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Lower power octal alternative to 6V6 ??

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10/20/1999 5:03 PM

Lower power octal alternative to 6V6 ??

I'd like to build a single-ended 2 power tube
guitar amp based on all octal tubes. I am going to use
6SL7s as the pre and drivers and would like to find
some commonly available tetrode to use as the output
tubes.

Optimally, I would just use 6V6s but they are not as cheap
as I'd like, and I already have a 6V6 based amp.

But, I am not aware of other tubes that might work
in this application. Anyone got suggestions?

10/20/1999 5:15 PM
R.G.	6K6, 6W6

10/20/1999 5:50 PM

I wasn't aware that there was any octal power tube as cheap as a sovtek 6V6, but you may be excluding that version and intending to use only NOS tubes.

So you want to keep it octal? And you want a parallel single ended output stage. With 6V6s, the output transformer needs to handle 70ma of DC.

You could use a pair of chinese 6L6s. They're cheap, they don't last terribly long but they actually have decent blues tone. For that circuit, you'll need a 120ma SE transformer. With 6SL7s, you should be able to meet the grid swing requirement, which is maybe 50% higher than 6V6's.

There are other octal beam tubes used as TV horizontal amplifiers These would be relatively cheap, but you'd have to experiment to see how their audio performance measures up.

If you can keep the plate voltage low, like 250 vdc, you could use 6Y6G or 6Y6GA tubes. These are good for audio, are larger bottles than 6V6GT (are about the same size as 6L6), are relatively cheap (sometimes on clearance at AES), and reqire grid drive voltages same as 6V6. They would require a transformer with DC current capability of the 6L6, but only half the impedance. If you had a 6L6 op trans you could use it with an 8ohm speaker load on 16ohm tap (or 4ohm load on the 8ohm tap) which will result in half the normal reflected load.

There are 7-pin and 9-pin beam tubes in the 6V6 class, but they're not octal. A 6AQ5 is a little 6V6 in a 7-pin bottle. These wouldn't take up much space, they're cheap & plentiful, and they wouldn't require a doorstop-size transformer. Just design for a 6V6 and you're set.

The 6K6 is a nice low power octal tube, but because they're used in vintage reverb units their price is getting higher all the time.

(I presume you are aware of the dual SE amp project on Angela's site, which uses 6V6s.)

How about a single ended guitar amp with one tube, a 6L6. You'd get 10 watts. Check the Champ-12 circuit, and a Glass Audio article called "Remaking of a Champ".

10/20/1999 6:09 PM

If price is important, there are a lot of tubes that were used in TV's that are cheap. Get a GE Essential Characteristics tube manual from Antique Electronic Supply to analyze all possibilities. A good way to save money is to use tubes with non-standard filaments. Here are a few options to think about:

1. Use a 12V6GT or 5V6GT and make the necessary filament arrangements. Another possibility is 12AB5.

2. The 6EZ5/6EY6 is a good sub for the 6V6 at a much lower price. Note that it requires more filament power than 6V6. Also available as a 7EY6.

3. Use a 6DZ7, which is a dual pentode and wire in parallel.

4. If you were interested in a triode output stage, use a 6CK4, or a 6BX7 in parallel.

There are many other TV pentodes that will work, but I don't have my tube manual with me as I type this. The above is a good start.

One more thing, 12SL7's are less than half the price of 6SN7's.

12V filament can be supplied by a seperate tranny of use a voltage doubler DC filament off 6.3 VAC winding.

10/20/1999 6:51 PM
Doc	I like your idea about the 5V6. I'd forgotten that. If the power transformer has a filament winding for a 5-volt rectifier, you could use a solid state rectifier for HV supply, then have the unused 5v winding available for the 5V6s.

10/20/1999 8:26 PM

I built a amp using that concept, 5V winding for the 5V6's AND used a 6AX5GT TUBE rectifier. This works because the 6AX5 filament is isolated from the cathode of the diode sections. The 6V winding powered the rectifier and preamp tube filaments. Another bargain rectifier tube is the 6BY5. This is a like a 5V4 with an isolated 6V heater that can be used in the same manner as a 6AX5GT.

Another good rectifier concept is to use damper diodes like the 6AX4, 6AU4. These have a low voltage drop like a 5AR4, but since they are single diodes it takes two. Two of these are $6, instead of $50-$70 for a 5AR4.

10/21/1999 1:00 PM
Graywater	Scott, Have you actually built an amp using the 6CK4? I seem to remember some non-linearity in it's characteristics that make is a poor choice for audio. GW

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