Sorry if the topic is already covered somewhere but just couldn’t find it. 
Being a bit cheapskate here, I am wondering what would be the penalties (performance, output power, sonic wise...etc.) if trying to halve the OPT's primary impedance by connecting 8ohm load to 16ohm tap?
argo
Being a bit cheapskate here, I am wondering what would be the penalties (performance, output power, sonic wise...etc.) if trying to halve the OPT's primary impedance by connecting 8ohm load to 16ohm tap?
argo
Loss increases, and damping factor drops slightly if not using gNFB. Bandwidth is about twice as wide (if not necessarily at high power, low frequency).
Tim
Tim
Oops, I forgot to mention – for SE amp
It has been difficult to locate/obtain a good SE OPT for 6C33C which requires 600 to 800 ohm primary impedance. Now I thought, if for an example using One Electron's UBT-1 (1600ohm pri) in this configuration (which should make 800ohm pri.?) for 6C33C SE amp with, lets say 300V plate voltage and 150mA on tube , how much power and low frequency extension will be sacrificed. Can you predict approximately how much the damping factor would decrease without using gNFB.
regards,
argo
It has been difficult to locate/obtain a good SE OPT for 6C33C which requires 600 to 800 ohm primary impedance. Now I thought, if for an example using One Electron's UBT-1 (1600ohm pri) in this configuration (which should make 800ohm pri.?) for 6C33C SE amp with, lets say 300V plate voltage and 150mA on tube , how much power and low frequency extension will be sacrificed. Can you predict approximately how much the damping factor would decrease without using gNFB.
regards,
argo
Argo,
>> Oops, I forgot to mention for SE amp
>> It has been difficult to locate/obtain a good SE OPT for 6C33C which
>> requires 600 to 800 ohm primary impedance. Now I thought, if for an >> example using One Electron's UBT-1 (1600ohm pri) in this configuration >>(which should make 800ohm pri.?) for 6C33C SE amp with, lets say
>> 300V plate voltage and 150mA on tube , how much power and low
>> frequency extension will be sacrificed. Can you predict approximately
>> how much the damping factor would decrease without using gNFB.
It's a good combination I already seen schematics with Hammond OPT 1640 SE and I have asked them about this, "it's allright" lower power output, wider bandwidth ; but a little less bass for that you need current...
And don't go to 300 volts keep it at 250/280volts
I have found these:
http://www.thlaudio.com/indexE.htm
PS: I am building a SE 6C33C-B myself..
Regards.
Alain.
>> Oops, I forgot to mention for SE amp
>> It has been difficult to locate/obtain a good SE OPT for 6C33C which
>> requires 600 to 800 ohm primary impedance. Now I thought, if for an >> example using One Electron's UBT-1 (1600ohm pri) in this configuration >>(which should make 800ohm pri.?) for 6C33C SE amp with, lets say
>> 300V plate voltage and 150mA on tube , how much power and low
>> frequency extension will be sacrificed. Can you predict approximately
>> how much the damping factor would decrease without using gNFB.
It's a good combination I already seen schematics with Hammond OPT 1640 SE and I have asked them about this, "it's allright" lower power output, wider bandwidth ; but a little less bass for that you need current...
And don't go to 300 volts keep it at 250/280volts
I have found these:
http://www.thlaudio.com/indexE.htm
PS: I am building a SE 6C33C-B myself..
Regards.
Alain.
Alain,
thanks for the input.
Why not 300V? 😕
The UBT-1 is for 160mA max DC. 160mA and 280V comes out aprox. 45W disipation. Not bad 😎 but why to limit to this figure?
I try to avoid Hammond like the plague. If their OPTs are built twice as good as their mains and chokes I wouldn’t still consider them as good buy.
Not long I go bombarded thlaudio with e-mails for price request on James opt but they never answered back. 🙁
How much did you pay for yours if it’s not secret?
argo
thanks for the input.
Why not 300V? 😕
The UBT-1 is for 160mA max DC. 160mA and 280V comes out aprox. 45W disipation. Not bad 😎 but why to limit to this figure?
I try to avoid Hammond like the plague. If their OPTs are built twice as good as their mains and chokes I wouldn’t still consider them as good buy.
Not long I go bombarded thlaudio with e-mails for price request on James opt but they never answered back. 🙁
How much did you pay for yours if it’s not secret?
argo
6C33c O/P Tx
Im in the process of doing exactly the same, SE 6c33c but Ive wound my own Tx with the help of Yves, and his calc from this Forum.
