World Designs KT88 amplifier

[Murphyv310]

Hi.
I got one of these really nice looking amplifiers in for repair today. I took the usual £30 deposit from the customer and showed him the shed and what I do, he was impressed and after demonstrating my Mullard 3-3 and 5-20 said they both sounded better than the World Designs amp.
I eventually got the bottom off the amp and had a look. Now he told me he had spent £1500 on a modification to the amp last year, afterward it kept breaking down and even blew the mains transformer.
Well I've never seen such a mess, the mods included a Veroboard home made HT rectifier circuit, a DC feed for the output valves, additional transformer, lots of zeners and other audio foolery that bear no resemblance to a valve amp. The workmanship is abysmal.
The amp itself although pretty is far from great in its virgin state, rubbish layout and not even ultra linear. I told the customer that to improve it, it needs a UL output stage and someting like the 5-20 output stage.
I've quoted him £400 plus labour to do the repair.

[Murphyv310]

                               

[tiv48]

Something wrong here trevor i built one back in the day and it was ultra linear the layout looked nothing like the one you have there .
i may still have the original build instructions and circuit i will see if i can find it.
Rob T

[Murphyv310]

Something wrong here trevor i built one back in the day and it was ultra linear the layout looked nothing like the one you have there .
i may still have the original build instructions and circuit i will see if i can find it.
Rob T

Hi Rob.
The early versions were just Anode class A-B not ultra linear. What you see in the piccys is the mods with DC fed KT88 heaters, Grid bias and silicon HT. Its a total pigs breakfast.

[tiv48]

I may well of had a later version .
Rob T

[Nick]

If it's not already got UL output transformers, that's going to cost a pretty penny on its own...

The early WAD KT88 designs were often heavily modded, sometimes by folk who didn't have a clue. There's a lot of information out there on this.

Nothing wrong per se with DC heater supplies etc.

[Dick.1]

That's criminal.............nothing less.

£1500 as well ...........

Dick.

[Murphyv310]

If it's not already got UL output transformers, that's going to cost a pretty penny on its own...

The early WAD KT88 designs were often heavily modded, sometimes by folk who didn't have a clue. There's a lot of information out there on this.

Nothing wrong per se with DC heater supplies etc.

Hi Nick.
Yes new UL transformers wont be cheap but Ed has PM'd me and offered rewinds of the existing transformers so that should cut the cost down.
I also know that DC heaters in the early stages can be beneficial for hum reduction but to have a separate DC feed to the KT88 heaters then dropped to 4.5v via the big gold resistor mounted on the rear of the chassis is ludicrous! I wonder if matey boy doesn't understand the heating effect of an RMS 6.3v. vs 6.3vdc.
The mods blew two output valves within minutes of each other, caused a valve base to track over and also blew the mains transformer.

[Mark Hennessy]

I wonder if matey boy doesn't understand the heating effect of an RMS 6.3v. vs 6.3vdc.

Given that the definition of RMS is "equivalent DC heating effect", or words to that effect, I'd say 6.3V RMS is exactly the same as 6.3V DC.

Presumably here, you're getting at the fact he's rectified and smoothed 6.3V AC to give a higher DC voltage that he has then had to drop via the big resistor?

You're a braver man than I to take this on - especially for just £400

Good luck,

Mark

[pwdrive]

If it's not already got UL output transformers, that's going to cost a pretty penny on its own...

The early WAD KT88 designs were often heavily modded, sometimes by folk who didn't have a clue. There's a lot of information out there on this.

Nothing wrong per se with DC heater supplies etc.

Hi Nick.
Yes new UL transformers wont be cheap but Ed has PM'd me and offered rewinds of the existing transformers so that should cut the cost down.
I also know that DC heaters in the early stages can be beneficial for hum reduction but to have a separate DC feed to the KT88 heaters then dropped to 4.5v via the big gold resistor mounted on the rear of the chassis is ludicrous! I wonder if matey boy doesn't understand the heating effect of an RMS 6.3v. vs 6.3vdc.
The mods blew two output valves within minutes of each other, caused a valve base to track over and also blew the mains transformer.

Out of interest what's the value of that resistor and which heaters does it feed?

" a DC feed for the output valves" is that the heaters?

Lawrence.