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I found some inexpensive transformers from Magnetic Components which should suit my Princeton Reverb build but I have a couple questions. A lot of folks recommend using a Deluxe OT but I found them to have 6.6k primaries verses 8k for the Princeton. Now I know impedance will vary and isn't constant but wouldn't the 8k be more efficient? Magnetic Components offers a vintage tweed deluxe 20W OT at 8k which seems to be perfect for me. What do you think? Also, several companies offer a beefed up PT with 100ma vs. 70-75. However, the heater currents for all of them are 2 amps. I added up the specs for (3) 12ax7s, (1) 12AT7 and (2) 6v6s and come up with 2.1 amps. Does it matter? Thanks in advance for your advice...frankeg
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The PR 'clone' I built from the Weber 6A14 kit has one of those 6k6 Pr 'deluxe' OTs and a 12" speaker - it does sound good (The plate voltage is about 430). Cleanish amp, starts to break up at around 6 or 7, but needs to be mic'd in a live situation to keep up with loud as hell tweed bassman and tweed twin amps. I built a 5G9 Clone with ~35-40W PP 5K Pr OT and plates at 370, and it sounds louder and can keep up pretty well unmic'd - it remains my favourite gigging amp. I think physical size of the OT has a bit to do with how they cope with volume.
As far as the PT goes, don't forget to allow for some juice for the lamp (if that is wired into the heater circuit). Also I think those 70mA rated High Tension windings, whilst probably fairly accurately reflecting the original 'stock' BFPR value, probably aren't as reliable as 100mA or 120mA (70Ma is about what 2 x 6V6s use, let alone the 10mA for the 12AT7 and the ~2mA each for the 3 x 12AX7)Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo -
Thanks Tubeswell! My EET training kicks in whenever I'm considering specs which usually means more is better... I was thinking about the "maximum power transfer" theory when referring to the 6.6k verses 8k primary. I realize that this is a "nominal" value as impedance varies through the cycle. I also realize that it seems to work fine (using 6.6K) from a practicle standpoint. I was just looking for a better understanding of the "why"... I agree with you and others about the smallish PT. However, the current rating for the heater circuit is higher for the Fender and I was wondering if a rating of 2 amps which is for the beefier PT, is suitable for a spec of 2.1 amps (when I added up the specs I got 2.1 amps). I like learning and seeing where my professional training fits in with my hobby. Thanks, againComment
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Allen amps sells an upgraded PT (though I wish it had a 3A rating for the 5V) for a Princeton that fits the same footprint but is an inch taller. (TP 25)
Allen Amplification - Parts Order Page"In theory, there is no difference between theory and practice. In practice there is."
- Yogi BerraComment
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Hey Joe, I just recieved my trannys from David Allen and they are really nice! Thanks again for the suggestion. I'll let you know how they sound when all is completed. frankComment
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That's great! I'm interested in how these will work for you."In theory, there is no difference between theory and practice. In practice there is."
- Yogi BerraComment
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Tubeswell, What speaker did you choose for your PR?
Joe, the Allen trannys are great. I bought all three - the upgraded PT, deluxe style OT and the reverb tranny. I'm trying to decide now what speaker to use. The cleans are awesome but overdrive is too dark for my tastes through the two speakers I have on hand - Eminence Black Powder and Jensen C12R. Like so many others, I wish there was a place you could go to tryout speakers before you buy....Comment
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Great news! BTW, I have an Eminence Swamp Thang that's going to go in my Princeton. Should be big improvement over a Fender OEM that in there now.Originally posted by frankeg View Post"In theory, there is no difference between theory and practice. In practice there is."
- Yogi BerraComment
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Weber Sig 12S (value for money)Originally posted by frankeg View PostBuilding a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- EnzoComment
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Joe, I take it you're keeping with the original designs 10" speaker? Since I'm building, I'm going for a 12". I like the way my friends PR sounds which was modded for a 12". He has a celestion green-back in his which sounds really good but I think that too will be to dark for me.Comment
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Maximum power transfer theorm does not apply with tube amp outputs, although it's confusingly similar wording.Originally posted by frankeg View Post
The thing is that the maximum power transfer theorm applies to a power source with an internal impedance and an external load. That is not what is happening in a tube amp because transformer does not match the speaker load to the plate impedance of the tubes. Pentodes and power beam tubes have a plate impedance that's much, much higher than the transformed speaker load. The "matching" speaker loads idea was noticed when back in the Tube Golden Age, the savants noticed that by varying the plate load on tube outputs, the power had a broad, sloppy maximum at some value. The value was not the tube plate impedance, but was determined by power supply, tube current capability, and other esoterica.
So tubes do have a power maximum, but it's not the simple one predicted by the maximum power transfer theorm.Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.Comment
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Thank you my friend! I like weber speakers but wasn't sure which would be brighter. To me, both the Weber and Eminence sound samples on their web sites are too generic - I usually can't tell the difference. Of course that could be my computer too for all I know....BTW, are you digging your PR? I like what you did with your trem - I just changed some component values (mostly recommended) and it turned out pretty good as well...Originally posted by tubeswell View PostComment
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Is it true to the extent that it matches or encompasses that broad maximum?Comment
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No, my Princeton has the baffle cut for a 12" speaker. (The Swamp Thang is a 12")Originally posted by frankeg View Post"In theory, there is no difference between theory and practice. In practice there is."
- Yogi BerraComment
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