DC in grids of output tubes in Rocket.

[Mark]

I noticed that I have 12mV and 8mV on the grids of the EL-84's in my Rocket. I have isolated the output stage by removing the PI stage entirely.

One way around the issue is to increase cathode bias resister from 50 to 62 or higher.

Any other thoughts or options?

[martin manning]

You could reduce the grid leak resistors.

[Mark]

Thanks Martin.

[hans-jörg]

Hi,
Martin, do you mean to reduce the 5k6 grid leaks at the EL84. Maybe a stupid question, but I`m not shur if I understand - although I´m not the main questioner. But very interesting.

Best,
Hans-Jörg

[martin manning]

No, the 220k's going from the grids to ground.

[hans-jörg]

No, the 220k's going from the grids to ground.

Ups got it.

Thanks

[Andy Le Blanc]

that family of output tubes has a G1 current, 6v6 and 6l6 don't.
I ran into it on a non-related build, it took me a while to figure it out and then I found statements in a
Crowhurst ref. about it, I think a HIFI book. I was using a paraphase inverter,
the slightest grid DC would play hell with the inverter...

Its natural to the tube type, el84, el34... and a few others under some operating conditions.

[Mark]

So what is your recommendation?

[gingertube]

8mV and 12mv - just ignore it.

With a cathode (auto) biased amp anything up to say 300mV is acceptable.
In a fixed biased amp you want say no more than 100mV

These figures are my estimates for EL84 with a total bias voltage of around 12V. You can scale them (for other tubes) according to total bias volts. The values are on the CONSERVATIVE side.

This voltage is developed across the grid leak resistor Rg1, as a result of grid current flowing to ground thru' that resistor.

The slight +ve voltage at the grid SUBTRACTS from the total bias voltage (Vg1k) causing a little more idle current.

In a cathode (auto) biased circuit this is partly opposed by an increase in bias with the increase in idle current.

In a fixed biased amp there is no opposing mechanism, that is why I said 100mV max. above and is exactly why the max Rg1 allowable value for cathode(auto) bias is always higher than the max Rg1 value for fixed bias.

Using a higher cathode resistor reduced the anode current which also results in a reduction in the grid current, hence lower grid voltages.
In this case the original cathode bias resistor is fine.

Cheers,
Ian

[hans-jörg]

Hi,
just as a drop in: with a PPIMV you can raise or lower the grid voltage perfect

Hans-Jörg

[Mark]

Hi Ian

Sorry to take so long to get back to you, I've just returned from holidays.

The DC voltage on the 50 ohm resistor is only about 9 volts. Perhaps I have it wrong, but my perception is that a DC voltage on the grids is a sign that the bias isn't working as it should. If I were to have a set of valves in the amp that would have 12VDC across the 50 ohm resistor then my voltage on the grids would be much higher.

I had thought of using a 62 ohm resistor as the amp would run a bit cooler and have a little more headroom to boot. I know this isn't per the Rocket schematic.

The thoughts that cross my mind with the build is the grid earth connection okay, and is the cathode cap a little leaky?

EDIT: here is more of the story as it develops.

http://ampgarage.com/forum/viewtopic.php?t=23368