Hammond Organ Amp to Guitar Amp Conversion Project

[skyboltone]

I tried using the Duncan PS calculator and when I plug in my values of full wave tube rectifier, 4H choke, 40uf cap and a 6K load, it errors with
" The rectifier PIV of 1,500 has been with a value of 1,593 at time 0.0428."

What is wrong with my values.
I'm not sure what to enter for the resistance on the tranny and capacitor.

ignore the warning. The valve recto conducts slowly. Look at the .0438 seconds. It's inrush current to the caps. Ignore it. Tell the software to chart voltage at C1.

[skyboltone]

You can do the same load testing with the software by putting in a constant current load of 75ma after the first filter cap.

[Structo]

Yeah, I don't know what's wrong with this PSII software, although it's probably operator error.....
It doesn't seem to graph anything intelligibly.

OK, so the Hammond amp had to drop that voltage down to 305v which is what it shows on the plates in the schematic.

I see the 6K resistor in the power supply in the schematic.

There were a couple large wirewound resistors that I think went into the trash....cause I can't find them now.
That is probably what those were.

Question, why would they design this amp with such high voltage on the transformer when I don't see anywhere on the schematic where there is anything over 310v used?

Is the 6K resistor the only way I am going to be able to drop this voltage to a usable amount?

I'm just about ready scrap this thing and buy a blank chassis and PT.

[skyboltone]

Yeah, I don't know what's wrong with this PSII software, although it's probably operator error.....
It doesn't seem to graph anything intelligibly.

OK, so the Hammond amp had to drop that voltage down to 305v which is what it shows on the plates in the schematic.

I see the 6K resistor in the power supply in the schematic.

There were a couple large wirewound resistors that I think went into the trash....cause I can't find them now.
That is probably what those were.

Question, why would they design this amp with such high voltage on the transformer when I don't see anywhere on the schematic where there is anything over 310v used?

Is the 6K resistor the only way I am going to be able to drop this voltage to a usable amount?

I'm just about ready scrap this thing and buy a blank chassis and PT.

I guess what I 'm saying is that this is a soft transformer. When we think of a pretty regular PT we are looking at (say) 33 ohms from center tap to each HT when using a full wave recto. So yours is in the neighborhood of 125 ohms. When you load that sort of winding it burdens. That's the proper word. So when you put a 75ma load on the PS what I'm saying is that the voltage will drop around 100 volts. All of Hammonds organ PTs are like that. The tone cabinet PTs and of course Leslie transformers are not. It must have had to do with the kind of attack that Hammond wanted on the tone wheel outputs. Trust me on this. Don't back up, after all this will smarten you up as you go. Either tube it up, or test the PS with a real world load and you'll see that you won't be running anybody out of spec. When I model the transformer with 125 ohms ct to ht and 120 volts in with 320-0-320 and a 5U4GB with 80uf filter and 75 ma of idle current I get 362.6VDC with 5 volts ripple.

[Structo]

Thanks.

I don't understand what to set the resistance on C1 at.
Is the the ESR? Or load resistance?

Don't I enter 380v for the tranny?

I figured out the graph not that that tells me much.

Does the modeler assume 120v supply to the tranny?

OK, I set it for 380v-0-380, 5U4GB, 4H@100 ohm choke, C1 is 40uf (not sure what to set the resistance at but on this sim I used 20ohm), 75ma load.

I get 307v on 495v Max and 307 RMS.
310v Max on C1.

Is the Diff voltage the ripple?

[Structo]

For some reason whenever I check the specs on T1 it says 240 volts for primary supply.
Is that peak to peak or what? Because if I set that to 120 it reverts back to 240.

[Structo]

So I guess what you are saying is load the tubes and see what it does?

They tube load will bring the voltage down?

[skyboltone]

Thanks.

I don't understand what to set the resistance on C1 at.
Is the the ESR? Or load resistance? I dunno. I just leave it at 2 ohms default which I assume is the ESR
Don't I enter 380v for the tranny?

I figured out the graph not that that tells me much.

Does the modeler assume 120v supply to the tranny? It asks you, you can change it to 1110,115,117,120,125 or whatever the heck you want. It's a design parameter. Whatever the manufacturer says it is.
OK, I set it for 380v-0-380, 5U4GB, 4H@100 ohm choke, C1 is 40uf (not sure what to set the resistance at but on this sim I used 20ohm), 75ma load.

