Another 1st, Another Big Project - JMI Vox AC30 (x2)

[Yeatzee]

I'm back with another big project! Same as before with the Vibrolux series, this thread will serve to record my vintage AC30 journey and hopefully along the way I'll get some of that stellar advice you guys so graciously offered me in the past.

Long story short, I've been passively after an AC30 Super Twin for some time now. Reason being weight / ease of transport, cool factor, and flexibility (to use other cabs). They are not a common amp to see around, and not cheap when they do come up for sale! #1 thing for me was the matching 2x12 cab with original celestion alnico speakers. I've heard time and time again that is so much of the sound, so that was key for me.

Fast forward a bit and to my shock an AC30 ST popped up on the local CL, claiming to be all original with a couple exterior photo's and nothing of the inside. After some communication and really low res photo's we determined it was nowhere near all original so my interest waned significantly BUT if the bones were good I was still interested as I know I'll never see another local again. Jump forward a bit and eventually we agreed on a price that felt fair and due to some existing major question marks concerning the transformers if it all ended up being swapped I wouldn't be totally screwed. After hearing the brilliant channel w/ Top Boost I was sold and moved forward with the deal (despite discovering some other issues that weren't disclosed originally). So que, the first amp!



Then, while I was in talks for that amp I found another local AC30 but this time a combo. What drew me to it was the original transformers and original alnico blues with supposed original cones. If the ST was a basketcase, I wanted to have a straight one to fall back on, have something to compare it against. This one took a lot of back and forth but eventually we landed on an agreement.



So just like that I went from dreaming about a Super Twin, to owning two copper top JMI AC30's in the span of a week!

The goal is to get both up and running and healthy to compare the two, then start gigging the Super Twin! This will be my first time diving into a vintage Vox amp, I've worked on many vintage fenders now at this point but obviously vox's are a different thing so LOTS to learn. There's also a great deal of history, and determining what's straight / vintage and what's not for me to do as I dive into both. I would love for the vox experts to chime in as much as you'd like, I can tell you a lot about vintage fenders but I am very new to these amps.

I am, like I did with my Vibrolux Reverb series, going to document everything on YT the good and the bad and make it a detailed series. The first part I just uploaded, which reviews both in detail on the exterior, and then to end it I open up the Super Twin to try and determine what's what and how much work there really is going to be to get it up and running strong.

i=iFvWPfWzM09EC1jR

[pdf64]

Though the build style is different to Fender, circuit wise, AC30 are pretty close to mid 50s Fender tweeds, with a brown era harmonic vibrato added.

In this era of UK electronics gear, black cylindrical Hunts Moldseal coupling etc caps are often found.
They’re very prone to failure, so if something doesn’t work right, look for those in that area of the circuit.

VintageAC30s invariably hum, but much of that is due to the unbalanced / single ended heater circuit. Thankfully the heater wiring is well twisted and routed ok, so it’s the easiest thing to balance it with a couple of 100R resistors.
If the balance node is referenced to the output valves cathodes (rather than chassis common), yet a further source of buzzy hum will be mitigated.

[Yeatzee]

Though the build style is different to Fender, circuit wise, AC30 are pretty close to mid 50s Fender tweeds, with a brown era harmonic vibrato added.

In this era of UK electronics gear, black cylindrical Hunts Moldseal coupling etc caps are often found.
They’re very prone to failure, so if something doesn’t work right, look for those in that area of the circuit.

VintageAC30s invariably hum, but much of that is due to the unbalanced / single ended heater circuit. Thankfully the heater wiring is well twisted and routed ok, so it’s the easiest thing to balance it with a couple of 100R resistors.
If the balance node is referenced to the output valves cathodes (rather than chassis common), yet a further source of buzzy hum will be mitigated.

I'd heard of doing that mod with the 100R resistors before, but wasn't clear to me what the steps were apart from cutting the connection to chassis ground at V1 off pin 4.

Here's where I'm at now, I found a layout online and mapped mine out ontop of it and noted all of the differences. Video with all of this to follow, but I wanted to get some insights before I make an order of parts.

Anything with the teal pen mark is being replaced, anything else is a note for example of how my amp differs from the schematic (original solder / QC paint intact).


Direct URL if it's not big enough to zoom in: https://i.imgur.com/I85gGnQ.png

So pretty interesting. The layout is for a normal model, mine is supposed to be a treble model so I don't yet know what differences come down to that vs something else. Some things, like the .047uF and 4700pF caps on the lower board next to eachother I think were factory mistakes if they're not a result of the treble model vs normal model. Other things like the 330k's + 220pF caps off the 3 volume pots with factory paint still on the solder joints I'm not sure about... hoping to get some insights from others what they're doing exactly.

Here's the current plan:

• Swap cathode bypass caps just to be safe
• Find a 500k trimpot to replace the 300k one in there now for the trem circuit
• Replace all other electrolytics
• Replace the few resistors way out of spec and swap the carbon films with CC's
• New 3 prong
• Mod trem to be always on on that channel

Here's my main questions at the moment:

• How the heck do I reach the bypass cap on the top board, there is zero room to reach the top side closest to the pots
• All of the different caps on mine vs the layout, how many are because of the treble vs normal and how many are something else (and what does that mean)
• Where do I find a 250uF 63v cap to replace the one in the amp? Is 25v enough?
• Are the 33uF 450 caps ok to swap with 22uF 500v F&T's I already have?
• The extra stuff on mine not on the layout (10k resistor, jumper lead on lower board, extra 33uF) all due to the topboost card install.. and if so was it done correctly?
• Are all of those little odd caps in the top board important to swap? Those little black hunts caps and other weird shaped ones all in the 100-750pF range).
• The resistors over the power tubes, do people typically install more heat resistant resistors there?
• Anything else I'm missing?

