World Designs KT88 amplifier

[AlanBeckett]

You don't want to retire Trevor, it's not all it's cracked up to be.

Alan

[Murphyv310]

You don't want to retire Trevor, it's not all it's cracked up to be.

Alan

Now that made me chuckle

[Murphyv310]

Hi.
I've now fully removed all the audiofoolery and cleaned everything up and have started on the rebuild.
I've fitted two new octal bases for the HT rectifiers which will be two GZ34's I've mounted two new tag strips, wired up the rectifiers and made an earthing bus. I also have started on the Right Hand channel, the Left one will be a mirror image. All the resistors are new and of 5% or better tolerance the caps are a mix of the new yellow axial types rated at 630v or above and of course new electrolytics. The smoothing will be 550v rated but are on order.
I plumped for two Mullard 5-20's modified for the use of the 6550 output valves.

I did make a fatal error in that this amp does have the UL output transformers, how I missed on that I'll never know, serious oversight on my part, I've explained the error to the customer and he thanked me for my honesty.
I'll post up some pictures in a day or so.

[Ed_Dinning]

Hi Trevor, Mullard 5-20 will run as Ul, it is so described in the "Circuits foe Audio Amps book". They even go on about 43% or 20% taps.

Ed

[Nick]

..."Circuits for Audio Amps book"....

58MB, so I didn't attach it!
 
http://www.sowter.co.uk/pdf/Mullard-Circ...ifiers.pdf

[Murphyv310]

   
Hi.
I've placed an order for some new caps that I have run out of. I have 90% completed the Right hand side for enough for a test.
What is needed is suitably rated smoothers, the coupling caps (0.22uf 400v) tidy the feedback wire and the busbar and DC feed the EF86 heaters.
First test was impressive with the amp (on one channel as that is the only one near completion) working first time. On 400Hz, 1kHz and 10kHz there is no clipping till 23 watts, the amp was running a little low on 200v AC as I didn't want to push the tacked in smoothers above their 450v rating. The KT88 cathode resistors are temporary with new wirewounds on order.
The Chassis will get a good clean with a stiff brush once all the work is complete.
There is a trace of hum which is coming from the EF86 heaters so as I said I'll DC feed them. 
The little piece of modern junk will need to stay as it will leave a gaping hole in the chassis.

[Murphyv310]

       
Couple of more pictures.

[valves4amps]

Looking good Trevor not only in looks but function too. I have tried all the various hum reducing techniques in the past, most more trouble to wire than the reduction they give. One of the simplest was a 'hum dinger' where a 100 ohm pot connected between the floating heater wires and the slider down to the star point earth. I must admit never tried them with an EF86 renown for being fickle where noise is concerned especially microphony.

Dave.

[Murphyv310]

Looking good Trevor not only in looks but function too.  I have tried all the various hum reducing techniques in the past, most more trouble to wire than the reduction they give.  One of the simplest was a 'hum dinger' where a 100 ohm pot connected between the floating heater wires and the slider down to the star point earth.  I must admit never tried them with an EF86 renown for being fickle where noise is concerned especially microphony.

Dave.

Thanks Dave.
I must say that I've never had any problems with the EF86 and like the EF37A it is a low micro-phonic valve.
I have tried DC feeding the heaters, that along with different smoothing values have resulted in virtually imperceptible hum. I have two GZ34's each rectifying the opposite phases from the mains TX, the anodes are strapped with 50 ohm limiters, the Reservoir is 47 uf and the smoother 100uf both rated at 500v.
The changes on the 5-20 circuit are:
C5, 50uf. C4, 10uf. R14 & 15 500 ohm otherwise as the Mullard diagram

[Murphyv310]

Hi.
The PSU follows standard circuitry with the exception of using two rectifiers, nu-necessary TBH but the original had two rectifiers and the look would be out of place with one and there would be empty holes.
The DC feed for the EF86's needs Schottky Diodes as a silicon bridge is too lossy at 6.3v.