Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yeatzee on August 20, 2023, 07:56:05 pm
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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!
(https://i.imgur.com/AMEwJFW.jpg)
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.
(https://i.imgur.com/pPBS1XX.jpg)
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
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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.
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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).
(https://i.imgur.com/I85gGnQ.png)
Direct URL if it's not big enough to zoom in: https://i.imgur.com/I85gGnQ.png (http://'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.
(https://i.imgur.com/ZDvl8IH.jpg)
(https://i.imgur.com/gOTnNyr.jpg)
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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/
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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.
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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 :dontknow: 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.
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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 :thumbsup:
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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.
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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.
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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:
(https://i.imgur.com/cA02hVG.jpg)
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):
(https://i.imgur.com/80qCcn4.jpg)
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.
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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 :thumbsup:
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.
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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.
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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.
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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/ (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 (https://www.thevoxac30guide.com//) is the way to go for schematics & layouts that won't cost a fortune.
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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/ (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 (https://www.thevoxac30guide.com//) 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.
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... 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 (https://el34world.com/charts/Schematics/files/Vox/Vox_Schematics.htm) of this forum's schematic library (https://el34world.com/charts/TubeAmpSchematics.htm). "Vox_ac301960 (https://el34world.com/charts/Schematics/files/Vox/Vox_ac301960.pdf)" probably matches the part numbers at voxshowroom.com.
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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.
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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.
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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.
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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!
(https://i.imgur.com/AMEwJFW.jpg)
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.
(https://i.imgur.com/pPBS1XX.jpg)
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.
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I would love to have a hi-rez scan of that AC-30 schematic from your Jim Elyea book.
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I would love to have a hi-rez scan of that AC-30 schematic from your Jim Elyea book.
Which version?
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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 :)
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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.
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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
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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.
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To bypass the footswitch simply put a jumper between lugs 4 and 6 in this pic...
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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...
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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:
(https://i.imgur.com/YQotKk2.jpg)
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.
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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.
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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??? :BangHead:
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.
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i=pz1dbfZCPYZYGQ5o
Screens and grids! Quite a pain on these
(https://i.imgur.com/yHGwG5W.jpg)
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Looks good. Why did you choose 470Ω screen resistors and 3K grid stoppers?
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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.
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Nothing wrong with those values. I've just never seen either of those values used with EL84 amps and was curious.
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Chipping away, just about wrapped up with the lower chassis.
(https://i.imgur.com/YQotKk2.jpg)
i=oV0rpkvp_RDldi0X
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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
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Top Boost card semi-rebuilt. Kicked my butt to say the least, that damn shielded wire is garbage.
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And at last...
i=bxvmvcmTTRSqsnDs
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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 :icon_biggrin:
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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 :icon_biggrin:
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 ;)
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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
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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.
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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)
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Here is a topboost module schematic. Not sure if you have this.
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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
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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.
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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.
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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.
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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.
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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.
(https://i.imgur.com/GtlT4hD.jpg)
Yeah I did that already, not a peep from anything. I think it might be something on the lower chassis though someone else said it might be a plate resistor that's only acting up when it gets hot. That is obviously a tough thing to test, because by nature of having the back plate off to be able to test things means it's not getting as hot as it would normally. I may just let it sit for like 30min sealed up and see if the sound returns or not and go from there.
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Just a thought on popping from vibration when turned way up. Have you tried it through a different set of speakers?
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Just a thought on popping from vibration when turned way up. Have you tried it through a different set of speakers?
Not yet, but I cranked it same settings when I moved it to the garage and took that photo and could not get the noise to show itself.
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Well that may be a good thing! Moving to a different location can sometime do the trick for unidentified freaky noises. So beside the distraction of the popping noise, what are you thinking about the sound and response of the amp? So is it living up to your expectations?
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i=_jfCOH6ecwLjhcAm
Next video up, all about checking the bias in the amp at a couple different voltages from the wall as well as show the PT buzz I'm getting for some reason.
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Well that may be a good thing! Moving to a different location can sometime do the trick for unidentified freaky noises. So beside the distraction of the popping noise, what are you thinking about the sound and response of the amp? So is it living up to your expectations?
Video coming out tomorrow with some recorded tones! I have no doubt it'll live up to or exceed my expectations based off the brief amount of time I've spent with it so far. It's fabulous!
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Annnnnd finally some playing... plus another speedbump. Getting there!
i=0HCgB532LJy_KYZl
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So left on the list:
- More tubes
- New resistor to set the bias a bit hotter
- HT fuse
- 100ohm resistors elevating the heaters to the cathode
- Maybe change the bright cap value on the normal channel to make it brighter?
- Eventually setup the 8ohm output so I have both available
- Swap the celestion blues into the cab and re-do it's wiring
Probably other things I'm forgetting right now but that's a good start
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So left on the list:
- More tubes
- New resistor to set the bias a bit hotter
- HT fuse
- 100ohm resistors elevating the heaters to the cathode
- Maybe change the bright cap value on the normal channel to make it brighter?
