Power pentodes as preamp tubes?

[J M Fahey]

Same here
Think of this as a collaborative effort
Love this Forum.

[Austin]

So say I configure my input pentodes as triodes by connecting the screens to the plates with 100 ohm resistors just to get it working, if I add screen supply afterwards would I then maybe have too much gain?

[Alan0354]

Thank you Alan, I am familiar with noise theory. I was just adding to the bulk of suggestions, not trying to out-do yours.

Sorry I came out wrong. I talked about this mainly because you specifically mention base spread resistance which has nothing to do with 1/f noise that is the main concern at audio frequency.

[J M Fahey]

So say I configure my input pentodes as triodes by connecting the screens to the plates with 100 ohm resistors just to get it working, if I add screen supply afterwards would I then maybe have too much gain?

Dear Austin.
If it's a "Metal" amp, there is *never* such a thing as "too much gain".

[Austin]

Dear Austin.
If it's a "Metal" amp, there is *never* such a thing as "too much gain".

So I should stick with triode mode. Just kidding I like metal sometimes when I hit "play all" and the "random" button is ticked... I crank it up a little bit just to test out my system before hitting the "next" button.

[J M Fahey]

Because of your nickname and Avatar, I *never* thought you were a "metal" guy

[jbefumo]

A bit late to this thread, but I encountered it by wondering if I could use a 6BM8 as a preamp tube. Not trying just to be different. the 6BM8 is, to my knowledge, the only pentode/triode back in current production. I'm using a pair of them in a lunchbox amp with the triode sections (5751 Equiv, I believe) for phase inverter, and the pentode sections (EL82 equiv) in push pull, fixed bias, for around 12W power section. Have used EF86s with good results in several builds (following Merlin's guidelines closely), but obviously, pentode and triode in a single socket would be an advantage. Hence, using the 6BM8 would be advantageous if it sounds good, and can be configured so as not to stress the power supply too much. Were it not for the space issue, I would be inclined to use them in multiple spots, i.e., in the first stage for the touch sensitivity, in the OD for its driven tone.

Joe

[jbefumo]

I actually have a reason for investigating this question, other than just trying to be different 8^) I've used a variation on the Dumble OD scheme (i.e., a 2-stage bypassable overdrive circuit), in which the second stage is an EF86 (configured for relatively low gain), and like the sound. A bunch of guys in another forum do something similar with triode/small-signal-pentode tubes, to reduce part count. Unfortunately, there are no such tubes in current production, and I prefer to avoid designing to obsolete components. The only current production triode-pentode I know of is the 6BM8, which is a high-mu triode and a power pentode (they work great as a 1-tube reverb-driver/recovery, btw). I guess the only way to judge the tone will be to go ahead and try, but would be interested in any contraindications before wasting time ....

[Enzo]

Tube circuits are SOOOO lacking in criticality. Try it, even if it doesn't "work" on paper, it may well work fine in your application. If my mom liked driving around in a cement mixer, she could use one to go shopping, even if it didn't "make sense". We have working circuits with 12v on triode plates, we have TV sweep tubes in hifi amps, you can "abuse" tubes all over the place and they still function. it either sounds pleasing or it doesn't. Give yourself permission to screw around with it.

As to obsolete components, there are plenty of small pentodes NOS that will be in large supply for years to come. Certain ones commonly used in amplifiers may have been snatched up already, but unless you are going into production, a box of say 10 of some odd type ought to be a lifetime supply for a one-off amp, obsolete or not.

I haven't bought tubes in a while, apparently Sovtek gave up on the 7199. triode-pentode.

Considering cost of the new 6BM8, I might trade a couple parts counts for saving $20 per tube.

[Gnobuddy]

A bunch of guys in another forum do something similar with triode/small-signal-pentode tubes, to reduce part count.

Would you be willing to share more details - name of forum, name of thread, or even better, a direct link to relevant thread(s)? It would be much appreciated.

I have been tinkering with 6JW8 (triode pentodes) in guitar preamps, with promising results, but there are still some issues to iron out. It would be interesting to see if anyone else is using the same valve, and if so, what sort of circuits they're building around it.

