Vox AC30 Top Boost clone finished

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Vox AC30 Top Boost clone finished

Postby autvox on Wed May 03, 2017 10:50 am

Hi,

At first i want to introduce myself. I build a VOX AC30 tb (built in) clone. I read a lot of posts in this forum, it was really helpful to build the amp and search for the right parts. It´s not my first amp build. Previously i built a few things like a Fender 5e3 and a few Effect Pedals. This amp was the first really big and, i think, very complicated amp. At first i had to search for the right schematic, because there are a few outside in the internet. The biggest problem was that i couldn´t even read most schematics because of the bad condition of the scans. So i drawed my own schematic like the old one.

I wanted to build an exact clone of the original, but with “exact” i mean the schematic and layout, not the chassis and boards. For the electric parts i used 2w carbon comp resistors and 630 volt foil capacitors. I read a lot about the old mustard caps and thought that there is no reason to use them (sound, voltage rating, some nearly 60 years old). In addition to that they were a huge cost factor and i didn’t want to build an amp where 20 capacitors are as expensive as the rest of the amp! An other advantage of the caps i used is that they are rated at 630v, so they are built in a way you don´t have to put them in i the right direction (outer/inner foil). But the most parts you will see in the photos below.

A word to the transformers. This topic was really hard, because there are a lot of trannys out there and most of them deliver different voltages. Another tricky part was the choke, it´s a Hammond 20h with a little more resistance than the original one and much heavier. I wanted to get the right B+ voltage as it is shown in the schematic with 320 volts DC. Now actually i get 330 volts DC which is right in the middle of what i read about voltages in the forum.

The chassis (upper part 2mm alloy, lower part stainless 1,5mm, really heavy!) is self drawed and laser cut, it´s a little bit bigger than the original chassis, because i had to fit in the turret boards (selfbuilt) and it´s better for the heat problem of the power amp.

here the finished amp:
Image


I´m sure you want to see some pictures. will follow soon...
Last edited by autvox on Thu Sep 14, 2017 8:20 pm, edited 1 time in total.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Thu May 04, 2017 7:24 am

here the tagboards i used...
the gap between the turrets is 8mm but wehen there is a large part it´s 10mm!
Image

Image

the finished boards...
Image
Last edited by autvox on Thu Sep 14, 2017 8:46 pm, edited 1 time in total.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Thu May 04, 2017 7:36 am

heater wires installed...
Image
I used 0,5mm² wire. The wires of the power amp and the preamp are separately connectet so there is no problem with the amperes flowing.
On the left side ther are the speaker cable connections. I used 3 XLR (4/8/16 ohm, 2 pins ground, 1 pin +) because they have a secure connection and can handle the power better flowing through.
Last edited by autvox on Thu Sep 14, 2017 8:49 pm, edited 1 time in total.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Thu May 04, 2017 9:48 am

Image
Image
the top boost section...
Image
Last edited by autvox on Thu Sep 14, 2017 8:52 pm, edited 1 time in total.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Thu May 04, 2017 9:55 am

the finished amp...preamp section with integrated top boost...
Image

power amp...cathode resistor 68ohm...seems to work fine...
Image
Last edited by autvox on Thu Sep 14, 2017 8:56 pm, edited 1 time in total.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Thu May 04, 2017 12:15 pm

I hope you like what you see in the photos...

I´m not sure about the power amp and if the EL84s are getting the right bias and voltages.

As i said i have about 330VDC B+ and measured across the cathode resistor (68R) i get 10,2VDC. cathode current is 10,2/68=150mA
Plate voltage is 328,6VDC.
I measured the voltage drop across the screen resistor (100R) which are all around 0,352VDC so i have 0,352/100r = 35,2mA screen current per tube.
Icathode - Iscreen = Iplate (?)....so its Cathode current/4 (0,15A/4) minus screen current (0,00352A) = 33,98mA
33,98mA * 328,6 = 11,17 Watt per tube....thats a lot!!??

