WAD Kit88 Output Transformers

[Mike Prime]

Hi all, this is my first post here, I hope someone doesn't mind helping a newbie.

I have a Kit88 that I built a few years ago, now one of the output transformers has died after the amp was left on with a speaker unplugged. I've heard mention of replacing the original transformers with Sowter UO82s. Does anyone know if this is the same model they still offer? What other modifications to the circuit would be necessary if using the Sowters? Any help would be gratefully received, I am anxious to get my amp working again.

[Tony Moore]

Hi Mike,

Richard can probably comment better than I can on the Kit88 mods necessary to use the Sowters. I installed U082s in my Kit6550 amp a few years ago and I did not _have_ to do any circuit mods to accommodate them. What I _did_ do was to reduce feedback (on Richards recommendation) and that certainly improved the sound once again over the extra quality gained from the Sowters. U082s is still available according to the Sowter website.

I will say that the U082s is larger than the stock OPTs and so I had to cut the case metalwork to suit them.

http://www.btinternet.com/~antony_s_...wad/Inside.jpg

http://www.btinternet.com/~antony_s_...g_1642copy.jpg

Cheers,
Tony

[Wieslaw Lipowski]

Hello Mike,

Where did you leave (lose) the green WAD PCBoard that you got with the kit? If you don't need it anymore why don't you...?

[John Caswell]

Hi all,
As I was the original instigator of this mod I can say it does indeed work very well with the standard 8K primary load and the ultra linear taps wired out of the way. You may need to tweak the feedback capacitor for best square wave response but that is about all.
Instead of ordering the drop through types order the upright types (which I prefer aesthetically) and all you may have to do is drill 4 or 5 holes.

John - Dr John

[Mike Prime]

Tony, thanks for the quick reply and the photos. Weislaw, sorry, I'm still using the pcb.

Dr. John, your gris-gris is too strong for me. Would you mind spelling out which are the ultra-linear taps, and how I should wire them out of the way? Here is a schematic of the Sowter transformers. Presumably, the 2 orange wires go to centre tap. Which of the other wires on the primary winding are the anode connections, and what do I do with the other two?

[pre65]

Hi-normally on a PP output transformer the B+ goes to the primary centre tap (orange) and the anodes of each valve to the white + yellow with the screen taps being the violet + pink.

Sowter seem not to stick to the "norm" with centre tap red,and the anode wires blue and brown.


Philip

[John Caswell]

Hi Mike,
The 2 Orange leads are connected together and are taken to the HT line. The Yellow and White leads are the anode feeds and the Violet and Pink leads are the Ultra linear taps which normally go to g2, but in this case insulate the ends coil them neatly and tie them out of the way.
Because the Sowter tx is different to the WAD tx you may get "motorboating", due to positive feedback, in which case swap over either the anode leads to the two anodes or the secondary leads (green and brown) BUT NOT BOTH, so leave them all long initially then tidy up when the amp is working OK

John - Dr John

[Mike Prime]

Many thanks for the information, I have ordered the Sowters.

[jazlichy]

Hi John and all

Merry Christmas! Hope you're all enjoying the break.

I've finally got around to ordering a pair of Sowter (now Carnhill) U082s for my KiT88 amp.

Many thanks for the wiring information here, without this I'd be struggled!

Before fitting can I check on the "motorboating" scenario? Is this an audio aberration or something I can detect with my scope?

Thanks in advance,

All the best

Jaz

[John Caswell]

Hi Jaz,
Very easy to check if you have a cheap pair of loudspeakers or drive units connected up.
If when you power up with the L/S connected you get a "howl" or a sound like a motorboat engine than then the feedback is the wrong way around, so reverse either the primary wires or the secondary wires of the optx BUT not both..
On a 'scope, first put a pair of small R's say 10R/3watts across the L/S sockets, make sure the scope probe is set correctly with the earth clip to chassis, set 'scope to say 20V/div and you will either see the trace galloping about = wrong phase or a flat trace = correct phase.

John