Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JB on June 19, 2017, 12:37:36 pm
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I'm planning a 5E3 build. Shipping transformers from the US to here in the UK is prohibitively expensive, but there are some Hammond dealers.
For PT it looks like either 290/291AEX or 290/291BEX - 240V version of the 290/91AX and 290/91BX. The BEX has higher current capacity for HT, 6.3V and 5V windings, and slightly higher voltage: 330-0-330 instead of 325-0-325. Could maybe run 6L6's, or bolt on tube reverb or tremelo units.
Any downside to using the BEX version though? HT secondary is rated 138ma instead of 100mA, is that going to result in any less desirable squish under load than the AEX? I played a phenomally good 5E3 clone last year, which I think had MM iron but was otherwise true to the original. I don't want to veer away from that tone.
And much difference between the 290/91 ? Hammond says the 291 is an upgrade - lower loss, more surge output power. Again, is that going to reduce wanted compression?
For OT it looks like 1750E or 1760E - both 8K:8R but the 1760 has additional 4 and 16 windings. All the other spec figures are the same so I'm guessing no tonal difference?
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is that going to reduce wanted compression?
Your compression will probably come from the tube rectifier, and lower value power rail caps rather than what a tranny is capable of. biasing, PA B+, bigger than "design" OT's, PA bypass caps, those will effect "compression" more
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True enough.
I guess I'm just wondering if using one of these will give good results, and if one model is a better choice than another.
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If it is the Sag you are referring to, it comes from undercurrent. I have built a 5E3 using Iron that would run 6L6 and it is not the same. I installed a 6K OT and left it as a Tweed Pro.
I borrowed a friends 57 Tweed Deluxe and tried to match the tone. Got very close. I used a 290-0-290 VAC, 70ma and the voltages were very close to the original with a variac set to 110 VAC. His original measured 325 plate voltage which is lower than I have heard the originals are, but I am not sure if other use a variac. One of the things I have never seen documented.
The reissue from Fender had plate voltages of 370Vdc.
The amp I ended up with I also used a 10K OT from a Hammond Tone Cabinet and a heppner Alnico 12" speaker. The speaker is not very efficient. I tried a Celestion Blue and a Weber 12A125, but both were a little too articulate and loud. There are always ways to get sag if you want. You can install a resistor that will switch in and out prior to the plates or increase the Screen Resistor size. Either will create more sag.
In old Fender amp builds I normally try to go with lower voltages on the power tubes.
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The AEX is rated for the smaller 5 to 15W amps while the BEX is rated for the 20W Deluxe amps. The AEX is physically smaller than the BEX. They will not fit the same chassis cutout. Not a problem if you are gonna start with a blank chassis.
I would use the BEX.
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325V is lower than I've seen on various schematics. Interesting. Also a 70mA rating, that's roughly half what the BEX is rated for, bound to have some tonal impact.
I've seen a pre-cut 5E3 chassis for sale over here and they do say it's for the BEX not the AEX.
On paper I'd say go for the BEX too. And that's probably what I'll do. But part of me is tempted for the lower voltage/current supply ability option!
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Get one of each. According to Hammond's website the AEX is rated for 100mA, not 70mA. It also has taps for 275-0-275 and that's popular for a lot of other 6V6 or EL84 amps. Could be a very useful PT even if it doesn't work out for the 5E3.
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If it is the Sag you are referring to, it comes from undercurrent. I have built a 5E3 using Iron that would run 6L6 and it is not the same. I installed a 6K OT and left it as a Tweed Pro.
I've been doing more reading and researching and that view does seem to keep coming up.
Lazy J make a well regarded 5E3. This interview with Jesse is interesting:
http://www.lazyjprojects.com/TQRJan-Feb13_Web.pdf (http://www.lazyjprojects.com/TQRJan-Feb13_Web.pdf)
There's a section in there where he talks about how much of an influence the power transformer has, not just for sag but for harmonic content. He says using more efficient iron is detrimental to the tone.
Think I'll go with a 290AEX!