Hammond AO-29 amp convert to Plexi 6V6?

[frpax]

Of course I kept the serial # plate from the original organ, and I mounted it on the bottom back slat.

[frpax]

I'm close to ordering my turret board material an turrets... do I need one of those staking tools to install the turrets? (I do not want to make my own)

[sluckey]

yes

[frpax]

yes

Thank you, sluckey!

Also, the Hoffman Plexi 6V6 schematic and layout do not use a tube rectifier, and I plan on doing so. What changes to the layout do I need to make?

[sluckey]

Don't install the two diodes in the lower right corner of the board. Connect the PT red wires to pins 4 and 6 of the rectifier socket. Run a wire from pin 4 of the rectifier socket to the bottom of that 220K/3W resistor on the right side of the board. Run a wire from pin 8 of the rectifier socket to the turret in the lower left corner of the board (the turret that no longer has a diode on it). If you want a STBY switch, put it between pin 8 of the rectifier socket and the board. That should be all.

[frpax]

Don't install the two diodes in the lower right corner of the board. Connect the PT red wires to pins 4 and 6 of the rectifier socket. Run a wire from pin 4 of the rectifier socket to the bottom of that 220K/3W resistor on the right side of the board. Run a wire from pin 8 of the rectifier socket to the turret in the lower left corner of the board (the turret that no longer has a diode on it). If you want a STBY switch, put it between pin 8 of the rectifier socket and the board. That should be all.

sluckey, like this?

[sluckey]

yes

[Paul1453]

Remember to make your cap can connections at ABCD, which are not shown here but are elsewhere.

What rectifier tube are you planning on using?

Some tube rectifiers can be sensitive to the value of the A node E-cap.
You want to make sure your rectifier is OK with that A node cap's uF value.
The other nodes should be good with Doug's cap can values.

This is a very nice sounding amp, IMO. 

[frpax]

Remember to make your cap can connections at ABCD, which are not shown here but are elsewhere.

What rectifier tube are you planning on using?

Some tube rectifiers can be sensitive to the value of the A node E-cap.
You want to make sure your rectifier is OK with that A node cap's uF value.
The other nodes should be good with Doug's cap can values.

This is a very nice sounding amp, IMO. 

I'm planning on using the same 5U4 rectifier tube that the original AO-29 had. I'm also going to use the original PT & OT, with the addition of that Mercury Magnetics Fender choke.

I'm dropping the "bright volume" pot from the Plexi 6V6 layout as well, and will run the 470k resistor from the volume middle lug straight to pin 2 of V2. So, since I am doing that and therefore eliminating the shielded wire that goes to the turret board and hooks up to a .0022 capacitor, I can eliminate that cap also, correct?

[Paul1453]

For me, I just went with Doug's 1 input 2 volume control and Master volume plan exactly.

I like the flexible tone you can get mixing the 2 input signals.

I definitely wouldn't modify that just to drop a shielded wire, pot, and cap.
But it is your amp, and you can make it how you want.
As long as you can deal with any issues your modifications may cause.   

Attached is a snip from the 5U4GB Data Sheet.
It indicates 40 uF as the limit for the input cap.
I think Doug's cap can is all 50 uF if I recall correctly.

Maybe one of the more experienced moderators can give you better guidance on the 5U4's input cap specification?
I wouldn't want you to burn up any 5U4s charging up an over value input E-cap. 
I know a SS rectifier doesn't have problems with higher value input E-caps.
I'm not certain on what a 5U4 will let you get away with using as an input cap.   

[sluckey]

Neither Doug's cap can nor his layout has any 50µF filter caps.

[Paul1453]

Sluckey is right, Doug's cap can is 40/20/20/20.
Too many amp PS's getting confused in my mind.

I should have checked the plans 1st.

So the 40uF is fine for the 5U4 or 5AR4.

But swapping in a 5Y3, 5R4 or 5V4 instead, might not be good.   
Those do seem to want a lower value input cap.
Maybe it's still OK, I tend to use a SS rectifier.
Cheaper, easier, less space used.

[frpax]

For me, I just went with Doug's 1 input 2 volume control and Master volume plan exactly.

I like the flexible tone you can get mixing the 2 input signals.

I definitely wouldn't modify that just to drop a shielded wire, pot, and cap.
But it is your amp, and you can make it how you want.
As long as you can deal with any issues your modifications may cause.   

Couple of reasons for dropping the Bright Volume.
1. In order to have sufficient space between the knobs and with the OT being where it is, I don't want to jam all the knobs too close to each other.
2. Never had one before, so I won't really miss it.
3. Since deleting it, I know I'm losing 1/2 of a 12AX7 (V1), so I'm going to wire that half in parallel with the other half by jumpering pins 2 to 7, 1 to 6, and 3 to 8 (4 & 5 are already jumpered), while keeping everything else the same. I should get more gain this way. Correct?

[Paul1453]

1. It seems as if you have the space to follow the plans.
    I moved the presence pot to the back near the output jack, that seemed easier to me.

