Decision time: upgrade KAT88 or new WD88VA?

[MarkL]

I have been meaning to upgrade my KAT88 for a while now - I've done the black gates and the Jensens bit, but stopped short of the full list which Richard posted once upon a time(anyone know where that is by the way?)

The sticking point was the sowter output transformers - it didn't look to be a drop-in replacement(I think some other components needed replacing too) and they were expensive.(£553 a pair currently)

So to my questions...

Would replacing the transformers improve the slightly vague and "plummy" bass that I get at the moment?

Is it worth the effort and expense? (The amp is cool otherwise, but I do want more and better bass)

Would I be better starting with a new WD88VA?

any advice would be appreciated

kind regards

Mark

[Cobblers]

Hi Mark,

Perhaps you could start by telling the forum what speakers you are using with the amp.


Even a fairly soggy bass valve amp will be greatly improved with WD25's for instance.
I expect the WD25T's are best in this area since the cab is optimally damped by the closed chamber. Also the T version offers more bass.

P'raps you do not need to change the amp at this moment.

Just my Tuppence.

[Greg.]

I have hard copies of many of all Richards upgrade stages as he progressed. If he cant email to you I'm happy to scan and send on if you're interested.

Best wishes,

Greg

[Richard]

Hi Mark, The Sowters are a definite improvement on this amp. I suspect it's partly due to the originals not being that good, as well as these particular Sowters being very good, that makes the difference.

Everything is better with those TXs and I've not heard its equal in the bass area with any amp since. That said, with smaller speakers on a quick listen many folk might wonder if it was worth the expense. We are into the law of deminishing returns but in the context of a good system they are the only way to achieve what they do. I sold my KIt88 but, looking back, I'd have it back again if it was offered.

Some of the things on the list I kept updating were subjective. The TXs were objective and could be measured and heard clearly. By all means try the parts I used. Each contributed a little and was audible.

I'm using the original WAD 300B p-p at the mo. It's a great amp and has qualities that I think come partly from the 300B valves and partly from being able to run without any global feedback. I sometimes think it's coloured. I've had 2 different 300B p-p amps and 4 different sets of valves and all give me that impression, which makes me think it's the valve type. It's also much poorer than Kit88 when run with feedback. That said, both 300B amps have tiny OPTXs, so it's not like for like, and lower distortion and that last octave of bass may be possible with better ones. They do have strengths in air, space and realism that counter this. Not heard the WD88 so no comment there I'm afraid!

I'll put the final spec in the next post due to word limit. Cheers, Rich.

[Richard]

KIT88 Spec 2004 Sowter OPTs 17dB feedback 250mV sensitivity 43W/8R/channel

Panasonic for Audio pot
Individually screened input cable Van Damme Maplin VU45 (3 inputs only and delete tape switch)

BR1 HT to 4 1200v Schottky diodes
BR2 Bias to MBR160 x 4 schottky diodes RS 642 222

* BR3 6AU6 Heater to 1N5820RL x 4 schottky diodes RS 183 7667 (check v after)
* C3 to BC/Philips 4700uF 25v RS 324-5228 (plus a second same cap and 3w 2R2 series resistor RS 214 2409 to make pi filter/adjust v) for 6AU6 heater smoothing
* Note, use these together or heater v will be too high. BR2/3 are as per circuit board markings but are marked wrong way round on some circuit drawings

C1/2 HT smoothing cap to Rifa PEH300XZ3470M2 470uF 385v RS 325 1009
C4 grid bias smoothing to BG FK type 100uF 100v
C7/8/15/16 signal coupling caps orange BC/Philips polyester .47uF 630v Farnell 966-538
C9/10/11/12 cathode by-pass to BG FK type 220uF 25v
C13/14 ground coupling caps orange BC/Philips polyester .47uF 630v Farnell 966-538
C17/18 6AU6 cathode network caps to BG standard type 4700uF 16v

Sowter Output Transformers UO82S 8k/8R
C19/20 feedback caps to Farnel polystyrene 2700pF with Sowter OPTs and reduced feedback Feedback reduced 5dB by changing R38/39 to 680R .6W RS 132-450 and another 680R from cathode 6AU6 to ground

R3 to 13k (bias)
R13/14/15/16 cathode R`s to Welwyn wirewound 6W 220R (60mA bias change) RS 159 455
R8/10/31/27 gridstoppers to 0R wire links
R34/35 gridstoppers to 0R wire links
R23/25 phasesplitter to 1M 1w carbon RS 131 924
R7/20/24/19 anode R`s to 2W carbon (reliability upgrade) 47k`s are RS 131 671, 36k`s are 47k paralleled with 150k RS 135 443.

5A T fuse instead of 1.6A T once tested
HT caps C5/6 came in my kit as Philips (2222) 159 47151 150uF 450v.
For RF suppression,
2 x RS "Y class" mains suppression caps .047uF 210-617 between L>N and N>E and
1 x VDR 250V spike suppressor Maplin HW13, RS 238-621 for 230v mains or RS 238-592 for 130v mains L>N soldered to back of mains IEC socket.

5 x RF by-pass caps on terminals of C1/2/4/5/6 ceramic disc 1kV 10,000pF RS 107 791

Valves, Sylvania 6AU6WC, Mullard CV4003 (ECC82) , Svetlana 6550C

Standard rectifiers were;
HT SKB B500 RS 183-2173 (singles @ £3.05)
Round W02G Maplin AQ96E 14p RS 659-826 (5s @ 30p ea)
Small square 2KBP02M RS 183-4078 (5s @ 47p ea)

[leo:)]

You know what Rich? IMHO your Sowtered KIt88 produced some of the best quality bass I've heard from any system! How many times would I comment on it after listening to your system
Your 300b's are lovely but to me the Sowtered Kit88 was the best in bass area, just my opinion of course

[MartinJones]

Richard,

Have you ever considered replacing the OPTX's on 300B's. And what would you (or anyone else) go for?

Martin

[rickychance]

Excuse my ignorance & numerous questions Richard (still learning!) but why did you choose 8K output tx's from Sowter? The new WD88VA I think uses 5K. What governs this choice? Does it make a big difference with PP?

[MarkL]

Many thanks for the replies everyone. I am so glad you kept the list of mods Richard! It will take me a while to work out which ones I have done so far!
I was pleased to hear that the sowters made a significant difference to the bass.

I have been trying to work out what each component was contributing to the sound. I have tried different pre-amps(I have a preII) and have put my pre into other systems to see what character it added. (Some of the plummy character is from the pre, but so is the fabulous midrange)

I have borrowed a full set of Nordost Valhalla cables to take poor interconnects + speaker cabling out of the equation. (probably a mistake with hindsight as the difference was far more than I expected)

Cobblers, you asked about speakers. I have a pair of KSL gold mkII's, but am currently using these which I built.


So the result of my testing was that the KAT88 needed a bit more in the bass department but was fine otherwise, hence the post.

So maybe sowters it is...

regards

Mark

[MarkL]

http://www.humblehomemadehifi.com/Soup.html

Link to the speakers, sorry - not tried a link before