UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 120W PP EL34 amplifier re-born.

bikerhifinut · 24th Oct 2018, 12:57 pm

OK Andy,
That ECC83 input stage had a familiar look about it and I remembered using it in an adapation of the Mullard mixing preamplifier circuit. From the Mullard Valve circuits book. I have reminded myself of the circuit and the local feedback on the ECC83 is applied exactly as yours is, a low value capacitor in series with a large resistor from anode to grid. There's no grid leak, but the circuit shows a large grid stopper of 470k in series with a 500k pot so the effect is pretty much the same as having a grid leak of some description. There's no dc blocking cap on the input but obviously the previous stage is capacity coupled.
Interesting though that you applied a bit of local feedback around the first stage rather than rely on the global feedback which is also coupled in via the cathode in the usual way.
Gives me an idea for reducing the overall gain a bit on the new design as i don't need to use an EF86 or similar pentode strapped as a triode and can use a double triode valve in the first stage, either ECC81/2/3 or 6SN7/6SL7.
On the other hand I do have lots of small signal pentodes doing nowt....................
Yeah the challenge of designing your own circuit, I started down that path a few years ago only to discover I had reinvented the wheel! But I was keeping the circuit as simple as possible and decided against things like CCS on the cathodes and once you do that there's not really that many ways of wiring a valve to amplify.
I love my valve amps, theres no logic to it at all. It is very much an emotional rather than rational thing. And if i want a very high power, low distortion power amplifier that will provide low end thrutch down to DC then really only silicon will do it at any sane cost! But wheres the romance?
I've been using that 1980's mitsubishi power amp a lot in the stereo and given that its got, even by todays standards, a good sound (pun?) design and seems built like a battleship, I have been very pleased and surprised with it's performance. But i will still use the KT88 35watt amp even if the reasons are more sentimental than logical!
Love to see the finished job when you get there.
A.

24th Oct 2018, 12:57 pm	#31
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: 120W PP EL34 amplifier re-born. OK Andy, That ECC83 input stage had a familiar look about it and I remembered using it in an adapation of the Mullard mixing preamplifier circuit. From the Mullard Valve circuits book. I have reminded myself of the circuit and the local feedback on the ECC83 is applied exactly as yours is, a low value capacitor in series with a large resistor from anode to grid. There's no grid leak, but the circuit shows a large grid stopper of 470k in series with a 500k pot so the effect is pretty much the same as having a grid leak of some description. There's no dc blocking cap on the input but obviously the previous stage is capacity coupled. Interesting though that you applied a bit of local feedback around the first stage rather than rely on the global feedback which is also coupled in via the cathode in the usual way. Gives me an idea for reducing the overall gain a bit on the new design as i don't need to use an EF86 or similar pentode strapped as a triode and can use a double triode valve in the first stage, either ECC81/2/3 or 6SN7/6SL7. On the other hand I do have lots of small signal pentodes doing nowt.................... Yeah the challenge of designing your own circuit, I started down that path a few years ago only to discover I had reinvented the wheel! But I was keeping the circuit as simple as possible and decided against things like CCS on the cathodes and once you do that there's not really that many ways of wiring a valve to amplify. I love my valve amps, theres no logic to it at all. It is very much an emotional rather than rational thing. And if i want a very high power, low distortion power amplifier that will provide low end thrutch down to DC then really only silicon will do it at any sane cost! But wheres the romance? I've been using that 1980's mitsubishi power amp a lot in the stereo and given that its got, even by todays standards, a good sound (pun?) design and seems built like a battleship, I have been very pleased and surprised with it's performance. But i will still use the KT88 35watt amp even if the reasons are more sentimental than logical! Love to see the finished job when you get there. A.