Kit6550 Grid stopper question

[Tony Moore]

Hi,

This section was looking a little bare so I thought I ask a quick question. (the question is quick but I may ramble on anyway )

After four years of experimentation, much advice from BB'ers, building and component tweaking I have finally almost arrived at a system, that for _my_ ears and room, suits me well for all the types of music that I listen to. That's not to say I won't keep on hankering after ways to improve it further! That's the nature of the "disease". However, I can now sit and listen and not be troubled by missing this or too much that. I've thought on more than one occasion that I'd been here before but it seems I was mistaken. You just don't know what you're missing out on when you haven't heard it yet!

There is just one area though that I would like to improve if at all possible and that is the really high top end. My ears are not what they were and I need all the advantage I can get! Since my amp, Kit6550, has the Sowter U082 fitted, long ago I reduced feedback as instructed by Richard but didn't at the time get around to removing grid stoppers. I have now removed (well, only bypassed as yet) the four 6550 gridstoppers and that has certainly given a little more treble detail. I was wondering if I should also remove the gridstoppers from the ECF80 inputs? They are not easy to get to so I would really like to know it's worthwhile before attempting it.

Are there any other tips you could give me on tweaking anything that might be contributing to a roll-off of HF? I have yet to put it on the bench and test it with scope and signal gen.

Also of course my speakers are not overly top endy. I have the IPL S5TL Gold built as standard. Any tweaking tips for those would also be gratefully received. I _have_ already bypassed the 1ohm series resistor to the matching transformer pcb.

I think all this new found desire for highs comes from knowing that my new CD source (TDA1541A NOS DAC and CDPro2M) is producing some great detail up there that is not being allowed to shine as well as it might.

All in all though a minor niggle, I am very happy with the overall effect. Stop me now, quick bring a straightjacket.

Happy New Year to All!
Tony

[NickG]

I would be carefull you don't introduce instability by removing the prntode grid stoppers.

I would have though the feedback cap would be one area to look for if you think you are missing HF response.

[Tony Moore]

Thanks Nick,

I wondered about affecting the stability. I will leave those alone then.

I _did_ change the feedback components at the time but I've lost the info on the values I used so it's time to "pop the hood" again and do some reverse engineering. (The Kit6550 was "put out to pasture" for a long time as I dabbled with the WAD 2A3PSE, I've only recently gone back to the 6550 and so it's a rediscovery time!)

I'll report back later.
Thanks,
Tony

[Tony Moore]

Hi Nick,

I can say with some certainly (without opening up the amp...I'd rather enjoy listening to it than fiddling with it! ) that my feedback components are as shown on the old WAD Faq for Kit6550 feedback reduction. ie. quoted from the WAD BB:

"Change R45 and R46 from 560R to 1k2. Connect 2 more resistors each of 1k from both Pin 7s to ground. Change C21 and C22 to 2.2nF"

I know that I used metal films for the resistors and silver mica for the caps.

Thanks,
Tony

[NickG]

Its possible though that with the different transformers a better/different feedback setup may be possible. Just a thought, I don't know.

[Richard]

Hi Tony

I didn't need a stopper on the first valve using those TXs with Kit88 so worth a try so long as you can check for stability on the bench and into a speaker load. There does seem to be an improvement in clarity but nothing that shows on the scope.

Other ideas would include changing certain resistors for metal film or metal oxide type. Those stoppers would be an option as would anode loads.

Polyprop coupling caps are worth a go.

What vol control are you using? A metal film R used as series with both Pan and Alps gave much higher level of detail when I tried them. Used D below, use a value around 80-90% the pot value if you use a 10k or 20k pot and note the pot is inverted; pin 1 would usually be the signal in and is now to ground. Pin4 is Pan pot only. Not sure if you were in that mad thread

Polyprop caps in the tweeter and mid sections at least are worthwhile over electros if you're not already using.

Regards
Rich

[Tony Moore]

Hi Richard,

Thanks for your comments. I'm actually using polyprops already for coupling; Dynamicaps. I'm also using a stepped attenuator based on Welwyn RC55 resistors (Y for series and C for shunts). So there's not really a lot of scope for change I guess.

I will research possible resistor changes.

When I get chance I will put it on the scope and sig gen and see if I can plot the frequency response.

I don't want to lose the tone that I have at the moment by doing anything too radical though. It does sound very nice.

Thanks,
Tony
ps. The IPLs HD-3P has it's own little crossover board with matching transformer, I can't remember what it looked like now though as it's buried inside.