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Bought 6AS5, got 6AS6 instead. Any audio use?

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I recently purchased what I thought was a small lot of 6AS5 NOS tubes. The listing was for 6AS5, the boxes had 6AS5 on them, and I didn't think much of it after they arrived.

However, now that I opened the boxes, the contents are all 6AS6, not 6AS5. It actually looks as though a mistake was made at the factory - the boxes were literally unopened, the tubes were brand new inside. Just the wrong tubes.

I'm not going to go back on the seller; it's not his fault, and I'd have trouble proving it anyway. And it's not a lot of money, so I'm not worried about that.

But I'd like to find some audio use for these 6AS6 tubes now that I have them.

They physically resemble 6AK5 tubes and some sources suggest they are similar. Any thoughts?



The 6AS6 has a good screen control over current plate, and apparently was designed to make functions as a Sanatron or a Phantastron tube. Google for this two words.

6AS5 is unknown for me.

Very interesting information, but I don't think building a working radar set is on my to-do list. ;) I was thinking more of using them as input tubes or perhaps output tubes on something like a headphone amp. Not that I was planning on that, but since I have them...

The 6AS5 is a pentode that I thought I could use as an output tube; but it was not to be as there are none in the boxes I bought.
Sorry, I did a mistake. I must say "good suppressor control".

In a Phantaston or Sanatron circuits, the pentode works in a very different way respect to an audio or any linear amplifier: the suppressor is made several volts negative respect to cathode, and grid is slightly positive, under static conditions, so all space charge is diverted to the screen, and no plate current at all. When a pulse external to the circuit changes this stable condition, then temporarily the tube behaves as an amplifier. Finished the cycle, it returns to this strange biased way. Both, the Phantastron and the Sanatron are monostable circuits governed by a linear plate ramp rundown, although the tube isn't biased linearly. The tube is positive and negative feed forward at the same time.

You can use it as a normal sharp cutoff pentode as tha 6CB6, for example, simply ground or bias the suppressor as usualy you do.
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6AS6 is a dual control pentode. Which means g3 has a fine wire pitch instead of the course g3 pitch typical for pentodes as suppressor grid. That makes the g3 overly effective when grounded, returning most of the plate directed current to g2.

The 6BV11 datasheet well illustrates the plate curve shape variation (top of page 4) for variation of grid 3 voltage. Positive 2 volts on g3 returns the plate curve shape to normal for a pentode. (allows electrons through to the plate)

From the 6AS6 datasheet (top right graph of page 4), it looks like +10V on g3 would restore normal pentode operation. (current arriving at plate)
(too much +V on g3 will cause g3 to start absorbing the plate current instead, so some experimentation required to optimize +Vg3)

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Although you could use these for audio, you might be better selling them to someone who actually needs a 6AS6 and then use the money to buy what you need. Or perhaps sit on them and hope they become sufficiently rare that people who need them to restore old communications receivers will pay serious money.

Yeah, I could do that, but we're talking about very little money here. Not really worth my time to list and try to sell, I'm afraid. I sincerely doubt they'll ever be worth anything. If they are of no value in a typical audio amplifier, I suppose I will just throw them in the plinker box and not worry about it anymore.
The 6AS6 is pretty common in military surplus. I have a few hundred used pullouts, including some Western Electric versions.

They are actually useful in audio compressor circuits. In an audio compressor they must be used in push pull circuits so that the predominately second harmonic distortion cancels in a manner similar to the old Fairchilds that sued the 6386 dual triode. Put audio on G1, control voltage on G3. Limit gain control range to about 20 db per stage.

6AS5 is unknown for me.

The 6AS5 is a small audio output pentode somewhat similar to the 6AQ5, but intended for line powered equipment. Maximum plate voltage 150 volts (can be ignored) maximum screen voltage 117 volts (must be respected!). A pair in push pull can make 8 to 10 watts, but so can a dozen more common tubes.
If they are of no value in a typical audio amplifier, I suppose I will just throw them in the plinker box and not worry about it anymore.
It looks as though you could use them as generic small-signal pentodes in a typical audio amp if you want too (see Smoking Amp's post about biasing g3 slightly positive).

I have another idea as well. In some guitar amps, there is what is called an "effects loop". These can be serial loops (more common), or parallel loops (more versatile, less likely to damage the guitar tone).

In a nutshell, a parallel effects loop splits the preamp output into two paths, one going direct to the power amp input, the other attenuated down to 1 volt or less, then going through the effects loop (which might contain one or more guitar effects pedals.) The two signals then have to be recombined at the input to the power amp.

And that's where I think one of your 6AS6's could play a role: the bigger direct signal could go to g3, the smaller signal coming from the effects loop to g1, and the 6AS6 could do the job of mixing the two signals.

Even the "flaw" that the gain from g1 to anode is bigger than the gain from g3 to anode, would be an asset in this use case!

could it be used as an electronic attenuator ( volume control )?

I was playing with them 5 to 10 years ago as I was deciding how to reduce my collection of over 100,000 tubes down to a reasonable number. At the time I realized that I had already outlived my corporate life expectancy by a dozen years or so. Over the course of several years 100,000 tubes became 10000 to 15000.

A little over 2 years ago, after 41 years in the same place, my layoff lottery number came up and I had to pack up and get out of Florida on 3 weeks notice. Another large number of tubes were sold or given away. I'm not totally sure the 6AS6's made the cut, but I believe that they did.

I was experimenting with them and a few other tubes for use as a vacuum tube Voltage Controlled Amplifier. The VCA is a key component in a music synthesizer, and an audio compressor.

There are several old vacuum tube audio compressors that have become cult classics. The Fairchild 670 is the king of the classics, and good clean used ones can fetch up to $50,000 today. The 670 used a chain of transformer coupled VCA's. Each stage was a differential pair made with a variable Mu dual triode, the 6386. NOS 6386 tubes are rare, and can go for $200 each. There have been several "adapter boards" made to use common tubes in old Fairchilds, or clones. Most use TV tuner tubes, but at least one board used the 6AS6. That's the application I was looking at.

The 670 is not a "squeaky clean" device, intended for broadcast (radio and TV) audio compression and limiting, it imparts a "tube sound" that has found cult status in certain recording studios. I don't know if the design is clean enough for HiFi audio.

The 670 schematics can be found on the web. My experiments used Edcor transformers.
I remark the fact that the suppressor is sharp cut off, then lower voltages at this electrode will cut current plate, an important difference with pentagrid converters or standard pentodes. Also check that with full negative bias at suppressor, screen power is below the maximum admissible (full cathode current diverted to screen).
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