After consideration of all factors, I have altered the plan. The most important element is the bias voltage. I planned for a VR tube bias on the cathode. The voltage of the filament it 20, so grounding the negative pole to circuit ground puting a resistor divider between both poles of the filament for the VR kathode adds +10v so a VR75 in that position gives -85v bias to fit the intetional operating point. Roughly speaking,because which voltage the VR settles at is not an exact science, the design is forgiving of variation, should the valves match well, VR tube rolling addresses the other variable.

20 watts is an accessible power and all I require. I have capacitors for the power supply with 1,500vdc rating. The output Traansformers are made by Pierre Tribute for the demands of V1505. The power transformers and chokes are similarly designed. 8k ohm load seems the norm for GM70 aswell as V1505.

All change again, after a tidy up I studied my Tribute Output transformers. They are 10K ohm into either 4ohm or 8ohm. But you have to faf around with a lot of soldering. Anyway when Pieter brought them to Eggborough for me he had set them up for 4 ohm. I must have requested that. I was using the V1505’s at that time, I must have had-loudspeakers more near 4 ohm than 8 ohm at the time.

Anyway turns out perfect for the Celestion 4 ohm and probably good for the Quad 57’s in the two areas where the impedance is very low.

So now I am back on a 10k ohm load and yet another operating point. Primarily the monoblocks will live as GM70 amplifier. But I have two valve bases for the V1505’s. So I shall make up an alternative filament supply to plug in for the V1505 for special occasions.

This project is going to take an age if Im the only person holding the soldering iron!


bias point arrived at with 10v of the filament and a 105v VR tube. Obviously it wont settle bang on, but whatever it settles at will do! Thats the beauty of vales , they dont expire of you are 1 volt watt or amp. Outside youre target. Also they bring their heatsink with them.

As you can see the GM70 doesn’t require as much real estate.

Time to mention again the best sound I ever heard at home was a single field coil driver on a Briggs Dimensions Open Baffle. But important to note the amplifier in those days was resistorless and Capacitorless.

The 57’s are very close to that, so close they will do.

Ref speakers. The only good sounding speakers at vsac 2003 were the late Bruce Edgar’s. It was at VSAC I found out the ear of people across the pond is nothing like my ear. Apart from Bruce. His ear and his knowledge were of the highest quality anywhere in the world.

I remember saying when I got back to these shores’ never to leave again. “I would buy anything of Bruce Edgar’s sight unseen!.”

In this industry most people buy hifi stuff site un seen and regret it like a promiscuous fool regrets one night stands looking back.