Preamp with DHT

[dejanm]

Hi,

I am thinking of building a preamp with following spec:

- only one or two gain stages
- tube preamp with DHT tube
- no feedback
- transformer coupled

To be honest, this idea I've got during examination of Sakuma's work. He believes that smaller tubes do not sound as good as DHT, normally used for output. He made several such preamps, but he is using Tamura transformers, which are not accessible for me here in Europe. Or at least, not easily accessible ...

Does anybody know a schematic for a such preamp ?

Regards
Dejan

[arnoldc]

www.vt52.com will give you some ideas, in particular, look at the Assault and Battery project.

I can't remember is Kevin's 26 is transformer coupled, but you may want to visit the Kennedy Audio website for that project.

[clivetjm]

I've been wondering about the circuit below that was described to me for use with a TVC. The 600V won't be easy, spending money of a plate tx will mean it only needs 300V (I believe). It's for a 2a3.

Thoughts, anyone?

[andyjevans]

I strongly recommend the Raleigh line stage.

http://www.raleighaudio.com/figure_18.htm

I'd substitute an appropriate DHT. I've tried most of the small DHTs and I'd recommend something great sounding but easy to get hold of on ebay like 31 ST, 201A, 26 or 5676. Maybe 3a5 - and if so use a bigger grid stopper like 8.2K. This resistor can form part of the differential shunt volume control, since there is one resistor in each leg. the shunt attenuator should then be wired directly to the grids of the tubes.

You can experiment with a cheap parafeed output transformer like the Edcor ones. The RAKK dac goes beautifully well into this circuit, with 75 ohm resistors to earth from each leg before the shunt attenuator. This is what I use myself (with a 5676 into a LL1540) and I'm very happy with it. K&K (Dave from Raleigh and Kevin from K&K) really have come up with a rather special combination here. One of the best solutions on the market for a front end.

[andyjevans]

Sorry - schematic here. Note the IXYS 10LM45 chip also works and may be easier to get.

[dejanm]

Thanks guys. These links are really amazing. Raleighaudio, with its explanations of building preamp is really extremly interesting. VT52 site is also in its own way amazing.

I am reading now ....

[gingertube]

If you are in experimenting mode you could try this on the cheap. There are a heap of of old battery radio DHT and DHP tubes out there that no one seems to want. Look at 1H5 for high mu and 1H4 for low mu higher current as just 2 off the cuff examples.
Cheers,
Ian

[kevinkr]

Originally posted by arnoldc
www.vt52.com will give you some ideas, in particular, look at the Assault and Battery project.

I can't remember is Kevin's 26 is transformer coupled, but you may want to visit the Kennedy Audio website for that project.

Yes it is transformer coupled and I have been refining the design for about the last 6 years or so. It is still the heart of my system. Can also use 12A/112 and 01/01A with just minor changes. Versions have also been built with the 10.

[dejanm]

I have seen on VT52 a lot of talking about 26 tube. Can you put some additional comments on this ? What confuses me a bit is the list of 10 best tubes that the guy provided there - and there is no 26 on that list, although he did several projects with it. I do not know this tube, so any info would be interesting ...

[andyjevans]

The 26 is right at the top of the list. You will have to get acqainted with the numbering - 26 is ST shape, 226, 326, 426 are globe shape. globes usually but not always sound better.

Also there are equivalents with different pinouts. The 1H4 mentioned is a 30 with octal base.

Then you get into microphonics. Worst is the 1G4, probably. Others vary.

There's quite a lot to find out about these little DHTs but it's well worth while. One thing is that they're just about all low mu. there's a bunch around mu=5 like the 31. Then a bunch around mu=9 like 26, 01A, 30, and then a few with mu=15 like 3A5. So gain is one thing that may affect choice.

Also, since you're going to lose gain in any step-down transformer, you may want to keep the ratio nearer to 1:1 if you want to preserve your gain, and use short cables. Many designs match up stepdown transformers of 4:1 or so with tubes of mu=40 and over, but with DHTs you just don't have that choice unless you use several stages, which isn't ideal if you're after that super-clarity you can get from DHTs.