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In Reply to: Re: I worship the DHT electric posted by Ian L on March 8, 2007 at 17:19:08:
Hi ,
To choose a driver , first you need to know the requirements of the output stage and if you're going to use an active linestage . Then you can work backwards to find suitable driver candidates . With DHT's you don't have a lot of choice , typical DHT driver valves have mu=3-6 Ra 2-5K , mu=8-12 / Ra 6-8k . There are also valves which have mu=30 but much higher Ra and a few unusual thoriated types such as 15E and 3C24 which have mu=20 Ra 15k . Try to avoid multi stage amps using battery valves , these have sub 200mA filaments and can be lead to microphony . Also on the same note ensure that there is not a hideous excess of gain as microphony on a two stage driver DHT amp can be a real problem . For your driver you will probably need to use DC , I would recommend LCL filaments , which is the bulkiest but IMO best sounding . Alternatively the Ronan reg style setups are ok (I think Andy uses these) , with valves with sub 250mA filaments a DN2540 depletion mosfet makes a good current source for filament heating . Other recommendations : use rubber isolation bushes to suspend the valves under the chassis , also make the chassis heavy and damp/brace extensively as a top-plate that rings also makes the valves ring . Once you go with DHT's , nail the quirks and enjoy the music , there's no turning back ;)cheers
Follow Ups:
Try to avoid multi stage amps using battery valves , these have sub 200mA filaments and can be lead to microphony .> >My answer to this is use balanced throughout, with constant current sinks under each stage.
Also on the same note ensure that there is not a hideous excess of gain as microphony on a two stage driver DHT amp can be a real problem.> >
Three stages should do it. Fatbottle's right - just use the amplification you need and no more. I'm OK with 3 stages, 3a5=mu 15, 1H4=mu10 and 2a3 outputs. Try and maximise the gain in each stage, e.g. active loads (haven't tried this with a CCCs under, but Morgan Jones got it to work, it's in his book), transformer coupling.
For your driver you will probably need to use DC , I would recommend LCL filaments , which is the bulkiest but IMO best sounding . Alternatively the Ronan reg style setups are ok (I think Andy uses these)> >
Yes - filament supply is essential. I don't think there are any really good published designs on the Net - the Ronan reg style setups work well. I found the circuit somewhere - forget where. You need a current source - sounds audibly better than a voltage source.
use rubber isolation bushes to suspend the valves under the chassis> >
I haven't done this. I did put rubber rings (plummers ones) round the 3A5.
also make the chassis heavy and damp/brace extensively as a top-plate that rings also makes the valves ring> >
Yes, yes, yes. My chassis plates are 4mm alu. That's pretty massive. Forget thin alu - must be rigid and not ring at all. Otherwise you have a bell.
Once you go with DHT's , nail the quirks and enjoy the music , there's no turning back ;)> >Amen
A word on small DHT tubes. My findings:
Doubles:
1G6G - mellow, but rather bland. Quite usable. Available cheap
3B7 - didn't like this one - harsh in my setup. Available cheap
3A5 - surprisingly good with real sparkle - top is bright, however. Tame the top and you have a very good sound. Available very cheapSingles:
30, 1H4 - lighter but detailed sound, good treble. Plentiful
VT-67 - 30 on steroids. Uncommon and pricey.
31 - lighter, detailed, useful lower anode impedence. Available
26 fat warm sound but detailed, some complain of dull extreme treble. Very plentiful
1G4 - mellow, warm verging on a bit tubby. Available
5676 - balanced, rather bland but very usable miniature. Available
12A, 01A - good allround favourites - 01A is a bit lighter sounding. Available
X199 - unusual, haven't tried it. PriceyDrivers:
49 strapped as triode. Available, good value
46 strapped as triode. Available
71A. Available, pricey.
Haven't tried the above enough to comment.I'd recommend pairing a light sounding tube like the 30 or 3A5 with a warm sounding tube like the 26. I haven't gone into this in detail, but I can see it's the way to go. I presently have 3a5 into 30 and it's a bit too light and bright - you need some warmth lower down to round out the sound. There again, with the 26 you need to brighten it up a touch in the treble. Consider this with your choice of output tube - with 300b you might want brighter drivers, with 2a3 you'd be looking for at least one stage with more body and warmth. And ultimately this depends on your personal preference - I like a lighter more transparent sound, others like a full warm sound with the detail still there but less in your face.