Unfortunately, there are no such tubes in current production

In the USA you can still get some NOS triode-pentodes for $1 USD each. Often they have much-sought-after brand names on them. But these valves were usually originally intended for use in (valve) TVs, so the Hi-Fi and guitarist crowds don't want them.

But a small-signal triode is a small-signal triode; it doesn't matter what it was originally designed to do, it will amplify audio! And a properly used preamp tube will probably last a couple of decades, so it's pretty likely that if I have one or two in stock, they will outlast me.

Another way to look at it: buy twenty NOS $1 triode-pentodes, and you can have twenty times as much fun experimenting as you would get from one new-production $20 valve!

-Gnobuddy

[Mick Bailey]

The 6BM8 works fine configured as a small signal amplifier. I've used this tube a fair bit in hi-fi builds as a preamp/driver tube. There's also a commercial amp from the 60s, the Telewatt V-112, that used the tube as the final stage of a preamp (pentode side) and PI (triode side).

The real bargains are the TV versions of tubes. Here in the UK the 6BM8 is known as an ECL82. The same tube with 16v heater voltage is PCL82 and are way cheaper for Mullard/Brimar NOS.

[Old Tele man]

ANY multi-grid tube can be used as a preamp as long as it produces gain, but there are reasons why some tubes are better than others:

1) Multi-grid tubes tend to generate more odd-order harmonics as the number of grids increase; so triodes are preferred for lowest distortion.
2) Multi-grid tubes can be configured to provide "adjustable" gain, triodes can't; so for amplification flexibility a multi-grid tube is preferred.
3) Multi-grid tubes have much higher dynamic plate load values than triodes and thus present LESS loading to subsequent circuits.
4) Multi-grid tubes behave much like constant-current sources making them easier to interface to / from.
5) Multi-grid tubes can utilize much more of their Eb-Ib curve and thus are more efficient at power utilization.
6) Multi-grid tubes require more support components than triodes; so triodes are simpler to use, circuit-component wise.

[jbefumo]

Tube circuits are SOOOO lacking in criticality. Try it, even if it doesn't "work" on paper, it may well work fine in your application. If my mom liked driving around in a cement mixer, she could use one to go shopping, even if it didn't "make sense". We have working circuits with 12v on triode plates, we have TV sweep tubes in hifi amps, you can "abuse" tubes all over the place and they still function. it either sounds pleasing or it doesn't. Give yourself permission to screw around with it.

As to obsolete components, there are plenty of small pentodes NOS that will be in large supply for years to come. Certain ones commonly used in amplifiers may have been snatched up already, but unless you are going into production, a box of say 10 of some odd type ought to be a lifetime supply for a one-off amp, obsolete or not.

I haven't bought tubes in a while, apparently Sovtek gave up on the 7199. triode-pentode.

Considering cost of the new 6BM8, I might trade a couple parts counts for saving $20 per tube.

Well, I decided to try it in MultiSim first. I took the basic Dumble OD Special schematic for the overdrive section as a baseline (green), then repeated the same circuit, using a 6V6 for the second stage (don't have an EL82 or EL84 model, which would be closer to the 6BM8 pentode section I plan on using). The test circuit plot is shown in red. I adjusted using the load value from the 6V6 datasheet, and the various other components more or less seat-of-the-pants. From this I conclude that trying it with real components is probably justified.

Joe

[J M Fahey]

Just as a side note: during WW2 here in Argentina we had *terrible* supply problems on a lot of stuff.
NO car/truck tires because Japanese grabbed Far east rubber plantations, all of Brazilian production was bought in bulk by USA and UK, for "some reason" we had no access to German synthetic rubber (BUNA) made at their advanced plant in Auschwitz , so in emergency, some cars and trucks even run on wood or steel wheels, go figure.

For the same reason, NO tubes available, neither European nor USA nor Japanese, all were fighting and had none to spare, so we being resourceful people used *any* tube available. anywhere as needed.
I remember yellowing Electronics magazines saying so, specially that any tube could be used, except rectifiers.

[Mick Bailey]

I guess at a pinch an output tube could also be pressed into service as a rectifier. You'd need two for full-wave, though one of my contacts has some dual 30W triodes that would do nicely.