Single Channel, No Reverb, No tremolo AB763 Amp Design

[robrob]

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 tube.

https://robrobinette.com/RR763_BLACKVIBE.htm
The webpage explains my design decisions and fully documents the amps.

[xtian]

Awesome, Rob!

You left in a 220K mixing resistor? Isn't that unnecessary?

Also might want to add 1R bias sense resistors to the power tube cathodes. Simplest way to check bias.

Finally, you did not spec the bias Balance pot.

Cheers!

[robrob]

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 schematic.

If a builder wants 1 ohm bias resistors they can add them.

[M Fowler]

Two more different flavors:

AB763 small chassis-Layout resize.GIF

dumble_mod_princeton_showman_otpwr_V3 modified MFowler.gif

[Roe]

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 schematic.

If a builder wants 1 ohm bias resistors they can add them.

For a faster response and tad more high-end and gain, try rather 47k grid resistors on both sides of the phase inverter. Personally, I prefer a ss rectifier and at least a 47uf 500v mains filter cap and 20-33uf at the screens. ALso, el34s are worth trying as is a 100+ka mid pot

[Stevem]

If the amp is for 6L6 usage I would up the bias cap to 200 UF.
I just did such when working on a guys Superreverb that he plays cranked and wants more punch!

The mod took the bias voltage from -54 volts up to -72 and added near 2 more volts RMS to the amps output since that voltage sags a bunch slower now.

[Roe]

big bias caps are risky since they charge slowly, compared to small 8uf caps

[Stevem]

On a amp with a standby switch by the time the tube filiments are up to proper temp a big bias filter is fully ready to rock!

You can incorporate the on/ off and the standby on one switch if you like , but it's far more reliable to use a single pole dubble throw switch for the standby and wire up both sides of it for longer contact life.

If the amp is going to be used to play near cranked out then I like the far less boomy usage of .022 caps out of the PI section which also delays nasty crossover distortion!

[sirbergs]

I have been thinking about a single channel with reverb but no trem. Anyone know where a schematic and or layout of that could be found?


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[sluckey]

Here's mine. Schematic and layout are at the bottom of the page...

http://sluckeyamps.com/tdr/tdr.htm

[sirbergs]

Here's mine. Schematic and layout are at the bottom of the page...

http://sluckeyamps.com/tdr/tdr.htm

Thanks. Working in the yard but I’ll check that out later.


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[tubeswell]

Here's mine

The James TS gives classic fender mid scoop when all pot rotations are at mid-point. (Bass and treble controls have wide range adjustment with these tone stack values).

Sans the 'channel mixer' voltage divider - for really loud volumes.

With a Heyboer 40W PT driving 2 x 12" 100W celestions for really clean guitar with heaps of bandwidth. Great for surfing, caintry, rockabilly, rock'n'roll, blues.