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VT-25/10Y tubes

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I just came across 7 pairs of VT-25/10Y tubes. OK, what is the best way to use them? I am thinking I want a SET amp, but my speakers are only 93db, so SET amp with them at the output doesn't likely make sense. But, well, I think it could make sense as a driver maybe. What would you build if you had some of these tubes I have:

Hundreds of 12AU7/variants
25 pairs or so 12SN7/12SX7s
20 pairs or so 6BL7s
10 pairs or so 2C50s
10 pairs of 201A/301As
2 pairs of globe 45s
2 pairs WE 300Bs
3 pairs WE 275As
2-3 pairs of 6A3s
1 pair RCA 2A3s

Also have about 50 pairs of 1625 tubes.

I can get some cheap 813 tubes.

What would you build?

Joined 2004
Paid Member
Hi Shawn,
I'd fix my old radios that use p-p 45's! I take donations!

Seriously, back in the roaring 20's they had already figured out that push - pull output stages had way more output power and lower distortion. So I would put some of those lovely tubes to P-P service. Take it from me, even old amplifiers and speakers from way back then sound better in push - pull.

Those 1625's are related to the 6L6 at higher plate voltage ratings. You are one lucky fella to have a bunch of those! If you really want to try single ended, use a 1625 or two. That would drive your speakers. A pair in P-P could blow your speakers with clean music power.

The 201 types (you are so lucky!!!), they are common in 11920's radios. Guess why I think you are so lucky. They are a low mu triode that modern tubes leave in the dust. They also take a 2.5 V filament supply, odd these days. They are difficult to use in modern gear because of the low voltage / high current filament. Then there is the fact that it isn't a cathode, the heater wire is the cathode. Use them for the radios they are meant for.

12SN7 / 12SL7. Again, you lucky guy! These are excellent signal tubes that would be lovely. Often you will see the 6 V version (6SN7 / 6SL7) in use. These were replaced by the 12 (AX / AT / AU) 7 lineup at half the heater power. Very good for linear signal stages.

You have a very desirable lot of tubes there. Please don't waste the ones that are hard to find so that more old equipment may live. I have a number of floor standing radios that need 45's in pairs, and you might restore one sometime later on. Use a 6L6 variant in triode mode for your amplifier projects. You will get more output from them, and the 6L6 types are more efficient tubes that take lower drive levels and bias voltages.

12AU7's by the 100's. If you were me, you would feel like you died and went to heaven. Those are the low mu versions I talked about earlier. They have about 20 as an amplification factor. The 201 types are a lot lower, but in the ballpark. For comparison, a 12AX7 amplification factor runs about 100. That's a big difference.

Do you have an RCA tube data book? They area available for free download on the net. If you need, I can send you a PDF, or a link to one on the web if the size is too large for your incoming message size limits.

Best, Chris
I actually use the 1625s in a push pull amp. 4 of them. With a 6bl7 phase inverter and a 6bl7 driver. Probably is about 110 watts per side. I was looking for some direct heated sound in an amp. I think the 2c50 may be the rarest of the lot. The ones I have are brown base, stamped KR, have the German army symbol and came in Valvo boxes. Would love to find a great preamp schematic for it. It is 1/2 the mu of a 12sn7.

I have the data sheets, etc. but was just looking for ideas.
Joined 2004
Paid Member
Hi Shawn,
Well, there is never a shortage of ideas around here. Tubelab is often a very good source of information, and he is typically right on the money for experimenting.

110 watts per channel is insane! Wow. I was thinking more along the lines of an 80 wpc using KT-88s. Something around 60 wpc would also make a fine music amplifier. I'll have to give that a try if I run into any of those. I really like the idea of a high voltage 6L6 that isn't an 807. Heater current is more sane at 12 V. I really like the idea of single pair output designs, and I think they sound better.

There are ways to linearize the transfer characteristic of newer tubes to be more like the old 201's, etc. There is even a method to return the screen current back to the plate circuit so it still runs in ultra-linear, but you get back that 20% of otherwise wasted screen grid current. You basically use a transistor. Rough concept is to return the base (gate) to the UL tap, the emitter (source) goes to the screen grid and the collector (drain) connects to the plate circuit. I have not tried this myself yet, but the idea stuck in my head. I can't remember where I read about it. Very probably a thread here. Something like a 6BQ5 might really benefit from this concept. So would a real EL34.

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