Red plating Vox AC

[Robert M. Martinelli]

Hi,

there are several schematics on the internet, try searching on google with "vox ac30 schematics" as keywords.....

having a solid state rectifier, and no dropper resistor, your ac30 is a post '70 model, if you can post some pictures of the details ( control panel, upper ventilation grids, model / S-N plate on the back, speakers etc ) I could help you in dating the amplifier and stating the version to match it with the right schematic.

Having said that, if you' re using the "shunt method" ( i thought you were using the 1- ohm resistor method ), i agree that the tubes are biased hot, so I also agree on the use of a dropper resistor on the +B, in the early 70s the dropper resistor was 47 Ohms, and the rectifier sported two silicon diodes, in 1986 the dropper was 22 Ohms and there was a full bridge rectifier ( 4 diodes ). I have also seen AC30s from the 60s modified with a couple of diodes in place of the original GZ34, but the countermeasure was simple then, as they had a voltage selector which went up to 245 V, so I advised the owner to keep the selector on the 245V primary tap ( at 220 V ) to bring voltages down some.

Oh, I forgot to say....If you still don' t find the schematics, drop me a message, I have some....

Hope this helps

Regards

Bob

[Valvehead]

the 1986 scem almost matches . except mine does not have a standby switch and has only 1 fuse.

Ill try that 22ohm and see what happens-

edit - the B+ is now 363V after the 22 ohm dropping res.

[Robert M. Martinelli]

Ill try that 22ohm and see what happens-

edit - the B+ is now 363V after the 22 ohm dropping res.

Hi Valvehead,
I' m a little puzzled by the voltages you' re getting.....you stated before you had 330 VDC with the tubes in and no dropping resistor.... how is it possible you' re now getting 363 VDC with a dropping resistor in series?
Am i missing something?
With no tubes in you' re obviously getting no drop across the resistor, since no current is flowing. With the tubes in and the dropping resistor in place I would expect the B+ to fall down some from the previous 330 VDC you measured before; assuming 200 mAmps ( quiescent ) are flowing through the resistor you should have a drop of about 4,5 VDC and so the +B after the resistor should be around 325 VDC ( to bring it closer to the design voltage, which is 320 VDC, you might like to try a 39 Ohm dropping resistor.
Regards
Bob

[Bruce / Mission Amps]

thanks bruce , 2 questions.

1. whats the best way to lower the b+ , a resistor just after the rectifiers , or is that too much for say , a 5 watt resistor
?

2. can anyone find a scem for this one ? I cant. The only thing i can tell you is it does say AC30 on the back , 6 pretubes , 4 -el84 , 6 input jacks , SS rectifier, normal/bright/trem channnels. 2x12 combo .

What I do is find a place near the power transformer where two small 3 lug terminal strips can be installed... then I use two or three 5w zener diodes, wired with their cathodes pointing to ground, in the center tap of the high voltage winding.
Or, if a FWB rectifier is used, I lift the common anodes of the FWB diodes and install the zener diodes there.
I never really use more then 12v zeners as each will have to handle all of the PT current and it can get pretty high.
Figure 200ma is possible.
200ma at 12v drop is about 2.5 watts, so each 5w zener is running at half power.
Also, each zener will not not really drop it's rated voltage. It will be something less then that so that is why I use two or three in series to make up a wanted vdc drop.

[Valvehead]

Hi Valvehead,
I' m a little puzzled by the voltages you' re getting.....you stated before you had 330 VDC with the tubes in and no dropping resistor.... how is it possible you' re now getting 363 VDC with a dropping resistor in series?
Am i missing something?
With no tubes in you' re obviously getting no drop across the resistor, since no current is flowing. With the tubes in and the dropping resistor in place I would expect the B+ to fall down some from the previous 330 VDC you measured before; assuming 200 mAmps ( quiescent ) are flowing through the resistor you should have a drop of about 4,5 VDC and so the +B after the resistor should be around 325 VDC ( to bring it closer to the design voltage, which is 320 VDC, you might like to try a 39 Ohm dropping resistor.
Regards
Bob

i dont know--may have mis- posted.

after the drop resistor , i get 363v

with NO power tubes i get 442v

[Valvehead]

Thanks very much guys.

I dropped the B+ a bit ( 50 ohm/ 10W ) and got the bias in a nice range with an 80 ohm / 10w .


Amp is performing nicely . Sounds really good---spanky clean.