Revised ?: Power Tubes Battle for Current Through Shared Cathode Resistor

[SingingCoil]

Good Morning Purveyors of Tone,

Guessing my last post( jtm EL34s to mismatched 6L6s) was somewhat pointless.
Hadn't finished my morning cup o coffee and was a tad frantic about my readings.

So, went back and did more homework.
Pulled four 6L6s from an old pawnshop Bassman I had recently purchased,
and gathered up four more from a friend's ebay Twin that he was going to scrap
for a new matched set he had gotten.
Using my jtm build as a guneapig testor, I spent a whole day juggling the eight of them around
and meticulously charting the PlateVoltages & Plate Current using the ShuntMethod.

Seven of these old 6L6s reflected a more familiar B+voltage of 392-395v,
(previously 407v with my EL34s). Of these seven I found two pairs that were pulling
44mA-46mA. ( Quite surprised, considering the ugly state of these pawnshop rejects).

The exercise brought me to the realizations(very roughly so) that:
1) I had been unaware of the battle for Current because my ELs had been matched from
the vendors( KCA NOS & BrentJesse ).
2) That of a mismatched pair, one tube will "steal" current from the ( weaker?) tube, so that
the CurrentDraw of an untested tube cannot be determined without
either applying a fixed bias voltage or splitting the cathode.(Assumptions of course).
Please apply a correction here if I've missed the point again.

Back to my previous post:
I installed the "weaker"(?) RCA that I had purchased ( 12mA / 60mA), (tested only on an Eico),
and it matched up pretty well with all four of the two pawnshop pairs I had found,
showing voltages in the low 390s and mA draw in the mid 40s.
I concluded(again roughly so) that this RCA was somewhat normal.

Emailed the old HiFi guy about my findings and told him I had used a 300R because I didn't
have a 350R, and that I had found recommendations online indicating 300R was not inappropriate.
His response: "Whoa there cowboy! What do ya think you're doin with that 300R?!"
( I almost took offense, but I know that HiFi guys run 'em way cooler than guitar guys).

So I installed both RCAs I had purchased & bumped the CathResistor to 350R/100uF/100v as
recomended by the old HiFi guy I got the from.
Readings: 12mA again on the "normal" tube, and now only 50mA(60mA/400v with 300R), but
the voltage on the "crazy" tube was now 600v - YOW!!!

Just starting to build my knowledge in the dark on this matter, but I wouldn't say I'm bein a cowboy about it.
Again,I think something is seriously WRONG with this one tube.
The other RCA and the two pairs of pawnshop "unknowns" all seem to play well together.

This 350R/50mA/600v tube seems to be the maniac here.
What might be going on inside this little device that sets it so far apart from the other five tubes???

Hope I'm making more sense this time.
Thanks for any light on the matter.

[SingingCoil]

Good Morning Purveyors of Tone,

Guessing my last post( jtm EL34s to mismatched 6L6s) was somewhat pointless.
Hadn't finished my morning cup o coffee and was a tad frantic about my readings.

So, went back and did more homework.
Pulled four 6L6s from an old pawnshop Bassman I had recently purchased,
and gathered up four more from a friend's ebay Twin that he was going to scrap
for a new matched set he had gotten.
Using my jtm build as a guneapig testor, I spent a whole day juggling the eight of them around
and meticulously charting the PlateVoltages & Plate Current using the ShuntMethod.

Seven of these old 6L6s reflected a more familiar B+voltage of 392-395v,
(previously 407v with my EL34s). Of these seven I found two pairs that were pulling
44mA-46mA. ( Quite surprised, considering the ugly state of these pawnshop rejects).

The exercise brought me to the realizations(very roughly so) that:
1) I had been unaware of the battle for Current because my ELs had been matched from
the vendors( KCA NOS & BrentJesse ).
2) That of a mismatched pair, one tube will "steal" current from the ( weaker?) tube, so that
the CurrentDraw of an untested tube cannot be determined without
either applying a fixed bias voltage or splitting the cathode.(Assumptions of course).
Please apply a correction here if I've missed the point again.

Back to my previous post:
I installed the "weaker"(?) RCA that I had purchased ( 12mA / 60mA), (tested only on an Eico),
and it matched up pretty well with all four of the two pawnshop pairs I had found,
showing voltages in the low 390s and mA draw in the mid 40s.
I concluded(again roughly so) that this RCA was somewhat normal.

