What do DHT (direct heated triodes) REALLY sound like? Yes we've heard VT25, 45, 2A3, 205d, 300B etc in many an amplifier, enshrouded by driver tubes and output transformers. And we can plainly hear the differences between DHT types in that context, tainted though the sound must be with so many extraneous elements.
The only way to really hear the sound though is to go spud. And what better place to do that than right at the beginning - the linestage. Many will say that the linestage contributes more than the amplifier, so...
UX4 sockets: this would allow 10/10Y/VT25, 26, 45, 71A, 2A3, 801A/VT62, 300B, 101D, 205D if we can design in enough control to supply the variety of filament, and B+ voltages needed. Used spud we'd have a friendly gain structure of between 9 and 18db. Most have lowish anode impedance; no transformer needed as in the range ideally suited for preamp duty. So...OTL it is!
With adjustable CCS for the anode we can control the anode current. Easy done.
Since this is an all-out attempt to hear the real DHT, and all of it, we don't want un-necessary caps in the signal path. So let's go fixed bias, thus avoiding the cathode bypass cap. To make this easy we have our ABS module feeding the grid which can be set with one resistor for the desired operating point.
Power supply: to stay with the theme lets use a DH rectifier tube. We don't want ANY electrolytic caps blowing smog into our sonic landscape so we will let the all film-cap FCUPS module provide the P:USH power we need. This has the added advantage of supplying the bias voltage needed by the ABS and bonus: jumper setting for virtual battery or virtual choke operation. And it is super quiet.
The filament supply also needs to be quiet, be a brick wall to the signal (DHT, remember?) and be adjustable: DIYHFS filament supply just happens to be on our shelf.
Transformer noise will be an issue so let's order a potted (3mm steel) torroid which has the advantage of radiating EMF axially thus avoiding noise in the chassis. Our goal is to do this all on one chassis. We'll mount tx forward, to keep it away from the signal end of the chassis.
We'll need one pair of 2.2uf/250v coupling caps for output and 1 pair 0.1uf on the input grid.
Chassis will be the Projekt Box, also at hand.
Layout will look something like the attached.
Here we go!
DIY - Done Right!
Follow Ups:
- RE: DHT OTL Linestage - Tram 2 - driguy 14:33:06 12/16/09
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- In Reply to: RE: DHT OTL Linestage - Tram 2 posted by bcherry on December 16, 2009 at 00:36:09
Very nice concept Brian. I built a similar unit using a 26 tube and a cap coupled output. You may want to leave some room for folks to experiment with different output caps as they make a large difference as one would expect. I use a battery for the filaments but your supplies should work as long as they are really quiet. Any noise will be heard as you already know especially with high efficiency speakers.
I do not have any experience with your FCUPS module but if it sounds like the specs read it should be very nice. The power supply needs to be really good to get the best out of the DHT tubes. I used a tube regulated supply after trying it without it. The soundstage really opened up after putting in the regulating tubes.
The CCS on the tube works very well just as it did on my LadyDay amps. After building a unit like this I think a lot of people will really be blown away by what it will do to their systems overall sound. It is truly revelatory especially for those who have been used to SS, TVC's or even units based on the smaller dual triodes like the 12AX7 and even the 6SL7's. Good luck with the unit. I would encourage anyone who has not heard a DHT driven preamp to give this one a shot.
Happy Holidays to all!
Tony
- RE: DHT OTL Linestage - Tram 2 - bcherry 16:46:17 12/23/09
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- In Reply to: RE: DHT OTL Linestage - Tram 2 posted by driguy on December 16, 2009 at 14:33:06
Hi Tony
Yes we will use a largish chassis with suspended subplate. As is our standard now, this will be all film caps.
I'm interested to know what noise levels you were able to achieve at the output of your 26 linestage. Is it all in one chassis?
I agree, the sound of the DHT linestage is mostly undiscovered and only the brave dare go there.
regards
Brian
DIY - Done Right!
