Why Altec chose beam power tubes over triodes

**RonSSS** · November 1st, 2007, 05:57 AM

Dave,

You ought to investigate DHT triode output stages. True, Pentodes or beam power tubes can be wired as triodes, but it's sort of like having a bunch of extra stuff in the way. (screen grid connected to the plate) Pentodes are better than beam power tubes when wired this way. But still not a real triode.

I have 4 Heath W5M's converted to 6B4G's. They make about 9 watts. I converted the driver stages to low impedance 5687's. Tons of bandwidth. (Better than 100khz) Perfect square wave response. And they are oh so sweet. But don't have enough power to get the model 19's cranking in the low end. At least not when you want to really rock and roll the blues, or big band.

The 300B PP amps on the other hand have plenty of power. The drawback of the 300B's? The long grid and plate structures are michrophonic! The output from the speakers tend to modulate the output. In fact, the structures are michrophonic w/o even being near the speakers. Something I doubt ever gets noticed with low efficiency speakers. It might be why folks find 300B's so sexy.

Still, they sound **** good.

Both sets of amps sound even better with the duplexes.

I have had SET amps in my system...borrowed from friends. All 300B. I personally didn't hear any real benefit over the PP amps. But I didn't have them long enough to give a fair comparrison.
Hum of course is tougher to tame in an SET, especially when my buds don't have high efficiency Altecs.

I'll never trade my tube stuff. I'm from that era. And although I'm sure there is high end SS stuff that sounds great, I would have to buy, not build it. And building my own, is just the way I am. Tubes sound great, I can roll my own, and shoot, they are just wy more sexy than SS amps...they glow!

Ron

**selmerdave** · November 2nd, 2007, 03:13 AM

Hi Ron,

I've heard some DHT amps, 300B, 845, 2A3 and PX25 or whatever that one is. Those were all single-ended. I'm not saying I don't prefer them, or that I think beam-power tubes are better (or triode-wired pentodes for that matter). Most triode amps are above my budget as are the tubes, and most of the equipment that I have has higher impedance OPT's so not really triode-friendly.

So I'm not passing judgement on triodes, the ones I heard sounded great, particularly the 845 which was Sakuma's amp and was really impressive. I happen to have a few amps that use beam-power tubes and found the above article interesting with respect to their application.

Dave

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**amandarae** · October 17th, 2010, 01:08 AM

I am on the SET camp!

I had tried some commercial SS amps in my system. All I can say is that in my experience, nothing comes close to the clarity of DHT's (I prefer 2A3 and 45) using my Altec drivers.

As for hum, an educated layout of the components and circuit design is the key. I have 1.9 mV of hum on my 2A3 with the amp vol pot fully open and preamp volume fully open! Around 1mV when using 45. For 300B, I use Pete Millet's DC filament circuit after trying many schemes to make the hum less than 2.5mV on the amp. With Pete's circuit, I have 0.4 mV maximum hum, not even audible, just measurable.

If you have not tried DHT SET (not triode strapped pentode!!) amp, then I would suggest to borrow one and see for yourself especially an AC heated 2A3.

Abe

**analog addict** · October 17th, 2010, 12:44 PM

I'm a believer in the flea amp too. Of course it helps when you have speakers that can sing with 1-2 watts. I'm not running anything too fancy but the simplicity of a SE 45 amp on some vintage Altecs is magic.

I'm still figgering out what the best pre amp might be, but the current one is nothing more complicated than a modded Bottlehead FP II.

**amandarae** · October 17th, 2010, 02:56 PM

analog addict,

I have a preamp for you! I mean, a suggestion only of course. I am thinking of saying a type 26 preamp, but it is too complicated to make it quiet.

The next best thing , for me is a type 76. I can send you the schematic as published in Sound Practices magazine way back, or the improvised schematic from a personal friend of mine(Joseph Esmilla of JE Labs). It will cost around $700 to build. I have, and some of my friends, compared this to a Supratek Syrah/Chenin(I own one), Manley, Aikido, Bottlehead, Cary, AES, CJ and a S&B TX series passive preamp, etc. in the under $5000 range and to us, it comes on top everytime, especialy when using Altec drivers for speakers.

Let me know.

Abe

**martyh45** · October 17th, 2010, 07:15 PM

26s are cool and sound fantastic? but yeah, they are hard to get quiet. 71A sounds great and the filament is easy to supply. With a choke load on the 71A you get about the same gain as a 26 with a proper output transformer which is about none. Zout is reasonable and the right plate choke is considerably less expensive than a good output transformer for the 26.

I?m not down with the whole IMD as be all and end all measurement or THD for that matter. I?ve heard really good and really bad amps and distortion specifications don?t correlate that well. But if you are into that type of thing triodes are pretty hard to beat as linear low distortion devices.