Vox AC50 OT

[hotrodder]

Hi!

I ahve an early AC50 that seems to need a new OT... The windings from the EL34's have two different readings - 1.8k and 1.4k to ct, and there is an unbelievable amount of wax melted onto the base board. The amp still works, but I'm afraid that the old OT might be close to the end of its life. What is recommended as a good replacement brand, (and perhaps model), that will let the amp sound like it does now. The chime is fantastic, but the amp seems a little down on power. Oh yeah, and this unit has had its PT replaced - the new one is putting out 408v. instead of 375v. Should I worry that it's running with the PT center tap fuse blown? I've been told that this is impossible, but this amp does work in this state. This unit does have a rectifier tube (5U4) and the PT seems to have the proper wiring....

Any help is appreciated - Thanks...

[olddawg]

Do you have a variac, a signal denerator, and a scope? I would reduce the AC mains until you have the desired B+. Then I would measure the output at clipping with a 1KH input signal. If it looks alright and is what it is supposed to be in output I wouldn't replace the OT. That resistance variance is probably close to spec for the old beast. The wax is from overheating. Are you sure it isn't from the old PT or an old filter cap? If you want to bring the B+ down after you take the amp off of the variac, a couple of 10v 5watt zener diodes in series on a tag board will do it. Just mount them where they can breath. I wold also consider installing a whisper fan, they're cheap.

[JHow]

I think the ohms readings on the different taps is not that helpful, and a sliht difference is not necessarily abig concern. One side is probably wound closer to the outside of the transformer, hence a longer length of wire for the same number of turns.

[soundmasterg]

That amp should be using a GZ34 rectifier tube instead of the 5U4. The 5U4 needs a higher rating for the filament current than the GZ34, 3a vs 2a, so that could cause a problem with the power transformer running hot. Given that the GZ34 will have a smaller votlage drop, then the power output of the amp should go up. B+ in those should be in the 450v - 460v range if I remember right. Anytime a fuse is blown that indicates a problem. Why did the fuse blow? You should figure that out before you operate the amp on a regular basis. The fact that the resistance is different from each side of the OT does not indicate that it is bad. The wax melting indicates that the OT got really hot, which should never happen in normal operation. Has the amp been rebuilt otherwise? Are the filter caps still original? If so, they should be replaced with modern equvilants. Make sure the power tubes are in working order also.

Heyboer makes excellent Vox clone output transformers that will bolt right in and have the correct specs. I had them make me one for an AC100. Just email them or call them. Mercury Magnetics makes one too but they are very expensive.

Greg

[Steve Conner]

Which transformer did the wax come out of, the PT or the OT?

If the center tap fuse is blown, maybe the current is running through the bias tap. Or maybe the new PT was installed by a monkey who committed some wiring blooper, bypassing the fuse. Would definitely take a closer look at that as it might be unsafe.

Are you sure about the 1.4 and 1.8k readings? They're extremely high for DC resistance of OT primaries in a 50 watt amp. Is it the correct OT, or did someone bodge in one from a smaller amp? That would also explain the overheating.

A 50 watt OT is usually about the size of a coffee mug (not a giant Starbucks one though! ) and weighs a couple of pounds.

[hotrodder]

Hi!

Thanks for the responses. Sorry it took a while to get back to the forum -
I rechecked the OT impedences readings, and found that I misread the meter - mybad! The proper readings are 154 ohms and 179 ohms. The wax is definitely from the OT. I'm including a couple of pictures of the evidence. I have a variac, but need to get a hold of a scope and generator to do any further wattage tests. If I change out the 5u4 to a gz34, do I have to make any adjustments to the circuitry to compensate, or are they directly interchangeable?

There is no label on either transformer, but the PT has definitely been changed. The caps look original, with perhaps the exception of the outboard caps. If I do a filter cap job on this, how much do you think it will affect the tone? As I said before, this is one sweet sounding puppy, and I'm just concerned about its reliabilty if I start gigging it.

Thanks again.

[olddawg]

It's been a while since I've been inside an AC50, but it looks to me like the OT has been replaced also. Those 4 holes probably mounted the original and the 2 holes with the bolts were drilled to mount the replacement. Do you actually see any wax on the OT? It looks like it has been a little toasty, but a lot of OTs look like that.

