Future plan for 300B XLS amp

[IslandPink]

Here's my status as of about 10 days ago . I'm calling this 'Phase 3A' , it's three stage, but RC coupled and R-loaded . Got the basics there for phase 3B when the chokes arrive . See below :

[IslandPink]

For anyone who's a connoisseur of breadboards, here's how the signal section looks right now . Filament supplies are a bit of pain on a DHT/DHT amp like this !
Yes, there is some exposed 465V quite near the volume pot . I'm going to do something about that .
MJ

[Nick]

Looks nice, specially those c-cores...

Valve porn, and why not

[Paul Barker]

Nice

[IslandPink]

Hello Paul
It's quite nice at present , but I don't think the upper mids and treble are as good as with the pentode driver , for the moment . Probably the extra coupling cap, or the R-load on the 45 . On the other hand the timing is better ( all three stages are fed direct from the reg supply ) and the dynamics are solid , and 'easy' too, very natural . Also the bass tone is excellent .
In 3-4 weeks I hope to have Bud's chokes and then we can go to the intended circuit ( see page 1 ) which I'm sure will be a big step up .

Mark

[Andrew]

Hi Mark,

Looking on with interest and its good to hear progress. Currently, I use a transformer coupled Aikido to drive the 300B, so no caps, great freq resp and good timing with this circuit, no 300B stodgy bass. So moderately pleased....

However, and there's always a 'but' isn't there? Getting a single gain stage to swing 180v p to p is very difficult when the gain of the driver is mu/2, so I'm basically using an 83 or a 6072 for my more sombre moods.

The tx-coupled Aikido works really well but I can help feeling, if you don't use a pentode, then having 3 gain stages is the way to go, hence the interest in your circuit, and your comments relating to the sound versus the pentode. A pentode would give me plenty of gain, of course.

I have been experimenting on paper with the idea of 6072 cap coupled into interstage driven 12B4 - just 'cos I have some. The down side is the need for coupling caps etc. Compromises, compromises....

Nick had success with a standard Aikido using an extra gain stage before that, which might be another approach.

-- Andrew

[Nick]

Personally (so please ignore) I can't help thinking there are more cons resulting from trying to drive a 300b in one stage than the pros.

[Andrew]

Personally (so please ignore) I can't help thinking there are more cons resulting from trying to drive a 300b in one stage than the pros.

Well, yes, I think that's a very valid viewpoint, and I'm mostly in agreement, in fact.

If we assume an interstage then using it in an Aikido format, is, in one context at least, not making the most of the inductive load to help swing the extra voltage required.

Ok but then its back to a cap coupled driver valve

-- Andrew

[Nick]

Yes, thats the problem. There is always direct coupling to the 300b grid though. I haven't tried that myself yet.

[Andrew]

Well yes, but with an interstage its the extra stage before that needs the cap coupling..to give the required gain over two easier stages....I suppose that could be direct-coupled but then I always find direct coupling a pain with operating points etc unless the valve was designed for it.

[Nick]

Yes, again, I think you often sacrifice the best op point to just avoide that cap.

Personally though, I think a good amp needs at least (and maybe no more) one very good cap in the signal path, its that combination of sounds again. Too much of anything being a bad thing.

[IslandPink]

This recent change has surprisingly shown me how good the C3m driver was , it's an easy way to drive a 300B pretty well .

Caps, I always think that despite the best will in the world they take out some colour somewhere in the range ; the Aurora showed me that - it had the widest 'colour gamut' of any amp I've used ; but had it's own issues as built .

However, as you say, the 300B is not easy to supply - swing + Miller cap is not trivial .
I think the 300B is still best thought of as a small transmitter valve, with the driver issues to suit .

Maybe a suitably set-up E55L would do a very good job - that's the only alternative simpler possibility I think - but the two stage amp has that issue always with 2nd-harmonic cancellation , which needs some thought to get more 2nd out of the driver to overcome the cancellation effect .

MJ

[Andrew]

Yes....the TX-coupled Aikdo with the self loaded 83 for gain does a very good job too, but I kept asking myself is there more there? I suspect I'm asking too much out of the 83...

[Andrew]

This recent change has surprisingly shown me how good the C3m driver was , it's an easy way to drive a 300B pretty well .

MJ

Yes, indeed, I heard the C3m or was it the 12GN7 at Eggborough, either way a testament to James D and his pentode drivers.

-- Andrew

[Andrew]

Well I got hold of a 6DE7 to try the compactron driver route when I get chance. Sort of a noval 6EM7 but not quite. The ideal is to load the bigger of the two triodes with the interstage and use the smaller triode as a gain stage; they could be DC coupled.

I still can't get over how good the Aikido sounded, just the lack of gain lets it down for a single gain stage, as Nick found it really needs another triode before it.

Then again the pentodes sounded pretty good too and no shortage of gain. I reckon we're gonna have to work quite a bit harder to beat either of these two.

I wonder what a pentode Aikido using high gm pentode would be like? Dave Dove used a lower gm pentode in Aikido format to great success.

cheers,

-- Andrew