Vox transformer build

marc88

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Hey all,
I love this forum and visit all the time and have gotten so much great info; so thank you! I’ve recently gotten into amp building and modifying and have successfully built a 5e3, mojotone nc3015, and a 6v6 blackvibe. I love those amps but now with my tinkering and modifying I have a few parts lying around that I’d love to use for something. What I have on hand is the power transformer from my Vox ac15c1 (300-0-300 HT, 6.3 heaters, and an accessory 25v that I’d probably cap), the output transformer from my 65 DRRI, and the choke from the DRRI. My thoughts are scattered and preliminary now but I’m thinking of trying to make a 2x 6v6 push pull amp with these parts. I’m leaning toward cathode biased to simplify things a bit. The PT doesn’t have a 5v winding so I think I’ll do a bridge rectifier and use that to maximize my voltage for the 6v6’s. Maybe similar power section to the 5e3, but since I already have a 5e3, I’d like to make some changes to the preamp and phase inverter. Maybe like a 5f6a preamp with a similar tone stack and a LTP and global negative feedback. Sorry to be vague and all over the place, but the three amps I have built so far have been pretty close to the layouts I’ve found online. I’ve made some minor adjustments, master volume and back rectifier diodes, stuff like that. But mostly it’s been following established circuits. Now I’m trying to design my own mix and I’m unsure where to start. So I figured I’d run it by the community here and see if anyone has done something similar and can offer some advice about what works and doesn’t. Thanks in advance!
 

andrewRneumann

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Mar 22, 2020
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Cincinnati, OH, USA
Hey all,
I love this forum and visit all the time and have gotten so much great info; so thank you! I’ve recently gotten into amp building and modifying and have successfully built a 5e3, mojotone nc3015, and a 6v6 blackvibe. I love those amps but now with my tinkering and modifying I have a few parts lying around that I’d love to use for something. What I have on hand is the power transformer from my Vox ac15c1 (300-0-300 HT, 6.3 heaters, and an accessory 25v that I’d probably cap), the output transformer from my 65 DRRI, and the choke from the DRRI. My thoughts are scattered and preliminary now but I’m thinking of trying to make a 2x 6v6 push pull amp with these parts. I’m leaning toward cathode biased to simplify things a bit. The PT doesn’t have a 5v winding so I think I’ll do a bridge rectifier and use that to maximize my voltage for the 6v6’s. Maybe similar power section to the 5e3, but since I already have a 5e3, I’d like to make some changes to the preamp and phase inverter. Maybe like a 5f6a preamp with a similar tone stack and a LTP and global negative feedback. Sorry to be vague and all over the place, but the three amps I have built so far have been pretty close to the layouts I’ve found online. I’ve made some minor adjustments, master volume and back rectifier diodes, stuff like that. But mostly it’s been following established circuits. Now I’m trying to design my own mix and I’m unsure where to start. So I figured I’d run it by the community here and see if anyone has done something similar and can offer some advice about what works and doesn’t. Thanks in advance!

Hello and thanks for your post. It seems like you have a good idea of what you want. Now is the time to start putting it all on paper. Do you have access to schematic creation software? The preamp of a 5F6A can go with the power amp of a 5E3. There will be a need to tweak some things, but it’s best to put it down on paper and start working your way through the circuit—usually from power amp back through the preamp.
 

marc88

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lynn ma
Thanks for the reply and the confirmation that my goal here makes some sense! I haven’t done anything with schematic software up until now, but I can check it out. Any recommendations? I was embarrassed to mention it in my first post, but I’m still pretty shaky at reading schematics. I’ve gotten a bit better at it and have used them to locate/fix some small troubleshooting, but I’m definitely more comfortable with layouts. Software may help me bridge that gap, so thank you so much for the suggestion!
 