Is it right what i figured out ?
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Re: Vox AC30 Top Boost clone finished

Postby pdf64 on Thu May 04, 2017 1:01 pm

Yes I think you have calculated the average idle current per tube correctly, and it's a lot, but would be a higher higher in a true copy (50 ohm cathode resistor).
Bear in mind that the HT / plate voltage will rise as the cathode resistor value is increased, due to the reduced current draw.
For me, 68 ohms is a bit high, above 62 ohms the overdriven tone suffers a little due to bias shift.
I prefer individual 240 ohm resistor (and bypass cap) per cathode, which allows unmatched tubes to properly self bias to broadly similar idle currents.

I don't think that a fuse on the OT CT is a good idea; if it blows, it looks like the screen grids would still be energised, which would lead them to conduct heavily and overdissipate. And the use of 2 watt screen grid resistors would make them unlikely to fuse, eg should the screen grid short to another electrode with a tube.
Merlin suggests a fuse on each leg of the PT HT winding.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Thu May 04, 2017 4:02 pm

Thanks for your advice!!

ok i wasn´t sure if its right.
I took the 68R as cathode resistor because i had only a 56R and a 68R resistor by hand. Other values that can handle 10W are difficult to find. At first i will listen to it fully cranked/overdriven and maybe i switch them and make new measurements.

Yeah that´s right, but the fuse is right after the rectifier tube, so, when it blows there is no high voltage at all. (the red cable goes from the rect tube under the black board right to the fuse) Would that be a problem for the rectifier tube when it runs disconnected (without the 16uF caps, load, etc)?

At the tube sockets (EL84) i took large resistors, glass emaille 5W/100r/5% and 2W cc 1k5, mostly because of the heat in that area.

I was also very surprised that the amp has no hum problem at all. Many times i read about hum issues in old voxes. In the preamp i grounded everything in one point and changed a few cables and where they run.
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Re: Vox AC30 Top Boost clone finished

Postby pdf64 on Thu May 04, 2017 9:33 pm

OK, that all looks good then.
There's no problem is putting a fuse between the rectifier and reservoir cap.
Kudos for a very high build standard :not_worthy:
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Re: Vox AC30 Top Boost clone finished

Postby LD50 on Fri May 05, 2017 9:20 pm

Looks like a very nice build 8)
All bleeding stops.......eventually.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Sat May 06, 2017 10:05 pm

Ok perfect! Then it´s a save solution.
Thanks for those kind words! It´s nice to hear such things after all that work!! :)
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Re: Vox AC30 Top Boost clone finished

Postby Beus on Sun May 07, 2017 10:53 am

very nice!!

what are the two switches on the top?
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Re: Vox AC30 Top Boost clone finished

Postby Unit_1 on Sun May 07, 2017 12:35 pm

Nice work! :thumbsup:
Image
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Sun May 07, 2017 8:42 pm

The left switch (next to the inputs) is the "footswitch", because i wanted to have the switch on the top of the amp. It´s connected with a shielded cable. If you want to change the speed of the vibrato/tremolo or make other adjustments you have to stand in front of the amp anyway, so i placed the switch on the top of the amp. Maybe i will install a 6,3mm jack somewhere later, so i can connect any footswitch pedal i want, but for now this is the best solution for me.

Then there is the Vib/Trem speed - rotary switch (3 resistors), and then the switch to change between vibrato and tremolo!

The whole channel is a bit quieter than the normal and the top boost channel. However...it sounds very good.

here a detailed picture...
Image
Last edited by autvox on Thu Sep 14, 2017 8:57 pm, edited 1 time in total.
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Re: Vox AC30 Top Boost clone finished

Postby autvox on Sun May 07, 2017 9:40 pm

made a few tests and measurements and here is the clipping of the amp when overdriven....

Image

At very high output you hear ah little bit of noise (same frequency as the input signal) out of the output transformer. At first i thougt it is a tube but it´s the OT. I think that´s normal at such a power level because the iron is in saturation and the windings also vibrate a bit...?!
Last edited by autvox on Thu Sep 14, 2017 8:58 pm, edited 1 time in total.
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