Its easy enough to get the parts for a Tx. and winding although tedious is achievable by hand--Ive got a hand-winder, but it has a few problems that Ive not sorted yet so had to resort to doing it by hand.
Took around 5 hours in all, tested it last night, and sounds quite good. If you decide to 'Roll Your Own' I can let you have the details and turns etc. 😀
Im using Cathode Bias, with 320 odd volts, dropping 90 odd over cathode resistor Anode Dissipation in the regeon of 43W
I chose Cathode Bias, as the 6C33C likes to change its charateristics over a long period of burn-in, and I wanted an amp that doesnt involve fiddling with the bias every couple of hours!
Im in the process of doing exactly the same, SE 6c33c but Ive wound my own Tx with the help of Yves, and his calc from this Forum.
Its easy enough to get the parts for a Tx. and winding although tedious is achievable by hand--Ive got a hand-winder, but it has a few problems that Ive not sorted yet so had to resort to doing it by hand.
Took around 5 hours in all, tested it last night, and sounds quite good. If you decide to 'Roll Your Own' I can let you have the details and turns etc. 😀
Im using Cathode Bias, with 320 odd volts, dropping 90 odd over cathode resistor Anode Dissipation in the regeon of 43W
I chose Cathode Bias, as the 6C33C likes to change its charateristics over a long period of burn-in, and I wanted an amp that doesnt involve fiddling with the bias every couple of hours!
Argo,
Well I dont know anything about Hammond quality production.
But I have seen a schematic for 6C33C-B with a 1640 SE connecting the 16 ohms for 8 ohms output. {And have asked their tech support about it}
I will pay a total of: NT$11026 ~ 447.00 Canadian Dollar
for the 2*James 6130H including shipping .... {I'll tell you about the custom taxes when received}
The base price is NT$7400 for the pair.
It was the best price I had for SE OPT's for 1* 6C33C-B ; otherwise
I got around 400 to 500 US $ for the pair and had to add the shipping charges + customs taxes {Lundahl, etc , even custom made in USA}
I have asked for price to a total of 10 companies for a SE Opt
Mine are shipped today, it took at least 1.5 month after the order
and they don't quickly respond to emails, I ordered on 29 march and expect have the OPT's in 2 weeks... not fast!
For the 300 volts as I remember, from schematic I have seen B+ is between 190 to 270 volts ; I am working on the Simplex from Ari Polisois
he recommends not going over 270 volts in SE ; about the same with
Erno Borbely ... Ari is on line called: G.Cesar in the tube forum
he is active in:
http://www.diyaudio.com/forums showthread.php?s=&threadid=57071
I think he will be able to answer about this point ; he has built several SE
with 1 or 3 * 6C33C-B all published in AudioXpress magazine
.
http://www.valvediy.com/simplexpg1.html
http://www.borbelyaudio.com/special_articles.asp
{there is a glitch in the link http://www.borbelyaudio.com/adobe/15wse.pdf}
I am waiting now for 2 power supply 350 VA toroids to have all components
to make a integrated stereo SE 2*20 watts with 6C33C-B tubes.
I raise a warning, it's a costly amplifier if you want to build one.
Feel free to ask if you need anything.
Best regards.
Alain.
Well I dont know anything about Hammond quality production.
But I have seen a schematic for 6C33C-B with a 1640 SE connecting the 16 ohms for 8 ohms output. {And have asked their tech support about it}
I will pay a total of: NT$11026 ~ 447.00 Canadian Dollar
for the 2*James 6130H including shipping .... {I'll tell you about the custom taxes when received}
The base price is NT$7400 for the pair.
It was the best price I had for SE OPT's for 1* 6C33C-B ; otherwise
I got around 400 to 500 US $ for the pair and had to add the shipping charges + customs taxes {Lundahl, etc , even custom made in USA}
I have asked for price to a total of 10 companies for a SE Opt
Mine are shipped today, it took at least 1.5 month after the order
and they don't quickly respond to emails, I ordered on 29 march and expect have the OPT's in 2 weeks... not fast!
