Brook 12a clone

[Briney]

Greetings All,

Well I tried to build a Brook 12a clone using the schematic attached.

I've run into several problems and have run out of ideas. So I turn to this forum where I'm consistently amazed at the depth of knowledge. Help?

I had my transformers built by Heyboer and they did a nice job, however they threw me a curve when they added a center tap to the 2.5v filament supply.(2A3s)

If I ground the center tap I get music at a normal volume but about 150mv of hum. The 700ohm resister R17 get insanely hot and voltages are very low, 165vdc at pin 2 of the 5U4G instead of 340vdc.

If I don't ground the center tap the amp is quiet but so is the music, no volume and very tinny sounding. The voltages go much too high at 444vdc at pin 2 of the 5U4.

The only change I made to the construction was leaving out resistors R20,R21 and capacitor C6. (I did try adding these guys in to the circuit using the 2.5 center tap but it didn't make much difference.)

What am I missing? Is this one of those misguiding schematics like the Hafler-Keroes Ultralinear?

Anyhow I'm at the point where I'll listen to any suggestions short of scrapping the whole project.

Thanks in advance.
Brian

[llwhtt]

Something is not correct on that schematic, the cathodes (filaments) of the output tubes need to go to ground some how, usually a center tapped winding. Also what's funny are the voltage readings for pins1 & 4 of the output tubes. You have half the voltage on pin1 and the other half on pin4, that implies a center tap since all of the measurements in the chart are referenced to ground. The * note has the full voltage measured between pins 1 & 4. What are the voltages on pin3 of the 2A3s. Are all of your voltages OK otherwise?

Craig

[llwhtt]

Just noticed something else, if there is supposed to be -67VDC on pin3 of the 2A3s, C3 has the wrong polarity, the + should be grounded. Still lookin' for more errors.

Craig

[Briney]

with the center tap grounded pin 3 gives me -9.92v.

all the other voltages are lower than the chart.

I'll try changing C3

[llwhtt]

Since the amp works more or less with the grounded C.T. let's assume it needs to be grounded. The voltage/resistance chart implies that anyway. Now why is your bias voltage so low? C3 in backwards? Also with the bias so low the outputs are running very very hot, therefore hot parts and low voltages are the result.

Craig

[Briney]

reversed C3, that brought all voltages within spec. However I'm getting about 150mv of hum and music sounds like it's coming through a 3" speaker.

also have the center tap grounded.

[llwhtt]

What test equipment do you have? O'scope and signal gen. sure would help here.

Craig

[sgerus]

Still need help with this?

The parts you left out (R20,R21,C6) make a voltage divider to bias up the 6.3 VAC heaters.... You need add this or try the 6.3V ct to ground. That should help with the hum problem.

I agree that the polarity of C3 is incorrect on the schematic you posted
(The plus side should go to ground)

Yes, R17 will get hot... 67V/700R= 95ma (the total current the amp draws)
67V * 95ma = 6.4Watts (Hope you used a 25 or 50watt resistor here)
This 700R is how you get the -67V for the 2A3 grid)

I also agree that the 2.5VAC CT should go to ground.

For the 2A3, You should end up with about 330V at the plate, -67V on the grid and 0 V on the cathode.

Please provide an update.... I'm thinking of building one of these.

[Briney]

Greetings Sgerus,

I had set the project aside for a while until this fall. What I did change when I went back was I ran one side of the 6v filaments to ground.(floated?) Just like the schematic!
Amps are working now and sound pretty good. Here's a picture of the one of the completed amps.

[kevinkr]

I have worked on and listened to a number of Brooke 12A, definitely worth cloning IMHO.. I'd go with the best iron I could reasonably afford.. The Heyboer stuff ought to be more than good enough..

Errors as noted in schematics notwithstanding..