Cathode bias 125% of max dissipation :·O

[deci belle]

That's 17.5w …jj 6v6s are rated at 14w.

I've never seen red-plating, and I don't know if I'm seeing red-plating.

I have a 380v 5E3 PT. I just put in a 5ar4 and the jj 6v6s. The rectifier is putting out 422v. Plates are at 410v.

I made measurements and did the calculations …then shut it off quick!! haha!!

Any sage advice? I know I can go over the dissipation rating with cathode bias— but this seems a bit much!

Seems like I need to wait until I hear from the experts before I turn it on again!!

Thank you!

[pdf64]

If it's based on a 5E3, the rectifier should be a 5Y3 rather than a 5AR4. With the correct rectifier, plate voltage / dissipation should fall into line.
Pete.

[Enzo]

Perhaps if you told us what the amp was...

For example, if it is cathode biased, did you subtract the cathode voltage from the plate voltage before calculating?

Is the amp one that is meant to run in class A?

"Red plating" is simple, it means the plate structure is physically getting red hot. Not the glowing orange heater in the middle, they all have that. I mean the gray metal structure of the tube innards gets red hot. Not the whole thing, but usually a vertical strip along the sides. I know ther are pictures of it on the internet.

[deci belle]

Ah… thanks for the description, Enzo! It's class AB and it wasn't red-plating idling at 17.5w dissipation (I didn't dare play the amp, though)!

It is a 5e3 through and through. It does sound properly delirious with NOS 5y3 and USA-made 6v6s— and I even found an RCA ay7, Pete! I've been playing it this way for about 6 weeks (373v @ plate; 13.4w dissipation= 95.7%).

I put those jj6v6s in because I knew they could take the voltage the 5ar4 would make. I wanted to see what those voltages would be with that rather high 380v tweed deluxe PT. (I'm just fiddling around in anticipation of getting a 350v 200ma PT, so I can also run 6L6s …I just wanted to see what the Sovtek 5ar4 would do).

Since It's my first cathode-bias amp build, I had no idea where the bias would be with those high voltages. I thought I'd have around 465v~ and was surprised to have only 422v at the rectifier output (I have 100uF caps in series switched in when the 5ar4 is fitted).

With the 380v PT, the 5ar4 and the JJs: cathode resistor is 267.5Ω and the volts across it are 24.3v, resulting in .091 idle current.

410v on the plates (–) 24.3v across cathode resistor = 385.7 plate to cathode volts.

385.7v x .091 idle current = 35.1 ÷ 2 = 17.55w dissipation. Ooops! I forgot to subtract 1w for the screen… so that's really 16.55w.

So that's still about 118% of the JJ's 14w max dissipation. I don't need to be daring, nor do I believe running this hot is even something to hear~ I just don't know if I should hear it!

That's why I was wondering if sane folk would have ever bothered to bias the JJ6v6s that hot on purpose. If that's not the case, it's fine with me. I won't go there too!

[Jazz P Bass]

You can set the idle bias wherever you want.
But you need to answer a few questions.
Do you want the tubes to last?
(If yes, run them cold.)
Do you want asymetrical clipping? (or some other odd tube behaviour.)
(If yes, run them hot.)
Ideally, the whole idea behind bias is to set the tube(s) on the linear portion of the characteristic curve.
(if that is what you want)
It used to be referred to as 'operating point'.
But alas, somehow that particular term got lost.

[kleuck]

JJ 6V6 can probably take even more dissipation, as their plates are 6L6 plates.
They are not 6V6, but 6L6 modified for a lower heater current.I suppose that -except for the max voltage- they just copied the datas from an old RCA datasheet , just like they all do (and have been doing for years : compare 6L6GT -19 watts- and 6L6GC -25 to 30 watts- you'll see that maximas are different, but curves are the same -too short to be useful most of the times- examples are just the same -way lower power than that can actually be achieved-)

[deci belle]

Oh ok! I bet I can run the JJs with the 5ar4 then. Compared to the USA kind 6v6 tubes I have, when I have the 5y3 in~ the JJ6v6s don't …pretty poor~ well, just boomy and prone to feedback, which I don't prefer. So I won't feel too bad just letting the JJs run with the 5ar4 for laughs.

They'll be running cool at 92.5% (19w) or frigid at 70% (25w) max dissipation for a cathode biased 6L6. ick!

Thank you kleuck!

[deci belle]

It used to be referred to as 'operating point'.
But alas, somehow that particular term got lost.

haha!! Thanks, Jazz!!

[kleuck]

Note that it's not necessarily fully safe : i do not understand the whole structure of the tube, but higher dissipation can perhaps stress the cathode (after all, it's the cathode that's provides electrons) and i don't know the true max dissipation of the screen-grid for the JJ (2 watts for a true 6V6) but with such voltages, you need big limiters on the screens.

[defaced]

(after all, it's the cathode that's provides electrons)

During emission, the cathode gets about 30% of the heat, the anode (plate) gets about 70% of the heat. The energy of the electrons smashing into the anode is what causes most of the heat. The same basic physics that governs a tube is what governs a welding arc which is where the 30/70 heat distribution numbers come from.

[deci belle]

defaced said:

The energy of the electrons smashing into the anode is what causes most of the heat. The same basic physics that governs a tube is what governs a welding arc which is where the 30/70 heat distribution numbers come from.

I'm keeping an eye and a temp-gun handy and I like the basic physics you and kleuck described!!

Nothing's gotten hot yet, kleuck~ what you told me just gave me enough to take it a little further than I would've on my own— much appreciated! I have 470Ω screen resistors on the sockets.

I tried the 5ar4 with the JJ6v6s and it sounds GREAT!! Lots of chime, but nothing as subtle that can be achieved with the 5y3 and USA 6v6 tubes. The thick raunch-factor is really on tap with the JJ/ar4, but I can still dial down to get a nice ragtime jazz piece articulated on an es175. So it's not bad at all.

I'm glad to know it seems to be do-able, after all. But if something fails, I'll bring this thread up to date.

Thanks for all the responses everyone