2. You never know what you might be missing if you don't try it.

3. I'm just a rusty old repair tech trying my best to learn this design stuff.
    Most modifications I make are to try to use my old vintage junk parts safely and efficiently.
    I've got many more years of trial and error experimentation to do before I could answer that question properly.   
    There are many other members here that could answer most all our questions with authority.   

[frpax]

Couple of update pics.
Got the new grill cloth put on:



And a nice Weber Silver Bell:



It's looking fairly good so far!
I rounded all the squared corners with my router. Stained it as close as I could to match the rest.
I've got a few minor details to do to button up the cabinet, and by the time that's done, I'll be ready to tackle the chassis.

[frpax]

I've been busy again!

I got the stuff to make my board and several items for the chassis.

So, I went from Doug's original board drill guide:



And because the dimensions of the board are different from his to what I need, I converted that to this:




I taped that drill guide to my board material and after center punching and drilling, I ended up with this:



Not totally perfect, I know, but close enough! Eh?

I decided to go with eyelets, rather than turrets. A local amp builder that I know very well said he prefers eyelets, so that's the way I went.


So far, so good!

[sluckey]

Two of the lugs for the bias pot are definitely close enough. 

I'm surprised that you chose eyelets over turrets. I bet if you did a poll on this board that turrets would be favored over eyelets.

[frpax]

Two of the lugs for the bias pot are definitely close enough. 

I'm surprised that you chose eyelets over turrets. I bet if you did a poll on this board that turrets would be favored over eyelets.

I was very torn on the turrets/eyelets decision. I believe, hands down, that turrets would be easier to work with. But my amp builder friend said that he prefers the sound of eyelets over turrets. That's why I went the eyelet route. He must have hearing like Eric Johnson. I doubt that anyone, other than those two, would be able to tell any difference!

At this point, I'm almost done with the cabinet. I'm going to start assembling the main chassis components next.

I will then order all new caps and resistors from Doug and get it all wired up.

Hopefully, it will actually work!   

[sluckey]

But my amp builder friend said that he prefers the sound of eyelets over turrets.

No wonder I can never quite get the sound right. I'm changing out my turrets this winter. Is it just the ones in the signal path or should I just change all of them?   

[Willabe]

    All that work down the drain. 

(One very famous tube amp guru said he could hear the difference in sound between different colored PVC wire insulation.)
 

[sluckey]

I've heard that clear insulation has the most transparent sound. Adds zero coloration to the sound.

[frpax]

OK. OK....

You jokey jokesters...   

[sluckey]

[Paul1453]

I think you will find the turrets easier to solder on and place the components.

Especially if you want to tweak things and or change/replace failed components.

With those type of eyelets the board tends to burn and the eyelet loosen from multiple solder applications.   

[frpax]

Major components are installed:







Its starting to look like an amplifier!

[Paul1453]

Looking really good!

Still planning on dropping the bright pot?

There is room for your presence pot next to your output jack.   

[Willabe]

I think you will find the turrets easier to solder on and place the components.

Especially if you want to tweak things and or change/replace failed components.

With those type of eyelets the board tends to burn and the eyelet loosen from multiple solder applications.   

Actually eyelets are easier to tweak/change parts than turrets.

And the boards don't tend to burn and the eyelets don't loosen from multiple re-heating's any more than with turrets.

[Paul1453]

OK, so that's your opinion and I'm wrong again.

If you say so.

[frpax]

Looking really good!

Still planning on dropping the bright pot?

There is room for your presence pot next to your output jack.   

Yes, dropped the Bright pot.   

[sluckey]

Yes, dropped the Bright pot.

You don't know what you're giving up.

[frpax]

Yes, dropped the Bright pot.

You don't know what you're giving up.

You're right, but I've already committed down the path of 6 pots.
Would you give up the Presence pot in lieu of the Bright? I could go that way.

[Paul1453]

Presence pot can go on the back near your output jack, and still keep your 6 pots on front.   

[frpax]

Presence pot can go on the back near your output jack, and still keep your 6 pots on front.   

I suppose I could do that.

[sluckey]

Would you give up the Presence pot in lieu of the Bright? I could go that way.

Absolutely! Being able to blend the bright channel with the normal (dark) channel is a big part of the plexi mojo. You'll probably diddle with the presence pot for about a week then it will likely be left alone.

However, you could probably put the presence pot on the front panel if you like. The presence pot is in the NFB loop which comes from the OT, so no worries about the pot being mounted right over the OT. I'd do it in a heartbeat.

Or, you could put it on the rear panel next to the speaker jack and even simplify the wiring by mounting R26 directly between the speaker jack and the pot.

Bottom line, that bright channel has much more tone value than that presence pot.

[frpax]

Would you give up the Presence pot in lieu of the Bright? I could go that way.

Absolutely! Being able to blend the bright channel with the normal (dark) channel is a big part of the plexi mojo. You'll probably diddle with the presence pot for about a week then it will likely be left alone.

