Used WAD KAT88 Issues and Circuit Diagram Request?

[BUTUZ]

Hi all thanks for having me.

I bought my first valve power amp on ebay a WAD KAT88 - it was described as having a couple of issues in particular noise in the right hand channel whilst it was warming up. I can confirm that this is the case I get a fairly loud crackly weird noise like rubbing a towel on a microphone for about a minute then it stops.

Also a fair bit of transformer hum, some through the speakers and some physically from the transformer (sounds to me like it's coming from the toroid but hard to tell)

It has been modified in the past - there are traces cut on the board and what look like zobel networks wired in and it has had caps and resistors changed around the place (all nice stuff though). The soldering was not so nice.

Could I get the circuit diagram and manual for it please?

Any any suggestions as to the issues I have please let me know!

Many Thanks

[bob orbell]

Hi and welcome, our man "Greg" keeps all the Manuals and diagrams, give him a Private Message, also Moderator "Richard" may be able to help. I would say your rustling noise is possibly dirty valve pins and or sockets. Bob

[BUTUZ]

Hi Thanks I'll send Greg a PM.

Well I did more experimenting tonight and I've identified a dud KT88 valve.

If I move that valve to the left I get the rustle on the left hand channel. If I move that valve to the right I get the rustle on the right hand channel!!!

[bob orbell]

Glad to be of some help, now, if you don't know the age of the KT88's, I would suggest replacing all four and my choice would be JJ's from Hot-Rox, they are what I use in my WA88, the small tubes should last almost a life time. Bob

[Greg.]

Build instructions and diagrams together with reliability upgrade (R3, 15/16, 19/24, 7/20) notes forwarded to BUTUZ.

[BUTUZ]

Thanks Bob and Greg for your help. I am trying to keep costs down at the mo and valves are so expensive - the valves look to be from March 2020 so not so old - they were from Hot Rox and have measurements printed on them so I have asked hotbox if they can source me one valve with similar measurements.

If I have to buy 4 valves, ideally in the future I could do with a bit more power can 4 x KT120 be used in this amp I have seen some amps on YouTube which seemed to be 6550/KT88/KT120 interchangeable?

Just wondering!

Thanks!!!

[Greg.]

Why do you need more power? Are your speakers so inefficient? You need to give us more detail about your system to allow us to be helpful.

[BUTUZ]

Hi Greg, yes my speakers are not efficient sadly but I always wanted to try valve amps anyway. PMC EB1 speakers about 15 years old. The specs state 89 dB 1w 1m and 4 Ohms nominal.

https://pmc-speakers.com/products/archive/archive/eb1

Thanks!

[bob orbell]

I use speakers of 86dB powered by a WA88 (30 watts max) more than enough volume. Bob

[John Caswell]

Hi,
Replacing the KT88 with either KT120 or the like in the Kit/Kat 88 will not increase the power output, you will have to increase the HT etc. to get more. The KT120/150/170 are basically up rated anode dissipation versions of the KT88 and as such will tend to last longer in a given amplifier due being run at lower dissipation rate.
I have a pair of home made GEC 88/50 (HT 500V) amplifiers and have measured them with the following valves GEC KT88, Gold Lion KT88, and Tung-Sol KT120 and KT150 and the power output is as near as dammit the same. I have KT150 in as standard and I suspect that they will last for ages as they are being run at approx half anode dissipation of about 40Watts as opposed to their max rated 75 watts.

John