- Eventually setup the 8ohm output so I have both available
- Swap the celestion blues into the cab and re-do it's wiring
Probably other things I'm forgetting right now but that's a good start
The Treble model and Normal variants of the AC30/6 have some notable variations in capacitor values and arrangements prior to and after the PI tube in the Normal Channel. Taking this approach help you brightening things up. Take note of C5 and C6 as well as the 330K Resistor and 220pf cap around VR1.
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So left on the list:
- More tubes
- New resistor to set the bias a bit hotter
- HT fuse
- 100ohm resistors elevating the heaters to the cathode
- Maybe change the bright cap value on the normal channel to make it brighter?
- Eventually setup the 8ohm output so I have both available
- Swap the celestion blues into the cab and re-do it's wiring
Probably other things I'm forgetting right now but that's a good start
The Treble model and Normal variants of the AC30/6 have some notable variations in capacitor values and arrangements prior to and after the PI tube in the Normal Channel. Taking this approach help you brightening things up. Take note of C5 and C6 as well as the 330K Resistor and 220pf cap around VR1.
Mines the treble model so it's got those specs already. I've heard from others they're way too bright but honestly it sounds great to me, just want a bit more out of the normal.
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Mines the treble model so it's got those specs already. I've heard from others they're way too bright but honestly it sounds great to me ...
It's the earplugs, and your ability to crank the amp up when you use it.
The bright caps & 330kΩ resistors make those amps intensely-bright for anyone trying to play a Treble model at apartment-friendly volumes (without an attenuator/reactive-load).
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Mines the treble model so it's got those specs already. I've heard from others they're way too bright but honestly it sounds great to me ...
It's the earplugs, and your ability to crank the amp up when you use it.
The bright caps & 330kΩ resistors make those amps intensely-bright for anyone trying to play a Treble model at apartment-friendly volumes (without an attenuator/reactive-load).
I guess that's fair (not the earplugs bit, I only do that for the video recording ;) )! But maybe not the best idea to buy an ac30 if you only play quiet at home lol
Yesterday in prep for a gig I played it side by side with my vibrolux reverb for something like 2 hrs straight and it performed flawlessly and sounded insanely good. No issues, sounded massive. Excellent amount of grind at gig volumes, way more than I would have expected which is a pleasant surprise.
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Glad to hear your pleased with the outcome. You have put a lot of effort into in and great documentation. Up to this point I didn't even know that Vox made a separate AC30 head/cab model. I'll be interested to hear your report when you play it live with your band. To me that's where the rubber meets the road.
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Glad to hear your pleased with the outcome. You have put a lot of effort into in and great documentation. Up to this point I didn't even know that Vox made a separate AC30 head/cab model. I'll be interested to hear your report when you play it live with your band. To me that's where the rubber meets the road.
For sure! I've got a gig this upcoming weekend, I'm contemplating taking it and the Vibrolux Reverb but running the AC30 as primary... TBD!
(https://i.imgur.com/0WZFSSJ.jpg)
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What size fuse should I use for the inline HT fuse when I add it in? 1A?
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I see 500mA (1/2A) used in a lot of 4xEL84 amps.
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I used F315mA in my AC30, in the HT winding CT return.
In conjunction with 1N4007 in series with each GZ34 anode, that should cover almost any significant fault.
The max HT DC current that the anodes can draw is the (loaded) anode to cathode voltage (maybe 280V) divided by the OT primary impedance under class B conditions (1k) = 0.28A.
Maybe go up to 400mA to eliminate the chance of spurious fusing.
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Well, I decided to go for it and play the AC30 at yesterdays 3hr gig. Amp performed flawlessly! We're a loud band so I was able to crank it up... honestly 30w is borderline with us, had her cooking for sure!
(https://i.imgur.com/KBE5j5q.jpg)
Top Boost channel at noon, cut at noon, bass at noon, treble around 9:30. Sounded excellent!
I did install a longer power cord since the one I first installed was just too short for a head that sits that high off the ground, as well as a 68r bias resistor to replace the 75. That got me to about 103% dissipation at idle, so still a touch cool when subtracting the 5% for screens. I did buy a 60r as well, but ran out of time before the gig to experiment. Realistically, what's the most I'd want to hit at idle assuming I'm using run of the mill modern power tubes? 115%? 110%?
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And a couple shots back at home for fun :)
(https://i.imgur.com/jEJux9K.jpg)
(https://i.imgur.com/KWOyVAM.jpg)
and next to my '64 AC10 Twin that's going to be an upcoming project. Just bought some celestion gold's so I can preserve the stock elacs :)
(https://i.imgur.com/xaImQjR.jpg)
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And a couple shots back at home for fun :)
And next to my '64 AC10 Twin that's going to be an upcoming project. Just bought some celestion gold's so I can preserve the stock elacs :)
Those stock AC10 Elacs(and goodmans) sound pretty great. I have some in my AC10 clone.
