ac50, joe piazza schematic questions

[mols]

Hi.
I am building an ac50 amp using the joe piazza schematics. It is the tuberectified version (gz34). Has anyone of you used this schematics? I have not been able to find any comments on it.
For example i am a little bit confused about the 5V heater going to pin nr.seven on the rectifier tube. Shouldn´t that be pin nr.eight? (As far as i understand there is no pin seven on the gz34 tube)Also i vould like to skip the neon light in the bias suply ,because the old chassies that i am using already has a mounted lightbulb for the 6,3V heater winding, that i would like to use instead. Should i put a resistor instead of the neonlight?  Or can i just skip it?

Mårten

[Willabe]

Here's a link to the schemo from Doug's library.

http://www.el34world.com/charts/Schematics/files/Vox/ac50.pdf


            Brad   

[Willabe]

Welcome!

For example i am a little bit confused about the 5V heater going to pin nr.seven on the rectifier tube. Shouldn´t that be pin nr.eight?

Yes, I think that's a typo, should be pin 8 not pin 7.

Also i vould like to skip the neon light in the bias suply ,because the old chassies that i am using already has a mounted lightbulb for the 6,3V heater winding, that i would like to use instead. Should i put a resistor instead of the neonlight?  Or can i just skip it?

Yes, I think you can just leave it out no need for a R.


              Brad     

[stratele52]

Neon, this circuit do not need it , just don't put neon if you don't want one .

GZ34 ; you have the answer .

Is it the first time you build tube amp ?

[kagliostro]

Here a lot of very good documentation if you are restoring or cloning the AC50

also with schematics and layout

http://www.backfromthesixties.co.uk/Pages/Vox_AC50.aspx

K

[jazbo8]

Very nice site! Lots of great information on old amps. BTW, please stop using the Piazza re-drawn schematics, they often contain errors, why did guy bother to re-draw so many schematics when factory drawings were available? 

[mols]

Thank´s for help ,and links. I will not be using the neon.
The back from the sixties site is really nice.
This is my first scratchbuilt amp. I ave built a kit from ampmaker (uk) before. Wanted to try something more challenging to learn more. It is allmost finished now. Soon time to test it. Using a lightbulb limiter, thick gloves and a mc helmet! Wanted to be outdoors to in case of fire, but there is thirty centimeters of snow right now.
By the way the error with the gz34 is in the schematic on Chris Devines site to.
Mårten

[kagliostro]

By the way the error with the GZ34 is in the schematic on Chris Devines site to

OK that is true, here attached you can see an image with the real GZ34 connection

I suppose that when the schematic was draw the author confused the number of the pin with the pin of the EL34 tube

here is the link where I've find the image of the GZ34 Vox AC50

http://hvid.it/voxac50restoration.html

K

[sluckey]

Aha! Thanks for that picture K. That confirms my suspicion. That neon is not being used as an indicator lamp. It is being used as a regulator in the bias circuit.

To mols... I recommend you use the NE-2 neon lamp since it's part of the bias circuit. They're cheap and small. That bias circuit will work without the NE-2, but the bias voltage will be different. Probably still have enough adjustment range, but using the lamp keeps the circuit like the original, if that's important to you.

[Willabe]

That confirms my suspicion. That neon is not being used as an indicator lamp. It is being used as a regulator in the bias circuit.

Interesting.


           Brad     

[kagliostro]

Ne-2 is correct that is a 65v lamp ?

K

[sluckey]

This is a quote from Wikipedia...

Most small neon (indicator-sized) lamps, such as the common NE-2, break down at between 90 and 110 volts. This feature enables their use as very simple voltage regulators or overvoltage protection devices.

[Willabe]

So that's like the avalanche voltage on a zener diode?


             Brad     

[sluckey]

Yes. Or like the strike voltage of a gas regulator tube such as an 0B2.

[Willabe]

Ok, so Vox used the NE-2 instead of a gas tube or zener because it was cheaper and zener's weren't well developed yet or where expensive back then?


