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Old 20th December 2008, 08:26 AM   #1
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Default Hammond 1650P for KT88 PP - any experience?

Hello all,

I am looking to build a PP amp using the Williamson circuit ( KT88 valves ). The recommended impedance of the Williamson is 6k. I am planning to use the 1650P transf from Hammond which has an impedance of 6.6k. Is this going to be a problem and if yes, which model should I use instead of the 1650P?

If anybody has built PP amps using Hammond transformers, how do they sound and measure?

Later on I will probably substitute them with Tango transformers but for now, due to my budget, i have to use Hammonds. SAC thailand offered a quite high price, the same as the price for a pair of Hashimoto trans so I would pass on his offer. James trans measures poorly and I would prefer not to use any.
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Old 20th December 2008, 11:52 AM   #2
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Although I have not used any of the Hammond 1650 series transformers, I've seen mention that the 1650N performs better than the 1650P. The 1650N has a 4.3K primary, which is a bit lower than you are seeking. I'd suspect the transformer's performance is going to be somewhat dependent on the rest of the amplifier's circuit, so your results may be different.

I used a 1620 on a push/pull EL84 project. It had a tendency to saturate at relatively low power levels on the bass notes. At 40 Hz I could only get a couple watts out, while the amp was good for over 12 watts over the rest of the spectrum. In casual listening, the 1620 sounded just fine. The price certainly was reasonable, so no complaints there. I think the 1620 would probably be better suited for a 6V6 amp instead of a 6BQ5. Finally, I'm not certain that the performance of one particular transformer in the Hammond line is indicative of what one might expect from all of their products.

Good luck. Let us know what you find.
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Old 20th December 2008, 12:26 PM   #3
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Hi

I've used the 1650P with SED6550 in both triode and UL with excellent results.
I do not have much experience with the up-market brands though.
However they measured very good and I had no indications of saturation pushing some 35-40W down to at least 30Hz. It is after all rated for 60W.

To me they sounded very good.

/Olof
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Old 20th December 2008, 02:08 PM   #4
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Not related to the Hammonds, but worth mentioning anyway... the Williamson is not the king of stability with the multiple RC couplings and the rather high amount of NFB used. Therefore the original transformers were quite specially wound...therefore I think that using the Hammonds may cause problems if you use the exact williamson circuit.
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Old 20th December 2008, 05:15 PM   #5
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Thanks for your replies, Erik - I know about the custom wound transformers for the Williamson circuit, the transformers are built by Milleroux and I missed a pair of them, sold on ebay.com, just a few weeks ago. As far as I know, the Williamson circuit has not changed much over the years and is pointed to be the maximum reached as a PP schematics for the KT88.
It is quite unique to find the rare Milleroux trans, I am looking for something which could substitute them in the schematics. What I am afraid of with the Hammonds is the quality of the iron used in them, I have red a few posts on this forum that they use low quality cores.
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