DIY 1624 from Hammond M3 Parts

[Smokebreak]

Oh I was merely pointing those out, as I'm trying to get a grasp on how so much voltage is dropped, myself, from the PT to the plates in the organ amp.

[dawsonaudio]

I will ask over at the organ forum that question. Any other things I should find out while I'm over there?

[Smokebreak]

You'll get some answers here from more experienced folks than me soon. All in all this looks like it may be a "build it and see what you get " scenario, as happens with these organ amps sometimes with the voltages.

[dawsonaudio]

Here are some answers from the organ forum:

"What you comparing the voltages with/to?
Are you using the saying that your voltages don't match up with the test point voltages in the schematics?

http://www.archive.org/download/Hamm...m2m3SvcMan.pdf

If you're saying you don't have 300+ Volts D.C. where you should - the first suspect would be 50 year old electrolytic can caps.
After that the 500K to 2Meg ohm resistors also have a tendency to fail or go out of spec. "

and

"I'm not sure I understand the question.

Looking at the schematic, the power supply node for the output stage is 310V, with 305V at the plates. That's not a huge drop.

Hammond had a habit of using choke or resistor-input power supplies, and those supplies have a much lower B+ voltage output for a given power transformer secondary winding compared to more common capacitor-input power supplies.

Choke or resistor input power supplies have somewhat better regulation and subject the PT and rectifier tube to lower peak charging pulses. "

[dawsonaudio]

So what should the correct plate voltage be at the 6973's?

[Smokebreak]

Ok let's backup for a second and I'll try and clarify the best I can.

Voltages on the 6973s in the Supro should be around ~350 on the plates, so I've read. At least this is what I was shooting for, and is also subjective, to a point.

I used a 275-0-275 PT and got 350V in my 1624.

Your PT is 380-0-380, so if you used your PT, using the same power supply as I did(the one in the 1624 schematic), I would expect your voltages to be much higher, my guess with a 5U4GB rectifier around 450 B+.....again, using the 1624 power supply.

However, your PT in the Hammond supply circuit is putting out only ~310V for B+, which surprised me. Then again, I'm not familiar with the Hammond's PS, so I'm trying to learn something here as well


The design of the Hammond power supply is responsible for the B+ that is considerably lower than each side of the PT secondary . I'm trying to figure out which element(s) of that PS are responsible(and why they aren't used more frequently, as high B+ is a common problem/concern).

Looks like some confusion between your question and your first response, as you don't have a working Hammmond PS.

The second response is what I was looking for. the Hammond PS uses a resistor input, which apparently yields lower voltages. I need to do some reading on this subject.

So basically, if you built back up the Hammond PS, you'd get around 330V (with modern wall voltage)as Martin and Phil explained, which would be just fine.
Using this PT with the 1624 PS, I would expect much higher voltages. It will change the tone, and some 6V6s or 6973s may not be able to handle it.

If it were me, I'd probably just build the Hammond PS. I'm not sure how else you are going to get 330V out of that PT. You know exactly where the field coil goes too

[JazzGuitarGimp]

I think trial and error is the letter of the day here. There are so many variables to consider, not the least of which is the fact that the original circuit had so many more valves than your average guitar amp. I would say if you have a variac, put it to good use when bringing the amp up for the first time. A zener diode between the PT center tap and the field coil may be necessary to bring the B+ down to a usable level. But you won't know until you try...

[martin manning]

The Hammond power supply has the speaker field coil and back bias resistors between the PT CT and ground. Per the schematic the CT is at -100V, and that's the main reason for the low plate voltage. The 30 ohm resistor at the input to the reservoir will drop some voltage too, maybe 10V or so. I'm thinking its main purpose is limiting rectifier current.

[JazzGuitarGimp]

Hi Martin,

I didn't notice the -100V designation at the left end of the field coil until now, after reading your last post. I'm having a hard time understanding why this point is negative, rather than positive. Can you school me?

Cheers,
Lou

[martin manning]

Imagine a simple loop from the PT HT through the rectifier, the load, the field coil, and finally back to the PT at the CT. The only place where voltage increases in that loop is through the PT secondary coil, so the CT side of the field coil must be at a lower potential than the load side. If the ground reference is placed at the load side then the CT side must be below ground. The same thing happens when you put a Zener between the CT and ground.

[Smokebreak]

Thank you Martin. I came across back biasing a few months ago in a 40's Epiphone and couldn't figure out how it was generating negative voltage, so I traced it out and eventually realized it was back biased. Now I see that it pulls down the B+ because the reference is to a negative potential instead of 0. Here's that circuit https://tubeamparchive.com/download/file.php?id=35062

I think that usually the back bias resistor is usually much smaller to simply generate negative bias voltage, and only moderately pull down the B+, but in this case the relative large resistance of the field coil puts the CT at such a larger negative potential.

I wonder why this isn't utilized more often, with simply a resistor, though I imagine it would have to have pretty large power handling capability.

Nate, I'll give a shot at modifying the organ supply.

[Smokebreak]

I think this will do it. I took out the voltage dividers for the various lower negative voltage nodes but kept the one at the first cap. Is this correct?

[JazzGuitarGimp]

Thanks, Martin -I'm going to draw this with a zener in the CT to see if I can fully wrap my head around the concept.

Smokebreak, you are correct about power (regarding doing it with a resistor). If you calculate how much power that field coil is dissipating: 100^2 / 700, you'll see it is 14.29 watts. And I agree, I don't know why this technique isn't used more often. Seems to me it's a good way to trim some voltage out of an overly high B+ while at the same time, generating a bias supply.

Cheers,
Lou

[Smokebreak]

Lou, Aiken has a nice piece on the subject, but don't look until you're done !
It's got a Zener back-bias circuit in there.
http://www.aikenamps.com/index.php/what-is-back-biasing

[dawsonaudio]

Thank you for helping me with all this. I am still wrapping my head around all of it and doing some research to understand how all of this stuff works. I know just a little about tube amp circuits and could use a little more education on the matter before I start assembling things.

Smokebreak...should that 10000 resistor be fairly large? 5 watt, 10 watt?