Who said it was a good idea to try pentode!

[alexmoose]

hey, I just yesterday tried to wire my EL34 amp as a pentode amplfier, and I was a little puzzled by the resuslts.

I let the amp heat-up in standby move for a good minute while I took a picture of the amp, I flicked the switch to let the B+ into the amplifier, and I heard a pop through the speakers it wasnt a bad sounding pop, but it was rather loud, although I think it was the same pop I hear everytime I turn it off standby, even when the amp was triode-strapped.

well I flicked the switch, and saw blue glow where indicated in the picture on both power tubes, there was no arching, and no sound coming out of the speaker (the volume was turned al the way down).

I saw this, and got the crap scared out of me and turned the amp off after about 5 seconds, I didn't dare turn up the volume in that time

is this in anyway, shape or form, normal?

[ethermion]

Blue glow is fine. No worries. Flick back the juice and give it some tunes.

[Giaime]

I assume that you DID the usual measures when converting an amp from triode to pentode mode:

• re-set the bias, pentode mode will draw higher current
• protect the screen grid with a 1k resistor

I can't understand from your picture if the glow was near the screen grid (bad) or near the glass (normal).

[tubelab.com]

Every one of the new production EL-34's that I have has some blue glow inside them. The JJ's (I have tried 4) have a blue glow on the inside of the plate that is visible through the holes in the plate. I am running them near (but not over) the dissipation limit. The glow is present in triode, UL and pentode mode. If I run the amp at full volume the glow dances to the music. The Shuguang tubes also glow, but it is not as obvious. Two of the Svetlanas have a relatively bright blue glow, the third also has an orange glow from some of the grid wires (bad) and the fourth has both the blue glow, and the orange glow, and it randomly arcs (real bad).

I have an old pair of Mullards, that have no visible blue in them in the same amp.

[alexmoose]

Originally posted by Giaime
I assume that you DID the usual measures when converting an amp from triode to pentode mode:

• re-set the bias, pentode mode will draw higher current
• protect the screen grid with a 1k resistor

I can't understand from your picture if the glow was near the screen grid (bad) or near the glass (normal).

It was near the screen grid....

I am pretty sure my bias was set for pentode mode, but I will double check

I didn't protect the screen gird... uh oh!
what exactly do i do? put a 1k resistor between the B+ and the grid?

[tubelab.com]

what exactly do i do? put a 1k resistor between the B+ and the grid?

YES. The original Mullard literature states that 1K ohms in series with the screen grid gives the lowest distortion in Ultralinear mode. I found that I like the sound better with a lower value, like 330 ohms. I have the resistor mounted right at the socket pins. It is in the circuit in all 3 modes.

[alexmoose]

alright, should be easy enough, what does the resistor actually do? keep it from oscillating? or does it just simply lower the potential?

[ghpicard]

It limits screen grid current to keep screen grid dissipation within the maximum limits.

[Tubes4e4]

Hi ghpicard,

regarding the screen resistor in UL mode:

Originally posted by ghpicard
It limits screen grid current to keep screen grid dissipation within the maximum limits.

Nope. Just apply Ohms law to your statement in the context of the the circuit and you will see that your statement is wrong.

Except, you mis-use an intentionally Pd-underated resistor as some sort of fuse

Tom

[alexmoose]

Originally posted by Tubes4e4
Hi ghpicard,

regarding the screen resistor in UL mode:



Nope. Just apply Ohms law to your statement in the context of the the circuit and you will see that your statement is wrong.

Except, you mis-use an intentionally Pd-underated resistor as some sort of fuse

Tom

W=IV
V=IR

any drop in V will be made up in more I, right?