Bias Circuit needed for 6L6 pp amp using toroidal PT w/ no CT

[shaun]

Heyo,
I've built a pp 6L6GC amp using a toroidal PT that has no secondary center tap and uses full wave bridge diode rectification. As far as I can tell from my research to date, the lack of the toroidal CT may change how typical -52v grid biasing would work as it does, say, in a typical Fender 50w amplifier. Actually, I tried installing bias circuits as diagrammed by Mr. Luckey and others that mimic Marshall bias circuits, running off one secondary tap, but toroidals get weird compared to the EL PTs that I'm more familiar with. In using the toroidal, I will have 200VAC on each secondary at full power, and around 400v B+ at the first node - probably end up with about 440-460v on the plates.

I'm unable to get the bias circuit to work correctly - I've been using my variac to dial in about 70vac going in to the amp, rather than the usual 120 from the wall, and it's clear from plate currents at that lower voltage that the bias circuit is not working as intended - the tubes ain't happy, and plate current wants to run away with the spoon. There is little or no bias current on the grids. I conclude that I'm going about this ass-backwards due to lack of understanding.

This is the first time I've tried building a grid biased amp. In fact, I've bitten off more than I can chew with this one, but a local guitarist asked me to make him one with everything, and like the hot dog vendor, I couldn't resist. Schematic attached. It shows cathode biasing, but I'm hoping for more headroom by using grid biasing - the cathode bias got a bit mushy.

I need help in figuring out a biasing circuit that'll give me around -52v when using a toroidal PT. Thanks for any guidance - always much appreciated.

[shaun]

Here's the schematic...

[AlNewman]

I think you need to do something like this.

[sluckey]

That bias circuit won't work with a bridge rectifier. You must use a cap coupled bias circuit as shown on pages 9 and 10 of my Amp Scrapbook. I don't see any reason that this circuit won't work with a toroid.

CAUTION!!! Take the 6L6s out of the amp and don't put them back in until you have proper negative bias voltage on pin 5 of all 6L6 sockets.

Another option is to use a separate 12vac transformer to produce ≈60vac to feed a standard bias supply. Just connect the 12v secondary to your 6.3v filament string. This will produce 60vac on the primary leads. Here's a Hammond 166F12D available from hawkusa.com. Only cost $10.30.

     https://www.hammfg.com/files/parts/pdf/166F12D.pdf?v=1777575579

[passaloutre]

Sluckey’s circuit should get you there, but here’s some more reading at the bottom of this page: https://valvewizard.co.uk/bias.html

[shaun]

Thank you Mr. Luckey! Great info. I'll try the cap coupled circuit first. It's also good to know that 12v trans trick to get 60v bias current - very smart idea, and I'll try that if I have no joy with the first option. Your amp scrapbook rocks.

AlNewman - I tried that circuit 6 ways to Sunday before realizing there is something about the toroid PT that disallows it - I haven't figured out the details yet, but it concerns the full wave bridge and positive voltage only. But thank you.

Now to go read up on what the Wizard says...not that I often understand his language :).

[sluckey]

AlNewman - I tried that circuit 6 ways to Sunday before realizing there is something about the toroid PT that disallows it - I haven't figured out the details yet, but it concerns the full wave bridge and positive voltage only. But thank you.

That circuit absolutely will not work with ***ANY*** PT using a full wave bridge. Toroid is not a factor. If you study and understand page 10 of my scrapbook you will see that the crazy AC voltage waveform applied to the bias circuit is always positive. Therefore the bias diode blocks the entire waveform and produces no negative voltage. You must have the cap and resistor to shift the zero volt reference baseline of that waveform such that it swings positive and negative. (Look at the scope pics, paying attention to the zero volt baseline) Now the bias diode will block the positive part of the waveform and pass the negative portion so it can charge up the bias filter cap(s).

[shaun]

Aha - thank you for those details, Steve. I get what you're saying now.

I installed your cap-coupled Bias circuit and it works beautifully. After looking at the V Wiz, I also added a 50KL pot for adjustment, plus the second reversed filter cap etc that he suggests. Things dialed in immediately.

At 120VAC in to the amp, I'm at 40mA plate current with around 490v on the plates, and that's with a ceramic dropping resistor between the diode rec and the first node. I've played with various R values, and they get pretty hot, which I don't like. I'd like to drop about 40v off the B+ first node, and I believe the choices are either to use dropping res, zeners, or a bucking transformer. Unfortunately, I'm kinda shoe-horned into the chassis and don't have a lot of room to move (this chassis fits a particular cab my guitarist friend wants to use.) I've got some 12v 5w zeners, so three of those might do the job, but I've read that they get also hot and could use some sort of heat sink.

Anyhoo, I'm open to suggestions on that topic too. I'm including some pics of my workbench just for grins. You'll see I've got a 5v supply in there too, for an eventual 3-way footswitch.

Thanks again for the help.

[shaun]

Rather than spin my wheels on the voltage dropping issue, think I'm gonna try aluminum housed 10w resistors and see how that goes.

Cheers all.

[sluckey]

Rather than spin my wheels on the voltage dropping issue, think I'm gonna try aluminum housed 10w resistors and see how that goes.

I suggest 25W and bolt them to the chassis.