DHT preamps

[pmcuk]

By now there are a few of us on the Wam using DHT preamps. DHT means a "directly heated triode" valve, like a 2a3 or 45 or 300b or 26 etc. As opposed to the usual type of valve which is heated "indirectly" by a separate heater circuit. Those include all the common types from 12AX7 to EL34. Those of us who have DHT preamps or DHT outputs in DACs can get quite addicted to the sound. It's typically transparent and detailed, but what it adds is a sense of life, vitality and directness which is very involving and makes most ordinary valves sound rather flat in comparison.

So hopefully we can use this thread to make comments and compare notes. So post your experiences here.

I've personally built preamps over the last 14 years with all kinds of DHTs so I know them well, and I've done a few shootouts with Ale Moglia of Bartola Valves who's a friend. He has a great website full of information on DHTs. Another great resource is Thomas Mayer of Vinylsavor and he has a lot of information on DHTs on his website.

In my own listening I like 01A but it's not practical, I like 26 but it's a little warm and euphonic, and I like 2P29L which is transparent and good all round and easy to work with. But the best results for me come with the bigger output type valves which add a larger sense of scale - more dynamic and even more involving. Here the list of contenders include 300b, 2a3, 6B4G/6C4C, 10Y, 45, 46 and 47. Maybe some others of the larger size DHTs. It's pretty hard obtaining a preamp with these kind of valves. Bottlehead do a 300b preamp kit in the USA. And there are some boutique makers with high prices making small quantities.

This is mostly the kind of preamp you have to build yourself, and it demands some good quality design and iron in the filament and power supplies. So it would be good to hear from Wammers who have built their own DHT preamps.

 

[rabski]

I have a lash-up chassis for DHTs, but I really need to try a 'final' build of one to iron out the niggles. I've tried a few of the usual suspects: 01A, 26, 2P29L, 30, etc. All my experimental builds so far have used Rod Coleman boards for filament bias and either straightforward plate resistor loading or your idea of choke loading using 'standard' Hammond iron in series.

All of them have shown serious promise, but I haven't got round to sorting the issues for 'real world' use. Grounding and layout are a bit of a pig in avoiding hum issues, but even then - and with serious work at getting power supply noise down - I've yet to manage a really low noisefloor. Microphony seems somewhat hit and miss, as some valves are prone and others less so. I have just about enough to be able to swap around and find the quietest ones. What I really need to do is optimise a suitable HT supply, as I'm not convinced what Ive tried is ideal.

Getting valves that match reasonably in terms of plate current seems more tricky, and also getting valves that stay stable in that regard. Mine so far seem prone to drifting after a while, though as above, I really need to do a 'full and final' build to see what happens over a longer time.

I haven't tried any of the larger DHTs yet, but there are 300B, 2a3 and 46 in the pile here (and doubtless some other suitable) so that may be something to work on.

Too many options, not enough time...

 

[Jules_S]

Call me stupid (most people do...) but I didn't realise that the 300B was directly heated, so I've learned something already from this thread. Thank you for that!

 

[THOMO]

Pmcuk,
You might be interested in looking at the Supratek blog or talking to Mick Maloney about DHT preamps.I believe he was one of the first people to start making DHT preamps and has a lot of experience in that area and I am guessing going back at least 20 years.
I have been using a Suprtatek Cabernet 300B/45/PX4 preamp for about 14 years.I have compared it with plenty of other preamps both valve and SS in that time and have always preferred it.That has mostly been with SS power amps.I think using a DHT preamp is a very effective way of getting valve sound into your system without having to use valve power amps.The main thing is that the SS power amps let the valve character through and not all of them do and you will only know that by trying them.For example two power amps I owned were the Rotel Michi and Musical Fidelity A5 which are good sounding amps in a SS sort of way but they imposed too much of their own sound.And yet a cheap 3876T gainclone type chip amp lets the preamp character through and sounds excellent.As does several other power amps I use.The Nelson Pass amps seem to be transparent and respond positively to the DHT preamp.The best combination I have heard was the Supratek with an Audio Flight Strumento 4 power amp which is their flagship power amp.That driving Magico S5s produced an amazing sound and transformed that system from sounding good using the matching Audio Flight Strumento preamp [which costs 5 times as much] to something really special with the Supratek.A shame I can't afford one of those Audio Flight power amps!

