RH2A3

[Paul Barker]

Good result now.

[Alex Kitic]

The day after... heater voltage is definitely OK, I guess.

It has striked me today how loud the music actually is, and how many new details are now emerging from the background (musicians moaning while playing, now more audible, and some "incidents" I have actually not heard before). Sounds like a phrase, but it reflects reality. After all, my loudspeakers are not particularly sensitive, and the additional power this amplifier offers is obviously important.

The difference at low levels is negligible - it sounds very similar the RH-TTA with 1619 tubes (definitely the most similar tone, the richness and warmth combined with great highs). But it can go louder than any of my previous amps (well, no wonder, if we assume 35W output power per channel, that's 3x more than the RH300B and at least twice the power of the Universal with 6550 or 8417 tubes).

This would not be as important with, i.e. 100dB/W/m speakers, where I would not be using all this power... but with my speakers, and any "normal commercial speakers", this seems to be "the way to go all the way". While the problem for a DIYer might be the relatively high B+, my experience so far is without glitches at voltages approximating 800V. I am mostly using it with 5U4 rectifiers, with approximately 705V across the tube.

In this regard, the complete absence of hum/noise/buzz from the heaters is not as important in the comparison, since the RH-TTA with 2A3 or 1619 meets basically the same criteria, and even the RH300B is very near such levels of inaudibility.

I guess I must try a similar schematics in the near future with GM70: their main advantage is being "current production" tubes, relatively inexpensive. Of course, the same applies to GK71, and I guess that the schematics of the RH813 will be directly applicable to the GK71 (i.e. treating it as a substitute with different heater voltage). As for the higher heater voltage needed, now we know that by re-winding the secondary of the electronic transformer output transformer it might be even possible to squeeze both GM70 heaters (or GK71 heaters) in one unit, which might be rated 150W, just in case (although, I guess eventual heating issues might be improved by adding heatsinks to the transistors - and it might also increase long-term reliability).

As for the schematics, I will be publishing everything soon on my blog... I still have to check a few details regarding VR tubes combination and the effects of different drivers on the behavior of the amplifier (safety and troubleshooting issues mostly). My idea is that the DIYer does not need to think about the 800V inside... if everything is connected properly, and all the fail-safe mechanisms are active. The only time he/she (although female builders of the RH813 are rather improbable) would "see" the 800V DC would be when measuring and checking the various points. In further use, it should be just blliss and abandonment to music...

[Alex Kitic]

We have been comparing anode structure to gm70 in another place, which inspired me to add the 813.

this reminded me of the gritty sound of the 813 triode connected compared to the 211. There is a vast difference.

I am wandering whether the GK71 will also lack finesse against the GM70. Just conjecture AFAIK non if us has tried the GK71 triode connected yet.

But if it does have the same issue as the 813 have you set plans in your mind to pentode connect it with either the anode resistor feedback or output transformer feedback?

This is a quote from another thread... but I wanted to stress the fact that the 813 in pentode mode is not at all sounding gritty.

Not having any means to compare it right now with a 211, I can only compare it with a 1619 used in a similar amplifier, and than compare the 1619 with 2A3 and 6B4G. Well, based on this, the 813 is not at all gritty when compared with a 2A3 (Chinese, OK) and NOS 6B4G. Actually, while the 6B4G is slightly warmer than the 1619, the latter has better resolution of mids and highs, so to speak, and is maybe a winner in overall performance. Now you can do your own comparison between what you remember of one of these tubes and the 211.

Just as said above, I believe the GK71 might be used in this circuit as a direct substitute, heater voltage and pinout (? did not check) aside.

[Paul Barker]

Yes the 211 is a very different sound to the lower powered dht's. All of them.

It is alone in it's type of sound.

Not easy to put to words though.

Very silky midrange lighter than the other valves but in the sense that it shows them up (to my taste). Negatively speaking the negative is the other valves the midrange to top end of the 211 is more what I like to hear. The fattest sounding of the lower powered ones is the badly designed 300b amp. Seductive midrange tone to some people. Fatness to me. In this respect IFHT's have a lot going for them. Sometimes such things as soundstage and imaging and neutrality across the frequency range are better with idht;s than with these dht's mentioned. But the dht is obviously less distorted, that is it's trademark. But these subtle sound characteristics won't appear on a scope, the distortion of the idht will imediately show up on a scope. The ear tells a more detailed story.

Bass response suffers on the 211 if the OPT isn't adequate.

Of course now that I have committed an opinion to paper, the snipers will start shooting from their safety position of not committing themselves but shooting down the brave after the event.

[Alex Kitic]

The fattest sounding of the lower powered ones is the badly designed 300b amp. Seductive midrange tone to some people. Fatness to me.

This reminds me of an exchange of taughts with a DIY friend. He dislikes the sound of 300B tubes - but would like to try if the RH300B is any different.

I commented that as much as I tried, I was never able to design a tube amp with that "fabled" warmth and fatness of tone - although I have heard more of those than of any other variety... and I was always wondering "how do they do it"

Jokes aside, I dislike fatness (of tone, no pun intended) and none of my amps shows this characteristic. While the 300B indeed has more "weight" in its sound compared to most other tubes, nevertheless it sounds utterly precise and fast in the RH300B. Particularly the previously mentioned "fake mesh" globe (Shuguang production, most probably) tube marketed as Full Music, Sophia... etc. It is the EH300B and other Russian 300B tubes that tend to exhibit this "weight" - but it can be controlled and exploited to an advantage.

[Paul Barker]

I have started a new thread on farting about with lighting transformers.