GT6L6C Power dissipation and max plate voltage

[shattered]

Hello!

I got a JCA50H amp that, although working, the previous owner swapped the tubes and I noticed they have considerable cracks due to the spring retainers, so I decided to replace them with a pair of Groove Tubes I had lying around.

I can't find the datasheet of these tubes, only GT-6L6-C(HP), dunno if they are the same. For what I've read, in the GT6L6C tubes the "C" corresponds to China. The stock tubes for this amp are also chinese, specifically Shuguang 6L6-WGB with max plate voltage rating of 400V.

I wonder if these ones will be probably fine for my amp, currently biased at ~24mA (via voltage drop measurement through 1-ohm resistor using a BiasMaster probe). Plate voltage should be around 485V, I don't know if they dissipate 19W, 25W or 30W. I'm sattisfied with the sound, just want to know if I'm in the safe side.

Any help is appreciated.

[Stevem]

The screen voltage on pin 4 is the main controlling factor in dissapation.

If you have 485 on pins 3 and 420 or more on pin 4 then it’s likely that when the amp is played at 75% or more the output tubes will red plate .

[shattered]

But first I need to know if these tubes dissipate 19W, 25W or 30W, right?

I don't find myself comfortable enough to use a probe directly on the solderpoints since I can't attach an alligator clip in this amp - the sockets are directly soldered to the PCB.

So my biasing method is flawed? I just divide tube power dissipation between plate voltage (pin 3 to ground), then multiply by 0.5 and 0.7 to get the range between 50% and 70%. As an example, for an EL34 with 25W power dissipation and 425V plate voltage, I calculate (25/425)*0.6 to get the mA needed for 60% dissipation.

[nuke]

This is the problem with re-marked tubes that don't reflect a standard type number. We just don't know what they really are, and 20 years or more after they were distributing them, can't find anything concrete about what they are, if any concrete information was ever available.

Maybe post a photo of the tubes in question, perhaps someone might recognize them.

Take a peep at them at idle in a darkened room and see if there's any red hot spots on the plates. If not, figure you're good to go.

The voltage the tube "sees" is between its plate and cathode. If the cathode is grounded, (usually the case in 6l6 amps) then the plate to ground is the correct measurement.

If you're using a bias probe to measure current, these usually have a 1-ohm resistor between cathode and to the cathode pin in the amp (usually ground in a fixed bias amp). The resulting voltage drop on the resistor in mv tells you the *cathode* current, which is the sum of the plate and screen current, which is a little more than the plate current alone.

The figure you're expressing of 485 volts and 24ma, is 11.64 watts of dissipation, and if using a resistor in the cathode, is the sum of plate and screen dissipation. That's on the cold side of things, but if it sounds OK then it is fine.

[shattered]

I fully agree, that's a problem.

I took the photos but forgot to post them, so here they are. I observed when I used the amp during the day and they didn't redplate, I turned it again now (no volume) in a darken room and I don't see any sign of red plating. Thank you for the tip.

Yeah, I contacted TAD and confirmed they work using the voltage drop via resistor method. Other probes break the circuit to measure mA using a current meter, which is more accurate but can cause oscilations and other problems, so the former method is preferred, am I right? I've also contacted Fender asking for information about these tubes.

I prefer to set bias tending to the cold side specially when I'm using tubes I don't specially trust or know enough. Biasing it hotter I feel more compression and sometimes it's too much, in my limited experience it depends on the amp and the tubes. I think I prefer 6L6 to run colder than EL34, but I've not experimented enough... just try to make them sound right being in a safe range.

[Stevem]

They're 30 watts .

[shattered]

So GT6L6CHP is the same tube as GT6L6C? Did you identify them by their construction?

In any case I'll write back I get an answer from Fender.

[wpaulvogel]

Here’s what I do when biasing an output section. I use my oscilloscope to view the output waveform. The maximum dissipation rating of the tube is used only to determine the maximum current I have to work with in an attempt to achieve full clean sine RMS output. Meaning that I want the output waveform to be lacking crossover distortion at the point of clipping. Sometimes this scenario is impossible to achieve and most notably occurs with high plate voltage. I use two meters set up to measure plate voltage on one and cathode current on another. I already have a sheet of predicted dissipation based on current and voltage so that I have a rough estimate of where I need to stop without overheating the plates. There are times that my goal is achieved at dissipation before 70% and at whatever point that occurs, I stop. It’s unnecessary to bias hotter. Sometimes it can occur at 80% in say a 100 watt Marshall with EL34’s and I have to compromise the waveform for longevity and reliability but just to set bias to a hard number of % of maximum dissipation seems to me like a mistake. I understand that not everyone has an oscilloscope but to me it’s a necessary tool for maintaining a good sounding amp.

[pdf64]

Here’s what I do when biasing an output section. I use my oscilloscope to view the output waveform. The maximum dissipation rating of the tube is used only to determine the maximum current I have to work with in an attempt to achieve full clean sine RMS output. Meaning that I want the output waveform to be lacking crossover distortion at the point of clipping. ...

My experience is that signal levels that high will tend to cause the onset of grid rectification, thereby shifting the instantaneous bias cooler than at the idle level.
Hence to achieve best linearity up near clipping, the valves will be running hotter than necessary. And when pushed harder, to and beyond clipping, grid rectification will push the instantaneous bias even cooler and you'll still get the crossover kink as well as clipping.
Aspen Pittman's The Tube Amp Book isn't perfect by any means, but he advises to set the level on the scope for about 70% of maximum output when setting bias by the crossover distortion method (70% of the maximum clean output voltage works out to 1/2 the max clean power output).
I surmise that level is suggested to avoid the above described grid rectification effect.

[nuke]

The -C and -C (HP) are not the same.

The HP versions were made with USA tooling and black plates. The -C is supposed to have grey plates.

These were OEM'd to Fender and bear a Fender part number, which all their 6L6GC tubes have in common, since the 1970's.

If the plates are black they are probably the HP version.

[shattered]

Thank you for your insight.

I got a reply from Fender and after sending them some photos of the tubes and their part number (073314) they referred me to this datasheet: https://tdsl.duncanamps.com/show.php?des=6L6GC