questions for KT66 P-P amp

[spy]

I'm planning to build a new guitar amplifier and I'm thinking to go with KT66s.
I dig the sound of blonde Fenders like a Bandmaster or Showman. So I need a clean with a lot of headroom sound!
I have put some KT66s in Fender clones and the results were more than pleased! But these tubes need their own biasing to give their best.

Does anyone have any experience with KT66? Any advice before ordering the parts?

To start the discussion I'll write down what I have in mind.

First of all, I'm planning to use one pair of Genalex Gold Lion. A good choice from a lot of builders.

I'm planning to use an OT with 6K6 primary impedance. It's something between 4K and 8K. Neither too low or too high. Marshalls with KT66 are using 8K but the results with 4K in Fenders are great, too. So, I'd like to go somewhere in the middle. Any opinions over here?

Also, I'm not sure with the anode voltage. From KT66's data sheet, they don't have any problem with 500V or a little bit higher. But I don't like to go at these levels. What about 480V?
And what do you say for the Screen voltages? I read somewhere that with lower screen voltages like 400V are much more better. Sweet clean sound with much more headroom. I can feed the center tap before the choke and the screens after the choke with 1K screen resistors and CRC filtering. Then, I can choose the right resistor for 400V on the screens or experimenting with different voltages at this range.

But I'm pretty curious for lower HT voltages, too. As watts are not my main goal I don't mind if I'll use a lower HT. Let's say, 400V or even lower and have the advantage to drive the output tubes with higher currents. If KT66s sound better like this it's no problem to lose some power. I'll be ok, even with 30W total power!

I'll use a well filtered fixed bias to drive the tubes hotter or colder and keep the bias I prefer. Maybe with separate pots so I can balance them or unbalance if I like! Two 1 Ohm resistors will be permanently at the cathodes so I can adjust the bias with an external DMM.

Please, any advice will be great! I have almost all the parts except the transformers!

I'm waiting your opinions! Thanks for your time!

[spy]

No one?

[EFK]

I've got one which consistently runs 480-490 on pin 3 and about the same on pin 4. No problems so far with tubes, I'm currently using Valve Art. I use 1K screen resistors and 3.9K grid stoppers. The amp is cathode bias, though. Basically a JTM45 normal channel circuit but I only wanted to use one preamp tube so I drive the tone stack off the plate. No problems there either. I;d be curious to know the differences in OT impedance; mine is about 7.5/8K but I'd be VERY curious to know how a 4K OT would affect tone. ??

[spy]

Seems like KT66s sound better at 480-490V but I'm pretty curious how they sound in lower voltages, like 400V. Anyone?

Have you tried grid stoppers at 1K? Why do you use 3.9K? I was planning to use 1K....

8K is better than 4K, AFAIK, but if you'd like to check the sonic differences you can connect a 4 Ohm speaker at the 8 Ohm tap!

[MWJB]

Marshal 1962 2x12" used a 4K primary with KT66.

[pdf64]

See what info the data sheet has re grid stoppers, screen grid supply (I think they are implying the use of an OC2 to reduce the screen grid supply 100V or so) and power band (p10).
http://www.mif.pg.gda.pl/homepages/f...126/k/KT66.pdf
But I think that they way they were operated in Marshalls was very different to what the tube manufacturer intended, especially re screen supply.
The Hammond 1750Q is intended for JTM45 and has a primary impedance of 7371 ohms (!).

[spy]

The Hammond 1750Q is intended for JTM45 and has a primary impedance of 7371 ohms (!).

Maybe they're following the same ratio with the original...

0C2 is a 75V regulator... 0B2 is better for 105V. But have you ever seen anything like this? If you mean to reduce the screen grid supply 100V from the anode voltage, IMHO it'll be better with a simple resistor.

[pdf64]

'0C2 is a 75V regulator... 0B2 is better for 105V. But have you ever seen anything like this?'
See
http://www.captain-foldback.com/Lesl...matics/122.GIF
OC3 used in Leslie 122 amp

'If you mean to reduce the screen grid supply 100V from the anode voltage, IMHO it'll be better with a simple resistor'
No, as that would introduce significant power compression. The screens will draw more current with signal amplitude. With a saggy supply, that would then reduce the amplification factor of the tube.
The screen supply needs to be stable, not saggy. These days a mosfet regulated supply would be best.

[spy]

Yes you're right! Sorry but I didn't understand it.

0B2 with 0C3 have the same Va. I like this idea!
I'll think about it.... or a Mosfet, why not.

Any advice/opinion on my other questions?

[pdf64]

Steve Conner has been through this loop, and kindly documented and posted it on his site -
Stabilised power supply
Toaster top page

[spy]

Great! Thanks for the links! That's the ultimate PSU.

But I'd like to stay more "traditional". Although, I dig the idea of adding the 0C3. Only one extra socket, stabilized screen voltage and one cool tube on the row!

[Michael Allen]

Lot of guys doing Trainwreck Rocket preamps into a KT66 power section, myself included. It is glorious! I built one for a buddy, I think the plates are at around 390V. Sounds real good. The Rocket is much like a Top Boost channel, head over to ampgarage and search for topics like 66 Rockster.

You will love love your amp!

And I think my screens were maybe 10 or 20V lower than my plates.

[spy]

Thanks for that Michael! I'll check it out!