Here is a design I worked up that avoids those expen$ive audiophool VTs. The prices for the DHT triodes (45, 300B, 2A3, and to a lesser extent the 845, 6A3, and even the 811A) are stratosphereic for new production, and let's not even discuss NOS. The prices for the audio pentodes has also been creeping up there lately. So, what to do about that? How about avoiding trendy types and circuit topologies?
So I designed this amp that makes use of the 6BQ6GTB horizontal deflection pentode as PP finals. As with any other HD pentode, the 6BQ6GTB has a characteristic that allows for some very high Ip's at lowish Vpk's. This means that it can't be used single-ended, since the most linear part of the characteristic is well within red plate territory. However, a Class AB1 loadline can reach this part of the characteristic without any spec-busting. With an Rl of 1K1, the resulting loadline looks pretty good, with an acceptable h3 of 5.0%, which can be improved by loacl NFB.
Though rated like a 6V6 (Pd= 12W for both) the 6BQ6GTB can supply more than twice the output power (37W as opposed to 18W for the 6V6). Another differenceis that the 6BQ6GTB isn't so amenable to either trioding or Ultralinear since the screen voltage rating is so low (Vsgsg= 150Vdc). That, however, may not be such a disadvantage since both of the latter are "trendy", and this might keep the expense of the 6BQ6 down and the availability up.
Loadline
The first preamp/phase splitter is a cascoded LTP with active tail loading, and based on the 6BQ7A small signal, VHF triode. This particular type was designed with cascoding in mind. As an audio triode, the linearity isn't bad, though it's doubtful you'd want to use it as a straight audio voltage amp. There are more linear triodes for that. However, since this is a differential, the balance cancels the even harmonics to improve the overall THD. Cascoding also gives enough gain margin to allow for gNFB without requiring an additional gain stage.
The LTP splitter drives a 6FQ7 dual triode with each section being used as a cathode follower to drive the final grids to charge up the Ci + Cmiller + Cstray, and to allow for DC coupling to avoid the blocking problem that occurs with capacitor coupling to fixed bias finals. This is critical since gNFB will exaggerate an already bad clipping behaviour if capacitor coupling is used. This also makes for an easy bias adjustment to reduce any DC magnetization of the OPT core.
A further aid to sonic improvement is an active screen voltage regulator. Stable screen voltage and a Lo-Z screen source helps to improve the sonics of any pentode final. The screen regulator is based on the 6KE8 (another TV t00b) which combines a high gain pentode (error amp) with a medium gain triode, both connected in an unbalanced differential stage. Reference voltage comes from a 0A3/VR75 gas discharge regulator tube. The series pass element is a trioded 6AQ5. This regulator holds the screen voltage rock steady regardless of the volume.
Main Schemo
Since this has just been completed, I ran it with no NFB connected whatsoever. This sounds quite good already. I couldn't hear any of that pentode harshness at all. The most obvious sonic defect was sloppy bass which becomes most noticeable when playing bass-heavy techno. I'd say that the 6BQ6GTB is a more heavy duty 6V6. It really does sound surprisingly good!
The cascoded differential also isn't contributing any noticeable distortion either. It looks like the main design objective has been definitely met: a good performing open loop implementation that can be further improved by adding NFB to correct what few sonic defects there are. (Unforch, I'm still waiting on Mouser for some parts I need to complete this right. I had to power up the heaters with AC, and the active tail load with batteries. Once those parts arrive, I can take care of that, and run more tests.)
Pretty good for tubes that don't cost more than $3.00 a pop.
So I designed this amp that makes use of the 6BQ6GTB horizontal deflection pentode as PP finals. As with any other HD pentode, the 6BQ6GTB has a characteristic that allows for some very high Ip's at lowish Vpk's. This means that it can't be used single-ended, since the most linear part of the characteristic is well within red plate territory. However, a Class AB1 loadline can reach this part of the characteristic without any spec-busting. With an Rl of 1K1, the resulting loadline looks pretty good, with an acceptable h3 of 5.0%, which can be improved by loacl NFB.
Though rated like a 6V6 (Pd= 12W for both) the 6BQ6GTB can supply more than twice the output power (37W as opposed to 18W for the 6V6). Another differenceis that the 6BQ6GTB isn't so amenable to either trioding or Ultralinear since the screen voltage rating is so low (Vsgsg= 150Vdc). That, however, may not be such a disadvantage since both of the latter are "trendy", and this might keep the expense of the 6BQ6 down and the availability up.
Loadline
The first preamp/phase splitter is a cascoded LTP with active tail loading, and based on the 6BQ7A small signal, VHF triode. This particular type was designed with cascoding in mind. As an audio triode, the linearity isn't bad, though it's doubtful you'd want to use it as a straight audio voltage amp. There are more linear triodes for that. However, since this is a differential, the balance cancels the even harmonics to improve the overall THD. Cascoding also gives enough gain margin to allow for gNFB without requiring an additional gain stage.
