I want to convert a vintage Amp into a VOX AC4.

[Avraxas]

Hello all!

Recently I got a vintage Amp with the following tubes inside:

EZ80, EL84, 2xEF86.

Of course it is in a bad shape and I am thinking as the transformers are in working order, to convert it to a VOX AC4. (Replace one EF86 with 12AX7 tube).

I find online a circuit for V1.1 but I don’t know if it is accurate.

Can I have please a circuit and a layout for the AC4 if it is possible?

In the following Link you can see the Amp:

https://www.dropbox.com/scl/fo/2xdlyhfb98abmpxlkxf17/AG_Vv6JdM5589HwQMOx45H0?rlkey=gn5irzx3120s2g244vr2eaxkx&st=qkbduk47&dl=0

Thank you!

[mresistor]

here is one layout 

[Avraxas]

That was quick! :)

[bmccowan]

You will find schematics here: https://el34world.com/charts/Schematics/files/Vox/Vox_Schematics.htm
But that chassis configuration would worry me. The exposed transformer connections on the same side of the chassis as the tubes! I would think about reconfiguring for safety.

[kagliostro]

Do a search on the forum for Vox AC4 by author Dwinstonwood

There are some interesting thread on the matter

See here for what I'm talking about

https://el34world.com/Forum/index.php?action=dlattach;topic=29599.0;attach=102175

Franco


p.s.: I've seen the photo of your amp, very nice score 

[Avraxas]

Do a search on the forum for Vox AC4 by autor Dwinstonwood

There are some interesting thread on the matter

See here for what I'm talking about

https://el34world.com/Forum/index.php?action=dlattach;topic=29599.0;attach=102175

Franco


p.s.: I've seen the photo of your amp, very nice score 

I like very much this work! I go for that I believe!

[dwinstonwood]

Thanks for posting that Franco!

I actually have updated and improved drawings. I added a small choke and now it's dead quiet.

Here's the schematic, and a PDF of all the drawings:

David

[Avraxas]

You will find schematics here: https://el34world.com/charts/Schematics/files/Vox/Vox_Schematics.htm
But that chassis configuration would worry me. The exposed transformer connections on the same side of the chassis as the tubes! I would think about reconfiguring for safety.

Hello!

Can you explain a bit further please. What you worry about. I mean what is the problem with the exposed connections? This amp have a cover so all this circuit it will be, let’s say in a box.

[Avraxas]

Thanks for posting that Franco!

I actually have updated and improved drawings. I added a small choke and now it's dead quiet.



Here's the schematic, and a PDF of all the drawings:

David

Thank you David!

Well this will be my first attempt to build an Amp from scratch and to be honest I am not experienced enough. But I want to try of course with safety steps.

I am glad your are active cause maybe I will need your help, if it’s not a problem. :)

[bmccowan]

You will find schematics here: https://el34world.com/charts/Schematics/files/Vox/Vox_Schematics.htm
But that chassis configuration would worry me. The exposed transformer connections on the same side of the chassis as the tubes! I would think about reconfiguring for safety.

Hello!

Can you explain a bit further please. What you worry about. I mean what is the problem with the exposed connections? This amp have a cover so all this circuit it will be, let’s say in a box.

I did not note a cover that would protect those tranny connections. So that helps for sure. But its my preference to be able to access the tubes without exposing high voltage connections.

[Avraxas]

You will find schematics here: https://el34world.com/charts/Schematics/files/Vox/Vox_Schematics.htm
But that chassis configuration would worry me. The exposed transformer connections on the same side of the chassis as the tubes! I would think about reconfiguring for safety.

Hello!

Can you explain a bit further please. What you worry about. I mean what is the problem with the exposed connections? This amp have a cover so all this circuit it will be, let’s say in a box.

I did not note a cover that would protect those tranny connections. So that helps for sure. But its my preference to be able to access the tubes without exposing high voltage connections.

No, it wasn't your fault. I didn't mention there is a cover. So your observation was absolutely correct!

