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DHT driver for triode wired SE EL84, 6V6 or EL34

Hello All.

I have a question regarding the use of DHTs as driver tubes for triode wired pentodes in a single ended amp. I have had a look online, but have not found anything about this topic. Why is this the case? Are DHTs unsuitable as drivers in this case?

If a DHT is able to properly drive a pentode wired as a triode, wouldn't the goodness of the DHT driver come through if that power tube was properly driven?

I have a reasonably good stash of 01A, 26, 71A, 30, and 31 tubes (and dozens of IDHT 27s), and I'm looking to build a small amp for use with the HF section of my horn two way speakers. The amp would do 500 HZ and up and would not require more that 2 watts of output. I can't find a single schematic that uses a DHT in the driver position, and a triode wired pentode in the power tube position.

Thanks in advance for your input on this.
 
As it happens, this is exactly what I've been experimenting with. It's a kind of back-to-front DHT amplifier or "Inverted DHT Amp" to give it a catchy name. I've been trying out quite a few output tubes with mu of 15-20. In medium power, you should look at EL33 in particular, but also KT61 and EL41, in that order. I'm talking triode connected here. EL84 is OK but the EL33 in particular is better and the KT61 is nice. In higher power look at EL12n or 7591.

For the DHT the 3a5 as described has the highest gain at around 15. Then the 4P1L at around 11-12, though I might prefer the 2P29L. 26 is good. 01A would be best, and the one I use myself, but it may need a cathode follower. Try it and see! The grid leak on your output tube should be as large as can safely be used in self-bias to make it easier to drive. Also, have a look at Ale's gyrator on Bartola Valves as an anode load for the DHT. I like plate chokes myself, but the gyrator is good and will give you a low output impedance. Use a Rod Coleman reg and filament bias for best results.

I've knocked up a specimen circuit to have a look at. Haven't built it. Hope there are no obvious mistakes in it!!
 

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I tried the 3A5 and the 3B7 as drivers for a 2A3 and vastly preferred the sound of a 12AT7 for whatever that might be worth to you. If you want to build something on a piece of plywood, a D cell battery is a decent filament supply for experimentation purposes.

This doesn't surprise me too much. I really disliked the 3B7 and thought the 3a5 was a pretty average DHT. 26, 01A and 10Y are a totally different story.
 
As it happens, this is exactly what I've been experimenting with. It's a kind of back-to-front DHT amplifier or "Inverted DHT Amp" to give it a catchy name.

"Inverted DHT Amp" sounds like a plan. For my specific use, 1-2 watts output is perfect. Would a EL84 do in that case? I don't know if I have any EL33, KT61 or EL41 on hand, but I'll have a look. With respect to the driver tube, I'll check through my stash for the tubes you recommend that are not ones mentioned in my first post.

The 27 is not a DHT, but I would be interested in using it somewhere as I have dozens of them, with more than half being of the globe type. It has an amplification factor of 9, so a bit more than a 01A. How does it sound compared to a 26? It being a IDHT should make the heater supply a lot easier to do properly.
 
Both section in parallel 3a5 should drive triode connected el84 and 6v6. I am not sure about el34. This is a good idea, please go ahead. If you need help please don't hesitate to ask.

Regards

I'll check to see if I have any 3A5 or the 3B7 also mentioned by audiowize. I have several boxes I bought a while back of what I thought were "reject" tubes that I have since gone back into and pulled some good stuff out of. Any other viable options for DHT drivers, and any other power tube suggestions would be much appreciated. I will go through the boxes again and look for all the ones mentioned in this thread. 6V6 or EL84 should be fine given my low wattage requirements.
 
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"Inverted DHT Amp" sounds like a plan. For my specific use, 1-2 watts output is perfect. Would a EL84 do in that case? I don't know if I have any EL33, KT61 or EL41 on hand, but I'll have a look. With respect to the driver tube, I'll check through my stash for the tubes you recommend that are not ones mentioned in my first post.

