6V6 glowing

[Ramsay]

I have a handmade tube amp based in the Gibson Electar 10 schematic whose images are shown below:

http://i223.photobucket.com/albums/d...ps0ec0d5c9.jpg
http://i223.photobucket.com/albums/d...ps7022c6ad.jpg

The amp works fine with a good sound with 6L6 or EL34, no humm or oscilations here, even with all pots full clockwise.

Only for testing, recently I purchased a brand new 6V6GT tube (Electro-Harmonix) and put it in the amp.

After 5 minutes heating the filaments, I turn on the standby switch and for my surprise, the plate of the tube is glowing a few and overheating the 6V6.

I took some measures of the amp with the 6V6:
B+ (with tubes): 383V
Vplate: 371.4V
Vrk (470 ohms): 24.5V
Vg2: 364.3V
Ik: 52.1 mA

I saw in the internet several tube amp schematics with a single 6V6 that works with more than 400V at the B+ and has a 470 ohms cathode resistor.

So I don't understand why this tube is glowing in my amp.
Using the Handy Calculator, it seems that a 1000 ohms value would be a good choice, because the cathode current drops to 24.5 mA and the plate dissipation drops to 8.9W (74%).

What do you suggest to me to solve this issue??
PS: I don't know what is the impedance of the output transformer primary.

[Justin Thomas]

The problem is that 52mA of current, making for 18W. You may just have an oddball tube; a 6V6 w. ~35mA might work with your 470R. Some have had the experience that once a tube redplates, itt will always do it because it's compromised; I don&t know, try it and see. An older USA 6V6 might get away with that kind of abuse, but prolly not a new one. The 1000R, going a bit on the cold side, is a better starting point, and maybe you can find something in the middle. If I keep any less than a 680 in my Champ, it redplates.

No trying to be snide here - just call it a lesson in considering the system as a whole... and always trust your measurements and your ears more than "common knowledge." Good luck getting it worked out, and hope your 6V6 is okay!

Justin

[GregS]

The reason a 470R cathode resistor may work in other circuits with a 6V6 is that the voltage off the PT secondary may be actually lower in the other circuits than yours. It may not appear this way on the surface but your B+ is likely being dragged down by the high current loading. It could also simply be a bad tube as mentioned above.

Greg

[Ramsay]

Thanks for the replys but:
- The 6L6 is brand new, in box, and I bought it at a shop and it works nice and don't introduces any noise on the sound.

- the glow in the plate is not so intense, but noticeable in the dark.


Perhaps, the best solution is to use a serial resistor with B+ and a additional filter capacitor to reduce voltage, But what would be the value and the dissipation of this resistor?

[GregS]

If you want to reduce the B+ the best solution is to use a zener diode to drop the voltage a fixed amount. If you use a resistor the voltage drop will vary according to the current drawn by the power amp in response to the signal. This will introduce "sag" which may not be what you want. You can use an appropriately rated zener between ground and the centre tap of your power transformer. The Weber site used to sell a kit for exactly this purpose and probably still does. If you do a search you will find many threads on how to drop B+ with a zener. It's a bit awkward responding with my phone so I haven't linked any.

Greg

[Ramsay]

If you want to reduce the B+ the best solution is to use a zener diode to drop the voltage a fixed amount. If you use a resistor the voltage drop will vary according to the current drawn by the power amp in response to the signal. This will introduce "sag" which may not be what you want. You can use an appropriately rated zener between ground and the centre tap of your power transformer. The Weber site used to sell a kit for exactly this purpose and probably still does. If you do a search you will find many threads on how to drop B+ with a zener. It's a bit awkward responding with my phone so I haven't linked any.

Greg

That is a good idea, but the 5W zeners must use heatsinks, or not?

Today I bought 2 resistors, 820/5W and 1K/5W and I will test both in the cathode to see if the glowing in the plate vanishes.

[GregS]

The type of zener I'm talking about is housed in a metal case and attached to a heatsink which is bolted to the chassis. Thermal grease is also used to dissipate heat more effectively to the chassis. The Weber site sold these kits that contained all the necessary items. I don't recall the wattage rating off hand but I think it would have been considerably more than 5W. I had to use one of these kits in a build I did a number of years ago where the output tubes were red plating.

[GregS]

The zener kit on the Weber site is called the V-Dump Zener B+ Dropper. It is a 25W zener and there is a choice of 20, 30, or 40V drop. Sorry for not providing a link but I'm not home and am struggling to reply with my phone.

Greg

[Ramsay]

Thank you, and based on what you said, I will not use zener diodes to lower the voltage because I think it is not worth it.