upgrades to Kat6550

[Ali Tait]

Hi Rich,

Are you talking about the WAD PP monos? Not heard a pair of those.How do they compare to the original 300b PP?

[Richard]

Hi Ali,

Yes, a very similar sound to the 300Bpse monoblocs! First time I heard them was with NealG. We only had a brief listen but they sounded so similar we actually took the bottom off one to check it was actually pp! (The print on the chassis still said "300BPSE".)

Playing with them later (borrowed from Mark at WAD) I found different rect valves made a big difference and the standard ones may have been masking the differences in the amps. That lead to trying a couple of ss diodes instead of the valve rect, increasing HT by 50v or so, and a much more dynamic sound.

The ones I've bought have some nice bits in them such as BG HT caps etc but they sound, fundamentally, just the same as the originals. Again, I'm thinking the HT supply is masking things. The idea, now I have a pair myself, is to explore the HT a bit more as they showed a lot of promise and should be very good indeed.

Sorry, forgot your original question in my ramblings. I thought of the PSEs to describe them as you had a pair. As standard the 300Bpp monos are very different to the original 300BPP - more, err, PSE-like.

Rich

[Ali Tait]

Thanks for that Richard.Interesting stuff.I thought about buying the conversion kit for my pse's when they brought it out.Never got round to it though.Still regret selling them in some ways.I have better amps now but I did have a soft spot for them.

[Yellow]

Hi,

Ok - back to my 6550.

I was going to upgrade things step-by-step and my first port of call was going to be the PSU.
I remember a lot of discussion about the values of the C1/C2 caps being too high at 100uF each as the 5U4 should only see 40uF maximum.

What caps are you guys using and if you have fancy PP PSU caps, I'd be grateful if you could tell me the details. I remember Greg had Solens and had dropped the C1/C2 value.
Can C5 be increased from 120uF to 200uF to increase smoothing? Any sonic benefits?

I also remember a tip from Richard that adding a 47ohm/3W resistor in series to each of the pins 4 and 6 of the 5U4 was a good thing - but I can't remember what that was for! Did it reduce temperatures?

Lastly C6 (4700uF for the ECF80 heaters) - I guess a BG Standard would slot in there nicely?

Regards,

Neil.

[Greg.]

Originally Posted by Yellow

I remember Greg had Solens and had dropped the C1/C2 value.

Hmmm, I've never owned a WAD KaT6550. Maybe you are thinking of what I did in 2A3PSE or mistaking me with Tony Moore or Richard Higgins who both did some serious stuff in this area on their 6550's.

Best wishes,

Greg

[JerryT]

Originally Posted by Yellow

Hi,

I remember a lot of discussion about the values of the C1/C2 caps being too high at 100uF each as the 5U4 should only see 40uF maximum.

Can C5 be increased from 120uF to 200uF to increase smoothing? Any sonic benefits?

I also remember a tip from Richard that adding a 47ohm/3W resistor in series to each of the pins 4 and 6 of the 5U4 was a good thing - but I can't remember what that was for! Did it reduce temperatures?

Lastly C6 (4700uF for the ECF80 heaters) - I guess a BG Standard would slot in there nicely?

Hi Neil

C1, C2 are in series to give the voltage rating. Therefore first cap value seen by 5U4 is 50uF, not 100. Better caps here make a difference but as long as the V rating is high enough (750 V to allow for switch on/off surges) you can replace two caps with one. The trouble is that anything other than electrolytics are somewhat large!

The 47 ohm 3 W resistors are marginally rated - I'd try 68 ohm 7W. The intention is to balance the current through each section of the 5U4. It also prevents rectifier self destruction on switch on, and for me gives greater clarity to the sound.

C5 benefits from a good quality capacitor - I use black gate WKZ here as it feeds the ECF 80 in the input. Before WKZ I added capacitance at this point and didn't notice much change. Playing with bypass caps or swapping to polyprops from ICW or whoever is another option I think.

C6 - Yes - but you'll get more mileage from replacing the bridge rectifier with discrete schottky diodes feeding a CRC network in my opinion. The schottkys give you more voltage so the R needs to be chosen to bring the output back to 6.3 volts. I'm using a 7 W wirewound R (1.5 ohm Mills from memory) with each C being a 1000uF BG. I did use a bigger BG with the original bridge but found this arrangement better. The BG found a home in PSU of my DAC instead!

