31-08-2016, 06:23 PM
(This post was last modified: 31-08-2016, 06:35 PM by Murphyv310.)
Hi.
Thanks for your input David.
Valve amp design of course can be very basic, conversely complex. Some of the circuitry seen today that appears on the net is a load of tosh to be honest.
The Mullard designs take a lot of beating, the fact that the 3-3, 5-10 & 5-20 are so good is the use of DC coupling to the phase splitter (in the P-P versions and output in the 3-3) this reduces phase errors and improves frequency response.
The 5-20 also had 0.5uf coupling caps to the output valves, this too reduces phase errors. The NFB can also be "adjusted" with an RC network to "Doctor" the response curve at the expense of slight phase errors, these are just about impossible to detect with the ear though.
What I do though is to try to move most "Push-Pull" amps out of class A, this can be achieved fairly well if there is enough HT headroom. The World Design amp had low cathode resistor values of 200 ohms, the HT was approaching 500v and the 6550 anodes were glowing red, valve life was horrendously short and after the previous "engineer
" had a go with fixed bias the scenario was worse.
When I rebuilt the amp all the fixed bias stuff, zeners, pots all over the place and the low DC heater feed to the outputs were binned, veroboard and other nasties were chucked out.
Now the cathodes sport 500 ohm bolt on gold resistors, decoupled by 330uf 63v. There is 55v across the 25 watt cathode resistors that are attached by heat conductive epoxy, running temperature is around 50c on the resistors, the amp is now not running in class A, P-P but more like AB.
With the higher value of cathode resistor the standing current dropped but I was unsure if distortion would be a problem. Fortunately a thorough test showed that there was virtually untraceable distortion till near maximum output. The owner of the amp rarely uses the amp at anywhere near its maximum so the reduction in anode current will improve reliability, don't get me wrong the amp is still an excellent central heating system.
This bring me to another amp. I got this amp from Mike (Radiomike) a few years ago, it was a home constructed amp (nicely made too). It's line up was EF39, ECC35, 2X 6L6, GZ32.
The output was low and severely distorted, the first thing to do was to swap out the EF39 with a EF37A, This made a marked improvement but the amp lacked punch and bottom end. The circuit was loosely based on the Mullard 5-20 but the EF37A was biased wrongly and was upsetting the DC conditions of the ECC35. The anode load and screen of the EF37A were too low, a 270k on the screen and another 270k decoupled with a 10uf on the HT feed sorted out the valves voltage, the HT on the EF37A anode is now 44v and it biasses the ECC35 correctly, mind this is DC coupled, I replaced the 0.1uf couplers to the 6L6 grids from 0.1 to 1uf, upped the cathode resistor values on the 6L6 and did a couple of other small mods. This amp now does a genuine 12 watts and sounds amazing.
I think if you know valve theory, have time and are happy to deviate just a little from the norm then you can make a seriously good amp, far cheaper and with a great satisfaction it is home built.
My next project is to have a go at a Williamson amp, mind you I have two orders for stereo Mullard 3-3 amps and another repair has come in!
Thanks for your input David.
Valve amp design of course can be very basic, conversely complex. Some of the circuitry seen today that appears on the net is a load of tosh to be honest.
The Mullard designs take a lot of beating, the fact that the 3-3, 5-10 & 5-20 are so good is the use of DC coupling to the phase splitter (in the P-P versions and output in the 3-3) this reduces phase errors and improves frequency response.
The 5-20 also had 0.5uf coupling caps to the output valves, this too reduces phase errors. The NFB can also be "adjusted" with an RC network to "Doctor" the response curve at the expense of slight phase errors, these are just about impossible to detect with the ear though.
What I do though is to try to move most "Push-Pull" amps out of class A, this can be achieved fairly well if there is enough HT headroom. The World Design amp had low cathode resistor values of 200 ohms, the HT was approaching 500v and the 6550 anodes were glowing red, valve life was horrendously short and after the previous "engineer
" had a go with fixed bias the scenario was worse. When I rebuilt the amp all the fixed bias stuff, zeners, pots all over the place and the low DC heater feed to the outputs were binned, veroboard and other nasties were chucked out.
Now the cathodes sport 500 ohm bolt on gold resistors, decoupled by 330uf 63v. There is 55v across the 25 watt cathode resistors that are attached by heat conductive epoxy, running temperature is around 50c on the resistors, the amp is now not running in class A, P-P but more like AB.
With the higher value of cathode resistor the standing current dropped but I was unsure if distortion would be a problem. Fortunately a thorough test showed that there was virtually untraceable distortion till near maximum output. The owner of the amp rarely uses the amp at anywhere near its maximum so the reduction in anode current will improve reliability, don't get me wrong the amp is still an excellent central heating system.
This bring me to another amp. I got this amp from Mike (Radiomike) a few years ago, it was a home constructed amp (nicely made too). It's line up was EF39, ECC35, 2X 6L6, GZ32.
The output was low and severely distorted, the first thing to do was to swap out the EF39 with a EF37A, This made a marked improvement but the amp lacked punch and bottom end. The circuit was loosely based on the Mullard 5-20 but the EF37A was biased wrongly and was upsetting the DC conditions of the ECC35. The anode load and screen of the EF37A were too low, a 270k on the screen and another 270k decoupled with a 10uf on the HT feed sorted out the valves voltage, the HT on the EF37A anode is now 44v and it biasses the ECC35 correctly, mind this is DC coupled, I replaced the 0.1uf couplers to the 6L6 grids from 0.1 to 1uf, upped the cathode resistor values on the 6L6 and did a couple of other small mods. This amp now does a genuine 12 watts and sounds amazing.
I think if you know valve theory, have time and are happy to deviate just a little from the norm then you can make a seriously good amp, far cheaper and with a great satisfaction it is home built.
My next project is to have a go at a Williamson amp, mind you I have two orders for stereo Mullard 3-3 amps and another repair has come in!
