OPT: Known Z ratio, what about the valves?

[paggerman]

Hi,

I have been acquiring more gear. This time it's the guts from a stereo radiogram. It works, but I'm interested in a conversion job. I plan to use the PT and chassis to make a PP using the 'Lamington' design by Grant Wills. It will be my second such build. The first is a beauty, albeit a bit buzzy (more work required). with the SE OPTs from the stereo stages I want to build a pair of small amps, similar to a Champ design and fit them into little 10W SS practice cabs which are widely available for next to nothing.

Anyhow, the circuit diagram for the stereo shows the speaker impedance to be 15ohm (yes, 15ohm) and the primary impedance to be 7000ohm. The power valves are 6M5 (~EL80?) which sound awful if I swap them into my Lamington. Now if I was to design my 'Champ-a-like' (or AA4 or whatever) to have an 8ohm speaker, that means that I would have a primary impedance of 3k7ohm, wouldn't I? If so, how do I match an OP valve to the OPT? If I wanted an EL84/6BQ5 OPV, do i just build it and tweak it or can I apply a little science to my choice of OPV? Is there something in the valve datasheet which should be obvious? Plate resistance, perhaps? Does it matter?

Thanks is advance for all your help, it's a pleasure to read these pages.

[martin manning]

1xEL34 w/ 3k7 Zpri at ~300V should be fine. See The Valve Wizard -Single Ended

[Wilder Amplification]

Hi,

I have been acquiring more gear. This time it's the guts from a stereo radiogram. It works, but I'm interested in a conversion job. I plan to use the PT and chassis to make a PP using the 'Lamington' design by Grant Wills. It will be my second such build. The first is a beauty, albeit a bit buzzy (more work required). with the SE OPTs from the stereo stages I want to build a pair of small amps, similar to a Champ design and fit them into little 10W SS practice cabs which are widely available for next to nothing.

Anyhow, the circuit diagram for the stereo shows the speaker impedance to be 15ohm (yes, 15ohm) and the primary impedance to be 7000ohm. The power valves are 6M5 (~EL80?) which sound awful if I swap them into my Lamington. Now if I was to design my 'Champ-a-like' (or AA4 or whatever) to have an 8ohm speaker, that means that I would have a primary impedance of 3k7ohm, wouldn't I? If so, how do I match an OP valve to the OPT? If I wanted an EL84/6BQ5 OPV, do i just build it and tweak it or can I apply a little science to my choice of OPV? Is there something in the valve datasheet which should be obvious? Plate resistance, perhaps? Does it matter?

Thanks is advance for all your help, it's a pleasure to read these pages.

Above you mentioned you wanted to build a "PP" which I'm assuming you mean "push-pull"? Will this be Class A or Class AB push pull? Or did I misinterpret and this is going to be an SE dealio?

[paggerman]

Hi Jon, sorry for the confusion. I have two projects in mind. the first is to convert the stereo chassis into a Push Pull following the Lamington design but using the original PT. with the SE OPTs that I take out of the stereo I want to make little SE amps to put into the carcasses of 10W SS amps. Recycling at its best.

[walkman]

Hi paggerman.

I have used a 7k OT from one of these kind of grams for a champ style SE amp with el84 but triode strapped. HT voltage round 250v Cathode bias resistor 180 ohm.Originally i used the 6m5 which I found quite good , but wanted to use a current production valve. When using the El84 i found the overdrive too harsh. The triode sound is different much smoother and warmer.