Need help with Vox AC4 top boost ideas

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a_whichello
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Need help with Vox AC4 top boost ideas

Post by a_whichello »

I have an old radiogram based around an EL80 powertube, and after looking at the power tranny and output tranny it seems that it would be quite good for a SE EL84 build.

So owning a couple of fender Deluxe reverbs (mkII and 65 reissue) I wanted something different and thought Vox. So using the classic AC4 power amp with the classic AC30 TOP BOOST seems to be my idea, because I've always loved the top boost sound. The only thing I cant decide on is whether in order to compensate for the lack of phase inverter boost, I need to add a tone stack recovery (plus it would give me a use for the other half of the first 12ax7).

My first schematic is just knitting the two together (there should be a 220k resistor on the way into the power amp that I forgot to add).

The second schematic is actually an adaptation of a jfet based topboost emulator - I was running on the assumption that considering the rest of the circuit was almost identical that the tone stack recovery would also be. PLEASE correct me if I am wrong.
** I feel I should also add that it is very similar to the recovery that they use in the AC4TV
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a_whichello
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Re: Need help with Vox AC4 top boost ideas

Post by a_whichello »

I just logged on with another computer - can anyone else even see the attachments? Because I cant on other computers

Here are some photobucket links for those that cant see them

[IMG:1024:504]http://i1189.photobucket.com/albums/z43 ... pboost.png[/img]

[IMG:1022:385]http://i1189.photobucket.com/albums/z43 ... ecover.jpg[/img]
Last edited by a_whichello on Wed Sep 26, 2012 12:03 am, edited 1 time in total.
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Colossal
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Re: Need help with Vox AC4 top boost ideas

Post by Colossal »

If you cannot see pictures in your thread, then you are probably visiting AmpGarage as a guest and not actually logged in as a user. I can see the schematics in both your threads.
a_whichello
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Re: Need help with Vox AC4 top boost ideas

Post by a_whichello »

Ok thanks for that clarification I wasn't aware that thats how this forum operated. I'll edit out the 3rd set of images.


So does no one have any ideas on the design?

I have another question actually in the original topboost design they have a cap (32uf) hanging off the B+ going to the preamp - this seems pretty common on a great deal of amps and I'm assuming it's to smooth out the voltage going into the preamp after it was knocked down by a power resistor?

I know that these seem like beginner questions, and to be fair I am, but I like to discuss things with people in the know and this forum has always seemed to have a great deal of knowledge.
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Colossal
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Re: Need help with Vox AC4 top boost ideas

Post by Colossal »

a_whichello wrote:So does no one have any ideas on the design?
I like the design and think it's a good start. A mini Vox mated to a Celestion Blue would be killer. Done right, you'd get the chime and grind in one package. To the design I might add:

1) More filtering. It's a Single Ended amp and you will need better decoupling. Not necessarily higher capacitance, add another B+ cap to filter the preamp. Too many triodes operating off one node can result in sympathetic behavior between stages and it might oscillate or motorboat.

2) Add a grid resistor on the output tube. 1k5 to 10k is typical.

3) Consider choking the whole amp. The whole amp won't draw much current so a choke to handle it won't be expensive. This will dramatically reduce the ripple which will get the SE hum down significantly.

4) You might need to reduce the size of the bypass caps on V1a and V1b. 25uF might be too woofy and overload the next stages. It won't take much to drive that EL84 into the zone. 0.68uF - 2.2uF (or maybe 4.7uF) should be fine. Do not worry about not having enough bass. It will be plenty of low end. You want clarity and punch not woof, blat, and mud.

5) If you go with adding the 1/2 phase inverter, you show the 220k in series with 50-something on the input (not sure what value that is meant to be). Consider adding another 220k to ground to create a voltage divider to simulate the load of the 'other' channel and bleed some signal going to the fake PI. There is going to be loads of gain, no need to overdo it.

6) The 220k grid leak on the output could be made into a 250kA pot for a master volume if desired. You might find that 100k is plenty of drive for the EL84.

7) Consider changing the 220k plate load on the fake 1/2- PI to 47k (more headroom). You don't need to crush the EL84, it takes very little to drive it into ugly blocking distortion (add that grid resistor).

8 ) Consider 7-9k primary for an OT!

9) Use a HIGH quality 1M metal film resistor on the input (1-3W). Any noise going in gets louder.

10) Consider adjustable cathode bias on the EL84 for more flexibility.

Just some ideas. I'm quite curious about this one myself! I hope you build it! I wish I had time right now as I would. I have a lonely Celestion Blue that needs a buddy :cry:
a_whichello
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Re: Need help with Vox AC4 top boost ideas

Post by a_whichello »

WOW thats exactly the kind of feed back I wanted/needed. Thank you!

The master volume does sound like a good idea for cutting down some of the preamp gain and getting some cleaner tones.

I really wasn't sure about how much gain the el84 would need as the original used an EF86 (not a tube I'm too familiar with) but I had read that it was low gain and unsuitable for the kind of crunch I'd like to have. I'd also read that they are notoriously monophonic and I really didn't want that kind of headache.

If I didnt go for the fake PI I could do the matchless thing and use both stages of the 1st 12ax7 for the first gain stage - like on the lightning. But I'm worried about making it sound unlike a Vox.

Do you have any thoughts about what type of caps to use and where? its another point where I'm a little lost. I think Vox used Mallory instead of the American favoritism for Orangedrop.