Here's some gut shots.

[pdf64]

…
I'd heard of doing that mod with the 100R resistors before, but wasn't clear to me what the steps were apart from cutting the connection to chassis ground at V1 off pin 4.

Yes, remove that link.
The 100R resistors can be added at the tagstrip between the mains transformer and rectifier socket.
It’s easy to connect their node to the tagstrip lug attached to the chassis.

Alternatively, for reasons unknown, the yellow heater wire is fastened to 2 lugs on the existing strip. That can be amended to create a spare lug for the resistor node. Or just fit an extra bit of tagstrip, or heatshrink the node.
That can be connected to the output valves cathodes, to elevate the heater circuit a bit.
That’s particularly beneficial on your treble model version, as the brilliant channel input stage is only partially bypassed.

Some things, like the .047uF and 4700pF caps on the lower board next to eachother I think were factory mistakes if they're not a result of the treble model vs normal model. Other things like the 330k's + 220pF caps off the 3 volume pots with factory paint still on the solder joints I'm not sure about... hoping to get some insights from others what they're doing exactly.

That all seems standard for treble models. The 220pF are bright caps, the 330k allow some bright boost even with the volume control set to max CW.

Here's the current plan:

• Swap cathode bypass caps just to be safe
• Find a 500k trimpot to replace the 300k one in there now for the trem circuit
• Replace all other electrolytics
• Replace the few resistors way out of spec and swap the carbon films with CC's
• New 3 prong
• Mod trem to be always on on that channel

It’s useful to be able to switch the modulation off, to completely kill any thumping it might cause.

I find that a very useful couple of mods is to fit CTS pull switch pots to the speed and vib/trem positions.
Use a 3M3 RA CTS track to get variable speed, and use its switch to get a front panel switch for the modulation oscillator.

Use a CTS 500k linear track to get variable depth (ie replacing the preset), use its switch to change between vib and trem.

As the brilliant channel sounds so good, make sure to ‘blueprint’ and document what you’ve got, before changing anything.

Here's my main questions at the moment:

[*1]How the heck do I reach the bypass cap on the top board, there is zero room to reach the top side closest to the pots
[*2]All of the different caps on mine vs the layout, how many are because of the treble vs normal and how many are something else (and what does that mean)
[*3]Where do I find a 250uF 63v cap to replace the one in the amp? Is 25v enough?
[*4]Are the 33uF 450 caps ok to swap with 22uF 500v F&T's I already have?
[*5]The extra stuff on mine not on the layout (10k resistor, jumper lead on lower board, extra 33uF) all due to the topboost card install.. and if so was it done correctly?
[*6]Are all of those little odd caps in the top board important to swap? Those little black hunts caps and other weird shaped ones all in the 100-750pF range).
[*7]The resistors over the power tubes, do people typically install more heat resistant resistors there?
[*8]Anything else I'm missing?
[/list]

1 one option is to remove all the pots. But it seems like you may need to replace or refurbish the pots anyway (I suggest to try the latter before condemning them).
2 from what I can tell, they’re all due to being the treble version. I think Jim Elyea’s book details a lot of the schematic variations, with great quality schematics.
3 25V should be ok, but higher voltage 105C type would be better, even just 35 or 50V (if it fails it can wreck the amp).
4 I suggest to use 33uF; it’s going to hum and thump a bit anyway. And you’ve got 2 amps to refurb, so it’s worth ordering in the right parts.
5 as far as I can tell, it’s good.
6 definitely replace the black Hunts caps; they’re a very likely cause of at least some of the problem with the vib/trem channel.
7 if they measure ok it’d be tempting to leave them. Maybe consider replacing the 100R screen grid resistors with a flameproof type, in case a valve shorts.
8 I agree to add the back up silicon diodes to the rectifier socket.
Also consider adding a fuseholder for a HT fuse, to help protect the transformers from shorts. Connect between the HT CT and chassis common. A fast acting 250mA can be used.
I understand that Jim Elyea’s book is worth tracking down if you really want to get into the weeds with vintage Vox.
But there’s also loads of excellent info at https://www.voxac30.org.uk/

[sluckey]

I'd heard of doing that mod with the 100R resistors before, but wasn't clear to me what the steps were apart from cutting the connection to chassis ground at V1 off pin 4.

Speaking of this, I noticed on the layout you posted, V1-4 (filament) is connected to ground. ***BUT***, there is also a green filament center tap wire also connected from the PT to ground! They can't both be right. I suspect the green wire is a drawing error.

How the heck do I reach the bypass cap on the top board, there is zero room to reach the top side closest to the pots

I would unsolder the three resistors on the speed switch then remove the switch from the control panel. Leave the yellow wire attached to the switch. Neatly replace the cap then reinstall the speed switch.

The resistors over the power tubes, do people typically install more heat resistant resistors there?

The 1.5K 1/2W CCs are fine. I would replace the 100Ω screen resistors with 100Ω 2W metal oxide because they are flame proof.

Mod trem to be always on on that channel

Not sure what you mean. Please explain. You should be able to turn on and off with the footswitch. Why would you want it to always be on?

From your note on the layout concerning the OT secondary impedance... Green is 16Ω.

