[AdrianH]

It has been a while on this project. I received the transformers over a week ago, but was playing with other things and this project tends to make noise, (this annoys the wife) so I was going to wait until she went back to work, but I got bored.

I think I have to make another chassis as the transformer is really too close to the output, but that will wait for a little while. Very please with the transformers, look nice and shiny.

Drilled the chassis and mounted the transformer, added the screen supply and bolted things together. Not using the g2 taps on this set-up as the valve has a single g2 pin for both tetrodes.

I did have a lot of initial issues:

Without any negative feedback the amp was noisy, picked up hum, hissed, whined, pinged etc. If I tapped the EF91 it would ring like a bell out of the speakers. I added some negative feedback and that quietened things down somewhat, but still hiss and a micro-phonic EF81. Out with the scope and found that the whole amp was singing at around 1.25MHz. It turned out the EF81 was oscillating and with very little space to move things around was difficult to determine what was causing it apart from my untidy wiring. A 100pf cap across the anode resistor has solved it for now.

Now with the oscillation gone, the amp is quiet, the EF81 no longer a problem in ringing out of the speaker when tapped and the amp seems to sound reasonable, up to loud! All subjective I know but it is good when something starts to work.

The HT is a bit low for the amp it is at 275 Volts with 21 Volts on the cathodes it could do with around the 320 Volt mark.

Now trying to build a 4 Ohm 10 load with a dozen 12 Ohm resistors.

One fortunate thing recently that did occur was a purchase of a batch of valves from Ebay around 26/27 Valves plus screening cans for £13.50.

I was please to find three EF86's in there and two Mazda valves look to be new with non tarnished pins.

Adrian