I will definitely be building this but its going to take a couple of months and I'm still in the design stage.

I notice that in bigger amps they add filtering caps to almost every valve of the preamp before sending it to the Anode - would this be a good idea say adding 2 more 16uf caps to ground on the B+ before the 2nd anode resistors of each stage?

I think that you are right and that a Celestion Blue would be killer but there's no way I'll be able to afford it in this amp - maybe an upgrade at a later date though.
rock_mumbles
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Re: Need help with Vox AC4 top boost ideas

Post by rock_mumbles »

http://www.rh-tech.org/public/Rock_SE_6V6_RevA_1.pdf

Here's a Plexi-SE amp design that you could use for some guidelines ...
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Re: Need help with Vox AC4 top boost ideas

Post by surfsup »

I notice that in bigger amps they add filtering caps to almost every valve of the preamp before sending it to the Anode - would this be a good idea say adding 2 more 16uf caps to ground on the B+ before the 2nd anode resistors of each stage?

Yea but you'll probably want to put a dropping resistor before each new cap to separate the nodes.
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Re: Need help with Vox AC4 top boost ideas

Post by a_whichello »

Ok so I've been tinkering with the design based off Colossal's suggestions and looking at the Vox AC4TV schematic. There is:
1) More filtering
2) a 5H 90ma choke
3) grid resistor on EL84
4) Master Volume
5) Reduced Bypass Caps

So thoughts on the improved schematic? is it improved?
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surfsup
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Re: Need help with Vox AC4 top boost ideas

Post by surfsup »

1k/16u filters will get you reduced factor of ripple:

F=753x1000Rx16u+1=13

the decibel equivalent would be:

20*log(13)=22.3dBv

Stacking those two RC filters like that should let you use a 25u cap no problem. I did a similar thing on one of my SE EL34 amps (two 25u on the first two stages) and there is zero hum. EDIT, I used 33u not 16u on the two stages as well...

I also used a mosfet for a CF instead of half a tube which allowed me to use that triode for other stuff...
Last edited by surfsup on Wed Sep 26, 2012 12:34 pm, edited 1 time in total.
a_whichello
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Re: Need help with Vox AC4 top boost ideas

Post by a_whichello »

Sorry Surfsup, if you hadn't already guessed I'm very new at this.

So what you're suggesting is upping the value of the preamp filter caps to 25uF.

Would there be any advantage in upping the value of all of them? I only used 16uf because the original with a valve rectifier used them - and in an effort to try to keep things Vox-y I didn't want to change too much; or more importantly I didn't want it to sound un-Vox-y.
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Re: Need help with Vox AC4 top boost ideas

Post by surfsup »

Ignore what I said about the 25u...and I should state I am not an expert either. These other guys like colossal are way more knowledgeable.
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Colossal
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Re: Need help with Vox AC4 top boost ideas

Post by Colossal »

a_whichello wrote:Would there be any advantage in upping the value of all of them? I only used 16uf because the original with a valve rectifier used them - and in an effort to try to keep things Vox-y I didn't want to change too much; or more importantly I didn't want it to sound un-Vox-y.
As Surfsup showed mathematically, upping the capacitance straight across the board will have the advantage of reducing the decoupling frequency way down and this is typical of SE amps where you want to get the most hum-free conditions you can. But that may or may not be what your ear likes. Just substitute larger cap values into the equations Surfsup provided to see the effects.

That added CLC filter (cap-inductor/choke-cap) in front of everything will help significantly though so you might not need (or want) to increase the preamp caps. You might find that you like the sound of 16uF in the preamp stages but you could certainly go up. The Voxyness will certainly come from the circuit itself, the EL84 and its unique tonal qualities, as well as getting the voltages in the preamp in the right spot.

Personally, I would add a third B+ so that your V1a input stage and maybe V1b is isolated and not shared with anything else. If you don't want to use a series string, you could consider parallel RC filtering. With parallel, you can greatly increase the dropping resistor value while decreasing the filter cap value to get to the same end frequency-wise. This might give a little more freedom with respect to getting the exact voltages you desire for each stage.

Oh, if a Celestion Blue is out of reach financially at the moment, a Greenback is a great alternative and will still bring the magic.
a_whichello
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Re: Need help with Vox AC4 top boost ideas

Post by a_whichello »

I've had a crack at a layout...

Any glaring errors? (be kind I've never done one before)
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Re: Need help with Vox AC4 top boost ideas

Post by crbowman »

a_whichello wrote:
I really wasn't sure about how much gain the el84 would need as the original used an EF86 (not a tube I'm too familiar with) but I had read that it was low gain and unsuitable for the kind of crunch I'd like to have. I'd also read that they are notoriously monophonic and I really didn't want that kind of headache.
Sorry if I'm late on this but.....
I did a Vox AC15 type head using an EF86 and it absolutely crunches like nobodies business!
Don't know where you heard that it was low gain, so long as you use the Vox style trim control and don't try to use a standard tone stack the EF86 will give you LOTS of gain and will sound very Voxy. They CAN be microphonic so some tube dampers/ tube rolling might be required, but honestly I haven't had any trouble with that. The other guitarist in my band has basically made off with my AC15/EF86 head and given notice that I'm not getting it back anytime soon.
<i> "I've suffered for my music. Now it's your turn."</i>
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