Cool Information About The Vox AC4TV Series

[SnorkelMonkey]

I picked up a Vox AC4TV8 last week and I made an interesting discovery regarding this amp series. Before I bought one the thing that stuck out in my mind as strange was Vox’s use of a 16 ohm speaker in these 4TV series combos. Also all the photos I’ve seen of 10’s, 8’s, and heads show the same fairly large (for a 4 something watt amp) output transformer (OT). It left me scratching my head… So I had to take it apart to find out what makes it tick.

I pulled the OT, checked its reflective load on the primary and found that with a 16 ohm load on the secondary there was about a 10k impedance on the primary. With an 8 ohm load on the secondary there was about a 5k impedance shown on the primary.

What does that mean if you own an AC4TV?

Pros? By putting a 16 ohm speaker on the secondary using this OT it halves the output power to ≈ 4-5 watts. By putting an 8 ohm speaker on the secondary you increase this to ≈ 8-9 watts. When using the external speaker jack it disconnects the internal speaker from the circuit not puts them in parallel like in some amps. So if you were to use an efficient external speaker cabinet loaded with for example a G12H30 @ 8 ohms you could plug the cab into the 4TVx to get the full output power, increase headroom, and frequency range from the amp for rehearsals/small club dates, unplug the cab to halve the output power for at home/studio recording, and use the attenuator for bedroom practice levels etc..

This makes the 4TV8/10’s pretty attractive for use as heads and practice amps. Maybe better the TV8 as it’s smaller? I ran it into a cab loaded with a G12H30 and it was very loud with lots of low end and highs. I also ran it through a Jensen C8R and it too was loud (not as loud as the G12H30) and had an increase in volume, headroom, frequency range.

The OT for these amps are very robust (as is the power transformer) at about 2 lbs in weight, dimensions 2”x 3” x 2.5”. I would say the OT is rated between 15-20 watts, way over spec’d. I would say it can handle/translate the increased watts and then some.

Cons? When using an 8 ohm speaker I do not recommend using the built in attenuator circuit i.e. 1W, 1/4W (or 1/10W as on the 4TV8) settings. They use a 2W resistor network to attenuate (lower) the signal. In 1W mode it would actually be somewhere around 2-3W with an 8 ohm load. Those resistors would get hot and could possibly fail. If you leave the amp in its 4W setting and add an 8 ohm external speaker you’re most likely looking to increase output power not looking to cut the power so that should not be an issue. But if you plan to install an 8 ohm speaker into the combo I would recommend upgrading those resistors to at least 5W. They use large 2W power resistors so I’m pretty sure you could replace those with those black wirewound Vishay/Dale 5W and get it all to fit. Personally I would pull that attenuator and install a VVR. I might post how to do that later. You should be good with the stock 2W cathode resistor on the EL84 but I might upgrade that also to a 5W while I’m in there. I had the amp running continuously for a 24 hour period with no issues.

Also for 4TV8 owners you might have found that the 1/10W setting is not as useful as you may like it to be. You can replace the 3.3 ohm resistor @ R14 with a 5.6 ohm 2W (or better 5W and replace them all) power resistor to get the 1/4W mode found on the other series. Besides speaker and cabinet configurations that one resistor is the only thing I could see that sets the 4TV8 apart from the rest in the series.

Remember, tube amplifiers can sometimes hold lethal voltages that can kill you even after they are shut off. Disclaimer, if you blow up your amp or yourself don’t come crying to me you’re on your own. Well happy hunting and don’t shoot your eye out.

image removed

 

[soulpatch]

Wow, that's very cool. I'd like to see how you replace the attenuator.

 

[SnorkelMonkey]

I forgot to mention in the original post that the EL84 output tube is actually looking for a 5K impedance load. By putting a 10K impedance (16 ohm speaker on the secondary) on the primary they are purposely mismatching the impedance to half the output to 4-5 watts.

What that means in layman's terms is (and based off the size of the output transformer) the Vox AC4TV series are actually 8-9 watt amplifiers stock.

 

[KevinB]

That's very interesting stuff.