[pdf64]

I'd heard of doing that mod with the 100R resistors before, but wasn't clear to me what the steps were apart from cutting the connection to chassis ground at V1 off pin 4.

Speaking of this, I noticed on the layout you posted, V1-4 (filament) is connected to ground. ***BUT***, there is also a green filament center tap wire also connected from the PT to ground! They can't both be right. I suspect the green wire is a drawing error.

Yes, AC30 don’t generally have a heater CT. Maybe some aftermarket transformers do  As drawn, the heater winding would burn out

How the heck do I reach the bypass cap on the top board, there is zero room to reach the top side closest to the pots

I would unsolder the three resistors on the speed switch then remove the switch from the control panel. Leave the yellow wire attached to the switch. Neatly replace the cap then reinstall the speed switch.

I thought that, but there’s another cathode bypass ecap further along, under the volume pots.

The resistors over the power tubes, do people typically install more heat resistant resistors there?

The 1.5K 1/2W CCs are fine. I would replace the 100Ω screen resistors with 100Ω 2W metal oxide because they are flame proof.

I’d be reluctant to increase the wattage, especially on a vintage amp, if not adding a HT fuse.
Mouser sell fusible MO resistors, if available I’d consider 100R 1W.

[Platefire]

OH My! Great Project and documentation. All my experience has been mostly Fenders and VOX's are great mystery. I'll be watching with much interest. Carry On

[pdf64]

I’d really appreciate an ‘edited highlights’ version of the videos.
I’m ill today, so watched and quite enjoyed it, but generally they move along rather too slowly for me to bother with.

[Yeatzee]

Apologies for the delayed reply, I've been deep in research mode and definitely feel like as each day passes I'm grasping more and more with these amps.


Finding all of the parts I need from one source has been a massive pain, as it stands I'll be needing to make orders from Tubesandmore, TubeDepot, and mouser. Yikes! Mouser is quite the intimidating site to shop on, talk about option paralysis holy cow. My current parts list is as follows:

• 20GA PTFE wire in various colors to have some on hand that semi-matches the stock wire in case I need to redo any runs or something - tubedepot
• Various carbon comps I don't already have (10k, 15k, 47k) - Tubedepot
• Spare 1n4007 1A diodes
• JJ 12AU7 to have a spare since I don't have any / no amps that use that tube - tubedepot
• JJ GZ34 spare - tubedepot
• Various Vishay ceramic disc caps for the trem circuit to have on hand incase the dogbone/black hunts caps are bad - Mouser
• 470 ohm or 1k ohm 3w Vishay/Dale metal film resistors for the screens - Mouser (470 seems like a good middle ground vs the 100 stock and the common 1k... but I'm not 100%)
• 3k ohm 2w Vishay/Dale metal film resistors for the grids - Mouser (rec I received /  have seen Psionic do)
• Vishay 220uF 63v bias cap - Mouser
• 500k Piher trim pot - Tubesandmore
• 25uF 50v Sprague Atoms - Tubesandmore
• 16/16 F&T can cap (already have, but buying another since it was originally destined for my AC10) - Tubesandmore
• 30uF 500v F&T x2 (why are these so expensive?!?) - Tubesandmore
• Vishay coupling caps to convert to normal model to see how I like it vs being a treble model: 2x .047uF, 1x .0047uF, and 2x .15uF - Tubesandmore
• Fuse block + 250v 0.25amp fast blow fuse - Tubesandmore
• 3 prong

I might be forgetting something, but I think that covers the order (at least the first). One thing that is potentially going to throw a (good) wrench in things is I have a line on two copper top chassis sans cabs that I can use for parts. One is nearly all original and untouched with a bad OT, the other has a bunch of replaced parts but critically it still has the albion PT which is what I need to make the ST whole again. Funny enough that chassis has a Lemark OT so swapping over the Lemark PT from the Super Twin would make that set more complete on the other chassis. I'm stopping by the shop selling it this weekend, TBD if we can make a deal on both but when I was there two weeks ago the owner wanted to move them as a package deal because they've just been gather dust so we'll see what happens. IF that works out, some of the parts above like the trim pot and the coupling caps I might be able to just take from one of those chassis. They've seen much better days, from there I'd probably just part them out or sell whats left for someone to make a cool project out of.

[Yeatzee]

Speaking of this, I noticed on the layout you posted, V1-4 (filament) is connected to ground. ***BUT***, there is also a green filament center tap wire also connected from the PT to ground! They can't both be right. I suspect the green wire is a drawing error.

Here's the Super Twin, with it's replaced Lemark (early 70's AC30) PT:





And here's an angle of V1-4 (and also the bypass cap with zero room to reach the backside of with a soldering iron that I was talking about earlier):

Mod trem to be always on on that channel

Not sure what you mean. Please explain. You should be able to turn on and off with the footswitch. Why would you want it to always be on?

From your note on the layout concerning the OT secondary impedance... Green is 16Ω.

Jumper the tags as if the footswitch was turned on, without the footswitch connected. The other AC30 (combo) has that, so trem is always working on that channel. I can't see why I would ever use that channel vs normal or brilliant/TB unless it was specifically for the trem so it made sense to me not having the footswitch for either amps.

I checked a few different sources for the OT impedance, seems like blue is actually 16ohm btw. Which would line up since that's what was hooked up on the amp and the ST cab is outputting 16ohm.

