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Old 1st March 2004, 12:18 AM   #1
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Default 845 PP and Lundahl 1620

Hi all,

Anyone care to give me their thoughts on the compatibility of 845s in low to mid voltage push-pull using the Lundahl 1620 as the output transformer? Or compare and contrast that choice with say 2A3s or 300Bs? I am suddenly besotted by the big beasts and wonder how they would work. Or need someone to talk some sense to me.

Michael
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Old 2nd March 2004, 06:16 AM   #2
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An 8-10K Ra-a load would do the trick. How "low" of a voltage? I consider
500V low, 750V medium and anything about 1000V to be utterly out of ones
mind.

This will be a big and expensive project. It will look utterly cool and will
drive anything you could throw at it. You'd basically be reproducing a late
1940s theatre amp, or, the modulator section of a 50kW AM transmitter. Just
replace the modulation transformer with an output transformer. So, yes,
it would be fun, cool and yes you are TOTALLY OUT OF YOUR MIND!

Here's a design for PP 813 amp... You could pretty much use the same
ckt for an 845 with a few minor changes... That being the cathode bias
resistors would need to be changed to about 130V for 845s @ 1kV rather
than 81V for the 813s.

click here for schematic

You could even drop the B+ down to a nice friendly 750V. You'd want a
cathode bias of about 93V in that case... At 750V, this is what you are
facing for transformer purchases:

You need two 10V 10A transformers just to light the 845s.
Another 10V 10A transformer to light the 872B filaments - I'd use that as your
rectifier, big, pretty when running, heavy duty.
Now, you need 750V @ 400mA. The Hammonds are only good for at best
half of that... Because you really want to do choke input, you need a nice
890-0-890V with 500mA CCS rating to be on the safe side.. I'd check Peter Dahl & Company on that.... Oh.. Then the chokes... Some Big Hammonds will work there.
We're now pushing 80lbs of transformers for the PSU... Get the idea?

-- Jim
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Old 2nd March 2004, 03:17 PM   #3
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Default Off to the races?

Jim,

Don't you know it's not nice to encourage the insane. I have a non center tapped transformer that is built for 240V to 1750V at 750mA. Think this would work with a bridge and run from 120V? I just weighed it this morning (something I had always wondered about) and it came in at 40lbs.

I was thinking something in the 550-700V range, plate to cathode. Maybe fixed bias, maybe cathode. I have a front end designed that would feed the 845 through a Lundahl 1660PP from 12B4s. Think I could do anything in the A2 arena with that?

Michael
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Old 2nd March 2004, 04:14 PM   #4
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A project of this magnitude will require massive amounts of time.
Parts acquisition takes alot of time. Breadboarding it out could easily take a month and chew into all your weekend time. Putting it into a beautiful set of chassis that you can live with would take forever. This was my experience with a simple 450V 845 SET
that I just finished making "liveable" with. It's SET, but it is still MASSIVE. Major time suck. I'm actually going to take some time off from this hobby in the spring and come back to it late in the summer.

This is a Class A1 amp by the way... Not A2. I dont think the
1660S can do PP A2, not at least unless the grid currents are entirely balanced on both phases... A2 is an entirely different beast. You should dig up the schematic for the Altec 1570B for a study in push pull A2 ; it uses the 811A transmitting tube. Like the 845, also an eye candy tube.

This is a substantial project... Keeping it around 400-475V like I did for my SET has many advantages. One advantage is that the power supply is infinitely re-usable for manifold other projects. If you have quality high current, clean power from 400 to 475 you can power many many projects. That way, if you take an exit strategy from the 845, your time put into the PSU is not thrown away.
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Old 2nd March 2004, 07:21 PM   #5
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Default Power supply

Hi Jim,

Besides the transformer mentioned previously, which I think would be OK with a hybrid bridge and 866s (?), I have a 1050VCT at 400mA potted UTC that might work either as low voltage choke input or mid voltage semi cap input, ie. a 1-5uF input cap followed by the large choke. I also have a boatload of 20uF 660VAC poly/oil caps. Would need a Hammond choke or three plus the filament transformers, but they are off the shelf.

The chassis could be killer hard work though. Do you have any pics of yours?

Unfortunately for me, I am building this for a friend of mine so I would only get to live with it for a month or two while building and tuning.