I get 307v on 495v Max and 307 RMS.
310v Max on C1. Ignoring the graphing function for a minute. When you simulate, you'll get a table of numbers including voltage and current at any point on the schematic. So as you read across VC1 (where your plate takeoff would be) you'll see Min 443, Max 453, Diff 10.51, Mean 448.52, RMS 448.44, The ripple will be 10.51, the actual plate voltage is 448.44. I did not know you ended up wth the 380-0-380 transformer. I was using 320-0-320 like the schematic says but I got a pile of the 380-0-380 so I know that's the way they came out of the factory. Ok, the cure is the center tap mosfet or a honking resisor like a 700 ohm 25 watt or so. You are not driving a field coil so you don't need the 115 ma figured in the original PS calculations for the field coil. Just the current to the screens and the balance of the circuit. Maybe 20ma tops. So 380 * .020 = 7.6 watts. That's gonna sink the ship plenty and will not differ much from an express PS. Put the choke on the center tap as part of the 700 ohms. Is the Diff voltage the ripple?

Dan

[Structo]

Thanks Dan, yeah I thought you knew this was a 380 volt tranny.

Being that I don' know squat about Zener diodes.

Is this what I would use? It is cathode to case, 100v 50w.

http://www.mouser.com/Search/ProductDet ... g3SI1ZA%3d

Data sheet:
http://www.mouser.com/catalog/specsheets/DO5%20SPEC.pdf

Does that mean it would drop the voltage by 100v?

And since it mounts to the chassis, is cathode to case the way to go?

[Structo]

***

[Structo]

I went to my local electronics house and bought a 130v 50w zener diode.
I wanted a 150v one but they didn't have it.

This brought the plate voltage down to 322v.

I probably need to drop it further and 150v zener is probably the ticket.

I have 470R screen resistors, the screens are at the same, 322v.
Not sure why the screens are that high but I may have to adjust the dropping string. The screens are right after the choke so apparently it isn't dropping it enough.

Heater voltage is 6.4v

V1 plates are 167 and V2 the PI is 189.

I plugged a guitar into it and it works but it is having some oscillations.
The volume pot is extremely touchy, not gradual at all so I'll have to look into that. I wasn't real clear on the way I wired the tone control so that may be part of it.

I'll swap the OT wires on the 6v6 to see if that is the source of the howling.
But hey, I got sound out of it.

[Structo]

Switching the wires made it worse.

The Hammond came with a 12AU7 so I tried that in V1.
Nope, made the howling much worse, strange...

So then I swapped it for the 12AX7 and put the 12AU7 in V2 the PI spot.
Much better!

I'll say this, that 50 watt zener gets mighty hot!

I also tried it with the V1b cathode bypass cap off, that cuts the gain a bit in a good way. May have to put that on a switch.

Now I have to figure out where the bias is at on the 6V6's, I went with a 430R cathode resistor up from the stock 250R value.

[Structo]

Changed the cathode bias resistor to 500R which sets the bias to around 24ma per tube, still a bit high.

[skyboltone]

Sounds like you're on your way Tom. The zener trick is one that I've used in a variety of other ways; mostly around screen supplys in transmitters. Anyway, the downside is that that heat is power, power that would otherwise go to the speaker. Somebody on the Organ Forum told me that the field coil on the old speakers devoured 115ma; so you've maybe got that to spare. Personally I dunno. I've used one of those transformers for a pair of KT-66 and seen the voltage drop from 460 or so to 410 after biasing the power tubes. One of these saturdays, I want to take that exact same set of iron that you have and make an Allen Class Act with it using one big tube. Second order harmonics etc. Champ on steroids.

[Structo]

It sounds pretty good.

I'm having one problem that so far I can't figure out.
If I dime the tone control it oscillates like crazy. Very high pitch.
If I back it off a little it goes away.

I've messed with the feedback resistor value to no joy.
If I try to put a 12AX7 in V2 (PI) it oscillates uncontrollably.

So I have a 12AU7 there and it doesn't do it.



Take a look at the schematic.
See the 470K resistor on the V1b grid, the grid to ground resistor?
I have that up on the volume pot, wiper to ground.
Should I move that to the pin 7 on the socket?

Also the grid stopper on the input is a 33K, would it help to increase that to 68K like I see in other amps?