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Old 23rd July 2007, 04:05 PM   #1
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Cool A Tee-Vee T00b Amp

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.
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Old 23rd July 2007, 07:34 PM   #2
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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
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Old 23rd July 2007, 07:51 PM   #3
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Since Image Shack isn't being very cooperative today, I'll try attaching the schemos for the main power supply and the screen regulator here.

Main Power Supply
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Old 23rd July 2007, 07:55 PM   #4
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Screen Regulator

There are a couple of modifications to this schemo.

1.0K screen stopper, and 4K7 grid stopper is added to the pentode section of the 6KE8. These being 0.5W, metal film resistors.
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Old 23rd July 2007, 08:03 PM   #5
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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)
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Old 23rd July 2007, 08:07 PM   #6
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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,

Quote:
Originally posted by Giulio Maiocco
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!
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!
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Old 23rd July 2007, 08:13 PM   #7
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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
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Old 23rd July 2007, 08:22 PM   #8
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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.
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Old 23rd July 2007, 08:23 PM   #9
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No, no, no. Nobody is interested in anything that doesn't require exotic (and peer-approved) parts. You can't go using cheap valves just because they do the job. You need some special hand-wound bifilar oxygen-free capacitors specially selected to complement your selected output valves.
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Old 23rd July 2007, 08:27 PM   #10
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Quote:
Originally posted by EC8010
No, no, no. Nobody is interested in anything that doesn't require exotic (and peer-approved) parts. You can't go using cheap valves just because they do the job. You need some special hand-wound bifilar oxygen-free capacitors specially selected to complement your selected output valves.
Nice grammar!
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