World Designs KT88 amplifier

[valves4amps]

Hi Trevor yes the microphony bit comes from Vox AC30 4 hole version. Mullard designed the EF86 for low microphony, personally EF37A was better, the vibtation in a combo amp used to kill EF86's so the 6 hole AC30 came out using the ECC83 which gave an extra preamp stage for 2 more inputs. Just saying they are not particularly strong mechanically and I've seen a lot rubber mounted for that reason. Probably not a problem in a free standing power amp.

Dave.

[Murphyv310]

Hi Dave.
Yes I remember the rubber mounted EF86, the ECC83 in my Pye Black box is rubber mounted too.
I've a goodly stock of EF86's and when the amp is complete I'll do some testing and select the quietest valves for the job. Personally I prefer the EF37A as well.
I think the amp though is well isolated from vibration where the lad has it installed so I'm not expecting a problem.
Ran the completed channel for a couple of hours into a dummy load yesterday and all went fine, going to finish the job during the week as I now have all the components to hand as the remaining stuff is now here.

[Murphyv310]

Today's progress.

[tiv48]

looking good
Did you end up using the WAD transformers?
Rob T

[Murphyv310]

looking good
Did you end up using the WAD transformers?
Rob T

Hi Rob.
Yes I've retained the original transformers. I'm not so sure as to their quality but we'll see how it all sounds once I've the new cathode resistors fitted.
I'm getting the customers speakers so I'll tweek the NFB if necessary for best quality.

[Murphyv310]

Hi.
Well the amp is finished. I have fitted the new cathode resistors that arrived early. I had to move the centre tap for the HT winding from the mains transformer to a different point on the busbar due to hum.
The cheap Chinese 6550's are far from good with them being wildly out in spec. Four brand new GEC KT88's make this amp rock........ totally amazing!!!
I don't think the customer will pay the £800 I could sell them for though.
I have suggested that two new 6550's would do the trick.
I have changed the cathode bypass caps for different values with an RC network to give a little bottom end boost.
Overall a magnificent outcome that is well within budget of the customer. Mind you I have been building valve amps for 50 years!

[AlanBeckett]

Well done Sir 


Alan

[valves4amps]

I second that with Alan. Well done though it's probably second nature to you.

Dave

[Murphyv310]

Hi.
Thanks to all for the nice comments.
I think I made my first amp when around 13, in 1967. It used a 6SN7, 6V6 & 5Z4. most of the bits were cannibalised out of old radios and the chassis was one I sawed in half! I used it for a record player amp and it did me for a few years. I mucked bout with transistorised amps too but built a few valve amps for people in the 70's & 80's. One notable one had 807's in Class AB1 but doubled up, so four outputs running at 650v anode volts. I had to get an output transformer made that cost me a fortune. It works well, I say works well as it eventually landed up in a local church to replace the amp in the old valve organ they have. I throttled back the HT in it though to 550v, made very little difference in output but It still is going strong and the last time I did anything to it was 1991, the organ itself gives more trouble with contact problems and I have to get these made every so often.
More recently I have made a Mullard 3-3 and have two orders for stereo versions, which is my next jobs.

[Terry]

I think I made my first amp when around 13, in 1967. It used a 6SN7, 6V6 & 5Z4.

Snap! Although, in my case, that would have been 1957!

I don't think I bought anything for it except the mains transformer, which came from Duke's in Manor Park* - who used to advertise regularly in PW, etc - and were only a short detour - by trolleybus! - on my way home from school.

I found a metal rectifier and a largish electrolytic in a box of stuff behind the shop where I eventually went to work. Obviously faulty and removed from a TV but worked fine in my amplifier where the demands weren't so high ...

The chassis was made from plywood covered in aluminium foil.

As for Mullard amplifiers, I've lost count of the number of 5-10s I built over the years.

* I was surprised to see an advert for Duke's in the Daily Mirror one day! This would have been in 1958 when the law changed so that all cars had to be fitted with two rear lights, one at each side, from 1st October.

Duke's seemed to have scooped the surplus pool with vast numbers of, presumably government surplus, rear lights with a diameter that matched the minimum diameter under the new regulations. If my memory is right, he knocked them out for 6d each - and, from the prolific adverts which popped up in all the newspapers and other normally unlikely places, I suspect he made a killing!