#1
So in the summer I was planning on building a reverb unit. It is going to be Fender-style, 3 tubes, Dwell, Mix & Tone controls. All the component values and the circuit design I've sourced are fine, but I needed help with the reverb-driving Transformer.

I was planning on building it into the VJ chassis using my VJ PT and maybe even OT, but I need to work out what the impedance ratios work out at.

I was either going to use an EL84 or a 6V6 as the reverb driver after the 12AT7.

Thing is, I was thinking about using the VJ OT (7.5k:8/4) but I've read in many places lots of different values for the classic Fender reverb-driving Transformer. What should the primary/secondary ratio/value be? I am planning on using an Accutronics 8AB2C1B which has a 10 ohm Input Impedance and 2575 ohm output impedance.

I was also considering adding a switch that allows you to switch the transformer's output from the Reverb tank to a speaker out, so it can also be used as a 5w SE amp as well. This is why I'd like to be able to use a 'conventional' OT.
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Last edited by MrCarrot at Apr 19, 2008,
#2
Interesting project.
I'd probably use the 6G15 circuit as a starting point.
Here's a link to one of the cleaner drawings I've seen:

http://www.ampwares.com/ffg/schem/reverb_6g15_schem.gif

I'd suspect the VJ's OT should match up well with a 6V6, if it's Version 1 or 2.
The most noteworthy comment I can make would be about the PS.
The reverb unit uses a much lower B+ supply than a VJ. (It's also a half wave rectifier, not a bridge)

This will present a small problem if you're planning on using the PT from the VJ.
You might consider using a EZ81/6CA4 rectifier. This will cause a considerable voltage drop. It should put you right about where you need to be. You'll need to look up the specs, but I think you can use just one anode and still not exceed the ratings of the rectifier. If you use both in parallel, that will decrease the voltage dropped across the rectifier. Do this, if the B+ voltage ends up being too low.

Also note that the cathode resistor is much larger. 1K.
And you only have 24mA of idle current in the output tube, not 60 ish like in the VJ.

The 6G15 circuit is also going to reduce the maximum power you can deliver to the speaker or the reverb tank. (1~2W) I see that as a good thing.
Meadows
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#3
Yeah, I was gonna go with the 6G15.

So how does the half-wave rectification differ from full-wave? I know the technical reasons but I don't understand why you wouldn't use full-wave.

And yeah, that lower output power would be great. Of course it would be a bonus if the amp section sounded nice as well but the reverb is just as important tonally.

Are you sure the VJ PT can handle 2.1A heater current, also?

And also, you're suggesting I should wire one of the Rectifying tube anodes to one of the HV windings, and ground the other? Just to make sure i'm not getting this all wrong.
The Laney Thread are big and clever. No exceptions.
#4
Quote by MrCarrot
So how does the half-wave rectification differ from full-wave? I know the technical reasons but I don't understand why you wouldn't use full-wave.
The only real advantage to half-wave with solid state rectifiers is 1/4 as many diodes compared to a bridge.

Quote by MrCarrot
Are you sure the VJ PT can handle 2.1A heater current, also?
No, I wasn't even thinking about heater current. tbh, I have no idea how much current the PT can deliver. That GZ81 requires a lot. It might not be such a good idea after all.

Quote by MrCarrot
And also, you're suggesting I should wire one of the Rectifying tube anodes to one of the HV windings, and ground the other? Just to make sure i'm not getting this all wrong.
You would connect one wire of the HV winding to ground. The other gets connected to the anode of the rectifier. (or both anodes, to decrease the "resistance") The cathode is the output. But as we were discussing, the heater current might be a problem. A series resistor or a zener diode might be an alternate way to decrease the voltage.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#5
Interesting project. I've been wanting to build a 6G15 for quite some time, but it'll have to wait.

Have you checked out the Vibroking schematic? It's slightly different, so it might give you some ideas.
#6
^ Yup, checked out the Vibroking stuff which is where I originally got the idea for using the EL84.

^^ Y'know, I think I remember reading somewhere that the VJ PT can handle just about dead on 6.3v/3A... I'll try and source it.
The Laney Thread are big and clever. No exceptions.