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Those stock AC10 Elacs(and goodmans) sound pretty great. I have some in my AC10 clone.
Yep, great sounding but fragile. I'd like to try gigging the AC10 for the smaller venues so we'll give the golds a shot. It might keep up pretty well since golds are presumably a lot more efficient too. We'll see!
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Yep, great sounding but fragile. I'd like to try gigging the AC10 for the smaller venues so we'll give the golds a shot. It might keep up pretty well since golds are presumably a lot more efficient too. We'll see!
The weakness in my experience with them is the magnets are screwed together as a ring and the center slug is swaged into the back plate. And when these were assembled they relied on the screws solely to hold the center slug centered in the speaker frame. And they don’t. The center slug migrates and the voice coil jams in there. The goodmans and Elac are almost identical in assembly and their issues. Really they need to be glued together to not shift. If they are good now you can pull the bell cover off and glue them. Technically if you take the magnets apart then you can loose the charge and need to recharge. I have done so without and couldn’t tell the difference. But the cone and voicecoil are available still. Weber can recharge magnets(if needed).
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The weakness in my experience with them is the magnets are screwed together as a ring and the center slug is swaged into the back plate. And when these were assembled they relied on the screws solely to hold the center slug centered in the speaker frame. And they don’t. The center slug migrates and the voice coil jams in there. The goodmans and Elac are almost identical in assembly and their issues. Really they need to be glued together to not shift. If they are good now you can pull the bell cover off and glue them. Technically if you take the magnets apart then you can loose the charge and need to recharge. I have done so without and couldn’t tell the difference. But the cone and voicecoil are available still. Weber can recharge magnets(if needed).
Yeah I'm more concerned about blowing the speaker with them being rated so low. I'd rather hang onto them untouched since they're fine and just swap in some modern speakers, assuming they sound good. I like the gold 12" and have heard even better about the 10
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If their 50+ year old speakers, then the glue that holds the voice coil and cone together is pretty well dried up and they will let loose of each other, sooner or later, probably sooner, as you play them loud. Just the way it is.
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True. Although there are also alot of vintage 50-60 year old speakers still kicking in vintage amps.
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I reached out to bygonetones to see what he thought about the celestion blues since when I took them off the combo they looked a little odd to me on the other side but still have pulsonic writing on them. He believes they are old recones, still pulsonic cones but not the original cone. He also believes they will have a higher power rating than the originals so should be a bit sturdier which is obviously good for me!
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I reached out to bygonetones to see what he thought about the celestion blues since when I took them off the combo they looked a little odd to me on the other side but still have pulsonic writing on them. He believes they are old recones, still pulsonic cones but not the original cone. He also believes they will have a higher power rating than the originals so should be a bit sturdier which is obviously good for me!
That’s super cool. And Pulsonics too to boot!! I have had that done, unfortunately not with pulsonics cones. I had picked up some of the late 60s Vox silvers in need of recones and had them reconed with H1777 Mueller cones and 30w wattage voicecoils so that I could use my vox 2x12 with these for my AC50 head too.
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I reached out to bygonetones to see what he thought about the celestion blues since when I took them off the combo they looked a little odd to me on the other side but still have pulsonic writing on them. He believes they are old recones, still pulsonic cones but not the original cone. He also believes they will have a higher power rating than the originals so should be a bit sturdier which is obviously good for me!
That’s super cool. And Pulsonics too to boot!! I have had that done, unfortunately not with pulsonics cones. I had picked up some of the late 60s Vox silvers in need of recones and had them reconed with H1777 Mueller cones and 30w wattage voicecoils so that I could use my vox 2x12 with these for my AC50 head too.
I've always wondered what AC50 owners do if they want a vox sound but don't have those weird original cabinets with that third speaker!
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I reached out to bygonetones to see what he thought about the celestion blues since when I took them off the combo they looked a little odd to me on the other side but still have pulsonic writing on them. He believes they are old recones, still pulsonic cones but not the original cone. He also believes they will have a higher power rating than the originals so should be a bit sturdier which is obviously good for me!
That’s super cool. And Pulsonics too to boot!! I have had that done, unfortunately not with pulsonics cones. I had picked up some of the late 60s Vox silvers in need of recones and had them reconed with H1777 Mueller cones and 30w wattage voicecoils so that I could use my vox 2x12 with these for my AC50 head too.
I've always wondered what AC50 owners do if they want a vox sound but don't have those weird original cabinets with that third speaker!
Yeah, those original AC50 speaker cabs are kinda rare. They had a goodmans horn as a means to add more chime and because the original lower wattage silver alnicos. When you do see the original AC50 cabs, often times the horn is removed. The latter AC50 had Goodman ceramic magnet speakers, so not sure what wattage those will handle. One this is for sure though. The EL34s in the AC50 give it a different tone from the EL84 amps through the Silver alnicos. A lot darker sound. I would love to A/B an AC50 head from an original cab with horn and without. I have heard the horns were pretty harsh.
