12E1 SE Amplifier

[NickG]

The 1meg grid leak resistor on the 76 is quite a high value, although 9 pin valves are OK with this I don't recall the 76 details, but reducing it to 500k wouldn't harm anything else, but would suit the valve better.
Reply With Quote

Just FYI, the RCA specs specify 1M as the maximum value. As Paul says changing to 500k wouldn't hurt, especially as the RCA specs probabbly didn't expect the valve to be used 60 years later.

[The Shadow]

Hi Nick/Paul,
I have reduced the input Z to 470K as I had run out of 1M resistors to parallel up to get the 500K. Near enough I hope.

Just about to transfer the Sowters to the new chassis so should be up and running in a couple of hours.

[NickG]

I have reduced the input Z to 470K as I had run out of 1M resistors to parallel up to get the 500K. Near enough I hope.

Unless you have a preamp with a single ecc83 as a output stage, 500k could meant anything from 100k to 999k

:-)

[The Shadow]

So it's not that critical as long as it is not too near 1Meg then

The amp has been successfully tested and fired up in the system. I'll leave commenting on the sound until I have let it settle for a couple of days but it is very pleasant and easy to listen to.

The finish however I will talk about. The whole amp is built on a 6mm thick clear polycarbonate subchassis decoupled from the wooden cabinet by rubber washers. More rubber washers sit on top of the subchassis in each corner and midway along each long side. Atop these washers sits another 6mm polycarbonate top plate to which are attached the output transformers. Thus the only visible fixings are the two front screws holding the transformers and the two front screws holding the top plate

The top plate has been sprayed with a Rover charcoal metallic finish on the underside. This method of spraying the back of a transparent material was copied from the way the tuning dial glass was done on our old 1950s Ferguson radiogram. The thing about this method of painting is that you are guaranteed a virtually flawless finish.

It looks nice.
The preamp rebuild is next on the cards with the same finish.

Steve

[The Shadow]

A few teething problems today.

Sat listening this morning when the music gently faded out leaving nothing but the sub chugging away. The 76s had gone out dammit!

Lifting the hood and the problem soon revealed itself. It was a dry joint in the heater cct where the run to the 76s teed off from the last 12E14. It must have been sparking away merrily as it had melted the insulation on the wire. So that heater run had to be replaced. A right bl***y job. That was just the beginning.

After much cursing the offending heater run was replaced. Checking the voltages at the 76s there were only 9 volts at the cathodes where there should have been 13; ominous.

Checking the 12E14s I had 40V at one cathode and 44V at the other and the HT was down. There was only 254 volts of HT coming into the B+ rail. I had lost 80V of HT somewhere; so according to the curves for the 12E14s, with the anodes at 245V and 40V at the cathodes they were cramped up to hell on the negative going signal, not good.

My eyes fell on the God knows how old used Sheldon 5U4G rectifier feeding the power amp supply. Swapped it for a NOS GZ37 and bingo! We were back in business....
Except for one problem. I had done the calculations for the potential divider that fed the 76s from measurements taken with a fading rectifier in, so the 76s were now way too high! God knows how I didn't notice that the HT was down and twig that something might be up when I did the calculations. Wouldn't have stopped the heaters going down but would have saved a lot of grief elsewhere.

Redoing the potential divider brought the 76s back into line but the 12E14s were still wrong. So the cathode resistors ended up being reduced to get the cathode voltages back away from the cramped up region.

Jeez! The things the Thermionic Gods send to try our patience.
Serves me right for playing with obscure tubes and old used rectifiers.

Steve.

[dave dove]

sounds like you're having great fun steve

dave

[Paul Barker]

Yes old rectifiers are an issue.

I've been using solid state for some time now out of convenience, and don't find it a major issue.

I believe if you use the Cree diodes (voltage allowing, as you can't voltage share with them) there is no loss in quality against your gz37.

Yes it is always nice to use valves, but if we don't compromise in a few areas we end up building a giant unreliable monster.

[The Shadow]

Hi Dave/Paul,

Yes.....took the best part of yesterday to sort the b****y thing out but it's now running sweet as a nut and back to being a seductive old smoothie in the Leslie Phillips class of valve amps

Paul.
Power supplies of the solid state regulated variety are definitely something I would like to look at in the future. Once the preamp has been rebuilt this holiday, I will probably embark on a marathon reading exercise starting with Morgan Jones huge chapter on PSUs.

The dual power supply I currently have is serving me well but I could do with one that puts out a range of voltages say 100, 250, 350, and 500VDC plus 2.0, 2.5. 5 6.3 and 12.6VDC for heaters.

Such a supply could probably fit into a case about the size of a 19" rack mount or a computer server shelf.
Now this is not a project to be undertaken lightly (could be expensive for one thing) but would with careful auditioning at each stage be something that could be an eventual fit and forget solution.

Steve

[IslandPink]

Paul

"Yes it is always nice to use valves, but if we don't compromise in a few areas we end up building a giant unreliable monster."

Wow , a shocking change of engineering approach for Mr. Barker then


MJ

[slowmotion]

Originally Posted by Paul Barker

Yes it is always nice to use valves, but if we don't compromise in a few areas we end up building a giant unreliable monster.

Originally Posted by IslandPink

Wow , a shocking change of engineering approach for Mr. Barker then


MJ

That was funny