You can use your multimeter to measure the output in the absence of a scope. You will need some kind of calibrated signal source. You can find one on the internet if you seach. Just seach for online signal generator.

Since this amp has been hacked already, I wouldn't be worried about "collector value" if you fix a few things. Replacing the filter caps shouldn't degrade your tone. It might even improve it. Coupling caps are very subjective. I wouldn't replace those unless they are leaky.

Reliability.....what a concept. I love my old tube amps but always have a backup.

[Steve Conner]

Yeah, that's a good point olddawg. I wouldn't be surprised if that big puddle of wax came out of the previous OT, and someone replaced it without cleaning the wax up. If the OT lost that much wax, you should see stalactites of wax dripping out of it, and it would probably smell pretty burnt too.

The PT looks like a Fender one. If you can find a Fender part number on it, you could figure out what amp it came from and what the ratings are. A GZ34 should be fine on it. It'll raise the B+ a little more, but EL34s should be cool with that.

I'd definitely go ahead and replace the filter caps, this is a "player's amp" now! But sometimes hum from crappy filter capacitors adds to the feeling of "chime".

[pdf64]

From the pics, I'd say that the OT is original (due to the type of flying leads, colour codes and dress, similarity of paper wrappings to the choke, and the fixings on the pre-amp chassis), though like Steve says, the PT looks a replacement.
If the big bolts on the OT power amp chassis are a US thread, then that would be a good indication that it's a replacement. If original they'll be the obsolete UK BA thread (finer pitch than US thread), probably 2BA like the fixings on the choke and other chassis bits.
Old UK transformers do ooze wax when running hot. I'd guess that an el34 shorted, ran the OT very hot and killed the original PT. As it still works without crackling I wouldn't worry about it.
With a tube rectifier and no cathode bias, that's a very old ac50.
http://www.schematicheaven.com/voxamps/ac50.pdf
If the PT B+ CT fuse is blown, there should be no B+?
Find the the actual B+ CT of the replacement PT - the wrong wire must have gone to the fuse.
All the electro caps must be replaced, especially the 8uF ones in the bias supply.
Check resistors for drift, many will be miles off, possibly > 2x indicated value.
Some old UK coupling caps go open circuit rather than leaky, so if you're losing signal that's worth checking for (though I've not known those mustards to do that).
Check the current through the el34s (ie bias). If ok (~40mA) then try a GZ34, but monitor the currents as the increase in B+ will increase the current, so requiring the bias pot to be adjusted.
If you are low on output, are you sure you've got the correct impedance load? Should be 8 and 16 ohm outputs.
If all the above is good, and the B+ is still low, then you could short out the 22ohm resistor between the rectifier and reservoir cap (big green clarostat), that might get you a little more by removing some sag.

[soundmasterg]

Hi!

Thanks for the responses. Sorry it took a while to get back to the forum -
I rechecked the OT impedences readings, and found that I misread the meter - mybad! The proper readings are 154 ohms and 179 ohms. The wax is definitely from the OT. I'm including a couple of pictures of the evidence. I have a variac, but need to get a hold of a scope and generator to do any further wattage tests. If I change out the 5u4 to a gz34, do I have to make any adjustments to the circuitry to compensate, or are they directly interchangeable?

There is no label on either transformer, but the PT has definitely been changed. The caps look original, with perhaps the exception of the outboard caps. If I do a filter cap job on this, how much do you think it will affect the tone? As I said before, this is one sweet sounding puppy, and I'm just concerned about its reliabilty if I start gigging it.

Thanks again.

The amp was originally made for a GZ34, so IF the circuit is still original, then you should be able to drop in a GZ34, readjust the bias, and be good to go. If they changed the circuitry though, who knows. The other thing to consider is that if the PT is not original, then that could be why they used the 5U4. The B+ should be around 460v-480v with vintage wall voltages. With today's AC out of the wall, it could be higher. Measuring that will tell you if the replacement power transformer is close or not. With the 5U4 in there, the rectifier section drops around 50v, whereas with the GZ34, the rectifier section drops around 17v. If you're concerned about reliability, then replace all of the electrolytic caps in the amp. The amp will sound better, having better lows and highs, and having less hum too. I wouldn't replace any of the signal caps though unless they are leaking DC and bad. I'd check all of the resistors against their value and replace any that are outside of 20% as that is what their tolerance was when new. Those are a bit tricky to bias due to the mixed fixed bias andcathode bias setup.

