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running metal 6L6s in my Onix SP3?

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Posted on July 27, 2014 at 16:12:49

gkargreen
Audiophile

Posts: 1562
Location: DC
Joined: February 5, 2005

while I was re-tubing the SP3, I was wondering about the use of metal 6L6 tubes. I see in the tube manual that they, along with 6L6, 6L6G, 6L6GB are all rated at 360v max on the plates. My SP3, which calls for 5881 tubes, has 340 on the plates as measured with tubes in the socket. So, would it be taking it too close to the edge of operation to run metal 6L6 tubes? I have read that they can be among the best sounding of 6L6-type tubes... thanks

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if I measure the voltage drop across the cathode bias...

gkargreen

13:15:57 07/29/14

RE: if I measure the voltage drop across the cathode bias...

Jim McShane

14:18:49 07/29/14

RE: if I measure the voltage drop across the cathode bias...

gkargreen

18:09:06 07/29/14

...

Show full thread outline!

if I measure the voltage drop across the cathode bias...

gkargreen

13:15:57 07/29/14

RE: if I measure the voltage drop across the cathode bias...

Jim McShane

14:18:49 07/29/14

RE: if I measure the voltage drop across the cathode bias...

gkargreen

18:09:06 07/29/14

RE: if I measure the voltage drop across the cathode bias...

Jim McShane

09:17:37 07/30/14

RE: if I measure the voltage drop across the cathode bias...

gkargreen

10:11:32 07/30/14

RE: Measure your voltage on pin 5

Russ57

08:38:55 07/31/14

RE: Measure your voltage on pin 5

gkargreen

08:50:17 07/31/14

Russ57

14:46:28 08/01/14

gkargreen

14:55:29 08/01/14

RE: bias is set rather differently

Russ57

20:58:30 07/29/14

thanks, all! ...

gkargreen

17:28:46 07/28/14

RE: thanks, all! ...

drlowmu

22:03:35 07/28/14

RE: thanks, Jeff!

gkargreen

07:56:23 07/29/14

Russ57

08:33:09 07/29/14

gkargreen

13:26:16 07/29/14

Watch out for voltage on the metal case

Alpha Al

05:41:46 07/28/14

RE: Watch out for voltage on the metal case

BofService

19:38:02 07/28/14

RE: running metal 6L6s in my Onix SP3?

drlowmu

18:10:29 07/27/14

RE: running metal 6L6s in my Onix SP3?

Chip647

16:30:32 07/27/14

RE: running metal 6L6s in my Onix SP3?, posted on July 27, 2014 at 16:30:32
Chip647 Audiophile Posts: 2652 Location: The South Joined: December 24, 2012	Voltage is one spec, watt rating is another. h Ih VaMax Vg2Max PaMax Pg2Max 6.3 0.9 360 270 19.0 2.5 As long as your amp is running them below 50mA you will be just fine. Volt ratings can usually be easily violated, watt ratings can't.

RE: running metal 6L6s in my Onix SP3?, posted on July 27, 2014 at 18:10:29

drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005

The manufacturer provides you with a Maximum Plate dissipation figure, in the case of a 6L6, 19 Watts. There are two components of the plate dissipation, (the voltage and the current) "across" the tube's plate.

Across the tube's plate means just that, you measure BOTH the plate and the cathode voltage, and in your case it may be +340 VDC plate, perhaps +22 VDC at the cathode. SO, you are running 318 VDC "across the tube's plate", where 360 maximum is allowed. Fine !

What about current, the "other component" of plate dissipation? In the GE tube manual, for Class AB1 or AB2 push pull operation, we see it states 88 mA. current, and that is for TWO tubes, in Push-Pull.

So, you want to next define your 6L6's current.

It may have a milliampere meter built in, or jacks for same, so you can measure plate currents directly. If its a self-biased amp, you have a cathode self-bias resistor. Once you measure that resistor's Ohmic value, and the voltage on "top" of the resistor - not the grounded end, you can EASILY compute current by Ohm's Law. Amperes equals Voltage over Resistance.

Multiply VDC across the tube,( in your case 318 VDC ) times current, which, lets "just say" is 40 mA. ( or 0.04 A. ) per tube, and you now have the plate's actual Power dissipation, in Watts. 318 VDC times .040 A. is 12.72 Watts power dissipation across the tube's plate.

Now the fun part ....its up to YOU to decide how hard you wanna run those metal plate 6L6s !!

Run them hot, close to 19 Watts dissipation, and they will only last maybe 2,000 hours and sound thermally stressed. Run them at 40 mA. and you will have less stressed sound, and may get 10,000 hours out of them. So now, you need to ask yourself, how MANY metal 6L6s do I own, and how soon do I want to use them up?

Biasing your outputs is like the old song "Will You Still Love Me Tommorow", recall the phrase..... " Is this a lasting treasure, or just a moment's pleasure". The choice ... is yours.

