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valves in a Farfisa pre amp

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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Mon Mar 04, 2019 8:33 am

Thanks Dave.
That AC winding reads 207 volts cold, about 10 mins later reads 205 volts.

I pulled the reverb tank out and reversed the wiring, swapping the driver and receiver signals around - it is definitely a transducer issue. Swapping them, you can hear the audio in the springs, but it doesn't make it back to the audio bus. The 100 volts signal just disappears at the faulty transducer, although I can't detect a short to ground. The transducer does appear to be a gramophone pick up, I took a photo..
http://www.adambaby.com/tempdownloadfil ... sducer.jpg
Inside the black casing is a substance that looks like wet salt - I guess this is what's left of the piezo crystal!

All the above considered, your suggestion of a simple FX loop may be the most sensible advice of all. If that TX should fail, I would be up for a major re-design and rebuild to put a modern PSU in there. I'd probably need more than one, given the AC and DC requirements.

Many thanks all!
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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Mar 04, 2019 8:46 am

Ok Adam, the good news is that the HT supply is pretty firm, you could fit a bridge, maybe double the res' cap value and go for a valve and transformer stage but tis a lot of work.

To drive an external FX pedal you will need a low impedance drive of around neg 10, 300mV and the easiest way is to take a feed from the cathode of the existing drive valve (if it is decoupled, unhook the cap' and use it as the output coupler. The incumbent recovery amplifier should have enough gain but if not a TL072 or NE5534 is a fairly easy fix.

Dug out the map. The cathode cap' C515 is just 100nF so up that to 2.2mfd 63V and fit a 22k or so "tie down" R after it. The recovery valve does not have a cathode capacitor so ~ 10mfds 22V would give that stage more gain. You can play with the value to reduce the LF response.

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Re: valves in a Farfisa pre amp

Postby Folderol » Mon Mar 04, 2019 9:46 am

Have you thought of trying to get hold of another ceramic PU? That one looks like it suffered moisture damage, whereas one in an old record player will likely still be clean.
You might also be able to get results from a modern peizo device of some sort.
Maybe I'm being a bit unrealistic, but I hate the idea of an old bit of kit being dramatically changed and sort-of faked :cry:
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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Mar 04, 2019 10:13 am

Folderol wrote:Have you thought of trying to get hold of another ceramic PU? That one looks like it suffered moisture damage, whereas one in an old record player will likely still be clean.
You might also be able to get results from a modern peizo device of some sort.
Maybe I'm being a bit unrealistic, but I hate the idea of an old bit of kit being dramatically changed and sort-of faked :cry:

An idea but might be a huge fiddle? https://www.gettheneedle.co.uk/alba-cer ... ridge.html
Cheapest I could find. One of the best of the ceramics was the Sonotone 9ta. 75 quid!!

Mind you Will there is some money to be made refurbing old record players. You can pick them up for £30-40 and they sell at auctions when fully resorted for ten times that.

An idea now that you are retired?

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Re: valves in a Farfisa pre amp

Postby Hugh Robjohns » Mon Mar 04, 2019 12:01 pm

Adam Inglis wrote:The transducer does appear to be a gramophone pick up, I took a photo..

My dad and I built a spring reverb tank when I was about 10 years old, using electric fire elements and some old 78 record crystal pickup cartridges. I don't remember the constructional details well, but I think it was relatively straightforward modifying the pickups to attach the springs.

You can still get 'crystal' pickups for a modest outlay. If you fancy a bit of DIY experimentation you might be able to get the original tank working again without having to rebuild the electronics...

https://www.gettheneedle.co.uk/index.ph ... cartridges

EDIT -- oops -- just saw others have the same idea (and google skills! :-D )

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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Tue Mar 05, 2019 8:10 am

Learning all the time, as Benny Hill used to say.

My local emporium didn't have any cartridge pickups of course, but it did stock some cheap little piezo transducers they called "buzzers" so I grabbed a couple. One type only responded to DC, so that was no good, but I had more luck with the other one. I had to crack the case to get to the transducer
http://www.adambaby.com/tempdownloadfiles/piezoTD.jpg
It seemed to have no problem with the high voltage signals. I messed around with different positions and mountings, and tried it both as driver and receiver, at either end of the tank (the factory position has the receiver at the end where the 3 springs converge).
http://www.adambaby.com/tempdownloadfiles/tank.jpg
http://www.adambaby.com/tempdownloadfil ... ceiver.jpg
The results were certainly useable. The reverb level is a smidge lower than before, or perhaps it is just a bit darker.
For now I've put it as the driver, using the surviving PU as receiver, and superglued the wire anchor of the spring to the piezo element. This transducer is quoted as having a resonant frequency of 4.2 kHz +/- 500 hz. It might be interesting to try some different types, with different resonances...

but certainly for now, I would call this a RESULT!
Thank you gentlemen!
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Re: valves in a Farfisa pre amp

Postby ef37a » Tue Mar 05, 2019 8:29 am

Regarding frequency response Adam. I was charged to fit a reverb tank in a 100W valve head for a very famous guitarist.

