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Old 17th September 2011, 11:11 PM   #11
Cobra2 is offline Cobra2  Norway
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Old 20th February 2012, 02:46 AM   #12
rmyauck is offline rmyauck  Canada
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I'm helping someone who bought a good used pair as his first tube amps for a nice price with recently new boards and can caps which look like the stock circuit and are probably dynakit replacement boards.

The previous owner recommended he replace the output tubes as they were failing so he purchased reissue TS 6550's. One immediately red plated a bit and did the same when tried in different positions. He said the tube seller will take it back and a new one is on the way.

He put one of the best of the old KT-88's in it's place and the amp works biased at 1.40.
I told him I would have bought KT-88's as they can take a bit more.

I recommended that he bias it really low and turn it up till it sounds good. He found it's not bad at 1.36. Sounded best at the original factory setting he thought (1.56) Maybe as tubes break in they will get better sounding at a lower setting.

Will these tubes last at a lower setting? I read a comment a few years ago on this form from someone who owned a pair yrs. ago and thought they were hard on output tubes so got rid of them.

I realize it's a dissipation issue related to the high B+ (close to 500V) on these amps and lowering bias helps to a point.

Is he best to sell the 6550's for KT-88's or do as I thought and lower the B+ by using a inexpensive Antec toriod for about $40. They have more Amperage in that price range for better PS headroom too.

I thought he could keep stock PS trans for fil. heaters and Bias V making them run cool and run forever.

An available 350-0-350 250 mA or better high amp one 360-0-360 450 mA ( but out of stock for a while) would give a B+ of 430-450 V depending on line V too. Stock is 400-0-400 @ 200mA.

The big 450 mA 325-0-325 one would work with SS diodes in place of 5AR4.

The 6.8K R to the 6AN8A might have to be changed.

Toriods could be enclosed in boxes or round covers over metal plates and hidden behind amps.

Is the above a bad idea? It would allow him to roll many different tubes and even get longer life out of KT-88's.

After all HK CIt II had a lower B+ 0f about 460-470 I believe. Maybe a bit lower than this which one could end up with will sound good too?

Owner doesn't mind a bit less power if tubes last.

How dependent on B+ is sonics? I think from reading many new production tubes can't take what some of the NOS could take.

I thought of a big choke , but that wouldn't help much and would just raise the DCR of the PS. Maybe a better choke with lower DCR could be used too. Say 10-20 ohms, and it would add 5-7 volts too the B+ if needed.

Thanks very much in advance,

Randy
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Old 20th February 2012, 02:10 PM   #13
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hey-Hey!!!,
The 360-0-360 Antek would work but it has no 5V winding. I'd call Heyboer Trans and ask for a 360-0-360 HV with 6.3 and 5V with capacity based on how thick a lam stack you think can be squeezed in there. You can probably go a half inch thicker. I'd also ask for a 125V primary. capacity will probably come out on the high side of 250 mA...
cheers,
Douglas
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Old 20th February 2012, 08:44 PM   #14
rmyauck is offline rmyauck  Canada
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"I thought he could keep stock PS trans for fil. heaters and Bias V making them run cool and run forever."

I should have said rect. heater too. That should work leaving the stock PS trans using the Antec just for the B+. I would think? I think the antec's are a lot less $ than the new reproduction ones with all the heater & bias taps. The ST-70 clones from Triode Electronics & Dynakit At about $130-135 ea would be perfect with 360-0-360 @300mA and I don't think anyone else would be much less. The Mark III clones are closer to $100 and they would bolt in and if Heyboer could make them to spec maybe he would except that. The only thing as I mentioned is that the Antec's would have to sit in boxes etc. behind amps. This would save drilling new holes in amp base too.

He's just wondering what kind of life would he get out of KT-88's in the stock amp say he set bias @ 1.40 ?

The reissue Tungsol KT-120 was another thought for the stock amp! I bet circuit wouldn't be up to it and so maybe sonics would suffer. PS tans may not be up to them either!

Thanks very much Douglas!

Regards,

Randy

Last edited by rmyauck; 20th February 2012 at 08:59 PM.
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