EL34 SE pin 1 ground ?

[dobrota]

Is it possible to ground pin 1 (grid 3) on EL34 SE power amp instead of connects it to pin 8 (cathode) without problems ?

[ThermionicScott]

No harm in it, and the more negative the suppressor grid is in relation to the plate, the better it'll do its job.

(Of course, if you already had the cathode grounded, there will be no change, but I'm assuming your amp is cathode-biased.)

- Scott

[JHow]

I think in many pushpull designs, both the cathode and the suppressor grid are tied to ground. Is the question pertaining to a situation like cathode bias where cathode is elevated potential and you take suppressor to zero?

[daz]

Thats how i have mine on my PP el34 amp. But what happens if you DON'T tie 1 to the cathode in a fixed bias amp? Where would it then go?

[Wilder Amplification]

It would go to ground, and I feel that's the best place for it. In a cathode biased amp without a negative voltage supply, ground is about as negative as you'll get it (relative to the cathode). That's because ground = 0 volts whereas the cathode resistor lifts the cathode above ground by a certain amount of positive voltage via the voltage drop across the cathode resistor caused by the cathode current that flows through it.

[daz]

It would go to ground

Well, then it IS tied to the cathode either way since the cathode is grounded. So i guess the only time this issue is present is in a cathode biased amp. I wasn't sure.

[Wilder Amplification]

So i guess the only time this issue is present is in a cathode biased amp. I wasn't sure.

That is correct. However, you can also do what Traynor did and tie it to the bias supply, which makes it more negative than the cathode by whatever the bias supply voltage is. Supposedly it's supposed to make the tube operate more efficiently and increase tube life, but I've never seen any 100% factual test data that supports this theory.

[Joey Voltage]

That is correct. However, you can also do what Traynor did and tie it to the bias supply, which makes it more negative than the cathode by whatever the bias supply voltage is. Supposedly it's supposed to make the tube operate more efficiently and increase tube life, but I've never seen any 100% factual test data that supports this theory.

Well I *think* the idea is that tying the Suppressor grid to a negative voltage will possibly make it do a better job at preventing secondary electrons from reaching the Screen grid, which might be more significant in the case where the grid may be driven positive, and your anode voltage is low. Nothing I have given much thought to to be honest

It has also been suggested that in the case where the control grid lost it's bias voltage, that tying the suppressor grid to a - voltage may help "fill in" so to speak ( help repel the rushing electrons) more so than it would tied to ground. although it is a much coarser screen, and I'm not convinced or sure on how likely this scenario happens to be honest. (losing your bias voltage that is)

If I'm wrong I'm sure someone will pipe in.

[Steve Conner]

On the old Ampage, I did some experiments to test this. I used a variable voltage power supply to run an EL34 at conditions similar to the signal peak, with a nominal screen voltage, the control grid connected to the cathode, low plate voltage and heavy plate current. I then applied a variable voltage to the suppressor grid, to see what voltage gave the best suppression (highest plate current, lowest screen current)

The answer, believe it or not, was about 0 volts with respect to the cathode. Both positive and negative voltages reduced the performance slightly.

We concluded that Mullard/Philips probably knew a thing or two about designing pentodes, and there was really no point tying the suppressor grid to anything else but the cathode.

[dobrota]

Thanks for your answers. I want to build small guitar SE power amp for using various tubes in octal socket. This way, in my circuit pin 1 is connect to ground - NC or ground for 6L6, NC for 6V6, G3 for EL34 - all is OK. And I can use pin 1 as ground for solder other side resistor and capacitor from cathode, other NC pin 6 for solder grid 2 resistor.

[Merlinb]

Making the suppressor negative with respect to the cathode will increase screen current at the expense of anode current. I know some people have used this as a trick to stop redplating in amps which run the EL34s rather close to the limit. Of course, you gotta hope the screen doesn't burn out instead!

[Wilder Amplification]

Making the suppressor negative with respect to the cathode will increase screen current at the expense of anode current. I know some people have used this as a trick to stop redplating in amps which run the EL34s rather close to the limit. Of course, you gotta hope the screen doesn't burn out instead!

Merlin, thanks so much for clarifying that. I know the whole thing didn't make sense at all seeing as how electrons are trying to get to the plate and would then be encountering a much stronger repelling force at the suppressor grid, forcing more electrons to the screen than would go to the screen if it were tied to the cathode.

[jbrew73]

Making the suppressor negative with respect to the cathode will increase screen current at the expense of anode current. I know some people have used this as a trick to stop redplating in amps which run the EL34s rather close to the limit. Of course, you gotta hope the screen doesn't burn out instead!

merlin, if you don't mind could you go a little deeper in the theory involved here? i'm designing a tube bass amp and was thinking about tying pin 1 to neg. supply but now you have me rethinking my design.

[Joey Voltage]

Making the suppressor negative with respect to the cathode will increase screen current at the expense of anode current. I know some people have used this as a trick to stop redplating in amps which run the EL34s rather close to the limit. Of course, you gotta hope the screen doesn't burn out instead!

seems to be somewhat of a double edged sword I suppose

[Merlinb]

merlin, if you don't mind could you go a little deeper in the theory involved here? i'm designing a tube bass amp and was thinking about tying pin 1 to neg. supply but now you have me rethinking my design.

Not much more to say really! Essentially, the cathode current of a pentode is substantially constant for a given bias voltage. Most of it comes from the anode, but some comes from the screen, and while the anode voltage is greater than the knee voltage the ratio of anode current to screen current is more-or-less constant (typically about 5-7 for tetrodes, 8-10 for pentodes). This ratio is determined by the suppressor voltage. As you make the suppressor more negative you increase the ratio (no, I don't know by how much- I have never looked into it!) so you get less anode current but more screen current.
In terms of what comes out of the speaker, I would guess it is approximately equivalent to lowering the screen voltage by some proportional amount.

As Joey observed, although it reduces peak anode and screen currents, the quiescent screen current increases. With a new build you can take this into account by just using larger screen stoppers (1.5k?) or something. Eventually I will look into exactly how more "at risk" the screen is, but that's an experiment for another day.