If low on back-up metal 6L6s, I'd run them 38-40 mA. maximum. Hope this helps you. And I ask you "Will you still love me tommorow ??" Cheers.

Jeff Medwin

Watch out for voltage on the metal case, posted on July 28, 2014 at 05:41:46
Alpha Al Industry Professional Posts: 2958 Location: N. Carolina Joined: February 16, 2004 Contributor Since: December 3, 2015	Pin one is connected to the metal case for shielding in RF circuits. Lots of amps use pin one as a tie point for the screen resistor. In those designs, it will result in high voltage on the metal case. I found out the hard way. Ouch!

thanks, all! ..., posted on July 28, 2014 at 17:28:46

gkargreen
Audiophile

Posts: 1562
Location: DC
Joined: February 5, 2005

I went under the hood and this is what I have, the bias is fixed and should be 1.15v, which dials in at around 33.5 ohms, this gives me 34 ma, correct? Pin 1 is connected to pin 8 with a wire going to the bias adjustment, and a large (5 watt?) 33 ohm resistor from pin 6 (open) to the combined pin 1+8, bringing in the bias voltage, I am guessing. The voltage at pin 3 (plate) is around 340 v, I do not see any plate resistor as this is on a pcb, one schematic shows the plates connected directly to the output tranny, but I am not sure if this is the schematic for the MKII version I have. So it would appear that this amp is putting out around 12 watts for each tube, however it is rated at 38 watts per channel which would place the current at around 47 ma at max, so I am guessing I am OK with using the metal 6L6, although I am bumping up close to the max ratings of the metal 6L6, correct?

RE: Watch out for voltage on the metal case, posted on July 28, 2014 at 19:38:02
BofService Audiophile Posts: 741 Location: Atlanta Joined: February 28, 2003	+1 Woo Hooo!

RE: thanks, all! ..., posted on July 28, 2014 at 22:03:35

drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005

Hi,

Well, you are slightly mixed up. Not a problem.

"one schematic shows the plates connected directly to the output tranny, but I am not sure if this is the schematic for the MKII version I have. So it would appear that this amp is putting out around 12 watts for each tube, however it is rated at 38 watts per channel which would place the current at around 47 ma at max,"

(1) An Output or Finals tube ( a 6L6 ) does NOT have a plate resistor, the output transformer primary winding is its load, and ultimately, the speaker comnected to the secondary.

(2) Plate dissipation in Watts, (you estimated 12 Watts), is NOT ever the same as the amp's overall Power Output in Watts.

Others, far better than I, will fill in more to help you. Maybe I can pull Henry away from his piano, he is qualified. I only scratched the surface.

Glad to see you trying to measure and understand.

Jeff Medwin

RE: thanks, Jeff!, posted on July 29, 2014 at 07:56:23
gkargreen Audiophile Posts: 1562 Location: DC Joined: February 5, 2005	must have been a little late for me last night, of course the output transformer is the plate load for the output tubes, I don't know what I was thinking! I would love to know how the output of the amp is related to the plate dissipation of the output tubes to know whether I am measuring the right thing and getting correct information, thanks!

Try this, posted on July 29, 2014 at 08:33:09

Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001

http://www.recordcrate.net/diy/ClassRoom/ga400ac.pdf

It should help a lot. In push pull as one tube's current goes down (and so plate voltage goes up) the other tube is doing the opposite. It is only that change in voltage that is taken across the transformer into the load.

In many ways, with push-pull, the colder the tubes are biased, the closer to class B operation you are....and the more output watts you get.

if I measure the voltage drop across the cathode bias..., posted on July 29, 2014 at 13:15:57
gkargreen Audiophile Posts: 1562 Location: DC Joined: February 5, 2005	resistor, I can get the current flow thru the resistor, which would be the same as the current thru the plate and multiplying that current by the voltage across the plate will get me the wattage being put out by the tube, correct?

RE: Try this, posted on July 29, 2014 at 13:26:16
gkargreen Audiophile Posts: 1562 Location: DC Joined: February 5, 2005	thanks, Russ, BTW, is there other glass audio pdfs around? I have been trying to get some to augment my paper collection as well as have my paper copies on my computer, thanks, Randy

RE: if I measure the voltage drop across the cathode bias..., posted on July 29, 2014 at 14:18:49

Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003

Hi Gary,

Actually what you get is the cathode current which is the plate and screen currents combined (and grid current if any is present).

And the "wattage being put out" is not the same as the dissipation wattage. The tube will have to dissipate wattage (in the form of heat) of the plate voltage times the cathode current. The formula is:

Dissipation wattage = (Plate voltage - cathode voltage) x cathode current

The power output in wattage has nothing directly to do with any of this.
If this is unclear let me know and I'll try again for you.

