KT66 power tubes in Hotrod DeVille

[Micahstephens838]

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!

[Leo_Gnardo]

They sold you some "hot" ones didn't they?

If I'm not mistook, Fender combines the output tube cathodes and supplies their current thru a 1 ohm resistor, handy to measure bias current. 104 mV means 52 mA for each output tube, and that's combined plate and screen grid current, so you're very close to the plate power limit but apparently not beyond it. (You may take a sigh of relief now.) Let's take a look at the schematic, I'm sure you can slug on a parallel resistor in the bias supply, that will put your bias adjust in the ballpark for your KT66's. Can't do this immediately, so patientez, svp. Be back in a while, meantime I'm sure another member of our panel of experts may make a suggestion.

Just curious, how much did you have to shell out for your Genny KT66's?

[Micahstephens838]

Just curious, how much did you have to shell out for your Genny KT66's?

I paid 79.96 for the pair. Didn't seem crazy high but I haven't had my foot in the tube market for long. If I'm that close to being in the ballpark, do you think I should return them and get new ones? Or is it a toss up if they will too be a very hot pair? Thanks so much for taking the time do check this out!

[martin manning]

You want to reduce R76, which should be a 15k. Parallel another resistor across it as Leo suggests, just guessing, but maybe another 15k? Look for about 72mV on the bias test point.

[Leo_Gnardo]

You want to reduce R76, which should be a 15k. Parallel another resistor across it as Leo suggests, just guessing, but maybe another 15k? Look for about 72mV on the bias test point.

Thanks for pitching in Martin, I had to participate in Sunday Dinner. Yes that sounds about right, anything from say 27K to 10K ought to do in case you don't have a 15K resistor in your collection.

Price on those 66's sounds fair, they are premium tubes after all. The hi fi crowd seems very happy with New Sensor's Genalex KT 66, 77, 88 series tubes, to the point some of the more trustworthy commentators claim they're better than the originals that now sell for gold dust money.

[katopan]

Schematic I've got shows R76 as 10K, but yeah reducing or even bridging that resistor.

[Micahstephens838]

Thanks for pitching in Martin, I had to participate in Sunday Dinner. Yes that sounds about right, anything from say 27K to 10K ought to do in case you don't have a 15K resistor in your collection.

Price on those 66's sounds fair, they are premium tubes after all. The hi fi crowd seems very happy with New Sensor's Genalex KT 66, 77, 88 series tubes, to the point some of the more trustworthy commentators claim they're better than the originals that now sell for gold dust money.

You want to reduce R76, which should be a 15k. Parallel another resistor across it as Leo suggests, just guessing, but maybe another 15k? Look for about 72mV on the bias test point.

Schematic I've got shows R76 as 10K, but yeah reducing or even bridging that resistor.

Okay great so whether its a 10k resistor or 15k(which ill find out when I open it up after my last set of the day) reducing/paralleling/bridging by 27k-10k should do the trick. Seems simple enough! another question for you guys though just to cover all my bases...what is the difference between reducing, paralleling, and bridging resistors? and which is a better option? would just replacing it with say a 30k work?
thanks so much for all your guys' input. Posted this on 5 other forums and not a single person has responded. Not to mention the amp tech that offered to do it for $100

[martin manning]

The fix is reducing the effective value of R76 so you can get a lower (more negative) voltage out of the bias supply. You could just replace it with a resistor of say half the current value, but that would probably require lifting the circuit board. An easier way to accomplish the same thing is to put a second resistor of the same value in parallel with the existing one, which will reduce the effective value by half. This can be done from the top side of the board by slipping the leads of the second resistor under the exposed leads of the existing one and soldering it in place.

[Micahstephens838]

The fix is reducing the effective value of R76 so you can get a lower (more negative) voltage out of the bias supply. You could just replace it with a resistor of say half the current value, but that would probably require lifting the circuit board. An easier way to accomplish the same thing is to put a second resistor of the same value in parallel with the existing one, which will reduce the effective value by half. This can be done from the top side of the board by slipping the leads of the second resistor under the exposed leads of the existing one and soldering it in place.

Okay got it. In my head I was thinking if I needed to lower the voltage, then I would need to increase the resistance(Replace it with a higher value resistor) but I guess I have it backwards

[Leo_Gnardo]

Not to mention the amp tech that offered to do it for $100

He heard you have those Cadillac tubes, up goes the price, what a gouger. To. be. avoided.

Hell I'd do it for free just to hear some Genny 66's in a modern Fender.

[Micahstephens838]

Not to mention the amp tech that offered to do it for $100

He heard you have those Cadillac tubes, up goes the price, what a gouger. To. be. avoided.

Hell I'd do it for free just to hear some Genny 66's in a modern Fender.

Yup! Haha I asked him if I could just pay him for his time to show me how to do it and he said it's not something he does. Probably because he would have had to own up to how simple the job was.
Hey...my hotrod is as vintage as a hotrod gets! 1996 the tubes do sound epic though.

[Leo_Gnardo]

The fix is reducing the effective value of R76 so you can get a lower (more negative) voltage out of the bias supply.

Woops we wuz wrong:

Just had a HRD on the workbench and we have to re-address this solution. There's not much voltage to be gained by reducing the upper branch resistor in this bias voltage divider, only a volt or so - it would be much more effective to increase the value of the resistor between the adjustment pot and ground, that was 100K in the unit I worked on. Try 120K there, see if that doesn't put your bias in the ballpark for KT66.

[Micahstephens838]

The fix is reducing the effective value of R76 so you can get a lower (more negative) voltage out of the bias supply.

Woops we wuz wrong:

Just had a HRD on the workbench and we have to re-address this solution. There's not much voltage to be gained by reducing the upper branch resistor in this bias voltage divider, only a volt or so - it would be much more effective to increase the value of the resistor between the adjustment pot and ground, that was 100K in the unit I worked on. Try 120K there, see if that doesn't put your bias in the ballpark for KT66.

Hmm..I changed R76 from 10k to 4.7k and it put my bias at 70% with my bias pot at about 10:00. Should I put it back to 10k and change what you said?

[martin manning]

I see no reason to change it. I don't believe any of the resistors are running hot, and the current in that branch should be less than 2mA.

[Leo_Gnardo]

Should I put it back to 10k and change what you said?

No panic, if the bias current is in a happy zone, leave things be.

When I poked into the HRD's bias supply, the one on my workbench, I found only about 1.5V across that resistor that's why I spoke up.

And a good review on the KT66's?