Hammond AO-63 conversion

[jmccanna]

Hello,

I recently acquired a cosmetically damaged but functional Hammond organ with the AO-63 chassis. The amp worked but had a very noticeable hum and motor-boated too. If I recall correctly in messing around with prior to disassembly the reverb did not work either. So, I pulled the amp, preamp and reverb box to see what it needed. Given the condition of the organ I have decided to try to convert this to a guitar amp. I know there are several threads on the Hammond here and reached out to Sluckey via PM because of his very cool GemiHammond project. He kindly replied and suggested I start a new thread here so everyone could participate and learn.

The first thing I noticed was the power cord had generally disintegrated. And I had two very burned resistors to the point they both broke when I tugged on them. I studied the schematic from the service manual and the documents from Sluckey's website too. Near as I can tell, R381 was completely burned through. My reading of the schematic shows that R378/379/380/381 are connected in series with the end of 381 going to C341 and to the reverb rectifier. Perhaps I am wrong because the copy is a bit hard to read.

Anyway, physical examination of the circuits differs. R381 does not appear to be connected to 380 at all. The resistors are mounted in a ladder-like configuration crossways:

R378
R379
R380
R381

[jmccanna]

Wow, hit a button I did expect sorry for the double post. Continuing on:


Anyway, physical examination of the circuits differs. R381 does not appear to be connected to 380 at all. The resistors are mounted in a ladder-like configuration crossways:

R378 <------------ L300
<
R379
>
R380

R381

The ">" indicates a jumper/connection between the legs of the resistors. In my set, where the schematic shows the left side of 380 connecting to the left side of 381 it does not. Instead there is a wire going from the left side of 381 to what I believe is C341 and the right side goes to the reverb rectifier. Where the wire connects to C341 there is another very burned resistor that I believe is supposed to be 470 ohm. My reading of the schematic leads me to believe that the wire going to C341 should be coming from the other end of R381 and that R381 should be jumpered to R380.

I have uploaded pictures and the schematic set from Slucky for convenience.

I noticed in studying Sluckey's article on his conversion that his picture of the original wiring appears the same as mine. I do not see the red wire going to the far side of R381. I do notice however that his original R381 looks burned too. Mine was much worse though. This leads me to suspect that I am reading the schematic wrong.

I would like to convert this using the existing preamp, reverb and amplifier and employ the reverb amp that is already on the chassis. The concept of mixing in a reverb through a separate speaker intrigues me. Why? I think just because it is a fun idea.

Any thoughts, corrections or suggestions are welcome. I feel stuck because I am not quite sure I am reading this right.

[sluckey]

The AO-63 will make a very nice guitar amp. Lot's of possibilities. It's also a complex circuit and the PTP wiring makes it tough to work on. Gonna take a lot of patience.

The big power resistor at the bottom of the "ladder" that you think is R381 is really R373 and it does connect directly to pin 3 of the EZ81 rectifier. It does not connect to any of the other three power resistors in that "ladder". Look near the center of the chassis. You will see a 4.7K 1W carbon comp resistor just above the OT. That is R381. And C341 is a silver colored axial cap mounted directly above the reverb transformer near that lamp bulb.

Now, look at the original pic of my amp. Just below the OT and a bit to the left, you will see two terminal strips mounted to the side of the chassis. There are a few resistors, a cap, and a coil mounted on those terminal strips. Those components sample signal from the main speaker and the reverb speaker and sent them to a tone cabinet thru P308. Your amp has none of that! So, out two amps are slightly different. This could be the only difference between my amp and yours, but beware. There may be others! My documents are correct for my amp, but may be different from your amp.

Notice that your amp actually has two small chokes connected in series near the PT but your expandedschemo schematic only shows one choke. That's not the correct schematic for your amp. Differences are probably minor, but could lead you down the wrong rabbit hole. Look for the AO-63 model number stamp on the chassis. There will be a series letter following the AO-63. Use that to track down the correct schematic for your amp. Otherwise, use what you have and keep in mind there may be other differences between your schematic and your amp.

[jmccanna]

Sluckey,

I humbly bow to you. It did not occur to me that R381 was a smaller resistor. I feel like I have averted a disaster thanks to you. So, thank you. I will take a closer look as you described and see if I can find the proper schematic.

Any thoughts, up or down, on leaving it as stock as possible and use the reverb and preamp for a guitar amp?

Also, why do you suppose that the big resistor was so burned up?

Thanks again.

James

[sluckey]

Any thoughts, up or down, on leaving it as stock as possible and use the reverb and preamp for a guitar amp?

You can do that. However, the organ preamp will not have enough gain for a guitar and there are no tone controls. You will have to rip out V301 circuit and build a guitar friendly preamp using a 12AX7. I decided on the Ampeg preamp. Another good choice would be a Fender AB763 preamp. The main power amp sounds great just as it is. Very clean full bodied and powerful sound. The stock reverb circuit should be fine also. You may need change the pick off point for the reverb input. And you will have to jumper around the missing console switches between the reverb recovery amp and reverb power amp. That's basically all you have to do.

