Octal 6g3

[jbones1]

Hey guys,
So this amp has been sitting for a while. But it is off to a new home in a couple weeks and I am making a couple tweaks. One question I have is about setting up the bias one final time. I'm running a pair of NOS RCA 6l6gb tubes. After reading some Internet horror stories this morning I'm starting to wonder if I'm running them too hot. According to my specs below I've got 410v or so on the plates (I've read about 350v being max?) and I've landed on 40ma or so by ear. Am I in the ballpark here? Asking for trouble? Thoughts? I'd like this amp to run for a good long while for the new owner and not eat tubes for breakfast! :/
Thanks a bunch!
-Jon

Here are all the tube pin voltage readings:

V1 (6sl7)
1. 1mv
2. 191.7v
3. 1.65v
4. 3mv
5. 183.1v
6. 1.54v
7. 3.2vac
8. 3.2vac

V2 (6sc7)
1. NC
2. 243v
3. 16.7v
4. 17v
5. 237v
6. 23.6v
7. 3.2vac
8. 3.2vac

V3 (6l6gb)
1. 168mv
2. 3.2vac
3. 407v
4. 407v
5. 38.5v
6. 38.5v
7. 3.2vac
8. 46mv

V4 (6l6gb)
1. 65mv
2. 3.2vac
3. 403.2v
4. 406.8v
5. 38.5v
6. 38.5v
7. 3.2vac
8. 45mv

I'm getting about 720vac from the power transformer which rectified down and filtered is giving me 425v before it hits the power tubes. I have the bias set at 45mv at 410v = 18watts per tube?

Again, I am a rote beginner at some of this stuff although I do have a few kit builds under my belt! I'd really like to get this thing running better and at a better volume! Any thoughts?[/QUOTE]

[g1]

A real NOS 6L6GB is rated 19W max., you are running 18W so yes, those tubes are biased very hot. Considering you are already way over the max plate voltage, you should really run them a lot cooler, probably somewhere in the 10 to 12 watt range.

[jbones1]

Maybe bump it down to 35ma or so? Thats still 14w. If I get it down to cold it doesn't sound very good. But I don't want to be popping tubes all the time either...:/
Wondering if I'd better off dropping the plate voltage a bit? Perhaps a different rectifier tube?

[g1]

Yes, you will probably want to drop the plate voltage, because as you cool the idle current, the B+ will go up. So you will end up with even more than 410V on the plates.

[jbones1]

Yes, you will probably want to drop the plate voltage, because as you cool the idle current, the B+ will go up. So you will end up with even more than 410V on the plates.

Ok, yes, exactly.
So @35ma I'm getting a plate V around 450...
So what's my best bet for dropping the plate voltage?

[jbones1]

Any thoughts on dropping a 5u4gb in for the 5ar4 that is currently in the amp. More sag ( which would be ok).

[sgelectric]

A 5U4gb should be fine seeing as your 5V winding is rated at 3 amps.

SG

[jbones1]

A 5U4gb should be fine seeing as your 5V winding is rated at 3 amps.

SG

I tried the 5u4. It only gave me about 20v less B+ and I don't like the quick power up of this tube. :/
I'm thinking I'll go back to the 5ar4 and swap the power tubes out for 6l6gc's and be done with it.
-Jon