[yestertech]

Amazing ! who would have thought I could be bowled over by a 3 watt valve amp !
This concerns an unloved Mullard 3-3 which I bought in a job lot recently. Probably not fired up for decades and valve-less ,it looked rather unloved.
Like most of these, it has been well made from a kit of parts right down to the bus-bar earthing. ( the 'right' way to do things )
Just by way of a change, I thought I would try it out. But first a few minor repairs....
The dual 50 +50 electrolytic reformed fine from the bench HT PSU, and having found some valves I fired it up on t'variac as usual.
After a while the HT began to appear. There is no "that capacitor" to worry about as the circuit is DC coupled from Ef86 to El84. Most of the voltages appeared to be about right, perhaps a bit low. Those around the Ef86 were very low and I mean VERY, as this stage is configured at 'starvation' volts to gather maximum gain and to enable DC coupling.
A quick prod of the input tag produce a healthy "barp". I then set about measuring the output and distortion. It seemed happy to produce 3 watts into 15 ohms, but the distortion figures were way out of spec. A quick frequency squeak was none too impressive either, with both LF and HF ends drooping within the normally expected 20 Hz to 20 Khz.
On revisiting the schematic, I could see that the HT feed to the EF86 was low, as was the screen grid supply to the EL84. On measuring the relevant resistors, they were miles out. There was also a 220nF decoupler on the anode load feeder to the EF86, and, yes you guessed it, it was a waxy - and leaking like a sieve. Having checked most of the resistors and found them out of spec, they were all replaced except the green w/wounds which were OK.
Surprisingly, the 2 rather elderly 25 micro electrolytics checked out fine for ESR and value so these were left alone. For good measure, a NOS EL84 was fitted and the whole test sequence re-run.
Result ? A complete transformation ! Distortion now comes in at the quoted 1% at 3 watts and the response is virtually flat from 15 Hz to over 20 KHz. ( 1 watt )
Much of this is a result of the heavy feedback used - A "raw" EL84 single ended generates nearly 10% THD at 3 watts !
So onto a listening test, via my now veritably antique Rogers Exports ( well, one )
Now, these are 8 ohms and this amp only offers 3.75 or 15 , so there was a degree of mismatch...oh and they're not terribly efficient, due to mountains of iron in the crossovers
BUT ....Breathtakingly good ! More than adequate lounge-filling volume until the sound just begins to break up on the peaks.
I'm amazed it sounded so good. Just goes to show how less is sometimes more.

I'm off to look for my PX4 single ended transformer and my chassis cutters now !

Andy