But before you get too excited, be sure both power supplies and the bias supply are working. Check for any shorted filter caps and replace if you find any. I was able to slowly bring up the line voltage using a variac and got lucky. All my filter caps were still usable. If I was gonna play in a band though, I'd replace every filter cap with fresh ones.

Also, why do you suppose that the big resistor was so burned up?

Too much current. It does look like it has been really hot. But those wire wound resistors are tough and can take a lot of heat. If it still measures about 64Ω I'd say it is still good. BTW, my resistor has not been heated. It's just a lighter brown color than the others. Look at the cap can just to the left of the OT. The red wire is coming from that burnt resistor. Also on the cap lug with the red wire is another red wire from the little OT. And there is a 470Ω 1/2 watt resistor also connected to that cap lug and over to another cap lug. That resistor looks like it's toasted in your amp. I suspect one or more sections of that cap can are shorted to ground. Use your ohm meter to check.

If that cap can does not have any shorted sections, plug in an EZ81 rectifier tube. Don't plug in any other tubes. Put a jumper on P301 between pins 10 and 7 so you can power up. Use a light bulb current limiter! Monitor that power resistor to see if it's heating up. Check voltages on the cap can. Report your findings.

[jmccanna]

Hello,

Thank you for the helpful suggestions. Yep, that other 470 ohm resistor is completely toasted and broke when I touched it. I suspect that capacitor is shorted too.

I have not put any power to this since it was pulled from the organ and I will work through this as you suggested. Finding the connection points will be an effort in patience for sure but I am up to it and rather enjoy it.

I understand I will need to do something to boost the preamp section signal and have just the right Telefunken 12AX7 I pulled from a tube powered reel to reel.

Do you know what the wattage rating is for R373? The big one that burned?

Thanks again, and I will make judicious use of my variac.

James

[sluckey]

R373 is a 64Ω 10 watt wire-wound resistor. It has been hot. I doubt it is damaged. If it still measures 64Ω I would use it.

[jmccanna]

Thanks Sluckey. I figured it had to be at least 10 watts. Not only is it damaged, it actually broke apart when I was moving it trying to decipher its markings. Broke in half.

One thing I did not mention was that the person who was in here before had replaced the main rectifier with a solid-state rectifier equivalent rectifier - at least I think it was equivalent. I read online (don't recall where) that a SS rectifier without a standby circuit can cause damage to other circuits because it is instant on. Not sure that is true but I replaced that tube right off with a proper rectifier tube. Is what I read online considered true by the experienced minds here?

I could not easily find affordable 64 ohm resistors at 10W so I ordered 10 64 ohm 5W resistors that I will parallel together off of Ebay. Arriving Thursday or Friday. There is still a lot of cleanup to do, tracing circuits and all.

If it is okay I would like to continue posting updates as I work through this. Based on GemiVibe I am pretty excited about this.

On a side note, I previously converted an Akai M8 SE amp to a simple little guitar amp. I rerouted the circuits to employ the tone pot as level control between the EF86 and 12AU7 circuits. I can also just bypass the EF86 and go straight into the 12AU7 to get a slightly different tone. That little thing only puts out about 4-5 watts but the sound is amazing.

This project is much more daunting.

Thanks again.

[jmccanna]

Update: I just found 10W 68 ohm ceramic resistors on Ebay. Going to go with that.

[pompeiisneaks]

That's an old wives tale, about ss causing damage, there is 'some' design consideration for it if you put a ss rectifier in that increases the output voltage, and the filter capacitors are set close to the limit for the previous rectifier.

i.e. if you had 440v B+ in with the old tube rectifier, and 450V capacitors, and you put in SS rectification that raised the B+ voltage to say 470, yes it will destroy the capacitor. That's changing the design of the amp, though, so you need to take that kind of change into consideration. You may need to bump up the voltage tolerance of the first or more filter capacitors.

Otherwise it's not an issue. You may consider replacing that 64 ohm with a 15 Watter, so that it has a bit more breathing room, just in case.

~Phil

[sluckey]

68Ω/10W is a very common resistor. AES has them for $1.30.

https://www.tubesandmore.com/products/r ... ound-power

Since you intend to keep as much of the original circuit as possible, I'd like to suggest a divide and conquer approach...

1. Put a jumper on P301 between pins 10 and 7 so you can power up.

2. With no tubes plugged in power on with variac set to 120VAC and check the PT secondary windings for proper AC voltages. If AC voltages look good, check the bias supply for -22vdc at C344. (Now would be a good time to just replace C343 and C344. These bias caps are cheap but too important to gamble with.) Also check for negative voltage at pin 6 of each 7591 output tube socket. If all's well, move on.

3. Plug in only the GZ34 rectifier tube. Use your variac to set the line voltage to about 50VAC and power up. Check for dc voltages on all filter caps in the main power supply. None of the power resistors should get hot. None of the filter caps should get warm or hot. Don't forget that silver axial cap by the reverb transformer. Voltages will be low for now. Just let the amp run like this for about an hour. Then increase the variac so the line voltage is 60VAC. Repeat dc voltage checks and make sure all filter caps are still room temp. Let the amp run like this for another hour and increase the line voltage another 10VAC. Repeat voltage and temp checks. Continue this procedure until you have reached 120VAC line voltage. Let the amp run for several hours like this. I would not leave the amp unattended overnight! If at any time you notice DC voltages have decreased or you feel a filter cap getting warm, stop and investigate. Any warm filter cap will need to be replaced. If that's a can, replace the entire can. Remove the GZ34 and move on to the EZ81 power supply...