For the 300 volts as I remember, from schematic I have seen B+ is between 190 to 270 volts ; I am working on the Simplex from Ari Polisois
he recommends not going over 270 volts in SE ; about the same with
Erno Borbely ... Ari is on line called: G.Cesar in the tube forum
he is active in:
http://www.diyaudio.com/forums showthread.php?s=&threadid=57071
I think he will be able to answer about this point ; he has built several SE
with 1 or 3 * 6C33C-B all published in AudioXpress magazine
.
http://www.valvediy.com/simplexpg1.html
http://www.borbelyaudio.com/special_articles.asp
{there is a glitch in the link http://www.borbelyaudio.com/adobe/15wse.pdf}
I am waiting now for 2 power supply 350 VA toroids to have all components
to make a integrated stereo SE 2*20 watts with 6C33C-B tubes.
I raise a warning, it's a costly amplifier if you want to build one.
Feel free to ask if you need anything.
Best regards.
Alain.
Costly!
If you get really esoteric, there are silver-wire versions available costing literally thousands each!
(doubt if they sound that much better than a good copper version!)
I chose the cheapskate route and rolled my own!😎
Total Cost for three very large, custom wound transformers,--Less than $50!
If you get really esoteric, there are silver-wire versions available costing literally thousands each!
(doubt if they sound that much better than a good copper version!)
I chose the cheapskate route and rolled my own!😎
Total Cost for three very large, custom wound transformers,--Less than $50!
Alastair,
I am happy for you, that you got all the Tx parts and even a winder.
I have not been so lucky till now. I have been looking for lams and bobbins locally without much success. The best I have found so far is RSComponents selling a 200VA power transformer kit for about 70 euros. I called a commercial tube amp maker here today and asked for some laminations.He promised to look something suitably big but he wasn’t sure at all, if he can find them. We'll see. I will definitely be glad for your and Yves's help in providing the details of your OPTs.
Alain, I think I have seen that schematic with Hammond 1640SE as well and that’s why I asked for some kind of confirmation to the idea or possible pitfalls of this approach.
I have bought some Hammond chokes and power Txs and they were really lousy made ones. I don’t know about their OPTs, though I have read in several places an opinion of One Electrons sounding much better, than Hammonds for the same amount of money. It would be also interesting to hear from you, what the Hammond's tech support guys answered to you about using 16ohm tap instead 8ohm to lower the load impedance.
You got a good deal from thlaudio with these James. I checked for the price of the same OPTs as yours from another supplier in Asia and while the initial price was attractive, with shipping and customs added, it would have turned out as much as buying from Borbely Audio for an example. Lundahl doesn’t make OPTs for this impedance higher than 90mA, if I remember correctly? Did you ask them for custom tranny? It would be closest place to order from them and they are quite reputable quality makers.
You are right about the plate voltage -I checked the datasheet again and it says - no more than 250V if over ran over 30Watts.
I must have remembered incorrectly. Well it seems, that the One Electron’s UBT-1 is out. At the end, it's a quite small tranny anyhow - about 2,5 kg.
I kind a like Ari's “Simplex”, especially the direct coupled driver but I am not sure, if will I go with the modulated bias of the output tube though. Because after all, it still has at least one capacitor in the signal path (if not two).
And high voltage, an 1000uF electrolytic one
,which would be difficult to find of “Audio quality”. You see, the output tube doesn’t have a current path other way around, than to go through these big elcos. How does it affect sound, I have no authority to say, having not heard one at first.
I have also seen the Borbely's design and liked it very much first. Now, after reading more about the virtues of the SE triode amp’s characteristic harmonic distortion pattern and it's influence on a sound in several places, I have came into conclusion, that using a distortion cancellation mechanism and/or NFB, would defeat the purpose of using SE triode output at the first place
I am considering a fixed bias BUT ONLY with the regulated PS, using another 6C33C as a regulator tube - the dang thing was made for that, why not use it for the purpose! 😀
I have the output tubes, sockets and multitude of driver tubes at hand to choose, some power trafos, chokes, caps, bolts and nuts etc., only if I could get some decent output transformers for not going broke. Alastair's "Rolling Roll Your Own" idea seems tempting one, if proper laminations could be found. It will be long-time, more of an experimenting project anyhow.
As to the cost warning, you are most welcome. I need to keep myself on the budget for not paying outrageously high prices for the OPTs just for the "Big Brand Name's" sake and I have yet to finish my 6AS7 PP amp and the high efficiency speaker pair project going on for too long time.
