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 Post subject: Power tubes
PostPosted: Mon Dec 27, 2021 9:02 pm 

Joined: Tue Sep 21, 2021 5:45 am
Posts: 2
Finished my build in October and have been using this amp daily. My poor V4B feels neglected. Been trying different power tubes. Tempted to go the 6550 / KT88 route, but the point of this build was to have a lower wattage amp to my V4B. Certainly I'll do it at some point, just because!

I've been trying different 6L6GC variants. Started with NOS Sylvanias but became put-off with those given the difficulty finding a good matched set and the sometimes crazy price. Had more than my share of crap tubes that were sold as "tested NOS"... In the end, I have two truly decent sets of NOS Sylvanias on hand. Spent an uncomfortable amount of money getting there though. Honestly not sure it's worth it. My experience with new reissue tubes has been excellent, they also sound great and you can get a well matched set from a reputable tube vendor. No need to play tube roulette. I've tried JJ 6L6GC, Tung-Sol 6L6GC STR, Tung-Sol 7581A (boosted 6L6GC) and just recently a set of Tung-Sol KT66. The Tung-Sol KT66 is said to be a rebottled 6L6GC STR, and it may be, but there is bit of difference in tone. The TS 6L6GC STR is just a bit dark, maybe good for a bass amp, but the TS KT66 seems to have a bright high end, not harsh just brighter. It is a great tube and I think it's going to be the one I use going forward. Bonus, it's a KT66 but it only draws 900mA at the filaments like the 6L6GC and looks bad-ass with its fat coke bottle envelope hiding under the tube cage (sad metric, I know).

The JJs are great. Nothing to complain about. Good value and are widely used by OEM. Could be completely happy with them if there were no better choices readily available.

All the Tung-Sols variants I've tried I like better than the JJs. Something about them. I've gone back and forth between any of the Tung-Sols on hand and the JJs, each time I put a TS tube back in it's an "ahhh" moment. Current favorite being the KT66.

Preamp tubes: Got a solid set (with a few spares) of NOS Sylvanias and they will stay put. Tried NOS GE and RCA too. They all sound great. Haven't tried reissue 6SL7GTs.

All in all, this was a fun build and I am sad to see it looks to be discontinued. All the messing around with tubes is really just tweaking that last 10%. The amp sound tremendous no matter the tube compliment. Hoping there is another bass amp kit offered at some point. I'd love to see a low watt SVT style kit. SVT preamp circuit / tone stack driving two 6550s would be most cool...

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 Post subject: Re: Power tubes
PostPosted: Sat Jan 29, 2022 4:30 pm 

Joined: Tue Sep 21, 2021 5:45 am
Posts: 2
After starting a new HiFi amp project, basically rescuing a Dyancor ST70 from the trash heap, I have had to learn a bit more regarding impendence matching OPTs. This new knowledge raises a question about running KT66 or KT88 / 6550 tubes and speaker impedance matching.

The trip top OPT is 5500 ohms at the primary, and I haven't actually tested the transformer, but I assume since the amp is based on the 6L6GC that the 4, 8 and 16 output taps are a correct as labeled for the corresponding speaker. The 6L6GC in AB push pull has a plate resistance 5600 Ohms at the typical 450V plate voltage. Should be perfect as is.

The KT66, KT88 and 6550 has something more like 3500 ohms plate resistance under the same conditions. Again, I have not done the measurements / mapped out the values for the OPT in question. For those running alternate tubes with the Trip Top, are you using the 4, 8 and 16 ohm taps as labeled for a given speaker impendence, or are you doing something different??? Say using the 8 ohm tap for a 4 ohm speaker, or 16 ohm tap for an 8 ohm speaker?

Am I over thinking this???

By the way, nothing stays the same! I have been running Tung Sol 6550s, biased at about 55mA for a few weeks and they are wonderful, but now I am concerned I should be using the 4 or 16 ohm tap for my 8 ohm speaker.


EDIT... I used the data for the OPT in the builders guide to determine impedance ratios (ZR). With 5500 ohm primary and the 4, 8 and 16 ohm secondary taps, I come up with ZR of 344 for the 16 ohm, 688 for the 8 ohm and 1375 for the 4 ohm taps. If using alternate tubes, I've learned you can find the new secondary impedance with some simple math by using the plate resistance (from the tube data sheet) of the tubes in use. With 3500 ohm plate resistance of the KT88 / 6550, the secondary taps come out to 2.5 ohm, 5.2 ohm and 10 ohm if my math and understanding of this process is correct.

Hmmm, which tap to use and how critical is this? Kind of want to try the tap labeled 16 ohms with the 6550s as it's closer to my 8 ohm speaker load, but not sure...

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