technical justification of this 70% "rule".
At best it could be interpreted as an upper limit to avoid overdissipating at medium output with fixed bias.

Most fixed bias guitar amps are fine with 50% to 60%.
Biasing hotter than that is likely to reduce tube life without giving any benefit.

[g1]

Agree with Helmholtz, 70% is a max. limit for class A/B running at highish voltages.
Imagine if there were some internet 'rule' that we should set up our cars to burn more gas than they need to.

The 2 channels are interactive with each other and it seems that if I get channel 2 set then I can dial in channel 1.

This does not seem right. The interaction should be very minor, as the B+ may change a bit, but I would not expect the interaction to be very noticeable.
However, there is no filter cap at point E, and that might be a useful addition. What is the DC voltage at point E?
Also the voltages at pins 3 and 4 of the power tubes.

[SacredGroove]

This does not seem right. The interaction should be very minor, as the B+ may change a bit, but I would not expect the interaction to be very noticeable.
However, there is no filter cap at point E, and that might be a useful addition. What is the DC voltage at point E?
Also the voltages at pins 3 and 4 of the power tubes.

The 2 channels interaction is minimal. I played with the bias again last night and found that if I adjust one side, the other side will temporarily increase or decrease and then stabilize where I had set it. The good news is that I am able to bias 5881/6l6 tubes without any issues. I'm not sure why, other than having flicked the bias switch a bunch of times since swapping out tubes. The bad news is that I still cannot use el34.

The voltage at pins 3 and 4 are ~457.5V, from memory.