[Diabolical Artificer]

Thanks Andy. sorry to hear your house is still in bits, hope you get started on your amp build soon." I'm sticking with tried and trusted "conventional" circuitry" I can understand that approach, this amp has taken a hell of a lot of work, with many steps backwards, however if I'd built an amp from an existing schematic I'd have learned nowt.

This amp started from a Hammond 1650TA OPT I got for a good price on ebay, at that point I knew next to nothing about about electronics, valves etc. I had read quite a bit on valve amplifier's, but it wasn't clear in my head. Once you try and build an amplifying stage from scratch, EG draw a load line, calculate all the R's and C's, light dawn's and another hundred question's arise.

" My main bit of nervousness is the need to use DC heaters on the EL34" I think you from recent experience that a 9v AC winding won't be sufficiant for 6.3v DC heaters. 9v rectified will give you around 12.5v unloaded, once loaded it will drop around 2v, so say 10v. So now it needs regulating, lose 1.2v ish for the reg and lose another volt ish for series pass transistors and you have not much "headroom" I found you need a winding of 12v AC to be on the safe side. Joe will probably tell me I'm talking out of my hat, just saying what I've found from building a 6.3v reguated 2A PSU. Anyhoo, you don't "need" DC heaters really on a PP OP stage. You could run them series connected at 25v (25.2v) @ 2A. ( four in series) or 2 x 12.6v This is what I did on this amp, IE 18.9v. It makes wiring the heaters neater, no big thick twisted wires. Just a thought.

Don't talk to me about rebates, although I thought mine were mitre joints, it's been a nightmare trying to get bang on accurate, square, 45 deg joint's with an old saw and plane. I made a shooting board which didn't work, and have had to do them by hand. These joint's, were a bad choice, they slip when assembled, so I had to put temp nails in, leaving six holes on each side.

Re wood fettling, I'd advise getting someone with a work shop and prefably long experience of woodworking to do them for you. To get 100% solid, no gaps joint's is not easy and takes far too much time. Find the wood, draw some plans and pay someone is my advise.

Re the IP stage, the 100k grid R is part of the FB circuit, so can't be messed with. I'm not 100% aux fait with this circuit, so I might be wrong, but AFAIK, it doesn't need a resistor to ground, and the coupling C is needed to stop as you DC getting to the grid and removes the pot from the DC equation.

I was in two minds as to whether to include the pot, these are power amps, so strictly speaking one isn't needed, but I'd already drilled the hole for one on the front panel. Also I like having one as I don't have a preamp.

This last issue highlight's one aspect of this amplifier that has been a problem, IE as it's taken two years to build and my knowledge has evolved over those two years, certain circuit's have needed changing. The OP stage, phase splitter and cathode followers haven't changed much, but I need to change the IP stage as it's frequency response was dire; HF rolled off at below 5khz, the amps sounded muddy and bass heavy as a result. Therefore I've been unable to drastically change some aspects without undoing weeks of work, so there are some fudges.

I had a bog standard common cathode gain stage at first, 220k Ra, 1.1k Rk, 1M Rg with a 39k grid stopper. The first thing that improved FR (frequency response) was removing the grid stopper, it went from 5khz ish to 10khz ish straight away. After then trying 7 different ways to provide cathode bias and improve the IP stage, I found the circuit for the IP stage with FB, it knocks every other stage into a cocked hat. On it's own it has a perfectly flat FR from 10hz to over 30khz. It massively improves the amp.

The downside is stability, I found that it worsened stability with especially bad oscillation on switch off. After experimentation fitting the RC's on the OP stage, IE 2k2 and 2200pf, the 220n and 4R7 on the OP but especially the 4k7 and 680p across the IP stage anode resistor improved stability immensely.

However the Mullard 3-3 use local FB, as do quite a few other commercial valve amps, so unless I experience problem's it stays. At some point I will do a test loading the OP with a capacitive load only, to really test stability.

Thanks for your input mate, Andy.