Hammond AO 29 conversion

[JunkDr]

Good day All,

In reference to this thread https://el34world.com/Forum/index.php?topic=9077.msg81697#msg81697 , I am struggling with this field coil being in series with the PT center tap to ground thing. My particular FC shows about 740 Ohms DC resistance. I am converting this into a guitar amp. The AO 29 schematic is confusing where it shows a "jumper in speaker plug" between pins 2 & 4, the original plug is something I didn't get with this "bargain". I have removed all of the voltage dividers and negative bias RC circuits indicated in the original PS schematic as I have moved the 6V6's to cathode bias to accommodate a tremolo circuit on the grid leaks. This leaves the original 30 Ohm power resistor in series with the B+ out of the 5U4 with that output feeding a 40 uF cap to the input of the FC. As there is another 40 uF cap in parallel with the B+ out going to ground or the grounded end of the FC, is this a PI filter in reverse? I have never seen such an arrangement - the PI filter is always in the + side of B. As I want to use the FC speaker is it possible to move the FC as a PI filter into the conventional B+ side of things? The reason for asking is that, the way it is, I am getting a "motorboat" or more correctly a "RC clock" the way I have it hooked up as per schematic reference. Any help would be appreciated. JB 

[PRR]

> I have never seen such an arrangement - the PI filter is always in the + side of B.

The other way works fine, and has been done a lot, even if not where you have seen it.

I suggest you jury-rig ten 100 Ohm 10W resistors in series to go where the Field Coil isn't going to be. That will give excellent filtering with large voltage drop. Once it is smoke-tested and test-played, re-think the final B+ on the power tubes. Very likely you can go much higher B+ in a DIY g-amp than Hammond dared in a church-owned UN-maintained organ. They stuck at 300V and we often push g-amp 6V6 to 350V or 400V. Short-out the string of resistors until you get a good B+.

I do not see this "jumper". Normally if you lost (didn't get) accessories, and you are sure the main unit will be fine without them, you solder a wire where the jumper would be.

[sluckey]

The AO 29 schematic is confusing where it shows a "jumper in speaker plug" between pins 2 & 4,

The speaker voice coil needs two wires and the field coil needs 2 wires. So, at bare minimum you need a 4 pin connector to connect the speaker assembly to the chassis.

The top notch Hammond engineers rarely work with bare minimum though. So they used a 5 pin female socket on the chassis and a 5 pin male plug for the speaker assembly. The jumper between pin 2 and 4 is on the male plug. This is a safety feature. If you unplug the speaker, you also disconnect the PT center tap, thus killing all B+ voltage in the amp.

[tubenit]

Have you seen this thread on the "ultimate Hammond conversion/s":  https://ampgarage.com/forum/viewtopic.php?f=6&t=24331

Maybe there are some ideas of info there useful to you?

[JunkDr]

Thank you all for your replies.

PRR - to be clear, I would like to use the FC speaker in this amp, I've heard they sound cool. It is in good shape and I have applied the thinned out tung oil trick to the flex ring and it moves great. I will definitely keep your suggestion in mind though should I go PM speaker and I may have to. Maybe I'm interpreting the schematic wrong but it shows that first 40 uF cap positive side being connected to the first leg of B+ with the negative side going to the input of the FC with ground termination after or through the FC. When I first powered up wired like that I got a very strong "RC clock" effect, something like once a second or so through the speaker. After thinking about that one for a while I decided to move the - side of that cap to ground and that beating stopped. What I noticed was a sharp increase in B+ with the cap connected to FC in and the other way a significant decrease in B+. The very odd thing that happened then was the plate voltage on the first 12AX7 stage showed less than 6 VDC, all others being what I would expect. From Hammond, that 12AX7 has a 330K plate resistor on the "intermediate" side and a 560K on the "pre-amp" side as defined by Hammond. I moved both of those to 100K and after that the "RC clock" came back but at a slightly faster rate - that plate voltage is now normal though. Not sure on the beating at this time but will persevere.

Sluckey - thanks for explaining that jumper, this makes perfect sense. I found an appropriate male plug in my junk collection for this connection but it does not have the 5th pin that serves as ground in the original design. As I am aware that amps should not be run without a speaker I have wired it to work off of the 4 pin arrangement. The FC speaker is new to me and that humbucker kind of threw me but thank God for the internet I was able to figure it out without blowing something up!

And Tubenit - thanks for that suggestion, I just got home from work and am looking at that next.

Thanks again fellows.