Emailed the old HiFi guy about my findings and told him I had used a 300R because I didn't
have a 350R, and that I had found recommendations online indicating 300R was not inappropriate.
His response: "Whoa there cowboy! What do ya think you're doin with that 300R?!"
( I almost took offense, but I know that HiFi guys run 'em way cooler than guitar guys).

So I installed both RCAs I had purchased & bumped the CathResistor to 350R/100uF/100v as
recomended by the old HiFi guy I got the from.
Readings: 12mA again on the "normal" tube, and now only 50mA(60mA/400v with 300R), but
the voltage on the "crazy" tube was now 600v - YOW!!!

Just starting to build my knowledge in the dark on this matter, but I wouldn't say I'm bein a cowboy about it.
Again,I think something is seriously WRONG with this one tube.
The other RCA and the two pairs of pawnshop "unknowns" all seem to play well together.

This 350R/50mA/600v tube seems to be the maniac here.
What might be going on inside this little device that sets it so far apart from the other five tubes???

Hope I'm making more sense this time.
Thanks for any light on the matter.

Or might it be that the five tubes that seem to be idling "normally"
are WEAK compared to this "crazy" healthy and/or HOT tube?

Is an internal resistence issue involve here, and is it measurable with a multimeter?

The overall question points to: what one does with a bunch of unknown power tubes
that drop into yer lap when ya don't have access to a high end tester.

[SingingCoil]

Ah,
thinking I can split the cathode with some 20w resistors, finding the one each tube likes,
and using a dropper somewhere if the voltage doesn't even out.
OR
Install an auxilliary tranny for bias voltage, and having two seperate voltage adjustments
for each tube?
Is either or both of thesea workable solution?

[g1]

Generally, when a tube is running hot, the plate voltage drops, due to loading on the power supply. You have a tube that is conducting heavily (50mA), yet B+ is high (600V) so what gives? If you are using shunt method, you may be mesuring the opposite tube you think you are measuring. Ideally, measure one tube at a time, with one removed. Run them only long enough to get a reading. If the one is way off, it is probably bad.

[SingingCoil]

BTW: g-one, I gotta tell ya man, was much more Zep than anything, but seeing Lemmy's face everytime I visit this site
just makes me smile and laugh for the pure joy of rock&roll, thanks for that alone !!!

So, pulled a tube, put a 700R/100uF/100v in, hooked up the meter, backed away a bit, and hit the power.
416 Plate V
36.5 Cath V
380 Actual Plate V
48.5 mA
18.5 W @56%
Much more familiar number, no freaking out.

Only question remaining would be:
Is a 700R per 6L6gc far out, indicating some internal problem with the valve ?
Or do these numbers seem reasonable, even if uncommon ?

[g1]

Compare the other 5, one at a time, under same conditions. Post results.
Does this one match, or is it way different?

[frus]

I don't get it. If you have a B+ of, say, 450 V unloaded, then when you put some tubes in, they will pull some current, maybe the power supply will sag a little, but the (idle) plate voltage can never be HIGHER than B+. If you got 600 V on some tube pin, then you're measuring something wrong

[J M Fahey]

I spent a whole day juggling the eight of them around
and meticulously charting the PlateVoltages & Plate Current using the ShuntMethod.

Well, that is NOT the Shunt Method, you got it wrong.
You are measuring Plate or +B (not clear) voltage to ground; "Shunt" implies measuring voltage drop *across* transformer plate windings, an absolutely different deal.

Seven of these old 6L6s reflected a more familiar B+voltage of 392-395v,
(previously 407v with my EL34s).

You are measuring the wrong way.
And although in theory you *might* somehow use those values , for any useful results you must know the internal resistance of your power supply ... of which you have no clue.

1) I had been unaware of the battle for Current because my ELs had been matched from
the vendors( KCA NOS & BrentJesse ).

Matched or not, all tubes fed from the same supply will "battle for current" ..... for DC they are all in parallel.

2) That of a mismatched pair, one tube will "steal" current from the ( weaker?) tube, so that
the CurrentDraw of an untested tube cannot be determined without
either applying a fixed bias voltage or splitting the cathode.(Assumptions of course).
Please apply a correction here if I've missed the point again.

