6L6GC tube testing

[dscottguitars]

Hi,  I have a pair of GE green label 6L6GC tubes that I tried in my amp, somewhat of a bassman/marshall clone.  One got red plates immediately and measured around 100mA, the other 70mA.  Even putting -64v on the grid still had the one measuring almost 70mA.  I tested them on my EMC model 213 tester and the meter pegs out past the good mark.

Are they bad?  Or, just really strong?

I bought them as NOS awhile ago and thought the test meant they were good, but did not try them until today.

Thanks,

Daniel

[eleventeen]

Three possible scenarios come to mind:

1: Perhaps you are making a socket-basing diagram error; recall that in most Fender (just to use one example) builds, the "incoming bias feed" is a wire which connects to pin 1 of a 6L6 [both of them will get such a wire, of course] but pin 1 is normally a dead or unused or even missing pin, and a 1500 ohm resistor runs across the socket to pin 5 the REAL control grid. Pin 1 is only used as a tie point, it is dead, nothing, a zero as far as the tube itself is concerned. So, suppose you have the 1500 ohms on a turret board instead of mounted on the tube socket. You run the wire over to the tube, pin 1. No good. It isn't connected to anything as far as the tube is concerned. It would have to connect to pin 5.

2: Another way you could be having a problem is you have completely and totally misread the tube pin numbers on the socket. I don't care how many of these things you have built, sometimes you just read the numbers wrong or count them backwards from what you (incorrectly, that day) think is pin "1".  Or get the top side confused with the bottom side.

3: Is your bias supply referenced to the same ground as your B+? It automatically is if you have a tapped power transformer like a stock Fender. The little bias supply will reference the center tap of the bigger B+/HV winding because it is a "subset" of that bigger winding whose CT is grounded in the case of a full wave non-bridge rectifier. However, suppose you employ a bridge rectifier to produce your B+, and steal your "bias tap" from one of the bridge rectifier feeds. Or suppose you use a voltage doubler for your B+. There are situations where your bias supply might not have the same ground reference as your B+ supply, in which case it won't bias the 6L6 tubes properly. You will have your -64 volts but the opposite side of that -64 volts, the "+" has to be grounded, eg; referenced to the same "0) volts as is the conventional B+ of the amp. Check your wiring.

At the risk of overgeneralizing, -64 volts on the control grid of a 6L6 should darn near shut it off under any conditions.

[dscottguitars]

None of those, but I don't fully understand #3.  It works fine with another set of tubes, that's why I am wondering about these tubes.  The set in there now has about -45v on the grid.  Here is a schematic with voltages and plate current.

Also, I don't use pin 1 as a connection point, I solder directly to pin 5.

[HotBluePlates]

...  The set in there now has about -45v on the grid.  ...

Hi,  I have a pair of GE green label 6L6GC tubes that I tried in my amp ...  One got red plates immediately and measured around 100mA, the other 70mA.  Even putting -64v on the grid still had the one measuring almost 70mA.  I tested them on my EMC model 213 tester and the meter pegs out past the good mark.

Are they bad?  Or, just really strong?

They sound like they're either shorted internally or very gassy.

You could try them in a different amp, but if you have excessive current even with over -60v of bias, the tubes are probably unusable.

[dscottguitars]

I was afraid of that.  $50 down the drain...

[HotBluePlates]

Who'd you buy them from? Ask for a refund/exchange.

If it was ebay, well, that's the danger of bidding unless the seller stated he'd tested them (and offered returns or an evaluation period).

[dscottguitars]

It was ebay and too long ago to try to get a refund.  Next time I'll try them out after I check them on my tester.  But, I don't think I'll be buying any old tubes.  I like the new Electro-Harmonix tubes for an inexpensive good sounding set.