Author Topic: Hammond AO-43 build (Read 20670 times)

NO_H2O · « **on:** July 18, 2015, 07:54:18 pm »

I picked up a very clean Hammond AO-43 organ amp/reverb unit. After some reading I know the B+ will have to come down. I am hoping that swapping the RT to a 5Y3 will get me in the pall park.
So far I have run all the original tubes across my tube tester to check for shorts and grid leakage. They all test good so I plugged them in, wired up a 10 inch speaker (unmounted flopped on the bench top), dug up a 1/4 in jack to RCA adapter, brought it up slow with my variac and had my frien try it with a Tellicaster. It was clean, clean, clean. No hum, etc. But it was not real loud either.
In the end I would like to have a VOX AC preamp an AC15ish power section. I think tremolo and reverb would be sweet too. If I were able to pick my favorite Tremolo sound ever. It would be just like my Fender Vibro Champ. If I were to pick my favorite reverb ever it would be either from a Fender Princeton or an Ampeg Reverborocket.
I am no expert when it comes to designing tube amp circuits, but I can follow a schematic and layout drawing. I like a Hoffman layout found in an AC 30 build thread on this forum. It is nice and tidy, one normal and one top boost input, 3 12AX7's and 4 EL84's. The schematic is nice, but the layout drawing is flipped. I could deal with the flipped drawing but it would be cool if I could find one that did not require a mirror to read.
I am thinking of using that preamp and plug it into the existing power section of the Hammond with the 5Y3 and see were I am at with that. Then plan out the finished circuit and transfer everything into a new chassis.
Has anyone gone down the AO-43 road before. Are the any bumps in the road I need to know about or derailments I am headed for?
Thanks Dave.

sluckey · « **Reply #1 on:** July 18, 2015, 08:43:43 pm »

Several people here have used the AO-43. It's a good amp with excellent iron. However, if you build an EL84 amp you will likely have a lot more B+ than you'd like. My solution was to use 6V6s instead. I built a Plexi 6V6 this year and Toxophilite built two 6V6 AB763s. He used the original chassis but I just used the iron in a new blank chassis. You can find info on all these amps in this same forum. And there is another member about to start a Plexi 6V6 conversion.

Here's a link to my AO-43. And I have another AO-43 for a future project.

http://home.comcast.net/~seluckey/amps/6v6plexi/6v6plexi.htm

Toxophilite · « **Reply #2 on:** July 18, 2015, 08:49:20 pm »

Sluckey the's the main forum member who guided me through my builds and they turned out great considering my previous experience.

I will add

Any speaker not on a baffle will be really really quiet(phase cancellation)

Moving already soldered components around to a new chassis probably won't be too fun/easy or 100% successful(unless you mean just the transformers)..If you plan to upgrade your build anyway go for broke and build what you want right away

NO_H2O · « **Reply #3 on:** July 19, 2015, 08:28:57 pm »

I was not worried about the volume when I tested it. The fact that it was not mounted and they it was a 16 ohm speaker. I was wanting to see if everything was working and if it had any hum. The super clean sound with no hum or noise made me happy.
I don't think I would have a problem converting to 6V6's. But I really want to hear these old Sylvania Black plates in action. I think I owe it to them to stab in a 5Y3 or 5R4, turn the voltage down to 110 on my variac and see if the B+ is in the ballpark. If so, I will use the Hoffman AC30 preamp section coupled to the Hammonp output section and roll it with a bucking transformer.
If it does not sound good, 6V6's here I come.
I do currently have a sweet sounding Vibro Champ that has a few mods that has some very impressive 6V6 tone. My son has taken it over and the folks he plays with could not believe that much good tone could come from a 5 watt Vibro Camp. But hey, if you are going to mic everyone's amp, that little Champ will never know it is small.
I might be swimming upstream for a bit, but I have to hear these old tubes first.
I know I won't be able to reuse the old board or resisters/capacitors over but they are cheap and I have a pile of them. So what the hell. I have to try it.
You guy are probably right and by the time I get the B+ down enough, the heaters will be at 4 volts. But I got to try it once.

Toxophilite · « **Reply #4 on:** July 19, 2015, 09:35:04 pm »

by all means
Have at 'er and have fun!