However, you could probably put the presence pot on the front panel if you like. The presence pot is in the NFB loop which comes from the OT, so no worries about the pot being mounted right over the OT. I'd do it in a heartbeat.

Or, you could put it on the rear panel next to the speaker jack and even simplify the wiring by mounting R26 directly between the speaker jack and the pot.

Bottom line, that bright channel has much more tone value than that presence pot.

OK, you & Paul talked me into putting the presence pot on the back.

[Paul1453]

Sluckey is absolutely right about diddling that pot.   

The only time I ever adjust it is if someone want to hear just how rough and nasty my Plexi can get.
Otherwise it is set mid range and forgotten about.
The salvaged pots of that value I have are all screw driver type with a locking nut.

The bright pot seems to be the one that I adjust the most often.
It gives just the right edge to the distortion tones, IMHO.   

[frpax]

Now that I'm putting the finishing touches on the cabinet, I have a question for you guys.
To add a little detail to the cabinet, I decided to run some green piping around the perimeter of the speaker baffle.
Should I also run the piping around the actual speaker/grill cloth area? Or no?

With




Without

[frpax]

A sneak peek of what it will look like:

[Paul1453]

You are way past the best looking amp I have ever built.   

My 1st Plexi is still naked, looking like a homely hobbled together science fair project.   

But, everyone who has ever heard or played it has said it is a very nice sounding amp.   

[frpax]

You are way past the best looking amp I have ever built.   

My 1st Plexi is still naked, looking like a homely hobbled together science fair project.   

But, everyone who has ever heard or played it has said it is a very nice sounding amp.   

Thank you Paul!

This is the first time I've ever built an amp. I built a 2x12 extension cabinet before, but this is, by far, the most aggressive undertaking I've ever attempted.

[frpax]

A few more pics of it nearly done:

[Paul1453]

That looks great to me!   

I think you will be very pleased with what you have created!

As you get closer to firing her up the excitement builds.

Please don't let that excitement cause you to rush or overlook things,
as it has done to me before.

Take your time, and do the highlighter tracing verification of your work.  Use your meter to check every line on that diagram.
Ensure your - bias voltage is present at pin 5 of your 6V6 tube sockets before you ever put them in.
Set it at the highest - voltage your pot will adjust to at 1st.
That will keep you from red plating your tubes, and give you some time to verify your voltage readings throughout the amp.
I had a leaky cap on my PI when I 1st fired mine up.  That might have caused 1 of my 6V6s to red plate if I had not set the bias so negative.
I had time to record all my voltage readings and compare them to the schematic without causing any harm to my tube.
It was very obvious from the voltage reading at that 1 point, that something was not right.  The amp did make sound at this point.
But I was not tempted to just start playing it because I knew something was wrong.
I quickly identified and replaced that leaky cap and all voltages were then good.
Then I adjusted the bias down to what I wanted, and tried her out.
It sounded great and I haven't had any other problems with it yet.

If something doesn't seem right, or you need help.
Post your questions here, and someone will usually give you very good advice rather quickly.   

[drew]

 My 2c : it doesn't need the piping around the speaker.

[frpax]

OK, so since I am adding the Bright Volume in and moving the Presence pot to the back, the layout shows a 3 lug terminal strip for the Bright Volume (of which, only 2 of the 3 lugs are used).
Due to the lack of space in my narrow chassis, I could mount the terminal strip right onto the eyelet board where there is some room right next to the Bright pot. OR, I can add eyelets right in that same spot and that would accomplish the exact same thing, wouldn't it?

[Paul1453]

Terminal strip can be eliminated, sort of.

Take a look at this pic, and remember it's my 1st science fair salvaged parts Plexi.   

Those parts can kind of be joined together, floating in a sort of point to point wiring way.

I'm sure you will probably figure out a nicer prettier way than I did on this, but it works fine.   

[Willabe]

Paul I sent you a PM and have not received a reply. 

[frpax]

Question:

When wiring in the Bright Volume, I'll be using the RG174 shielded wire.
In the image below, it looks like at the pot, I hook the hot to the lug and ground the shield. YELLOW CIRCLE
Now at the board, to I just wire the hot to the turret (eyelet, in my case) or what? BLUE CIRCLE

[sluckey]

Now at the board, to I just wire the hot to the turret (eyelet, in my case) or what? BLUE CIRCLE

yes

[Paul1453]

You got it right.  Only signal wire at the board.

Signal wire at the pot, and shielding Gnd wire to what I call a Gnd bus wire at the pots.

If you look closely at that last pic, you will maybe see that.

And you will maybe notice I had already had swapped my Gnd and signal connections because I had wired it up backwards the 1st time.   

[frpax]

Very good on the signal to eyelet only.

Now, here's yet ANOTHER question!

In this image, this thing labeled "feedback" has me confused. Do I run a wire from the board to the speaker jack? I'm not using an impedance selector.



Also, by looking at my modifications/adaptations, does everything look copacetic?