Vibro Champ Voltages

[caesparza]

Sorry totally my mistake. I have another thread about a 64 Princeton Amp and got mixed out with the schematics. You are right.

[caesparza]

Got the parts to replace just to realize that I missed that the ground leg of the 220K resistor was disconnected.

Poor visual inspection from my part, sorry guys.

Now the grid on the power tube is around 0V and the voltages on the rest of the amplifier got closer to the schematic.

I checked again the BIAS values and got this:

Plate Voltage: 370 VDC (went up almost 100V).
Voltage Drop across cathode resistor: -13.58 VDC
Cathode resistor value: 241.1

Making the calculations I get Plate Current of 56.32mA and Dissipation of 20.48Watts (Very hot). Now the tube is not glowing blue, and the sound got cleaner without that abnormal distortion I was experimenting before.

I guess that now I can proceed to set the Bias for this fellow, should I aim to 29mA for the power tube?. What do you think about the values so far?.

Thanks for your kind help.

[The Dude]

I find this a good guide.

http://www.tedweber.com/webervst/tubes1/calcbias.htm

It recommends about 29mA for a 6V6 single ended with 370V plate. FWIW: A tube glowing blue is not an issue. Glowing red or red plating is another issue all together.

Edit: Also, when calculating plate voltage, don't forget to subtract the cathode voltage if you measured to ground.

[caesparza]

Replaced cathode resistor and cap for 1K5W and 47uf50V.

Plate voltage: 418VDC
Cathode resistance: 998 ohms
Voltage Drop: 24.57VDC
Plate Current: 24.61mA.
Dissipation: 10.36W (86.33%).

That look a lot better, I'll play with the amp and see how it sounds.

[The Dude]

Is that 418V plate to ground or to cathode?

[caesparza]

Plate to ground The Dude.

[The Dude]

You'll need to subtract the cathode voltage for a cathode biased amp. The spec is plate to cathode. The tube doesn't know if its cathode is grounded or not.

[caesparza]

You mean reading voltage from cathode (pin 8) to ground and then subtract that amount from Plate Voltage? That would be voltage from Plate to cathode? According to weber page, would that be equivalent to get the voltage from pin 3 and 8?

Sorry, everything is explained on that page.

Thanks a lot for the info!

[caesparza]

I don't get something here.

The cathode voltage would be the same Drop Voltage across the cathode resistor. According to that, the results would be like this:

Plate Voltage: 418
Cathode Voltage: 24.57
Plate Voltage for BIAS calculation: 418-24.57 = 393.43
Cathode Resistor: 998 ohms
Voltage Drop on cathode resistor: 24.57
Plate current: 24.57/998 = 24.62mA
Power Dissipation: 0.02462*393.43 = 9.69Watts. (80%).

I'll need to raise a little bit that dissipation.

Does it make sense?

If I measure the voltage between pin 3 and pin 8 I get 1.18 VDC I don't know exactly what to do with that value. That is supposed to be the plate to cathode voltage and is not similar to the subtraction of the cathode voltage from the plate voltage.

[Leo_Gnardo]

I don't get something here.

The cathode voltage would be the same Drop Voltage across the cathode resistor. According to that, the results would be like this:

Plate Voltage: 418
Cathode Voltage: 24.57
Plate Voltage for BIAS calculation: 418-24.57 = 393.43
Cathode Resistor: 998 ohms
Voltage Drop on cathode resistor: 24.57
Plate current: 24.57/998 = 24.62mA
Power Dissipation: 0.02462*393.43 = 9.69Watts. (80%).

This ^^^ all makes sense, no worries.

I'll need to raise a little bit that dissipation.

You can if you want to, by reducing the value of the cathode resistor. But if the amp sounds good why bother? As Enzo keeps reminding us, these are guitar amps, not precision lab equipment. And as Duke Ellington keeps reminding us "If it sounds good, it IS good."

If I measure the voltage between pin 3 and pin 8 I get 1.18 VDC I don't know exactly what to do with that value. That is supposed to be the plate to cathode voltage and is not similar to the subtraction of the cathode voltage from the plate voltage.

I don't understand how this can be. Something is way off about that measurement.

[caesparza]

Ok, with the parts available I put a 2K2 resistor in parallel with the 1K getting 685 ohms. I lifted the 330pf cap and tested again. Put a Sprage Atom 50/50 instead of the cheap 47/50 cathode cap.

This configuration gives me a Plate current of 31.73mA and dissipation of 12.19Watts (101%) with Plate voltage of 406 and voltage drop of 21.74. Now the reading plate to cathode is consistent (385). I think I prefer the sound of the colder BIAS setting and the 330pf cap in, its cleaner and breaks up almost at 10 which is something I like.

I'll go back to that setup and finish the amp. Thanks a lot guys for the help.

[Leo_Gnardo]

Ok, with the parts available I put a 2K2 resistor in parallel with the 1K getting 685 ohms. I lifted the 330pf cap and tested again. Put a Sprage Atom 50/50 instead of the cheap 47/50 cathode cap.

This configuration gives me a Plate current of 31.73mA and dissipation of 12.19Watts (101%) with Plate voltage of 406 and voltage drop of 21.74. Now the reading plate to cathode is consistent (385). I think I prefer the sound of the colder BIAS setting and the 330pf cap in, its cleaner and breaks up almost at 10 which is something I like.

I'll go back to that setup and finish the amp. Thanks a lot guys for the help.

Good report on your experiment. Cathode bypass cap whether "cheap" or Sprague Atoms, make sure there's some space between the cap and cathode resistor. Cooking the cap with heat from the resistor shortens its life. Factory Champ builds often ignore this factor. I've seen caps partially melted.

[caesparza]

I guess the last factor about the sound of this little amp is the speaker. I'm not very fond of the original Oxford, not a very efficient speaker, last year I had Fender Champ with a Warehouse speaker that was just amazing. I think a swap in the speaker makes a huge difference at this size and wattage.