2L man

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What I have on hand is the power transformer from my Vox ac15c1 (300-0-300 HT, 6.3 heaters, and an accessory 25v that I’d probably cap), the output transformer from my 65 DRRI, and the choke from the DRRI. My thoughts are scattered and preliminary now but I’m thinking of trying to make a 2x 6v6 push pull amp with these parts. I’m leaning toward cathode biased to simplify things a bit. The PT doesn’t have a 5v winding so I think I’ll do a bridge rectifier and use that to maximize my voltage for the 6v6’
You can't bridge rectify that PT for 6V6 if its secondary is internally connected to one coil which has a CT because filtered voltage would come even 840VDC. If you use only other half you would get 420VDC but only half the current output potential because there flow current 100% time.

Only when there are two separate secondaries they can be parallelled.

Use two Silicon diodes around CT and filtered DC voltage comes about 420VDC.
 

marc88

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Joined
Apr 19, 2012
Posts
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Location
lynn ma
You can't bridge rectify that PT for 6V6 if its secondary is internally connected to one coil which has a CT because filtered voltage would come even 840VDC. If you use only other half you would get 420VDC but only half the current output potential because there flow current 100% time.

Only when there are two separate secondaries they can be parallelled.

Use two Silicon diodes around CT and filtered DC voltage comes about 420VDC.
Oh wow thank you for that! I actually knew that through my research but completely blanked on it! I’ll make sure not to bridge rectify it, thank you again!
 

ArcticWhite

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Thanks for the reply and the confirmation that my goal here makes some sense! I haven’t done anything with schematic software up until now, but I can check it out. Any recommendations? I was embarrassed to mention it in my first post, but I’m still pretty shaky at reading schematics. I’ve gotten a bit better at it and have used them to locate/fix some small troubleshooting, but I’m definitely more comfortable with layouts. Software may help me bridge that gap, so thank you so much for the suggestion!
Schematics are way easier to understand than layouts. Layouts are just building instructions, while schematics show how an amp works, and what is actually happening inside one.
If you haven't yet, you should read @robrob s pages on how tube amps work, and how the 5E3 works.

Rob takes you through the signal path, and explains the power supply, and everything in between. Don't worry if you don't get it immediately. Just read it periodically.
 

marc88

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Joined
Apr 19, 2012
Posts
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Location
lynn ma
Schematics are way easier to understand than layouts. Layouts are just building instructions, while schematics show how an amp works, and what is actually happening inside one.
If you haven't yet, you should read @robrob s pages on how tube amps work, and how the 5E3 works.

Rob takes you through the signal path, and explains the power supply, and everything in between. Don't worry if you don't get it immediately. Just read it periodically.
Good point! I should have mentioned Rob in my first post; his site (particularly about the 5e3) is what inspired me to try amp building in the first place. Such a tremendous resource and it has helped me immensely! I have read through a lot of his explanations and have been re-reading them recently to improve my understanding of the workings of each part of the amp. I definitely plan on spending more time on them and really get it down. I think my eventual goal is to marry the 5f6a preamp and the 5e3 power amp, but I’m going to make sure I understand what tweaks to make and where to make that split. Thanks for the reply though, I appreciate you guys weighing in!
 

JamesAM

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if you're not wedded to 6v6, you could build an 18w or a spitfire clone with that PT. Or if you are, maybe a 6v6 18w, which sound great. i think those are around 290-0-290, which with the right rectifier and cathode resistor might get you to where you need to be depending on the current rating of the PT.

Just something different, but similar enough to the NC3015 - there are a few build threads for spitfires on here and a bunch of them for 18watts.
 

marc88

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Posts
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Location
lynn ma
if you're not wedded to 6v6, you could build an 18w or a spitfire clone with that PT. Or if you are, maybe a 6v6 18w, which sound great. i think those are around 290-0-290, which with the right rectifier and cathode resistor might get you to where you need to be depending on the current rating of the PT.

Just something different, but similar enough to the NC3015 - there are a few build threads for spitfires on here and a bunch of them for 18watts.
Hm thanks for the suggestion! I was thinking 6v6 since I have a decent supply on hand. I’ll do some searching and thinking!
 




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