questions for KT66 P-P amp

[electrocutioner]

Hello everybody! First post here! Please be kind with me!

I'm planning to build a new guitar amplifier and I'm thinking to go with KT66s. 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 I believe, these tubes need their own biasing to give their best.

Here's 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. Hope it's true!

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 exceed 500V as I'd like to use electrolytics @500V. 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 of the OT before the choke and the screens after the choke with 1K screen resistors and CRC filtering before. Then, I can choose the right resistor for 400V on the screens or experimenting with different voltages at this range.
Or, as discussed in another forum, by using an 0C3 as used in old Leslie 122 amp and have a stabilised voltage 105V lower than the anode voltage and feed the screens. I really like this idea!

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 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 wherever 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!

[Jana]

"Hello everybody! First post here!"

Welcome.

"Please be kind with me!"

I guess that eliminates me from this thread.

Personally I try for about 400 to 430 volts B+.

[electrocutioner]

I guess that eliminates me from this thread.

Oups!

Personally I try for about 400 to 430 volts B+.

Can you be more clear on this? Why not higher?

[ToneMerc]

Hello everybody! First post here! Please be kind with me!

I'm planning to build a new guitar amplifier and I'm thinking to go with KT66s. I need a clean with a lot of headroom sound!

If it was me I would run a pair of KT88's with about 475-490V on the plates. I would run the the KT66 at least 455-470.

I did try KT66 in an amp I built last year, the clean tone was great. The OT trans was 4.4K, around 475V on the plates with 1k screen resistor. I'm about to start a JTM45 clone, my plate voltages should fall in around 462-468, I'm either going to use 1k, 1.5K or 1.8K screen resistors.

TM

[electrocutioner]

If it was me I would run a pair of KT88's with about 475-490V on the plates. I would run the the KT66 at least 455-470.

Maybe you're right but I'd like to try KT66. They're great tubes!
I'm planning to run them at 480V. I don't think it's too high for these tubes.

[Zippy]

Check for threads by RJ Guitars regarding "Rockster" - they use KT66s with a Rocket pre and sound great.

[M Fowler]

I like the 6k6 OT and doing what Zippy said will get you to what your after in my opinion.

The JTM45, Dr Z route 66 and rjguitar's 66 Rockster are pretty much in the same boat.

I could be wrong so set me straight here guys and you to Jana.

[Jana]

I am going to assume (you know how dangerous that can be, lol) that the desire to build a KT66 amp is inspired by the Bluesbreaker amp and sound.

If that is the case then I think an OT with about a 6k6 primary and about 410 to 420 volts is what you want to shoot for. I am drawing a lot of memory here. The Marshall RI of the bluesbreaker has an OT with a primary of 7.4k. At least it did on the one that I had. This is too high, especially since they didn't even put KT66's in the amps. With an impedance this high the amps sound loose and sloppy--unfocused might be a better term. These amps are not a clone of the original--impedances wrong, filter cap values wrong, wrong type of output tubes. The output tubes I can understand since there were no decent/affordable KT66's when the RI came out.

For an OT I would suggest one of these TW transformers that has the 5k2 and 6k6 taps. I forget who has them but somebody on this forum sells them. RJ maybe?

The original post is contradictory:

"I need a clean with a lot of headroom sound!"

But then you say:

"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 it's no problem to lose some power. I'll be ok, even with 30W total power."

Either you want clean with a lot of headroom or you don't.

If you do, you get that with power. Kt88's at 480 volts will get you some clean headroom. Kt66's at 30 watts won't.

I suppose you also need to define the situation that requires this clean headroom. 30 watts in the living room by yourself will be enough for clean headroom. 30 watts in a band with bass, drums, another guitar? will not be clean headroom. Yeah, the amp will certainly hold its own but you will not have the power to get clean and powerful sound that cuts above the rest. It's not going to cut clean like a twin reverb would.

Part of the magic of the original bluesbreaker is that it didn't have tons of clean headroom.

just my .02, which adjusted for inflation isn't worth a rats 401K.

[M Fowler]

Jana, that was more than 2 cents worth but great post.

Yeah clean head room in this situation is not going to happen. How about 60w PT for reserve amperage and use the Pacific or Heyboer 35w 6k6 OT it could get the HT stiffer to get clean stage volume. What the hell do I know.

What ever you choose the KT66 will sound good.

Mark