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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps
Post Subject: Don't waste time being "creative"Posted by nl on: 12/3/2008
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There is no "one tube DHT amp." There are only amps with gain in one box or two boxes.
Your 6E5P tube is not so special. People have been making WE437 spud amps since the 1960s. There is a class of amps that use a high-gm tube driver and a DHT output. Gordon Rankin has done this for about twenty years, and still does. However, I would suggest moving away from the high-gm driver, simply because the point is to try something new.
I will reproduce Jim de Kort's findings with the 437 spud, which roughly mirrors my own results. He used an amorphous Tribute transformer.
17-05-2001
All the wiring was done, double checked and then hooked up the amp. Everything reacted as I though it would. The 437's bias themselves nicely at 205V/25mA. I did some listening even though it is not wise to make a judgement just yet. All the parts are brand-new and take a while to play in (200 hours minimum). First impression: WOW!!!! Very direct sound, right in the middle of the music. The first minute it sounded like crap but eventualy started to turn around. Sound is a bit closed and rough around the edges, but this should change after it has been playing for a few hours.
19-05-2001
I made some measurements on bandwidth and power output. Better than I expected really. Bandwidth is 18Hz - 225kHz at -3dB. The low end was expected to be higher as the OPT's were designed for 50Hz and up (midrange), they don't have the inductance to go lower. As the 437A was intended to power my horns, the 20Hz is better than I expected to get anyway. Power output is 0.7W before any visible rounding of the sine, 1W max with clear signs of clipping/distortion. Square wave response is excellent even at 1W output, down at 60Hz is really the limit on the square wave test, very rounded. For those building this amp and wanting more power, use a 3K or 3K5 OPT.
The sound is becoming ever more open and clear. Another experiment that has paid off. The compromise with DHT's is that I need three tubes and acompanying transformers/chokes to get the same level of output (0.5-1W). This project proves that an IHT with high mu might be a compromise by itself when used in an audio chain, but I think the right tube used on it's own can sound better than three great tubes strung together to perform the same task. The key factor with this amp is that I can do with 0.5W perfectly. I don't have 90dB speakers that need at least 3-5W to come to their rights. I think this amp might even stay in my system for a long time to come. I'm almost thinking of not continuing my Exodus amps 
Nah...
25-05-2001
Playing-in the parts sucks big time!!! Highs are getting smooth, low is getting tight, but mids are too woolly and agressive. I hope it will pass. I've only been playing for about 50 hours now, anybody have a time machine?! Soundstage is very good, precise placing of the voice etc. Depth is wonderfull.
03-06-2001
Visited Bert Doppenberg and took my "amps" with me, also a nice benefit of a small amp Bert is very sceptical on anything that is not his, so I knew he would complain as ussual (grapje!). I was kind of worried that he didn't say anything just after hearing the first notes coming out of the amp. After a few minutes it was obvious he liked this amp, or he would have honestly told me his thoughts. So, now I am not the only one to agree this little amp sounds good.
The amp still has a lot of hours ahead of it to burn in all the parts, but it is getting better each day. I didn't think this idea would be anything else than a nice little replacement amplifier that I could put on a shelf if I didn't need it anymore. It just may up becoming my favorite amp Funny how things work out.
06-09-2001
Ai, Cold Turkey... I hooked up my system this weekend (finally). It hadn't played since the ATF2001, yes yes, I am ashamed of myself. I didn't want to hook up the Exodus amp as it was a lot of hastle with the batteries etc, so I just used the 437A amp. A friend called that evening saying he would drop by and wanted to hear my new setup (just got a new CD transport). Heck, now I really had to hook up the Exodus amps, ah well, it was a good excuse to get my ass off of the couch and get everything playing.
The tubes were lit up again, the 10 sticking out ofcourse with it's bright glowing tungsten filament. Always a nice sight!!! In went the CD, click... out came the music. What?! Couldn't believe my ears. Was the 437A really that bad??? It was like the music opened up a door and let in the air. Everything wa so smooth compared to before, it was music again. I really didn't notice with the 437A as I hadn't heard the good old DHT's in a while (due to all the rebuilding). I have gotten a big slap in the face again waking me up to the difference between a directly and indirectly heated tube.
Even a single tube performing all the tasks can't come close to my three tubes per channel DHT amp system. There is no getting around the immense difference in sound. Yes, the 437A and 6S45P sound incredible and perform a unique task, but compared to a DHT it really can't hold water. I was doubtful about the comparisson before, but now I am fully convinced. Even three stages of (good) DHT's can't be beaten by a circuit using only one IHT.
Basta!
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