Biasing KT77

oliver95 · 1 Biasing KT77 on Wed May 20, 2009 3:03 pm

oliver95

Posts: 1
Join date: 2009-05-20

Hi

I live in France and i bought a Dynaco ST70 from Bob a few months ago.I'm very happy with.
As i read i can swap the power tubes, i'd like trying the JJ KT77.
However, which biasing voltage must i choose ?
Is it 0.4V or 0.5V ?
Thank you
Olivier

Bob Latino · 2 Re: Biasing KT77 on Wed May 20, 2009 6:41 pm

Bob Latino

Admin

Posts: 226
Join date: 2008-11-27
Location: Massachusetts

oliver95 wrote:Hi

I live in France and i bought a Dynaco ST70 from Bob a few months ago.I'm very happy with.
As i read i can swap the power tubes, i'd like trying the JJ KT77.
However, which biasing voltage must i choose ?
Is it 0.4V or 0.5V ?
Thank you
Olivier

Hi Olivier,

A KT77 can take a little more bias voltage than an EL34. The recommended bias voltage for an EL34 in a VTA ST-70 is .400 volts DC per tube. For a KT77 you can go up to .450 volts DC per tube.

A couple of things about the JJ KT77's. One - is that they are great sounding tubes and reasonably priced. Two - JJ KT77's sometimes have issues. A couple of years ago they had problems with pins that were smaller than normal. I had a quad with two tubes that had smaller pins but the vendor replaced them immediately. Supposedly JJ corrected the small pin issue but there is still the chance that some tube dealer may have a quad on his shelf with one or more tubes having small pins. The JJ KT77's are definitely a good buy at about $80 a quad.

A nice set of tubes for any ST-70 are the Genalex Gold Lion KT66's. I use these now in my own VTA ST-70. These tubes are (supposedly) exact copies of the original Genalex tubes and are extremely well built with thicker glass than most other tubes. The only downside is that they are expensive at $150 a quad. You can also bias these up a little higher than EL34's at the same .450 volt DC level as KT77's. Genalex also now makes KT77's but they cost almost the same as the Genalex KT88's > $190-$200 a quad and IMHO are not worth it in an ST-70. Understand that no matter what output tube you use the amount of power the amp puts out will be virtually the same. You will, however, notice subtle changes in the way the amp sounds as you change output tubes.

Bob

Wieslaw Lipowski · 3 bias or idle current on Thu May 21, 2009 9:19 am

Wieslaw Lipowski

Posts: 3
Join date: 2009-05-21

Hello,

I don't think one can mix the terms "bias" with "idle current".

Bias is normally about minus 30-60 volts measured between the grid and cathode of the power tube.

The .400V mentioned above IMHO is the cathode voltage, which with 10R shows 40mA of cathode idle current.

I think a correct advice should say: "Adjust the bias so that you obtain .400V at 10R".

Regards

Brinkman · 4 Re: Biasing KT77 on Wed May 27, 2009 12:17 am

Brinkman

Posts: 10
Join date: 2009-01-01

I'd even advocate a cathode voltage even higher -- 500mV. I didn't like the JJ KT77s until someone insisted I move the NFB tap from the 16-ohm to the 8-ohm taps (without any other changes to the FB network). This helped the voltage measurements at the cathode stabilize. Beyond that, increasing the cathode voltage to 500mV (50mA) really fleshed out the tubes sound immensely.

Changing FB taps as I did had no noticeable performance-related side effects. Whether it's kosher or not is for the designer to say.

Best.

1 Biasing KT77 on Wed May 20, 2009 3:03 pm

oliver95

2 Re: Biasing KT77 on Wed May 20, 2009 6:41 pm

Bob Latino

3 bias or idle current on Thu May 21, 2009 9:19 am

Wieslaw Lipowski

4 Re: Biasing KT77 on Wed May 27, 2009 12:17 am

Brinkman