[kagliostro]

Many tanks for the update David

in an SE the use of a choke is a good improvement

A choke or Gyrator against Humm are a must

if there is interest in a Gyrator this is a Layout I drawn following Merlin's Gyrator schematic
(not flour of my bag)




Franco

[bmccowan]

It'd be fun to try that two pentode (ef86s) circuit before you convert to the AC-4.
I have a DanElectro amp that uses multiple 6SJ7s - an octal pentode similar to an ef86. Always fun to explore what you have before you do the major surgery.

[glass54]

Small diversion here if possible 
Franco, Thank you for posting (reposting?) the Gyrator Schematic.
Do you or Merlin (if you want to chip in) have any more background/technical info.
I'm keen to try this in future projects. Looks like a great spacesaver and I have plenty IRF820/830 devices
Kind Regards
Mirek

[Avraxas]

Many tanks for the update David

in an SE the use of a choke is a good improvement

A choke or Gyrator against Humm are a must

if there is interest in a Gyrator this is a Layout I drawn following Merlin's Gyrator schematic
(not flour of my bag)




Franco

Hello kagliostro!

Just to understand please. This Merlin’s Gyrator is a replace of dwinstonwood’s choke for his AC4 Schematic in case you don’t want for some reason to put a choke? And if yes, I can add this AS-IS with the same components or need to be 4H/170Ω;

I prefer that Gyrator replacement for the only reason it is a build one and I like to building stuff.

Thank you

[kagliostro]

You can swap it as is

The principal difference between Choke and Gyrator is that Gyrator don't stock energy so it is good as noise reduction but it hasn't the benefit of storing energy

This difference will result in a more evident difference on PP amps (resulting in less SAG using a choke) but in an SE this thing has near no sense, so you can use it and obtain a low noise

About the values I think you don't here differences (if we exclude the minor noise) between the 4H choke and the 155H gyrator but it will be nice if Mr Merlin could explain any difference if present

K

[Avraxas]

I actually have updated and improved drawings.

David

Hello David.

Looking forward to make your Schematic in my EF86 amp. Looks really nice!

I have some questions to make please.

1) First about the Resistors. Some are marked 3W and 5w. All other can be 1W or better 2W?

2) About Filter Capacitors. Its ok if i put 22uf or 47uf instead of 30uf? I am asking because i have both values on F+T already from previous project.

3) About the 2x .01uf and .02uf capacitors what volt should i choose? Can you suggest a brand/type?

4)  Why you replace the speed pot with 3M one instead 1M you had in previous schematic?
 
5) Can you saw me a real photo of the work you done in the board? I love to see it!

Sorry if my questions are a bit silly (like a noob i am), but i want to move on and order the stuff i need and i am afraid don't do any mistake in parts and stuff.

Have a great day all! :)

[kagliostro]

About resistors

I usually use metal film 2W resistors (if a higher W is not required)

The official reason is because they are less noisy, the other unofficial reason is because I can see it  , also  2W resistors looks better in a vintage board, I consider 1W or less as unwatchable

Franco

[dwinstonwood]

Avraxas, I'll be glad to answer those questions when I get home this afternoon (on my way to work now).

In the meantime, I recommend browsing through Hoffman's web store here:

https://hoffmanamps.com/MyStore/catalog/partsenter.htm

David

[Avraxas]

I recommend browsing through Hoffman's web store here:

David

Take your time please. No Rush! I am just in the beginning.

Now about Hoffman's web store i will check for sure, but for me is difficult to order from the USA. Lots of shipping charges plus TAX. I need to order from the EU. My last order was from the Banzai Music GmbH. To be honest i don't know any other. 

[Avraxas]

also  2W resustor looks better in a vintage board, I consider 1W or less as unwatchable

Franco

I agree! In a previous restore i used Vishay PR Series Resistors. Great looking resistor's and vintage looking enough. ;)

[pdf64]

Their noise info isn't published. Their temperature coefficient is poor, dunno if that's indicative of the noise 
I'd save those for HT droppers etc, power applications.
https://www.vishay.com/docs/28729/pr010203.pdf

[Avraxas]

Their noise info isn't published. Their temperature coefficient is poor, dunno if that's indicative of the noise 
I'd save those for HT droppers etc, power applications.
https://www.vishay.com/docs/28729/pr010203.pdf

Well to be honest i never read the datasheets. I just read the following from sellers description:

[pdf64]

The description covers pretty much all the info from the manufacturer.

