4K Primary 18 Watt Output Transformer

[gktamps]

Howdy,
Several years ago I bought a custom-made Heyboer P-P OT that has 4K and 8K primary taps, and 4, 8 and 16 ohm secondaries from a fellow here on TAG.
I had intended originally to use the 8k primary for EL84s, but was wondering if there are some lower dissipation tubes that would be options for the 4k tap?
Thanks for your help,
Greg

[pompeiisneaks]

I think two 6L6's are in that 4k range, the OT that Doug Hoffman sells for the Fendery 6L6 amps is a 4k primary

[gktamps]

Thank you. Yes, but then we're looking at higher dissipation tubes, so I'd have to run those at low voltage and/or bias cold, correct?
I'm curious to know if there are lower dissipation tubes with ~4k impedance suitable for an 18 watt OT.
Thanks again,
Greg

[pompeiisneaks]

Well, if I understand it right, you don't necessarily need to bias the tubes differently, you can also adjust the plate voltage. That adjusts the output wattage as well, with the same type of biasing. Example:

http://www.radiostation.ru/tubes/6L6GC-GE.pdf

If you look at the section on Push Pull AB1, it shows two voltages at 360 V, biased at 11mA for the setup its at 3800 ohms, but at 450v at 5600 and 22 mA, but then over on the left 360V at 15 mA is 6600. So there are various ways to adjust and get what you're looking for I think.

~Phil

[brewdude]

I wonder if 18w at 4K primary would be a good choice for cathode biased 6L6's?

[gktamps]

Thank you, Phil and Brewdude.

[Stevem]

4 6AQ5 tubes would be fun, but they use 7 pin sockets.

[Ten Over]

I guess I don't see why you would want a lower dissipation tube when the load decreases. The slope of the load lines increase and the maximum dissipation curve moves down and to the left on the Average Plate Characteristics graphs with decreased load and decreased tube dissipation causing an undesirable intersection point.

There are some very good reasons why you don't see 18W transformers with 4K primary impedances. These will probably not become apparent until you start drawing lines and curves on graphs.

I considered the possibility that the 4K taps were actually meant for ultra-linear screen taps, but they would be at 70.7% instead of the more common 43%, so it isn't likely.

[Phil_S]

I suspect you'd do well with a pair of 6BM8's on the 4K primary. Maybe a quad of EL95's, too. I'm having some trouble imagining what was intended for the 4K.
The limiting factor I'm thinking about is 18W. A quad of 6AQ5's, for example, is possibly headed into the range of 22-25W, depending on exactly what you build.

[strelok]

A pair of 6k6's will give you about 8 watts with a b+ of 300v. Though they like to see a bit higher impedance, around 12k. Why not a pair of 6v6's? Cathode Biased and a B+ around 320-340 will give you around 12 watts of clean power.

[gktamps]

Thank you all for your input. It seems I am not alone with being puzzled about how to best use the 4K taps. I will just stick with the 8k taps and go with EL84 or 6V6 tubes as I've done in the past.
Thanks again.

[rp]

Matchless, supposedly, runs their 18W 2X EL84 amps, e.g. Lightning, Spitfire, etc. at 4k. Good amps, right? My Lightning clone has a TDS Matchless copy OT at 4k. I tried mis-matching to get 8k, horses for courses, both sounded good,I leave it at 4k, sounds a bit meaner and brighter. Their 4X EL84 amps they run 4K BTW.