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valves in a Farfisa pre amp

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Re: valves in a Farfisa pre amp

PostPosted: Tue Feb 19, 2019 11:07 am
by Hugh Robjohns
ef37a wrote: ... look at that g'awful HT supply?

There is a fairly widespread consensus that, popular and significant though the Farfisa Compact was to the popular music industry of the sixties, the in-house electronic design left quite a lot to be desired! :lol: It's all pretty marginal really -- at least when compared to its contemporaries -- but maybe that contributed to its character (much like many of the early studio console designs).

H

Re: valves in a Farfisa pre amp

PostPosted: Tue Feb 19, 2019 11:26 am
by ef37a
blinddrew wrote:
ef37a wrote:As for this all being G'gook to many readers? I am in my 74th year and Will is not that far behind me I don't think? Are there any non pension age valve savvy peeps about or will the technology become incomprehensible in ten years? I DO know valves will get ever more expensive and there will be fewer and fewer affordable valve amps. Some clever young buck could get clued up and make a fortune "roadying" for the Rock Gods!

Dave.
I did think about it* recently but you lost me at 'clever young buck'... ;)


* generally retraining rather than valve-specific.

Heh! Elsewhere in the forum is a chap asking if he has been ripped of for the repair of his amplifier? Tell you a stooooree (you will only get that if you are close to my age)

Around 2006 I went down to London to check and do some minor mods to a pair of A30 combos (won't name drop, Paul might be upset! No, didn't see him, daughter was sick as!)
I was there a good 3 hours and with the travelling, gone all of the working day. I would think a service tech to the Great & Good would cop at least 1/2 a bag for that jaunt?

It was an in warranty job so no charge. One of the great things about being a valve amp tech is that valves will ALWAYS fail eventually! I guess folks don't earn a lot of bread waiting for the likes of RME and Benchmark kit to go wrong?

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Wed Feb 20, 2019 2:30 am
by Adam Inglis
ef37a wrote:As for this all being G'gook to many readers? I am in my 74th year and Will is not that far behind me I don't think? Are there any non pension age valve savvy peeps about or will the technology become incomprehensible in ten years? I

Exactly, which is why I value the considered and comprehensive replies given in this forum. These are the documents we leave behind for those that come after!

ef37a wrote:AI DO know valves will get ever more expensive and there will be fewer and fewer affordable valve amps. .

Why so? Guitarists, bless 'em, seem a rather conservative lot when it comes to their technology, and they don't appear (at least from YT) to be becoming an endangered species.

Re: valves in a Farfisa pre amp

PostPosted: Wed Feb 20, 2019 8:41 am
by ef37a
Adam Inglis wrote:
ef37a wrote:As for this all being G'gook to many readers? I am in my 74th year and Will is not that far behind me I don't think? Are there any non pension age valve savvy peeps about or will the technology become incomprehensible in ten years? I

Exactly, which is why I value the considered and comprehensive replies given in this forum. These are the documents we leave behind for those that come after!

ef37a wrote:AI DO know valves will get ever more expensive and there will be fewer and fewer affordable valve amps. .

Why so? Guitarists, bless 'em, seem a rather conservative lot when it comes to their technology, and they don't appear (at least from YT) to be becoming an endangered species.

Because Adam, valves are a pretty dirty technology and the resulting equipment staggeringly inefficient. The Green lobby* does not seem to have woken up to the fact yet but it surely cannot be long before the Russian and Chinese factories are forced to clean up? The result of that will be much more expensive valves and a vastly reduced inventory.

*A movement I generally admire greatly. If valve equipment has one saving grace it is that properly designed and supported, it is eminently repairable and it is "fixing not flooding" that is one of the main ways we must go IMO if we are to avert catastrophe for our grandchildren.

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Wed Feb 20, 2019 11:13 am
by CS70
ef37a wrote:Adam, if you can poke a voltmeter at things and are 'scope cute you can do any of the work Folderol and I have suggested. I can talk/walk you through it if you PM me and do you shaky diagrams.

I will ammend Folderol's statement slightly? All ECC83s USED to be the same! There is some crap about. One that I have found very good for noise, hum and reliability is the TAD 7025 HG. Decent value as well at about £10.00.

The "bursting into life" MIGHT have been a bad joint or might be the result of heat or vibration on the transducers.

As for this all being G'gook to many readers? I am in my 74th year and Will is not that far behind me I don't think? Are there any non pension age valve savvy peeps about or will the technology become incomprehensible in ten years? I DO know valves will get ever more expensive and there will be fewer and fewer affordable valve amps. Some clever young buck could get clued up and make a fortune "roadying" for the Rock Gods!

Dave.

You guys should write a book! :D

Re: valves in a Farfisa pre amp

PostPosted: Wed Feb 20, 2019 11:27 am
by ef37a
CS70 wrote:
ef37a wrote:Adam, if you can poke a voltmeter at things and are 'scope cute you can do any of the work Folderol and I have suggested. I can talk/walk you through it if you PM me and do you shaky diagrams.

I will ammend Folderol's statement slightly? All ECC83s USED to be the same! There is some crap about. One that I have found very good for noise, hum and reliability is the TAD 7025 HG. Decent value as well at about £10.00.

The "bursting into life" MIGHT have been a bad joint or might be the result of heat or vibration on the transducers.

As for this all being G'gook to many readers? I am in my 74th year and Will is not that far behind me I don't think? Are there any non pension age valve savvy peeps about or will the technology become incomprehensible in ten years? I DO know valves will get ever more expensive and there will be fewer and fewer affordable valve amps. Some clever young buck could get clued up and make a fortune "roadying" for the Rock Gods!

Dave.

