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DHT driver for triode wired SE EL84, 6V6 or EL34
DHT driver for triode wired SE EL84, 6V6 or EL34
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Old 14th January 2021, 09:19 AM   #1
tizman is offline tizman  Canada
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Default 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.
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Old 14th January 2021, 11:18 AM   #2
minhaj is online now minhaj  Bangladesh
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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
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Old 14th January 2021, 03:31 PM   #3
andyjevans is offline andyjevans  United Kingdom
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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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File Type: png EL33+26 SE .png (80.9 KB, 1300 views)
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Old 14th January 2021, 05:42 PM   #4
audiowize is offline audiowize  United States
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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.
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Old 14th January 2021, 06:33 PM   #5
andyjevans is offline andyjevans  United Kingdom
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Quote:
Originally Posted by audiowize View Post
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.
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Old 14th January 2021, 07:37 PM   #6
tizman is offline tizman  Canada
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Quote:
Originally Posted by andyjevans View Post
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.
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Old 14th January 2021, 07:42 PM   #7
tizman is offline tizman  Canada
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Quote:
Originally Posted by minhaj View Post
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.

Last edited by tizman; 14th January 2021 at 07:50 PM.
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Old 14th January 2021, 08:03 PM   #8
andyjevans is offline andyjevans  United Kingdom
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Quote:
Originally Posted by tizman View Post
"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.
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Old 14th January 2021, 08:11 PM   #9
tizman is offline tizman  Canada
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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.
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Old 14th January 2021, 08:26 PM   #10
euro21 is online now euro21  Hungary
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DHT driver for triode wired SE EL84, 6V6 or EL34
71a as driver with EL84 SE is good 1.5W amp.

Sample: a little modified RH84
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File Type: jpg 71a, EL84 SE.jpg (132.1 KB, 1249 views)
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