Poor man´s VVR, will it work like this?

jodie · Post by **jodie** » Sat Mar 14, 2009 10:10 am

Hi there,

unfortunately Dana´s nice VVR only works for amps up to 50 watt.

I want to have voltage regulation (scaling the whole amp) for a showman amp ( 80 watts? It´s a AA 763, no more a collector´s item, because the board had many modifications that I reversed to original).
My plan is to use a chain of 10 volt 5 watt zeners attached to a 5 position rotary switch, each position having 4 zeners in series for a voltage drop of 40 Volt. That would be a maximum drop of 200 Volt, which I think will be enough for me, as I do not need bedroom levels.
What about heat dissipation, will the zeners desolder themselves? Are there other issues that I forgot?
I will use the other part of the rotary switch to adjust the bias accordingly, still I do not know, how to calculate the resistor values. I assume the idling current has to stay the same, so I will have to lower the negative voltage when I lower b+?

Any help or opinions appreciated

Regards

Jodie

Structo · Post by **Structo** » Sat Mar 14, 2009 3:54 pm

I don't know that much about power scaling but as I understand it, you have to scale the bias as well as the voltage to the power tubes.
Then what happens if the preamp is not scaled as well?
Also what will the power handling be for the zener's?

Things to consider.

txbluesboy · Post by **txbluesboy** » Sat Mar 14, 2009 5:59 pm

Actually idle current drops too, as b+ is decreased. Bias should decrease (increase? Become a smaller negative value) at the same proportion as the B+. So if full voltage is 400 volts and your tubes bias properly at -35 volts, when you decrease b+ to 300 volts you will want to see about -26 volts bias. at 200 volts B+ - 17.5 volts bias and so on.

Structo · Post by **Structo** » Sat Mar 14, 2009 6:23 pm

Are you sure about that.

I thought I had read that it does not track linearly.
I saw something about that at the power scaling forum.

Something about using a clamping circuit to make it track properly.

gldtp99 · Post by **gldtp99** » Sun Mar 15, 2009 9:21 am

If the Showman is too powerful, then why not just use a lower powered amp ?....................gldtp99

jodie · Post by **jodie** » Sun Mar 15, 2009 12:04 pm

Hi gldtp99,

people keep telling me, 4 power tubes do sound better then two, I want to find out...
and I have the amp anyway.

Jodie

UR12 · Post by **UR12** » Sun Mar 15, 2009 1:08 pm

A Showman or most 80 - 100watt amps that I know of all use Fixed bias and you will need to make the bias track along with the B+. It won't do it by itself unless you go with cathode bias. The comment regarding linearity of the tracking is interesting. The bias does need to track linearly (proportionaly) with the B+. If you drop the B+ by 10% you need to drop the bias by 10%. I would be interested in hearing comments to the contrary.

austinb · Post by **austinb** » Mon Mar 16, 2009 1:43 pm

You could us a 2 pole waffer switch and use the second pole to select bias resistors or pots. The wafer switch contacts will have to be able to handle the power also. Another solution would be to use 2 paralleled power fets for VVR circuit.

jodie · Post by **jodie** » Mon Mar 16, 2009 5:59 pm

Thanks for all the answers,

now there are only two questions left: Will the zeners ( 10 volt 5 watt axial type ) get very hot so that they maybe desolder themselves, and will I there be switching noise issues?

Jodie

Structo · Post by **Structo** » Mon Mar 16, 2009 6:48 pm

Yes, you must use ones that have the correct power handling capabilities or they will self destruct.
Not sure about getting so hot that it would melt the solder.
The one I used was a 50 watt zener that is a stud mount.
It uses the chassis as a heat sink.

It is a good idea to bypass each zener with a ceramic disc cap of the proper voltage rating around .1uf across each zener to help with the noise. If there is still high freq noise you may have to go with a .01uf cap.

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