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

[Yeatzee]

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

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

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

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

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

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

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


Hopefully this is of some help.

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

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.

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.

[Platefire]

Just a thought on popping from vibration when turned way up. Have you tried it through a different set of speakers?

[Yeatzee]

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.

[Platefire]

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?

[Yeatzee]

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.

[Yeatzee]

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!

[Yeatzee]

Annnnnd finally some playing... plus another speedbump. Getting there!

i=0HCgB532LJy_KYZl

[Yeatzee]

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

[Jalmeida]

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.

[Yeatzee]

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.

[HotBluePlates]

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).

[Yeatzee]

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.

[Platefire]

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.

[Yeatzee]

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!

[Yeatzee]

What size fuse should I use for the inline HT fuse when I add it in? 1A?

[sluckey]

I see 500mA (1/2A) used in a lot of 4xEL84 amps.

[pdf64]

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.

[Yeatzee]

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!



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%?

[Yeatzee]

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 :)

[Jalmeida]

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.

[Yeatzee]

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!

[Jalmeida]

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).

[Yeatzee]

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

[Willabe]

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. 

[Jalmeida]

True. Although there are also alot of vintage 50-60 year old speakers still kicking in vintage amps.

[Yeatzee]

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!

[Jalmeida]

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.

[Yeatzee]

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!

[Jalmeida]

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.

[Yeatzee]

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.

[Jalmeida]

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.

[Yeatzee]

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.

[Yeatzee]

Little Wet/Dry testing with the AC30 as the dry amp:

--------------

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

[Yeatzee]

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?

[sluckey]

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?

[Yeatzee]

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.

[sluckey]

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

[Yeatzee]

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.

[sluckey]

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.

[Yeatzee]

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?

[sluckey]

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

[sluckey]

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.

[Yeatzee]

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.

[sluckey]

They're out of phase unfortunately.

Not so. Did you try it?

[pdf64]

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.

[pdf64]

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?

[sluckey]

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.

[Yeatzee]

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!

[Yeatzee]

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.

[sluckey]

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