Any suggestions re; KiT6550 brightness?

[Intenso]

Hi all,

I wonder if anyone may have any suggestions for my KiT6550 amp that has developed a troubling brightness that is bothering me. The amp has all of the WAD recommended upgrades and also has Sowter Txs. The work has been completed by my friend Greg, so I can vouch for the quality of the work completed.

Since I’ve had the amp it has had a bit of an intermittent aspect to its sound quality. There have been times when it has sounded sublime with a very fulsome bloom to the bass and a sweet top end that I have very much enjoyed. At other times the bloom has disappeared, and the sound become toppy.

We recently changed the PSU caps to Mundorf Mylitcs and for about three weeks the sound was the best I have ever heard. The lovely bloom and sweetness were returned. However, since then the sound has again become toppy and lost the bloom. I am at a complete loss as to what the issue may be and wondered if this might be a burn in issue for the new caps. I am a little concerned as the amp has had at least a 100 hours use since the new PSU caps.

I’m wondering if the coupling capacitors may need replacing. They are currently Jensen PIO but are over 10 years old. However, they appear to be visually OK. Tubes are all relatively new.
I thought maybe the silver signal wiring. However, when the amp has been singing this hasn’t been an issue re; brightness.

Any ideas?

[Phil Y]

It does sound odd that the sound should change like that. Can you confirm that it is on all sources just to be sure it is the amp?
I think I would be inclined to suspect some outside cause, have you noticed whether it is at certain times of day, or on certain days of the week?

I would be inclined to change out 10 year old PIO caps but purely from a reliability point of view, I don't think they are likely to be the cause of your problem, unless the amp is going long periods without use, but I'm sure that isn't the case.

Regarding burn in of the Mundorf caps, my experience was not anything like your described problem, I just found a subtle improvement in clarity but not much else.

Phil.

P.S. Just to be clear, you said "intermittent". Has the sound settled in a bright condition or does it change from one day to another?

[Intenso]

Hi Phil

Yes, its on all sources and we tried the amp in another system the other day and it still sounded very bright.

Its very odd as previously it was intermittent and then following the cap change it was fantastic and now it seems to be stuck sounding very bright.

[Phil Y]

Mmmm, it is difficult to think what might cause this. Perhaps if the some cathode decoupling electrolytic caps were drying out and going low in value it would cause that but you might expect that to be constant rather than variable, and would probably be worse on one or other channel as two caps are unlikely to degrade at the same time.

If the PIOs were drying out this could cause similar but they would have to be very bad to be that obvious in sound balance.

Phil.

[Greg.]

Just to clarify, the signal wiring I used is silver plated oxygen free copper. Single multi strand core in PTFE sleeve with SPOFC braided shield and PTFE outer. It is not unlike the twin core wire used in WD kits.

We tried Tony’s amp in my system last week and whilst I acknowledge that Tony has the issue he describes, I personally would not describe it as a very bright sound. I think it is more about the amp being driven to the edge of sibilance. We tested with one LP that I particularly know will show this problem. To my ears the sound was within the bounds of acceptable, but Tony’s hearing appears to be more sensitive than mine in this area. I could not detect any problem in the bass.

I think it would be worth changing the coupling caps as this needs doing owing to age and reliability anyway. I would favour a pair of Mundorf M Cap Supremes, or even a return to the original orange drops. We could change the signal wiring (fiddly job) to simple OFC as this might take a sharp edge (if perceived) off the high frequency sound. What I find most mystifying is that the amp sounded so good to all who heard it and now Tony is experiencing problem.

[davebms]

I have the same amp ,with mine I got the opposite effect with the
Jensen it went ;slow and sloppy; sounding, I fitted soniqs sax poly ,
in my system they are superb !!! best so far by a long way,,, My hearing
is very sensitive at the top end and I find silver plated wire does NOT help
at all

all the best dave

[Richard]

Happy New Year everyone!
Yes I'd change the coupling caps. Vishay axial polyesters are natural sounding, google RS 185-4369,
or Soniqs SAX from Matthew are lively and open without being tinkly, http://www.world-designs.co.uk/capacitors.htm
both at real world prices.

[Intenso]

Thanks everyone, it is so puzzling as the amp has previously sounded very good but is currently to my ears sounding quite relentless.

I'll change the coupling caps as a start and see how things develop.

[Chris]

I noticed a drop of fullness of what you described as bloom with the Mundorf film caps in my speakers after about a week. I would go with Greg and put some Mundorf Supremes as coupling caps..

Chris.

[John Caswell]

Initially instead of spending lots of money on "boutique" capacitors , do as Richard says and buy some MKT 1813 from RS/ Cricklewood or some "Orange drops". Put them in and see what you think after some days/weeks, I think you might be surprised at the results.

John