[Yeatzee]

I’d really appreciate an ‘edited highlights’ version of the videos.
I’m ill today, so watched and quite enjoyed it, but generally they move along rather too slowly for me to bother with.

Haha yaaaaa I go back and forth on that. I take the time to timestamp EVERYTHING so that you can jump around as you'd like, but because I'm not an expert and am very much learning still I've tended to more film everything vs the standard "here's what I'm going to do /jump cut / ok here it is done". That way I can get more tips, and it's a better record of what I'm doing for myself down the line. Some videos I cut out more if it's redundant. It's a balance, but definitely not the most entertaining content in the world lol.

OH My! Great Project and documentation. All my experience has been mostly Fenders and VOX's are great mystery. I'll be watching with much interest. Carry On

Thanks! I feel like I'm finally starting to understand fenders, and yeah this vox has been incredibly confusing by contrast for me. Once I realized where V1 / V2 was and that 90% of the circuit is just for the dang trem it started to make more sense.

[pdf64]

Various Vishay ceramic disc caps for the trem circuit to have on hand incase the dogbone/black hunts caps are bad - Mouser

Just to note that those caps in the phase shift circuit should preferably be close tolerance, high stability types. Some ceramics are fine (class 1), they’ll have a material code starting with N or below (eg NP0, C0G). Some aren’t suited (class 2), they’ll have a dielectric material code at the end of the alphabet (eg X7R, Z5U).
Class 1 caps are typically low value, class 2 are typically used for higher values.

[pdf64]

Regarding the valves mentioned in the video, Pinnacle were a rebrander, typically of premium UK / European made stuff. If you’re lucky that ECC83 may be a Mullard, Brimar etc.

The 5963 valve in the V8 modulator circuit is a weird one, I can’t work out whether it’d function in there or not.
Whereas if you’re stuck for an ECC82, ECC83 work fine in that position, though it’ll draw less anode current, so the voltages will be different.

[HotBluePlates]

I understand that Jim Elyea’s book is worth tracking down if you really want to get into the weeds with vintage Vox.
But there’s also loads of excellent info at https://www.voxac30.org.uk/

Elyea's book is great, but it's about cosmetic features & dates.  The schematics were found in a special folio that was part of a special limited-run, and is very expensive when you can find one second-hand now.

voxac30.org.uk is great, but also read http://www.voxshowroom.com/uk/amp/ac30_6_hood.html for details on the Normal vs Treble capacitor changes.  And Grosvenor's book is the way to go for schematics & layouts that won't cost a fortune.

[Yeatzee]

I understand that Jim Elyea’s book is worth tracking down if you really want to get into the weeds with vintage Vox.
But there’s also loads of excellent info at https://www.voxac30.org.uk/

Elyea's book is great, but it's about cosmetic features & dates.  The schematics were found in a special folio that was part of a special limited-run, and is very expensive when you can find one second-hand now.

voxac30.org.uk is great, but also read http://www.voxshowroom.com/uk/amp/ac30_6_hood.html for details on the Normal vs Treble capacitor changes.  And Grosvenor's book is the way to go for schematics & layouts that won't cost a fortune.

Yep that's the book I bought, just hasn't arrived yet. I've been studying up on voxshowroom too, unfortunately their schematic links are all broken. On that page they list the capacitor changes between the models, but they name them by CX numbers.... which doesn't help if I don't have a layout telling me where CX is supposed to be and I struggle to equate what my eyes see with the very busy schematic. I ended up working it out on my layout eventually.

[HotBluePlates]

... unfortunately their schematic links are all broken. On that page they list the capacitor changes between the models, but they name them by CX numbers.... which doesn't help if I don't have a layout telling me where CX is supposed to be ...

That's the task if you're gonna be going inside amps much.  Look at the schematic ---> look at the physical amp ---> determine where the schematic-part resides in physical space.

From your post, you seem to have already been to the Vox section of this forum's schematic library.  "Vox_ac301960" probably matches the part numbers at voxshowroom.com.

[pdf64]

I’ve found a copy of the AC30 treble schematic. I was sent it awhile ago, it’s a photo from Jim Elyea’s publication, so subject to copyright.
Apologies, I thought the schematic folio it must be from was part of his book.
PM me your email and I’ll send it over.

As an aside, all JMI era AC30 schematics erroneously note C41 as (.)005, implying 5nF, whereas the value fitted was 47nF.
C41 is in the V8 modulation circuit area.

[Yeatzee]

I’ve found a copy of the AC30 treble schematic. I was sent it awhile ago, it’s a photo from Jim Elyea’s publication, so subject to copyright.
Apologies, I thought the schematic folio it must be from was part of his book.
PM me your email and I’ll send it over.

As an aside, all JMI era AC30 schematics erroneously note C41 as (.)005, implying 5nF, whereas the value fitted was 47nF.
C41 is in the V8 modulation circuit area.

Thanks, the Elyea book I am borrowing from a local shop is the special addition with the schematics so I've got that to reference right now. Good to know about the C41 mistake!

Parts should be arriving tomorrow so I can get back to working on this.

[AlNewman]

Almost always radial caps are more efficient than axials.  especially in the 500v range.  They may not look original, but neither do F&T's. 

Sites like Mouser may seem intimidating, but if you dial in the search engines, you can find parts for pennies on the dollar, especially when you search for different voltages and values than Leo installed on his amps in 1965.  I guess sometimes it's nice to preserve the vintage "feel" of an amp, but others it makes sense to have 50 of everything, just in case.

