Cathode biased kt66

[JoshBernstein]

After finishing my 1/2 watt build in one afternoon, im itching to build another amp, and im interested in using a pair of kt66's for the outputs. Im not so great with charts, so im wondering what cathode resistor value would be a good starting place. Im mostly familiar with el84's so im not sure what b+ voltage would be good. What kind of wattage should i expect? Finally, can i use a gz34 rectifier with the kt66's? I seem to like ss rectifiers, but i want to give the tube another shot. Thanks!

[xtian]

I think I was in your mind set about five years ago. I thought the shape of the bottle held a lot of interest, and I wanted to build "something" just to get to use that bottle. Blue JJ KT88! (Actually, that made a rippin' AX84 SEL.)

My point is, the bottle shape has MUCH less to do with the sound of the finished amp than does the circuit as a whole.

So my advice is, find a guitar AMP that you KNOW you would love to own, or are curious about because you can't get your hands on one, and build it using your choices of power tubes. JTM45 with KT66? Hells, yeah!

[Smokebreak]

Let's look at the data sheets , and I have a question as well hopefully someone can answer.

Short answer, looks like 400V plates and separate 500r cathode resistors( or 250r shared) is a good start.

On the bottom half of pg2, we have 2 charts for Push Pull AB1 Pentode. I'll assume that's how you want to run it. The one with Va(anode voltage) at 250V looks to be the minimum value chart, perhaps even the designed value?
The next one that starts with 450V seems to be the max suggested Va. In that chart we have 415V for the Va and 300V Vg2(screen), which no one seems to respect
If you look down to Rk, that's your cathode resistor, and it's listed as 500r. I saw another data sheet annoted that these MUST be separate. Not sure why, they certainly don't have to be. I'd make it 10W to be safe, 5W if separated. Bypass it with a big cap if you like.

My question concerns the Va (b) and Vg2 (b) @ 450V . What is that?

[stephenl]

400v, 8k primary OT, 250r shared resistor with no cathode bypass cap sounds really nice

[RWood]

Finally, can i use a gz34 rectifier with the kt66's? I seem to like ss rectifiers, but i want to give the tube another shot. Thanks!

JTM45 = Pair of KT66s fed by GZ34
http://www.classictubeamps.com/schemati ... layout.pdf

[JoshBernstein]

Thank you for the help.
How much output should i expect? Any specific ot recommendations?

[Littlewyan]

If you're going cathode bias 425V on plates, 390V on Screens, 8K Impedance with 500Ohm Cathode Resistor you're looking at 30W output.

JTM45 is usually about 30W as well, maybe bit higher.

What kind of amp do you want though? As I have a JTM50 which uses EL34s and kicks out 50W clean but roughly 90W Distorted and it is LOUD. I have to attenuate about -12dB and even then its a tad too much.

[JoshBernstein]

Right now im thinking about the ef86 side of a matchless dc30 for the preamp. I love that preamp and i thought it be an interesting one to use for experimentation

[M Fowler]

Or build a Dr Z DB-4 using 5879, 12AX7, four EL84's and 5AR4.

[tubeswell]

A Dr Z Route 666 uses an EF86 in V1, and LTP and a pair of KT66 (UL, fixed bias with 440 on the plates, grids @ -50V) with a GZ34 rectifier. The PT uses a 370-0-370 HT winding. There is no NFB.

Going by the GEC KT66 data sheet, a cathode-biased non-UL version could be designed running ~450 on the plates and ~300 on the screens with a 2 separate 500R cathode resistors running into an 8kPP OT. If you used a paraphrase inverter (instead on an LTP), you could get quite a funky sounding blues amp. Note that each KT66 draws 1.3A filament current.

(Edit - Oops, I see Smokebreak already looked at the data sheet).

[martin manning]

My question concerns the Va (b) and Vg2 (b) @ 450V . What is that?

No idea.

[Firestorm]

My question concerns the Va (b) and Vg2 (b) @ 450V . What is that?

No idea.

You see that in a lot of British sheets, sometimes opposite Vb(o) (zero signal). It might be supply voltage, but that doesn't make much sense.

[Malcolm Irving]

I read something in the DIY Audio forum regarding the (b), on the KT77 datasheet. Nobody was sure, but some thought it was 'b' for battery, but why that would be different I don't know.

[pdf64]

I've also asked about this on MEF and not got a response.
I suspect it may indicate an absolute max value, which never be exceeded, eg when the mains is at the top of its allowable range.

[JoshBernstein]

If i can expect about 30 watts out of this thing, would a 6.6k tw express output transformer be acceptable?