Hey thanks guys.
Still kicking tires. For the last couple of years I've been listening to a 6BX7 driving 6336 Parafeed. I love the sound and might be happy staying with it but the amps are huge (four channels for biamp) and hanging out all over the place and need a more contained rebuild if I want to make them keepers that I feel OK letting my friends near. Before investing the time and energy I'd like to give a linear DHT with about the same power output a try. My system setup is pretty well matched gainwise and to maintain that, it works out that with a PX25 as output, the driver mu would still be best at something around 10. For that, it would probably be easiest to use the 6BX7 again but you know how it is, new horizons beckon and all that, so a DHT driver IS tempting. . .
This is where I start to wonder though because with a biamped system the supply considerations are doubled.For a single stereo amp my leanings would be to something like your LCL fils Fatbottle, but eight DHT fils done that way would make amps I'd need a fork lift to get on and off the bench. I know there's got to be a good way of doing it, it's just taking a while to arrive at compromises I like the look of.
I've been searching around to see if anyone has tried using just a single choke as the L in an LR filter - the fil providing the R value - but haven't found anything more than a single hint and no clear indication of the degree of success obtainable. I'll give it a try on the breadboard.
Andy, thanks for your mention of the 12A. I ran across that tube a few years ago but never tried it. It's because of you that I got around to building a circuit with the 2C22 - which I really liked - so I'll probably give the 12A a go in the new amp - with or without DC fils.Anyhow, no hard data here, I really just signed on to say thanks for your replies.
If either of you happen to run across any interesting schemos or pages on DC fils would you please post a link?
Thanks, I'll post again with results once something gets built.
Lots of good information there. I especially like your comparisons
of the various tube types. I have a few of most of these types and
am looking forward to experimenting with them.I don't have an all DHT system yet but I am working on it. My next
design is going to be all DHT p-p amp to go with the all DHT pre
I have already built.DHT's gotta love em.
Glad to help. I should have mentioned 6J6G in the double triodes - balanced neutral sound, quite nice, bit bland, but very usable. Handy octal base, mu 14. Would probably go rather nicely with 1H4, another octal, which could be used as a driver, mu 9.5.
Any experience with the 3C6 Loctal? I have a few of these somewere.
Probably microphonic due to the low filament current but looks interesting.
Andy:I too liked your posts. Studied it and looked up a few tubes. Happy too that NJ7P tube database is back after a few days of hinky operation.
You mention 6J6G. I know the 6J6 to be an IDHT twin triode.
Do you mean 1J6G. I have been looking high and low for information on Class A operation of 1J6G. I finally bought some and tested on my RAT tube tester. However, I got a mu of 2- - 21. Now maybe I tested wrong -- you indicate a mu of 14, assuming you are talking 1J6. Do you have a data sheet on it?
However, I got a mu of 2- - 21
Sorry. Left hand broken - bad for typing.I meant 20 to 21. I have a Steve Bench design RAT tube tester I built three years ago. However, I did not build his supplemental power supply for DHTs, so the ony DHT I have tested was 1J6. I calibrated first with known IDHT tubes, but my mu readings were stable and consistent.
Anyway, my starting point was a search of the (now) many good web sources for datasheets to find some Class A info on 1j6, but to no avail.
This page links to a number of data sheets, but I didn't see any with class A.
Indeed - all the online data looks like class B. I don't know where I got mu of 14 from. Kurt Strain was using them - maybe something he wrote? Sounds about right, though. It's a smooth sound, but not holographic like the best DHTs.
"I don't know where I got mu of 14 from."I don't know for sure anymore, but 14 sounds about right to me, too. These are nice sounding, but a bit warm and fuzzy for a DHT, after looking into other DHT's out there. Very easy filament to heat, though, not a lot of current. They were designed for battery operated class B output stages drawing low power.
because he used 1J6s as Class A drivers. I studied a lot of his posts, but never saw a statement of mu for Class A purposes. I never asked him, concluding instead to procure and test. Have not tried them in a circuit yet.
I built a PP KT-88 amp using 1J6G as input and driver. Mu is indeed *about* 14, but it' hard to be accurate as there's no class-A data about. I just extrapolated the class-B data.
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