[Jalmeida]

I'm back with another big project! Same as before with the Vibrolux series, this thread will serve to record my vintage AC30 journey and hopefully along the way I'll get some of that stellar advice you guys so graciously offered me in the past.

Long story short, I've been passively after an AC30 Super Twin for some time now. Reason being weight / ease of transport, cool factor, and flexibility (to use other cabs). They are not a common amp to see around, and not cheap when they do come up for sale! #1 thing for me was the matching 2x12 cab with original celestion alnico speakers. I've heard time and time again that is so much of the sound, so that was key for me.

Fast forward a bit and to my shock an AC30 ST popped up on the local CL, claiming to be all original with a couple exterior photo's and nothing of the inside. After some communication and really low res photo's we determined it was nowhere near all original so my interest waned significantly BUT if the bones were good I was still interested as I know I'll never see another local again. Jump forward a bit and eventually we agreed on a price that felt fair and due to some existing major question marks concerning the transformers if it all ended up being swapped I wouldn't be totally screwed. After hearing the brilliant channel w/ Top Boost I was sold and moved forward with the deal (despite discovering some other issues that weren't disclosed originally). So que, the first amp!



Then, while I was in talks for that amp I found another local AC30 but this time a combo. What drew me to it was the original transformers and original alnico blues with supposed original cones. If the ST was a basketcase, I wanted to have a straight one to fall back on, have something to compare it against. This one took a lot of back and forth but eventually we landed on an agreement.



So just like that I went from dreaming about a Super Twin, to owning two copper top JMI AC30's in the span of a week!

The goal is to get both up and running and healthy to compare the two, then start gigging the Super Twin! This will be my first time diving into a vintage Vox amp, I've worked on many vintage fenders now at this point but obviously vox's are a different thing so LOTS to learn. There's also a great deal of history, and determining what's straight / vintage and what's not for me to do as I dive into both. I would love for the vox experts to chime in as much as you'd like, I can tell you a lot about vintage fenders but I am very new to these amps.

I am, like I did with my Vibrolux Reverb series, going to document everything on YT the good and the bad and make it a detailed series. The first part I just uploaded, which reviews both in detail on the exterior, and then to end it I open up the Super Twin to try and determine what's what and how much work there really is going to be to get it up and running strong.

i=iFvWPfWzM09EC1jR

Sweet amp!!

I have the special edition Jim Elyea book with the schematics. Let me know of you need something and I can scan to PDF for you. Super nice quality diagrams.

As for tracking down layout stuff versus the schematic, i am not a seasoned tech like the other guys here. But often times I find the nearest tube socket to that component an go forward or backwards from there. The Vox tagboards are busy and messy for sure.

Let me know if I can scan anything from the Jim Elyea book schematics. The Stephen Grosvenor book is great but has errors.

[sluckey]

I would love to have a hi-rez scan of that AC-30 schematic from your Jim Elyea book.

[Jalmeida]

I would love to have a hi-rez scan of that AC-30 schematic from your Jim Elyea book.

Which version?

[Yeatzee]

Almost always radial caps are more efficient than axials.  especially in the 500v range.  They may not look original, but neither do F&T's. 

Sites like Mouser may seem intimidating, but if you dial in the search engines, you can find parts for pennies on the dollar, especially when you search for different voltages and values than Leo installed on his amps in 1965.  I guess sometimes it's nice to preserve the vintage "feel" of an amp, but others it makes sense to have 50 of everything, just in case.

Totally fair!

Sweet amp!!

I have the special edition Jim Elyea book with the schematics. Let me know of you need something and I can scan to PDF for you. Super nice quality diagrams.

As for tracking down layout stuff versus the schematic, i am not a seasoned tech like the other guys here. But often times I find the nearest tube socket to that component an go forward or backwards from there. The Vox tagboards are busy and messy for sure.

Let me know if I can scan anything from the Jim Elyea book schematics. The Stephen Grosvenor book is great but has errors.

So kind of you to offer! I actually got hold of one with the schematics as well, and got my vox ac30 schematics/layout book in the other day so I think I'm good now. Yes the tagboards are so busy, and what makes it extra hard is how busy they are underneath as well... tracking what is tied to what has been a nightmare!
With that, I was hesitant to upload this because there's not a lot of action but it's me going through the entire amp and pointing out everything I found (both pre and post finding the layout drawing I've been referencing online)

i=yNv5ks3OZaF59yzy
Soldering has already begun, just behind on editing the videos as I have very little free time with the newborn so it's either work on the amp or edit! Goal is to pump out the next video of me working on the amp tomorrow if I can edit it tonight, that's where the rubber meets the road :)

[sluckey]

I would love to have a hi-rez scan of that AC-30 schematic from your Jim Elyea book.

Which version?

The version Yeatzee is working on and the AC30-4 (may be called AC-34 with the EF86 normal channel and Vibrato channel). But if it's not too much trouble maybe you could scan all the old AC-30 schematics that are older than about 1975, and I could put them in Hoffman's schematic library. The ones in the library are a pain to read.

[Yeatzee]

I'd heard of doing that mod with the 100R resistors before, but wasn't clear to me what the steps were apart from cutting the connection to chassis ground at V1 off pin 4.

Speaking of this, I noticed on the layout you posted, V1-4 (filament) is connected to ground. ***BUT***, there is also a green filament center tap wire also connected from the PT to ground! They can't both be right. I suspect the green wire is a drawing error.