The LTP splitter drives a 6FQ7 dual triode with each section being used as a cathode follower to drive the final grids to charge up the Ci + Cmiller + Cstray, and to allow for DC coupling to avoid the blocking problem that occurs with capacitor coupling to fixed bias finals. This is critical since gNFB will exaggerate an already bad clipping behaviour if capacitor coupling is used. This also makes for an easy bias adjustment to reduce any DC magnetization of the OPT core.
A further aid to sonic improvement is an active screen voltage regulator. Stable screen voltage and a Lo-Z screen source helps to improve the sonics of any pentode final. The screen regulator is based on the 6KE8 (another TV t00b) which combines a high gain pentode (error amp) with a medium gain triode, both connected in an unbalanced differential stage. Reference voltage comes from a 0A3/VR75 gas discharge regulator tube. The series pass element is a trioded 6AQ5. This regulator holds the screen voltage rock steady regardless of the volume.
Main Schemo
Since this has just been completed, I ran it with no NFB connected whatsoever. This sounds quite good already. I couldn't hear any of that pentode harshness at all. The most obvious sonic defect was sloppy bass which becomes most noticeable when playing bass-heavy techno. I'd say that the 6BQ6GTB is a more heavy duty 6V6. It really does sound surprisingly good!
The cascoded differential also isn't contributing any noticeable distortion either. It looks like the main design objective has been definitely met: a good performing open loop implementation that can be further improved by adding NFB to correct what few sonic defects there are. (Unforch, I'm still waiting on Mouser for some parts I need to complete this right. I had to power up the heaters with AC, and the active tail load with batteries. Once those parts arrive, I can take care of that, and run more tests.)
Pretty good for tubes that don't cost more than $3.00 a pop.
Hello,
Wow, your project and circuit diagram sounds interesting, congratulations! I may build a near copy of yours (if you don't mind, of course!), I love dearly the 6BQ6 tube. I would be very interested to see the whole schematic, power supply included
About 6 months ago I fixed-modified-rebuilt an amp for a customer, but he never showed again, so I gave it to my friend, who is a bass guitar player. I obtained a tad over 93 (yes, 93!!) W RMS from 2 EL36/6CM5 tubes, each with a nominal plate dissipation of 12W, but I pushed them far over the maximum stated, about 30W each, there is no sign of red glow! As a matter of facts the tubes are still at 100% after 6 months and my friend plays it quite frequently (2 or 3 1-hour gigs every week). It sounds very good as added bonus, even with a CD player as input source and the bass are very deep and pleasant (at least, for my ears and tastes...)
Cheers
Giulio Maiocco
Wow, your project and circuit diagram sounds interesting, congratulations! I may build a near copy of yours (if you don't mind, of course!), I love dearly the 6BQ6 tube. I would be very interested to see the whole schematic, power supply included
About 6 months ago I fixed-modified-rebuilt an amp for a customer, but he never showed again, so I gave it to my friend, who is a bass guitar player. I obtained a tad over 93 (yes, 93!!) W RMS from 2 EL36/6CM5 tubes, each with a nominal plate dissipation of 12W, but I pushed them far over the maximum stated, about 30W each, there is no sign of red glow! As a matter of facts the tubes are still at 100% after 6 months and my friend plays it quite frequently (2 or 3 1-hour gigs every week). It sounds very good as added bonus, even with a CD player as input source and the bass are very deep and pleasant (at least, for my ears and tastes...)
Cheers
Giulio Maiocco
Cool, thank you for sharing with us your precious work! And I'm not kidding 
When you have time, please could you post the values of the various resistors and capacitors?
Now I know what to do tomorrow in the morning: calculate the output transforrmer, as I like to wind them myself...
Best Regards
Giulio Maiocco (Italy)
When you have time, please could you post the values of the various resistors and capacitors? Now I know what to do tomorrow in the morning: calculate the output transforrmer, as I like to wind them myself...
Best Regards
Giulio Maiocco (Italy)
Hi Miles,
This is super work. Not only have you shown TV tubes can be used in audio, but taken seriously as well. Tubelab will be proud!
Hi Giulio,
EL36 is one of my favorite tubes. Tubes 4e4 did some work with this one a while back and found it quite tolerant of overload, but 93 watts?!?! You need a medal!
Cheers!
This is super work. Not only have you shown TV tubes can be used in audio, but taken seriously as well. Tubelab will be proud!
Hi Giulio,
Giulio Maiocco said:
EL36 is one of my favorite tubes. Tubes 4e4 did some work with this one a while back and found it quite tolerant of overload, but 93 watts?!?! You need a medal!
Cheers!