NO_H2O · « **Reply #5 on:** July 21, 2015, 12:56:07 pm »

Well after making a list of what I would need for the first stage of conversion, then digging thru a couple boxes of stuff I have hoarded over the years. I have most of the list covered. I will need 5 or 6 things so I think I will run up to Ack Radio and pick up my short list and get started. All I will need is some time.

NO_H2O · « **Reply #6 on:** July 11, 2016, 08:36:45 pm »

I am sure you thought I fell off the face of the earth. I had to put the AO-43 aside as life life happens. I have recovered and pulled that project out again. I thought I would just gut it and start over. After 2 or 3 plans I ran across the Hoffman AC30 layout and started down that path. Only I will do it as a 15 watt. More to follow.

plexi50 · « **Reply #7 on:** July 11, 2016, 09:09:20 pm »

This is an AO-43 i built i think in 2005. It was a great sounding amp. I just had to relocate the choke and add another PS cap next to the existing one. Plus strip that tiny board down and mount all the parts. It worked out. Very small board for sure.

http://www.triodeamplification.com/18Watt.html

NO_H2O · « **Reply #8 on:** July 12, 2016, 08:41:18 am »

That plexi a nice set up. Maybe I should have gone that way.
I started with the idea of using a clean (new) chassie and mod the layout. But it turned into a gut and start over job. I am shooting for a small 1-12 combo in the end. It will be a bit tight and busy inside but I think I can get it all in there if I take my time and keep it neet.

NO_H2O · « **Reply #9 on:** July 14, 2016, 02:45:20 pm »

Bought a box when The plan was to mod the AO-43 and just have a clean box to start with. But then I found the Hoffman AC30 no trem layout. I stuck with the narrow build idea to keep the size of the combo small. We will see if that was a good idea or not.

NO_H2O · « **Reply #10 on:** July 14, 2016, 02:48:25 pm »

I modified an off the shelf board and used a section of the original Hammond board for the filter caps.

NO_H2O · « **Reply #11 on:** July 14, 2016, 02:52:55 pm »

Filter cap board.

NO_H2O · « **Reply #12 on:** July 14, 2016, 02:55:25 pm »

Main board and ground buss/control pots in.

NO_H2O · « **Reply #13 on:** July 14, 2016, 02:58:34 pm »

The "NO LICK" power section is in.
Fell free to point out anything I have screwed up. No thin skin here.

sluckey · « **Reply #14 on:** July 14, 2016, 03:09:19 pm »

I've built 3 amps in a narrow 3 x 4 x 17 Hammond chassis. Is that the same as yours? They are a challenge to build in but it's doable. I had to really plan in layers for my TDR. They are not fun to work on later. Keep in mind how you will replace filter caps, etc. down the road.

NO_H2O · « **Reply #15 on:** July 14, 2016, 06:30:23 pm »

I think it is the same chassis. And yes, it is a bit of a stacking/layering build. I will give enough slack to be able to pull the filter cap board up and replace the caps. the main board will have to be un-soldered to remove it. I will have to take my time and keep it neat.

NO_H2O · « **Reply #16 on:** July 15, 2016, 02:31:03 pm »

Another trip to Ack Radio for the rest of my components and I finished and mounted the main board to the chassis. And mounted the output transformer and choke.

NO_H2O · « **Reply #17 on:** July 15, 2016, 02:35:54 pm »

Damn. It will be tight down it there. I think I will leave the heater wiring until last and keep the twisted pairs up toward the opening of the chassis and everything else down toward the closed face.

eleventeen · « **Reply #18 on:** July 15, 2016, 10:44:09 pm »

Definitely a tight chassis to work in. No two ways about it, especially with mounting vol/tone controls which the original did not have.

Willabe · « **Reply #19 on:** July 15, 2016, 10:51:05 pm »

That hook up wire is not the easiest type to work with in tight spaces.

The cloth push back hook up wire that Doug sells is much easier to work with. It bends easy, stays in place well.

And no striping the insulation. You just cut it to length, push the cloth braid back, solder the wire in place, push the braid back, done.

NO_H2O · « **Reply #20 on:** July 17, 2016, 08:16:42 pm »

Willabe, your are correct about this wire. I may have to order some of the good wire from Doug to do the twisted pairs for the heaters. I went as far as I could today. I stopped short at the heater wiring. Here are some pix so let me have it.