- RE: DHT OTL Linestage - Tram 2 - driguy 17:27:18 12/23/09
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- In Reply to: RE: DHT OTL Linestage - Tram 2 posted by bcherry on December 23, 2009 at 16:46:17
Hi Brian, I was able to reduce the noise to a nearly inaudible range even using a single chassis and a toroidal power transformer. I did use a shield between the AC and DC sections much as you do with your LadyDay amps. I did use a thicker plate (4mm)and I think that helps with low frequency problems better. I can barely hear any AC hum component at all and only if I put my ear within 1 foot of my 100bB sensitive speakers. I actually get more noise from a little tube "rush" than from the power supply. I think I would be able to completely eliminate what little hum I have by using an "R" type transformer as a supply especially since the need for current is so low. I accomplished my low noise by using Mundorf M Tube caps and chokes but as you can imagine this is expensive, bulky and heavy.
Getting rid of electrolytics in the power supply IME is almost as important as it is in the signal coupling area. Even Blackgates cannot reach the level of musicality that film caps seem to be able to reach with relative ease.
The largest problem as you may be aware with the DHT's in a preamp, is the filament heater supply. I could not get it quiet enough with a regulator and caps. If you have one of your filament heater modules set up for very low noise like the TentLabs "low noise" units, that in my mind would be the very best way to go especially from a product packaging point of view.
I do not want to push too hard on any particular point but as with the LadyDay amps, a CCS setup works extremely well with my 26 tubes. Of course it adds some power supply rejection as well but it does require additional volts to drop across it as well.
I was using a highly modified Audio Note M3 kit before I built the '26 and without any break-in and some (well, lots of) noise from the filament supply I could tell within a minute that I made the right choice by building it. Personally I feel that other than making the choice to go to tubes and especially single ended amps that your customers will look at choosing a unit like this will make the biggest difference in how music is presented in a system. Good luck with it.
Happy Holidays to you,
Tony
- RE: DHT OTL Linestage - Tram 2 - bcherry 23:16:00 12/23/09
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- In Reply to: RE: DHT OTL Linestage - Tram 2 posted by driguy on December 23, 2009 at 17:27:18
Thanks Tony
Appreciate you sharing your experiences.
Playing now with 45's and we're getting there. Agreed about film caps 100%.
Brian
DIY - Done Right!
- RE: DHT OTL Linestage - Tram 2 - bcherry 23:16:00 12/23/09
(0)
- RE: DHT OTL Linestage - Tram 2 - driguy 17:27:18 12/23/09
(1)
- RE: DHT OTL Linestage - Tram 2 - bcherry 16:46:17 12/23/09
(2)
- Now that is a brilliant concept Brian! - Bas Horneman 01:31:43 12/16/09
(8)
- In Reply to: RE: DHT OTL Linestage - Tram 2 posted by bcherry on December 16, 2009 at 00:36:09
I'm about to build a similar preamp. Unfortunately without the flexibility that fixed bias affords. But it all did cross my mind before to do something as you propose here. This should become a real hit...
To infinity and beyond!!!
- RE: Now that is a brilliant concept Brian! - bcherry 16:57:13 12/23/09
(7)
- In Reply to: RE: Now that is a brilliant concept Brian! posted by Bas Horneman on December 16, 2009 at 01:31:43
Hi Bas
Yes I'm quite excited after our first listen - it definitely raises the bar with the biggest boldest and sweetest sound I can remember.
Yesterday I tried about 10 different sets of NOS/new 2a3 and 45 (globe and ST). I found only 1 pair that didn't have a problem with microphonics in this linestage: NOS 45st. These were also the smallest tube and had the least mechanical resonance when dinged with a fingernail. The next best were NOS 2A3st.
The worst were the globes, except the baddest of them all (rang like the Liberty Bell) - VA 2A3B single plate. That one has coil springs across the top holding the filaments.
We've got the hum tamed and now just trying to track down the source of some sizzle.
regards
Brian
DIY - Done Right!
- RE: Now that is a brilliant concept Brian! - driguy 15:05:19 12/28/09
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- In Reply to: RE: Now that is a brilliant concept Brian! posted by bcherry on December 23, 2009 at 16:57:13
Hello Again Brian,
I just ran across an article in 6Moons about Emission Labs producing a run of 45's in case you are interested. Their 300B XLS tubes are fantastic IMO. The microphonics in the DHT's are a real problem. I mounted mine inside a tall case to help in this regard. I realize that some methods are a little difficult for a production run but I had good success with the following.
