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Old 30th November 2004, 05:27 PM   #1
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Default Vintage amp rebuild advice

Hi, I'm about to start to rebuild a pair of PYE HF25 amplifiers - an ultralinear KT66 design from the 1950's. Both are working, and one of them actually sounds really good right now, but I was intending to replace all the passive components and wiring whilst trying not to ruin the original character.

Luckily, one of the amps is completely untouched and I know its history from new, so I have a reference as well as circuits and service info, but I would appreciate some advice about the best replacements for the vintage parts:

Capacitors: These are TCC paper-in-oils with values from 0.5uf/500v to 50uf/12v. Would Jensen be a good substitue?
There are also some radial electrolytics (the aluminium-cased type) with values like 64uf/350v to 100uf/50v.
Finally little silicon-mica 150pf - what should I use instead?

Resistors: I haven't measured anything yet, but I'm sure all of the 50-year-old carbon resistors have drifted way off, so I reckoned on replacing them all. What's the best modern substitute? They are a mixture of 1/4 and 1/2w, mostly 10 or 20%, but a couple of 5%. Finally a pair of 600ohm-3w WWs

Some advice here would be much appreciated.
regards, Jeff
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Old 30th November 2004, 06:28 PM   #2
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Welcome to the forum. I think you mean silvered mica. They're still available. 50uF 12V will be an electrolytic. The TCC 0.5uF 500V are unlikely to be paper in oil, more likely paper (and horribly leaky). At that value, the plausible modern equivalent (unless you go for new paper in oil) is polypropylene.
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Old 2nd December 2004, 02:15 PM   #3
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Hi, thanks for that info - i see that I can get the little flat silver micas in a 500v version. The .25 and .5uf are the TCC "Metalpack" "super visconal" kind, which are paper-in-oils, so would Jensen be the best replacement? One of their advantages is that they are the right kind of size and would fit without changing the layout, where the Audio Note ones tend to be rather fat.

I would like to keep as close to the style of the original spec as possible, but I'm reluctant to use new-old-stock caps and reisistors even if I can find them.

regards, jeff
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Old 5th December 2004, 04:30 PM   #4
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My personal opinion is that you are right to avoid NOS 0.25uF and 0.5uF capacitors - they could easily be leakier than your existing ones! I have no experience of Jensen etc, so I can't comment.
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Old 5th December 2004, 08:04 PM   #5
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If these were my amps, I would be leaving the paper in oils well alone if they are working fine - they are usually stable and reliable. Electrolytics are a different story as they tend to dry out with age and go leaky. Try measuring some of the resistors - they may well be fine. I have a leak amp of a similar age which is still working very well with most of the original components.
If you do decide to change the PIOs, then don't throw out the old ones - mad people like me actually collect them because we reckon they sound nicer
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Old 6th December 2004, 02:25 PM   #6
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Hi, Thanks Martin - it's a hard choice but I think I am going to take your advice go for the very minimal change I can - just replacing anything that has really gone off spec and trying to hide the replacement parts if possible. These amps are so nicely laid out and made that it seems a shame to just rip everything out and replace it.

thanks, Jeff
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