NO_H2O · « **Reply #21 on:** July 17, 2016, 08:18:53 pm »

More pix.

eleventeen · « **Reply #22 on:** July 17, 2016, 08:29:33 pm »

Your build looks excellent, to me. I didn't quite notice at first you have the tubes mounted on the long narrow side of the chassis, like a tweed amp. Sharp. I think you're doing fine.

NO_H2O · « **Reply #23 on:** July 17, 2016, 08:56:55 pm »

Thanks. We will find out when I start to bring the power up on it. It is my first full build. I have done some mods and a couple repairs and recaps but this is ground up amp.

NO_H2O · « **Reply #24 on:** July 18, 2016, 02:08:13 pm »

Well I am not sure how "neat and tidy" it is but the heater wires are in. This wire is tricky on a single run. But a twisted pair is down right bulky. I was trying to keep it away from al the other stuff but that was tough. I wanted to finish up but came real close to ordering some new wire.

NO_H2O · « **Reply #25 on:** July 18, 2016, 04:37:48 pm »

I caught a mistake as I was double checking. I joined the heater wires to th grounded end of the simulated center tap resisters.

But it was a quick fix.

Willabe · « **Reply #26 on:** July 18, 2016, 06:49:17 pm »

Quote from: NO_H2O on July 18, 2016, 04:37:48 pm

I caught a mistake as I was double checking. I joined the heater wires to th grounded end of the simulated center tap resisters. But it was a quick fix.

Good catch!

And it all looks real good!

NO_H2O · « **Reply #27 on:** July 18, 2016, 11:10:42 pm »

I guess the next step is to stuff a rectifier tube in and bring the power up slowly and watch the amperage and see if I can get to full power without finding any more mistakes. Then take voltage readings on all the tube socket pins and report back here to the amp builders guild.
My variac is down at a friends place but he does have an 8 ohm cabinet to plug into while I power it up.

Willabe · « **Reply #28 on:** July 19, 2016, 12:29:11 am »

Do you have a light bulb limiter? If not there very simple to build, link below. Use it for 1st power ups.

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

NO_H2O · « **Reply #29 on:** July 19, 2016, 12:32:12 am »

I have thought I need to build one.

Paul1453 · « **Reply #30 on:** July 19, 2016, 02:43:08 pm »

I think you did a very good job with your heater wires in that confined space.

You've come in from the top and kept the signal wires and board down low and about as far away as you could.

Hopefully you will have very good results!

HotBluePlates · « **Reply #31 on:** July 19, 2016, 04:52:33 pm »

+1

I think you've done a damn fine job working in the tight space of that chassis.

Good luck on the power up!

(And yes, I fell victim to buying that exact same wire before... Never again!!)

mresistor · « **Reply #32 on:** July 19, 2016, 05:21:15 pm »

It looks very very nice... great work...

NO_H2O · « **Reply #33 on:** July 19, 2016, 05:29:51 pm »

Thanks guys. it will be a couple days before I get a limiter built and get with my friend and get to my variable power supply.

sluckey · « **Reply #34 on:** July 19, 2016, 07:08:28 pm »

Everything I've seen looks top notch. I bet it fires up just fine on the first try.

Willabe · « **Reply #35 on:** July 19, 2016, 07:23:14 pm »

You don't need the variac for 1st power up when using the lamp limiter. Just make sure you use a small wattage (60w to 100w) light bulb.

There have been a few guys who posted here that they were using a 200w and a 300w bulb and that's too large for even a 100w amp to give enough protection for a short.

The smaller the wattage of the bulb the more protection it will give.

I and others here have a variac but don't use it for 1st power ups, we use the lamp limiter alone with a small wattage bulb.

NO_H2O · « **Reply #36 on:** July 19, 2016, 08:01:51 pm »

Thanks. I will build one and fire it up and start recording my voltage at each pin.

PRR · « **Reply #37 on:** July 19, 2016, 10:01:06 pm »

> build one ... recording my voltage

Don't take voltages while on a lamp-limiter. Fire it up, watch lamp, wait for smoke, and quick check you have "some" light in heaters and "some" voltage at B+. You might even play a bit, but don't fret that it sounds lame or unhappy.

NO_H2O · « **Reply #38 on:** July 22, 2016, 03:25:34 pm »

Built the current limiter.

