Cheap TV Tube Pentode/Triodes in ODS Topology

[David Root]

For a while I have had a bee in my bonnet about these old combination pentode/triode tubes as potential alternatives to +$100 NOS 12AX7s. NOS NIB TV tubes can be had for $5-$10.

So I did some research. Object was to find tube(s) that had pentode plate and screen dissipation & plate resistance close to a 5879 and the triode be close to a 12AX7. I.E. close to a known good sounding small signal pentode and D-style triode.

I decided to use the ODS topology because I'm familiar with it and I know what it sounds like so I have a benchmark sound.

I'm posting this here instead of the Dumble Discussion because I think it is potentially of more general interest.

Instead of two triode stages I ran the pentode into the triode as a cathode follower for both clean and OD, but the OD pentode is bootstrapped by +ve feedback from the CF cathode. This works because the CF gain is <1. The pentode's gain boost is controlled by the OD drive pot. With the 68K gridstopper on the 6JW8 pentode, and the cathode follower & its feedback the idea was to get some good sustain going.

I built it in a CE three hole #124 chassis and damn it works pretty good! MAJOR kudos to Merlin Blencowe for his preamp design book, it made this exercise much quicker and easier and more accurate than it otherwise would have been. Not to mention revealing the bootstrapping which I had no prior knowledge of. I just couldn't resist doing it with a pentode because Merlin writes that it might be too much uncontrollable gain with a pentode!

I chose the 6CM8 for V1, clean stages, and the 6JW8/ECF802 for the OD stages. Both pinouts are the same except pins 3 & 7 functions are reversed on those pins. The 6CM8 is set up for clean centered wide swing with 325V B+, the 6JW8 is at 207V B+ with a bit lower screen and bias.

The PI and power end are straight D-style, sort of 4th gen all in all. I used an existing generic D-style main board and just modified the layout to suit. The TV tubes I oriented in the chassis so the heater pins were next to the chassis apron. This made the lead dress on these tubes less than optimal as you can see in the gut shot. Schem ofthe TV tubes setup also attached.

[martin manning]

Nice work David, and a very interesting experiment! One question, though: your V2 B+ is 118V lower than V1?

[David Root]

That's right Martin. The 6JW8 is 250V plate max, and I'm not looking for superwide swing anyway, although it's still good. So I picked about 200V B+ to keep the triode plate happy. Doesn't need to be higher than V1 to get the gain structure I was looking for from the bootstrapping with that tube.

Sure the convention is that V2 B+ is higher than V1 inthe dropping string but it's just a convention, not a commandment. So long as the filtering is adequate which it seems to be.

It sounds pretty Dumbley, in fact it doesn't seem to sound "pentodey" at all in the clean mode, and only maybe a little bit in OD mode.

I tried dropping the cathode resistor down to get the bias warmer on the 6JW8, went down to 0.86V, 390R (half the original 680R) and it dirtied up. But the gain went up and made it more difficult to balance the clean level with the OD level. Doesn't seem to need snubbers in the OD at 680R. If I crank the Drive pot to 10 it gets a bit gnarly but that just means I got the the entrance trimpot up maybe a bit high. Might try dropping the screen voltage some too.

I also checked out Jana's tip about a small bypass cap on the V1 supply node for more note clarity, hifi style. That's a .015 uF 715P 1200V tacked in there. I'll have to look at that again tomorrow, didn't notice anything at a quick look.

[Regan]

Ahhhh, that's the secret amp you been cooking up!
Sounds really interesting David, hope to hear it sometime.
I do believe I have several of both those tubes too, hmmmmm!

[gingertube]

For Info:

Cheers,
Ian

[RJ Guitars]

Very cool work David, thanks for posting it up. I have an affection for using odd tubes in guitar amps so this build is especially interesting to me. We'll see how long it takes for the market on these tubes to heat up??

[Phil_S]

Post of the week! And it's only Sunday!