 

[Psilonaught]

I invested in a type 45 DHT preamp and it has offered a worthwhile improvement in timbre. Based on several reviews I ordered the KR Audio P135.

I was tempted by a Supratek but I was informed by a few folks on here that Mick's preamp isn't a proper DHT design given the use of a 6SN7 in the path....

I use an Emission Labs 45 Mesh Globe.

Power amp and DAC also triode based.

Review

 

[Psilonaught]

On a separate note it has been fascinating to compare my 300B tubes to 45 (Ken-rad) in my DHT dac. I initially thought the 45 better and it is a smidge more organic than my Cossor WE300B replica but the 300B is fast and really punchy. Definitely an interesting comparison.

I want to compare a pair of GEC PX4 to the 300B/45 but summer holidays have cleared me out so I won't be buying any until the autumn

 

[pmcuk]

The 300b is one widely used preamp tube these days. Less interesting as just an output tube with 6SN7/12AU7 inputs as in some designs. Better as a pure 300b preamp, like the Bottlehead BeePree2. https://bottlehead.com/product/beepre2-300b-preamplifier-kit/

This is an interesting read, since it outlines some of the issues Bottlehead faced with the 300b. They ended up putting a 50K stepped attenuator after the 300b rather than before it to keep noise down. They could have chosen the 2a3 instead of the 300b but didn't - I prefer and use the 2a3 or its 6.3v versions the 6B4G and 6C4C, more focussed and less euphonic. I guess the 300b has a "name" that attracts punters, but the 2a3 is still made and available. One thing is that it's more tricky to use with DC filaments since it's 2.5 amp where the 300b is 1.25 amp. The 6B4G/6C4C is 1 amp filaments which is the most manageable for DC heating, but the 6B4G is obsolete apart from NOS versions. Still some 6C4C about but less than there were and it's Russian. So for ease of use and availability the 300b is an understandable choice I guess.

 

[steve 57]

Cossor WE300B replica but the 300B is fast and really punchy. Definitely an interesting comparison.

It's nice to see some of the old valve manufacturers names being used again.
Cosser actually never made the 300b in the day, only stc which was a subsidiary of WE, the only British company to produce them, mostly in their Paignton factory.

Cosser did make some really good valves in the day.
I've used cosser 41mp globe mesh plates, although indirectly heated they are every bit as good as an equivalent directly heated valve from the 30s.

My experence is that both anode size and the metals used are more important when it comes to sound quality.. But it's an interesting subject.

 

[Psilonaught]

It's nice to see some of the old valve manufacturers names being used again.
Cosser actually never made the 300b in the day, only stc which was a subsidiary of WE, the only British company to produce them, mostly in their Paignton factory.

Cosser did make some really good valves in the day.
I've used cosser 41mp globe mesh plates, although indirectly heated they are every bit as good as an equivalent directly heated valve from the 30s.

My experence is that both anode size and the metals used are more important when it comes to sound quality.. But it's an interesting subject.

https://www.stereophile.com/content/gramophone-dreams-48-venus-tube-western-electrics-300b-page-2
Interesting read and it talks about the Cossor WE300B that I have. Not cheap but exceptional. They are superb, probably because they are a Linlai product.

 

[MF 1000]

I love my type 26 DHT preamp …..can’t claim anything over its design & build - above my paygrade as they say. It’s an Ale basic design ….built by Rui Lorenco and then moved on to Simon Shliton (SJS) who removed the glow tubes from the psu, fitted quite a few foo components (clarity & hovland caps, Coleman regs etc) and got it to the level it’s remained at for the past 6 years.

there is a write up about its original build on Ale’s website

 

[steve 57]

I prefer and use the 2a3 or its 6.3v versions the 6B4G and 6C4C, more focussed and less euphonic.

Thats interesting,
I find those twin anode valves have the sound of parallel single ended amplifiers in general. They sort of thicken the sound up at the expense of some signal cancellation when 2 unmatched devices are working together in parallel
I suppose our view of the the actual description of the sound is the same though ?

 

[rabski]

Reading this, and having a good idea of some people's 'ears', I really need to look at trying some of the bigger DHTs. From the ones I've tried, sonically the 01 in various forms has been the best for my tastes, in terms of producing that superb 3D feel and correct tonal feel. However, it's also been the worst in terms of finding ones with reasonably close matching plate current and also so far near impossible to get quiet enough for my liking.

