KT66 power tubes in Hotrod DeVille

[tubeswell]

Looking at the schematic, the input voltage for the bias supply is about -74V.

http://www.blueguitar.org/new/schem/fen ... eville.pdf

The stock voltage divider is 10k in series with a 25k pot wired as variable resistor in series with a 100k tail resistor. The output is the pot wiper. Range of the output voltage is between 74% to 93% of the (-74V) input voltage. (about -55V to -69V). To get a range that gets closer to -74V you could just bypass the 10k altogether with a jumper wire.

But if -74V isn't negative enough, build a 1/2-wave voltage doubler for the bias supply, and then adjust the voltage divider for a lower (negative) output by paralleling the 100k tail resistor with whatever desired parallel resistance you need to get you into the ballpark bias voltage range.

[martin manning]

Pete, your numbers are off because you are ignoring the 51k resistor in parallel with the 25k pot & 100k. The minimum bias voltage (original circuit) is about -58V. Reducing the 10k to 4k7 should lower it ~6V, which is enough and working fine per the OP's note above. Also, look closer... it is already using a half-wave doubler circuit ;^).

[Micahstephens838]

And a good review on the KT66's?

I love the KT66s but I "had" to leave on vacation in the middle of my tone searching. Had a bit of volume drop with the mod some I'm just tweeking stuff around

My brain just exploded [/quote]

[renshen1957]

Hey people question about biasing KT66 tubes in my HRD 4x10.
I got a new set of Genelax Gold Lion Kt66 tubes to replace the 6L6s(was advised this would be okay by the place I bought them) the problem is with my bias adjustment pot all the way down, I'm getting a reading of 104mv at my bias test point. with a plate voltage of 470v-480v on the hotrod and the tube rating at 25w, I'm positive that these are running too hot and i cannot adjust it any lower. BUT I played through it regardless for about 30 seconds and I loved the tone of the tubes. Is there a way to Mod the bias circuit in order to accommodate the new tubes, or maybe I'm missing something. Thanks all!

Hi

How much 6V3 heater current does your Hot Rod Deville actually has? Real KT66 require as much current as an EL34, circa 1.2 amps (Rusky reissues data sheet 1.27 amps) vs 6L6 at 0.9 Amps.

Fender isn't generally known for having reserve heater capacity above the original tube configurations. (That would cost money and fender is a cheapskate penny pincher).

1.27+1.27+.9=3.44 amps

You may be okay as:

Fender Original Hot Rod DeVille Power Transformer.

Equivalent to #041752.

Specifications
Primary 117 volt, 60 Hz
Secondary 450 VDC, 270 mA after rectification
Filament Winding 6.3 VAC, 3.75 A
Mounting Centers 2" x 2.5" horizontal
Weight 7.4 lbs

https://www.tubesandmore.com/products/P-TF41752

However, to be on the safe side, have someone who knows how to test heater current check the actually current. Fender has a habit of substituting transformers or changing specifications without notice.

As a precaution might want a small fan to circulate air as Fender PCB copper traces are barely adequate size for the job and you will have an additional 5 watts of heat with nowhere to go but upwards into the chassis. Although 5 watts does sound like much, try holder a light 5 watt nightlight bulb in your hand.

Don't aim the fan's air flow directly at the tubes, just near by.

http://www.midlandhardware.com/844066.h ... bZvpPl53-A

Could be positioned outside the amp with no additional modifications to amp.

Another two pennies worth.

Best regards,

Steve