Converting 36w to a Vox AC30?

JohnnyCrash · Post by **JohnnyCrash** » Fri 03/28/08 3:17 pm

I've got several 18w variants, several fender tweed builds, and a few BF style Fender builds.

What I aint got is a Vox.

I'm thinking about converting my 36w TMB *editted by poster* to a Vox AC30 - 12AX7 Top Boost channel with MV perhaps.

Will the GDS/Heyboer PT and OT work for a Vox AC30? I'm mainly concerned about the PT/voltages/current and any schematics/layouts...

Think it's possible?

I'm planning on keeping the same board (*editted by poster*), chassis, and all of that - just rewiring the board for a AC30.

JohnnyCrash · Post by **JohnnyCrash** » Fri 03/28/08 3:31 pm

1. I'm guessing the OT is fine, right?

2. The PT I need to find specs on (having difficulty finding AC30TB voltage charts). I need to find and compare GDS' Heyboer 36w PTs' specs as well.

3. Having difficulty understanding the difference between a AC30 reissue and vintage style choke - one appears to be 5H (Hammond #194D) the other 30H (Hammond #194E)...

The guts/board is another matter, one not really suitable for this board (it would be off-topic), but I've got that handled - I just need specs/help with the 36w PT/OT and voltages for the AC30.

Franz1987x · Post by **Franz1987x** » Fri 03/28/08 3:56 pm

Hello JohnnyCrash,

The Choke is a 30H on AC30TB, the OT is 4K,CT on primary and secondaries at 8 and 16 Ohms.
PT is an Hammond 290MX with HT at 275V-CT-275V with 5V winding for GZ34.
But I have An AC30 TB6 reissue, i could check some voltages if you really need it.

Post by **zaphod_phil** » Fri 03/28/08 6:14 pm

JohnnyCrash wrote:I'm thinking about converting my 36w TMB (Mojo smallbox cab, Ceriatone chassis and faceplates) to a Vox AC30...

JohnnyCrash · Post by **JohnnyCrash** » Sat 03/29/08 11:12 am

zaphod_phil wrote:
JohnnyCrash wrote:I'm thinking about converting my 36w TMB *editted by poster* to a Vox AC30...

Ah, is that a "no no" to mentioning brand names or to the conversion in general?

I can edit brands out of my post if you'd like.

If it is about posting about a non-18w amp, I really think most of this is relevant and I am discussing 36w GDS/Heyboer PT/OT specs - the actual AC30 conversion and build I will refrain from posting anything about.

Let me know if I bent some rules and I will modify/edit my posts as appropriate.

Thanks for the AC30 specs, now GDS/Heyboer 36w specs would be helpful

JohnnyCrash · Post by **JohnnyCrash** » Sat 03/29/08 11:31 am

OK, I editted my posts to remove any other brand name issues. Sorry

Franz1987x wrote:Hello JohnnyCrash,

The Choke is a 30H on AC30TB, the OT is 4K,CT on primary and secondaries at 8 and 16 Ohms.
PT is an Hammond 290MX with HT at 275V-CT-275V with 5V winding for GZ34.
But I have An AC30 TB6 reissue, i could check some voltages if you really need it.

Thanks!!

No, the voltages I can figure out. I want to try to keep 18w off-topic posts down, in case. I just need info about a specific 18w vendor (and member), and a specific 36w PT.

The GDS FAQs look like they post no specs for the 36w PT. Of course the OT should be fine (30 watt-ish, 4k impedance).

I am realizing the choke may depend on the AC30 circuit I end up using.

So now, I am off to look at circuits, get the right choke, and figure out if the Heyboer/GDS 36w PT will cut it after seeing what the circuit will need.

Phil, if I bent any rules let me know... I apologize in advance. If you're just appaled at my "ruining" a 36w... don't be, I have three 18w builds to forgive this "sin" HAHA!

The 36w is killer - especially after the mods I made to it... I just don't have a Vox in my stable and another build is too expensive.

Post by **rjgtr** » Sun 03/30/08 9:51 pm

Why don't you consider adding a Post Phase inverter master volume? This will let you dial back the gain into the power amp, which will get you closer to a Vox sound with the PPIMV dialed back.

