Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: czgunner on December 10, 2021, 04:02:12 pm
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Got my Mojo Princeton Reverb kit assembled yesterday. I've got the instructions for startup testing and to check A/C voltage on pins 4,5 and 9 on the preamp tube socket pins. I'm getting around 3.5-3.7 VAC. Mojo doesn't include the number I should be looking for. The Stewmac kit instructions I looked at say 5-7 VAC. Is my voltage way wrong? If so, what should I be looking for to fix?
Found this: https://chuck.cranor.org/mpr/testin/xls/MOJO-PR-BIAS-4.pdf
And it seems to verify that I have something wrong.
I emailed was just told by Mojo I'm supposed to be reading across 4/5 a0nd 9. When I check that way, I get 0 volts. Any ideas?
ETA: LOL after calling Fluke and talking to the lady, I'm told my old 88 is obsolete and is common for the AC side failing. I'm sending it out to be replaced with the new model at a discounted price. In the mean time I pulled out a spare Fluke 115 and my voltage is 6.8 VAC. I'm hoping this is good and I can put the tubes in and check the high voltage side. This should be DC, I believe.
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if you measure 4/5 to ground it should be ~~ 3vac
same when you measure 9 to ground ~~3vac
when you measure 4/5 to 9 it should be ~~~ 6.3vac
If that's not the case;
start at the 1st socket you soldered the 2 filament wires to.
verify the above if that not correct;
unsolder the 2 wires from the socket and verify ACROSS the 2 wires there's 6-7vac
re-solder to the socket and recheck
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if you measure 4/5 to ground it should be ~~ 3vac
same when you measure 9 to ground ~~3vac
when you measure 4/5 to 9 it should be ~~~ 6.3vac
If that's not the case;
start at the 1st socket you soldered the 2 filament wires to.
verify the above if that not correct;
unsolder the 2 wires from the socket and verify ACROSS the 2 wires there's 6-7vac
re-solder to the socket and recheck
Thanks for the reply. I updated my original post. Tool malfunction! I'm hoping that the 6.8 VAC is good and I'll move on.
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Hopefully 6.8vac will drop a little closer to 6.3vac when the amp is finished and the pt has the full load on it. 6.8 is on the high end of OK so not to worry to much ..
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Got everything voltage tested (I thought). No sound from the amp. I've been going through and checking everything. Not sure what I missed.
Scratch that. I found my speaker cable shorted in the screw together housing. Put a piece of plastic in there to insulate it. This doesn't seem to be a very good solution. What keeps these wires from touching the housing?
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There is usually a piece of tubular plastic on the signal lead (center) that you slide over it after soldering to keep the connection from shorting to the metal. If there isn't one then use
a piece of heat shrink on it. An insulator like electrical tape on the one half of the clam will work to and sounds like what you have done. when you say "wires" makes me wonder what kind of
plug you are using, in the standard Fender type plug the one wire gets soldered to the shell and the other is the tip (signal) wire.
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There is usually a piece of tubular plastic on the signal lead (center) that you slide over it after soldering to keep the connection from shorting to the metal. If there isn't one then use
a piece of heat shrink on it. An insulator like electrical tape on the one half of the clam will work to and sounds like what you have done. when you say "wires" makes me wonder what kind of
plug you are using, in the standard Fender type plug the one wire gets soldered to the shell and the other is the tip (signal) wire.
This is what came with the kit: https://www.mojotone.com/Mojotone-1-4-Stereo-Flat-Right-Angle-Phono-Plug https://www.mojotone.com/Mojotone-Mono-Right-Angle-1-4-Plug
With the insulation, it works great. Very, very quiet (low noise. No hum) IMO for a tube amp. My Fender HRD is louder.
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That's the one. Not a very fair comparison of amp. HRDs are insanely loud. 6V6 vs 6L6.
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This is what came with the kit: https://www.mojotone.com/Mojotone-1-4-Stereo-Flat-Right-Angle-Phono-Plug
Are you sure? That's a stereo plug. This is the correct plug...
https://www.mojotone.com/Mojotone-Mono-Right-Angle-1-4-Plug
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This is what came with the kit: https://www.mojotone.com/Mojotone-1-4-Stereo-Flat-Right-Angle-Phono-Plug
Are you sure? That's a stereo plug. This is the correct plug...
https://www.mojotone.com/Mojotone-Mono-Right-Angle-1-4-Plug
Yeah, I linked the wrong one, but the concept is the same for what I was trying to get across. Thanks.
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That's the one. Not a very fair comparison of amp. HRDs are insanely loud. 6V6 vs 6L6.
Sorry, I meant "noise". Very little noise. It's practically silent sitting idle. Hopefully that means I did good with the wiring LOL.
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I didn't know they made a clamshell right angle plug in stereo.. sorry .. mono is what you need,
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What you need is VERY tidy compact solder joint so it fits inside the clamshell. This takes practice/experience. And small wires, and sometimes a file.
These things used to come with a bit of fishpaper stuck to the outer shell, but that always got lost, and I may not have seen one in decades.
The stereo "could" work but leaves less space for the joints. But you did get the MONO?
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What you need is VERY tidy compact solder joint so it fits inside the clamshell. This takes practice/experience. And small wires, and sometimes a file.
These things used to come with a bit of fishpaper stuck to the outer shell, but that always got lost, and I may not have seen one in decades.
The stereo "could" work but leaves less space for the joints. But you did get the MONO?
Yeah, the wire that came with the kit to build the speaker cable is quite thick. I cut off about 1/2 the strands and still had issues.
Like I mentioned above, I accidently linked the stereo plug. The kit came with a mono.
Thanks!
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Tinning those wires before making connections is always a good idea. Even more critical when using that style plug.
In the future you may consider the Switchcraft 226. They easily handle large wires up to 16AWG. No need to weaken the wire by cutting half the strands to make it fit.
https://www.tubesandmore.com/products/14-plug-switchcraft-cable-mount-right-angle