Greg

[Bruce / Mission Amps]

The OT looks like it could be a real vintage VOX OT but possible not from this amp. The plastic wire from it looks English.
The power transformer looks odd to me because I thought the filament wires in those old Vox PTs were all thick plastic wire and sometimes actually doubled wires to the same lugs on the first socket... well, that could be for the AC100.
The trannys usually have things written on them on the other side.
I don't see any other pictures.
Also, is the black tape on the inside, by the on off switch, covering the old voltage selection switch?
That could be an indication it is not the original PT too.
Regardless... what real 50 watt Fender PT has a 5v winding for a rectifier
tube... only the Super Reverb comes to mind and the PT should be marked with some kind of manufacturing number.
The AC50 when tuned up and running right at 120vac, is the killer 50 watt amp of all time by the way.
Replace all the E caps and redo the bias supply too.
Do what ever is needed to the bias supply in order to control the idle current of the power tubes.
Don't let the power tubes in this amp idle at more 16 to 18 watts each no matter what anyone tells you.

I've refurbished lots of old VOX amps over the years and have only have a couple AC50s and/or AC100s with bad OTs... but, Sergio over at Merecury Magnetcs rewound them and those amps sounded superb.
I use Heyboer ,Lenco, Mag Com, TMI, W.Schumacher and other very high end magnetics companies but I would not use any other but MM since the amps are highly desired and hard to find. They really nailed the VOX tranny.

Oh and it is not a real class A amp, which I've read on the NET frequently over the years too.

[soundmasterg]

>>Off topic for a moment<<

Hey Bruce,

With the AC100's that you've worked on in the past, did you ever measure the actual power output?

I've been building myself an AC100 clone with a 2nd higher gain channel added. For the Vox channel I went with what they used originally, and my higher gain channel is in phase with the Vox channel. I am mixing them both on the first section of the phase inverter...I added a parallel triode there, and the rest of the inverter is identical to the original AC100. It seems like the inverter is not driving the power tubes enough....I'm only getting about 50 watts out on the amp....I should think it should be about 80 watts in cathode bias and 100 watts in fixed bias right?

Thanks,

Greg

P.S. I have played a couple AC50's in the past and REALLY wish I had one. Very nice amps those are.......

[Bruce / Mission Amps]

Heck yeah!
A really good AC100, using fixed bias with the power tubes idling at about 15-16 watts each, will make about 105-115 watts output into a dummy load.
The cathode biased 50 watters ones are down in the 35-40 max watt range and run the power tubes way too hot at idle... but also sound glorious.

[soundmasterg]

Heck yeah!
A really good AC100, using fixed bias with the power tubes idling at about 15-16 watts each, will make about 105-115 watts output into a dummy load.
The cathode biased 50 watters ones are down in the 35-40 max watt range and run the power tubes way too hot at idle... but also sound glorious.

Hi Bruce,

I get 112 watts full out into a dummy load in fixed bias. I'll have to check again and see where I have the amp biased. If I check output power when the amp is clean, then it is only 50 watts, and both of those readings are for the higher gain channel that I added. The Vox preamp channel is lower than that by probably 10 watts. Seems low to me for 466v plate and four KT77's. Cathode bias output power is actually not too much lower in this amp.....Do you know where the cathode biased 100 watt Vox 80100 is as far as it's power out? I've got individual 330 ohm Rk's and 100uf cathode caps for each power tube when in cathode bias, just like the originals that lived are.

Greg

[soundmasterg]

Ok, I think I discovered my low power problem. The Heyboer output transformer that I used is about a 4600 ohm primary impedance, which is more suited to two EL34's than four I would gather. I wonder if they gave me a 50 watt output transformer instead of a 100 watt. It does sound good as it is, but I would guess maybe the higher load is working the tubes harder than needed?

Greg