I probably want to try to build a vacuum tube amplifier in nearby future. Let it be a medium powered (15-20W) SET amp. A significant part of existing designs is based on something like 300B or 2A3 which are just too expensive. I live in Eastern Europe where Soviet made tubes are readily available so I can search there. 6c33c is the obvious option. But what about more "classic" tubes like direct heated triodes? There is GM70 which is reasonably priced too but it means spending more on transformers. What other options do I have? With transistors for comparison it's quite simple, you have catalogs with parameters to choose from. Also they are more or less similar to each other in the same power handling group (especially BJTs and to a lesser degree FETs). But tubes... I'm just getting lost.
There are DHT SET amplifiers.
There are Indirect Heated (IDH) SET amplifiers. Just an alternative.
15 - 20 Watts solid state is not expensive.
15 - 20 Watts DHT SET is expensive. There are no cheap miracles here.
SET Parallel Triodes. DHT or IDH Just an alternative.
SET Parallel Triode wired Pentodes or Triode wired Beam Power tubes, just another alternative.
Just my opinions.
There are Indirect Heated (IDH) SET amplifiers. Just an alternative.
15 - 20 Watts solid state is not expensive.
15 - 20 Watts DHT SET is expensive. There are no cheap miracles here.
SET Parallel Triodes. DHT or IDH Just an alternative.
SET Parallel Triode wired Pentodes or Triode wired Beam Power tubes, just another alternative.
Just my opinions.
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If you want a true triode your choices are a little limited unless you go to crazy high voltages which is expensive and dangerous. The 6c33c is easier but it has some quality problems and needs an output transformer that can’t really be used for anything else easily like a 3.5k or 5k primary can. Triode wiring a KT120 will get you there and sounds very good. I used them in a switchable triode ultra linear amp to get the best of both worlds.
Yes, this is why I included 6c33c as an example of indirect heated triode and GM70 as an example of direct heated one. Just because heard about them. But I don't see the whole picture. Some model names appears here and there but there is no single list or table.
Unless you go with Transmitting tubes, power DHT tubes are on a pretty small 'table' (list).
For some types you might want to use, Obsolete-ium and Un-Obtanium come to mind.
For some types you might want to use, Obsolete-ium and Un-Obtanium come to mind.
15-20W is not 'medium power' for a SET.
I'd call 5-8W medium.
What are you after?
How much power do you really need? More power = bigger transformers = more expensive.
A tubelab simple SE (trioded 6L6/EL34/KT88) can be a great guided introduction into tube audio.
GM70 is definitely an advanced project with very high voltages, big expensive transformers and well designed heater supplies.
I'd call 5-8W medium.
What are you after?
How much power do you really need? More power = bigger transformers = more expensive.
A tubelab simple SE (trioded 6L6/EL34/KT88) can be a great guided introduction into tube audio.
GM70 is definitely an advanced project with very high voltages, big expensive transformers and well designed heater supplies.
Actually I'd like to build something that looks cool. So I'm looking for tubes with bakelite bases first. Not so sure about power. I just took my solid state amp power rating as a reference. I think I'd be ok with even lower power.
I'm about to build a pair of TL towers with something like Alpair-10 full range drivers. What I have now is just a crap. A pair or Chinese active speakers with built-in amp bypassed. And I'll definitely do speakers first.
15-20W SET is not an easy job, and -as wrote earlier- it's not cheap .. and not -only- the power triode is expensive.
The good SET amp OPT cost is -at least- 50..60% of total cost.
Good low impedance -for 6C33C- and high impedance -GM70- output transformers are rare and pricey toys.
If you choose "general" 2k5..3k5 transformer, the alternative power tube list is wider ... but the output power is limited, so the optional loudspeaker list is narrows.
The DHT amp "territory" is in the easy driving, high efficiency loudspeakers "land".
If you have 90-96dB efficiency "tube friendly" loudspeaker, the 10W ... 4W SET amp is acceptable.
For example if you have 94-96dB efficiency speaker (and not too large room), 6C4C is good alternative for 3..4W SET.
The good SET amp OPT cost is -at least- 50..60% of total cost.
Good low impedance -for 6C33C- and high impedance -GM70- output transformers are rare and pricey toys.
If you choose "general" 2k5..3k5 transformer, the alternative power tube list is wider ... but the output power is limited, so the optional loudspeaker list is narrows.
The DHT amp "territory" is in the easy driving, high efficiency loudspeakers "land".
