Some questions on a mid60s Vox AC50

[Coschi]

Hey,

I just scored a Vox AC-50 II, mid 60s with matching 212 cab

Only thing is, to get the great distortion sound it has to be far too loud. Besides a power attenuator on the speakers, I was thinking of either:

Adding a master volume control or
Changing to a smaller tube (currently EL34)


In my Fender blues deville i changed the 2 6L6s to 6V6s; this works great but it was easy because the operating voltages are mostly the same and the pinout and socket type are identical. This isnt' the case for EL34s switching to EL84s

So, what would you suggest?
I'd like to change to EL84s but this would require a major revamp on a classic amp, maybe not the best option?

Personally, I'd rather not add a master volume because I want power tube saturation, not just preamp overdrive


Also, this model has the solid state rectifier

Could I simply replace this with a GZ34 valve rectifier?

[olddawg]

Have you considered putting in a set of Yellow Jackets or similar devices?

[Robert M. Martinelli]

Hi,
well, if I was you I wouldn't touch your AC50, all the more so if it's completely original, it's getting more and more difficult to find originals in pristine conditions, it would be a pity to do anything exceeding normal maintenance, all the more so because the value of Vox vintage amps is increasing by the hour; modding/tweaking the amp would greatly decrease its value....

Having said that, the kind of sound you' re after requires extensive modifications, as you've already noted, EL84s aren't a choice, due to their much lower ratings and the different sockets, you could try to switch to 6V6s, which have a similar pinout, but you' ll need to lower the +HT voltage from 470 to, say, 350-380 VDC, re-bias the thing accordingly and, very likely, do some tweaking on the PI section as well.

Also, you have no chance of simply dropping in a tube rectifier ( 5AR4/GZ34 ) as this tube needs a separate 5VAC secondary ( your AC50, being probably a MK3 issue, lacks this secondary, having a SS rectifier ) with a current availability of 2 Amps for its heater ( it has to be separate mainly because the cathode is at a very high potential and one of the heater's terminal has to be put at cathode potential - otherwise the tube would arc - and also because of the different heater's voltage ), if you'd like the amp to "sag" at high volumes ( much like a tube rectifier would do due to its higher differential resistance ) you could try a Weber "copper cap", which it's a solid state device designed to simulate the "sag" of old tube rectifiers. I' ve never tried it personally, but I' ve read it does the job pretty well.

As to distortion, simply adding a MV wouldn't help, you need more gain to get distortion, and this would mean either extensively modify the existing preamp's stages ( with the risk of losing the amp's original sound ) or adding a gain stage with a channel switching circuit ( you should verify the 6,3 VAC secondary current capability to see if it can take the extra 0,3 Amp for the extra heater ).

All in all, given the above "sentimental" and "technical" reasons, I think you' d better leave the amp the way it is and rather spend your money on some good quality external effects ( fuzz or distortion boxes ) to get the distorted sound you' re after - BTW that's what I do with my 1964 AC30TB - and the results might even surprise you ( I play everything from metal to fusion and even blues with a rather simple setup, and I' m very satisfied with the sounds I'm getting ).

Hope this helps

Best regards

Bob

[soundmasterg]

Yah, I agree. Leave the nice vintage AC50 alone and either use pedals or get a different amp. The AC50 was known as a somewhat clean amp as compared to Fenders and Marshalls of the time. You definitly can't run EL84's in it, and anything else is going to change the sound for the worse. Use it for what it is or use pedals with it, or sell it/get something else more suited for what you are looking for.

Greg

[Coschi]

In fact I was thinking the same thing

I had a good jam with a few buddies the other day and most definitely it has to be at an unusably high volume to bring out the amps natural distortion, but I wouldn't dare change a thing at risk of losing the tone of it. I've build several valve preamps, and I know exactly the situation; change this, like it, go for a little more, tweak it, 10 changes later you hate the sound (the next day) and wouldn't have a clue how to get back.

In all honesty, modding it slightly wouldn't bother me as I never intend on selling it. It's more the risk of losing the tone that deters me now

In fact, I don't know what it is about this baby but it's so well balanced it's not funny. We play a range of styles from latin funk to acid drenched psychedelic jazz and rock, and no matter what, every note shimmers, can always be heard beautifully like it reserves a space in the room mix that is always dedicated vox. It never impacts on the bass or keys to create muddiness and it has an strange ability to allow perfectly clean notes to sustain via resonating with other tones in the room

Well that's enough of that

On that note, any suggestions on a distortion pedal or some?
I'm wanting the sound of a tube amp in power saturation but creamy, like the vox rather than the fender style break up (too crunchy not in a good way). And the ability to get heavy but not Metallica style (well, maybe a little), but perhaps more like At The Drive In.. I've got some research to do it seems

So far i've gotten The Rat, TS9, Timmy, Sunface

[Robert M. Martinelli]

For a warm, smooth "English" overdrive you might want to try a Colorsound Route 66, for a truly distorted sound try to get an old MXR Distortion+ or a Ross Distortion ( tan ), these last ones are also very easy to tweak so you can experiment with different sets of clipping diodes or "play" with gain. I have built myself a Distortion+ clone using a NE5534, metal oxide resistors ( to get a lower noise ) and three switchable sets of clipping diodes at the output ( red LEDs, Germanium, Silicon ) to shape the output waveform, and the thing works pretty well ( I also added an external gain switch ).

