Redplate 6AQ5?

[jeff]

 I have a problem I can't figure out and hope your experince can help me with. I built a nice little SE 6AQ5 amp. I noticed that there is a thin red stripe when the standby is on I take the amp out of standby mode  that dissappears when I switch it off it's in standby mode.
I thought OK I must be exceding the max dissapation wattage of the tube. I looked at the data sheets and the 6AQ5 is a lower max voltage of a 6V6. It's a 12W tube so I took some voltages and the numbers don't add up.

plate                  255V
screen                235V
cathode             10.92V
grid                     0V
Cathode resistor  250 Ohms(exactaly, measured with a ohm meter)

so plate to cathode is 244.08V and current is(10.92/250=)43.68mA. Some of that is the screen current but that's still below 12W (244.08 X .04368=10.66)

I can't figure this out, what am I missing?

 What else would cause a tube to redplate beside exceeding the max plate dissapation?

Do tubes redplate when they need to be replaced? I don't have a lot of experince so I don't know what happens when a tube is on its way out.

BTW this happens even if I'm not playing through it.
   Thanks
    Jeff

[DummyLoad]

so plate to cathode is 244.08V and current is(10.92/250=)43.68mA. Some of that is the screen current but that's still below 12W (244.08 X .04368=10.66)

no, plate + g2 current (Ik) is Vrk/Rk  - in this case 10.92V/Rk

then Va-k * Ik = Pdiss  - in this case (255V-10.92V)*Ik

dartboard - looking at datasheets w/ g2 @ 250V if g1~~=Vg-k, and is @ -10.92V then Ia = ~~ 55mA would be ~~ 13.5W Pdiss.

if grid is truly @ 0V then Ia = ~~115mA = very bad news and you have 25+ watt 6AQ5.

-ISO

check the grid leak circuit - that's the hundred K or more resistor from the grid to ground.

according to datasheets you should be running more like 15V Vk to be within 90% of 12Watts.

[jeff]

so plate to cathode is 244.08V and current is(10.92/250=)43.68mA. Some of that is the screen current but that's still below 12W (244.08 X .04368=10.66)

no, plate + g2 current (Ik) is Vrk/Rk  - in this case 10.92V/Rk

I don't understand the difference between what I said and what you said.

Isn't my 10.92/250 the same as your 10.92V/Rk? I assume Rk is the cathode resistor which is in my case 250

I said some of that is srceen current so if the plate and screen current is less than 12W than the plate alone must be under 12W, no?

then Va-k * Ik = Pdiss  - in this case (255V-10.92V)*Ik

 (244.08 X .04368=10.66)

I don't get what I'm missing here. Can you explain that a different way?

[DummyLoad]

i misinterpreted your explanation - you are correct. no coffee.   

[RicharD]

>I noticed that there is a thin red stripe when the standby is on that dissappears when I switch it off.

Are those voltages measured at idle or with the amp in standby?  What kind of standby?  If you're disconnecting B+, I don't see how it can red plate with no voltage.  Quick fix = never put the amp in standby.   

"Doctor, it hurts when I hold my arm like this."
"Then don't hold your arm like that."

[jeff]

 Sorry I may have been unclear.

 When I said the standby is off I guess I meant to say when the amp is in standby mode(the switch is off/no connection).

The red stripe happens when the standby is in the play(switch is on/B+ is connected)

I got the terms confused with the switch.
All voltages at idle

[jeff]

 Thanks for the help.

I got to use a tube tester and the tube tested weak, still above the min. but weak. I'll try a new tube. I just hope the problem was with the tube and not with the amp.

[Geezer]

I have an amp with a quad of cathode biased 6AQ5A/6005W & have found they don't like to be run like a "normal" 6V6-type (at, or anywhere even close to, 100% of max pdiss).

I started with them at 100% = red plated bad & quickly.
Went down to 70% = took longer to get there, but still red plating.
Ended up @ ~~60% (~~7.2w)  to get them to behave.

I have tried both NOS 6AQ5's (Tung Sol) and 6005W's (GE) with the exact same results.

I have ~~290v plate to cathode, and am currently using a 250R for the bias on the quad....I started with a 125R.

If it still red plates with the new/different tube(s), you may have to keep increasing the Rk value until you hit the lower pdiss that keeps the tube happy.......

