Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: toposeven on September 15, 2023, 05:16:57 am
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I need advise on what to do to add a 5u4 to a crate 3112 for switchable rectification id like to be able to turn off 2 of the el84s and eventually convert it to point to point since this is my first amp mods or build as it may turn out to be I could really use a bit of advice on the proper power transformer to order as im sure im going to need a 5 volt rectifier circuit or can it be done by adding a 5 volt transformer for this circuit or is this a workable idea ,how do you go about setting up switching to turn off 2 el84s to run at 15 watts and with the sise of the circuit board am I going to have to convert it to point to point to have the space to install the rectifier any one feel like helping on this one
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Are you experienced with working on amps safely? Can you read schematics well? Any chance you have a good photo of the chassis interior of the amp that you can share with us?
IF this schematic of the power amp section matches your Crate 3112, then you have 317v on the plates of the EL84's with solid state rectification. In light of that, you probably don't want to add a 5v transformer to the amp to add tube rectification and lower the voltage even more?
Crate_vc3112_poweramp.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Crate/Crate_vc3112_poweramp.pdf)
Crate_vc3112_preamp.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Crate/Crate_vc3112_preamp.pdf)
The amp in the schematic is cathode biased. To run two tubes for 15 watts, you'd pull two of the "outer" tubes and change the cathode resistor for the EL84's to something closer to 120ohms or 130ohms.
See Hoffman's example here for two EL84's : 18WattStout_8.gif (1296×675) (el34world.com) (https://el34world.com/projects/images/18WattStout_8.gif)
Not sure why you want tube rectification? Honestly, in a blind test .......... you may not be able to even hear a difference between the solid state rectification vs. the tube rectification? OR if you could hear a slight subtle difference, might not be able to tell which one is which? OR if you heard a slight subtle difference, you might end up prefering the solid state rectification?
Unless you are quite experienced in amp building and amp tweaking, then I would suggest NOT replacing the PT and NOT going with a tube rectifier.
IF it were my amp, I would likely pull two tubes and replace the cathode resistor and call it a day. Since 15 watts is plenty loud for my needs (even for playing out), I would not have the amp switchable from 15 watts (two tubes) to 30 watts (four tubes). That's just my personal preference. It could be done by using a switch to change from a 60ohm cathode resistor to a 120ohm cathode resistor. That is NOT something you can do on the fly. Switching from 15w to 30w would have to be done by turning the amp off, pulling two tubes and then switching the switch to 130ohms/10watt.
OR ............. I would completly gut the amp, build a turret board with a design that I'd be more happy with. In either case, I would NOT add tube rectification. I think tonally, you would likely find that a waste of time and money. Crate amps are OK, but they would be low on my list to invest very much money in such as buying a new PT.
With respect, Tubenit
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... need advise on what to do to add a 5u4 to a crate 3112 for switchable rectification ....
Simply don't do it, it is simply unnecessary
What you can do is to add a SAG resistor (with a parallel switch to bypass it when you want)
You'll have the same result in response and save a LOT of money
If you don't know the argument do a search and you'll find a lot of documentation about
Suggested read (for SAG and other)
https://tone-lizard.com/modifications/ (https://tone-lizard.com/modifications/)
http://hotroddiy.com/The-Tube-Rectifier-Sag-Mod.pdf (http://hotroddiy.com/The-Tube-Rectifier-Sag-Mod.pdf)
Franco