Driving an unusal Z=1k5-2k input spring reverb w/o tranny - diyAudio
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Old 18th September 2006, 01:30 PM   #1
bembel is offline bembel  Europe
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Lightbulb Driving an unusal Z=1k5-2k input spring reverb w/o tranny

reverb spring data is : Z input = 1k5-2k / Z output = 2k2-2k5 / both internaly grounded with the tank

Any ideas on topologies ?

What do you think of single (or paralleled) triode 12AT7/ECC81 as cathode follower with coupling cap to drive input ? then 1/2 12AX7/ECC83 for recover

Or maybe a single ECL8x or ECF8x tube including driving & recover.
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Old 19th September 2006, 12:05 AM   #2
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Don't bother trying the cathode follower. Although the calculated output impedance will be low it will fall on its face if it's asked to drive a load that low.

I would look at triodes with low plate resistance. 12b4 comes to mind. Could use a plate resistor and a big honking coupling cap. A 1:1 transformer might work better, but might not be necessary depending on the application.

A 25k pot on the recovery end will drive just about any small triode.

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Old 19th September 2006, 12:43 AM   #3
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There's a seller on eBay (vintagetubes) that sells new replacement driver transformers for Fenders that will work elsewhere too. About $16 USD plus shipping.

http://cgi.ebay.com/Fender-Reverb-Dr...QQcmdZViewItem

I hope this is clickable.

Victor
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Old 19th September 2006, 01:23 AM   #4
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bembel,

This seems like a job for a MOSFET source follower. Check out the IRFBC20; data sheet here. While you will not get the spec'd 1.4 A./V. transconductance at a rational drain current, you will get excellent drive capability and a low O/P impedance. Of course, that's what you need in front of the reverb tank.

If the electronics upstream of the buffer is wimpy, like a 'X7 section, use a cascade of FETs. The ZVN0545A presents a load that even the 'X7 can drive. Stick the little FET between the triode and the big FET. Useful information on setting up FETs as "spear carriers" for tubes can be found in MOSFET Follies.
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Old 19th September 2006, 01:45 AM   #5
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Go to Schematic Heaven, and look in the AMpeg section for model V-2 or G-12 as examples. very simply driven by a cap off the plate of the drive side.

Your high impedance input pan should be driven that way. The ones driven by a transformer will have low input impedance.
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Old 20th September 2006, 11:11 AM   #6
bembel is offline bembel  Europe
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Thank you all for your useful comments,
unfortunately, I realised that the spring unit won't fit in the chassis without big work!
Your ideas for driving this "transformerless" spring, will be used later.
Instead of that, I found an interesting article on a "springless reverb" circuit using only caps.
But I think, that it is no more reflecting the thread title, so I create a new one named "strange reverb circuit".
hope to read you there, thank you again.
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