The 27 is not a DHT, but I would be interested in using it somewhere as I have dozens of them, with more than half being of the globe type. It has an amplification factor of 9, so a bit more than a 01A. How does it sound compared to a 26? It being a IDHT should make the heater supply a lot easier to do properly.

A 27 into an EL84 in triode will get you there. Should be enough gain if you don't need anything too loud. I would recommend trying an EL33 or even a PL33 if you see those cheaper. A KT61 is nearly as good, not much in it. EL41 can be ignored really, though it's OK. If you use an octal socket for the output that gives you more flexibility. I did, however, look at EL33 on eBay in Canada and it was scarce and much more expensive than in Europe. Same with the KT61. You need a gain of around 15-20. Preferably 20 if driving it with a mu of 9.
 
Thanks andyjevans. I’ll go through my stash and see what I have. Also, and I should have mentioned this earlier, the source in this case is the output of an XTA DP200 acting as a crossover, so I’ll have plenty of signal available rather than the “normal“ home audio 2V output of a DAC or CD player.
 
A 27 into an EL84 in triode will get you there. Should be enough gain if you don't need anything too loud. I would recommend trying an EL33 or even a PL33 if you see those cheaper. A KT61 is nearly as good, not much in it. EL41 can be ignored really, though it's OK. If you use an octal socket for the output that gives you more flexibility. I did, however, look at EL33 on eBay in Canada and it was scarce and much more expensive than in Europe. Same with the KT61. You need a gain of around 15-20. Preferably 20 if driving it with a mu of 9.

I went through my entire tube stash, and found lots of good tubes I missed the previous 6 or 7 times, but not a single one of the ones you recommended. In my search, I found 25 or so 24A tubes as well. Given the output potential of my XTA DP200, shouldn’t any of the DHTs that I have, or the 27 tube, provide enough amplification to max out the EL84?
 
I went through my entire tube stash, and found lots of good tubes I missed the previous 6 or 7 times, but not a single one of the ones you recommended. In my search, I found 25 or so 24A tubes as well. Given the output potential of my XTA DP200, shouldn’t any of the DHTs that I have, or the 27 tube, provide enough amplification to max out the EL84?

Yes, you should be OK. Out of the 01A, 26, 71A, 30, and 31 tubes (and dozens of IDHT 27s), I'd try the 01A and 26. 01A has the best sound, followed by 26. 30 and 31 aren't as good and can be microphonic. 71A has lower gain. 27 is easier to implement but why not use a DHT since you have them. I recommend Bartola Valves for circuits and Rod Coleman regulators. Cheap enough and very good indeed (Lyrima website). I also recommend filament bias for 01A and 26, see Bartola. You should get great sound if you optimise the DHT in this way, and the 01A has the best high end of any tube I can think of.
 
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My 2 cents:

I think 6V6 is more linear and sounds nicer than El84. Within max Pd you will not get more than 1.5watt in triode mode, period. Also I don't think you need any more than 1.5watt for your HF driver. Since you have plenty of drive from your source/pre whatever, any of the above discussed dht will do the job. So far filament bias sounds best to my ears. You should try that. Since taste/listening room/system in the chain are always unique, you can not determine beforehand which solution you will like most.

Request you to bite the bullet and start building and report back, from this we will be benefited also. 100 pages of discussion is no use if no building the project. Start simple build the amp listen carefully, if the like it upgrade to:

Filament bias
filament regulator
plate chokes/CCs/Gyrator (for plate load)
Triode mode/Pentode mode for el84/6v6
FB/no FB
etc.

VT 52 once realized he is not listening to music rather complecating his building/projects. And he sadly refrain himself from the hobby for quite some time. I like your original idea of a small SE amp with dht driver.
Please pay attention to good grounding scheme and really really good power supply. if these two things are not good enough nothing will make your amp a good one.

Regards