845 Nuvistor Amp

[Les Reed]

Hi All

Thought you might like to see some pics of my 845SE with RCA 7586 Nuvistors used in the voltage gain stage. These are described as medium mu triodes but their amplification factor can be as high as 40. This is about double the mu of a 6SN7GT but nicely lower than a 6SL7GT both of which I had tried in the empty octal socket. An immediate advantage is that the slight amount of buzzy 100Hz hum I had experienced with 6SL7GT's has now disappeared.



This is just about the optimum amount of gain for this design and when I first listened to the amp I was completely knocked out by the amazing clarity of the sound. The bass sounds even more powerful and controlled than it was before - which was considerable (flat down to 15Hz). I haven't done any serious measurements yet, because I can;t stop listening to it.

I am intending to modify the wiring to allow switching between the Nuvistors and an octal double triode. At the moment the anode load resistors for the octal valve are disconnected but the coupling capacitors (Mundorf silver oil) remain in place and I have used additional caps from the Nuvistors to 300B's. I found some NOS Mullard mustard caps for this purpose and these really do sound amazing - I might not use anything else in future.

Will bring this to the next Eggfest, hopefully in its final form.

[NickG]

Very nice, look forward to hearing it at the next fest.

Have you done the woodwork yourself? I ask as I am musing over getting at least part of my 211 into something like that.

[andrew ivimey]

Les, it looks lovely but I can't think actually how nuvistors improve (change yes but ...) the sound quality - why could it?

[Global]

Lovely work Les!

What are the valves with the top connectors?

Is the circuit your own or is it published somewhere?

Mark

[BOONDI]

Greetings to all......

Superb casework.

845 driven by Nuvistor, now there is a novel idea......

What was the donor amp, or was this a scratch build?

[pre65]

Hi Les-it really does look nice,congratulations.I would also like to know which circuit you used and a few technical details.
regards Philip

[Les Reed]

Hi Nick - the wood plinth was professionally built by a cabinet maker. It was a bit expensive though and for future projects I think I will just use hardwood sides which I can cut myself with a mitre saw and aluminium rear and front plates (planning a phono stage in this style at present).

Hi Andrew - I agree that most of the sound quality comes from the output valve but using Nuvistors (especially the medium-mu types) does give a very linear and therefore low distortion front end. At the moment the amp definitely has more authority with the nuvistors fitted, but as a I said originally I am planning to rewire it to allow A/B comparison with an octal double triode.

Hi Global - the top connectors are on Svetlana 6D22S high voltage rectifiers in a bridge. I like these very much- ultra robust with very thick glass. The circuit is my own design but I suppose it is based on the Audionote Ankoru (but with a single output tube) and only one interstage between the 300B & 845. Obviouly the Nuvistor part is my own creation.

Hi Boondi - definitely scratchbuilt!

[Andrew]

Hi Les,

Looks great, I'd be interested in hearing your amp again at the next eggfest.

I'd be interested in your phono plans.

cheers,

-- Andrew