[kagliostro]

Banzai Is one, I can suggest to give a look also to Tube Town

Franco

[Kesi from Hungary]

My suggestion is also Tube Town in Germany and Tonefactory and Jukebox Revival in the Netherlands.
Good luck with Your project

[dwinstonwood]

Hey Avraxas,
I'll try to help out with your questions as best I can. I've learned the little I know from the experienced folks on this forum.

1) First about the Resistors. Some are marked 3W and 5w. All other can be 1W or better 2W?

My power tube cathode resistor is rated at 5W. That is actually overkill, as Fender used a 1W part in the Vibro-Champ. But, an overrated resistor won't hurt anything (higher-wattage resistors are said to be quieter in certain circuits).
I just always use 3W dropping resistors in the B+ string. A lot of people do. Again, Fender used 1W dropping resistors, even in the Twin Reverb.

2) About Filter Capacitors. Its ok if i put 22uf or 47uf instead of 30uf? I am asking because i have both values on F+T already from previous project.

Either should be fine. When using a tube rectifier, you don't want to exceed the datasheet limit by too much. The EZ81 datasheet states a 50uF limit for the first (reservoir) capacitor (capacitor input condition). The second capacitor - after the choke - can be a higher value, from what I understand.

3) About the 2x .01uf and .02uf capacitors what volt should i choose? Can you suggest a brand/type?

For the non-electrolytic caps like the coupling and tremolo circuit caps (yellow and orange in my layout) I just always use 600V or 630V caps. There are many brands to choose from. That's why I suggested looking at Hoffman's store, even if you buy your caps elsewhere. He stocks parts that are correct for tube amps. The Jupiter's, Dijon's, Mallory's, Sprague's, etc., that have those voltage ratings are all good. It's a personal choice thing. Coupling caps are typically connected on one side to the B+ high voltage via a plate resistor, or even directly to B+ in the case of one triode in the vibrato circuit (or with cathode followers), so they have to be rated to handle those higher voltages.

4)  Why you replace the speed pot with 3M one instead 1M you had in previous schematic?

I actually used the Vibro-Champ tremolo circuit instead of the AC4 circuit because I wanted to add the Intensity control. It worked really well:
https://el34world.com/charts/Schematics/files/Fender/Fender_vibrochamp_aa764_schematic.pdf

5) Can you saw me a real photo of the work you done in the board? I love to see it!

I don't have any pics of the final version of my AC4's chassis. I'll try to get around to taking some.  But, my amp is pretty much identical to my layout. My whole project started because a neighbor gave me a broken Fender Frontman 15 combo amp. I decided to gut it and use the cabinet to build the AC4. Because of that, I used a small Hammond aluminum chassis that fit into the Frontman cabinet. But, a Fender 6G2 Princeton chassis and cabinet would work great simply using a couple of 8 to 9-pin tube socket adapters:
https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=MiscHardware&ORDER_ID=188891520#:~:text=8%20Pin%20to%209%20Pin%20Tube%20socket%20adapter

I hope that helps!

David

[Avraxas]

Very helpful! Thank you very much David!

[Avraxas]

Hmmm, I am a bit confused! See that and I like it. Just need one EL84 more from tubes. I like very much the rotary impedance rotary switch.

Any opinions?

[kagliostro]

Note that the 18w is a PP amp not a SE so you need a PP Output Transformer

Franco

[Wonder2121]

Can you upload pics of this Vintage amp you have?
What make/model would be a start?
Have you thought about just restoring it...Seeing it's your first attempt?....You do realise high voltages can kill...You don't know the difference 3W resistor from a 0.01uF caps not a good sign to be prodding about..

[Avraxas]

Well I decided to stuck with the AC4 build as it is my first attempt to build an Amp and it is more easy.

It is a nice project too. I leave the rest I have in mind when I’ll be more experienced.

Soon I will order everything I need and the "Game” will begin!