I am eagerly waiting your and Alastair report on the amps you are making.
Cheers,
Argo
I am happy for you, that you got all the Tx parts and even a winder.
I have not been so lucky till now. I have been looking for lams and bobbins locally without much success. The best I have found so far is RSComponents selling a 200VA power transformer kit for about 70 euros. I called a commercial tube amp maker here today and asked for some laminations.He promised to look something suitably big but he wasn’t sure at all, if he can find them. We'll see. I will definitely be glad for your and Yves's help in providing the details of your OPTs.Alain, I think I have seen that schematic with Hammond 1640SE as well and that’s why I asked for some kind of confirmation to the idea or possible pitfalls of this approach.
I have bought some Hammond chokes and power Txs and they were really lousy made ones. I don’t know about their OPTs, though I have read in several places an opinion of One Electrons sounding much better, than Hammonds for the same amount of money. It would be also interesting to hear from you, what the Hammond's tech support guys answered to you about using 16ohm tap instead 8ohm to lower the load impedance.
You got a good deal from thlaudio with these James. I checked for the price of the same OPTs as yours from another supplier in Asia and while the initial price was attractive, with shipping and customs added, it would have turned out as much as buying from Borbely Audio for an example. Lundahl doesn’t make OPTs for this impedance higher than 90mA, if I remember correctly? Did you ask them for custom tranny? It would be closest place to order from them and they are quite reputable quality makers.
You are right about the plate voltage -I checked the datasheet again and it says - no more than 250V if over ran over 30Watts.
I must have remembered incorrectly. Well it seems, that the One Electron’s UBT-1 is out. At the end, it's a quite small tranny anyhow - about 2,5 kg.I kind a like Ari's “Simplex”, especially the direct coupled driver but I am not sure, if will I go with the modulated bias of the output tube though. Because after all, it still has at least one capacitor in the signal path (if not two).
And high voltage, an 1000uF electrolytic one ,which would be difficult to find of “Audio quality”. You see, the output tube doesn’t have a current path other way around, than to go through these big elcos. How does it affect sound, I have no authority to say, having not heard one at first. I have also seen the Borbely's design and liked it very much first. Now, after reading more about the virtues of the SE triode amp’s characteristic harmonic distortion pattern and it's influence on a sound in several places, I have came into conclusion, that using a distortion cancellation mechanism and/or NFB, would defeat the purpose of using SE triode output at the first place
I am considering a fixed bias BUT ONLY with the regulated PS, using another 6C33C as a regulator tube - the dang thing was made for that, why not use it for the purpose! 😀
I have the output tubes, sockets and multitude of driver tubes at hand to choose, some power trafos, chokes, caps, bolts and nuts etc., only if I could get some decent output transformers for not going broke. Alastair's "Rolling Roll Your Own" idea seems tempting one, if proper laminations could be found. It will be long-time, more of an experimenting project anyhow.
As to the cost warning, you are most welcome. I need to keep myself on the budget for not paying outrageously high prices for the OPTs just for the "Big Brand Name's" sake and I have yet to finish my 6AS7 PP amp and the high efficiency speaker pair project going on for too long time.
I am eagerly waiting your and Alastair report on the amps you are making.
Cheers,
Argo
I am in the same route as you guys but with one difference, I have it ready in my home as we speak, its a simple 6C33SE with Hammond 1640SE wired for 16Ohms and I can assure you it sounds marvelous. The design is based on 6N1P-ECC99-6C33.
the voltage is 220VDC at 200mA. The power supply is CLC. The sound is the best I have heard till now very dynamic, detailed and "human". I have to admit my speakers are 100dB sensitive(tannoy monitor gold horn loaded). The amp belongs to a friend and got it for audition now. Mine just started and will be exactly the same but with anoter 6C33 as a power supply regulator for both channels(already tried and sounwise is excellent).
The amp design can be found here:
http://audio.dynatech.gr/toppage4.htm
The speakers:
http://audio.dynatech.gr/horny.htm
the voltage is 220VDC at 200mA. The power supply is CLC. The sound is the best I have heard till now very dynamic, detailed and "human". I have to admit my speakers are 100dB sensitive(tannoy monitor gold horn loaded). The amp belongs to a friend and got it for audition now. Mine just started and will be exactly the same but with anoter 6C33 as a power supply regulator for both channels(already tried and sounwise is excellent).
