Blackface Champ

[dpierce05]

Hey everyone,

I'm getting ready to start my next project. I just have a few questions regarding the Blackface Champ.
1: Should I use a cap can and what value? or should I use 3 power supply caps instead. The schematic calls for 3, 20uf 450V caps. Could I use 22uf 500V? Sorry Im still new to amp building and just want to be sure.

2: Could someone explain to me how to wire in a 2nd 6V6 for more power. I think this is called dual single ended or putting a 2nd 6V6 in parallel. A Schematic or layout of how to do this would be great. Thanks for all your help.

[scole]

i used the jj 40,20,20,20. use the 40 for the first cap.

[Bruce / Mission Amps]

Hey everyone,

I'm getting ready to start my next project. I just have a few questions regarding the Blackface Champ.
1: Should I use a cap can and what value? or should I use 3 power supply caps instead. The schematic calls for 3, 20uf 450V caps. Could I use 22uf 500V? Sorry Im still new to amp building and just want to be sure.

2: Could someone explain to me how to wire in a 2nd 6V6 for more power. I think this is called dual single ended or putting a 2nd 6V6 in parallel. A Schematic or layout of how to do this would be great. Thanks for all your help.

1.:
Yes, absolutely you can use three discrete 22uF@500v caps for this project and it will be much cheaper too.
I would consider using a 33uF@500v to 47uF@500v cap for the first one though... especially with two class A power tubes running.
There will be no difference in the way the amp "works" as long all three caps have their negative leads grounded to the same place as the multicap would have been... which was right at the chassis where the multicap was installed.
Actually you don't need to ground those three discrete caps quite like that but at the least, be sure you connect the high voltage center tap secondary from your power transformer to the exact same spot as the first filter cap's negative lead is grounded.
This greatly helps eliminate a potential ground loop which can create annoying hum later.
Find a layout for a vintage 5F2A Princeton and use the power supply section as your starting point but ground your PT's Hi-V center tap as mentioned above, not randomly to a PT bolt or to the chassis just anywhere.
Modify the tone-volume section after the first triode of the 12AX7 to include the tone stack of the BF Champ to complete this project.
The BF tone stack has much more signal loss then the tweed tone control so pay attention to the way the BF Champ uses cathode bypass caps for extra stage gain and the way the negative feedback is connect to the second triode cathode for some stability and distortion reduction.
These items will aid you in tweaking the amp for a more modern and slightly aggressive tone if desired.

2,:
To run parallel power tubes in class A single ended designs you just wire the two sockets (lug numbers) in parallel... 3 and 3, 4 and 4, 5 and 5, 8 and 8, etc, etc.
You will have to use half the value of the cathode biasing resistor if you use two parallel 6V6s and it will need to be at least a 5 watt resistor... possibly a small 10 watter would be safer.
With classic B+ voltages built into this amp of yours, instead of a single power tube using a 470 ohm to 510 ohm biasing resistor on it's cathode, you would use half that value at twice the power rating with two tubes.
Another point you will need to address is the resistr between the first two filter caps.
You'll need to reduce it's value a bit too since it will be supplying current to the screens of two power tubes now instead of one.
I'd adjust that 10K value as I went along but I start with one about 25%-30% smaller and at 2 watts instead of 1 watt.
Also, you would use an output transformer with twice the power handling capability and half the primary impedance.
As an example, let's say your stock single ended 6V6 amp used an OT rated at 5 watts and 5,000 to 6,000 ohms when connected to a 4 or 8 ohm speaker, with two parallel 6V6s you would need a 10 watt OT at 3,000 ohms.
Now if you used a large enough OT, say a WeberVST SE 25W OT or equiv., it will work OK with a single 6V6, two 6V6s or a single 6L6, KT66 or EL35.... now you could switch between speaker taps to get the lower impedance to match all those tubes assuming your power supply is up to the much larger current load those tube combinations can demand.
How that works is by remembering that the primary of an OT is more like a reflection of the secondary load and this is called an "impedance ratio".
The impedance ratio is a function of a few things, most of which the turns ratio, ... that would be the ratio of the actual number of primary wire (power tube side) turns around the transformer's lamination center finger with respect to it's secondary wires (the speaker load side).
Pretty loose statement but I think you have the general idea.