I've only had my AC4TV8 for a week or so and while I like the tone, it does seem to break up a little early for me, even at 4W. I was thinking of an OT swap for something with a little more power but this seems a much better alternative. Again, I realize it's only a practice amp and I'm not looking for more volume, but more headroom. The little amp does sound better through a 12" (Epi) 16 ohm cabinet I have, but I think I'll try replacing that speaker with an 8 ohm one.

Thanks!

 

[tooncaster]

I played one of these in the store a couple months ago and was super impressed. It seemed to outperform any 4W competitor. Now I have a good idea why.

It's on my GAS list, really a debate between this and a more expensive Night Train. Just how loud is loud for this? What venues are 8-9W amps best for? (Yes, I'm a noob.)

 

[jefrs]

Whilst using an 8ohm load will indeed increase the output, you are stuck with a single EL84 which will max-out at 5.7 watts. Very few EL84 bottles will tolerate more than that in true Class-A mode (an SE amp has to be true, always-on, Class-A).

The reason you have such a large output transformer is because you are have got a single-ended Class-A amplifier which passes continuous D.C. current through the primary: these need a huge transformer, usually the same one fitted to a 30 watt 4xEL84 amplifier. If it is only 15-20 watts then it is under-specced. You may find the primary is centre-tapped.

Does the amp have negative feedback (NFB) from the speaker output ?
If so then it may be possible to safely use an 8ohm load on the normal output connection, the amp will self-correct for load.
If you do not have NFB then it might not be possible to safely do this unless the amp is built to tolerate the mis-match (many guitar amps are).
What can go wrong? - the amp can draw too much current which might overheat the output transformer or blow the EL84 valve, but chances are the HT fuse would go first - that's what it's there for.

 

[SnorkelMonkey]

Whilst using an 8ohm load will indeed increase the output, you are stuck with a single EL84 which will max-out at 5.7 watts. Very few EL84 bottles will tolerate more than that in true Class-A mode (an SE amp has to be true, always-on, Class-A).

Jefrs, you can push EL84's past that as done here. Here's a link with some more information on the very topic.

http://www.ax84.com/bbs/dm.php?thread=4177

The reason you have such a large output transformer is because you are have got a single-ended Class-A amplifier which passes continuous D.C. current through the primary: these need a huge transformer, usually the same one fitted to a 30 watt 4xEL84 amplifier. If it is only 15-20 watts then it is under-specced. You may find the primary is centre-tapped.

This is not correct in practice. For example see the Epiphone Valve Jr., Fender Champ, etc. for typical SE transformers used.

I suspect they used this OT was for one (or all) of several reasons...

1.) Purchased in bulk. Might be used or will be used in another amp with higher spec requirements? Or just got a great deal on them?

2.) With a 16 ohm load the amp is running cooler and also allows the use of (cheaper) smaller power resistors in the attenuator circuit that is basically already shoe horned into the chassis as it is.

3.) Fault tolerance. 16 ohm speaker/cabs is about the highest configuration you commonly see. If the user mistakenly uses a 8 ohm speaker/cab on output there is no mismatch as the amp. With a 4 ohm speaker/cab you have a mismatch but due to the size of the transformer it could possibly cope for a period of time (if not indefinitely), maybe not so much the tubes... That equals to a lot less repair calls do to impedance mismatches. Remember these are usually marketed as student amps for the first timer. :idea:

Does the amp have negative feedback (NFB) from the speaker output ?
If so then it may be possible to safely use an 8ohm load on the normal output connection, the amp will self-correct for load.
If you do not have NFB then it might not be possible to safely do this unless the amp is built to tolerate the mis-match (many guitar amps are).
What can go wrong? - the amp can draw too much current which might overheat the output transformer or blow the EL84 valve, but chances are the HT fuse would go first - that's what it's there for.

There is no mismatch with an 8 ohm load as I have described in the OP.

I ran the amp continuously for a 24 hour period and the transformers both were cool to the touch.

