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Old 6th January 2011, 02:31 PM   #1
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Default H. H. Scott 299C output transformers


I'm new to DIY audio. I've decided before I try to build something from scratch I'm going to attempt to rebuild an H. H. Scott 299C amplifier. The amp was in good condition when I found it except for one problem. Someone had scavenged/removed one of the output transformers. Chances of finding transformers for this amp seems to be slim. I've searched ebay for the last couple months with no luck. Has anyone done a similiar swap or have any other suggestions on what output transformers would be a good replacement for this amp?
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Old 6th January 2011, 03:52 PM   #2
kevinkr is offline kevinkr  United States
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Best bet would be to look for a parts donor unit in rougher shape. Before you go any further though you should make sure that the power transformer is OK. It is pretty unusual IMLE for someone to scavenge an OPT off of a properly working or restorable Scott amplifier.
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Old 7th January 2011, 12:51 AM   #3
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Kevin is correct about making sure the power trafo is operating properly. Assuming the worst and the power trafo is dead, all is not lost. Magnetics available in the marketplace will allow the amp to become functional again.

A look at the Scott 299C schematic shows the O/P tubes to be 7591s. It is widely known that 7591s work well into O/P trafos with a 6.6 KOhm primary. A pair of Edcor model CXPP60-MS-6.6K ($70.30 each) are electrically suitable. What has to be determined is how good a mechanical fit the Edcor "iron" is. BTW, the Edcor stuff allows experimentation with ultralinear (UL) mode "finals", should you so choose.

A reasonable candidate for the power trafo job is the Dynaclone PA-060-S intended for for use in ST70s. IIRC, the PA-060-S is a good mechanical fit for Scott units, but that needs verification. Comparatively minor changes to the OEM PSU are needed in adapting the DynaClone part to service in a 299C. One of the 6.3 VAC windings takes care of all AC heater requirements. The 2nd 6.3 VAC winding is Greinacher voltage doubled (Schottky diodes please) and 7812 3 terminal regulated to provide DC heater power. The bias supply for the 7591s is obtained by 1/2 wave rectifying the tap provided for that purpose on the B+ winding.

If the O/P trafo currently in the unit is functional, sell it on Flea Pay.
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Old 7th January 2011, 03:08 AM   #4
rmyauck is offline rmyauck  Canada
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Joe,

Another option is to get another wound by one of the high end transformer winders in the U.S. They may be able to make an exact copy as they may have the info. Might have to get a matching pair, but they will be noticeably better than low cost ones.

Your Scott would have had really good output transformers. Keep your old one to compare to for specs and sound quality.

Another option is to buy another or LK-72 (the kit version) and keep yours for parts. Anyway don't be quick too get rid of the old output transformer, as because soon as you do another will show up on a good parts unit.

I agree it will be hard to find that individual transformer alone, but I think you have a much better chance of getting one in a good parts unit and at a good price too if not working(warranty on all transformers being good).

Last edited by rmyauck; 7th January 2011 at 03:19 AM.
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Old 7th January 2011, 03:46 AM   #5
rmyauck is offline rmyauck  Canada
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Joe,

Another thought, besides the usual sales sites, look on the other audio forms and advertise your need if you haven't. Four I found are audioasylum, audiokarma, diytube, and audiogon. I think there is a Scott form out there too. There maybe others I missed and they all have sell & swap sections including this one. You never know who might have a good parts unit for a fair price.

I think I would keep that old output transformer as a spare if it was mine as they do get taken out by tubes , caps, accidents like a broken or disconnected speaker wire etc. more than you think, and as you know they aren't too common.

I think you will find a good parts unit!

Last edited by rmyauck; 7th January 2011 at 03:57 AM.
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Old 7th January 2011, 04:39 AM   #6
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Thanks for the valuable information guys. The first thing I will do is check the power transformer. Its good to know there are more options available than I originally thought. I went to the Edcor site and there transformers seem to be of very high quality. I think I may go that route if I grow impatient trying to find a donor unit and the dimension are the same or close. I'm going to add the LK-72 to the list of possible donors. I also read on the H.H. Scott site that the 299D had the same output transformers as well. I will post which direction I went and how it turned out.
Thanks again,
Joe
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Old 7th January 2011, 06:23 AM   #7
rmyauck is offline rmyauck  Canada
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Joe,

What's the part #'s stamped on your transformers as they used different ones on the 299C when it was manufactured. There may be other's that use the same ones.

You could also use a pair output transformers from 233 , 340, 340B, 380, 345. (the last 3 have same part number in this group so circuit determined power rating) as they all used the 7591 output tubes. These models all had 33, 30, 35, 35, 32 max power ratings respectively instead of 40W on 299C, 299D, and LK-72, LK-72B.

Edcor's won't have the extra interleaving layers which give the greater high and low frequency bandwidth. This of course adds to the cost. You will hear more things too you would otherwise miss in the music. Specs can be written to look good. Edcors are no doubt a great bang for the buck!

Bud @ Onetrics (doesn't have website) , Jack @ Electraprint , Dave Slagle @ Intact Audio, and Mike @Magnaquest I'm sure could help you. Maybe you could get just one from them for not much more than a price of a pair of Edcor's.

Last edited by rmyauck; 7th January 2011 at 06:48 AM.
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Old 7th January 2011, 03:14 PM   #8
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The remaining transformer is labeled TRA-11-2. I don't want to comprimise the sound quality of this amp. If the high and low frequency bandwidth on the Edcor transformer is less than the original I'm going to wait and find a donor or parts amp. Thanks for the info on the other sites. I'm going to email them and see what they can do. Thanks
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Old 28th January 2011, 03:16 PM   #9
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rmyauck,
So basically if I substituted output transformers from either a Scott 233, 340, 340b, 345, 380 the watts out of each channel would then be the same as the model I used the transformers from? Would the sound quality be changed as well?
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Old 28th January 2011, 04:31 PM   #10
pjanda1 is offline pjanda1  United States
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You might call this guy if you are interested in rewinding. I don't know much about him, but it looks like he specializes in that sort of thing and is nearby for you.

Paul
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