Heptode as mixer

[Arthur Dent]

I would really like to know how to properly set up a heptode as a mixer, specifically the 6BE6. When I tried it with both control grids at 0vdc and a normal cathode-bias arrangement, the signal associated with g3 came out grossly asymmetrical.

So, how is this *supposed* to be done?

[Enzo]

6BE6 is a pentagrid converter tube. Read the page in the RCA book about it.

What do you mean set it up as a mixer? And what do you mean the signal asociated with g3 came out. Grids are generally where something goes in rather than comes out.

This tube was used in frequency conversion stages of radio receivers.

Just what are you trying to do with it?


Draw us your circuit at least.

[Arthur Dent]

I mean using both g1 and g3 as control grids, each with its own input.

g1 gets the input from one preamp, g3 gets input from another preamp, g2 and g4 are both used as screen grids (+), g5 is the suppressor attached to cathode.

Oh, and I said the signal *associated* with g3. You put a normal signal into g3, you get an asymmetrically clipped signal out of the plate. Put the same signal into g1, and it comes out (at the plate) normal and pleasant-sounding.

The 6BE6 was designed to do the combined functions of a mixer and oscillator in radio stuff. I've found it also makes quite a nice sound when used in the same way as an EF86 or 6AU6, strapped as a pentode in a preamp. Similar tubes have been successfully used as mixers in audio amps (6L7 for example), and I know it's possible with this one as well. I would simply like to know how it's done.

I'm guessing I'll have to have different bias voltages at each control grid.

[Rob Mercure]

The 6L7 was specifically designed as a "pentagrid mixer" and not a pentagrid converter the intended use was audio mixing and control. The tube was featured in several compressor schematics and I've built a couple - one using a 6H6 and the other substituting SS diodes for the 6H6. For some reason, possibly pyschological, I likes the 6H6 action better but I couldn't easily double-blind myself <grin>. While the 6L7 is a bit larger and more awkward with the grid cap it really does a superior job of audio control.

Rob

[Enzo]

The 6H6 wins all battles for cool tube in my book. That short little metal body just makes it for me.

[truth_hz]

I ended up using a heptode in my most recent build. At first I tried a 6BE6, But I later switched to a 6CS6. I never bothered to scope the thing but it sounds sweet. I have grid1 being fed by a pentode pre. grid3 is just a second input now, but I'm going to try running a triode stage into it. not quite enough drive for the el-84 on its own.

If you look in the datasheets the various heptodes have quite a range of transcondance between grids. for example the grid1 to plate transcondance is much higher than grid3 to plate in the 6BE6. I have never worked with heptodes before this, but it must play a role in how they sound.

I would love to learn how to use 'em properly, but my result sounds good. they sound great strapped as a pentode too.

my build thread:
http://music-electronics-forum.com/s...ead.php?t=8055

[Arthur Dent]

Strange, your circuit is essentially the same as mine, but without the other preamp feeding g3. It didn't quite work for me.

Oh well, this amp sounds great now with simple resistive mixing. Each preamp feeds a volume control, a 220k series resistor, and then the grid on a 6BA6. in the future, I'll probably use a dual triode, connect plate and cathode at the socket, and have each preamp feed a triode's grid when I need to mix channels.

It's good to see someone else using awesome tubes like the 6AU6, 6BA6, and 6BE6. The 6X4 is a pretty cool recto, too. If you want to try something interesting, use grid-leak bias on that first stage. Put a DC blocking cap in series with the input jack, ground the cathode directly, and replace that 500k R with a 10M. It sounds with a pentode.