Suitable Transformer for Etude 12B4A

[Hewie]

I'm nearly finished with a Gainclone project, and am starting to collect components for what I ventured into DIY for in the first place: toobz! I've pretty much settled on the Etude 124BA with this power supply based on reading the forum for quite some time to keep things simple, cheap, and good listening. I really don't expect to get into this project in ernest for some time, but here are my first of many question:

I pulled an amplifier from a Hammond M3 organ at the dump over the weekend. The transfo is 380-0-380 w/ 5U4 rectifier. The organ schematic shows 140mA after the rectifier. I did my best to model this in PSUDII, and show 285V for HT. Looking at tube operating curves, I'm guessing I'll be ok, but I have just enough knowledge to be really dangerous. Will this transfo and rectifier work for this project or do I need to dump some excess voltage? What's the best way to do this simply?

Thanks ahead for what is probably the first of many many question.

Dave

 

[resident]

Hi Hewie
The load you have in your PSU is not 8K5.This the load of each 12B4A.
When I'm using PSUDII,I change the resistive load with a current source.
In your purpose use a current source about 30mA.It's 15mA at each tube,right?
If you'd like to use the tranny you allready have, you can use RCRC filtering and calculate the resistors to take the voltage you want.You can do it at PSUDII! 🙂

The rectifier is a little bit big for this purpose but you can use it if you don't want to buy another one.

5 ohm for the caps is too big.Change it to 1ohm.Not too much difference but better simulation.

Do you have allready the chokes?'Cause maybe yours are not 595ohm.If you have them measure them with a multimeter.
If you don't have them don't buy chokes and use Shindo style PSU.Do an RCRCRCRC network and you'll have very good results.With less money. 😀

Measure the power transformer with the multimetter,too.

 

[Geek]

Or make some SS slaves to the GOT 😀

 

[Burnedfingers]

Did you have the Hammond 157G's in mind? They are listed at 595 ohms and measure the same. They work good in this situation as I have them in my power supply on my 12B4.

I use a 5Y3 rectumfire in mine and have at my option a bridge I can use(installed 1N4007 diodes in old tube base)pull out 5Y3 and install bridge.

Do you have any other Hammond schematics? Or Wurlitzer's?

 

[Hewie]

Resident- Thanks for the PSUD hints. I was worried I was using the wrong load in PSUD, I’ll play with that further. I’ve also simulated additional RC filtering to drop the voltage further, but I’m still trying to understand if this is necessary.

Burnedfingers-

Did you have the Hammond 157G's in mind? They are listed at 595 ohms and measure the same. They work good in this situation as I have them in my power supply on my 12B4.

Yep, I was going to order the 157Gs. Much of my inspiration for this amp is from your postings.

Do you have any other Hammond schematics? Or Wurlitzer's?

Nope, but I googled “Hammond organ schematic” and got a number of hits, one of which had the M3 schematic as well as some others.

Thanks,
Dave

 

[resident]

I’ve also simulated additional RC filtering to drop the voltage further, but I’m still trying to understand if this is necessary.

It depends on how high you'd like to drive 12B4A.I drive mine at 300V with anode load of 6K and cathode resistor(bypassed) at 410R.
I use 10uF MKP after the rectifier and 4 RC (470R-150uF)after that.And exactly at the anode load of each 12B4A one 47uF MKP.

More RCRC networks-better filtering.The same happens with more LCLC.But chokes are more difficult to tune them.
Did you see at PSUDII the graph with the chokes?
Do a simulation only with RCRC.. and look then the graph.
Less ringing problems and more stability.
Do also an experiment with the current source.Use the stepped load.Apply another value of current bigger or smaller than needed.Say after some seconds,10 or 20,and you can see your PSU's behavior.