Bias Circuit Help

[wdelaney72]

I acquired a Peavey Dual Two Twelve they I decided to repurpose into my own build. Chassis and Transformers are getting re-purposed and I've removed all the existing circuit boards.


The Power Transformer has 24vac lines WITH a center tap, which I would like to use to drive a bias circuit. I've seen all kinds of ideas out there, but nothing based on winding with a CT. The HT does have a CT, which I'm locating in-between the Mains Caps, similar to 100W Marshall. The Heaters do NOT have CTY, so i have an artificial CT using Humdinger pot.


I've attempted to draw up a bias circuit, but likely need some guidance here. The little bit a read was to keep the CT on the 24vac source disconnected...I'm attempting to use a bridge rectifier with 1N4007 diodes and a voltage doubler. In the pic....I missed the symbol....the green dotted line is ground.

Any advice here?
https://drive.google.com/file/d/18UKJKIUnYgc0M7KAtW5YTws1aoDsstNo/view?usp=sharing

[Latole]

24 volts with a center line = 12-0-12 volts, not 24-0-24 volts.

Voltages are too low for a bias circuit .
Use one high voltage wire like do Marshall.

I use it for few amps built

[mresistor]

Have you measured each tap to the center tap? 

[wdelaney72]

I understand I can tap off the HT AC line like a Marshall KT66...but I would rather utilize the 24vac tap. I have not physically measured the tap, but I did find this schematic for a similar Peavey and it shows 24vac and it's used for bias and other parts...likely relays.


If I do not ground the center tap, wouldn't it stay 24vac on each peak?

[wdelaney72]

wait...I read that again. CT is grounded and result of the rectifier bridge is 25vdc....I am guessing the tap is 50vac? I guess I really need to measure this thing 


I was unable to find specs on the PT.

[mresistor]

Yes probably best to measure the leads of the supposed low Volt winding   tape off the HV leads first and be ULTRA CAREFUL.

[wdelaney72]

24 volts with a center line = 12-0-12 volts, not 24-0-24 volts.

Voltages are too low for a bias circuit .
Use one high voltage wire like do Marshall.

I use it for few amps built

I'm back to the drawing board here. I originally wired the mains / HT circuit with a full-wave bridge rectifier, similar to JMP100 Marshall Superlead. Power section is fine, but it with the tap on the one HT secondary, the voltage to bias circuit drops to zero. My HT Secondaries are 190vac, so I cannot do this like a JTM 45/100 rectifier....my HT only peaks out at 250vdc that way.


How do I tap the bias off of HT Secondary  when using full-wave bridge rectifier such as Ceriatone reference below?
https://ceriatone.com/wp-content/uploads/2016/10/Ceriatone-69-Plexi-Super-L.jpg

[stratomaster]

Check out the Valve Wizard bias page on how to tap off the HT with a bridge.

If you want to make the low voltage winding work you can use a voltage doubling rectifier.
https://www.electronics-tutorials.ws/blog/voltage-multiplier-circuit.html

In theory it's good to 2*V peak.  With 24vRMS peak should be around 34V.  Knock 1v off for diode drop and you should be able to get~-65v out of it if you don't load it down too much and orient the diodes properly.  You'll need a second filtering stage too.  I'd say dump into a voltage divider with a fairly high total resistance to minimize the drag on the voltage doubler and feed the bias voltage off the wiper. Maybe a 50k pot with an 82k to ground.

[pdf64]

Text descriptions of circuits are prone to misunderstanding, that's why schematics are a thing.

If s FWB rectifier is used for the HT supply and the same HT winding needs to supply the bias, capacitive coupling is required.

Roughly what bias voltage magnitude do you need, ie whats the intended screen grid voltage and output valve type?

[Merlin]

The Power Transformer has 24vac lines WITH a center tap, which I would like to use to drive a bias circuit. I've seen all kinds of ideas out there, but nothing based on winding with a CT.

So just leave the CT disconnected and use it as a single 48Vac winding. With bridge rectifier that will give you over 60Vdc at low load, more than enough for biasing!

