AB763 voltages

[Diverted]

Hi all,

Finished the Hoffman board AB763 (2x6L6GC) yesterday for the most part and powered up today. What a loud, clean amp, fun and low stress building on this board. So far all systems go with a few questions:

Anyway: B+ at the rectifier is about 477. Plates are at 475 and grids about the same. The tubes are biased at about 33ma for now. Bias voltage at -50. I'm trying to keep it low for the time being.
All those power rail voltages are pretty close, within a volt or two. Super Reverb schematic specifies 465 B+, 460 on the plates. Should I have a bit wider range of voltages on the plates and B+?
Thanks for the help!
http://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf
https://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

Edit: Bias capacitor has since been replaced since I didn't want to series two together.

[DummyLoad]

crank up the idle current to ~45mA that'll drop B+ somewhat. probably sound better at lower vol. as well.


--pete

[PRR]

> Should I have a bit wider range of voltages on the plates and B+?

Why?

The Fender voltages are all approximate.

The difference B+ and Plate depends on the OT losses and the idle current. Ideally zero losses, in practice we aim for very-small loss. Your OT may have less DC Ohms than Fender's sample; as DL says idle current affects the difference, but in any case your OT loss is small and that is fine/good.

[Willabe]

Your work, wiring/lead dress/soldering looks very good. 

You need to use rubber grommets in the chassis holes where the transformer wires come through for safety.   

[Willabe]

You might have already got these but.......

[Diverted]

Thanks for kind words and the suggestion! I thought the through chassis wiring would be fine as is as wires are not stressed/taut and otherwise aren't really touching the chassis heavily. But it's easy enough to put in grommets so I've done that.

The amp is playing great. More out of nervousness than anything (not used to building 6L6-based stuff) I had kept the tubes biased pretty cool but have bumped them up to about 38-40ma and it's sounding terrific. Super loud and what tone!

I also put the tone stacks on a bypass/raw switch and HOLY TOLEDO> I didn't realize the stacks cut so much gain. Incredible jump with the stacks disengaged.


Ted

[Diverted]

Another question's popped up. It seems like the tremolo intensity isn't all that intense. I would like to get a lot more thump to it; with intensity dimed, it's pretty underwhelming though there. Right now the amp is biased at about 60 percent and I can provide voltages. Any suggestions?

Thanks!

Ted

[sluckey]

Tremolo strength for that amp depends on the bias setting. Set the bias hotter and see if the trem gets stronger. If still not what you want try these suggestions. The LED mod will get your trem into overdrive mode.

     https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf

[Diverted]

Thanks Sluckey. I'll bias it a little hotter but probably not much more. That LED sounds cool .. did the trick for that Ampeg mod you suggested a few months ago!
One question: I used Doug Hoffman's schematic for tremolo. It's different from yours in that the tremolo switch is grounded after the 1M resistor out of the cathode instead of yours, off the input grid.
Also in Doug's the 1m grid resistor goes to ground; in your version there's a voltage divider 2.2m after it to ground, with the line to the foot switch at that junction. Will this affect the use of the LED off the cathode?
Also, just curious because I know nothing about LEDs: How does the LED increase intensity? Would I get similar results just taking the V5A cathode cap out?

Thank you for your help!

[sluckey]

That drawing is Hoffman's. Look at the circuit on the bottom, not the one on top with the red circle. If you simply remove the cathode cap the trem will get weaker and probably will drop out completely toward each end of the speed pot. If you've already done the LED mod on another amp and are happy with the results, maybe you should try that first and save a lot of time.

[Diverted]

Just tried it, thanks. LED increased the intensity a bit but not by a ton. I'll go through some of the other swaps you recommended and see how it goes. Thanks again for the help.

Edit: Just swapped out 470K for 100K and .1 to .047. MUCH stronger tremolo. Got it thumping right where I hoped it would be. Also slowed it down by swapping out .01 for .02.
Great advice, thank you very much.

[tubeswell]

Very tidy build. Well done!

[Diverted]

Thanks! These Hoffman boards are great. Such a nice change from using a big line of 10-lug terminal strips in two rows and shoehorning everything in. I'm sold!

[bnwitt]

Nice work Diverted.
Barry

[pompeiisneaks]

You might have already got these but.......

Just reading the thread, and I didn't see if you addressed these.  Make sure these get some solder :D  Otherwise it looks great! It may be working fine now because you have a good physical connection, but over time they may wiggle and come loose.

~Phil