Hi Geek,
Yes, exactly 93,5W from a pair, but, to be correct, most of the research work was done way before I discovered this by a dear friend of mine located in England, John Chambers (www.chambonino.com), I post the link not for commercial purposes... The conditions to obtain this kind of power are the following:
Va= 435/445V
Vg2= 200V, better if stabilized to this value, the power obtained is largely dependent by this voltage i.e with 185V I obtained about 80W RMS
Zaa= 2,4Kohm
-Vg1= around -35V.
Just try it...
Cheers
Giulio Maiocco
Yes, exactly 93,5W from a pair, but, to be correct, most of the research work was done way before I discovered this by a dear friend of mine located in England, John Chambers (www.chambonino.com), I post the link not for commercial purposes... The conditions to obtain this kind of power are the following:
Va= 435/445V
Vg2= 200V, better if stabilized to this value, the power obtained is largely dependent by this voltage i.e with 185V I obtained about 80W RMS
Zaa= 2,4Kohm
-Vg1= around -35V.
Just try it...
Cheers
Giulio Maiocco
Heater Supply
The heaters run off a DC supply, based on the FAN1084 integrated voltage regulator. This is a floating supply to protect the heater/cathode of the 6FQ7 since the negative rail comes up before the heaters get a chance to warm up. This could cause a flash-over since the Vhk rating isn't nearly that high.
DC on the heaters also helps to keep the amp quiet.
The heaters run off a DC supply, based on the FAN1084 integrated voltage regulator. This is a floating supply to protect the heater/cathode of the 6FQ7 since the negative rail comes up before the heaters get a chance to warm up. This could cause a flash-over since the Vhk rating isn't nearly that high.
DC on the heaters also helps to keep the amp quiet.
Attachments
poynton said:
May be I should have gotten a bunch of This instead? Might make the sound more "wooly".
An externally hosted image should be here but it was not working when we last tested it.
Tubelab approves of anything that makes clean power with unusual tubes. I remember the 6BQ6 from my teenage years. They were common in old TV sets. I used to make guitar amps out of old TV sets then. I remember that the 6DQ6 would rock harder than the 6BQ6, but the details are lost in the 40 years that have passed since then. The 6BQ7 is an RF amplifier tube used in VHF TV tuners. It was designed for AGC use so it can be somewhat nonlinear with large signal levels. It does do well as a LTP phase splitter in the first stage, although I have not tried it in cascode. I used it as the phase splitter in my 6AV5 P-P amp. The 6FQ7 - 6CG7 is well known in the audio world, but it was designed to be a horizontal oscillator.
I have collected several hundred pounds of "worthless TV tubes" over the years. I have not had the time to "test" all of them yet. I have acquired an even larger collection of WWII era military tubes. I have barely dented this collection. The 5670 - 2C51 is a little dual triode that works well as a LTP splitter. I have about 1000 of these. I also have a zillion 6AK5's, but haven't found anything that they excel at yet. So many tubes, so little time.
Not audio related,but 6AK5's make nice little VHF amps.
http://www.users.qwest.net/~ptaylor/Radio/vhf1watt.gif
tubelab.com said:
Give it a try sometime. I got some excellent results from cascoding. Lotsa gain, though the Vo swing is a bit thin for the Vpp (common cascode drawback) though it's more than enough to drive the 6BQ6s. Probably won't be sufficient to drive a power triode or trioded pentode, though.
6AK5s make good distributed amps if you need huge bandwidths. I also recall seeing a project where a whole bunch of 6AK5s were paralleled to make a power amp, but I can't recall where I saw that.
Miles Prower said:Since this has just been completed, I ran it with no NFB connected whatsoever. This sounds quite good already. I couldn't hear any of that pentode harshness at all. The most obvious sonic defect was sloppy bass which becomes most noticeable when playing bass-heavy techno. I'd say that the 6BQ6GTB is a more heavy duty 6V6. It really does sound surprisingly good!
Hi! This schematic looks like the one I'm looking for, but I wanted something different, with other tubes as phase splitter, like the well known 12ax7 or similar. I can get those tubes, their performance is really good. Can you replace 6DQ6 with them? They have the same plate resistance, although different transconductance and the BQ has lower rattings. I was looking for 30W or 40W (or the max power I could get from a pair of 6BQ6!!!!) What about the PP primary output tranny impedance?
I read you mention EL36, a guy I know told me he used to get 120W from 4 of those tubes (nothing in comparisson with 93W from a pair) and said these tubes really rock!!!!
DigitalJunkie said:Hey Miles,
One possibly dumb question.. Why the Resistor from pin 1 of the 5U4 to ground,isn't pin 1 N/C?
For this particular application, pin 1 can probably remain floating with no problems. However, it is a connection that discharges static charges that can build up when the 5U4 is used with TV sets where there's the voltage from the CRT HV supply. It's discharged via a 1M resistor. The voltages used here aren't that extreme, but keep it in mind if you get the occasional flash between plates, or from plate to filament. We are coming up on static electricity season here.