4. Replace the two B+ dropping resistors that have burned up. Find out why they burned up and correct that problem. I highly suspect the cap that the burned 470Ω/1W resistor is connected to. Not the cap with the two red leads but the cap at the other end of that burned resistor. Once all four caps in that can are verified non shorted or replaced, repeat step 3 for the EZ81 rectifier and filters.

That's all for now. Should keep you busy for a few days. Let us know the results. Don't get in a hurry at this point. This stuff has to be right before you move on.

[jmccanna]

Thanks for the response on the SS rectifier. How do all those old wives think of these things?

I went ahead and replaced it because I like the look of the tube rectifier anyway. It was an anomaly from someone who was in there before and I wanted to get to "stock" first before I make changes.

Thanks,

James
Kingston, WA

That's an old wives tale, about ss causing damage, there is 'some' design consideration for it if you put a ss rectifier in that increases the output voltage, and the filter capacitors are set close to the limit for the previous rectifier.

i.e. if you had 440v B+ in with the old tube rectifier, and 450V capacitors, and you put in SS rectification that raised the B+ voltage to say 470, yes it will destroy the capacitor. That's changing the design of the amp, though, so you need to take that kind of change into consideration. You may need to bump up the voltage tolerance of the first or more filter capacitors.

Otherwise it's not an issue. You may consider replacing that 64 ohm with a 15 Watter, so that it has a bit more breathing room, just in case.

~Phil

[jmccanna]

Hi Slucky,

Thanks for the thoughtful guide. I am awaiting my power resistors to arrive and then can get started. Meanwhile I installed a new power cord. I will go ahead and replace the C343/344 caps now. and ones where the resistor burned. This thing is old and it hasn't failed now it seems like it has to be close to failing soon anyway.

While I am waiting for parts I am going to study the layout and determine what can be removed, where I will put in potentiometers, power switch etc. The bottom of your GemiVibe is so clean looking. While I do not believe I have the same level of skill it makes sense to try right?

James

68Ω/10W is a very common resistor. AES has them for $1.30.

https://www.tubesandmore.com/products/r ... ound-power

Since you intend to keep as much of the original circuit as possible, I'd like to suggest a divide and conquer approach...

1. Put a jumper on P301 between pins 10 and 7 so you can power up.

2. With no tubes plugged in power on with variac set to 120VAC and check the PT secondary windings for proper AC voltages. If AC voltages look good, check the bias supply for -22vdc at C344. (Now would be a good time to just replace C343 and C344. These bias caps are cheap but too important to gamble with.) Also check for negative voltage at pin 6 of each 7591 output tube socket. If all's well, move on.

3. Plug in only the GZ34 rectifier tube. Use your variac to set the line voltage to about 50VAC and power up. Check for dc voltages on all filter caps in the main power supply. None of the power resistors should get hot. None of the filter caps should get warm or hot. Don't forget that silver axial cap by the reverb transformer. Voltages will be low for now. Just let the amp run like this for about an hour. Then increase the variac so the line voltage is 60VAC. Repeat dc voltage checks and make sure all filter caps are still room temp. Let the amp run like this for another hour and increase the line voltage another 10VAC. Repeat voltage and temp checks. Continue this procedure until you have reached 120VAC line voltage. Let the amp run for several hours like this. I would not leave the amp unattended overnight! If at any time you notice DC voltages have decreased or you feel a filter cap getting warm, stop and investigate. Any warm filter cap will need to be replaced. If that's a can, replace the entire can. Remove the GZ34 and move on to the EZ81 power supply...

4. Replace the two B+ dropping resistors that have burned up. Find out why they burned up and correct that problem. I highly suspect the cap that the burned 470Ω/1W resistor is connected to. Not the cap with the two red leads but the cap at the other end of that burned resistor. Once all four caps in that can are verified non shorted or replaced, repeat step 3 for the EZ81 rectifier and filters.

That's all for now. Should keep you busy for a few days. Let us know the results. Don't get in a hurry at this point. This stuff has to be right before you move on.

[jmccanna]

Hi,

I have quite a parts bin from prior projects and in going through them I have most of the replacement capacitors needed for the capacitor can where the burned resistor was and I have question about the bias circuit Sluckey recommends I just change now. The schematic calls for C343 to be 100uf/50v and C344 100uf/25v. I have 2 each of a 220uf/50v on hand. Would that adversely effect the bias circuit too much? I am not sure what capacitance does in a bias circuit.

[jmccanna]

Took a magnifying glass to the bias circuit and C343 on the amp is a 50uf/50 and C344 is a 100uf/25v. C343 looks to be a replacement because the prior tech put some shrink wrap on one of the legs where no other caps have that.