Diy valve driven spring reverb

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oltern8

Member
Joined 2013
#1
Hi


I want to build a valve driven spring reverb using 6J1cheap Chinese valve pre-amp. The only reverb tanks available to me are rated 150 ohm input impedance.
Is it possible to rewire the transducer so its impedance is 4-8 ohms so the valve amp can drive it properly.


What equipment would I need to do this accurately? and would it be possible to use a multi meter to get in the ball park?


thanks



Oli
 
#3
Hi

thanks for your reply.

So if I understand correctly I would have to unwind the coil until my multi meter reads 1 or 2 ohms (DCR).

Do I have to completely unwind the transducer and measure the cables resistance then rewire the transducer? or should I uncoil the transducer bit by bit and measure the resistance until I have the correct reading?

thanks
 
#4
oltern8 said:
Hi
So if I understand correctly I would have to unwind the coil until my multi meter reads 1 or 2 ohms (DCR).
Click to expand...
No. JM might explain fully later.

The easiest option is to put a 16ohms (a couple of watts) resistor in parallel with the tank input. Then your amp will see about the right impedance but you're going to loose about 10db of drive.

Actually, the easiest option is to buy a suitable reverb driver transformer 😀

Which I'm assuming you've already discounted for some reason. So you're left with finding a close substitute. That is, something with an appropriate turns ratio to present an acceptable load to your tube when loaded by the tank.

The usual place to go a-hunting is the PA transformer catalogue as (for guitar reverb) the loss of bass caused by running a wee bit of DC through them is acceptable

What tube are your trying to drive with?
 
#5
oltern8 said:
So if I understand correctly I would have to unwind the coil until my multi meter reads 1 or 2 ohms (DCR).
Click to expand...
Not exactly.
Drive transducer has a tiny coil which is *filled* (for efficiency) with proper diameter enamelled wire.
spring_reverbs_ideal_transducer_orientation.svg


different wires for different impedances.

Actual impedance is measured @ 1kHz but since "all bobbins are the same", knowing DCR already tells you which one it is.

For SS direct drive, typically from a TL072, the WIDELY used one is the 150 ohm impedance , 25 ohm DCR which apparently is the one you can get easily.

For Tube Driven ones, most famous is the one in Fender **** Reverb amps and which I guess you wantb to buulod uses an 8 ohm nominal, 1 ohm DCR coil.

You would need to remove transducer coil, (you can´t work with it mounted inn the tank) unwind original wire, measure it , (say, 0.05 mm wire) and use 3X diameter one (say, 0.15 mm diameter) to rewind it.

As I sais, doable, there´s people who make a living out of repairing dead tanks, but not easy, by any means.

WHERE do you live?
Easiest would just to order the proper tank from Belton (Korea/USA) or one of its distributors worldwide or plain EBay, they are around U$ 40 .

Sometimes I hear about somebody offering replacement bobbins, you order one 8 ohms impedance, search for it.

A tiny component, it can be delivered through regular Mail anywhere for very low cost.

Spring Reverb Tanks Explained and Compared | Amplified Parts
 
#6
Hi


Thanks for the explanation I now understand how I would go about rewiring the transducer bobbin. I am currently living in Guangzhou, China. It would definitely be easily to order what I need. I was just hoping to keep costs down and use what is easily available here.


I have been looking for a Fender tank which would have the correct input impedance but haven't been able to find a retailer. Most retailers are wholesale and even if I tracked them down they wouldn't be interested in selling one unit.



I was planning to use this 6J1 valve pre-amp to drive it as I already have a similar unit.



Douk Audio Mini 6J1 Valve And Vacuum Tube Pre-Amplifier Stereo HIFI Buffer Preamp - Newegg.com


If I did use this pre-amp and a 16 ohm resistor in parallel across the tank inputs as suggested by thoglette I could use the spare channel of the pre-amp to re-amplify the signal making up for the losses. Does this sound feasible or should I rethink this idea. May be a 600 ohm tank into the valve preamp?




thanks for your time and help


Oli
 
#9
oltern8 said:
I am currently living in Guangzhou, China. It would definitely be easily to order what I need. I was just hoping to keep costs down and use what is easily available here.

I have been looking for a Fender tank which would have the correct input impedance but haven't been able to find a retailer. Most retailers are wholesale and even if I tracked them down they wouldn't be interested in selling one unit.
Click to expand...

No idea about China but US suppliers will definitely sell you *one*.
Maybe some China/Hong Kong/Taiwan/Korea based seller can supply one so you save on shipment.

No need for "Fender Brand"; Belton, Accutronics and MOD all offer 8 ohm input tanks. Browse their pages for model numbers and then search retailers.
I was planning to use this 6J1 valve pre-amp to drive it as I already have a similar unit.
Click to expand...
Sorry but no.
You need to clone the Fender circuit which takes a 12AX7 to handle signals, both send and recovery, and a 12AT7 to drive the tank through a special transformer (high impedance to 8 ohm) with actual *power*, between 1 and 2 W RMS.

You must shake those springs well, in fact one bench test to confirm drive section is working fine is putting *a speaker* at transformer out and play a guitar, not kidding.

There are Guitar Tube mini amps using that circuit, go figure.

A regular preamp won´t drive it at all.

Download and check Fender Twin Reverb schematics to see how it´s done.

I guess you will have to US order both the tank and the transformer from the same supplier, you will need to pay *one* shipment anyway.

These are complete kits for you to build .
Admittedly expensive, use them as "inspiration" but specially to know what are you getting into 😱

This is the Nobel prize level kit:
Tube Reverb 6G15 Style Amp-Kit

Very good ones:

Tube-Town Store - TT Kit Tube-Reverb

Effects Kit - MOD(R) Kits, The Wave, Spring Reverb | Amplified Parts

This is what they deliver:

MOD(R) Kits DIY The Wave Effect Unit Demo (Tube-Driven Spring Reverb) - YouTube

Notice the actual tanks by themselves cost around 40-50 bucks, a transformer will be around $40-60, it´s "everything else" around them that makes it expensive, *specially* if you go the Tube route.


Douk Audio Mini 6J1 Valve And Vacuum Tube Pre-Amplifier Stereo HIFI Buffer Preamp - Newegg.com


If I did use this pre-amp and a 16 ohm resistor in parallel across the tank inputs as suggested by thoglette I could use the spare channel of the pre-amp to re-amplify the signal making up for the losses. Does this sound feasible or should I rethink this idea. May be a 600 ohm tank into the valve preamp?
Click to expand...
No, you need POWER diving the tank.
Fender circuit actually includes an almost 2W RMS power amp, single ended and tube driven, with output transformer.
A preamp can deliver voltage (to a high impedance load, think 50K ohm) but not power.
 
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