I wonder, with the combination of the ultralinear connection and cathode feedback, whether I’ve inadvertently and quite innocently, created a single-ended version of a “Super-Triode” amp?
See, Menno Van Der Veen http://www.next-tube.com/articles/Veen2/Veen2EN.pdf (p3) for diagram.
I did get the idea of taking cathode feedback off the OPT secondary from my Morgan Jones book, and VDV does show a triode with cathode feedback config in his paper, but looking at his super-triode config, it’s definitely what I’ve got on my output stage; at least I think it is.
The configuration VDV shows for super-triode, on p3 is virtually the same as mine, except that I, of course, use the OPT secondary, for the CFB/UL combination, whilst he uses a dedicated CFB winding for the same effect.
The subjective quality of the amp is exactly the same as VDV says he gets from his, on p8. I’m really quite chuffed.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
OK, a few observations of the qualities of super-triode mode:
After 5 days of living with the super-triode output stage, I have to say that there is no way that I will go back to single-ended triode, or ultralinear modes, with modern output pentodes/beam tetrodes.
If I could afford a custom wound pair of single-ended output transformers, with dedicated cathode windings, I would go for full fixed bias, super-triode output stages, in a heartbeat; ST mode really is that good.
EDIT. and hype removed. Let’s not get over-excited eh?
Sgt. Baker started talkin’ with a Bullhorn in his hand.
OK,
Lately I have been having one or two unsure moments with this amp. Eventually, the damn’ mechanical noise from the two mains transformers and the big 10H choke drove me to distraction and I put in the NVA A20 as a good solid state alternative. The silence was welcome and to be honest the sound was fine and the bass went deeper with the NVA in circuit.
I wrote on an HFS thread, that single ended amplification, using valves is a concept hard to realise fully, unless one is prepared to spend inordinate amounts of money on expensive iron, and from bitter experience, still stand by the assertion that one can make a good single ended valve amp using cheap iron, but a great one is simply not going to happen.
Anyway I spent a couple of days with the Super Rocky, tearing down the choke-input power supplies. The chokes I have, even the Sowter ones I acquired from Mr I for the preamp section, are not meant for LC supply duty. The large 10H choke in the power stage supply, for example, was making far too much noise under load, to be right.
So I finished rebuilding both power supplies as RCLC filters last night. It was possible, by using the 375V tap on the mains transformer feeding the output stage, to get the HT down to 400V using a GZ34 rectifier valve, which worked out perfectly for KT88 or KT120s in single ended ultralinear operation with a 5K load.
On the preamp stage supply, it was also a simple matter to increase the series resistor, feeding the shunt stabiliser so that the 410V of HT resulting from the RCLC filter could be dropped properly to feed the glow tubes their striking voltage. There was only 40mA being drawn by the shunt stack and MH4s combined, so no resistor, heat issues to contend with there. Let’s just say, thank God for valve rectifier drop. It meant I didn’t have to change out the mains transformers, ruining the look of the amplifier.
With cap input supplies, the mechanical noise from the iron is much reduced and the whole amp sounds better. It has a more relaxed air about the way it goes about its business, and the top end has come out of hiding. Why the top end is better I don’t know. I can’t think why this would be so, but I’ll take it)
I’m not going to get a truly great amplifier, because of the cheap Hammond OPTs and the non DHTs, but with the cap input supplies rather than the inadequately rated choke inputs, I now have something I can call a very good amp, rather than a good one. It’s probably the best SE valve amp I’m going to get, with the restricted budget I have.
I think that by having a proper solid state amp as a benchmark to measure your DIY valve efforts against, you avoid fooling yourself into believing that you are playing with the big boys, when you’re not. It becomes an incentive to get your act together and start to really think about what it is that you are trying to achieve.
Only downside is that I have put my back out, carrying the valve amp downstairs after I’d tested it, prior to putting it back in the system. I woke up this morning and it took me fifteen minutes to get out of bed. BAH!
Last edited by Cressy Snr on Tue Dec 08, 2020 2:57 pm, edited 1 time in total.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
simon wrote: ↑Tue Dec 08, 2020 1:28 pm
Did you have a small first cap Steve?...
Yep, I had all the normal anti-spike arrangements in place. It appears that it was simply the iron that was not designed to do what it was being asked to do.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Took the opportunity earlier this week, to order a couple of Mills 12W resistors for the output stage cathodes and a couple of F&T, 250V electrolytics for the bypasses. The output stage cathode resistors had been made up from a motley collection of wire wounds because it was what I had. Let's just say it was not ideal.
The amp was unfortunately a rats nest of scrappily wired choke input supplies and bodged output stages.
After rebuilding the power supplies as cap inputs with different capacitor, values and the output stages with the new components, the interior looks a damned sight better than it did previously:
Sgt. Baker started talkin’ with a Bullhorn in his hand.
OK,
At long last, after wrestling with my drawing software on and off, all this week, I give you the audio circuit and the new cap input power supplies:
Sgt. Baker started talkin’ with a Bullhorn in his hand.
and hype removed. Let’s not get over-excited eh?