Circling back to this, I shared a photo of my transformer. It seems to show two leads to the ground lug including a green one just like the drawing. And it has the v1 connection like the drawing. Amp functioned before I took it apart. Can you explain this to me more? What's the issue exactly?

In other news started the soldering, quick video removing resistors and learning the intricacies of the tag board

[sluckey]

Speaking of this, I noticed on the layout you posted, V1-4 (filament) is connected to ground. ***BUT***, there is also a green filament center tap wire also connected from the PT to ground! They can't both be right. I suspect the green wire is a drawing error.

Circling back to this, I shared a photo of my transformer. It seems to show two leads to the ground lug including a green one just like the drawing. And it has the v1 connection like the drawing. Amp functioned before I took it apart. Can you explain this to me more? What's the issue exactly?

The issue is a simple layout drawing error. The green wire from the PT is indicated to be a center tap for the filament winding. That's impossible! It's clear that V1-4 is clearly attached to ground at the socket on your amp and the layout. That's correct. If the PT green wire were really a filament center tap and it's also connected to ground, then you would have a dead short across half the filament winding and the amp would not work at all because the filaments would not light up. In fact, the PT would likely be toast if the line fuse did not blow. So, the green wire is most likely the center tap for the high voltage winding and the other ground wire is the shield. Notice on the layout, the high voltage center tap wire is missing? That's just another small error on the layout of the PT.

There is a simple way to prove what I'm saying, and it fits in with something you need to do to this amp anyway. Disconnect the PT green wire from the terminal strip. Now measure resistance between the dangling green wire to either of the 6.3VAC filament wires. If what I'm saying is correct, there will be infinite resistance between the green wire and the filament wires. I suspect there will be a resistance reading between the dangling green wire and the rectifier socket pins 4 and 6. This is a check you really need to verify.

That PT has been replaced. I'm sure you've noticed the sloppy wiring of that PT. All of it needs to be redone, tube socket and terminal strip.

I'm attaching an annotated pic of your PT. Let me know if I'm right or wrong.

[sluckey]

To bypass the footswitch simply put a jumper between lugs 4 and 6 in this pic...

[sluckey]

I'd heard of doing that mod with the 100R resistors before, but wasn't clear to me what the steps were apart from cutting the connection to chassis ground at V1 off pin 4.

Here's a pic showing an easy convenient way to improve the heater ground reference...

[Yeatzee]

Speaking of this, I noticed on the layout you posted, V1-4 (filament) is connected to ground. ***BUT***, there is also a green filament center tap wire also connected from the PT to ground! They can't both be right. I suspect the green wire is a drawing error.

Circling back to this, I shared a photo of my transformer. It seems to show two leads to the ground lug including a green one just like the drawing. And it has the v1 connection like the drawing. Amp functioned before I took it apart. Can you explain this to me more? What's the issue exactly?

The issue is a simple layout drawing error. The green wire from the PT is indicated to be a center tap for the filament winding. That's impossible! It's clear that V1-4 is clearly attached to ground at the socket on your amp and the layout. That's correct. If the PT green wire were really a filament center tap and it's also connected to ground, then you would have a dead short across half the filament winding and the amp would not work at all because the filaments would not light up. In fact, the PT would likely be toast if the line fuse did not blow. So, the green wire is most likely the center tap for the high voltage winding and the other ground wire is the shield. Notice on the layout, the high voltage center tap wire is missing? That's just another small error on the layout of the PT.

There is a simple way to prove what I'm saying, and it fits in with something you need to do to this amp anyway. Disconnect the PT green wire from the terminal strip. Now measure resistance between the dangling green wire to either of the 6.3VAC filament wires. If what I'm saying is correct, there will be infinite resistance between the green wire and the filament wires. I suspect there will be a resistance reading between the dangling green wire and the rectifier socket pins 4 and 6. This is a check you really need to verify.

That PT has been replaced. I'm sure you've noticed the sloppy wiring of that PT. All of it needs to be redone, tube socket and terminal strip.

I'm attaching an annotated pic of your PT. Let me know if I'm right or wrong.

Oooooh ok that makes sense. I was confused, center tap for filament vs high voltage. How do you know / tell the center tap would be for either based off the drawing? Is it because it's located inbetween the filament wires? So it's missing a center tap for the filaments, hence the artificial one people add?

I've already redone the wiring on the transformer / socket + installed a new terminal strip with SS hardware:



Behind on editing the videos! I added some diodes to the rectifier per someones suggestion. I also connected the ground off the main filter cap (dual 16uF) to that ground connection. Need to find what preamp ground I want to tie in the 30uF for the trem channel still, I really struggle picking out which tag is ground and which isn't since it seems to use both top and bottom for ground at different points (and all the wires underneath snake everywhere). Going to look at the layout and determine a good spot, then try and look under the tag board on the actual amp to confirm.

[Yeatzee]

i=v0wrGNkOOnGOiNLY

Here's the latest I've uploaded. Replaced the resistors with new carbon comps, swapped in a new trim pot for the trem depth, and then replaced the bypass caps. Not as bad as I was expecting, mainly because the new ones are a lot smaller so I could reach the back of the top board still.

[sluckey]

Oooooh ok that makes sense. I was confused, center tap for filament vs high voltage. How do you know / tell the center tap would be for either based off the drawing? Is it because it's located inbetween the filament wires? So it's missing a center tap for the filaments, hence the artificial one people add?

Most manufacturers' drawings made it obvious on the schematic as well as the layout. Vox was an odd duck and followed no generally accepted conventions. Even the color code for the PT wires is intentionally crazy! Pink for 5V filaments, Yellow for HV winding! Who the hell does that???   