 

[jefrs]

SnorkelMonkey, the Pa-max of an EL84 is 12W. That is the absolute maximum it will take on the cycle up-swing, above which it will not go, the minimum is of course zero. If you look at graphs of these you sometimes see a power line drawn with the area to the right greyed-out: there is no point arranging the valve to go there. Hence the EL84 is usually rated at half-way i.e. 5.7W, but 7 watts is possible with some brands: Mullards were designed to exceed the specification. One can certainty push more power out by going to push-pull: 17 watts is normal for a pair of EL84 in Class-AB1, 22 watts by going to Class-AB2. Another funny thing with valves, sometimes their power output goes up as their output load is increased: it changes the bias point.

A valve may be viewed as a variable resistor. For the EL84 Ra is 4.5k, with Va at 250V (even my old WEM runs them at 300V), that's 14W when fully on, that's already exceeded Pa-max=12W and it will be red-plating. But, and here's the thing, my old WEM is push-pull Class-AB (like wot an AC15 is), so they are turned off some of the time and can cool down, and we can turn the power up even more.

Transformers: I do have an EVJh, the output transformer is seriously under-specified: it is a very cheap amp. Class-A output, whether single-ended or push-pull is very greedy on the output primary. It is possible to build a true Class-A, always-on, push-pull amplifier, they are low output, need a gigantic OT, and as a guitar amp, sound like poo (so does UL). Class-A pushes continuous D.C. current through it which magnetizes the core, so a very much bigger core is needed than for Class-AB push-pull. Plus the core requires careful design of the laminations to prevent eddy currents. Not to mention various weird and wacky harmonics. Most modern output transformers use the same core laminations as for the mains transformer: this is wrong. My Laney L5T (three times the price of the AC4) uses the output transformer from their 30W 4xEL84 LC30 amplifier, this monster is just about the right size for a single-ended Class-A EL84. Designing an output transformer is a black art, impedance is not linear with frequency, and this applies to speakers as well as transformers. Class-A is greedy on output transformers, a good one should amount to half the cost of the amplifier, and most of the weight: you can see why manufacturers may be keen to skimp on the OT..

The EVJh has multiple output taps, 16/8/4. If I correctly load the output then the 16ohm tap sounds best (pair of 8ohm speakers series or parallel), this is mainly because the 16ohm tap uses the entire secondary, the 4ohm output is 'browner'. Same applies to 30+watts Epi BC30 but for some reason that came with a pair of 16ohm speakers in parallel into its 8ohm socket: I soon fixed that.

Incidently the most efficient secondary load for a valve amp is 25ohms i.e. with output transformer to suit: BBC valve equipment traditionally used 25ohm speakers, their equipment was/is better than most recording studios.

 

[SnorkelMonkey]

Jefrs, My post was not intended to argue the technology of SE amps with you. Did you read the AX84 link I posted, or are you saying they clearly they must be misinformed? They might be if you're going off of ancient tube spec data. Tube amps have pushed tubes way beyond their intended specs. Might you bring up your questions over at SEWATT.COM? There's a host of information specifically about the topic of pushing these single end amps. Some good ones to look into for starters is the the Valve Jr. threads or "The Beast" project (15W or so SE amp), or the "Octal One".

 

[1955]

Really interesting.

 

[celeste]

SnorkelMonkey, the Pa-max of an EL84 is 12W. That is the absolute maximum it will take on the cycle up-swing, above which it will not go, the minimum is of course zero. If you look at graphs of these you sometimes see a power line drawn with the area to the right greyed-out: there is no point arranging the valve to go there. Hence the EL84 is usually rated at half-way i.e. 5.7W, but 7 watts is possible with some brands: Mullards were designed to exceed the specification. One can certainty push more power out by going to push-pull: 17 watts is normal for a pair of EL84 in Class-AB1, 22 watts by going to Class-AB2. Another funny thing with valves, sometimes their power output goes up as their output load is increased: it changes the bias point.

A valve may be viewed as a variable resistor. For the EL84 Ra is 4.5k, with Va at 250V (even my old WEM runs them at 300V), that's 14W when fully on, that's already exceeded Pa-max=12W and it will be red-plating. But, and here's the thing, my old WEM is push-pull Class-AB (like wot an AC15 is), so they are turned off some of the time and can cool down, and we can turn the power up even more.

