The Amp Garage

Congrats Phil on getting the Blackvibe up and running. I'm very happy to hear it is being gigged on a regular basis.

The mixing resistor was left in because it affects the phase inverter's overdrive tone. It's there to keep the tone as close to standard AB763 as possible. Thanks for pointing out the missing schematic label for the bias balance pot but it is labeled as a 25KL pot in the layout. I'll update the sche...

I know this has been done before but here's my take on an easy to build, normal channel only, no reverb, no tremolo AB763 blackface amplifier. There is a 2x6L6 and 2x6V6 version along with a 1 watt micro version that uses the AB763 preamp and phase inverter but feeds a single 12AU7 push-pull power t...

Here's one way to add a presence control to a blackface/silverface amp. It uses the 5F6A Bassman presence control and the amp's NFB & PI Tail resistors are adjusted to compensate for the presence circuit's added resistance. https://robrobinette.com/AB763_Modifications.htm#Presence_Control https://ro...

This is fixed bias isn't it? Then the cathode is basically at 0V (well 10's of mV through the 1 ohm resistor). You are right, it is fixed bias so there is no difference between plate voltage and plate-to-cathode voltage. Sorry about that OP. With a fixed bias Class AB amp like yours most people use...

The simplest way to do it is to put the multimeter red probe on the plate pin and the black probe on the cathode probe and the measured voltage is the plate-to-cathode voltage with no math involved. You can measure the cathode voltage direct at the cathode pin to ground or from the tube side of the ...

You should also use the plate voltage - cathode voltage for your "plate voltage" in the dissipation calculations so that will drop your dissipation % too.

I have a tube bias calculator webpage that's pretty easy to use: https://robrobinette.com/Tube_Bias_Calculator.htm

sliberty, I did something similar in this modded version of the Deluxe Micro:

[img1377]https://robrobinette.com/images/Guitar/ ... ematic.png[/img]

martin manning wrote:Rob, what is the source for the quote above?

My 5F6A mod web page.

This is my 5F6A tweed Bassman "Lead" channel mod. It uses the Marshall 1987 first preamp cap and cathode resistor values. With this mod there's a real difference between the standard Normal and Bright channels. [img:400:752]https://robrobinette.com/images/Guitar/Bassman/Bassman_Lead_Channel.png[/img...

Thanks Martin.

Am I the only one getting a warning from Google that there's malware on the site? I have to click on the ominous "visit the unsafe site" link to see any content here.

I just noticed the 6G3 schematic doesn't show a cathode resistor. The standard would be a 250 ohm 5 watt.

Zep, I prefer the 6G3's long tail pair phase inverter for its own overdrive tone and the much larger voltage swing to push the power tubes to full overdrive. You can always use a solid state inverter in the 6G3. Since you're using one input the 6G3's first grid stopper should be 33k (like the Prince...

The black 'ground' wire from the output transformer secondary should be grounded at the speaker jack's sleeve (ground) terminal.

I would ground the variable NFB pot to the preamp bus. That's where the 5F6A Bassman's Presence control is grounded.