Cathode bias question

[dude]

I have "good" RCA Black Plate 6L6 that is a real "hard tube". 

All of my USA old stock 6L6's don't match with this tube, when I adjust to around 25 mA's  this black plate is at 40mA. So, rather than have dual bias pots, I'd like to use it my single ended Champ 12. But it doesn't sound anywhere as good as other used USA 6L6s in that amp.

I'm wondering if the standard bias R (470) in this Champ 12 could be changed to maybe get this 6L6 Black Plate to sound like they usually do? If so, would I go up or down? Or perhaps this tube is not up to specs...? It tests very good on my tester.

Thanks in advance,
al

[HotBluePlates]

... I'd like to use it my single ended Champ 12. But it doesn't sound anywhere as good as other used USA 6L6s in that amp.

"Doesn't sound as good" means what in this case?

I'm wondering if the standard bias R (470) in this Champ 12 could be changed to maybe get this 6L6 Black Plate to sound like they usually do?

You usually wouldn't change a cathode resistor to get a tube to sound differently. You'd change it to get correct idle current in the tube. Except that outside very unusual circumstances, the cathode resistor value that works for one tube of a given type is the same value used for any other tube of that type. Each tube will settle in at its own idle current level depending on the tube's Gm and whether or not it is gassy.

... I'd like to use it my single ended Champ 12. But it doesn't sound anywhere as good as other used USA 6L6s in that amp.

I'm wondering if the standard bias R (470) in this Champ 12 could be changed ...

If this Champ 12 schematic is correct, there isn't a 470Ω cathode bias resistor. In fact, the cathode resistor is split into 2 pieces and is used to change bias depending on whether the amp is set for clean or distortion. There is also a feedback loop around the output tube and output transformer. If the feedback does its job, there should be relatively little difference in sound regardless of what 6L6 you plug in.

All of my USA old stock 6L6's don't match with this tube, when I adjust to around 25 mA's  this black plate is at 40mA. ... perhaps this tube is not up to specs...? It tests very good on my tester.

What kind of tester? Do you have a gas test function on your tester?

If a tube is gassy, it could have a cascade of effects which make plate current higher than normal for a given set of electrode voltages. If you have other brand-new 6L6's which tend to measure 6,000 micromhos on a transconductance tester and this tube measures abnormally high (like 8-9,000 micromhos), then it could be a gas defect. Or it could be a tube with abnormally-high Gm; it may take careful comparison in-circuit against a known-good tube, or use of a testing method to identify whether a tube is gassy.

[PRR]

Don't want to speculate on tube bias until you say what current the tube is actually running.

If this is a true Champ 12, what is the voltage on the power tube cathode?

And if it isn't happy in a self-bias circuit, there may be something wrong with it. (NO tube-tester tests for all faults, or even all common faults.) Many tube testers test very little, and that perhaps abusively.

> cathode resistor is split into 2 pieces and is used to change bias depending on whether the amp is set for clean or distortion

I don't believe the power tube bias changes enough to notice.

220r to two 390r. One to ground. Other to LED+diode or LED. I suspect bias is 65mA with about 5% change depending on FS1.

Power tube better be >= 6L6GC because it idles at 29 Watts (maybe 28W in plate).

> a feedback loop around the output tube and output transformer.

Appears to do very little at nominal load. (May do more at bass resonance.)

NFB over just one stage. Tube bias is 450V and 15V, so gain is under 30. OT should be near 7K, or 29:1 ratio to 8r load. Amp gain about 30/29 or just 1. NFB resistors are 270K and ~~~200K, inverting, so while it looks like gain a little over 1 the amp really needs gain over 2 before NFB happens.

[dude]

Yeah, that's the schematic.

I meant to say 390 ohms, but forgive my ignorance this schematic is over my head. I'm not familiar with the two paths the bias takes....

I've tried this tube in a SE 6V6 amp and it just doesn't have the "tone", it's kind of sterile and harsh sounding same as in the champ. I suspect this tube is bad (I'll have to do some reading on gassy tube), the tester is my avatar which I take with a grain of salt. 6L6 RCA black plates are expensive so I was wanting it not to be the tube but probably is...?

Thanks for educating me a bit.

al

[HotBluePlates]

> cathode resistor is split into 2 pieces and is used to change bias depending on whether the amp is set for clean or distortion

I don't believe the power tube bias changes enough to notice.

220r to two 390r. One to ground. Other to LED+diode or LED.

Yep, I got cross-eyed looking at the LDR's. I was thinking the 390Ω to the LED was instead in the switched resistive path, switching cathode resistance from 610Ω to 415Ω. Oops... none of that is happening.

[HotBluePlates]

... the tester is my avatar ...

The avatar is too small for me to know what model it is. Looks like any of a hundred other testers (but maybe an EICO model?). I was trying to get at whether it was a Gm tester or an emission tester, and whether there is a gas test function on-board.

I've tried this tube in a SE 6V6 amp and it just doesn't have the "tone", it's kind of sterile and harsh sounding same as in the champ. I suspect this tube is bad ... 6L6 RCA black plates are expensive so I was wanting it not to be the tube but probably is...?

If you think the tube is okay but just doesn't work for you, then sell it. If you think it is bad, then do what your integrity guides you to do.

I've only had 1 output tube to have an outright failure. It was a blackplate RCA 6L6GC which shorted plate to heater internally. Felt somewhat sad to toss it in a garbage can, but that's where it went; fortunately I acquired it inexpensively.

[dude]

Indeed the tester is an EICO model 667/Dynamic Conductance Tube & Transistor Tester. Definitively made in the early sixties.

I did check most of the R's and caps, cleaned all contact points, etc. This tester has a line voltage and frequency of between 105-130 volts, so you can adjust the ac voltage before testing.

At least I can tell me if a cathode is leaking, manual says:

"Direct reading of inter-element and cathode heater leakage in ohms on a 0 to 20 megohms scale. DC test voltage always applied in correct polarity to eliminate emission effects from reading. Has a Merit test for diodes and rectifiers and a conductance mutual, plate and conductance, emissions".

The 6L6 RCA Black Plate, has no leaks, and rates high on all tests, but the dial for grid and plate only go from 0 up to 100vdc and is set according what the manual says for that particular tube.

I don't know, sure don't want to toss it if it's good, looks brand new, pins are very shinny new. Got it from a friends grandfather who was a TV repairman (in a new original box). Just always read very high in mA when I tried to bias it in a fixed bias amp. Just couldn't find any other 6L6 to get within 5 to 8 mA.  Just thought it was a very hard tube, the way Pittman used to rate NOS tubes, this would be a 9 or 10 I assumed ...?

Sure don't want to sell it, if it's toast.

I'm comparing the tone of it against an Allen Old Flame I build years ago, I got hold of some Lowery RCA black plates cheap, $30 on ebay and that amp never sounded so good.

thanks,
al