Assuming the cathode follower has a large load resistor of which only a small fraction is used for auto-biasing then its input impedance ought to be enormous i.e. it shouldn’t be loading the '81 significantly at all. So you should see a voltage gain of pretty much mu, shouldn’t you ? That should be about 60 for the '81 at 4.5mA and Va~180V (what you’d need to get 4.5mA with a 430ohm bias resistor). Can you measure the signal voltage at the grid ?
I’m sure the modelling will have allowed for the interaction between the cascode’s relatively high output impedance and the 6.2k filter impedance and also for the filter losses at whatever frequency you’re measuring (1kHz ?).
whistles surreptitiously while soldering in a couple of 10ks
Except now the power supply appears to have gone bonkers with the HT supply appearing to fluctuate by a total of 15V with a period of a couple of seconds. Am now confused…
And having isolated it to the cathode follower (now put as the last valve rather than the last two halves but with the same symptoms) and with valve changes having no effect, I can only assume that Ohm’s law has been disapplied.
I have checked the relevant cap in the power supply by taking the B+ feed from the sections that I know work, I have rewired and resoldered the valve base to make sure it works, I have checked that the resistances work as they should.
It is a phono stage. Overnight I thought of the last possible option which was the coupling cap into it. It now has a 0.1uF and works properly, a somewhat bigger one obviously didn’t…
Put it down to a reduction of personal idiocy…
Edit: I will sling up a circuit when I get to the computer.
Bought a cheap (£12) drill mount polishing kit off ebay and had another go at the SUT. Now looks even shinier and with a coat of halfords clear lacquer will hopefully stay that way
Yes I know there’s some black paint around the edges, I still want it to look a bit aged.
Just built a pair of these. They are Club-27 ‘Amy’. I bought the kit of drivers and the kit for the cabinet. The first cabinet was not my finest hour, but I learnt and the second one is pretty good. They use a 12" wideband driver and a horn tweeter. The crossover is just a resistor across the woofer and a cap and resistor in series with the tweeter. They are rated at 98dB which seems fair as they are currently going great guns on the ens of an AI 300. Really quite impressed with them so far.
Well they don’t have the tonal expression of the Snells, the Snells are seamless and sophisticated. Having said that the Amy is a lot closer than I was expecting. Also they need a fair bit of running in, they have already improved over the last few days. I am enjoying the dynamics and shear level that they can produce. I have never really had enough serious watts to put through the Snells.
I wil put the Snells back in, in a couple of weeks, should be interesting.
There’s a lot to be said for using good drive units with little in the way of crossover, that are efficient and allow good use of low power class A amplifiers.
