opamp driver stage - Page 2

Graham Maynard · 22nd February 2005, 08:22 AM

Hi Ian,

Okay, this is 30yr memory because I have not been able to find my notebook or the amplifier since the birth of my son, which meant that the roofspace became storage with a top layer of kiddies/teenage toys/books etc.

I used a solid state rectified HT rail with capacitve charging delay to the driver tube, which would still have strained the second halves of each ECC82 at start-up, but I did not observe any problems.
Possibly 350V, to give 150V p-p output voltage per ECC82.
I split the anode resistor (for the first half common cathode) and cannot remember the exact voltage drops. I would have had equal resistors to start with, but cannot remember whether I left them like that.
The first half had simple cathode self bias, but I don't remember using a decoupling capacitor.
I directly coupled the second triode grid to the first triode anode and had an appropriate cathode resistor, the second cathode voltage must have been between 120 and 150 volts. The three resistors were large 5% carbons.
The second anode went to stage HT while the second cathode was bootstrapped to the split anode resistor junction via a high voltage electrolytic.
There was one ECC82 per push-pull half, each driving 2x KT88 grids via separate 400V polystyrenes, possibly 1uF or 2,2uF (four in total) for I still see these in my parts boxes, and the KT88 grid resistors were either 22k or 47k.

This arrangement was surprisingly stable and linear, and there were not any reliability problems. I know I was on the limit for grid voltage during switch-on, and heater-cathode during operation, but it ran trouble-free.

For sure I'd have to increase the supply voltage very slowly if I ever do turn this amplifier on again.

Cheers ......... Graham.

gingertube · 22nd February 2005, 10:09 PM

Graham,
Thanks for the description of your bootstrap arrangement. I'll give it a try.

Acoustat,
I went thru' my stack of Eelctronics World last night. The article you would be looking for was called "Improved Hybrid Amplifier". It had 15dB of AC feedback plus an idle current balance DC feedback path so that a Amplimo (Van der Venne) Toroidal Output Transformer could be used. Specs were excellent. I meant to bring in in today so I could give you the exact volume and issue number and forgot BUT it was 2nd half of 2003.

Cheers,
Ian

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22nd February 2005, 08:22 AM	#11
Graham Maynard diyAudio Member Join Date: Dec 2003 Location: UK	Hi Ian, Okay, this is 30yr memory because I have not been able to find my notebook or the amplifier since the birth of my son, which meant that the roofspace became storage with a top layer of kiddies/teenage toys/books etc. I used a solid state rectified HT rail with capacitve charging delay to the driver tube, which would still have strained the second halves of each ECC82 at start-up, but I did not observe any problems. Possibly 350V, to give 150V p-p output voltage per ECC82. I split the anode resistor (for the first half common cathode) and cannot remember the exact voltage drops. I would have had equal resistors to start with, but cannot remember whether I left them like that. The first half had simple cathode self bias, but I don't remember using a decoupling capacitor. I directly coupled the second triode grid to the first triode anode and had an appropriate cathode resistor, the second cathode voltage must have been between 120 and 150 volts. The three resistors were large 5% carbons. The second anode went to stage HT while the second cathode was bootstrapped to the split anode resistor junction via a high voltage electrolytic. There was one ECC82 per push-pull half, each driving 2x KT88 grids via separate 400V polystyrenes, possibly 1uF or 2,2uF (four in total) for I still see these in my parts boxes, and the KT88 grid resistors were either 22k or 47k. This arrangement was surprisingly stable and linear, and there were not any reliability problems. I know I was on the limit for grid voltage during switch-on, and heater-cathode during operation, but it ran trouble-free. For sure I'd have to increase the supply voltage very slowly if I ever do turn this amplifier on again. Cheers ......... Graham.

22nd February 2005, 10:09 PM	#12
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	Graham, Thanks for the description of your bootstrap arrangement. I'll give it a try. Acoustat, I went thru' my stack of Eelctronics World last night. The article you would be looking for was called "Improved Hybrid Amplifier". It had 15dB of AC feedback plus an idle current balance DC feedback path so that a Amplimo (Van der Venne) Toroidal Output Transformer could be used. Specs were excellent. I meant to bring in in today so I could give you the exact volume and issue number and forgot BUT it was 2nd half of 2003. Cheers, Ian

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