Amp advice needed for a poor DIY er...

[Dxvideo]

Hi all,

I am from the chipamp forum.
Last months I had some discrete experiences, but I cannot hear much differences between chipamps and discrete amps. I think its not worth to make a discrete (bipolar or MOSFET) class AB amp in anycase. Except a single end class A project.
I made a single end 100mW headphone amp with just a IRF510 MOSFET and saw the diffierence..
After that experience, I tried to make a bigger Class A for my speakers however I coulndt solve the thermal problems and for now solid state high power class A idea is dead for me

Anyway,
Now coming my real question.

I want to try to make a tube amplifier. It must be a single end Class A one and 20W per channel at least. However I have no idea about the tubes.
May you advice a
- Really simple,
- Adjustment free (or minimal),
- Cheap, good sounding (better than a solid state AB)
and of course with more available parts tube amplifier pls?

Thanks a lot in advance for your helps.

[soundbrigade]

You end up with some kind of terrible tube crying for 1200V and loads of fresh air to heat up ....

To be serious - you may end up with tubes like 211, 845 or 813 that gives the required power. There's a rather unkinky Russian tube (hard to find) GU-72 that is suitable too.

OR you can go for PSE - parallelled tubes like EL34, 6550 or just a pair of good old 6L6 or 807.

I would recopmmend you the second solution as you can use rather kind stuff like transformers that isn't constructed to withstand 10kV or PSUs that gives Total Makeover a new meaning ....

[Dxvideo]

Yes thats fine.
But may you advice an application with one of these tubes?

[Andro_BG]

Hi!
I think that 20W output power SE is often too hard for a beginner in tubes. IMHO 5W is a good starting project. IMHO output transformer is a key component in a tube amp. Therefore I could recommend that you first check what OTs you can afford (they are typically expensive...) and then consider tubes selection. Maybe Hammond OTs is a good thing to start with. There are also Russian OTs like VR20CSE (25W out, 120mA DC max, plate resistance depends on windings connection 2.5 ... 10 kOhm, loads 4 / 8 / 16 Ohm, ~$90).

Some plate resistance - output power - primary voltage amplitude - current amplitude are (95% OT efficiency assumed):
2 500Ohm - 5W - 162V - 65mA
2500 - 10 - 229 - 92
2500 - 20 - 324 - 130
5 000 Ohm - 5W - 229V - 46mA
5000 - 10 - 324 - 65
5000 - 20 - 459 - 92

Note that you need both higher power supply and anode bias current than the one needed for primary swing. It depends already on the tube how much more... Anyway, a 400V 100mA means 40W anode dissipation (to hope for 10W output power). From Soviet tubes I know I could recommend 6p45s (6n45C in Bulgarian letters, EL509 equivalent), in Bulgaria they are ~$15/pc and you would not need to parallel tubes. Well, there are much more tubes that would work in 400V 100mA, like, for example, SU GU50, LS50 clone that is several times cheaper - I tested one just yesterday and I liked the sound I got at a glance. I think you will find out that the OT is much more expensive than many tubes. You might like to have a look at the prices of one of the most famous audio DHT, 300B and OTs from Tango and Tamura... And I think that results from "whatever" transformer and tube amp are likely to be unpleasant and confusing.

When selecting OT you need to keep in mind that for triodes (I assume triode SE class A operation everywhere herein...) Rp/Ri (primary impedance to internal triode impedance) ratio needs to be higher than 3 for good distortion and damping factor performance. 6p45s/EL509 has Ri of about 300 Ohm, while GU50 has about 1 kOhm, for example - so a Rp/Ri=5 would be 1.5 kOhm vs 5 kOhm. I myself like higher Rp, but if you are chasing higher output power you might need to consider other options, too...

Best regards,
Andrey

[Tom Bavis]

Patrick Turner has a 35W SE amp (4 x EL34) - in triode mode about half that. http://www.turneraudio.com.au/se35cfbmonobloc.html Not TERRIBLY complicated except for obtaining the output transformer (He makes his own).

5-6W would be MUCH easier, with a single EL34 or 6550 output tube. Search for "Tubelab simple SE".

[Andro_BG]

For cheap output transformers you might like to have a look at this article. Well, it is significantly below 10W (I would expect couple of decent watts) and I have not tested it, but you could use the transformers for filament supply in case you don't like the sound...
Best regards,
Andrey

[Dxvideo]

Hi Tom,

It doesnt look like terribly complicated. But I am newbie and I dont think I can have a success start up with it. Its is a bit risky for me I guess.
My first target is to have a real class A amplifier, however my experiences are on solid state and I expect to have a simple and ralitively "cold" amplifier.

Thanks a lot.

Dear Andro,

I also think like you on this matter. After a short internet searching, I found some OT projects. One of them is Andrea Ciuffoli's SEPP project. Its very simple and relatively cheap one. He uses 6C33C-B x 2 as output pairs, ECC82 to drive them and as input pairs...
Anyway, Its very look like a solid state amplifier.
This is it;

However I am confused on SEPP concept! How can a SE output become a Push Pull at the same time? Whats that? Is it possible to prevent crossover distortion with that way?
And a final question; tubes in this project widely available and cheap ones as Andrea says?

Thanks...

[Shoog]

Think of something like the RH84 or the RH807 , but with a 6550/KT88 as the output tube. This gives triode like performance but with pentode like power. You might expect to get about 10W of power from a KT88, but you are likely to be surprised at how far those 10W go compared to a SS amp.
This would also be one of the cheapest approaches as the RH designs place much less of a demand upon the Output transformer and so it can be cheaper. It will also produce one of the best bass responses of any SE design.

There is also the option of going PP - its easy to get your 20W this way.

Shoog

[Andro_BG]

Hi!
I did not recommend 6S33S (6C33C) for several reasons:
- I could not get a really nice sound from these myself - well, under some conditions I liked some music much, but not all...
- note that the filament is 44W per tube...
- they are hard to drive (you need to develop about 200 Vp-p on ~200pF input capacitance, and the grid resistor should be less than 100 kOhm for stable operation with higher voltage and/or plate dissipation).

Add: OTL approach is something I would not recommend - I see no normal way to ensure decent Rp/Ri value and therefore the distortion needs much more complicated schematic, NFB etc... which is not the best tubes can give us IMHO ;-) I like two stage SE designs. But the schematic is your last concern, I think - it is, IMHO, much more important to get the right concept for your case. My own opinion is that very few things can be done with tubes without output transformers - in other words, trying tubes OTL is not the real thing IMHO. However, here are couple of links with an interesting approach for OTL:
http://members.aol.com/sbench102/inv6as7.html
http://members.aol.com/sbench102/inverted.html

For the tube watts and SS watts - I didn't wanted to say that myself :-) Anyway, moving from SS to tube amps is often accompanied with new understanding of the output power needed...

Sorry, not much time now :-(

Best regards,
Andrey

[Dxvideo]

Is there TUBE WATT and SEMICONDUCTOR WATT exists?
At the moment I am using a lateral MOSFET output class AB amp for my living room. Its 150W rated per channel but I need only 10 or 15W per channel, why I use that much power is just for headroom I need.
Anyway,
I have a pair of Wharfedale Diamond 9.6, theyre not "hard to drive" speakers (91dB at 1 semiconductor watt ) but I dont think just 10 watt amplifier will be enough for me!! Or am I mistaken?