Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bnew63 on January 08, 2012, 03:28:54 pm
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I just ran the test from Geofex .
Here's what I got
the opt is a five wire PP having red,blue,brown,green&black wires
ohms between B+ & primaries were 73.4 & 88.3
ohms between secondaries is .6ohms
3.3 volts in gives 134.5V B+ and 67V both primaries
3.3 into secondary gives 3.3 out.
This is an OPT I'm looking to use for a Ceratone 36W ef86
which runs 4 el84's
From this I gather that the turn ratio is 40-1
I, assuming the secondary is an 8ohm tap but am unsure??
Can someone tell me if I've done the test correctly,is my 8ohm tap assumption correct and will this OPT work
for the 36W amp
Thanks Brian
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an OPT is not 8 or 16 ohm, it is ONLY A TURN RATIO. Let's say you have a 40:1 turn ratio. 40 squared = 1600. If you connect a 4 ohm speaker, it is now a 6400 primary load impedance. If your power tubes call for that kinda load, you're right on it. OPT with taps identified 4, 8 or 16 ohm mean that connecting corresponding load speakers to the taps don't change the impedance of the tranny, cause the taps correspond to different turn ratios, inside the same multitaps tranny.
Colas
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Thanks
I'm still new to this so it takes awhile for some things to sink in
According to what I've read el84's like an impedance of around 6.5-8k
So i'm assuming I'm going to be looking for a 4 ohm speaker?
I also have a PT that puts out 310-0-310 and the specs call for 290-0-290
Will this higher voltage present a problem,to the 4 x el84 design or would it be close enough to not worry about.
Brian
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I also have a PT that puts out 310-0-310 and the specs call for 290-0-290
Will this higher voltage present a problem,to the 4 x el84 design or would it be close enough to not worry about.
Hard to say if it will be a problem. But, I can tell you... "My B+ is too high!" is near the top of the list of complaints with DIY builds.
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SS rectified or tube rectified ? if ity is SS, you can calculate 1.4 X 310 = 430V approx., once rectified. Check the MAX operating voltages on a tube data chart for an EL84. If too much voltage for the class ( class A = less voltage ), you can use a VVR to lower the voltage accordingly. So, too much voltage is not really a problem, though it is always better to keep things as simple as posssible. If you are not familiar with power scaling ( VVR ), use the search engine just ahead of you, up north to the right of your screen, and you'll find lots of info.
Good luck !
Colas
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Here the schematic of the Power Supply of an old italian amp
it uses 4 x el84 and has a very high AC 366v-0-366v
as you can see they use two large resistors (150ohm 15W) between PT and diodes
the problem is at start up, the tubes don't draw current and receive a raw high voltage till they begin to draw current, than the voltage sag and B+ is lowered
from the Merlin's Power Supply Book one of the possible solution
(I recommend Merlin's Books, you can find many practical solution to amp building problems)
simply a Zener Amplifier between CT and Ground (you can omit the 10k trimmer if you don't need fine regulation)
that is a simply, effective and cheap solution (high W zener can be expensive)
Kagliostro
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The voltage readings smell a lot like 6,600:4
It could be 13.2K:8 or 3,300:2, but 6.6K:4 seems VERY likely.
On paper, P-P EL84 really-really favor 8KCT per pair. They don't flow well enuff to pull lower impedance, the factory rated voltage bans the higher voltage needed to work well at higher impedance.
Loudspeakers and guitar amps are not paper. If you won't spend for a new OT, just use it. It probably does favor 4-6 ohm loading per pair-EL84.