Here is an example of how to use that relationship to your advantage:
The given would be the OT you choose has an impedance ratio of 625:1 or 750:1.
I'll use the 750:1 example.
That means a single 1 ohm load on the secondary winding will create a 750 ohm zed (impedance) on the primary windings, which is the part of the OT the power tube'ss plate is connected to.
So if you connected the 750:1 zed to an 8 ohm speaker load, the new primary zed would be 6000 ohms.
Now, if you have a decent OT with 4, 8 and 16 ohm taps, you just slide the 8 ohm speaker load over to the 16 ohm tap (which would essentially cut the primary zed in half) to get 3000 ohms with two power tubes.
If you would have slid the 8 ohm load to the 4 ohm secondary tap. the primary impedance would have doubled to 12000 ohms!
I can't think of any amp scenario with multiple or single power tubes where that combination would be attractive but running the 8 ohm speaker on the 16 ohm tap opens up a lot of useful power tube combinations.

I think you'll find a good OT with a rating of around 25 watts should be OK for your build.
Check out the specs of a Hammond 125ESE or the WeberVST 25W SE OT.

[dpierce05]

Bruce, thank you so much for the wealth of information, it will help a lot and I've learned something new. I just have a couple more questions.

1: I think I'm going to follow the Blackface Champ Schematic as much as possible, I will add a 2nd 6V6 in parallel. The cathode resistor in this schematic calls for a 470ohm 1w should it be replaced with a 250ohm 5w since I will be using the 2nd 6V6?

2: The 1st resistor between the first 2 power supply caps in this schematic is a 1K 1W should I use a 1K 2W or should I lower the value even more. Thanks for your help once again.

[Bruce / Mission Amps]

Bruce, thank you so much for the wealth of information, it will help a lot and I've learned something new. I just have a couple more questions.

1: I think I'm going to follow the Blackface Champ Schematic as much as possible, I will add a 2nd 6V6 in parallel. The cathode resistor in this schematic calls for a 470ohm 1w should it be replaced with a 250ohm 5w since I will be using the 2nd 6V6?

Yes somewhere around 250 ohm to 300 ohms at 5 watts should be OK.

2: The 1st resistor between the first 2 power supply caps in this schematic is a 1K 1W should I use a 1K 2W or should I lower the value even more. Thanks for your help once again.

Well, 1K at 2 watts should be OK.
You might find that with say a 500 to 750 ohm resistor, the voltage drop across the output transformer, under load, might have your 6V6's plate and screen voltages very close.
In that case I'd still use the 1K@2 watt and install a 470@1W screen resistor on each 6V6 socket... like seen in black face 6L6 amps.

[TD_Madden]

here's a link you might find useful:

www.angela.com/catalog/how-to/Super.SE.6V6.html Look at the bottom of the page.....single-ended, with 2 6V6 in parallel.

or look at the old Gibsonette schematic (attached) for a very simple version....I've got one (white tolex) and it is a real screamer.

[Paul P]

In the Gibsonette, what is the purpose of the 470 ohm resistor to the
grid of output tube #2 ? I notice that in Fender amps with 2 x 2 tubes
in parallel they also have a 1500 resistor to the grid of the second
tube of a pair. Would there be anything wrong with using these resistors
on both tubes ?

Paul P

[TD_Madden]

don't want to sound dumb, but which Fender amps are you referring to with the paralleled 6V6s?

[Bruce / Mission Amps]

don't want to sound dumb, but which Fender amps are you referring to with the paralleled 6V6s?

I think he just meant two 6V6 as in your 5E3, which has a 1k5 grid stopper resistor on each tube socket...
The single 470 grid stopper on the one tube is probably what Gibson found that was needed to insure there would be no ultrasonic oscillations or RFI developed on the power tubes in parallel.
By the way, I have worked on a few of these old Gibson gronks and although interesting, (home use harp players really seem to like them), if I remember correctly, they only make about 6 - 7 watts, cleanish, as viewed by my O scope and driving a pure resistive dummy, speaker load.

[TD_Madden]

probably about right.....of course, I don't play them "cleanish"... ;-)

[Paul P]

don't want to sound dumb, but which Fender amps are you referring to with the paralleled 6V6s?

I was referring to the quad 6l6 amps like the Twin etc. Only one tube of a
pair has a 1500 grid stopper resistor. I was wondering why the other one
of the pair didn't have one but Bruce has cleared that up.

Edit : actually it was the Fenders with 5881's had only one resistor but I just
saw that later ones with 6l6's have a resistor for each tube.

Paul P

[TD_Madden]

thanks for clearing that up...I was locked onto the 2x6V6 idea.