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Old 25th August 2012, 03:23 PM   #11
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How about driving mosfet power follower, SE, like Cuffoli's 99c with tube preamp. One could use cathode follower to be sure mosfet's input capacitance is driven good enough. If you still want to taste tansformer's sound, probably HV mosfet is way to go.
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Old 25th August 2012, 06:05 PM   #12
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I built a SE tube + MOSFET follower parafeed amp with a fixed ratio of feedback to the tube' plate so it can interact with the load in a scaled way, as if the load was scaled up relative to the tube's plate resistance. It acts exactly like a SE amp using a power tube.

It also uses a transformer, but rather than use an expensive SE OPT it uses a low ratio matching transformer from 70V line to speaker.

In other words, the MOSFET follower outputs 70V line voltage and impedance directly to the matching transformer. This allows the tube to interact with the load through a real transformer. It's also a convenient voltage gain for a tube front end (2vrms=>70rms is Av of 35, doable with a D3A in triode or 417A, etc.)

The whole thing is very SE-like with predominant 2nd harmonic and soft asymmetric limiting.

Last edited by Michael Koster; 25th August 2012 at 06:08 PM.
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Old 25th August 2012, 06:21 PM   #13
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I recall seeing your Op amp controlled current booster; same one? Can't remember the details. Is the DC nulled out in the OT?
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Old 25th August 2012, 06:32 PM   #14
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I put this stuff on my web site back in 2007. Nobody expressed any interest except for the usual "Transistorlab" comments so I never posted any further info to the web site. I think I posted some of the experiments on this forum somewhere, but it was probably in some loosely related thread. I don't have time to look for it right now.....tropical storm Isaac is coming

Super Tubes

The tubes and solid state parts are wired like darlington pairs.

Since then I have experimented and built a few of these. The bottom circuits have not worked out too well usually resulting in smoking sand. Both of the top circuits work quite well and do sound much like pure tube outputs.

The BJT circuit seems to sound the best, yet measures the worst. The distortion numbers and profile look much like the tube alone. SOA limitations on the BJT limit the fun factor and plate voltage.

The mosfet design could put out all the power the little 600 ohm Chinese OPT's could eat. Over 50 watts at 1 KHz, about 8 at 40 Hz. The distortion numbers were very low up to OPT saturation and mostly second harmonic.

All experiments were done in SE using a hacked SSE board. Several tubes were tried with 6V6 working the best for the mosfet since I could run 300 volts and 6Y6 at 200 volts on the BJT since more voltage blew the transistor.

The only P-P experiment using a hacked SPP board with 6CW5's blew up on the launch pad, so I abandoned it without knowing what went wrong.
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Old 25th August 2012, 07:12 PM   #15
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Sounds like the N parts had better voltage ratings than the P parts. These current boost approaches do seem like the "bee's knees" for using power xfmrs or 70V speaker xfmrs as OTs.

Another attractive version is to use a pentode driving an N type SS follower with its output fed back (at somewhat reduced voltage from the plate; using a zener or R divider) to the pentode screen. Optionally a CCS pullup on the plate. Effectively a triode with super low Zout, high current ability, and constant Mu operation.
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Last edited by smoking-amp; 25th August 2012 at 07:21 PM.
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Old 25th August 2012, 07:21 PM   #16
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Quote:
Originally Posted by Michael Koster View Post
I built a SE tube + MOSFET follower parafeed amp with a fixed ratio of feedback to the tube' plate so it can interact with the load in a scaled way, as if the load was scaled up relative to the tube's plate resistance. It acts exactly like a SE amp using a power tube.

It also uses a transformer, but rather than use an expensive SE OPT it uses a low ratio matching transformer from 70V line to speaker.

In other words, the MOSFET follower outputs 70V line voltage and impedance directly to the matching transformer. This allows the tube to interact with the load through a real transformer. It's also a convenient voltage gain for a tube front end (2vrms=>70rms is Av of 35, doable with a D3A in triode or 417A, etc.)

The whole thing is very SE-like with predominant 2nd harmonic and soft asymmetric limiting.
My favorite, 70V transformers. A bit unsure how you fed back the tube transformer interaction thing sound cool. See, I knew you guys were a creative bunch.

Tubelab, absolutely no surprise you played around with it. Is there anything you haven't done?
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Old 26th August 2012, 12:18 AM   #17
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Quote:
Originally Posted by Headbanger View Post
How about driving mosfet power follower, SE, like Cuffoli's 99c with tube preamp. One could use cathode follower to be sure mosfet's input capacitance is driven good enough. If you still want to taste tansformer's sound, probably HV mosfet is way to go.
yes you can.....
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Old 26th August 2012, 12:59 AM   #18
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Quote:
Originally Posted by Printer2 View Post
Keep us posted on your progress.
sure thing, i am commited....i am building the power traffo and a pair of opts in the coming week....

i am using the 10EW7, a fat bottled 9pin tube....

i am using led bias in the first stage, fixed bias on the second stage.....

i am having second thoughts about regulating the B+, my reasoning is that since all stages are run classA and CCS used as loads, then the pssr should be high....

your thoughts are welcome, anyway i will have taps on the power traffo just in case....
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Old 27th August 2012, 12:54 AM   #19
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Got a 4p1l mosfet hybrid breadboarded , yet to measure it but it sounds very good . Low parts count but built tube-style with choke input , choke smoothed ps and choke loads for the mosfets . Currently using (and really abusing) IRL2703 mosfets , there is a cooling fan being used at present as the mosfets are puny TO220 devices disspating 30 watts each .

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Old 18th January 2013, 02:59 AM   #20
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I, for one, would like to know more about an FET source follower driving 6GK6 (actually 10GK6) output tubes using the triode of a 6GH8 in a cathodyne configuration. Is this worthwhile?

Quote:
Originally Posted by tubelab.com View Post
I put this stuff on my web site back in 2007. Nobody expressed any interest except for the usual "Transistorlab" comments so I never posted any further info to the web site. I think I posted some of the experiments on this forum somewhere, but it was probably in some loosely related thread. I don't have time to look for it right now.....tropical storm Isaac is coming

Super Tubes

The tubes and solid state parts are wired like darlington pairs.

Since then I have experimented and built a few of these. The bottom circuits have not worked out too well usually resulting in smoking sand. Both of the top circuits work quite well and do sound much like pure tube outputs.

The BJT circuit seems to sound the best, yet measures the worst. The distortion numbers and profile look much like the tube alone. SOA limitations on the BJT limit the fun factor and plate voltage.

The mosfet design could put out all the power the little 600 ohm Chinese OPT's could eat. Over 50 watts at 1 KHz, about 8 at 40 Hz. The distortion numbers were very low up to OPT saturation and mostly second harmonic.

All experiments were done in SE using a hacked SSE board. Several tubes were tried with 6V6 working the best for the mosfet since I could run 300 volts and 6Y6 at 200 volts on the BJT since more voltage blew the transistor.

The only P-P experiment using a hacked SPP board with 6CW5's blew up on the launch pad, so I abandoned it without knowing what went wrong.
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