Transformers: I do have an EVJh, the output transformer is seriously under-specified: it is a very cheap amp. Class-A output, whether single-ended or push-pull is very greedy on the output primary. It is possible to build a true Class-A, always-on, push-pull amplifier, they are low output, need a gigantic OT, and as a guitar amp, sound like poo (so does UL). Class-A pushes continuous D.C. current through it which magnetizes the core, so a very much bigger core is needed than for Class-AB push-pull. Plus the core requires careful design of the laminations to prevent eddy currents. Not to mention various weird and wacky harmonics. Most modern output transformers use the same core laminations as for the mains transformer: this is wrong. My Laney L5T (three times the price of the AC4) uses the output transformer from their 30W 4xEL84 LC30 amplifier, this monster is just about the right size for a single-ended Class-A EL84. Designing an output transformer is a black art, impedance is not linear with frequency, and this applies to speakers as well as transformers. Class-A is greedy on output transformers, a good one should amount to half the cost of the amplifier, and most of the weight: you can see why manufacturers may be keen to skimp on the OT..

The EVJh has multiple output taps, 16/8/4. If I correctly load the output then the 16ohm tap sounds best (pair of 8ohm speakers series or parallel), this is mainly because the 16ohm tap uses the entire secondary, the 4ohm output is 'browner'. Same applies to 30+watts Epi BC30 but for some reason that came with a pair of 16ohm speakers in parallel into its 8ohm socket: I soon fixed that.

Incidently the most efficient secondary load for a valve amp is 25ohms i.e. with output transformer to suit: BBC valve equipment traditionally used 25ohm speakers, their equipment was/is better than most recording studios.

Wow jefrs, must be past your bed time, your posts are usually technically correct, but you really missed a lot on this one. I guess transformers are not your thing.

Have you ever seen a AB2 EL84 amp? I know I have not and I really doubt you have either. Here is a hint in recognizing one, transformer or directly coupled control grids on the power tubes and a power tube as a driver. even the Fender PS300 with a 6V6 as a driver is just barely able to qualify as subscript 2 operation. The PS 400 with its 6L6 driver is likely truly AB2. You can not run subscript 2 with cap coupling.

Class of operation and core size are unrelated, A PP OT core has no magnet flux at idle if the tubes are balanced properly. Yes there may be a heavy DC current in classA, but both tubes are drawing the same amount, and the flux that that current induces cancel out because it is in opposite directions.

 

[jefrs]

Celeste, suggest you re-read my post, we seem to be in violent agreement on most points.

Core size and primary impedance are of importance for Class-A operation, particularly so for single-ended use. The problems associated are negated by using Class-AB push-pull output.

Suggested reading is Morgan Jones, Valve Amplifiers ISBN 978-0-7506-5694-8 Chapter 6, The Power Amplifier.

esp. Transformer Imperfection pp381-383, "... Unfortunately we pass a constant magnetizing current through the primary of the output transformer. In order for the core not to saturate, which would cause odd harmonic distortion, we need a large gapped one. ...which results in a physically large transformer... expensive and have compromised bandwidth in comparison with a push-pull transformer" - the core problem does not arise with a PP OT, yet Class-A still needs lower primary impedance than Class-AB, which in itself results in a large transformer.

Class-AB2 is fairly easy to achieve, this is where grid current is allowed to flow, increased efficiency at loss of linearity. Some of the old 4xEL84 AC30 were brutally modified to well over 50 watts by running Class-AB2 (the AC30/6 was 46 watts anyway). Class-AB1 is where grid current is not allowed to flow. Class-AB is there the valve transits from Class-A to Class-B. Class-A the valve is always conducting, Class-B the valve shuts-off.

Class-B and indeed Class-C are used for instrumentation and transmitters, and yes I have seen and used them, saving that the EL84 is really not a good choice of valve for the purpose.

 

[celeste]

Celeste, suggest you re-read my post, we seem to be in violent agreement on most points.

Core size and primary impedance are of importance for Class-A operation, particularly so for single-ended use. The problems associated are negated by using Class-AB push-pull output.

Then perhaps It was just poorly worded, because I have reread your post several times and it appears you are suggesting that a class A PP OT needs to be larger then one for class AB.

