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Input transformer negative bias schematic

martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
Am I likely to experience noise or other problems if I bias a preamp input as shown? I have some voltage to drop in the mains transformer so I’m thinking of generating the bias voltage with a resistor from the CT to ground.

Thanks,
Marty
 

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6A3sUMMER

Member
2016-06-07 6:50 am
You will have to have a lot of capacitance across the resistor that connects from the center tap to ground.
A cap input B+ circuit has very high transient currents, they will appear as voltage transients across the resistor. Just a cap across the resistor will probably not be enough.
A choke input B+ will have smoother current.
In both cases, you likely will have to have at least a 2 pole filter.
Parallel RC from CT to ground.
A second resistor from CT to a second capacitor to ground, take the bias off the second cap.
 

6A3sUMMER

Member
2016-06-07 6:50 am
For your circuit in post # 3:

You will notice that there is no steady bias voltage until the output tubes draw current.
Well, there is some transient current (and transient bias voltage) when the 2nd and 3rd B+ capacitors are charging, but that is only transient, until the output tube draws current.

I would be very careful of the order of the timing of the rise of the voltages, B+ and Bias.

Do you see that?
 

martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
You will notice that there is no steady bias voltage until the output tubes draw current.

...You might get away with a zener instead of that R in the CT, as the zener voltage is more independent of the current through it.
Jan

Thank you both for the excellent advice. It’s a stereo preamp so only one tube per channel with output transformer. I didn’t show a second voltage divider on the bias voltage for the other channel. Thinking about what you both wrote, a zener would be better just in case one of the wipers open or a tube is missing etc. Ill be sure to check the time on both the bias and B+
 

6A3sUMMER

Member
2016-06-07 6:50 am
Same circuit, same problem, the timing of the B+ and - Bias voltages.
Partly depends on the load.
Partly depends on the time constants of the B+ and - bias circuits.

You get initial surge current through R111 as diodes in B+ circuit charges C101. Then, you have to wait until there is a full load on B+, before the full -bias is developed from the steady B+ current through R111.

B+ load, is it DHT or slow warming power pentode/beam tetrode?
 

martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
The resistor option did not look that great when I started checking time constants. (Thanks for the tip) Here is a more fleshed out version of zener bias from the center tap (thanks for that tip too) that I think will avoid the problem of the bias lagging the B+ at start up. Any obvious problems with this?
 

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There are no vacuum diodes in the Electronic Workbench app so I've stuffed in SS diodes with 100R in series with each to sim the 6X5GT rectifier. A couple of simplifications in the simulation, one 15V Zener rather than three of 5.1V. And one 15K load to replace the two bias columns.:)
 

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martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
I forced myself to figure out how to model it in LT spice. My transformer model is pretty crude but I get the voltages I expected out of the circuit. Running the first filter cap as I show it gets less noise on the bias supply (0.91mV rms) but more complex. I don't have time to figure out the frequencies right now but it looks like line & 4x line or so
 

martyh

Member
Paid Member
2004-02-01 2:58 am
Wisconsin
So this was not the best idea I ever had. The attached is the simulated output of the bias supply as shown in my last drawing. The shorter period is line x2 and the longer is line/3. Plugging it into the circuit gets 5mV RMS of PS noise on the output of the pre-amp where a pure DC source for bias produces uV levels.
 

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PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
> not the best idea

I wondered.

The Zener current is all spikes so the Zener voltage will be all jaggy. 0.05u is not near enough filtering for this low-Z source. But your bias is a fairly hi-Z load. Add a stage of R-C filtering on the bias. Meanwhile take your main B+ filter cap from "common ground", not 3 Vz away.

Some values will need adjustment but this will give you something to try.
 

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waltube

Member
2005-02-19 1:46 pm
roma
Simply question.
But if you put a little transformer with 0-18 volt at 30-50 mA and the use of a 7915 (just an example) then two trimmer and two resistors you will have a perfect bias regulation.
Of course you need some space but the dimension of the trafo is little.

Walter