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Another Maggy Thread

stanbp64 said:
Hey Kegger, did you ever get a chance to look over this schematic (located in posts 189 and 190) ?? See any possible improvements? Thanks, Stan
Click to expand...

"Quote:
Originally Posted by jaymanaa
Keg, what improvements can/would you make to that schematic?"

Well you guy's asked so I did load the schem up in spice and check it out.

With what I got from spice and some of my thoughts on getting just a touch more
current to that bottom part of the 12AX7 I'm posting what I would believe to be an
update, with this schem both sections now draw roughly the same current, there is a
touch less gain (so I modded the feedback circuit a touch as well) and in the originals
design the phase splitter was highly sensitive to the gain of the 12AX7, not just on the
section to section but on the gain it'self, meaning if you had the amp balanced well but
dropped in a different 12AX7 with a tad more or less gain than the one pulled from that
amp it would make a fairly large change in the phase balance, in this new version that is
greatly reduced so most any (well matched sections) 12AX7 should perform nicely.

You'll notice a pair of 27k resistors in parallel to provide the "balance" as there is no 13.5k
resistors easily found, as I believe Gordon suggests you could put a pot there, but I think
with gain/balance deal working better in this it may not really be all that needed/wanted. :)
(13k will actually work fine here)

I also changed the 47k input resistor to something less obtrusive, an the input impedance
to 330K just to use all the same resistors for builds/buy purposes, added grid stoppers to
the outputs an [screen resistors (may want to tailor those to taste, down to say 100ohm]
(grid stoppers aren't a necessity on these tubes)


To mod the feedback circuit "perfectly" someone really would need to put it on a scope,
also the 2 caps on that cathode of the top 12AX7 section can be removed if you can get
away with less gain in the amp an have a pre with gain you otherwise couldn't really turn
it to far previously, you may even want to tweak the feedback more by upping the resistor
from say the 4.7k I have there now to something like 10K? (scope it out to be sure)


Without having one of these in front of me I surely can't tweak it, and I don't have one, so
it is up to you guy's from here, but I believe these changes are quite solid. :)

Hope that is what you guy's had in mind an I was successful on improving the original design.

Edit: Also the supply to that front end should work well with 250-350v on it, 300v looks nice.

-----------

Edit: Recently did a rebuild of one of these and the 4.7K feedback resistor worked quite well.
 

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I believe someone also asked about subbing a 12AT7 in here?
(here are values to use a 12AT7, I removed the feadback and bypass for myself)

Attached is a schem showing values to use a 12AT7, this is a version I would think
about building using some UL tranny's then trying it without loop feedback, plus I'd
removed the cathode bypass caps on the driver. :)
 

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Hey Stan, my scanner is buried under what looks like a Flat and Skruggs LP, a teddy bear of some sort, and a bunch of smaller misc. crap. So.....Here goes; the rectifier is a 5U4, the high joltage leaves it and goes straight into a Hammond Choke Part # 156R (it is 1.5hy by 200mA by 56ohms DCR) and a 20uf by 450wv lytic, it goes through the choke and then to 3 Nichicon Lytics, each one 330uf by 400wv. From those it goes to the OPTs and a resistor (68k 1 watt). After that resistor it gets another 105uf of lytic, then to the front end tubes.:thmbsp:
 
BTW Stan, I have this thing out now and listening, and it's better than I remembered. A very worthy project by any standards I think. I also have the bottom off doing some measuring so if you need any specs, just hollar.
 
Hey Jay, thanks alot!!! I'm waiting on some parts for this build... I've got original transformers from a 9300 and some 6kg6 tubes but am waiting on everything else. I think I may try implemeting the changes from Kegger if I have the necessary resistors. You gonna leave yours as is or maybe try the changes? Thanks again for info!:yes:
stan
 
stanbp64 said:
Hey Jay, thanks alot!!! I'm waiting on some parts for this build... I've got original transformers from a 9300 and some 6kg6 tubes but am waiting on everything else. I think I may try implemeting the changes from Kegger if I have the necessary resistors. You gonna leave yours as is or maybe try the changes? Thanks again for info!:yes:
stan
Click to expand...

I'll probably try Keg's circuit on the next one (which is patiently waiting).:thmbsp:
 
Sam Cogley said:
Markus, shouldn't you have a rubber grommet on that power cord hole?
Click to expand...

I totally should - I did file the edges so it's a smooth opening though . . .

I also want to get some little vacuum line 'caps' so the front RCA's can be covered - they are still 'in circuit' which allows me treat them as a line level output. I've been running it with my little German open baffles, and a small Yamaha powered sub :music:
 
Well you guy's asked so I did load the schem up in spice and check it out.



Kegger, what effect will upping the coupling caps to .22mfd have? Am wondering if I should maybe order some .22's.

stan
 
stanbp64 said:
Kegger, what effect will upping the coupling caps to .22mfd have? Am wondering if I should maybe order some .22's.

stan
Click to expand...


.22uf caps in that position, with the 330K grid resistors, will give around a 2 Hz rolloff point. That's OK, but I doubt anyone would hear any difference between that and say, .1uf (which would give between 4Hz and 5Hz)... with the .1uf caps, the rolloff point is still more than two octaves below the lowest frequency seen in almost any music (20 Hz)...

Won't hurt anything, but I don't think it'll make an appreciable gain, either.

Regards,
Gordon.
 
Basically what Gordon said..

I don't use any coupling cap of less then .1uf, I have found to often the sound to be
thin, not to mention when you drop the grid resistors of the output stage down you'd
need to up the cap to perform the same, the original spec'ed .047uf, I usually have a
stash of .22 and .33uf on hand as I find they get the job done well.
 
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