......
Class-AB2 is fairly easy to achieve, this is where grid current is allowed to flow, increased efficiency at loss of linearity. Some of the old 4xEL84 AC30 were brutally modified to well over 50 watts by running Class-AB2 (the AC30/6 was 46 watts anyway). Class-AB1 is where grid current is not allowed to flow. Class-AB is there the valve transits from Class-A to Class-B. Class-A the valve is always conducting, Class-B the valve shuts-off.

Class-B and indeed Class-C are used for instrumentation and transmitters, and yes I have seen and used them, saving that the EL84 is really not a good choice of valve for the purpose.

I strongly disagree that AB-2 is easy to achieve. Subscript 2 is not determined by grid current, that starts to flow well before the Vgk=0, but by the ability to drive the control grid positive WRT the cathode for a sustained period of time. My own designs tell me that to do so is a real Challenger, and will melt many control grids. At this point the only grids I trust are in 807's and 6N6P's

 

[SnorkelMonkey]

Based on what you're saying I should be able to hook up my external 16 Ohm cab in parallel with the internal 16 Ohm speaker and run them both? I realize I'd have to re-wire the jack, but that would be seriously cool!

I didn't even think of that. Yes. And yes you would need to rewire the jack to run the two cabs in parallel.

(For those that don't understand the question, two 16 ohm cabs in parallel equal 8 ohms on the secondary. That will allow you to use the stock speaker and a external 16 ohm cab but the external speaker jack will need to be rewired)

 

[soulpatch]

Gee, then how about this?

My daughter as the 8'", I have the 10".
How about rewiring the jacks to run them together as parallel speakers? How the heck would you do that? Is it even feasible or just plain dumbass?
Might sound kind of cool with the two different speakers. My 10" has a Weber blue alnico in it and hers has the stock Celestion.
Would that make it, in effect, an 8 ohm cabinet with the increased wattage?
Remember that old adage "there's no such thing as a stupid question"?.......I never quite believed that, myself......
Anyway, what say you?

 

[T Prior]

although I am not totally up to par on tubes and specs, it's been years since I pulled out a tube book and read it ( yawn)

here is the spec sheet for the EL84/6BQ5 in a single tube scenario , I am familiar with the concept of more gain but the I believe Vox knew exactly what they were doing here, which is securing the single ended 4.3 watts or so, maybe a couple more for robustness and reliability. They built the amp around common tube specs , maybe made it a bit more rugged or perhaps used common parts so they would NOT have to run out and buy specific OT's for this amp alone, who knows, I know I don't ! I do know this, it's already too loud to use in a hotel room even in the 1/4 watt mode !

(link removed)

 

[Hoodster]

Okay, you guys can argue the technical details forever, but how about we hear from some people who have actually run this amplifier at 8 ohms? If you have, please report on the results. My AC4 head is currently up for sale, but if I could run it safely at 8 ohms and it sounded good (or even better), I might reconsider.

 

[Necksnapper]

I just received #2 today. I'm thinkin I'm gonna make a 1X12 or maybe 2X12 cab and mount the amp in it. Sooo... do I make this 8 ohms for more power and don't use the attenuator? I'll be watchin this thread. Another thing, the 1st one I got is real growly and dirty and breaks up at less than 1/2 Volume. The one that came today isn't nearly as dirty. It doesn't break up until at least 3/4 volume. It's a whole different vibe. That surprised me. Which amp do I mod? More clean, More dirt.( As I type this, I look at the top of the screen and see the answer. Thanks Hoodster.)

 

[dqami]

Ok, so I hooked my Ac4tv8 up to a 4x12 cab. Jeeze! It's like a totally different amp!. Huge volume increase and all clean tone. To get some dirt now it is too loud (past 10 o'clock) so a pedal would be recommended. I was playing through a hot single coil (melody maker) so an HB guitar may get dirtier earlier. If I could change the setting to 1 watt I'm sure it would have been ideal but I didn't want to fry anything.

 

[l2yz]

Anyone try running an 8ohm extension cab with the ac4tv at 4w setting yet?