Willabe · « **Reply #39 on:** July 22, 2016, 03:42:40 pm »

Looks real nice.

If you can test it on a known working tube amp so you get a feel for the brightness/dim look of the bulb at 1st turn on, how it looks with a working amp with no shorts.

NO_H2O · « **Reply #40 on:** July 22, 2016, 03:48:36 pm »

Will do. Thanks for all the help.

jojokeo · « **Reply #41 on:** July 22, 2016, 11:28:03 pm »

Quote from: NO_H2O on July 19, 2016, 08:01:51 pm

Thanks. I will build one and fire it up and start recording my voltage at each pin.

To avoid frustrations later on I've gotten into the habit of testing connections for continuity or voltages - without any tubes in place for SS recto or only with the recto tube in place. This is without the limiter. You can test your heater wiring down the entire string, power tube plate & screen points, and B+ down the string - all unloaded of course. This can catch all voltage mis-wiring and get confirmation for proper voltages at all respective pins. It's not just for mis-wiring as you may also catch cold solder joints. I had this happen once on a preamp tube heater pin. It was found pretty fast after getting no sound upon 1st fire up. I take pride in getting things right the first time but this little diddy caught me by surprise, made me frustrated a bit at first, and caused me to laugh at myself for not practicing what I preach & know to do. Murphy's Law always finds a way to raise his ugly head.
Anyway, you can still have other mis-wiring issues but at least the basics and or fatal things are out of the way upon 1st fire-up and you can be reasonably sure you will not have any big issues BUT - it's highly recommended to still use the lamp limiter for your first fire up with all tubes in place.

NO_H2O · « **Reply #42 on:** July 23, 2016, 10:13:51 am »

Thanks. That was my plan for the first power up. I my have a chance to do that today. I will post results when I get it powered up.

NO_H2O · « **Reply #43 on:** July 23, 2016, 11:10:45 am »

I ran down to my buddy's place to borrow a pair of spare Russian EL84's and pick up my variac power supply. I remember now why I hate moving the damn thing. I opened it up to see about mounting a household outlet to plug into instead of using a pigtail with a ground jumper. Now I know why it weighs a ton. And it has a lamp limiter built in. You just can't see it when it is closed up. It has some huge transformers in it. Way more power supply than I need but they were throwing it out at work years back so I loaded in the car.
And I think I will stick with the pigtail.

NO_H2O · « **Reply #44 on:** July 23, 2016, 01:23:57 pm »

Power up with only a 5Y3 and an 8 ohm cabinet plugged in. No noise and no smoke and the limiter lamp behaved well.
Voltage check was as follows.
Node A 440 vdc
Node B 440 vdc
Node C 438 vdc
Node D 438 vdc
Node E 438 vdc
V1
Pin 1 430 vdc
Pin 2 0
Pin 3 0
Pin 4 7.3 vac
Pin 5 7.3 vac
Pin 6 430 vdc
Pin 7 0
Pin 8 0
Pin 9 7.3 vac
V2
Pin 1 435 vdc
Pin 2 0
Pin 3 0
Pin 4 7.3 vac
Pin 5 7.3 vac
Pin 6 435 vdc
Pin 7 435 vdc
Pin 8 0
Pin 9 7.3 vac
V3 PI
Pin 1 432 vdc
Pin 2 0
Pin 3 0
Pin 4 7.3 vac
Pin 5 7.3 vac
Pin 6 432 vdc
Pin 7 0
Pin 8 0
Pin 9 7.3 vac
V4 output
Pin 1 0
Pin 2 0
Pin 3 0
Pin 4 7.3 vac
Pin 5 7.3 vac
Pin 6 0
Pin 7 440 vdc
Pin 8 0
Pin 9 440 vdc
V5 output
Pin 1 0
Pin 2 0
Pin 3 0
Pin 4 7.3 vac
Pin 5 7.3 vac
Pin 6 0
Pin 7 440 vdc
Pin 8 0
Pin 9 440 vdc

Voltages seem high to me. Anyone catch something I missed. Ok I must admit. The first power up was a dud. Everything was a 0. No limiter lamp surge, on voltage no jewel lamp. It helps if you put in a jewel lamp bulb and a fuse before you star.

sluckey · « **Reply #45 on:** July 23, 2016, 01:30:11 pm »

All voltages will be high with no tubes plugged in. Now put all tubes in and power up on the lamp limiter again. If lamp limiter does not glow brightly then plug amp straight into the wall.

