Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on November 16, 2023, 08:08:17 am
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gonna start my jtm45/35 style build. everyone under the sun is telling me this is the wrong OT to use with EL34 (using because i have them) and i can only really use 6V6 or EL84's with it. Now this set was initially designed for a TW Express which uses EL34. Running the math, (plate voltage squared)divided by output tubes wattage). Ex, (400 x 400)/(25 +25) and this comes to 3200. This OT is 6.6k so the math says no but this OT was designed for an amp which uses EL34 (germino uses this exact OT in his Lead 35 which can use any power tubes so he says). I will be using a GZ34 rectifier. What power tubes can i use and what should grid leak resistors be, screen resistors, grid stoppers be?? i have no issues getting the right output tubes (5881, kt66, etc.)
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put your 8 ohm speaker on the 16 ohm tap, walla 3.3K
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my cabs are 16 ohms
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Not a crisis. Something you should commit to memory... AN OT HAS NO IMPEDANCE. IT HAS AN IMPEDANCE RATIO. Write "impedance ratio" 100 times.
So, having learned that, you can now realize that your OT impedance ratio is 6.6K:4/8/16Ω could just as easily be labeled 3.3K:2/4/8Ω. Ratio is still the same. Just as 1:5 is same as 2:10 is same as 4:20. Makes sense, right?
P/P 6V6s are often operated at 6.6K impedance so your OT would be a good match to a 4/8/16Ω speaker load.
P/P EL34s are often operated at 3.4K impedance so that same OT would be a good match for 2/4/8Ω speaker load.
See how easy that is?
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ok thanks. i will likely just hardwire the 16 ohm tap to one speaker jack and use it for 8 ohm or 16 ohm cabs . i never use anything lower than 8 ohm cabs. math is hard :l2:
NFB off which tap?
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NFB off which tap
math
:icon_biggrin:
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... everyone under the sun is telling me this is the wrong OT to use with EL34 …
Depending on what the other conditions affecting the operating point are, they’re almost certainly wrong / misguided.
Clearly, a 6k6 OT doesn’t preclude the use of EL34, as the Express demonstrates. You may just need to amend the JTM45 circuit to accommodate the higher screen grid current that EL34 will draw, especially so at high loading.
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i will copy the grid leaks, screen resistors and grid stoppers from the express. the express doesn't halve the speaker impedance though with this OT (16ohm goes to 16 ohm speaker, 8 ohm goes to 8 ohm speaker). PT is the same as express as well so my B+ likely to be the same. the one difference is i will be using the tube rectifier
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Probably best to forget about the express since you're building a different amp.