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Old 27th April 2011, 11:50 AM   #1
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Exclamation Vox AV 100 Mk2

Hey folks,

My names Mikey and I've been building this project for a few months now,
its a Vox AC 100 Mk2 and is a college project for my final year and my first valve project (quite a large project, for my first but it too late now).
My intention is to make the bias adjustable in the long run but time restrictions as its a college project are forcing me build the original bias.

Schematic

Here is a link to the schematic I've been working from. The problem i'm having at the moment is that when the amp is turn on fully and is idle one of the 100 ohm resistors on the anode of the EL34's starts smoking and then the HT fuse blows. I am thinking of increasing the power rating on these resistors but from reviewing out pictures of the amplifiers on websites it looks like the power rating on these resistors (R26, R27, R28, R29) or around 5 - 7 watts and the power rating of the resistors I have in place at the moment are 5 watt.
I measured the voltage across one the these resistors when I turned it on and it rose to 16 volts before the HT fuse blow. The bias produces -35V.

Any observations on this would be very welcome.

Thanks,
Mikey

Last edited by MickeyIRL; 27th April 2011 at 11:57 AM.
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Old 27th April 2011, 02:09 PM   #2
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Are tubes matched?
You can change tubes from one shoulder to another and see what will happen with "wrong" and next resistor.
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Old 27th April 2011, 04:20 PM   #3
llwhtt is offline llwhtt  United States
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I am rebuilding an original VOX AC100 at this very moment, well many many moments actually. Though I haven't finished it yet I can tell you that if the each output tubes is idling at 50ma (just a guess at this time but close, maybe high) the 100 Ohm plate resistors will have 5 Volts across them and only dissipating a 1/4 Watt. You may have a bad tube, make sure the -35VDC is on pin 5 of each output tube. I think the original plate resistors were 2 Watts. The Vox schematic really lacks in regards to Wattage specs of the resistors. Notice the 18K Ohm resistor coming off the choke, this one dissipates 11.25 Watts!!!!!!

Craig

Last edited by llwhtt; 27th April 2011 at 04:23 PM. Reason: mistake
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Old 27th April 2011, 05:30 PM   #4
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Quote:
Notice the 18K Ohm resistor coming off the choke, this one dissipates 11.25 Watts!!!!!!
That is insane, especially considering the limited power supply capacitance.

Checking the -Vg would be the first thing to do. 16V @ 100R is 160mA idle anode current, which is 60% above the maximum rating of the EL34, so you're probably doing something wrong. No oscillations, by any chance?
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Old 27th April 2011, 08:09 PM   #5
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@llwhtt Wow! how is the rebuild going? can you notice any discrepancy between what you see in the amplifier and whats in the schematic?
ha ha, yea that was a hidden surprise, not hard to work out though if you look at the D.C voltages on the schematic.

If it was a faulty tube how would you recommend testing tube to see if it was faulty?
Once I check the bias is correct, how would you recommend checking for oscillations?
The pre amp works with out the power tubes but when the power tubes are in the and the amp is turned on a hum grows then the HT blows and the hum dies away, this leads me to believe there oscillations build some where in the amp.
By removing the pre-amp and phase splitter tubes and then working back and replacing them would it be possible to isolate the oscillations?

As this is my first project this is quite new to me so bear with me please I don't mean to be annoying.
Thanks for all your replies!
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Old 27th April 2011, 08:49 PM   #6
llwhtt is offline llwhtt  United States
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The only major concern is the fact that it's in the US on 120VAC/60Hz. All Vox's of the period has a setting for 115VAC and usually that's wrong too. I have rebuilt many AC30s and sometimes all of the voltages are correct at 100VAC with the selector set to 115VAC. I assume you are using up to date xfmrs and the primary has been sorted out. Another minor thing is that damn Brimistor, probably end up using a piece of wire for that. And I will probably use adjustable fixed bias instead of the zeners. I also added screen resistors on the EL34s. If you Google "Brimistor CZ4" you'll find many pictures of AC100s, none of them have NOT had major work done to them!!!! At least the AC15,30 and 50s have some sort of resemblance to stock after all of these years. As for the schematic markings it just seems strange to label the Watts on some minor resistors but leave out some of the major ones, like 18K megawatt one and the output plate resistors.

Craig
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Old 27th April 2011, 08:53 PM   #7
llwhtt is offline llwhtt  United States
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Forgot to answer about testing tubes, a tube tester would help. For oscillations a scope is the only answer I can give you. Vox didn't use negative feedback so that's not your problem. Maybe get it built to the best of your ability and then take it to a shop that will help YOU sort it out. I don't like guessing on some else's $.

Craig
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Old 28th April 2011, 10:49 AM   #8
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As Craig/llwhtt indicated:
- Use a 'scope to check for oscillations
- The hum-thing might suggest an oscillation, but it may also indicate a bad tube

I always use g2 stoppers on triode-connected pentodes to prevent oscillations and excessive g2 dissipation. If your problem is indeed related to oscillations, then it is very likely it only occurs with all tubes installed. You can try removing the preamp and PI tubes and see if the problem persists.
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Old 16th May 2011, 03:36 PM   #9
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I've had exams so I haven't been working much on this but now that their out of the way i'm back working on this.

@Mastodon, what value of g2 stopper resistor would you use? I have removed all the tubes apart from the output tubes and the amplifier idled fine. Once I added the phase splitter tube back in the oscillations started building up again so added grid stoppers on the grid of the phase splitter but this had no effect.
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