If that isn't enough you could rebuild the TMB channel to be like the Vox Top boost channel.

what_wires · Post by **what_wires** » Sun 03/30/08 10:55 pm

I have some VOX iron. I am soooo suprised how large it is.

guitarmike2107 · Post by **guitarmike2107** » Mon 03/31/08 8:58 am

I think you will find both the PT and OT on the Vox are bigger, I was reading the Torres book this weekend and he had a thing or two to say about changing an amp to class A and how the continues current draw requires both the trannies to be bigger. I would say best to do some reading before going ahead with this

JohnnyCrash · Post by **JohnnyCrash** » Mon 03/31/08 9:46 am

guitarmike2107 wrote:I think you will find both the PT and OT on the Vox are bigger, I was reading the Torres book this weekend and he had a thing or two to say about changing an amp to class A and how the continues current draw requires both the trannies to be bigger. I would say best to do some reading before going ahead with this

The Vox AC30 is NOT a Class A amp.

http://www.aikenamps.com/VoxAC30classA_2.html

I would imagine if it really was, the OT would of course be very different than the 4k Vox OT... even more so if it were Single Ended running four EL84's parallel as opposed to two pairs Push/Pull would make for more than a few differences in that regard.

As far as the PT, Class A or otherwise, the voltage and current in the 36w PT were my only concern... makes no sense why a Class A PT would be that much larger.

As with most everything Torres, you have to take anything he says or writes with a grain of salt large enough to choke a horse.

Post by **zaphod_phil** » Mon 03/31/08 10:39 am

JohnnyCrash wrote:Ah, is that a "no no" to mentioning brand names or to the conversion in general? Let me know if I bent some rules and I will modify/edit my posts as appropriate.

Sorry for not replying. I somehow missed your posts. Well, of course what you're suggesting is sheer heresy and bordering on being OT for this board.

JohnnyCrash wrote: If you're just appalled at my "ruining" a 36w... don't be, I have three 18w builds to forgive this "sin" HAHA!

Whew!

Well, V*xes aren't my favorite amps anyway. However, you could convert a 36W power stage to a Vox one, basically by reducing the grid leaks to 220k and putting a choke in to feed the screens. Maybe you could even put them on some kind of switches, so that you could get both AC30 and 36W tones. Or rjgtr's suggestion to use an MV control to give you the full range of V*x to Marshall dirt levels.

The other thing of note is the 1.2k cathode resistor in the typical V*x PI, which gives somewhat fizzy distortion at certain gain/volume levels. I know I'm not the only person who finds this. Doug Hammond, for instance, changed the PI cathode resistor of his Voxy amp to 750 ohms IIRC to get rid of the fizz. One of Ken Fischer's Rocket schematics (drawn by Mark Abbot) also shows a pair of 1.5k resistors in parallel on the PI cathodes. So I guess he was de-fizzing that one as well.

You don't hear the PI fizziness in Brian May's tone, as he's running his AC30s dimed, so that the power amp distortion dominates. Come to think of it, the way he uses his AC30s, they're operating closer to Marshall 18W/36Ws, since he's only using the Normal channel and cranked right up.

JohnnyCrash wrote:As with most everything Torres, you have to take anything he says or writes with a grain of salt large enough to choke a horse.

pjb · Post by **pjb** » Mon 03/31/08 10:59 am

what_wires wrote:I have some VOX iron. I am soooo suprised how large it is.

Hey!!!... I resemble that remark

I have another set sitting on my work desk right now!!!!
Have you bailed on the idea?
-Paul

guitarmike2107 · Post by **guitarmike2107** » Mon 03/31/08 11:02 am

Spanked

worst thing i had read that article on the weekend!

JohnnyCrash · Post by **JohnnyCrash** » Mon 03/31/08 1:33 pm

guitarmike2107 wrote:Spanked worst thing i had read that article on the weekend!

Ah no, man I didn't mean to "spank" anyone's statements.

I woke up this morning and my water heater had broken AGAIN. Freezing cold showers = fun (wait, maybe it = no fun HAHA). I was in a hurry and slightly grumpy, sorry

JohnnyCrash · Post by **JohnnyCrash** » Mon 03/31/08 1:46 pm

zaphod_phil wrote:
JohnnyCrash wrote:Ah, is that a "no no" to mentioning brand names or to the conversion in general? Let me know if I bent some rules and I will modify/edit my posts as appropriate.
Sorry for not replying. I somehow missed your posts. Well, of course what you're suggesting is sheer heresy and bordering on being OT for this board.

HAHAHA!

I know. I should've said "AC30" and then immediately ducked for cover!

zaphod_phil wrote:
JohnnyCrash wrote: If you're just appalled at my "ruining" a 36w... don't be, I have three 18w builds to forgive this "sin" HAHA!
Whew!