They still sound stellar, even @ the colder bias....a VERY sweet tube, indeed.
One of the 1st amps I built years ago was a 5E3 variant using 6AQ5's. I gave the amp to a friend, but there was always "something special" about that amp...

$.02

G

[jeff]

 The sheets say the max voltage is 250V

[jeff]

 ISOtone, I meant no offence I just didn't understand. 
I read my post again and on the cold page it sounded like I was being a jerk, sorry. I didn't mean it like that, I just didn't get it. This is why I hate using computers instead of talking to people.

[Geezer]

The sheets say the max voltage is 250V

Oh my, I guess I'd better stop using the amp!  :shocked:  (just pullin' yer chain in fun!  )

I realize I'm pushin' the envelope, but I was just giving you some ideas that may help you get your tubes "happy".
Some tubes (6v6's or EL84's, for instance) can be pushed well over their rated specs with good results (as Fender, Vox & others have proven over many years).
Other tubes don't like to be pushed, or may not be "happy" when even close to the "max" rating.......6AQ5's may be one of those tubes(?).
As I said, the tubes don't seem to be bothered/adversely affected by the lower dissipation. If I had a B+ of 400v to work with & had to run the tubes @ 25% of max, it wouldn't bother me at all IF they still sounded this good.
I tend to use the data sheets as guidelines, not gospel, & if I can make what I have "in hand" work, I will, no matter whether the data sheets or popular thinking agrees.

Respectfully, Geezer

[Geezer]

One of the 1st amps I built years ago was a 5E3 variant using 6AQ5's. I gave the amp to a friend, but there was always "something special" about that amp...

Just remembered....That amp was a rebuild of a Bogen PA head that ran the tubes with well over 300v on the plates (as all bogens tend to do....push to get the max power out of the poor little tubes).
I just re-used the existing power amp section, so I don't remember what the Rk value was, but the amp is still going strong today.

G

[jeff]

 OK I'm still a noob and that kinda stuff scares me, But like you said Fenders been doing it for years.

I just meant maybe that's why yours was redplating and that's why you had to bias cooler. But mine is well within specs. I'll try a new tube and get back to ya.

           Thanks
            Jeff

[DummyLoad]

ISOtone, I meant no offence I just didn't understand.

i did not interpret your response as offensive.

This is why I hate using computers instead of talking to people.

i deal with resolving technical matters via written word as part of my job (network engineer) and this is not new to me, so no worries. :-)

perhaps the tube is defective or there is a problem with the grid circuit. i remember (re)building a bogen K-10 with them and they were running over 300V to the plates. 

EDIT: found old info:

some more info that may help. this is from the bogen K-10 i hacked a while back.

A = 323V
B = 312V
C = 272V
D = 173V

V2a bias = 1.62V
V2b bias = 1.62V

V4,5 bias =  22.2V

schematic:
http://www.el34world.com/Forum/index.php?action=dlattach;topic=7404.0;attach=10216

link 2 tread:
http://www.el34world.com/Forum/index.php?topic=7404.0

IIRC, i left the PS, paraphase inverter, and output stages untouched. with -22V VRk, bias seemed kind of cold but the amp worked very well. i remember the 6AQ5 as being a cantankerous tube. i sold it for a concert tix. + beer monies.

HTH

--ISO

[Geezer]

i remember (re)building a bogen K-10 with them and they were running over 300V to the plates.

Iso, that's the exact amp I re-did & gave to my friend, so it's good to confirm that I remembered correctly that the B+ was over 300v.

G

[jojokeo]

OK I'm still a noob and that kinda stuff scares me, But like you said Fenders been doing it for years.

I just meant maybe that's why yours was redplating and that's why you had to bias cooler. But mine is well within specs. I'll try a new tube and get back to ya.

           Thanks
            Jeff

Jeff, just curious if you figured out what your power dissipation is for the screen? It should be okay I would think but I'm curious. Going by max screen diss @ 2 watts/235v your current should be under 8.5mA if tested there.

[DummyLoad]

Iso, that's the exact amp I re-did & gave to my friend, so it's good to confirm that I remembered correctly that the B+ was over 300v.

i thought the same thing. i had to confirm - senility prevention & protection, doncha' kno.