Selecting Reverb Transformer

[harry]

Hi all,

I have just finished building a DR103 clone from scratch, using parts I had lying around. All I had to do was buy a capacitor; it cost me 4 british pounds. I could not believe it when I turned it on and it actually worked (and sounded great...)

In a similar mindset, I am going to try building an amp of my own design. I've never done this before, so bear with me!

I have an Accutronics reverb tank, code 8BC1C1B. Decoding this, it has a 190 ohm input impedance, and a 12k ohm output impedance. I want to drive this with a 12ax7. I understand that to do this, I should get a transformer that matches the output impedance of the 12ax7 with the input impedance of the tank.

So I've done some maths (please tell me if it's not correct!):

O/p impedance of 12ax7 is 70k in parallel with plate resistor.

Using a 100k plate resistor: 41k

Impedance Ratio is 41000:190, or about 215:1

To get the turns ratio, take the square root of this = 14.7

240V / 14.7 = 16.3



So if I found a 240V to 16V transformer, I could use this as a reverb driver?

Also, what kind of power should the transformer be able to handle?


Any suggestions are much appreciated!

Thanks,

Harry.

[imaradiostar]

I would encourage you to just get a Fender driver and try it! I bet it'll work just fine.

what about this?

I don't know that verb needs a super precise ratio...try some things and see how it works. You could always just buffer the 12ax7 with a mosfet to decrease the driving impedance and drive the tank using a cap.

Fender verb transformers are designed with parallel 12at7's in mind. The reflected impedance from a high impedance coil like that may favor the 12ax7 anyway.

jamie

[harry]

Hi Jamie,

Thanks for the suggestion... the Fender reverb drivers seem a little bit harder to come by in the UK! If it's just a transformer with a 15000:8 ohm impedance ratio then might be easier and cheaper to find my own.

I'll have a little search and see if I can find someone who stocks one over here.

[Merlinb]

Firstly, half an ECC83 makes a very poor reverb driver due to its low power, but it can be done. If you can though, I would recommend a more powerful device like an ECC81 or ECC82.

From your description you seem to be describing a parafeed design? i.e., a 100k load resistor,which is cap-coupled to a transformer. You'll get really puny power from an ECC83 in this mode. A conventional transformer-loaded stage is a must.
You'll want a primary impedance from about 40k to 60k (you're reasoning was wrong, but your answer was reasonable). Into 190 ohms, a 240V to 15V or 16V transformer would do. You will only get about 250mW audio power, but because there will be DC current in transformer, a 3VA tranny is probably necessary.
Otherwise you can find really small OTs in old radios and radiograms. A 100V line matching (PA) transformer can also be used and are quite cheap. One with a 2W to 5W tapping into 8 ohms would suit you.

However, you are fortunate in having a medium impedance tank, so you may be able to drive it without a transformer. An SRPP could save you a lot of space and money. See here for more info: The Valve Wizard