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valves in a Farfisa pre amp

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Re: valves in a Farfisa pre amp

Postby Watchmaker » Tue Feb 19, 2019 3:56 am

thar's people wot know wot they're on about about here
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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Tue Feb 19, 2019 5:26 am

Phew! Lots to follow up. Thanks for all the great info... although it would appear some are over-estimating my skills in this area!

So, I swapped the valves - no change. Took the opportunity to clean the pins and sockets with IPA - no corrosion evident.
Continuity-testing the related circuitry, including the reverb tank, was all OK. (The tank appears to be original, there's no name on it, but it has that tri-radiate arrangement of springs, and the transducers are physically very small. This organ has been "in the family" since about 1979 and has needed no major repairs apart from the odd capacitor replacement on an oscillator board).

Putting the PSU chassis on it's side to access the underside of the sockets I took some voltage measurements - when I touched pin 6 (anode) of V2, I heard a familiar echo of a clunk - the reverb was back! Maybe a dodgy solder joint? The reverb level is now much as I recall it, and with it's affinity for the high-mids.
However the voltage measurements weren't up to spec - the anodes showed between 130 to 150 volts, the cathodes were 0.7 - 1 volt.
There's clearly some amplification going on - on the scope a 200 mV signal from the osc/filter boards gets cranked to about 100 volts before it gets to the reverb.

I might get me some new valves.
"Tube noob" question: - are all ECC83 s the same, or do I have to watch out for voltage ratings etc?

cheers
A
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Re: valves in a Farfisa pre amp

Postby Folderol » Tue Feb 19, 2019 8:23 am

Low voltages all round are not likely to be due to aging valves, more likely the actual HT voltage. If the valves go low emmission, the cathode voltage drops and the anode rises. It might be low HT, so worth looking at those 16u & 32u caps, and the 22k resistor.

In theory all ECC83s should be the same. In practice they vary depending on manufacturing quality :(
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Re: valves in a Farfisa pre amp

Postby ef37a » Tue Feb 19, 2019 10:40 am

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.
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Re: valves in a Farfisa pre amp

Postby blinddrew » Tue Feb 19, 2019 10:57 am

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.
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Re: valves in a Farfisa pre amp

Postby Hugh Robjohns » Tue Feb 19, 2019 11:07 am

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
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Re: valves in a Farfisa pre amp

Postby ef37a » Tue Feb 19, 2019 11:26 am

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?

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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Wed Feb 20, 2019 2:30 am

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.
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Re: valves in a Farfisa pre amp

Postby ef37a » Wed Feb 20, 2019 8:41 am

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.
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Re: valves in a Farfisa pre amp

Postby CS70 » Wed Feb 20, 2019 11:13 am

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
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Re: valves in a Farfisa pre amp

Postby ef37a » Wed Feb 20, 2019 11:27 am

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.
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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Thu Feb 28, 2019 8:06 am

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!
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Re: valves in a Farfisa pre amp

Postby ef37a » Thu Feb 28, 2019 9:02 am

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?

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Re: valves in a Farfisa pre amp

Postby Adam Inglis » Sat Mar 02, 2019 3:16 am

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)
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Re: valves in a Farfisa pre amp

Postby ef37a » Sat Mar 02, 2019 6:39 am

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.

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