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450W bass amp: a sextet of GU50s with shunt&current feedback
450W bass amp: a sextet of GU50s with shunt&current feedback
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Old 16th July 2020, 07:09 AM   #171
zintolo is offline zintolo  Italy
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Quote:
Originally Posted by jhstewart9 View Post
Never built a Williamson. Too many stages & two many phase shifts, it had low frequency instability, sometimes difficult to cure.
I don't know if you read the previous part of the thread, but I decided (well... better to say "tried") to avoid the classical configuration with an extra stage between PI and output tubes. I'm very curious to know how it will sound... and also how it will become at the end, as it is changed alot since the beginning of the thread.
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Old 16th July 2020, 10:04 PM   #172
jhstewart9 is offline jhstewart9  Canada
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Default Driving an Audio Power Tube Cathode with a FET

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Originally Posted by smoking-amp View Post
John, do you recall if the Mosfets were used as followers (source connected to cathode, P-channel Mosfet) or as cascodes (drain connected to cathode, N-channel Mosfet)?

The N-channel cascode version would give "transistor sound" since the transistor gm determines the current flow, while the P-channel follower would give "tube sound" with the tube gm determining current flow.

By the way, one could use an N-channel Mosfet to drive the cathode as a follower using a CCS-tail with a differential (Mosfet with the tube) configuration. More parts, and class A only though. Not likely to get used much.

Here is one solution, there are most likely many others. This has as its origin in an article in the Philips Journal of around 1960 on improving a photo multiplier HV PS. The Stabilization Factor of a vacuum tube PS is given by

S = uAK, where u is the passer tube mu, A is the gain of the error amplifier & K is the portion of the output sampled referred to the error amp. The value of K would be small, high stability standards such as the 85A2 were common. So 85 Volts was compared to perhaps 1.5 KV.
The paper covered how an NPN transistor could be used to greatly increase the apparent u of the passer tube. That way S could have a reasonable value.
Much later I tried simulated various versions of regulated PSs using an IRF 840 in the cathode of various power tubes. A regulated PS circuit is a SE amplifier, it follows its standard voltage as an input. So the circuit posted was developed from one of those PS circuits.
Don't see any advantage, others might.
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File Type: jpg FET Driven Power Stage.JPG (55.0 KB, 125 views)
File Type: jpg Philips PS G.jpg (500.7 KB, 127 views)
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Old 19th July 2020, 01:05 AM   #173
jhstewart9 is offline jhstewart9  Canada
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Default Regulated PS, The Starting Point

For the curious & others this DC PS Regulating circuit was the starting point for that FET driving the cathode of a power amp. All based on the circuit published by Philips in 1964. I did several simulations in 2004 looking at the practicality of such an arrangement for a lab supply. But never built, I already had something that was more than adequate for what I was doing. Still is!

Still interesting, NTL.
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File Type: jpg FET Driven Power Stage Starting Point.JPG (38.6 KB, 108 views)
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Old 19th July 2020, 06:25 AM   #174
zintolo is offline zintolo  Italy
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Thank you jhstewart9, never seen that circuit before!
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Old 19th July 2020, 09:40 AM   #175
Kay Pirinha is offline Kay Pirinha  Germany
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May I ask what these 250 next to the diff amp triodes' designators mean?
Best regards!
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Old 19th July 2020, 05:23 PM   #176
jhstewart9 is offline jhstewart9  Canada
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The 250 is part of the 6SL7 label I put in Electronic Workbench, I used the data sheet specs at 250V. Often the data sheet includes specs at other voltages, say 100V. So I would include that model in the list, that way the simulation could be run at a condition closer to the real thing if necessary.


BTW, I enjoyed Munich several times while with R&S in Canada. Took a Mountain Bike with me one time, a great way to see the city. Took the S-Bahn down to Starnberger & rode the bike around Starnberger See. Passed by the Amperewerk on the other side. And brought my wife on one trip, so we were tourists for another week.
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Old 24th July 2020, 09:39 AM   #177
zintolo is offline zintolo  Italy
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Just as an update, I've done some simulations of the p-mosfet cathode-driven idea in a SE amp using some vacuun tubes I've found in my parent's house. Results are here: What can I do with them? 4x 1L4, 1x 6AQ5, 3x 6BQ5, 1x 6SN7

Still doing some simulations with this GU50 amp trying to improve it.
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Old 24th July 2020, 06:32 PM   #178
jhstewart9 is offline jhstewart9  Canada
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Default Another Look

The GU50 has a max dissipation rating of 40 watts. So with six in PP the total would be 240 watts. 450 watts of audio is unlikely without exceeding the dissipation max. Much of the performance data given on the specification sheets for the GU50 relates to Class C operation at radio frequencies.
Off the top a Class B PP audio amp gets about 60% efficiency.
Please excuse if someone has already made this point.
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Old 24th July 2020, 07:07 PM   #179
zintolo is offline zintolo  Italy
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Thanks jhstewart9, Wavebourn uses GU50s with 850V B+ and Raa 10k, while es345 used them with 950V B+ and Raa 6k. I'm using them with a value inbetween: 900V B+ and Raa 7k5. Yes, it goes above dissipation limits for part of the swing, but as far as I've read from other users, GU50s can support it well, and it's quite typical for instrument amplification to have lower Raa than the ones suggested in the datasheet.
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