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i=1juoBFtq3TWTnRx8
Some final details wrapped up before I got to do a string of gigs with the amp. So far so good, it's been rock solid and sounding great! I still plan to try the blues and I also need to lift the heater ground connection. It's noisy, but also that doesn't matter when you're playing so hasn't been a priority.
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i=1juoBFtq3TWTnRx8
Some final details wrapped up before I got to do a string of gigs with the amp. So far so good, it's been rock solid and sounding great! I still plan to try the blues and I also need to lift the heater ground connection. It's noisy, but also that doesn't matter when you're playing so hasn't been a priority.
You may also try Silvers too if you don’t already have some blues. I have both. I honestly am not sure of the real differences between the blues and silvers. Some swear the silvers have more midrange, but I believe that they were identical from an individual component breakdown both being Alnico and Celestion. And you can score Vox Silver alnicos often cheaper than blues because of less hype. I am sure someone with better ears than me can claim they sound different. But I am not sure they sound any more different blue to silver than the natural variances speaker to speaker on the blues. And to me they both sound like the same speakers.
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i=1juoBFtq3TWTnRx8
Some final details wrapped up before I got to do a string of gigs with the amp. So far so good, it's been rock solid and sounding great! I still plan to try the blues and I also need to lift the heater ground connection. It's noisy, but also that doesn't matter when you're playing so hasn't been a priority.
You may also try Silvers too if you don’t already have some blues. I have both. I honestly am not sure of the real differences between the blues and silvers. Some swear the silvers have more midrange, but I believe that they were identical from an individual component breakdown both being Alnico and Celestion. And you can score Vox Silver alnicos often cheaper than blues because of less hype. I am sure someone with better ears than me can claim they sound different. But I am not sure they sound any more different blue to silver than the natural variances speaker to speaker on the blues. And to me they both sound like the same speakers.
The amp already has silvers in it now (one 444 recone, one old pulsonic recone). The combo I have has pulsonic reconed blues that I want to compare. The amp originally would have had blues more likely.
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Little Wet/Dry testing with the AC30 as the dry amp:
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And in other news, I'm getting a ton of extra idle noise/hum when I have the top boost card enabled (the treble pot is a switching pot so I can take it [mostly] out of circuit if I want). Any ideas? This is the video I did about the card rebuild.
i=xNGhhR-4B0GzbMsv
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Another question. I removed the 220pf bright cap and 330k resistor off the brilliant channel volume pot to try and tame a little brightness the treble model has inherently. The results were not what I expected, it basically sounded identical, I couldn't tell any difference.
Why is that?
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A typical bright cap is usually only effective at lower volumes. The higher you turn up the volume, the less effect it has. And if you turn the volume to max the bright cap has no effect. So, were you comparing at low volume or high volume?
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A typical bright cap is usually only effective at lower volumes. The higher you turn up the volume, the less effect it has. And if you turn the volume to max the bright cap has no effect. So, were you comparing at low volume or high volume?
I figured that might be the case. Volume was on half. What is the 330k is doing here? It's only present on the treble model, and removing it didn't change anything that I can tell.
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That 330K allows the 220pf bright cap to remain in effect even at high volume. Notice that the normal volume pot has this exact same configuration with the 330K and 220pf. Messing with these components is not gonna affect brightness much.
To really affect the brightness look at coupling cap C3. It's a tiny 500pf cap and has a huge effect on the brightness. Compare to the normal channel's coupling cap (C2). It's value is .047µF. That's 100X bigger than C3!
So, to decrease the brightness, increase the size of that 500pf coupling cap (C3). Easy to quickly experiment with. Just connect a couple gator clip leads across C3 and patch in another cap until you are satisfied with the tone. I suggest using a 500pf, .001, .0022, .0047, and .01. I suspect one of those values will make you happy. When you find your magic tone just permanently solder the magic cap across the 500pF. ***CAUTION*** There's 170V on one side of C3. Be careful.
I modified my AC-15 with a 6 position switch to select different value caps for that coupling cap. A very effect tone control. Look at page 2 of this pdf...
http://sluckeyamps.com/VAC15/Vox_AC15.pdf
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That 330K allows the 220pf bright cap to remain in effect even at high volume. Notice that the normal volume pot has this exact same configuration with the 330K and 220pf. Messing with these components is not gonna affect brightness much.
To really affect the brightness look at coupling cap C3. It's a tiny 500pf cap and has a huge effect on the brightness. Compare to the normal channel's coupling cap (C2). It's value is .047µF. That's 100X bigger than C3!
So, to decrease the brightness, increase the size of that 500pf coupling cap (C3). Easy to quickly experiment with. Just connect a couple gator clip leads across C3 and patch in another cap until you are satisfied with the tone. I suggest using a 500pf, .001, .0022, .0047, and .01. I suspect one of those values will make you happy. When you find your magic tone just permanently solder the magic cap across the 500pF. ***CAUTION*** There's 170V on one side of C3. Be careful.