NO_H2O · « **Reply #46 on:** July 23, 2016, 03:42:14 pm »

Ok. All tubes in. Powered up with the limiter and bulb went back to a dim glow. No smoke no funky noises. Plugged it in the wall and powered it up. Warmed it up a bit. No smoke only a faint tube amp hum. I don't have an ave here so I passed a 500 MHz tone thru it and it makes noise thru both channels.
Voltages as follows.

V1 12AX7
Pin 1 1.3 vdc
Pin 2 0
Pin 3 143 vdc
Pin 4 6.8 vac
Pin 5 6.8 vac
Pin 7 143 vdc
Pin 8 1.3 vdc
Pin 9 6.8 vac

V2 12AX7
Pin 1 183 vdc
Pin 2 0
Pin 3 1.3 vdc
Pin 4 6.8 vac
Pin 5 6.8 vac
Pin 6 297 vdc
Pin 7 183 vdc
Pin 8 183 vdc
Pin 9 6.8 vac

V3 PI 12AX7
Pin 1 235 vdc
Pin 2 45.5 vdc
Pin3 67.9 vdc
Pin 4 6.8 vac
Pin 5 6.8 vac
Pin 6 238 vdc
Pin 7 46 vdc
Pin 8 67.5 vdc
Pin 9 6.8 vac

V4 EL84
Pin 1 0
Pin 2 12 vdc
Pin 3 16.6 vdc
Pin 4 6.8 vac
Pin 5 6.8 vac
Pin 6 26.8 vdc
Pin 7 323 vdc
Pin 8 21.5 vdc
Pin 9 341 vdc

V5 EL84
Pin 1 0
Pin 2 0
Pin 3 15.5
Pin 4 6.8 vac
Pin 5 6.8 vac
Pin 6 2 vdc
Pin 7 347 vdc
Pin 8 2 vdc
Pin 9 346 vdc

Not sure why small voltages are on pins 2, 6 and 8 of V 5 and not V 6.
Looking over the numbers, what do you think. I need to haul it down to my buddy's place and run it thru the 8 ohm greenbacks and see what it sounds like. I need the check the place disapation first to see if it is in the ballpark.

sluckey · « **Reply #47 on:** July 23, 2016, 04:15:18 pm »

Tell me about your EL84 tubes. Are they old tubes that may have come with that AO-43 amp? Or new, modern tubes?

NO_H2O · « **Reply #48 on:** July 23, 2016, 04:39:36 pm »

They are older tubes pulled from a modern H and K combo. I used some Russian /China tubes to power it up and once I am sure about it, I will put the original black tubes back in it. Didn't want to risk them.

sluckey · « **Reply #49 on:** July 23, 2016, 04:47:50 pm »

There is a bit of non standardization with EL84 tubes. They don't all have the same pin out. For instance, I have some that have pins 1 and 2 tied together inside the tube. Can't use those on Doug's AC30 layout. Maybe your Russian/Chinese tubes have some peculiarities that might account for voltages on pins that don't even have anything connected? Put some good tube in and recheck voltages. Oh, and recheck V1 voltages too. I think you dropped a stitch when you were typing them.

Welcome To the Hoffman Amplifiers Forum September 08, 2025, 10:03:17 am

Welcome, Guest. Please login or register. -User Name -Password
Home Help Search Login Register
Hoffman Amplifiers Tube Amplifier Forum » Amp Stuff » Tube Amp Building - Tweaks - Repairs » Hammond AO-43 build

Choose a link from the Hoffman Amplifiers parts catalog
Mobile Device Catalog Link	Yard Sale Discontinued	Misc. Hardware
What's New	Board Building Parts	Amp trim Handles
Lamps Diodes	Hoffman Turret Boards	Channel Switching
Resistors	Fender Eyelet Boards	Screws/Nuts Washers
Jacks/Plugs Connectors	Misc Eyelet Boards	Tools
Capacitors	Custom Boards	Tubes Valves
Pots Knobs	Fuses/Cords	Chassis
Tube Sockets	Switches	Wire Cable

Welcome To the Hoffman Amplifiers Forum

Author Topic: Hammond AO-43 build (Read 20670 times)