RE: if I measure the voltage drop across the cathode bias..., posted on July 29, 2014 at 18:09:06

gkargreen
Audiophile

Posts: 1562
Location: DC
Joined: February 5, 2005

thanks, Jim, I took another look under the hood, there is a large 33 ohm resistor connected from pin 6 to joint pins 1 & 8, pin 6 is grounded to the pcb which is also grounded to the ground connection on the separate bias board. The bias board has a wire connected to the joined pin 1 & 8, the bias board has a multi-turn trim pot that is used to set the bias (1.15v) I am a bit confused by this arrangement, rather than a large bias pot, like the Cit II has, there is this little trimpot, probably a 1/2 watt max, to set the bias with, and it appears to be in parallel with the large 33 ohm cathode resistor on joined pin 1 & 8, any idea?

RE: bias is set rather differently, posted on July 29, 2014 at 20:58:30

Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001

Most amps set the output tubes bias point through one of two methods.

A) There is a resistor from cathode to ground. It may be shared between both tubes in one channel, or even all four tubes. Or there may be one for each tube. Typically there will be a bypass cap across said resistor. This is "cathode bias" and it does create some feedback so amp will have a little less power output.

B) The cathode is grounded (in some cases a low value resistor is inserted between ground and cathode to measure bias point) and negative voltage is feed to the grid of the output tube. This is called fixed bias despite the fact that it is often adjustable. You might have a pot for each tube or a pot for each pair of tubes.

Now what is different about your amp? In your amp the tubes immediately before your output tubes have their cathode's referenced to a negative voltage supply. They do have cathode resistors. The signal is taken off the cathode of those tubes (they are cathode followers). Because of the negative voltage supply this AC signal voltage is below ground potential (it is negative voltage). This keeps the grid of the output tubes negative relative to their (grounded) cathodes and sets the bias (idle current) point for the output tubes. This is different than most amps.

I don't know if any of this made sense. I could try and explain differently. It would also help if I could find an up to date schematic.

RE: if I measure the voltage drop across the cathode bias..., posted on July 30, 2014 at 09:17:37

Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003

That 33 Ohm resistor is between the tube cathode (pin 8) and pin 6 - pin 6 is not connected inside the tube, they just used it as a tie point. From there the cathode goes to ground. So it goes tube cathode pin 8 to 33 Ohm resistor to pin 6 tie point to ground.

Pin 1 is tied to the metal shell so by connecting it to pin 8 it will have just the cathode voltage (a bit over 1 volt) on it - so it's safe.

The bias adjustment setup is unclear, a schematic would be needed to completely dope it out. But while a .5 watt pot is almost always just fine for conventional fixed bias adjustment, what we don't know is what changes when we adjust the pot in your amp.

RE: if I measure the voltage drop across the cathode bias..., posted on July 30, 2014 at 10:11:32

gkargreen
Audiophile

Posts: 1562
Location: DC
Joined: February 5, 2005

thanks, Jim, that is the problem, no real schematic on this version, the current one online is I believe the V1 build. About the only thing I can guess is that the two resistors are in parallel, however if that is the case, then there is too much current passing thru the cathode to use even the recommended 5881! (1.15v across 37 ohms in parallel = 1.15/16 = .075 amps, 340v * .075 amps = 25.5 watts; someone else recommended no more than 40 ma thru the plate circuit...)

RE: Measure your voltage on pin 5, posted on July 31, 2014 at 08:38:55
Russ57 Audiophile Posts: 3754 Location: South Florida Joined: November 16, 2001	Verify adjustment pot changes pin 5 voltage. Suspect you will see a range from negative high teens to mid twenties. I assume you read my post below?

RE: Measure your voltage on pin 5, posted on July 31, 2014 at 08:50:17
gkargreen Audiophile Posts: 1562 Location: DC Joined: February 5, 2005	yes, Russ, and you are correct, the bias is coming from the grid, pin 5 shows -33v, if I remember correctly, thanks!

RE: -33??????, posted on August 1, 2014 at 14:46:28

Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001

If true that is biased really cold. On course it would depend on plate to cathode voltage. But given the 400 VDC max plate voltage rating it seems very cold to me. Perhaps they were going for maximum output wattage?

FWIW classic operating point was 360 vdc plate and 270 vdc grid 2 into a 6K6 transformer. At a grid one voltage of -22.5 each tube ran at 46.5 milliamps. Output power was 26.5 watts with a THD of 1.8%.

Easy enough to measure all of these things yourself....up to and including output transformer impedance.

RE: -33??????, posted on August 1, 2014 at 14:55:29
gkargreen Audiophile Posts: 1562 Location: DC Joined: February 5, 2005	thanks, Russ, that is very informative, I will give it another measuring this weekend and see what I have and report back. I should note that the output is rated at 38 watts/channel...

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