If you have 90-96dB efficiency "tube friendly" loudspeaker, the 10W ... 4W SET amp is acceptable.
For example if you have 94-96dB efficiency speaker (and not too large room), 6C4C is good alternative for 3..4W SET.
The 6C4C is a great sounding tube and would be my choice. I have a 6C4C amp and a 2a3 amp and there is no difference in quality. Easily available 3.5K output transformers. For DHT tubes I use Rod Coleman filament regulators. Now the V9 version.
I use all-DHT tubes these days, nothing indirectly heated. For a driver tube you can use a cheap 2P29L or 4P1L. Both excellent. I prefer the 2P29L. I use a 1:4 step-up transformer on the input, Hammond 1140-LN-C. Not expensive.
You will have a wonderful amplifier. Plenty of schematics here. Check out the All-DHT amplifier thread.
I use all-DHT tubes these days, nothing indirectly heated. For a driver tube you can use a cheap 2P29L or 4P1L. Both excellent. I prefer the 2P29L. I use a 1:4 step-up transformer on the input, Hammond 1140-LN-C. Not expensive.
You will have a wonderful amplifier. Plenty of schematics here. Check out the All-DHT amplifier thread.
With like 4 watts and an Alpair 10 you hit like 95Db if you’re lucky. That is loudish but not Loud. The most common way of getting 8 watts of DHT sound is the 300B but that costs a lot to just take a chance you’ll like it. Like I said before, just triode strap a KT88 or KT120 and give it a whirl. If you use like a 3.5k output transformer you can try the 300B later using the same transformer. If you put in the UL switch you can get over 15 watts. Dual personality amp with two power levels. It’s a good place to start and lots of popular schematics are out there.
It's true the 6C4C has its limits, but I've used it successfully with Alpair 10s in a smallish room. I had plenty of sound. I think it sounds better than the 300b so it's my output tube of choice. Svetlana quality - it's really well made and stocks still about. No problem running at 2a3 operating points - 250V, 60mA, -45V. It's all a question of power - I wouldn't sacrifice the sound quality of a 6C4C for a KT88 if the power is enough. And like I said above, I'd use a DHT like 2P29L to drive it with a 1:4 step-up. This is my daily system and it's very hard to improve on this kind of sound quality. In fact you couldn't even buy an amp like this with a DHT driver - it's beyond what's commercially available.
The more efficient the speakers, the less wattage required, the more choices you have, at a lower cost.
The speakers will cost more, but will open many more options….being you are replacing speakers.
The speakers will cost more, but will open many more options….being you are replacing speakers.
Then Class A PP is a whole pile easier. Triode strapped beam tetrodes (or KTs) are cheap and so is the iron you need, compared to SET iron and 300Bs. At a pinch you can use PA line transformers as your OPTs but there's a pile of old iron intended for this sort of thing.
Heck, follow Lynn Olson's prescription and just triode-strap an ST-70.
For ultra-cheap, if you are lucky, look for an old valve powered stereo console.
Strip out the amp and sell the rest as furniture. Polish up for lots of vintage cool (replace the electrolitics)
Another option is to search out designs using 6AS7 as the power valve. It has two triodes, with parameters close to 2A3 so could give you the effect you are looking for?
I started a thread earlier to see if anyone had ideas about using a single 6AS7G and pre-amp double-triode as driver to make a two valve stereo SET amp.
I started a thread earlier to see if anyone had ideas about using a single 6AS7G and pre-amp double-triode as driver to make a two valve stereo SET amp.
But do you really need a 15-20Watt SET amplifier? How big is your room? What speakers are you going to use?
Since it sounds like you have the luxury of a bit of time to think about options, it can be useful to subscribe to a different sources of second hand parts and see what comes along. I picked up a cheap pair of Lundahl 1628 transformers, gapped for 120mA in SE, so that nails on a great sounding amp for a reasonable budget.
Maybe you will see an abandoned project or cheap amp that needs TLC.
If you read all of the notes on Tubelab for the SSE and TSE amps, you will get the picture for what traffos suit what tubes, and the sort of operating points you will need with output power. Maximum theoretical power from a tube in SE is only 50%, hence the output is probably less than you expected. However people say tube-watts are louder ;-)
Maybe you will see an abandoned project or cheap amp that needs TLC.
If you read all of the notes on Tubelab for the SSE and TSE amps, you will get the picture for what traffos suit what tubes, and the sort of operating points you will need with output power. Maximum theoretical power from a tube in SE is only 50%, hence the output is probably less than you expected. However people say tube-watts are louder ;-)