The circuit was a shameless rip of Fender (why reinvent the wheel?) and all worked pretty well except the sound had a nasty honk at around 400Hz iirc. This was tamed with a parallel T filter in the recovery amp path. These have a very simple formula to calculate R&C once you have identified the offending frequency and almost all DAWs have a spectroscope function easily good enough for rock n roll! Though the thought just occurs? You have a keyboard!

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Re: valves in a Farfisa pre amp

Postby Folderol » Tue Mar 05, 2019 9:10 am

@Adam
Very pleased to see... hear? this. We all learned a bit I think - one of the things I most enjoy about SOS.
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Re: valves in a Farfisa pre amp

Postby ef37a » Tue Mar 05, 2019 10:57 am

Folderol wrote:@Adam
Very pleased to see... hear? this. We all learned a bit I think - one of the things I most enjoy about SOS.

Agreed matey. I get really hissed off on forums when someone shouts "OT! OT!" For me it is the avenues and byways that make forums interesting and if I can add even just a tiny bit to the sum total of audio knowledge that is my "fee" as it were.

Those that want a down the line, bish-bash-bosh answer to their questions can PAY an expert! Of course, we must never lose sight of the OP's orignal problem and try to solve that as well and that brings me to my other dislike? People on forums that denigrate the newbs kit and say "X is no *&^%g use. You need Y" (at many times the money).

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Re: valves in a Farfisa pre amp

Postby Hugh Robjohns » Tue Mar 05, 2019 11:30 am

Adam Inglis wrote:For now I've put it as the driver, using the surviving PU as receiver, and superglued the wire anchor of the spring to the piezo element. .... but certainly for now, I would call this a RESULT!

Brilliant. Well done that man! I'm sure a bit of fettling will fine-tune the results.

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Re: valves in a Farfisa pre amp

Postby Martin Walker » Wed Mar 06, 2019 5:54 pm

Adam Inglis wrote:Learning all the time, as Benny Hill used to say.

My local emporium didn't have any cartridge pickups of course, but it did stock some cheap little piezo transducers they called "buzzers" so I grabbed a couple. One type only responded to DC, so that was no good, but I had more luck with the other one. I had to crack the case to get to the transducer

If you're prepared to go mail order, you don't have to crack the cases ;)

I've bought some of these in the past:

http://spiratronics.com/product-39448.html

Image


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Re: valves in a Farfisa pre amp

Postby James Perrett » Wed Mar 06, 2019 9:50 pm

Folderol wrote:Have you thought of trying to get hold of another ceramic PU? That one looks like it suffered moisture damage, whereas one in an old record player will likely still be clean.

I realise that I'm a bit late to this but I thought I'd mention that obtaining working crystal/ceramic pickups is becoming a big problem for the vintage record player fraternity. High output types are especially rare. The only new ones available are the ubiquitous red ones seen in the cheap record players which can be obtained very cheaply if you can wait for them to arrive from China.
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Re: valves in a Farfisa pre amp

Postby Sam Spoons » Wed Mar 06, 2019 11:34 pm

Martin Walker wrote:If you're prepared to go mail order, you don't have to crack the cases ;)

I've bought some of these in the past:

http://spiratronics.com/product-39448.html

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Martin

Maplin was the goto for those but I'd expect them to be easily available elsewhere.
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Re: valves in a Farfisa pre amp

Postby Martin Walker » Thu Mar 07, 2019 1:49 am

True, but I've actually since remembered that I bought mine via Ebay, via this link:

https://www.ebay.co.uk/itm/360719728982 ... rsta=en_GB

The different sizes have differing resonance frequencies.


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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Mon Mar 25, 2019 4:46 am

Thanks Martin.
That vendor quotes a "resonant impedance" as measured in watts. Not sure what that is... but those larger 35 mm ones with resonant frequency around 2 kHz could be well worth experimenting with.

A couple of observations:
messing around with the current setup, I found several "sweet spots" for the transducer, in contact with the aluminium baseplate (this is isolated by foam blocks inside the aluminium chassis of the tank). I suppose there is probably some amount of "plate" resonance going on here along with the spring vibration (some will recall the DIY plate reverb on youtube recently using piezos).
In addition to the reverberation, there is also quite a bit of "direct" sound, which, I guess because of two state changes and a bit of metal in between, has a rather distorted edgy sound.
The overall effect has buckets of character!
I’m enjoying it so much that I’m going to look at installing an external input to the circuit so that I can use it as a send from the mixer.
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