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Re:anyone has built a spitfire?


 
12/16/1999 10:11 PM
dave rutherford
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Re:anyone has built a spitfire?
I know hammond offers a line of 'universal primary' PT' for eurpoean voltages, but i don;t know if it includes the 272. Try there web site and look up their line of Universal Primary PTs.  
 
The resistor values are all in ohms. The first 12ax7 is connected in parallel. For a choke, try any replacement choke for an amp of similar power rating or design, like fender deluxe or vox. AC15 (do they have a choke? i don;t know..)  
 
I wouldn't have the PS filtering so high, you might lose a lot of warmth in the tone. I used LCR 32ufx32uf and 16x16uf cans in mine, mounting them inside the chassis on the sidewall.  
 
Best of luck to you with your project.  
 
Dave Rutherford
 
12/18/1999 4:57 PM
Bruce
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Re: Me too, what power tranny?
Actually Benjamin, since the Hammonds are wound with a 115vac primary in mind, and are so good, you'll see something like 1.06% more secondary output even when the PT is loaded down a little.  
A 270FX will probably show you closer to 290vac on the secondary and, depending on your rectifier, about 355vdc to 385vdc with a pair of EL84s in moderate bias current level.  
If you are cathode biasing EL84s and can live with relative short power tube life, this PT will work hard on them, but great with a pair of 6V6GTs.  
Could be just fine with four nine pin preamp tubes and reverb, but, in my opinion, this one might be just a little big to be used with a small amp using only two EL84s and two 12AX7s.  
I don't think I would use it like this with a solid state recitifer unless you are going to use something to drop the B+ a bit, like the zener you mentioned.  
Using a couple cheater 150 ohm 10 watt power resistors on each AC leg, to drop some AC going into the rectifer, will get the B+ down a little after filtering too.  
 
Bruce
 
 
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12/18/1999 6:12 PM
Benjamin Fargen
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Bruce,  
 
Your absolutely right! I forgot about the efficiency of those bad boys! I had forgotten all about it after I dropped the B+ down with some zeners.........I'm having to rethink my transformer choices for "the epic" so that I don't have to rely on the zeners for the correct B+.  
That's a good idea to use them for a 2 X 6v6Gt type amp.  
 
Take care,  
Benjamin.  
 
 
12/19/1999 3:47 PM
Lee
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So is there a commercially available power tranny more suitable for 2xEL84 power amp with 3 12AX7s? I really don't want to go the zener route. I used the 272 series for my 5E3 and got about 370v B+ with a 5Y3. I got 333vac out of the power tranny with 122vac line in. If I ratio everything out for the 270 series, I should see around 340v B+.  
 
On the STF Electronics site, he recommends a Hammond 261M6 which is 215v out. Theoretically that should supply 300v B+ with a s.s. rectifier.
 
12/19/1999 6:06 PM
Bruce
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I need to back up a bit here Lee.  
Are you building an EL84 amp or trying to do something with the 6V6s?  
 
The 272DX might be decent choice for the EL84 amp because the current draw will cause it to sag down a little bit.  
Since these amps are not hugely powerfull amps, you should have NO trouble using a couple $1.00 5 watt zeners in the power supply to drop overall B+ when using a larger current capacity PT like the 272FX or 272HZ, and if you want a little extra cushion the power supply rail, I see no reason to not use a 10 watt few hundred ohm resistor in the center tap of the PT, or both.  
 
I guess what I'm saying is, when using a good piece of iron like a Hammond, keep in mind (according to my source at Hammond), the ratings are based on their gaurantee that the secondary AC voltage will stay at the advertised voltage when current drawn from he secondary of the PT is at it's rated value... 275vac@150ma load.  
That means if the thing is a 115vac primary with 275v@150ma secondary PT, and we know it will put out almost 6%-8% more then that at 122vac primary, then it will be putting out more high voltage under any load and "much more" secondary voltage then you expect when there is a light load.  
When using a cheap PT, you have to make sure the current rating is much higher then what you think you'll realkly need.  
 
But, a pair of EL84s @ 36ma and a two or three 12AX7s @ 2ma each, is not that much of a load for a "real" 150ma PT.  
To be truthfull, earlier this year, I was experimenting with a little amp design for the AX84 gang, and ran a pair of El84s in PP (fixed bias) at about 24ma each and a pair of 12AX7s with an inexpensive hot running 265vac@50ma PT using SS rectifiers, a 100uF first filter cap and a cheap reissue Princeton OT, with the intended results being a baby 14 watt, Voxie/Marshall sounding shreder.  
FWIW:  
I doubt if a Fender Pro Jr., w/two 12AX7s and a pair of EL84s at about 18 watts output, has a transformer that is much over 70ma.  
Bruce  
 
12/20/1999 1:15 AM
jason
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quote:
"I was experimenting with a little amp design for the AX84 gang, and ran a pair of El84s in PP (fixed bias) at about 24ma each and a pair of 12AX7s with an inexpensive hot running 265vac@50ma PT using SS rectifiers, a 100uF first filter cap and a cheap reissue Princeton OT, with the intended results being a baby 14 watt, Voxie/Marshall sounding shreder. "
 
Is the info on this amp currently posred on the AX84 site?  
jason
 
12/21/1999 6:59 PM
Bruce
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I sent Chris the schematic for it quite a few months ago and he redrew it for the site but I've made some other changes for the better and haven't had a chance to update it for him yet.  
I'll be getting to it pretty quick though.  
 
Bruce
 
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