AB763/BLJr mash-up

[lonote]

Just finishing up an amp for a friend with a AB763 TMB pre, an AB763 2-tube reverb (+dwell), coupled to an LTPI EL84 PP output w/a LARMAR MV. He wasn't interested in having trem, which I likely would have added if it were mine.

Feeding the EL84s was initially fussier than I expected, but there was a lot of useful information in this linked TAG thread https://ampgarage.com/forum/viewtopic.php?f=6&t=37166 that I implemented in the final design, so I thought I would share the final project, as opposed to adding it to the existing thread.

Bringing down the signal going into the PI was the key & I used the marked-up drawing for the pre-PI voltage divider that Martin Manning had provided in the linked thread, as well as some of Sluckey's ideas for the bias circuit & heater elevation (gathered at TAG & Hoffman), as well as a few VOX component values around the output tubes.

So far it has passed the smoke test, sounds nice, owner was happy after a test run on the bench, but it has yet to be gigged.


I just finished mapping out the circuit(s) & drawing the "AS-BUILT" schematic, hopefully with no errors.



Edit: corrected drawing

[cdemike]

Really cool build! What kind of voltages are you getting at node "D"? I'm underway with a build with a similar power supply, and I was considering direct-coupling my phase inverter since the 100K and 47k resistors form a divider for B+ in addition to the signal.

[lonote]

I didn't actually check the wall voltage when I measured, but it tends to run right at 120V where I am.


Nodes were reading:

A- 369V
B- 350V
C- 323V
D- 304V

That put all the D-node tubes about 203V-206V at the plates (w/100K plate resistors)

I am pretty happy with SS rectification & have been using that for the last few builds.

This was a first for me using for elevated heaters. Not a complicated thing, plus I really like that the voltage divider across the B+ lines drains down the filter caps as soon as it is powered off. A nice bonus.

[stratomaster]

Do you have a coupling cap after the 3rd gain stage? As it is right now there's basically 147k to ground from that node.

Also I'd recommend going the other way with the reverb driver cathode resistor since it's got a MV. Try 1.5k to start.

[lonote]

Do you have a coupling cap after the 3rd gain stage? As it is right now there's basically 147k to ground from that node.

Also I'd recommend going the other way with the reverb driver cathode resistor since it's got a MV. Try 1.5k to start.

There is indeed a coupling cap between those resistors & the plate, thanks for pointing that out. I expected a drawing error was in there somewhere. Revised the attached drawing above.

Thoughts behind the driver cathode value?

[stratomaster]

Essentially even the typical 2.2k biased the tube cold enough in that application to run out of headroom in the AB763 style amps. The fact that the amp itself gets dirty around the same point as when this happens masks that issue. If you add a master volume control however the issue becomes more prominent. The AB763 also runs that tube 80-100v higher than you currently are, so you've reduced voltage AND biased the tube colder. There is likely not much headroom in your driver at all. Your saving grace, however, is the Dwell control which is likely never full up.

With your reduced voltage, a cathode resistor of 1.5k is a decent place to be to at least retain some headroom under higher Dwell and preamp volumes with low master volume settings. You may find 1.2k is even better. As always, take voltage measurements, calculate current and make sure to avoid overdissipation. But I think you'll be fine down to about 820ohms at your voltages.

[sluckey]

Some of the later Fender amps even had a 470Ω cathode resistor.

[Roe]

Some of the later Fender amps even had a 470Ω cathode resistor.

some also used 680R

[lonote]

Thanks all for the insights. I have the amp here, as I am still working on the cabinet.

I will revisit that section & try out some alternate values.