I modified my AC-15 with a 6 position switch to select different value caps for that coupling cap. A very effect tone control. Look at page 2 of this pdf...
http://sluckeyamps.com/VAC15/Vox_AC15.pdf
I am extremely appreciative for the insights you offer on here! Thank you! Makes perfect sense. I'm not sure how I didn't catch how drastically different C2 is vs C3. I'd love to brighten up the normal channel a bit, it's just too dark to be usable without a treble booster or extremely bright guitar running into it so I think I'll experiment with replacing both. I honestly didn't even recognize C3 as a coupling cap, in the amp as you said it's a tiny little guy unlike any other cap I've seen before.
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I'd love to brighten up the normal channel a bit, it's just too dark
And now you know how to easily do that. But there's another way to brighten the normal channel too. Make the 25µF cathode bypass cap smaller (V1 pin 8). I would suggest 2µF to 5µF.
And look at how small the bright channel bypass cap is (V1 pin 3). It's only .1µF. Increasing the value of that cap will also help remove some brightness.
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I'd love to brighten up the normal channel a bit, it's just too dark
And now you know how to easily do that. But there's another way to brighten the normal channel too. Make the 25µF cathode bypass cap smaller (V1 pin 8). I would suggest 2µF to 5µF.
And look at how small the bright channel bypass cap is (V1 pin 3). It's only .1µF. Increasing the value of that cap will also help remove some brightness.
Wait wait wait. So the 0.1uF cap that's on tag #27 on the layout drawing (doesn't exist on the drawing but mine has it so I added it in) is a bypass cap of the bright channel?? I didn't even catch that difference on the layout vs my amp when I was combing through the thing.... on the layout pin 3 and pin 8 are jumpered but you're right on my amp they have separate wires going to tags. Wow, ok that's a something big I missed. I need to look at the amp in person and re-orient myself and make sure I'm following / seeing things correctly because as I look at it further in photo's I have of the board the 0.1uF and 25uF caps are not side by side like I have them in the layout drawing modification, there is a tag between them but I'm not sure what's there. I need to look in person and figure out what the heck is going on.
Edit: Just looking at one of my videos since I'm not near the amp, yeah the 25uF bypass cap is 9 tags from the right which matches the layout. Then there's the 1.5k resistor on tag 10 from the right which is correct, and then from what I can tell the missing 0.1uF cap is on it's own tag #11 from the right and then the .047uF C2 is on tag #12 from the right... but on the layout it's on tag #11. Still there appears to be the same number of total tags, so there's got to be more differences down the chain that I didn't notice. Very interesting...
And just to clarify, if I modify the C1 25uF cap to say a 5uF sprague I know I have lying around that will brighten up the normal channel? Will it affect the bright channel?
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So the 0.1uF cap that's on tag #27 on the layout drawing (doesn't exist on the drawing but mine has it so I added it in) is a bypass cap of the bright channel??
Well I don't know about tag #27 but the schematic you gave me shows a .1µF bypass cap on V1 pin 3.
And just to clarify, if I modify the C1 25uF cap to say a 5uF sprague I know I have lying around that will brighten up the normal channel?
yes
Will it affect the bright channel?
no
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Here's an idea... Don't change anything in the either channel. Instead, jump the channels together just like many Plexi people jump their bright and normal channels.
Plug guitar into J2. Connect a short patch cable between J1 and J4. Now you can use the two volume pots to blend the normal (dark) channel with the bright channel. Give this a try before changing caps. I think you will be pleasantly surprised.
I have the bright and dark channels of my Plexi style amps permanently jumped and really like it like that. I've always found the bright channel to be too shrill and the normal channel to be too muddy. But jumped together they sound fantastic.
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Here's an idea... Don't change anything in the either channel. Instead, jump the channels together just like many Plexi people jump their bright and normal channels.
Plug guitar into J2. Connect a short patch cable between J1 and J4. Now you can use the two volume pots to blend the normal (dark) channel with the bright channel. Give this a try before changing caps. I think you will be pleasantly surprised.
I have the bright and dark channels of my Plexi style amps permanently jumped and really like it like that. I've always found the bright channel to be too shrill and the normal channel to be too muddy. But jumped together they sound fantastic.
They're out of phase unfortunately.
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They're out of phase unfortunately.
Not so. Did you try it?
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They're out of phase unfortunately.
Not so. Did you try it?
Bear in mind that a TB AC30 bright channel has an additional common cathode, hence inverting, gain stage, compared to the normal channel.
It a different arrangement to an AC15, non TB AC30, or 5F6A based amp architecture.
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Another question. I removed the 220pf bright cap and 330k resistor off the brilliant channel volume pot to try and tame a little brightness the treble model has inherently. The results were not what I expected, it basically sounded identical, I couldn't tell any difference.
Why is that?
I wonder if that bright cap is dead?
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Bear in mind that a TB AC30 bright channel has an additional common cathode, hence inverting, gain stage, compared to the normal channel.