1.Herbie Audio Labs tube dampers. 2. Mounting the tube sockets using rubber grommets or washers. If you can make up some washers made of sheet sorbothane it should help even more. 3. Last and perhaps most effective in eliminating both microphonics and RFI may be to have the tubes mounted inside a small size transformer cover (1 each) with vent holes. As the 45 does not generate that much heat a few holes at the top and bottom rear of the cover would do the trick and keep aesthetics intact. No, it is not the same as having them exposed but you still have the rectifier tube there and the possibility of removing the cover and mount if the customers' system will allow it. A good test for RFI seems to be my cell phone. I hope this helps in some way.
Regards,
Tony
- RE: Now that is a brilliant concept Brian! - bcherry 03:31:58 12/29/09
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- In Reply to: RE: Now that is a brilliant concept Brian! posted by driguy on December 28, 2009 at 15:05:19
BTW, with inputs shorted would you know output noise? With NOS 45 I'm getting 0.3 - 0.4mv. Anything else is more. TJ Globe 2A3 and VA 2A3B are the worst > 2mv.
So far all the discussion i'm seeing on DHT linestage only references subjective noise levels.
regards
Brian
DIY - Done Right!
- RE: Now that is a brilliant concept Brian! - driguy 16:38:31 12/29/09
(3)
- In Reply to: RE: Now that is a brilliant concept Brian! posted by bcherry on December 29, 2009 at 03:31:58
I just measured the output noise and it comes in at 0.2 to 0.3 mV. Not too bad I guess. I should have measured it previously but the system is pretty revealing of any noise. The LadyDay amps are dead quiet now so they contribute pretty close to nothing in the way of noise.
I wish that more folks could really see what it takes to get some of this stuff right. It's not just the parts! The moment when it all comes together is pretty satisfying though. Thankfully your DIY guys know first hand what any change can bring about. I can't wait to see what your end result is!
Happy New Year,
Tony
- RE: Now that is a brilliant concept Brian! - bcherry 16:29:33 12/30/09
(2)
- In Reply to: RE: Now that is a brilliant concept Brian! posted by driguy on December 29, 2009 at 16:38:31
Hi Tony
Thanks for that. Gives us something to shoot for.
You're right about it being more than the parts. How many times we build something and take it apart and build it again with the same parts, just to get it right.
Brian
DIY - Done Right!
- RE: Now that is a brilliant concept Brian! - monch 06:04:39 12/31/09
(1)
- In Reply to: RE: Now that is a brilliant concept Brian! posted by bcherry on December 30, 2009 at 16:29:33
Thinking along those lines, what's the state of development of the LD Mono Max?
Are you any closer to a final design and its pricing?
TIA
- RE: Now that is a brilliant concept Brian! - bcherry 17:01:56 01/02/10
(0)
- In Reply to: RE: Now that is a brilliant concept Brian! posted by monch on December 31, 2009 at 06:04:39
Yes! Power transformers due to arrive Monday so we'll have the new pricing a day or 2 later and should be ready to go by the end of the week.
Thanks for waiting.
regards
Brian
DIY - Done Right!
- RE: Now that is a brilliant concept Brian! - bcherry 17:01:56 01/02/10
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- RE: Now that is a brilliant concept Brian! - monch 06:04:39 12/31/09
(1)
- RE: Now that is a brilliant concept Brian! - bcherry 16:29:33 12/30/09
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- RE: Now that is a brilliant concept Brian! - driguy 16:38:31 12/29/09
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- RE: Now that is a brilliant concept Brian! - bcherry 17:34:01 12/28/09
(0)
- In Reply to: RE: Now that is a brilliant concept Brian! posted by driguy on December 28, 2009 at 15:05:19
Thanks for the tips. We are testing with NOS 45. So far I've found, the bigger the bulb, the worse the microphonics. I've mounted the tube sockets with 2 very compliant rubber mounts and seems to handle chassis pickup quite well. Also been trying various tube covers. We're getting there.
regards
Brian
DIY - Done Right!
- RE: Now that is a brilliant concept Brian! - bcherry 03:31:58 12/29/09
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- RE: Now that is a brilliant concept Brian! - driguy 15:05:19 12/28/09
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- RE: Now that is a brilliant concept Brian! - bcherry 16:57:13 12/23/09
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