            Brad     

[sluckey]

Definitely much, much cheaper. I don't think it's necessary at all. Looks like some 'Rube' engineer's idea. Looks good on paper. Don't do too much in the real world. Think about all the major brand amps that take the bias supply AC source from the HT winding. Fender and Marshall come to mind immediately. This Vox AC50 is the only time I've ever seen a NE-2 in a bias supply. And Vox eliminated that lamp in later editions of the AC-50.

This lamp is used on the AC side of the rectifier, so it's more like a limiter than a regulator, although you could call it both or either. It's being fed from a 375VAC_rms source. That's 530VAC peak!. Using round numbers let's say the neon fires at 100V. So once the neon fires, it prevents that voltage from rising higher. The excess voltage is dropped across the series 330K resistor. As the AC sinewave continues to rise to it's peak, then begins it's descent toward zero, it passes thru 100V again. The neon quits glowing and looks like an open circuit. The same action happens on the negative half cycle. So, the AC applied to the rectifier is limited (regulated) to 100VAC peak and will probably more resemble a 200V_pp square wave with slow rise/fall times. Good idea on paper.

Just a little side note... Notice there are two poles inside the neon lamp. Only one will be glowing at a time. If you could decrease the frequency of the applied voltage to say 5Hz, you could see the poles alternately firing, but never both on at the same time.

[Willabe]

Ok, thanks.


            Brad   

[mols]

Quote from Sluckey:
To mols... I recommend you use the NE-2 neon lamp since it's part of the bias circuit. They're cheap and small. That bias circuit will work without the NE-2, but the bias voltage will be different. Probably still have enough adjustment range, but using the lamp keeps the circuit like the original, if that's important to you.
[/quote]

I really like the sound of the ac50,but my amp will allready be different from original.
I am using a powertransformer from classictone (40-1829) that is lower in voltage then what is specified (355-0- 355v) compared to (375-0-375). And a different outputtransformer to. So if the neon drops the voltage in the bias circuit i probably sholdn´t use it?
Would you recomend a standby switch? I have read that its not necessary whit the tube rectified version.

Thank`s again for your coments!

[sluckey]

So if the neon drops the voltage in the bias circuit i probably sholdn´t use it?
Would you recomend a standby switch? I have read that its not necessary whit the tube rectified version.

The neon doesn't drop the voltage in the bias circuit. It limits the voltage to the input of the bias circuit to whatever the firing voltage of the neon is. I used 100V as an example. What that means is... you could use a PT that puts out 200VAC or 300VAC or 400VAC and the bias voltage would remain the same because the neon limits the input to the bias supply to 100V, regardless of how much voltage is on the HT winding.

Make sense?

Oh, and standby switches are nice on bigger amps, regardless of which type rectifier is used. Is it necessary? Maybe not, but it sure is handy.

[kagliostro]

I was asking about the 65v because I found something that make me to suppose that

a further research give me this result in Mouser catalog

http://it.mouser.com/ProductDetail/Chicago-Miniature/A9A/?qs=sGAEpiMZZMsEIsGLxVc9kMj9z82SfnH7


Datasheet
http://vcclite.com/_pdf/Neon%20Indicator%20Lamps_7_&_8.pdf

so the voltage of 65v I've read in some places refers to the voltage in AC and 90v DC is the voltage at which the lamp acts on the Vox circuit

K

[sluckey]

65VAC_rms is the same as 91.9VAC_peak. So, everyone is correct.  In my examples above I was sometimes talking abour RMS voltages and sometimes talking about peak voltages. And I didn't always say so. Sorry for the confusion.

[kagliostro]

I was in confusion, not what you said Steve

K

[SoundmasterG]

I haven't built any AC50's, but I did build an AC100 clone, and I've played real AC50's before. The big Vox amps like these sound great! Even though your voltages may be off, I think with some tweaking of your amp you will be very happy with the circuit!

Greg