But, you can tell where the center tap is connected if you look at the Vox schematic.

I've already redone the wiring on the transformer / socket + installed a new terminal strip with SS hardware:

Still room to neatly install the resistors on the terminal strip. But honestly, if you're happy with the hum level from the amp, just leave the ground jumper on V1-4 and call it done. Lot's of old Fender amps had one side of the filament circuit connected to chassis rather than use a center tap filament winding. Never heard any complaints.

[Yeatzee]

i=pz1dbfZCPYZYGQ5o

Screens and grids! Quite a pain on these

[sluckey]

Looks good. Why did you choose 470Ω screen resistors and 3K grid stoppers?

[Yeatzee]

Looks good. Why did you choose 470Ω screen resistors and 3K grid stoppers?

Thanks!

I had a few recommendations for what values to go with there, lots of opinions! Screens I knew I didn't want 100ohm, most suggested 1k but in talking to some people 470 seemed like a good middle ground. Same with the grid stoppers, that was a suggestion and something I've seen others do on theirs including Psionic Audio on his JMI restoration.

[sluckey]

Nothing wrong with those values. I've just never seen either of those values used with EL84 amps and was curious.

[Yeatzee]

Chipping away, just about wrapped up with the lower chassis.

i=oV0rpkvp_RDldi0X

[Yeatzee]

Moving on to the last of the major electrolytics the amp requires. Not a lot of space to mount them, but fairly happy with how it came out overall. Using the existing clamp + the heaters which are tight + silicone to hold the dual 8uF and 30uF

[Yeatzee]

Top Boost card semi-rebuilt. Kicked my butt to say the least, that damn shielded wire is garbage.

[Yeatzee]

And at last...

i=bxvmvcmTTRSqsnDs

[Platefire]

Well Congratulations!! Glad it came up with no problems. Now if you will just put it back together and send it to my address, everything will be fine

[Yeatzee]

Well Congratulations!! Glad it came up with no problems. Now if you will just put it back together and send it to my address, everything will be fine

Thank you! I'm not totally out of the woods, I did fix the trem channel so it's working fine but I'm getting a weird occasional popping noise when the amp's turned up loud on the Brilliant and normal channels. Thinking the vibrations are causing an issue somewhere, so I need to diagnose that still. Once that's done I'll send it on over ;)

[Platefire]

Well my first thought was a preamp tube, but since it's occurring on two channels, if it is a tube it would have to be the phase inverter or power tubes. I think you've already cleaned the tube sockets with contact cleaner, haven't you?

Edit: I see bright and normal channel share V1 stages a & b, so that would be the first check for me on the popping, trying a known good tube

[Yeatzee]

Well my first thought was a preamp tube, but since it's occurring on two channels, if it is a tube it would have to be the phase inverter or power tubes. I think you've already cleaned the tube sockets with contact cleaner, haven't you?

Edit: I see bright and normal channel share V1 stages a & b, so that would be the first check for me on the popping, trying a known good tube

Yeah I'm not sure! I did a little tube rolling in V1 after getting it running before installing it back in the chassis and the one it came with sounded the best to me so I kept it in there. I didn't get up to the volume levels I did for the demo then, so maybe? It was not a typical popping noise I've heard before though, hard to describe. I didn't even notice it (I do the demo with in ears in to block out noise since it's deafening inside my home when I do it) until I was testing with my 3rd guitar.

Someone suggested it could be a plate resistor as well, I swapped the one for V1 already but haven't for the PI yet so that's also on the list. I already checked values and they measure fine, but really one seemed to have an issue when the amp was loud and vibrating a certain way.

[Jalmeida]

Apologies for the delayed reply, I've been deep in research mode and definitely feel like as each day passes I'm grasping more and more with these amps.


Finding all of the parts I need from one source has been a massive pain, as it stands I'll be needing to make orders from Tubesandmore, TubeDepot, and mouser. Yikes! Mouser is quite the intimidating site to shop on, talk about option paralysis holy cow. My current parts list is as follows:

• 20GA PTFE wire in various colors to have some on hand that semi-matches the stock wire in case I need to redo any runs or something - tubedepot
• Various carbon comps I don't already have (10k, 15k, 47k) - Tubedepot
• Spare 1n4007 1A diodes
• JJ 12AU7 to have a spare since I don't have any / no amps that use that tube - tubedepot
• JJ GZ34 spare - tubedepot
• Various Vishay ceramic disc caps for the trem circuit to have on hand incase the dogbone/black hunts caps are bad - Mouser
• 470 ohm or 1k ohm 3w Vishay/Dale metal film resistors for the screens - Mouser (470 seems like a good middle ground vs the 100 stock and the common 1k... but I'm not 100%)
• 3k ohm 2w Vishay/Dale metal film resistors for the grids - Mouser (rec I received /  have seen Psionic do)
• Vishay 220uF 63v bias cap - Mouser
• 500k Piher trim pot - Tubesandmore
• 25uF 50v Sprague Atoms - Tubesandmore
• 16/16 F&T can cap (already have, but buying another since it was originally destined for my AC10) - Tubesandmore
• 30uF 500v F&T x2 (why are these so expensive?!?) - Tubesandmore
• Vishay coupling caps to convert to normal model to see how I like it vs being a treble model: 2x .047uF, 1x .0047uF, and 2x .15uF - Tubesandmore
• Fuse block + 250v 0.25amp fast blow fuse - Tubesandmore
• 3 prong

