Need help with Vox AC4 top boost ideas

[a_whichello]

That’s interesting, and it could be just complete novices like me spoiling their good rep all over the internet but as I stated in the first post I've always loved the sound of the top boost preamp, so maybe I'll have a crack at the original Vox AC4 with its ef86 channel at a later date.


(Just because this is now on page 2 I thought should re-mention I just posted a layout diagram on page 1 near the bottom for critiquing)

[crbowman]

Sorry, don't mean to confuse the issue.
I too love the AC30 top boost sound with the extra triode.
You certainly can't miss going that route.
Just be aware that the EF86 stage is a viable option for what I understand, is what you're trying to achieve.
Now I could be totally off base here, but it is my understanding that the AC30 first came out with the EF86 preamp stage. They quit using the EF86 and went to a 12AX7 based preamp because good 12AX7's were much more viable from a manufacturing standpoint, but people noticed the drop in gain. Vox then came out with the 'top-boost' module to try to recover some of the gain lost from the EF86 stage. That and to keep Hank Marvin happy.
You really can't go wrong either way as they both sound excellent.
Good Luck, and cheers,
CB

[surfsup]

(Just because this is now on page 2 I thought should re-mention I just posted a layout diagram on page 1 near the bottom for critiquing)

I don't understand your layout regarding the grounding. Commenting on what I see regarding the changes I would make (again my opinion only), the PT HV CT is not connected and should be, to the first cap. There should be a lead from there to chassis ground.

Since I have never "cap stacked", I guess the other three caps can be as you have them, to chassis ground. Every amp I built had the caps located at the preamp stage they filtered though.

for the pots I would have a ground lead from each pot's ground to the buss bar matching the following stage grid/cathode.

On v2, put the lead straight across the socked from pins 1 to 6.

I would probably elevate the heaters to 50v since you have a cathode follower.

I would probably use a fake 100R/100R Filament CT as well instead of the 6.3CT.

Its a simple amp so looks fine to me.

[a_whichello]

Sorry I should have clarified that I haven't yet sorted out all the windings on the transformer that I have so I haven't filed in the specifics of the taps yet.

So you think I should move the preamp filters to the preamp side of the board and ground them near the preamp?

What do you mean up the filament voltage to 50v? I don't really understand that whole idea - if you dont want to explain it could you point me in the right direction to read up on the idea?

So you would do away with the ground bar for the pots and instead ground them to a corresponding position on the ground bus?

[Colossal]

So you would do away with the ground bar for the pots and instead ground them to a corresponding position on the ground bus?

Yes, I would do away with a pot ground bus bar. It 'works' but it's also a possible way to introduce a ground loop IMO and there's better grounding methods. I would route your pot grounds (one for Volume and one for the Tone Stack) back to the respective nodes that they serve.

By node, I mean B+. Each section of an amp has an associated B+ filter cap. Route grounds within each of those sections back to their respective B+ filter cap negative/ground, "closing the loop".

[surfsup]

In a car...copy paste below in google:

"Heater elevation" site:ampgarage.com

[a_whichello]

I just thought I'd keep a running update so every one knows the project isnt dead -

I've decided to drop the cathode caps to 2.2uf as is seems like a more common design and I dont want to over do it with the gain.

changing the layout to facilitate the aforementioned grounding scheme

I measured the power transformer I'm going to use and its a 280-0-280 (according to the spec sheet I found for it I measured the secondaries and that seems about right after I took into account the unloaded rise). It also has a center tapped 6.3v and a 6.3 with a 5v center tap - would there be any advantage to run the heaters for the power tube and the preamp tubes separately.

I cant find any info on vox using elevated heaters in their circuits so I'm still not convinced on this idea.

[Colossal]

I cant find any info on vox using elevated heaters in their circuits so I'm still not convinced on this idea.

You won't. And there won't be any Fender or Marshall amps with elevated heaters either. It is a concept not unique to any brand but a tool to be used to improve the noise floor in an amp. If you think about what it does, it is very useful for amps with cathode followers. Modern production 12AX7s have a heater-to-cathode voltage limit of about 100V where old tubes were about 200V. So, elevating the heaters to DC (they are still referencing AC as normal) offsets the cathode voltage on a cathode follower. In modern Marshall builds for example, this is very useful since Marshall cathode followers run >100V and that can be a source of hum.

You don't have to use it, but if your PT has a center tap on the heater winding, you simply connect the heater filament center tap to the cathode bias resistor on the power tube. It's just a freebie and won't do any harm.

[a_whichello]

I'm currently looking at where to buy parts - I've priced things from antique electronic supply. Does anyone have any suggestions? baring in mind that I have to have it sent to Australia (the only reason I don't buy local is because its about +100% to +%400 on each component cost).


does anyone have any experience with the Celestion Greenback G10M? I like the idea that its a 10" and a Celestion (as at the moment I'm still thinking of this as a high class (recording quality hopefully) practice amp)

I must say again thanks for baring with me and giving me pointers ets. its been invaluable so far.

[dorrisant]

I have been doing some research about the 10" greenback you have in mind...

Seems the consensus is that it is pretty close in tone to the 12" but with slightly attenuated highs by comparison. It doesn't seem to be too bad of a thing... the guys who point out this difference also say that it can be compensated for in the tone controls easily and they don't mind it at all.

I'm building an X10 into a donor combo with an "8 in it... Gonna require a little surgery but I'm putting a greenback 10" in there for sure.

Tony

[a_whichello]

Hi Tony,
Yeah that seems to be what I've been reading/thinking from youtube videos etc.

Unfortunately for me in Australia (I dont know what its like for you guys elsewhere in the world) its really difficult to go and try out speakers so I have to pretty much research and take the plunge. I do have a little experience with the Greenback 12" in an AC15 and it seems to be pretty much what I want tone wise so if this 10" can get something like that it'll be bang on the money.

[Colossal]

I do have a little experience with the Greenback 12" in an AC15 and it seems to be pretty much what I want tone wise

Was that an old Greenback, 25W GB Reissue (made at Celestion China), or 20W GB Heritage (Made in Ipswich)?

[a_whichello]

It was some kind of new release but it had a pretty good celestion/vox tone (within my budget constraints).

out of curiosity is there a simple way to throw in an effects loop in this design? I'm a reverb fan and in a worst case scenario I can just throw it in the front end.

[Colossal]

It was some kind of new release but it had a pretty good celestion/vox tone (within my budget constraints).

So a reissue then. Good sounding speakers when they get a few hours on them. Little more sizzle in the midrange.

out of curiosity is there a simple way to throw in an effects loop in this design? I'm a reverb fan and in a worst case scenario I can just throw it in the front end.

Sure, you could easily add a passive series or parallel loop; it's just a couple of jacks between the preamp and the power tube. However, keep in mind though the implications of this in terms of the loading effect on the amp and the tone. You could borrow the reverb circuit from the Reverb Rocket design (search the Trainwreck Discussion for this) if you want to build a reverb into the amp. My personal take though would be to keep the amp as pure and simple as possible, preserving the Vox content, focus on a quiet and well filtered power supply, and add effects up front after the fact. Even better, leave the amp dead stock as designed, mic it, and add your reverb and effects in parallel through a second small amp set to clean (dry/wet).

[a_whichello]

Yeah you're probably right.

Well now I'm just waitin on a bit of cash to order the parts. So hopefully get them ordered in the next couple weeks.

Just on a previous question what shop does everyone use for parts?