6K:8 25W SE OPT, now what tube should I use?

[dnewma04]

I picked up a quad of SE 6k:8 open frame OPTs recently and have 1 project in mind and need some ideas for a second. The first will be a mono block variation of the RH807 using a 6AB4 driver and 5B/255M in place of a single 12AT7 and 807.

What I am looking for is ideas for something with a little more power to better utilize the 25w capabilities of the second pair of OPTs. Any suggestions on a tube to use that might give me 15-20w? I don't have any aversion to running in triode, pentode or UL mode, I just want to play around.

Thanks for any input.

[ja2dhc]

One of my friends made an 801A triple paralleled SET. The driver is cathode follower of 5998 to swing plus domain of the grid of 801A, resulting the output power is about 15W. He said Good Sound.

Another idea is triode connected 813. I think Pete Millett's schematic is beautiful.

http://www.pmillett.com/813_se_triode_amps.htm

'73 de JA2DHC

[Francois G]

How about a low voltage GM-70 - around 600 V on the plate.

[dnewma04]

Those are both intruiging options, I'll do some searching on both to see what I can find. Thanks for the tips.

[tubelab.com]

I'll vote for a KT88 running near maximum voltage in UL mode. The 6K ohm load will reduce the output power compared with the usual load, but the distortion and damping factor will improve.

I have a SimpleSE board running with 6K ohm transformers and B+ around 500 volts. The output power is about 7 watts in triode and 13 watts in UL mode. You could probably get 15+ watts in pentode, but I haven't tried it.

The power ratings on output transformers are often inflated. It all depends on what kind of low frequency response you want. A 2 or 3 pound (weight) transformer can do over 50 watts at 1 KHz, but only a watt or so at 20 Hz. The typical size for many SE OPT's is around 7 pounds. These can do 10 or 15 watts at 40 Hz but about 5 watts at 20 Hz. The big Edcors or Hammonds are in the 10 to 12 pound range and are rated at 25 watts. Some saturation related distortion can be seen at 15 watts at 20 Hz, so even these published ratings are optimistic. I recently purchased some Chinese SE OPT's that are rated for "50 watts" they only weigh 6 pounds. Saturation related distortion is seen with my 8 watt amp at frequencies below 40 Hz.

If your transformers are less than 10 pounds each, the 25 watt rating may be optimistic. Whether this is an issue or not depends on your speakers and your musical tastes. I have 6 pound transformers in the amp mentioned above. It is used with 7 inch speakers that have very little response below 65Hz so the low frequency distortion is never heard. The combination works well. My other system uses 15 inch woofers in an open baffle configuration. These speakers have response down into the mid 20Hz range. I need the biggest OPT that I can find here, so it is the Hammond 1628SEA (11 pounds, 5K ohm). I can shake my neighbors walls with a 14 watt KT88 amp with this system.

[dnewma04]

Tubelab, I have been looking at your SimpleSE board for the last week, thinking it would be a good idea to try it. KT88/6550 would be interesting. This would be an Edcor in the 5 lb range, I believe. Maybe I'm trying to shoot for too much power. Maybe the 5w or so I get out of the 5B/255M is about right and I should be looking for a lower power SET amp, instead.

I'll have to sleep on it and decide what direction to take.

[Steven-H]

Dave,
As I have told you; I have one of Tubelab's SimpleSE boards - I assure you you will not be disappointed. Go with the KT88 route - that way you could sub in any of the tubes within that family for different rolling.
The board is very inexpensive; and Im sure that you can mange to do a sub $200 build.