The best sounding 300B SET amp I have ever heard.

[sser2]

Then after that, he changed to single resistor and dropped the driver's power supply.

 

[6A3sUMMER]

Most of us . . . who heard various designs of Gary Pimm's amplifiers, and his various designs of loudspeakers;
we had one thing in common . . . we enjoyed the musical experience.

I was blessed, I heard them at Gary's home, at various years of VSAC conferences in Silverdale, and at another Puget Sound location at a 3rd party's home.

 

[56085]

I would also look for Pimm’s Tabor amplifier
View attachment 1040363

Thank you for the schematic of the "Tabor local feedback amplifier". It indeed only has one pair of resistors, acting both as the load resistors for the pre-stage and as the feedback resistors.

I'm out of my depth when it comes to understanding how this circuit exactly works but after some reading of older threads about this amplifier, I get the impression that the gain of this amplifier is low and that it's essential that the pre-stage has pentodes in it.

How does this topology 'translate' to the attached circuit? To drive the 300B to full power something like 75 V of signal is needed on its grid. Can the triode connected D3a provide this? And what signal voltage would the D3a need at its control grid to drive the 300B to full power?

 

[sser2]

D3a triode has mu of about 50. RC-coupled D3a stage of your schematic will have gain of about 30. With 2 VRMS input signal, the output of 60 VRMS is sufficient to drive 300B to full power.

The weak point is driver's low plate voltage relative to its output, which will cause significant distortion at high output levels. This can be remedied by choke loading instead of resistor loading. Gain will increase with choke loading. Same applies to interstage transformer coupling.

 

[56085]

So 2Vrms at the input of this amplifier is enough, eventhough the load resistor of the D3a is connected to the plate of the 300B?

 

[sser2]

With Schade feedback, gain will be significantly less and driver's output insufficient.

 

[sser2]

Gary Pimm got away with this because both his driver and power tubes are pentodes. For the same reason he didn't need high DC current in the driver stage.

 

[56085]

With Schade feedback, gain will be significantly less and driver's output insufficient.

OK, thanks. So I conclude that the amplifier (schematic in post #6 and post #23) will be a very poor performer (hardly any power output).

 

[Herrmann]

OK, thanks. So I conclude that the amplifier (schematic in post #6 and post #23) will be a very poor performer (hardly any power output).

Those schematics are not in accordance with the original design, which can be found here (scroll down a little): Schematic

 

[56085]

Thanks, but I knew that. The schematic that represents the amplifier of TS is in post #1 and follows the original design you linked to.

 

[jwags81818]

I'm in Medford, I might try to get up to hear it. Can you send me a PM? Thanks!

still interested? I have a nice dac and a decent turntable now too.

 

[jwags81818]

1. For Newbies, Be careful if you wire according to the schematic . . .

Inside the glass envelope of the rectifier, it shows a very unusual drawing of the plates and the cathode.
The cathode is drawn as the traditional straight line of a plate.
It also draws the plates as slanted straight lines.
Look up almost any 5AR4 data sheet for a more proper and traditional way of drawing the elements.

2. Schade negative feedback:
When using a triode wired pentode (as this circuit does), or when using a true triode for the input tube . . .
If you bypass the cathode bias resistor (as this circuit does), the plate resistance, rp, is reduced; That reduces the amount of negative feedback.
That is why most Schade negative feedback circuits do not bypass the cathode bias resistor of the input tube.

My conclusion is that there is effectively very little Schade negative feedback in this amplifier, and I am not surprised that it sounds excellent with very little or no Schade negative feedback (after all, it uses a very low distortion output tube, the 300B).

Just my opinions.

Your Mileage May Vary

Correct , there is no neg feedback in this amp and it does sound excellent.

 

[NickKUK]

Am I missing something - where is the input signal injected?

I see the grid of the pentode is connected to the volume control and to earth. However there's no input signal in the schematic? I assume that's the coming in on that grid?

 

[grovergardner]

still interested? I have a nice dac and a decent turntable now too.

Yes! In fact I'm driving up to Portland in early June. Send me a PM!

 

[jwags81818]

Am I missing something - where is the input signal injected?

I see the grid of the pentode is connected to the volume control and to earth. However there's no input signal in the schematic? I assume that's the coming in on that grid?

its on the input of the volume control which is feeding the grid of the D3a.

 

[jwags81818]

Yes! In fact I'm driving up to Portland in early June. Send me a PM!

I live in corvallis now but you are welcome anytime. My room is really small but it still sounds great.

 

[marcomariutto]

@jwags81818
I am also planning to build the same amplifier and I am glad to hear you liked it.
Iam modifying the schematic to use fixed bias on the power stage, and a hybrid LED + filament bias on the driver stage.
Also, I am having some custom OPTs built by monolith magnetics instead of the LLs.


I am also a big fan of Duelunds capacitors, and I will most likely used PIO Cast Cu/Ag for the coupling of the 2 stages.

I will soon post the full schematic.

 

[marcomariutto]

@jwags81818

These are the mods to the biasing circuitry that I am making to the original design from A. Ciuffoli

D3a filament/LED bias

300B gate bias + DC Filament

 

[marcomariutto]

@jwags81818 and this is how I expect the PSUs (one for each monoblocks) to look like..

 

[stocktrader200]

Interesting drive , if the driver tube dies the outputs go into meltdown