[TD_Madden]

I've got a '66 AC50 that kicks major ass...you could aways rebias the amp lower to get a sooner breakup.

By the way, which 2x12 does it have..the "bigbox" that's about 31 inches wide(and has plastic corners) or the smaller 27-incher without the corners? Does it still have the "midax" horn?

[Coschi]

I've never played around with a variable bias before, only fixed bias amps by calculation rather than measuring voltage. Something about measuring the VDC at the grid with the inputs to ground?

What would I be aiming for to achieve the desired effect?

It's this exact amp:



Plastic corners, no horn unfortunately, and it has reissue celestion greenbacks in it


Thanks Voxrules! i'll check out the Colorsound Route 66

[Robert M. Martinelli]

I've never played around with a variable bias before, only fixed bias amps by calculation rather than measuring voltage.
Something about measuring the VDC at the grid with the inputs to ground?

What would I be aiming for to achieve the desired effect?

well, both fixed and variable bias have the same purpose, which is to make the grid negative with respect to the cathode, so it's all about thinking to the cathode as the reference point....

Rather than biasing the tubes colder (i.e. with a "more negative" voltage ) you could try to lower the +HT voltage, this should result in the amp starting to distort earlier, and probably this distortion will be more pleasant to the ear than the one obtained adopting a colder bias ( it should have more even harmonics, the other method enhances odd harmonics ). If memory serves me well Eddie Van Halen used this kind of distortion on some early VH records.

If you need help with variable bias amps' biasing procedures, let us know....

Hope this helps
Best regards
Bob

[TD_Madden]

I forgot to mention that I use a Weber Browner to bring the wall-voltage down to 115...that also lines-up the filament voltage to 6.3v.

Even THAT might help.

[soundmasterg]

The AC50 uses a combination of cathode bias and fixed bias, with a resistor to ground from the cathode (47 ohms??, can't remember offhand and can't see a schemo right now) and a negative grid voltage. There is a specific procedure to set the bias on them, though I can't remember where I saw it. Ken Fischer used to have some info about these amps.

Greg

[Coschi]

maybe I'll just leave the bias alone
I guess what I meant was that with fixed bias you have more visibility of what you've done, and easier for record keeping to go back and forth. Since i've never really played around with variable bias I don't really want to go changing anything now at risk of losing the sound

If you google "Vox AC 50 schematic", click the i'm feeling lucky button, you'll go straight to the schematic of the amp

I wish I was more familiar with power amp design.. but i'm guessing i'd either want to lower the voltage to the centre tap of the output transformer, or lower the voltage coming into the centre grid?

Argh, nah I really don't want to go messing around too much..
although, a variable HT to the power section could be awesome..

[Robert M. Martinelli]

The AC50 uses a combination of cathode bias and fixed bias, with a resistor to ground from the cathode (47 ohms??, can't remember offhand and can't see a schemo right now) and a negative grid voltage.

Hi all,
Just a minor footnote:
AC50s used the bias network Greg described only from versions from MKII onwards, the AC50 MKI had a simple fixed bias which unfortunately raised some reliability issues, this is the reason why Dick Denney and Triumph Electronics ( which was building AC50s for vox at the time ) switched to a different biasing network on later versions.

As to the fear of losing sound messing with bias, you should not have any fear as long as you keep records of the initial voltages/conditions ( and the changes you make as well ), this way you can always go back whenever you want.

About the amp pics, plastic corners only have one pin so the amp is most likely a pre-1967 version - this can be confirmed looking at the rear model/serial number plate, if it has two lines the amp is a pre-1967.

Hope this helps
Best regards
Bob

[pdf64]

Coshi, what do you mean with fixed bias and variable bias? I understand them to be the same thing, as far as the tube is concerned, in that the tube has a bias voltage applied to the grid.
The fixed term just means that the bias voltage is set by potential divider (usually resistive), the variable term is used to describe circuits in which the bias voltage can be easily adjusted on a pot. But they're essentially the same thing.
The other type of bias is cathode bias, where the grid is held to DC ground, with the cathode current generating a bias voltage which kind of balances itself.
As soundmasterg says, the ac50 uses a combination of both. As there's a cathode resistor and a control grid bias pot on each el34, you can easily measure and record your exact settings. In fact it's one of the easiest amps ever made for doing that!
To reduce the B+ / output power, just increase the value of the high wattage sag resistor that already sits on terminals on the tube side of the chassis. Stock is 22ohms (or the dreaded brimistor - don't ask). 47ohms 10 +watts, or 100ohms 15+watts will bring the power amp saturation levels down. That resistor and terminals also make the ac50 one of the easist amps ever for getting a shock off the B+.
Peter.

[Coschi]

Thanks for the info Peter
yeah, fixed, variable, same of course, just a different approach
no more on that matter we understand eachother

I'll see if i've got a brimristor in there,
Unfortunately much of panel on the back of the amp is damaged, but i'll take a pic