SE Class A, 6L6GC to KT66 (Fender Ramparte)

[GERPUD]

Hi everyone,

I'm a newbie and new to this forum. I have a technical question that I think you would have an answer.

I have a Fender Rampate (Single ended Class A, design for 1x6L6GC). For the pure goal of modifying it for fun, I swaped the 6L6GC with a KT66 (Gold Lion)
I read that KT66 can be a direct replacement for 6L6GC, if the the power transformer (PT) could handler the extra current (1.3A instead of 0.9 for 6L6GC).

1- When I put the KT66, everything seemed fine, except for the grid resistor that seems to get a bit hotter (~135degC instead of ~115degC). This resistor is 390Ohm/5W.
2- the KT66 is supposed to draw more power. I measure the power input to the transformer with the 6L6GC and with the KT66. The power consumed by the amp was the same. The temperature of the PT was also the same.

Questions:
1- Should I change the grid resistor value when using KT66 in this amp? Will it damage prematurely the tube with this resistor value?
2- People were saying that KT66 will draw more power from the PT, but it doesn't seem to be the case (same voltage but higher current). Is my transformer limiting the current draw? Is the design is limiting the current draw?
Any other advice or comments will be greatly appreciated.

Thanks!

[matt h]

(deleted)

[GERPUD]

Thank you very much for your quick reply! It is very very appreciated.

To try answer your questions to my questions:

- The amp is Cathode Bias
- The grid resistor is 390RJ/5W, (so 390ohm?)
- See the picture for the voltage reading relative to the ground.

Basically, from my newbie understanding, when switching from 6L6GC to KT66,
pin 4 remains constant at 406V,
pin 3 decrease from 399V to 390V
pin 8 increase from 30V to 34V
You should check the drawing because I'm not sure of the pin numbers.

Thanks!

[matt h]

(deleted)

[GERPUD]

Thanks! Wow!
I'm saying it again, I'm a newbie, so sorry for the error.

I've recheck the power draw from the PT. It is accually 0.06A more for the KT66. That make 7.2W more power. So this question is answered by myself...

It is also possible that it is a little more power for the KT66 because the voltage was fluctuating and I tried to make an average reading.
Also, no multimeter is perfect...

So I found the "real" screen grid resistor: yellow, purple, Brown and gold.
So I guess it is 470ohm +/-5%.

What should I change to safely run the KT66?
What impact will it cause to the sound? (Level, break up point, etc)

Thank you for your time and knowledge!

[matt h]

(deleted)

[Stevem]

What is the voltage rating of that bypass cap from pin 8 to ground, if with 34 volts across it now I would change it to a 50 volt cap if it is not such already!

[GERPUD]

Stevem: The cap from pin 8 is rated 63V, so no problem there

matt h:
Voltage drop across the 470ohm screen grid resistance:
1.2V with the KT66 (2.55mA)
2.0V with the 6L6GC (4.26mA)
Voltage drop across the 390ohm cathode resistance:
33.7V with the KT66 (86.4mA)
30V with the 6L6GC (76.9mA)

That means that the current is less through the screen grid of the KT66?
By increasing the cathode resistance as you seems to suggest, will it change the current through the screen grid?
In the spec sheet of the KT66 (single valve, class a, penthode), I see Ia and Iga, which are 85mA and 6.3mA. Are they refering to Cathode and Screen current? If yes, that would mean that I could draw much more on the screen grid and I take a little too much on the cathode. The current screen is safe enough. Is it too safe? What do you think?

The spec sheet of the KT66 tube:
http://www.thetubestore.com/lib/thetube ... x-kt66.pdf

Thanks for your valuable information!

[matt h]

(deleted)

[Stevem]

The circuit traces on those fender boards to take to resoldering more than about 2 times, as such you would do your self a favor to get 6 terminal bolt down strip and wire it in where the stock resistor was and then mount your new resistor to the strip, be it a screw down terminal type or a solder type!!

[GERPUD]

I'll do that!
Thank you very much!

[GERPUD]

Could you give me a link of the exact kind of terminal you suggest.
I have no idea which one the choose among the 100,000,000 choices!
Is ceramic resistor 5W exist in the variable model? It yes, would it be a good idea?
Would it be feasible to have a variable resistor for the grid resistance?

[Structo]

Is it a Fender Ramparte amp?

Like this?


[img:375:375]http://content.andertons.co.uk/2/1/imag ... _super.jpg[/img]

[GERPUD]

yes it is!