For measurement purposes, don't guess, just measure them one by one and write results down.

Emailed the old HiFi guy about my findings and told him I had used a 300R because I didn't
have a 350R, and that I had found recommendations online indicating 300R was not inappropriate.

Following random recommendations found online is not exactly the best.
By the way, earlier you mentioned a "JTM build" as your "gunea pig" .
I'm baffled .... that's a fixed bias amp, what's all this talk about 300/350/700 ohms cathode resistors?

So I installed both RCAs I had purchased & bumped the CathResistor to 350R/100uF/100v as
recomended by the old HiFi guy I got the from.

Readings: 12mA again on the "normal" tube, and now only 50mA(60mA/400v with 300R),

12mA is "normal" but 50/60 mA (5X *larger*) deserves the qualification of "only"?????

but the voltage on the "crazy" tube was now 600v - YOW!!!

Plain impossible

This 350R/50mA/600v tube seems to be the maniac here.
What might be going on inside this little device that sets it so far apart from the other five tubes???

Probably a case of


Sorry, don't want to seem rude, but please try to focus on one subject at a time and provide data in a usable form.
If you have to bump your own questions time and again, even self quote , because "nobody answers" .... think again about what might be the cause.
The way many of your questions are worded makes them basically unanswerable.
Or kind Forum members, in good faith, will answer whatever lesser point seems to be clearer, but the main one will still be obscure.

Good luck

[SingingCoil]

Ahh, pardon the scatterbrained-ness,

Shunt method used: B+ to Plate(pin3) measuring range in mA.
Voltage method: Plate(pin3) to GND.
Actual Plate V: Plate voltage minus Cath V.

And ya, 600V??? Changed batteries in meter. Same thing, numbers keep climbing.
Hit the kill switch 'cause I don't even wanna see how high they're gonna get.

Build started as jtm, but I like the cathode thang, along with other usual basic
preamp mods. Not sure what to call it yet.

This is my first experience with 6L6s, so I was expecting to be working with
B+ voltages around 400, give or take a twenty, (measured B+ to GND).
Then shooting for 70% Dissipation.
So expecting PlateCurrent around mid-40s.

I shall run the other tubes through resistor values and take readings after
I get my backlog of real-life duties alleviated a bit.

No worry bout appearing rude. I grew up learning to restore classic cars in my
grandfather's shop. Cut my teeth on paying attention and not screwing around
with the good ol' slap-in-the-back-of-the-head-method. Also cleaned the grease
off with leaded gasoline, so between that and the muse, sometimes I'm a bit out
of focus. And pardon the extra blabla, I'm a context guy.

Totally appreciate all the input guys!

[frus]

Is it possible that there is some kind of ultrasonic oscillation with that tube?

Meter measuring the plate voltage would show the average value, that could be higher than idle (since there IS a signal), and the average current would also be much higher than we would expect in idle

[frus]

Is it possible that there is some kind of ultrasonic oscillation with that tube?

Meter measuring the plate voltage would show the average value, that could be higher than idle (since there IS a signal), and the average current would also be much higher than we would expect in idle

[SingingCoil]

Ah, Good Man frus!

That's the kind of intelligent supposition I was expecting from a tech forum!
I shall do some reading up on this tube oscillation,
but this does ring a bell seeing as how my meter was climbing up to 600,
but was definitely not settling on a number. It is as if the meter was misbehaving...
kind of "oscillating" as it were.

I had been trying to get to: "possible problems within a tube,"
though apparently not clearly enough for some.
Whether or not you nailed it, it's a great point of direction for my continuing study.

Thanks Dude!

[g1]

I had been trying to get to: "possible problems within a tube,"
though apparently not clearly enough for some.

Sorry, I was under the impression you wanted to "prove" out the bad tube, possibly for purpose of return/refund. This is why I suggested you test them all one at a time and record the numbers you get. If the one is way off compared to the others, then it is obviously defective or grossly unmatched and you should be able to return the pair.
And again, if using shunt method to measure with a pair installed, the tube you are "shunting" is shorted out, and you are reading the current of the other tube.
If you have definitely isolated the bad one, and it is giving high current and weird voltage readings, then as frus suggested, oscillation is a good suspect. But if only that one tube oscillates, you still have only the same conclusion, a bad tube.