HTH

Jerry

[Richard]

Hi Neil,

I never had this amp but have heard it quite a bit in my system as a friend had it and tried quite a few tweaks.

It's good that Jerry, John and David can offer first hand advice.

The balancing resistors go here, and if you use wirewounds anything over 3W should be fine as they are rated at 10x for 5 secs so the average current can be used for calc.



They will,

help balance the current through each valve section,

increase the charging conduction angle lowering ht slightly,

increase impedance therebye increasing filtration with first cap and lowering ht slightly further.

All this will help the rects cope better with the 50uF first cap partic at switch on when they will also act as surge resistors. The first cap value is only one parameter, the supply impedance is equally important and it's quite possible to use large first cap if the supply has high impedance (but resulting ht will be lower).

All the usual things have been covered. I found the amp smooth and open but perhaps lacking a little presence and immediacy (I know this is subjective and others will find it beguiling). Not done it yet but something I'll be trying on the 300BPP monoblocs (which have a similar trait) is a big polyprop cap across the ht after the choke parallel with C3/4. It would be easy and cheap to try Maplin's 30uF 440v here (ac rating so should be fine) RG69A.

Rich

[Yellow]

Originally Posted by Greg.

Hmmm, I've never owned a WAD KaT6550. Maybe you are thinking of what I did in 2A3PSE......

Hi Greg,

Maybe I was thinking that: I was just referring to a general upgrade you made with Solens. Or was it not you after all, irrespective of amplifier!!??
I'm sure someone dropped a pair of 39uF PP Solens in C1 & C2 to great effect and I thought it was you! Sorry if I got it wrong - it's been a few years!

Neil.

[Yellow]

Chaps,

Great info - it's all coming back to me now - slowly!
I haven't spent any time with my amp for the last couple of years, other than listening, and I feel it has great potential. I've been away a lot too and have had no time for upgrading or for learning more about valve amps which I'm keen to do. So thanks for all the tips and advice.
I'll be coming back on this Board much more often I guess and it is nice to see all the familiar names once again.

Best wishes,

Neil.

[Yellow]

Originally Posted by JerryT

Hi Neil

C1, C2 are in series to give the voltage rating. Therefore first cap value seen by 5U4 is 50uF, not 100. Better caps here make a difference but as long as the V rating is high enough (750 V to allow for switch on/off surges) you can replace two caps with one. The trouble is that anything other than electrolytics are somewhat large!

The 47 ohm 3 W resistors are marginally rated - I'd try 68 ohm 7W. The intention is to balance the current through each section of the 5U4. It also prevents rectifier self destruction on switch on, and for me gives greater clarity to the sound.

C5 benefits from a good quality capacitor - I use black gate WKZ here as it feeds the ECF 80 in the input. Before WKZ I added capacitance at this point and didn't notice much change. Playing with bypass caps or swapping to polyprops from ICW or whoever is another option I think.

C6 - Yes - but you'll get more mileage from replacing the bridge rectifier with discrete schottky diodes feeding a CRC network in my opinion. The schottkys give you more voltage so the R needs to be chosen to bring the output back to 6.3 volts. I'm using a 7 W wirewound R (1.5 ohm Mills from memory) with each C being a 1000uF BG. I did use a bigger BG with the original bridge but found this arrangement better. The BG found a home in PSU of my DAC instead!

HTH

Jerry

Jerry,

Thanks for all of this info.
Concerning the 5U4's and what they 'see'. Even though the first effective cap value is 50uF, this is still 25% over the rated spec for a 5U4, which is 40uF MAX if I remember rightly. I thought people had lowered the cap values so the rects did see about 40uF instead?

On switch-on, I get a quite long, deep metallic 'twang' sound as everything is shocked into life and if I can do anything to reduce this, it would make me feel better. Maybe Richard's mod to add the 47R to balance the inputs plus a lowereing of the 1st cap will do the trick.

All of your other info is being inwardly digested right now!

Cheers,

Neil.