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P/TRANS QUEST? 180-0-180

4/3/1998 11:51 AM
P/TRANS QUEST? 180-0-180
AND I WANT TO KNOW IF I CAN POWER 3 12AX7 for my preamp AND 1(SE) OR A PAIR(PP) OF 6AQ5A TUBES OR 1(SE) OR A PAIR(pp) OF 6AR5A TUBES for the output. I DONT KNOW HOW MUCH current this puupy is putting out. can anybody help.  
thanx christian sverchek
4/3/1998 4:55 PM

It's always easier if you know what the PT  
came out of, then you can just add up what  
it was supplying.  
But if you don't know, you need to figure  
that you'll need about 1.5 - 2 amps for the  
filaments (1A for the 3 12AX7, 1/2A per  
6AQ5). Probably need about 75-100ma from the  
180-0-180 secondary depending on you're  
output section.  
Best way to guess is size and weight. If  
it's at least as big as an average sized  
apple, it'll probably be ok! Are the wires  
coming out of it average size or really  
light gauge? Thicker wires usually imply  
larger current rating.  
When it's all up and running, if you can  
hold your hand on it then it's fine, but  
if it's too hot to touch, then it's too small  
to use.  
Worst that can happen then is that you'll  
need to find a bigger PT...  
Really like your e-mail address BTW...  
... Whit
4/5/1998 7:15 AM

Hi Christian,  
What Whit said is good advise.  
But, I've had class A amps were the power transformer was a bit too small and the idle current through those PTs made them too hot to touch.  
I never had one blow up though, so, it probably does depend on the transformer and how long between on and aff times. ( Duty Cycle)  
Still, it is sound advise though.  
I did not see where you mention if the power transformer has secondary taps for filaments.  
If not the that could be a good thing if the transformer has a very low current rating.  
It means you will need at least one 6.3vac/2amp  
filament transformer ( $10 or less) mounted on the chassis with the PT you now have.  
The reason this might be good is that with the number of tubes you mentioned, they will draw a lot of current through their respective heater elements and therefore pull a fair amount of power from the power transformer too.  
If this current came from a seperate filament transformer, it would take that much longer to heat up the PT because now you would only be pulling only plate and screen current from the tubes in idle. It could make a small diff in the heat.  
As for the 360VCT power transformer you do have.  
Well if you use the transformer with solid state rectifiers in full wave configuration and a 100uF to 200uF capacitor filter, you will have well over 250VDC tfrom the caps with no load.  
That would be perfect for all the 12AX7s and a litlle low for a pair of 6V6 but would still work OK.  
It would be very good for a pair of 6AQ5As though and th e12AX7s  
If you used the SS rectifier in full wave bridge configuration ( no center tap), then you would have way too much plate voltage and only half of the  
max current capability.  
Now you would be looking at over 500V DC with no load on the supply. I don't think 6AQ5s could last very long at that level.  
I think all and all, you have a good start to a nice little Pocket Rocket amp at 250V DC..  
I'd try this setup using an old Tweed Deluxe circuit using only two 12AX7s and the two 6AQ5s, but build it class AB by using a slightly larger resistance in the cathodes of the 6AQ5 finals.  
Also you'll need a PP output trannie that has a slightly lower primary Zed then is normally found in guitar amps.... not too worry though.  
A Hammond 125E OT will work fine but there are ways to get around that too.  
This schematic is on Steve's schematic link.  
Go back a page or two untill you can find it.  
I think there was a small error in the drawing and I don't know if Steve fixed it or not yet.  
I'll go look and see if that was one I spotted a typo in and repost to you.  
Make sure you come back and post how your project is going Christian, we are all interested in homebrewing.  
4/6/1998 8:45 AM

Thanks for the embellishment, I learned  
a few things myself. I like the notion  
of doing the filaments on a seperate (cheap)  
tranny to lighten the load on the HT one...  
That idea just saved a few of my dumpster  
specials from returning from whence they  
... Whit
4/6/1998 9:20 AM

Hi Whit,  
Yeah, and listen to this 'cause this is scarey...  
I'm really gonna get flamed on this because it "can't work" but what the heck!  
I am experimenting with a free old 5v/3amp filament tranny but running it as a 15 watt class A SE OT... Oh brother! Katie bar the doors if this works out !  
Like me and others. how many of those gronk filament trannies have you tossed out?  
117v/5v= 23.4 .... 23.4 x 23.4=547.57....  
547.57 x 8 ohm speaker = 4K4 ohm OT !  
Try that with a 6.3v/3a filament trannie and two 8 ohm speakers in series.  
117v/6.3=18.57.... 18.57x 18.57=344.9....  
344.9 x 16 ohms = 5K5ohms with a 16ohm speaker.  
Oh yeah, the iron mix is probably all wrong of course but if it works at 60 cycles at 3 amps it should have enough iron and X sub L to work OK at guitar freqs.  
The tone and frequency response is different.  
Guess that's the trouble with being a DIY'r, I'll try anything if it's free.  
4/6/1998 9:32 AM

Nothin' wrong with a free experiment  
in my books...  
Wouldn't catch an audiophile bragging  
about it I suspect, but for guitar it might  
just be ok, or even great, who knows...  
By all means post the results...  
BTW, did you get my e-mail last week,  
or did I forget to click the send button  
... Whit
4/6/1998 12:41 PM

I always wanted to do that! Another "idea" would be run a 110/220V to 5V transformer as push-pull output. Oooops! I didn't say that...  
Sure I want to read how you went with it.  
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