The amp design can be found here:
http://audio.dynatech.gr/toppage4.htm
The speakers:
http://audio.dynatech.gr/horny.htm
argo, and all
What I got searching for OPT's for the SE 6C33C-B single tubes
BARTOLUCCI TRANSFORMERS Italy Model 51: 227 Euro pc
shipping: one pair Model 51 to Canada is 180,00 euro.
addr: http://audiomarketing.net
XE-20-600s was out of production already at diyaudiocraft.com
James # JS-6130H at octave-electronics.com
JS 6130 RM 1200.00/Pair ( US$ 316.00)
Shipping by Federal Express 10Kgs RM 365.00 ( US$ 96.00)
From hammond at hammfg.com
The closest we would have is the 1640SE. The impedance is 1250 ohms with bias current of 200 mA. Response to 20K Hz
You coud use the 16 Ohms for 8 Ohms output to halve the primary impedance ; Your price would be cad $97.50. pc
TRAU SE transformers at Selected Audio Components
The quote for a pair of 25W SE transformers is 640 Euro ( some 850 USD ).
Shipping costs: Air Mail 174 Euro Delivery 10-12 days
From Lundahl - K&K Audio at: www.kandkaudio.com
You should order the LL1627/250mA model. It is a special order unit, but at the standard price (US$195 each). It takes about 4-6 weeks to get these, with payment in advance.
ELECTRA sud/ouest <ELECTRASUDOUEST@wanadoo.fr>
6C33 SE C5 176.00 Euros pc or 154.00 Euros T5 format
For QUEBEC shipping charges for 2 transfos are 44 Euros
bestellung@roehrentechnik.de Model: 53.33 for a 6C33C-B
Didn't get any answer!!!
Finaly THLAudio offered: 2*James JS-6130H weigkt 4.5 Kg each!!!
at: NT$11026 or about 470 cad $ shipping included
In a nutshell, that's all I got ; I lost a lot more because I crashed my HD at home...
I have done the same thing for Power Supply transformers,
Custom from 250U$ pc = shipping to 450 U$ pc + shipping
C_Cores, E-core, specials toroids etc...
got a good offer from sumr.com in Toronto for 2 Audio quality 330 VA
toroids with : Price $ 85/ each Shipping post $ 14.00 est.
Total :$ 196.88 Cdn {Richard Sumner is a great guy, good advise !}
Each 330VA toroids; OD: 122mm HT: 58mm
220V load: @ .9 amp 238V off-load
350V load @ .08 amp 370V off-load
160V load @ .05 amp 167.2V off-load
15 V load @ 2.25 Amp 16.1V off-load *
15 V load @ 2.25 Amp 16.1V off-load * parallel for 4.5 Amps
15 V load @ 1.00 Amp 16.1V off-load
15 V load @ 1.00 Amp 16.1V off-load
9V load @ .50 Amp 9.5V off-load
I will have bridges and regulators {LT1083/84 or MosFet's} on each supply for :
B+: 270 V DC for 6C33C-B
B+ 6SN7GTB's 420 Volts DC
B+ 6SN7EH preamp 200 V DC
12.6 V DC serie heaters of 6SN7GTB's
12.6 V DC serie heaters of 6SN7EH's
12.6 V DC serie Heaters 6C33C-B {2 parallel windings for 4.5 Amps}
5 VDC for LCD display for all plate current and B+ voltages on a 12 pos switch...
Thanks to all, you are making great amplifiers, keep this thread alive
with more informations.
Regards.
Alain.
What I got searching for OPT's for the SE 6C33C-B single tubes
BARTOLUCCI TRANSFORMERS Italy Model 51: 227 Euro pc
shipping: one pair Model 51 to Canada is 180,00 euro.
addr: http://audiomarketing.net
XE-20-600s was out of production already at diyaudiocraft.com
James # JS-6130H at octave-electronics.com
JS 6130 RM 1200.00/Pair ( US$ 316.00)
Shipping by Federal Express 10Kgs RM 365.00 ( US$ 96.00)
From hammond at hammfg.com
The closest we would have is the 1640SE. The impedance is 1250 ohms with bias current of 200 mA. Response to 20K Hz
You coud use the 16 Ohms for 8 Ohms output to halve the primary impedance ; Your price would be cad $97.50. pc
TRAU SE transformers at Selected Audio Components
The quote for a pair of 25W SE transformers is 640 Euro ( some 850 USD ).
