Hammond AO-63 conversion

[sluckey]

I would get two fresh 100µF @ 100V bias caps. They're cheap.

[jmccanna]

Corrected my prior post.

[jmccanna]

Thanks Sluckey: I know patience and not moving fast is important but I do not want to wait until I get started for more parts.

I dug around some more in my bin and found 4 220uf/25v caps. Two each in series should give me 110/50v. I figure that is close enough for this? What I do not really need is another batch of parts I will have to add to the already large parts bin.

[sluckey]

Take 5 minutes to power up with no tubes and check the bias supply. You may not actually need any caps to begin. Then put the proper caps on a parts list for later. Add parts as needed. Make the order down the road.

If you decide to go with four 220µF caps, then you will also need four 100K to 220K resistors placed in parallel with the caps to equalize the voltage across them. I would rather just use two 220µF caps. I don't keep electrolytic caps on hand because they will dry up just as fast in a parts bin as they will in an amp. I rather trust new that are bought from a vendor that probably has a high turnover on these. Better chance of getting fresh ones this way.

What you propose will certainly work, but if you follow this path you will quickly clutter up a chassis that is already quite full. Just wanting you to get off on a good start. That's what I'd do. But you do what makes you happy.

[jmccanna]

Hi,

I want to power up but was concerned because I do not have that replacement resistor that burned up installed yet. It is supposed to arrive tomorrow. Here is where my theoretical/fundamental understanding of things lacks and my relative newbieness compared to you is exposed. But, if I do not ask I will never learn right?

Will the absence of the replacement resistor R373 affect the bias test you suggest? The reason I ask is that even with the burned resistor, the amplifier did produce rather robust volume despite the hum and motorboating. No reverb though.

As to my happiness, I am usually happier when I follow the advice of those smarter, more knowledgeable and more experienced. So, if I can go ahead and do the test without that resistor installed I will. If not, then I will wait and fiddle around with other things in the meantime.

Thanks again for responses, they are very helpful and I learn a lot from them.

[sluckey]

R373 has nothing to do with the bias circuit. That resistor is in the power supply for the REVERB AMP. That's why you have no reverb. I would not ask you to check the bias if it were not possible.

Reading and understanding this schematic is gonna be very important to restoring/modifying this amp. If you are having trouble with this please say so.

[jmccanna]

deleted double post

[jmccanna]

Hi,

Well, as is probably abundantly clear there is quite a lot I am not getting and it is good that you asked about that. The schematic is very complicated with all the other things that went on in the organ. I am trying to break it down to just the preamp, reverb and power circuits. It is definitely a work in progress.

I do not know how the bias circuit, in and of itself, affects the other parts of the amp. I have worked previously mostly on SS amps and preamps which are easier to map out for me. I have a pretty good basic understanding of how tubes work in relation to other tubes, voltages and stages. I have a good understanding of how to be safe in working on these. I can check for voltages at the rights spots.

However, it is clear that there is much I need to learn. I try to learn before I ask questions.

R373 has nothing to do with the bias circuit. That resistor is in the power supply for the REVERB AMP. That's why you have no reverb. I would not ask you to check the bias if it were not possible.

Reading and understanding this schematic is gonna be very important to restoring/modifying this amp. If you are having trouble with this please say so.

[sluckey]

I do not know how the bias circuit, in and of itself, affects the other parts of the amp.

Doesn't matter at this point. I want you to pull every tube out of the amp. So the bias circuit can't affect anything. I just want you to check to see if you have -22VDC out of the bias supply. That will tell you if you need to be in a hurry to replace the bias caps. The bias supply consists of one diode, two caps, and two resistors. It's the simplest circuit in this amp. But it's very important. No different than something you may have seen in a SS schematic.

Look at page one of my documents...

http://sluckeyamps.com/hammond_2/AO-63.pdf

I have already cut/pasted the AO-63 schematic from the entire organ schematic. And I reassembled the schematic in a very logical method that should be easy to follow. The bias supply is at the bottom of the page.

So, take 5 minutes and check it. Does it work?

[jmccanna]

Hi,

Found five minutes and pulled all tubes and jumped the power wire and let her warm up for about five minutes. I got -22.6 volts at R384 (5600) where it is connected to C344. Thank you for your patience on this.

James

[jmccanna]

I went ahead and tested all four capacitors in the can for shorts using the continuity function on my DMM. (C336-A, B, C and D I think - can't quite make out the lettering on the last number). There were no shorts. I then went back and put the probes on each prong (+/-)of the caps and let the continuity function charge them up for a little bit. Then, I changed the DMM to voltage and each capacitor had taken a small charge (approx. 2.6v) and when I put the voltage tester on it they slowly discharged. I suspect that the caps where the smaller burned resistor was might be okay at least for now.

[sluckey]

Perfect! Now just put a couple caps on your want list. Actually, I would only replace that big brown one. The black one will probably last a lifetime.

I'm just trying to get you to divide and conquer so this project doesn't overwhelm you. There really is a lot of stuff to do to get to the end of your goal. And there is some logic in the plan I laid out, even if it's not apparent yet.

I suggest moving on to step 3 and start reforming the filter caps in the main power supply. By the time you're through your parts should have arrived. BTW, I have that original 64Ω power resistor from my AO-63 right here by my keyboard. You can have it if you want it. Just send me a pm with your address info.

[sluckey]

I went ahead and tested all four capacitors in the can for shorts using the continuity function on my DMM.

Don't use the continuity function. Use the resistance (ohms) function. A cap that measures 500Ω might not beep for the continuity test but would still burn up stuff in the amp. All those caps should have a fairly high resistance reading. The readings will be affected by the condition of the cap and also by the circuitry around the cap. But you don't want any low resistance readings.

Something caused those two resistors to burn up. It takes a lot of current to burn up a 64Ω 10 watt resistor. If not a failed filter cap then something else, like a shorted 6GW8.

[jmccanna]

Hi, I do not have a tube tester at the moment but I checked for shorts on the 2 6GW8 tubes with my DMM. I just put a probe on each pin and tested it across all other pins. I only got continuity on pins 4/5 (heaters) but no others. Thanks on the offer of the resistor but I have replacement being delivered tomorrow via USPS.

Thanks again, I feel like I made progress today and managed to fit it in between work obligations.

James

[jmccanna]

I actually have the filter caps for that particular can already from another project. They are just sitting here and are not going to be used. As to reforming the other caps, this organ was played about a month ago and it worked okay but motor-boated. So, I think I need order caps probably for the rest of the circuit too.