Re: Class of operation. I know the 845 is designed for A1 or AB1, I was only wondering if the driver stage with the 12B4-1660S would be able to transistion to positive grid voltage for smooth overload characteristic and possibly a little of extra power. Looking at a datasheet for the 845 it doesn't seem to draw much grid current until you go more than +50V on the grid or take the plate down to <100V. If I read that right , that is. The output impedance of the 12B4-1660 may still be too high though. Just want to consider any facet of the design I can think of before solder fumes happen. Much easier to say "that won't work" now than after it's built.

Thanks,
Michael
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Old 3rd March 2004, 03:55 AM   #6
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Check this out, from Sowter, a Williamson style PP OPT with
10K plate to plate. 30 Watts. Massive. High primary L too.
Run your 845 at the 60mA/side operating point...

Sowter 8950

Another nice design of theirs is a general purpose PP 8K
tranny of EL34, 6L6 etc.

Sowter u067

Either of these OPTs leave you with an exit strategy if 845 PP
isnt your sonic cup of tea. The LL1660S is a low risk purchase
given the multiple applications available for this device.

Part of what you are chasing is the "look". You could certainly
get just a nice sound with PP 300Bs or 2A3s. You're not paying
that much extra for the "look" in this case. You already own the most expensive PSU components. You really just need one good large choke to get going. I Like the Hammond 193M 10H@300ma, 63 ohm DCR. Big. I'd use that for a nice choke input supply around 450V. At that voltage, each 845 needs 60mA around -50V grid bias. So, you're using 4*60=240mA just to bias the 845s. That leaves 60mA left for drivers... Or, a 30mA current budget per channel for all the drivers (!!) Use this sparingly!

Now, at this operating point, you need 100V between the grids at full power. This is easy. Use a low Ra, low mu tube to avoid
Miller capacitance issues with the 845... 6BX7 would be good.
So would a pair of triode strapped 6V6-GT ; both have a mu about 6. Problem is, in both cases, you'd use up all of that 30mA budget just for the driver. Thats not a problem if a 12V input sensitivity is OK with you. Otherwise, you need one more stage of gain. Mu less than 20. 6SN7, 5687, ECC99 what not.

So, you need a bigger power tranny... At least 500mA so that you
can support a 6BX7 type solution as well as another gain stage.

The 12B4 certainly has low mu and low Ra as well. It's also
a dual triode. Worth a try.

I'd go for this:

5687 -> LL1660S --> 6BX7 or 6V6 triode -> LL1660S -> 845

-- Jim
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Old 3rd March 2004, 10:09 PM   #7
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Hi,

Quote:
It's also a dual triode. Worth a try.
Actually the 12B4A is a single triode.

Just thought I'd set the record straight.
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Frank
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Old 4th March 2004, 01:47 PM   #8
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Default PSU

Does this seem to be drawn right? Suggestions for diodes. Yes it is missing the preamp/driver supply.

Michael[IMG]http://www.msnusers.com/_Secure/0TwAUGQAX9n6MC2qydoiZ7OmrlyJVaDT2*j2UU2yGq8eCLBUUe MZ2bZRpQ7bW7o!hako1BaIu0LHfWJro*Z3ArHv4npdF1t84CmO bLx7wFFqAOJevS9tkPw/PSU%20rev%201.0.jpg?dc=4675462403733935803[/IMG]
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Old 4th March 2004, 01:49 PM   #9
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Default Oops

Image doesn't seem to work, but I don't have time to deal with it now.

Sorry,
Michael
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Old 4th March 2004, 08:13 PM   #10
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Default Try again

Schematic

If you try this address?

http://www.msnusers.com/_Secure/0TwCz*wAX9n6MC2qydoiZ7OmrlyJVaDT2*j2UU2yGq8eCLBUUe MZ2bZRpQ7bW7o!hako1BaIu0LHfWJro*Z3ArHv4npdF1t84CmO bLx7wFFqAOJevS9tkPw/PSU%20rev%201.0.jpg?dc=4675462403733935803

It appears to work sometimes, if you can get the whole address pasted into your browser. For some reason when I try to link to the image or give the url the forum editor inserts characters which screw up the address. Even when you copy it and paste it into the address bar some extra characters show up???

Could someone tell me the cure for this?

Thanks,
Michael
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