Well, V*xes aren't my favorite amps anyway. However, you could convert a 36W power stage to a Vox one, basically by reducing the grid leaks to 220k and putting a choke in to feed the screens. Maybe you could even put them on some kind of switches, so that you could get both AC30 and 36W tones. Or rjgtr's suggestion to use an MV control to give you the full range of V*x to Marshall dirt levels.

rjgtr and you are always pragmatic when it comes to solutions... which is great because it keeps me conservative enough to avoid my usual habit of doing things the harder and more roundabout way.

I'm definitely putting in a MV

I need some schooling on chokes... there are two different ones used depending on schematic variation. I know enough about chokes to be dangerous.

Any helpfull tech info/links on chokes?

zaphod_phil wrote:The other thing of note is the 1.2k cathode resistor in the typical V*x PI, which gives somewhat fizzy distortion at certain gain/volume levels. I know I'm not the only person who finds this. Doug Hammond, for instance, changed the PI cathode resistor of his Voxy amp to 750 ohms IIRC to get rid of the fizz. One of Ken Fischer's Rocket schematics (drawn by Mark Abbot) also shows a pair of 1.5k resistors in parallel on the PI cathodes. So I guess he was de-fizzing that one as well.

I think I'm going to start with typical V@x values first.

There is so much hype about the AC30 and I've never experienced one before. I'll go about tweaking it after I've given the stock circuit a few weeks of my Les Paul and Telecaster... if only I owned a Ric

zaphod_phil wrote:You don't hear the PI fizziness in Brian May's tone, as he's running his AC30s dimed, so that the power amp distortion dominates. Come to think of it, the way he uses his AC30s, they're operating closer to Marshall 18W/36Ws, since he's only using the Normal channel and cranked right up. :lol:

Brian May is one of the big reasons I decided to gut my "extra 18w" head, 36w...

Actually, Tom Petty and Mike Campbell as well as U2's Edge are even bigger reasons for me. The tonal spectrum (clean jangle, to May's dirty and saturated) of these guys alone is enough for me to at least explore an AC30.

Who knows, I might go back to a stock 36w after this experiment

Hoffman's 18w Stout circuit is still making me smile bigtime - as is my main rehearsal amp, my 6V6 modded 18w TMB build. My 6V6 18w may be getting a Fixed Bias mod/switch soon too... I need to order a bias trim pot for that project though

When I do that, I'm sure I'll post a thread with some photos.

Alexo · Post by **Alexo** » Mon 03/31/08 2:45 pm

Just curious - do you own any Blue Celestions? I've never owned an AC30 either, but according to many, and I'm inclined to believe them, those speakers are a huge part of the AC30 sound, might be worth checking out if you really want to see what that amp is all about.

JohnnyCrash · Post by **JohnnyCrash** » Mon 03/31/08 3:41 pm

Alexo wrote:Just curious - do you own any Blue Celestions? I've never owned an AC30 either, but according to many, and I'm inclined to believe them, those speakers are a huge part of the AC30 sound, might be worth checking out if you really want to see what that amp is all about.

This has been on my mind a great deal as well.

I'd love a pair of Blues... need to find the cash for that though - they are dang expensive!

Wayne_Alexander · Post by **Wayne_Alexander** » Mon 03/31/08 9:43 pm

As many of you know, I usually play through AC30s and I've had a bunch of them. Well-broken in Celestion Blues (or Golds, the same voice) are absolutely essential for the "classic" voice of that amp. It's a large percentage of the sound (50%?). The other elements are the tone stack, the lack of negative feedback, cathode bias, running EL84s hot, the phase inverter design, etc. Johnny, the type of master volume that sounds best on an AC30 is a simple "crossline" one that just connects one side of the grid wires coming out of the phase inverter to the other through a 500K pot. The same as the "cut" control on an AC30, but with no cap. You need both.

Post by **zaphod_phil** » Tue 04/01/08 4:10 am

All kidding etc aside, if we're going to be getting into the details of how to build an AC30, then this thread will need to continue on ppwatt.com "Da rulez is da rulez" I'm afraid... even if we can forgive Johnny because of the number of 18Ws he has...

JohnnyCrash · Post by **JohnnyCrash** » Tue 04/01/08 10:17 am

zaphod_phil wrote:All kidding etc aside, if we're going to be getting into the details of how to build an AC30, then this thread will need to continue on ppwatt.com "Da rulez is da rulez" I'm afraid... even if we can forgive Johnny because of the number of 18Ws he has...

I have all of the info I needed about the 36w trannies. As far as the AC30, I am all good on my own... we can talk about something else now

Donations

Links

Welcome

Converting 36w to a Vox AC30?

Converting 36w to a Vox AC30?

Re: Converting 36w to a Vox AC30?

Re: Converting 36w to a Vox AC30?