Ahh, yes. The TB circuit would put the channels out of phase. I was only looking at the AC-30.36 Treble schematic. I had not seen the Top Boost mod schematic until you just mentioned it. Sorry for my confusion Yeatzee.
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Bear in mind that a TB AC30 bright channel has an additional common cathode, hence inverting, gain stage, compared to the normal channel.
Ahh, yes. The TB circuit would put the channels out of phase. I was only looking at the AC-30.36 Treble schematic. I had not seen the Top Boost mod schematic until you just mentioned it. Sorry for my confusion Yeatzee.
All good! Hoping to do some testing tonight!
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Alright just got done swapping the 25uF bypass cap for the normal channel with a 5uF and then for the brilliant / TB channel I added a .0022uF in parallel with the 500pF coupling cap and recorded some quick side by sides with the Ox Box. Quick and dirty recordings, it's getting late so I'll have to review maybe tomorrow but initial impressions...
Normal channel feels a tad less bloated, but only a touch. Not a big difference going from 25uF to 5uF for the bypass cap. Think I might want to explore this one a bit more, but ultimately the true test will be through the cab so I'll reserve judgement.
TB / Brilliant channel, pretty significant difference here! I had some other values on hand but I tried the .0022uF first and felt like it was a solid change so I didn't try any others. Seemed to add in more lows/low mids and cut out some of the top top end. Through the OX with the volume on half it felt gainier and too much low mids at the same settings, but simply dialing back the cut control from 1 o'clock back to 11 o'clock seemed to get it in a very nice place. Looking forward to hearing it through the cab tomorrow, and i've got a gig tomorrow night so should learn a lot in a quick period of time.
(https://i.imgur.com/8bfGHlY.jpg)
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There's not much exploring to be done with the normal channel. Maybe experiment with smaller coupling caps for C2. You would need to remove the .047µF in order to experiment.
Take a look at Hoffman's AC30 schematic. It is drawn in a much more organized/logical way that makes it easy to see the signal flow and understand the circuit. Vox has never been able to draw a decent schematic. Ignore Hoffman's component reference designators because they have no relationship with Vox reference designators.
https://el34world.com/Hoffman/files/Hoffman_AC30.pdf
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Alright just got done swapping the 25uF bypass cap for the normal channel with a 5uF and then for the brilliant / TB channel I added a .0022uF in parallel with the 500pF coupling cap and recorded some quick side by sides with the Ox Box. Quick and dirty recordings, it's getting late so I'll have to review maybe tomorrow but initial impressions...
Normal channel feels a tad less bloated, but only a touch. Not a big difference going from 25uF to 5uF for the bypass cap. Think I might want to explore this one a bit more, but ultimately the true test will be through the cab so I'll reserve judgement.
TB / Brilliant channel, pretty significant difference here! I had some other values on hand but I tried the .0022uF first and felt like it was a solid change so I didn't try any others. Seemed to add in more lows/low mids and cut out some of the top top end. Through the OX with the volume on half it felt gainier and too much low mids at the same settings, but simply dialing back the cut control from 1 o'clock back to 11 o'clock seemed to get it in a very nice place. Looking forward to hearing it through the cab tomorrow, and i've got a gig tomorrow night so should learn a lot in a quick period of time.
(https://i.imgur.com/8bfGHlY.jpg)
What value is C5 on your amp? Just comparing the various AC30 and AC15 schematics and you will see at some point between the early 60s and the mid 60s the capacitors at the phase inverter changed from .01 to .047. Which while not the coupling cap, could still impact tone. In fact there is a note on the original AC15 Schematic noting that the capacitor value at the Phase Inverter changes on bass models.
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Alright just got done swapping the 25uF bypass cap for the normal channel with a 5uF and then for the brilliant / TB channel I added a .0022uF in parallel with the 500pF coupling cap and recorded some quick side by sides with the Ox Box. Quick and dirty recordings, it's getting late so I'll have to review maybe tomorrow but initial impressions...
Normal channel feels a tad less bloated, but only a touch. Not a big difference going from 25uF to 5uF for the bypass cap. Think I might want to explore this one a bit more, but ultimately the true test will be through the cab so I'll reserve judgement.
TB / Brilliant channel, pretty significant difference here! I had some other values on hand but I tried the .0022uF first and felt like it was a solid change so I didn't try any others. Seemed to add in more lows/low mids and cut out some of the top top end. Through the OX with the volume on half it felt gainier and too much low mids at the same settings, but simply dialing back the cut control from 1 o'clock back to 11 o'clock seemed to get it in a very nice place. Looking forward to hearing it through the cab tomorrow, and i've got a gig tomorrow night so should learn a lot in a quick period of time.
(https://i.imgur.com/8bfGHlY.jpg)
What value is C5 on your amp? Just comparing the various AC30 and AC15 schematics and you will see at some point between the early 60s and the mid 60s the capacitors at the phase inverter changed from .01 to .047. Which while not the coupling cap, could still impact tone. In fact there is a note on the original AC15 Schematic noting that the capacitor value at the Phase Inverter changes on bass models.