The obvious solution I suppose is a CCS, but I've yet to try CCS anode loads that don't seem somehow to flatten the sound and take things away just a little.

Andy and Steve, what are you using for B+ and plate load with 2a3, 300B, etc?

 

[JamieMcC]

Cossor WE300B replica but the 300B is fast and really punchy.

I'm finding this with the Cossor WE300B replica as well. I can also run 2A3 in the same amp which sounds superb after rolling but after a week I start to miss that speed and punch particularly in the lower registers in comparison to the 2A3 and swap back.

More on topic about 10 years back I had a Bottlehead Quickie which is a battery powered single ended dht preamp (3S4 tube a Pentode but wired as triode ) probably as stripped down and as minimalistic as possible two resistors and one 3S4 tube per channel. The kit used to sell for $99 there was a constant current upgrade which was a nice addition.

It was my introduction to diy very easy for a first time noob to assemble. Later I used it as the pre in front of the Nelson Pass ACA monoblocks which were built up from a PCB with secondhand heatsinks and home built enclosure.

I've come a long way on the journey since but that little combo around £200 total blew my mind coming from budget integrated's

Im tempted to build it again sometime.

 

[steve 57]

Reading this, and having a good idea of some people's 'ears', I really need to look at trying some of the bigger DHTs. From the ones I've tried, sonically the 01 in various forms has been the best for my tastes, in terms of producing that superb 3D feel and correct tonal feel. However, it's also been the worst in terms of finding ones with reasonably close matching plate current and also so far near impossible to get quiet enough for my liking.

The obvious solution I suppose is a CCS, but I've yet to try CCS anode loads that don't seem somehow to flatten the sound and take things away just a little.

Andy and Steve, what are you using for B+ and plate load with 2a3, 300B, etc?

I tend to use all my old valves conservatively 10 to 50 volts below recommended max
My stc 4300a have never been in one of my my amps. I took them to one of my audio talk meets and they had an airing in one of nick gorham's amps.
He told me a few months later it put him off using modern 300b's as all he could hear was what was missing.. although there have been some much better modern valves available since then.
I don't actually use a pre amp as I have not found one that does not take something away with my set up. But most of my amps the source is feeding straight into an output valve so there's little difference except the extra box.

I'm finding this with the Cossor WE300B replica as well. I can also run 2A3 in the same amp which sounds superb after rolling but after a week I start to miss that speed and punch particularly in the lower registers in comparison to the 2A3 and swap back.

More on topic about 10 years back I had a Bottlehead Quickie which is a battery powered single ended dht preamp (3S4 tube a Pentode but wired as triode ) probably as stripped down and as minimalistic as possible two resistors and one 3S4 tube per channel. The kit used to sell for $99 there was a constant current upgrade which was a nice addition.

It was my introduction to diy very easy for a first time noob to assemble. Later I used it as the pre in front of the Nelson Pass ACA monoblocks which were built up from a PCB with secondhand heatsinks and home built enclosure.

I've come a long way on the journey since but that little combo around £200 total blew my mind coming from budget integrated's

Im tempted to build it again sometime.

In audio simple can usually turn out better results. Valve pre's into solid state is good combination for many systems.
Most sceptics blame valve distortion for that, but I firmly believe that not the reason
Many old output valves work well because they can amplify just a whiff of signal and keep it all in tact. They don't need a high current input.
Add to that most output valves are designed to amplify 20 to 80 volts input, are very low distortion when presented with just a couple of volts to amplify at the control grid, you can easily understand what the fuss is about when used as a pre.

 

[Psilonaught]

The obvious solution I suppose is a CCS, but I've yet to try CCS anode loads that don't seem somehow to flatten the sound and take things away just a little.

My atma-sphere OTL use CCS via a 6SN7, I don't fully understand its purpose.

 

[pmcuk]

Reading this, and having a good idea of some people's 'ears', I really need to look at trying some of the bigger DHTs. The obvious solution I suppose is a CCS, but I've yet to try CCS anode loads that don't seem somehow to flatten the sound and take things away just a little.

Andy and Steve, what are you using for B+ and plate load with 2a3, 300B, etc?

