12E1 SE Amplifier

[masterpg]

Originally Posted by The Shadow

OK to recap

Using a choke loaded C3g as a driver, the amp gave a good account of itself at Steve's last Saturday and drove Ed's Vofos, Nick's open baffles and Steve's Lowthers nice and smooth.
Bass was tight fast and clean but lacked ultimate extension. Normally I use a sub driven from the preamp so I had not noticed this at home but on big full- range speakers the deficiencies at the bottom end were all too evident.
Also there was a slight hardness to vocals through Ed's Vofos on some recordings especially The Beatles "Let it Be (Naked)" album that had Paul and I wincing.

The cause of this turned out to be too much current drive from the triode strapped C3gs to the power valves. Dropping the quiescent current from 17mA to 8mA cured the problem. However the new o/p dropped the loadline so low we were nearly scraping the bottom of the graph, right where the grid lines were curving. A waste of the C3g's capabilities IMO.

Choke loaded 76s are what I am currently wiring in after experiments on a breadboard proved them to be far more appropriate in this application than the hard driving C3g.

Experimenting with a 10K:10K isolator/balancer transformer on the input proved inconclusive. The transformer coupled input smoothed out the already smooth sound a bit too much for my liking but the soundstage was enormous. Swings and roundabouts there then.

I had a thought that the primary of the IPTX might have been forming a low pass filter with the 470nF output capacitance of the preceding preamp.But since I did not know the inductance of the IPTX and had no means of measuring it there was no point messing about. A change of output cap in the pre might have restored the sparkle and still given the huge soundstage but I was now allowing myself to get sidetracked into complication as is my wont if not restrained, so I shelved the IPTX idea for another day.

I finally settled on the circuit below and am currently building it into a case. The power amp case will then be used for the preamp rebuild as the shiny aluminium top surface is starting to look decidedly grotty and the pre has had so much tweaking about that it looks like a rats nest, something I can't stand.
The output caps will be changed to 100nF Auricaps to suit the traditional 1M input on the power amp. Might have to watch out for hum with that input impedance though, so we'll see what happens.

I have two weeks off as the schools have broken up for Easter so I have a bit of time to get things sorted properly.

Steve

pic. not open

[bob orbell]

This happens in old posts, it was posted some 24 years ago. Bob

[Greg.]

Blimey. As long ago as that?

[masterpg]

how can I see the diagram on 12 E14 ?
if someone has it preserved, then please publish it.