Shipping costs: Air Mail 174 Euro Delivery 10-12 days
From Lundahl - K&K Audio at: www.kandkaudio.com
You should order the LL1627/250mA model. It is a special order unit, but at the standard price (US$195 each). It takes about 4-6 weeks to get these, with payment in advance.
ELECTRA sud/ouest <ELECTRASUDOUEST@wanadoo.fr>
6C33 SE C5 176.00 Euros pc or 154.00 Euros T5 format
For QUEBEC shipping charges for 2 transfos are 44 Euros
bestellung@roehrentechnik.de Model: 53.33 for a 6C33C-B
Didn't get any answer!!!
Finaly THLAudio offered: 2*James JS-6130H weigkt 4.5 Kg each!!!
at: NT$11026 or about 470 cad $ shipping included
In a nutshell, that's all I got ; I lost a lot more because I crashed my HD at home...
I have done the same thing for Power Supply transformers,
Custom from 250U$ pc = shipping to 450 U$ pc + shipping
C_Cores, E-core, specials toroids etc...
got a good offer from sumr.com in Toronto for 2 Audio quality 330 VA
toroids with : Price $ 85/ each Shipping post $ 14.00 est.
Total :$ 196.88 Cdn {Richard Sumner is a great guy, good advise !}
Each 330VA toroids; OD: 122mm HT: 58mm
220V load: @ .9 amp 238V off-load
350V load @ .08 amp 370V off-load
160V load @ .05 amp 167.2V off-load
15 V load @ 2.25 Amp 16.1V off-load *
15 V load @ 2.25 Amp 16.1V off-load * parallel for 4.5 Amps
15 V load @ 1.00 Amp 16.1V off-load
15 V load @ 1.00 Amp 16.1V off-load
9V load @ .50 Amp 9.5V off-load
I will have bridges and regulators {LT1083/84 or MosFet's} on each supply for :
B+: 270 V DC for 6C33C-B
B+ 6SN7GTB's 420 Volts DC
B+ 6SN7EH preamp 200 V DC
12.6 V DC serie heaters of 6SN7GTB's
12.6 V DC serie heaters of 6SN7EH's
12.6 V DC serie Heaters 6C33C-B {2 parallel windings for 4.5 Amps}
5 VDC for LCD display for all plate current and B+ voltages on a 12 pos switch...
Thanks to all, you are making great amplifiers, keep this thread alive
with more informations.
Regards.
Alain.
Sorry for such a long delay. Thank you Alain for such a comprehensive list - makes the buying much easier. This is good news that Lundahl also makes some higher current versions as well, even if only for special order. I have couple of more OPT suppliers bookmarked at my home computer and will post the info soon, if anybody is interested in.
Shouldn’t this thread renamed or moved by the way, as it has developed into a bit different direction from original topic?
Sorry Alain that I touched the Simplex capacitor issue - just couldn’t resist. Well, now as the Genie has let out from a jar...
I am not an EE and I am lousy at explaining circuits, so I tried to redraw the Simplex schematic, for maybe easier view for you.
Note that no component connections have been changed. Important thing to keep in mind with this circuit (as with all circuits) is that it is the Current (flow of electrons) that circulates in circuit and Voltage (differences in potential) only helps this to happen.
One more brainier considering the schematic, can you predict the plate voltage on 6C33C?
Cheers,
Argo
Shouldn’t this thread renamed or moved by the way, as it has developed into a bit different direction from original topic?
Sorry Alain that I touched the Simplex capacitor issue - just couldn’t resist. Well, now as the Genie has let out from a jar...
I am not an EE and I am lousy at explaining circuits, so I tried to redraw the Simplex schematic, for maybe easier view for you.
Note that no component connections have been changed. Important thing to keep in mind with this circuit (as with all circuits) is that it is the Current (flow of electrons) that circulates in circuit and Voltage (differences in potential) only helps this to happen.
One more brainier considering the schematic, can you predict the plate voltage on 6C33C?
Cheers,
Argo
Attachments
Dear Argo, the reason why I use 6AU6 is simply because I got many in hand and its cheap. However, any similar pentode used, gave same results, for example 6AK5(5654W) (this one is also used in the famous Shusurin design in somilar PS arrangement) gave exactly the same performance and also got the same pin arrangement, even EL84 performed well.
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