Picked a good time to find out UL Taps :) Doh...Help

a 3W single ended EL84 amp for home, work & school

Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Sun Feb 15, 2015 10:14 am

So I have 2 issues....

1 that i don't have UL taps on the output trannies :) Old magnavox power/output trannies.
I plan to pull her apart and do the below mod..
Easiest would be to just hook up the plates as normal in the schematic and solder two 1W, 100 ohm resistors between pins 7 and 9 on V2 and V3. Also omit R9, R10, C5, and C6 (i.e. zero feedback in triode mode).
My 6.3V supply doesn't have a CT on it, but I assume this is ok since it will not be used w/no feedback?

2. My power tranny is sitting at 540 VCT but I used a bit higher resistance choke ~90 ohms to compensate more or less. My question is I am seeing 360V at J6. Is this because the above condition has the power supply unloaded or do I need to triple check the board when I pull it out and go through my wiring again?

Here is how it will look once I pull it out fix it and shove it all back together....man I am never trying to use that small of a chassis again. It looked like a good idea in the planning phase.

Image

J
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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Thu Feb 19, 2015 3:37 pm

So good news is it plays....after adding the resistor in it works. bad news is that its still at 360 volts at B+ tranny puts out 260V...any ideas?
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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Thu Feb 19, 2015 5:59 pm

So we are now sitting at 316V for B+ and the gain seems low. I have to turn it up all the way on the headphones to get it loud enough. It sounds good though.

Anybody have any ideas?
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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby Shannon Parks » Thu Feb 19, 2015 6:00 pm

lord_glum wrote:Easiest would be to just hook up the plates as normal in the schematic and solder two 1W, 100 ohm resistors between pins 7 and 9 on V2 and V3. Also omit R9, R10, C5, and C6 (i.e. zero feedback in triode mode).
My 6.3V supply doesn't have a CT on it, but I assume this is ok since it will not be used w/no feedback?


Correct. No big deal and a perfectly good route to go: triode strapped with no feedback. You'll be around 1.5W instead of 3W. Big deal. ;)

lord_glum wrote:2. My power tranny is sitting at 540 VCT but I used a bit higher resistance choke ~90 ohms to compensate more or less. My question is I am seeing 360V at J6. Is this because the above condition has the power supply unloaded or do I need to triple check the board when I pull it out and go through my wiring again?


Just change the two EL84 cathode resistors to something around 400 ohms, 2W. If you have 100 ohm, 1/2W, just series four of them each.

With no filament CT, use another two 100 ohm (or 1k) resistors as a virtual center tap to audio ground (or eyelet A or B), wired across the 6.3VAC taps.

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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Thu Feb 19, 2015 6:16 pm

Shannon,
Thanks for the info.

When i simmed it w/PSU designer I was looking to add a 360 ohm 7 watt resistor in series with the choke. More or less the same thing you suggest?

J
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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Thu Feb 19, 2015 6:52 pm

with the 360 ohm resistor i am hitting 290 B+ now.

Gonna wire 2 resistors to A for the virtual ground now. i tried it to normal ground and got 7.1

thanks,
j
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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby Shannon Parks » Fri Feb 20, 2015 7:32 am

Actually, I was thinking you should leave your B+ alone, and just change the cathode resistors at the EL84s (i.e. re-bias for lower current). That approach makes more sense.

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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Fri Feb 20, 2015 6:33 pm

So i am clear its resistors 17 & 18 correct?

no worries about the voltage to the 12ax7 if the main voltage is 25V high?

i added 1/2 ohm of resistors to the 6.3 tap to get about 6.5 vac for the moment.

Thanks for the help!
J
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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby Shannon Parks » Sun Feb 22, 2015 2:10 pm

lord_glum wrote:So i am clear its resistors 17 & 18 correct?


Correct.

lord_glum wrote:no worries about the voltage to the 12ax7 if the main voltage is 25V high?


No worries. ;)

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Re: Picked a good time to find out UL Taps :) Doh...Help

Postby lord_glum » Sun Mar 01, 2015 1:33 pm

Parts are on order....i'm a bit slow. I suffer from having too many projects and not enough me.

Thanks for the help.
Jeremy
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