Read up on the Bottlehead BeePree2 which is the nearest thing to what one would build. I found it very informative and I'm going through the Bottlehead Forum posts on it as well. The original BeePree used a 3K anode resistor, but the revised version uses a C4S CCS as load. You can easily substitute Ale's gyrator here. I agree that I don't like CCS loads because they mess with the tonality of acoustic instruments. I'm starting with a resistor load and might try a plate ckoke as well. I'm going to make a prototype 300b pre and will start with 20mA, 125V and -26V with a 5K anode resistor and B+ around 250V. I'll get that from a 230-230V isolation transformer, e.g. Airlink do some cheap, and 1200V SIC diode bridge. Rod Coleman filament regs. A very familiar build for me. It could be adapted for a 10Y as well, which would be a bonus.

In terms of noise, Bottlehead put the volume control after the 300B, and they make a 50K 36 step attenuator from 2x 6 step ones - course and fine adjustment. That makes sense in terms of switch availability. The better solution is a TVC or an autoformer AVC like Dave Slagle's. It's not a hugely complicated build. I put big 2A chokes in the filament supply but that's a luxury.

Bottlehead have had their 300B preamp going for 10 years and have upgraded it as a result. It's actually not too expensive compared with boutique jobs. That is if you don't go nuts with the suggested 10uF output caps and buy teflon ones. There are a lot of 300B preamps which actually aren't because they use 6SN7 or 6SL7 as well and the 300B just as outputs. Supratek, Transcendent, Ming-Da, Audio Space, Manley..... Thrax Libra is a bonkers build, all balanced with just the 4x 300B tubes.

 

[JamieMcC]

In terms of noise, Bottlehead put the volume control after the 300B, and they make a 50K 36 step attenuator from 2x 6 step ones - course and fine adjustment. That makes sense in terms of switch availability.

I have one of the same 36 step Bottlehead attenuators in my Bottlehead Mainline (plus some 10uf teflon capacitors ). The attenuator is a absolute joy to use pretty sure I used Vishey Dale RN60 resistors.

It's worth noting that Bottlehead charge $349 plus shipping and import fees to the UK add probably another £100 on top.

If you use a pair of Elma 01-1263 switches £25 each you can build a high quality version of the same attenuator well about a third of the cost of Bottleheads.

https://www.ebay.co.uk/itm/26427063...rentrq:de6a74f41810a69d24b4ce92fff82214|iid:1
Glassware also do a DIY version on a PCB called the A5 for $42 including resistors which and is easier to assemble but the switches are more agricultural in action it's clunk clunk rather than the lovely click click of the BH one. The switch contacts of the two I've built tend to I'm guessing tarnish if not used regularly which can lead to a bit of crackling on first use when switching after sitting for a while this does disappear after a bit of use.

https://glass-ware.stores.yahoo.net/a5stat.html
There is also a more premium 66 position version with higher quality switch gear.

https://glass-ware.stores.yahoo.net/atstat2.html

 

[pmcuk]

Very useful post about attenuators - something to consider.

I've been making prototype DHT preamps recently with my modular construction system, and the latest one is a 300b, inspired by Bottlehead and others. I'm not a big fan of the 300b, I find it euphonic and rather veiled compared with other more focussed and lively DHTs including the 2a3 or the Russian 6.3V version, Svetlana 6C4C. So I was curious to hear it in a preamp, and it was exactly the same. Smooth, pleasant, euphonic but rather lifeless. So I'm going to convert the preamp to a 10Y which should give better results. It's Thomas Mayer's favourite preamp tube.

https://www.emporiumhifi.com/head-office-norfolk-suffolk/thomas-mayer-10y-line-preamp/
I can understand why the 300b is used in commercial products - first it's the most available of the larger tubes, and then it's not over demanding to implement. But I don't think it's the best choice sound wise. Sounds very like the 26 in fact, and of my recent builds I'd prefer a 2P29L or best of all the 6C4C. Still to come 10Y, 46 and 47.

Update: I now have the 10Y preamp up and running. It's clearly better than the 300b as a preamp tube. Smooth, detailed, refined, involving, very nice. A sound you can live with. I'm going to build a few of these. I still like the 6C4C a lot - it has boogie factor and it's very detailed. So definitely a result here. Going through all my favourite tracks with the 10Y.

 

[Jazid]

I'm slightly at a loss to understand why people are now choosing big bottle DHTs for their pre amp circuits? What am I missing here?

 

[Psilonaught]

What makes a 6S4S /6C4C superior to 2A3? They are identical apart from the octal and heater voltage?