.01uF
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Good story, great score. Random occasional popping noise is most likely a loose socket pin clamp or cold/bad solder joint. Give the component leads a good wiggle with some needle-nose pliers- one way to find the culprit.
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RE: bias
You can put your 75 ohm back in and put various 2 watt resistors that you may already have in your stash in the 220 to 330 ohm range in parallel with it. This will allow you to dial in quickly without having to pony up for a single 10 watt for experimenting. The way the current splits in parallel the 2 watt will be perfectly fine in the amp.
You can even put the larger value resistor on a switch like some modern AC 30s have. Your 75 ohm would be employed on hot stages or outdoor gigs. You can switch a 330 ohm in parallel for roughly 62 ohms (which seems to be the modern compromise for torturing EL84s just shy of death at 120Vac+ from the wall in AC 30 style circuits). Current splitting is roughly 80/20 with that config, so 2 watts power rating is as good as 10 watts.
RE: Screen resistors
It seemed like not much technical decision making went into your choice of 470 ohm.
This isn't a knock on you, but an opportunity to put some more rigor into your choices on this in the future. Datasheet max screen voltage is laughably below where just about any guitar amp runs them. Study the function of the screen in a power tube and you'll quickly see why this is a huge area of concern. Tubes aren't cheap anymore, not like even just 5 years ago. So you can attempt to extend the life of your tubes at the expense of a bit of perceived increased compression "under the fingers" (and measurable on a scope if you're ever curious) by reducing the voltage on the screens, even at elevated plate voltages. There are multiple schools of thought on how this is done best: increasing the 4 screen resistors at each tube, replacing the 4 screen resistors with a single larger value and power rated resistor, or a bit of both, a larger value resistor of higher power rating feeding 4 individual screen resistors at the tubes. I prefer the latter. I like this because I can do significant voltage cutting with the resistor up front and retain the benefits of discrete screen resistors. For more info there are discussions at length at the 18watt forum. Those guys live and breathe EL84s. All that to say, if you find yourself wanting to experiment with other screen voltages it's perfectly valid and very easy to put a 5w resistor between the existing screen resistors and the screen node. On AC 30 style circuits (read most Dr Zs, Matchless, Bad Cats) I find 1.2k before the split (@100 ohm) gets me screen voltages around 320v, the common schematic value for vintage Vox. Play around with this to determine if the change in tone is worth replacing your tubes every 6 months instead of every 4--assuming a busy gig schedule. Combine this with the hot/cool bias switch detailed above and you have an amp that is capable of prolonging tube life and growling with the best of the AC30s at the flip of a switch.
RE: coupling and cathode bypass caps
This is valuable to understand if you're going to be seriously working on and voicing tube amps. Think of cathode bypass capacitors as high pass filters with gain and coupling caps as passive low cut filters. The bypass cap allows you to select which frequencies you want to boost and a coupling cap sets which frequencies you want to allow to pass to the next stage. There is a lot more going on technically including destructive feedback, frequency dependent impedance, DC coupling, and phase shifting... But not necessary to fully understand to make useful voicing changes in an existing circuit. Both capacitors have the effect of reducing low frequency content, but they aren't doing the same thing. Also know that these are essentially flat filters, but the rolloff isn't instant. It gently rolls and extends over more of an octave in the low frequency range than the high. See log frequency plots. This is why you'll see factors of 10 or even 100 differences in the values of these capacitors. In lowish gain vintage amps like AC30s and Black/Silver Fenders I prefer to do my low frequency shaping by cathode bypass capacitor value tweaking and leave my coupling capacitors as full frequency if they were initially selected to be so. I find this leaves more of the character of the amp intact and gives me just the shaping I was looking for--just my opinion/preference though. But try a 3.3uF in place of the 22uF on v2a of your Vibrolux sometime and be amazed at the ability to turn up your bass knob and not flub out--but it still sounds like a Fender Vibrolux. Just remember you're shaping/cutting the low end and this gives you the impression of increased brightness. Tweaking the values of either or both of these capacitors alone will not add brightness. For that you'd have to increase the gain of the triode along with the capacitor changes or add a cathode bypass cap where none previously existed.
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Good story, great score. Random occasional popping noise is most likely a loose socket pin clamp or cold/bad solder joint. Give the component leads a good wiggle with some needle-nose pliers- one way to find the culprit.
Thanks! It ended up being a bad tube
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Great info, cut because message was too long to respond to
Very interesting, I didn't know that's what vox did in their more modern ac30's! I'm pretty happy with the bias at the moment, right around 100% dissipation feels like a good spot to be and it sounds fine. No discoloration of the tubes either, and it's been used for several long gigs at this point and been rock solid.
Appreciate all of the tips and detail! I did the 470R after some conversations with people that pointed out 100R is too brutal on modern tubes and some think 1k+ is chokes them out a bit too much. 470 felt like a decent middle ground, but you're right I was shooting from the hip a bit there!