You guys should write a book! :D

Merlin Blencowe is very good. He has done guitar and bass preamps, gitamp PSUs and I wish he would get on with his guitar power amps volume. I may not have that long!

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Thu Feb 28, 2019 8:06 am
by Adam Inglis
My reverb holiday didn’t last long, as I feared. The tank still echoes when struck, but no useful amount of organ sound gets into it, so I assume it is the drive transducer that has failed.
I replaced the valves - no change.

So, it looks like I have some homework to do.
I have reviewed the above information kindly provided, and chased those links.

Option 1: Repair the transducer? - no info around on this
Option 2: Replace the transducer? Apparently these were basically ceramic phono stylus transducers that haven’t been available since the 1970s, so that’s probably not going to happen.
Option 3: modify the current circuitry to drive a new-style reverb tank, as favoured above by the cognoscenti. The current piezo transducers apparently are high impedance (can’t find much data on exact numbers) while modern reverb tank systems vary from about 8 to 2000 ohms.

https://www.tubesandmore.com/tech-corne ... d-compared

To modify the circuit, there are several possibilities.
Hugh favours re-wiring the valve circuit along the lines of a Fender amp reverb driver.
Dave favours adding a MOSFET to the drive valve - that Rod Elliot article explains the context
http://sound.whsites.net/project167.htm
Will suggests using a transformer and replacing the type of valve to ECC82 to drive the highest impedance tank available (which appears to be around 1.5kohm)

The problem with all of those could be that there won’t be enough current available from the 210 volt winding on the main TX to power the new circuitry, but this could be improved by adding a diode bridge to replace the half-wave rectification that exists at present.
Schematic again
http://www.adambaby.com/tempdownloadfil ... mboPSU.pdf

There is a big Rod Elliot article that covers a lot of this including transformer drive options and recovery circuits, that I am attempting to get my head around…
http://sound.whsites.net/articles/reverb.htm
Phew! - who knew there was so much to consider when replacing a little ol reverb tank!

Re: valves in a Farfisa pre amp

PostPosted: Thu Feb 28, 2019 9:02 am
by ef37a
I would say Adam the first thing is to get some idea of the rating of the mains transformer?

With the valves stone cold and mains off, croc an AC voltmeter to the HT AC winding. Switch on and note the voltage at that instant. Let the valves warm up, a minute say, then take another reading, if the latter reading is more than 10% below the first the transformer is likely running at about its limit and drawing more power from it might be a bad idea. The use of a bridge rectifier will make things more efficient but you don't want to risk burning out that traff! GAKnows where you would get another!

I think I have come to the conclusion that an IC driver is the best option? The LM386 can put a watt or so into 8 Ohms and I think 8R tanks are one of the most common types?

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Sat Mar 02, 2019 3:16 am
by Adam Inglis
I will do that test and report back.

Tanks with 8 ohms input impedance are available, up to about 1.5 k ohms. But in this case, wouldn't you want the highest impedance possible, so as to minimise current draw?

See Table 2 about a third the way down this page
http://sound.whsites.net/articles/reverb.htm
(Still getting my head around all that info, but those quoted current figures stuck out)

Re: valves in a Farfisa pre amp

PostPosted: Sat Mar 02, 2019 6:39 am
by ef37a
Adam Inglis wrote:I will do that test and report back.

Tanks with 8 ohms input impedance are available, up to about 1.5 k ohms. But in this case, wouldn't you want the highest impedance possible, so as to minimise current draw?

See Table 2 about a third the way down this page
http://sound.whsites.net/articles/reverb.htm
(Still getting my head around all that info, but those quoted current figures stuck out)

I have had quick look at that article and it seems good advice. The point about "current drive" being in theory a good idea is one that I had thought of, used to drive high inductance tape heads, but in this application it seems that in reality, voltage drive gives better results.

He quotes a drive level of about 3V rms for an 8R tank and that is a smell over 1W so the 386 would just cope but if you wanted a bit more headroom one of the more complex circuits he suggests could be used. Now Maplin are no more, look for Vellman amp kits. There is a 7W amp kit that should suit run off that 20V supply.

Whatever drive circuit you chose do not DC couple the drive coil to the transistors! Amps have been known to fail and putting 2 amps through the coil will surely burn it out. A 220 mfd cap will save it.

Whatever you do it involves quite a bit of work, not only building a drive amp but beefing up the power supply to feed it. Have you considered an external reverb pedal and two jacks as an FX loop?

https://www.velleman.eu/products/view/?id=352708
Note, you would need a 7815 regulator to feed the above but that seems the simplest solution to me.

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Mon Mar 04, 2019 8:33 am
by Adam Inglis
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!

Re: valves in a Farfisa pre amp

PostPosted: Mon Mar 04, 2019 8:46 am
by ef37a
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.

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Mon Mar 04, 2019 9:46 am
by Folderol
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:

Re: valves in a Farfisa pre amp

PostPosted: Mon Mar 04, 2019 10:13 am
by ef37a
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?

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Mon Mar 04, 2019 12:01 pm
by Hugh Robjohns
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 )

H

Re: valves in a Farfisa pre amp

PostPosted: Tue Mar 05, 2019 8:10 am
by Adam Inglis
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!

Re: valves in a Farfisa pre amp

PostPosted: Tue Mar 05, 2019 8:29 am
by ef37a
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!

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Tue Mar 05, 2019 9:10 am
by Folderol
@Adam
Very pleased to see... hear? this. We all learned a bit I think - one of the things I most enjoy about SOS.

Re: valves in a Farfisa pre amp

PostPosted: Tue Mar 05, 2019 10:57 am
by ef37a
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).

Dave.

Re: valves in a Farfisa pre amp

PostPosted: Tue Mar 05, 2019 11:30 am
by Hugh Robjohns
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.

H