I might be forgetting something, but I think that covers the order (at least the first). One thing that is potentially going to throw a (good) wrench in things is I have a line on two copper top chassis sans cabs that I can use for parts. One is nearly all original and untouched with a bad OT, the other has a bunch of replaced parts but critically it still has the albion PT which is what I need to make the ST whole again. Funny enough that chassis has a Lemark OT so swapping over the Lemark PT from the Super Twin would make that set more complete on the other chassis. I'm stopping by the shop selling it this weekend, TBD if we can make a deal on both but when I was there two weeks ago the owner wanted to move them as a package deal because they've just been gather dust so we'll see what happens. IF that works out, some of the parts above like the trim pot and the coupling caps I might be able to just take from one of those chassis. They've seen much better days, from there I'd probably just part them out or sell whats left for someone to make a cool project out of.

For wire, get Tefzel from here: https://www.aircraftspruce.com/catalog/elpages/unshieldlwire.php?clickkey=3524

There is also a guy on ebay that sells the same wire in assorted colors. What you need is part number M22759/16-XX-YY

XX=AWG(22 for 22AWG etc)
YY=color(0 for black, 9 for white etc)

[Jalmeida]

Here is a topboost module schematic. Not sure if you have this.

[Jalmeida]

Oddly, I don’t see a Super Twin or super twin reverb specific schematic. So it is just the two separate schematics with the add-on Too Boost and/or reverb CCT module.

The forum won’t let me attach the AC30/6 schematic I do have due to size. But, I have scanned and sent a bunch into this forum to get added onto the folder so that it will be of use to anyone.

But in the interim, here is a googledrive link with some loaded..

 https://drive.google.com/drive/folders/1115fswsKfzievtlC0tvghPlXk3Z9c29v

[Yeatzee]

Oddly, I don’t see a Super Twin or super twin reverb specific schematic. So it is just the two separate schematics with the add-on Too Boost and/or reverb CCT module.

The forum won’t let me attach the AC30/6 schematic I do have due to size. But, I have scanned and sent a bunch into this forum to get added onto the folder so that it will be of use to anyone.

But in the interim, here is a googledrive link with some loaded..

 https://drive.google.com/drive/folders/1115fswsKfzievtlC0tvghPlXk3Z9c29v

Thanks for sharing! Yeah it's just the regular AC30/6 with the add on card.

[Jalmeida]

Oddly, I don’t see a Super Twin or super twin reverb specific schematic. So it is just the two separate schematics with the add-on Too Boost and/or reverb CCT module.

The forum won’t let me attach the AC30/6 schematic I do have due to size. But, I have scanned and sent a bunch into this forum to get added onto the folder so that it will be of use to anyone.

But in the interim, here is a googledrive link with some loaded..

 https://drive.google.com/drive/folders/1115fswsKfzievtlC0tvghPlXk3Z9c29v

Thanks for sharing! Yeah it's just the regular AC30/6 with the add on card.

All of my submitted schematics have been loaded on the amp schematics section. Here you go:

https://el34world.com/charts/Schematics/files/Vox/Vox_AC36_Schematics.pdf


Hopefully this is of some help.

[Yeatzee]

Oddly, I don’t see a Super Twin or super twin reverb specific schematic. So it is just the two separate schematics with the add-on Too Boost and/or reverb CCT module.

The forum won’t let me attach the AC30/6 schematic I do have due to size. But, I have scanned and sent a bunch into this forum to get added onto the folder so that it will be of use to anyone.

But in the interim, here is a googledrive link with some loaded..

 https://drive.google.com/drive/folders/1115fswsKfzievtlC0tvghPlXk3Z9c29v

Thanks for sharing! Yeah it's just the regular AC30/6 with the add on card.

All of my submitted schematics have been loaded on the amp schematics section. Here you go:

https://el34world.com/charts/Schematics/files/Vox/Vox_AC36_Schematics.pdf


Hopefully this is of some help.

Nice! Actually done with the project, more less. Fired up, I fixed the trem not work. I started to record a playing demo but noticed as I was getting some base tones set this weird popping noise that seemed to be vibration related when the amp was turned way up. I stopped the recording but have not been able to replicate the issue so still trying to figure it out.

[Jalmeida]

Oddly, I don’t see a Super Twin or super twin reverb specific schematic. So it is just the two separate schematics with the add-on Too Boost and/or reverb CCT module.

The forum won’t let me attach the AC30/6 schematic I do have due to size. But, I have scanned and sent a bunch into this forum to get added onto the folder so that it will be of use to anyone.

But in the interim, here is a googledrive link with some loaded..

 https://drive.google.com/drive/folders/1115fswsKfzievtlC0tvghPlXk3Z9c29v

Thanks for sharing! Yeah it's just the regular AC30/6 with the add on card.

All of my submitted schematics have been loaded on the amp schematics section. Here you go:

https://el34world.com/charts/Schematics/files/Vox/Vox_AC36_Schematics.pdf


Hopefully this is of some help.

Nice! Actually done with the project, more less. Fired up, I fixed the trem not work. I started to record a playing demo but noticed as I was getting some base tones set this weird popping noise that seemed to be vibration related when the amp was turned way up. I stopped the recording but have not been able to replicate the issue so still trying to figure it out.

Might need a good chopsticking to check for cracked solder joints. 50+ year old solder joints can start to act up on the slightest disturbance of other wires around.