[wdelaney72]

The Power Transformer has 24vac lines WITH a center tap, which I would like to use to drive a bias circuit. I've seen all kinds of ideas out there, but nothing based on winding with a CT.

So just leave the CT disconnected and use it as a single 48Vac winding. With bridge rectifier that will give you over 60Vdc at low load, more than enough for biasing!

I like this idea....will this layout work as long as I keep the CT from the 24vac winding disconnected?
https://drive.google.com/file/d/18UKJKIUnYgc0M7KAtW5YTws1aoDsstNo/view?usp=sharing


I am driving 4 6LGGC power tubes for Bias, unloaded the HT is measuring approximately 505vdc, so I would imagine bias range should be -25vdc - 40vdc

[pdf64]

....will this layout work as long as I keep the CT from the 24vac winding disconnected?
https://drive.google.com/file/d/18UKJKIUnYgc0M7KAtW5YTws1aoDsstNo/view?usp=sharing

That layout seems weird, eg why 2 trimmers when there's only one bias supply output?

I am driving 4 6LGGC power tubes for Bias, unloaded the HT is measuring approximately 505vdc, so I would imagine bias range should be -25vdc - 40vdc

Unless that 505V is super saggy, that bias range is inadequate. Assuming the screen grids will be up around 470, the bias supply range needs to be more like -40 to -60V.

[wdelaney72]

....will this layout work as long as I keep the CT from the 24vac winding disconnected?
https://drive.google.com/file/d/18UKJKIUnYgc0M7KAtW5YTws1aoDsstNo/view?usp=sharing

That layout seems weird, eg why 2 trimmers when there's only one bias supply output?

I am driving 4 6LGGC power tubes for Bias, unloaded the HT is measuring approximately 505vdc, so I would imagine bias range should be -25vdc - 40vdc

Unless that 505V is super saggy, that bias range is inadequate. Assuming the screen grids will be up around 470, the bias supply range needs to be more like -40 to -60V.

I was completely guessing on the bais range, so your -40 to -60 range may be more accurate.


Two trimmers allows biasing by pair v. quartet....I'd prefer to buy matched pairs of tubes over matched quartet...splitting hairs, but I just always use this type of setup.

[Merlin]

I like this idea....will this layout work as long as I keep the CT from the 24vac winding disconnected?
https://drive.google.com/file/d/18UKJKIUnYgc0M7KAtW5YTws1aoDsstNo/view?usp=sharing

Qualified yes. You probably want to put one of those caps before the 15k resistor, and one cap after, for extra smoothing. Since they're already 100V rated this should work fine.
Also, your green wire is ground, so it needs to be connected to the rest of the amp ground.
EDIT: Your trimmers need rethinking. Currently both bias outputs are connected together.

I am driving 4 6LGGC power tubes for Bias, unloaded the HT is measuring approximately 505vdc, so I would imagine bias range should be -25vdc - 40vdc

Rule of thumb is to take your screen voltage (or just your B+ voltage) and divide by ten; that is now your target max bias voltage. This will be enough to bias most any power tube pretty much to cut-off. You can tweak the minimum voltage to whatever you like.

[pdf64]

...
Two trimmers allows biasing by pair v. quartet....I'd prefer to buy matched pairs of tubes over matched quartet...splitting hairs, but I just always use this type of setup.

There may be 2 trimmers and 2 output wires but the trimmers both seem to be in parallel, both output wires connect to the same output node.
ie the design seems to be nonsensical.

[Willabe]

Two trimmers allows biasing by pair v. quartet....I'd prefer to buy matched pairs of tubes over matched quartet...

Why?

You can use 2 bias pots 1 for each pair with a matched quartet of power tubes. 

[Willabe]

Members really need to post a schematic drawing of the circuit not a layout drawing! 

[Merlin]

the trimmers both seem to be in parallel, both output wires connect to the same output node.
ie the design seems to be nonsensical.

You're right I missed that. You need to reconfigure your trimmers.