Finally for the different cap affects, it's fascinating to learn and is making the amp functionality make much more sense for me going forward. I can totally see tweaking based off tone impressions. I'm not totally convinced with what I did to the ac30 after my gig with it, TBD.
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Elevated the heaters with 100R resistors for the cathode bus wire... Unclear how big of a difference that made because I used the top boost channel as the before and after... And realized how insanely noisy it is in comparison to the other channels I haven't really used.
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Elevated the heaters with 100R resistors for the cathode bus wire... Unclear how big of a difference that made ...
Longer answer here (https://www.thegearpage.net/board/index.php?threads/1964-jmi-ac30-super-twin-my-first-vox.2492645/post-37967727).
Short Answer: there was no improvement in noise, because problem that an "elevated heater reference" cures wasn't present in your amp.
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Providing a ground reference or reference to output tube cathode voltage is not supposed to cure the type of noise you have coming out of the TB module. It's only effect at reducing 60Hz filament hum. There's a much better way to check to see if you get an improvement. Your ears are not a good method unless you are quickly switching between no reference and ground reference. Use a meter set to measure small AC volts on the speaker. Should decrease when you connect the ground reference or cathode voltage reference.
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Elevated the heaters with 100R resistors for the cathode bus wire... Unclear how big of a difference that made ...
Longer answer here (https://www.thegearpage.net/board/index.php?threads/1964-jmi-ac30-super-twin-my-first-vox.2492645/post-37967727).
Short Answer: there was no improvement in noise, because problem that an "elevated heater reference" cures wasn't present in your amp.
Hey HBP appreciate the thorough response. Here's my reply copied over:
ALWAYS appreciate when you chime in friend! A ton to soak in, but I think I'm following. Even with knowledge it'd barely affect the noise level, I still would have done it for the added safety benefit / solder reps so it's all good there! The process is fun either way.
Interesting thought about a resistor causing that, would a jump in hiss be that drastic going from Brilliant channel to Top Boost brilliant channel? Is the amplification of that additional gain stage that much? The Brilliant channel without the TB is what I'd consider pretty quiet for a vintage amp, with the TB it's quite loud (but not loud enough for me to freak out about it, I gig it and we're loud enough that none of that will ever ever be heard so who cares).
I have tried swapping the TB 12ax7 and V1's 12Ax7 in the past, neither made any discernable change to the "hiss". Another thought was the lead dress of the TB card, perhaps I didn't route the wires in the most optimal way?
Providing a ground reference or reference to output tube cathode voltage is not supposed to cure the type of noise you have coming out of the TB module. It's only effect at reducing 60Hz filament hum. There's a much better way to check to see if you get an improvement. Your ears are not a good method unless you are quickly switching between no reference and ground reference. Use a meter set to measure small AC volts on the speaker. Should decrease when you connect the ground reference or cathode voltage reference.
Cool tip!
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i=IG11rFnzwSrjDlBN
I'll circle back to the noise issue in a bit, but here's the video companion to the earlier bright cap discussion. I compared stock with the 220pF / 330k resistor, no resistor and 100pF cap, and neither cap nor resistor. To my ears, they all sounded just about identical which I wasn't expecting. Editing the cap change video today to share tomorrow.
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You had bridged a .0022µF cap across the 500pF coupling cap to reduce the brightness. Is that mod now permanent?
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You had bridged a .0022µF cap across the 500pF coupling cap to reduce the brightness. Is that mod now permanent?
That's the video I'm editing today hopefully to be done tomorrow or the next day. I did recordings before and after for that one as well
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And here it is, installing and comparing the coupling cap mod in the top boost channel and the Bypass cap mod in the normal channel. Super fascinating.
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Regarding your noise issue…if it’s mains hum frequency then make an L shaped shield/bracket and mount it on your on/off switch to shield it from the very close by Cut and PI circuitry. That really worked for me in one of my JMI AC30s.
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Regarding your noise issue…if it’s mains hum frequency then make an L shaped shield/bracket and mount it on your on/off switch to shield it from the very close by Cut and PI circuitry. That really worked for me in one of my JMI AC30s.
I'm not sure what it is to be honest, I feel like grounds are a big issue for me right now. Have any photo's?
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Regarding your noise issue…if it’s mains hum frequency then make an L shaped shield/bracket and mount it on your on/off switch to shield it from the very close by Cut and PI circuitry. That really worked for me in one of my JMI AC30s.
I'm not sure what it is to be honest, I feel like grounds are a big issue for me right now. Have any photo's?
Stop feeling and start measuring. Buy a cheap o-scope or build an audio probe and start tracing through the schematic. There are instructions here on this site for a "listening amp". Look into that. It will also make for a compelling video to find exactly where in the signal your noise is coming from. Visually and audibly using both the scope and the audio probe. You can also quickly identify the problem areas by using an alligator clip to ground out the grids of triodes. The alligator clips can then be used for auditioning ground points if you're looking to redo the ground scheme.