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valves in a Farfisa pre amp

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valves in a Farfisa pre amp

Postby Adam Inglis » Mon Feb 18, 2019 11:01 am

OK, I'll fess up at the start - I know next to nothing about valves and have not had to repair anything that contains them.
On the bench at the moment is a Farfisa Compact organ circa 1968.
I've so far "fixed" a lot of troubles just by replacing the obvious electrolytic caps that were leaking and corroded, in the filter boards and the PSU, followed by an excruciatingly tedious and finicky job of cleaning the key contacts (3 equally-inaccessible contacts per key, yay!).

Anyway, there are two twin-triode tubes in the PSU (ECC83). Their duties are pre amp for the audio output, and amplification of the audio buss to and from the reverb tank transducers.
See here:
http://www.adambaby.com/tempdownloadfil ... mboPSU.pdf

The problem: the reverb is barely audible, but definitely still present on the main output.
My question: in valves/tubes such as these, can one half fail while the other is still functional?

cheers
A
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Re: valves in a Farfisa pre amp

Postby Wonks » Mon Feb 18, 2019 11:07 am

Have you tried swapping the two valves around? If one is faulty, then you'll end up with a different issue.
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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Feb 18, 2019 12:11 pm

Yes, it is possible for one section of a double triode to fail but since they are invariably in cascade i.e. one section drives the next, you cannot tell. Pretty rare though.

Usefully, the DC conditions around a valve virtually completely determine if it is working properly or not. Check the voltages on the anodes and especially the cathodes. The ECC83 almost always runs with a cathode voltage of 1 -2volts and an anode current of around 1mA. The value of the anode loads will determine the anode voltage so if you can check the value of those resistors a voltage check will tell you with almost complete certainty if the valve is ok.
Triodes in low power circuits have an incredibly long life, decades and this is especially true of original old Mullards.

Reverb tanks on the other hand are not so reliable! Again it seems the newer ones are worse but if you can find the make and model (Accutronics?) the Web will get you the coil impedances.

Can you link to any schematics? Oh you did. Sorry! (oil be bek)

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Re: valves in a Farfisa pre amp

Postby Wonks » Mon Feb 18, 2019 12:14 pm

ef37a wrote:Can you link to any schematics? .

Have you simply tried the link to the schematic in his post? :headbang:
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Re: valves in a Farfisa pre amp

Postby Hugh Robjohns » Mon Feb 18, 2019 12:16 pm

I'd second all that. Unlikely to be the valve, but it is possible. Swap valves around and see if the faults move as a first check. And/or mess with the voltmeter as Dave suggests.

My experience is mostly of Hammonds, but from that I'd say it's more likely to be an issue with the connectors to/from the reverb tank, or the tank itself (driver/receiver), or possibly those reverb length and level controls.

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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Feb 18, 2019 12:28 pm

Wonks wrote:
ef37a wrote:Can you link to any schematics? .

Have you simply tried the link to the schematic in his post? :headbang:

Shurrup. Got up late, only had one coffee so far and brekky and pills.

The Verb! Looks like a very special high impedance tank coil there. Does the tank give a loud CLANK if you thump it? If so that proves the output coil and valve so as Wonks says, swap them. If still no joy check the DC resistance of the drive coil. My guess it will be around 10k.

If it seems continuous try driving it from another source. The line out of a desk, AI, even a phone HP jack. If it is borked it might be hard to find a replacement but there are ways to modd the circuit for a modern tank.

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Re: valves in a Farfisa pre amp

Postby Watchmaker » Mon Feb 18, 2019 1:56 pm

ECC83s are cheap and generally reliable. I almost always simply replace the lot. Maybe these links help?

A Farfisa engineer quoted "To overcome Hammond's patent we made our own units, using piezoelectric transducers instead of magnets, and this required 100V plus drivers, at the time not easy to have with transistors."

http://www.combo-organ.com/Farfisa/Compact/#Compact

https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared
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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Feb 18, 2019 4:54 pm

Watchmaker wrote:ECC83s are cheap and generally reliable. I almost always simply replace the lot. Maybe these links help?

A Farfisa engineer quoted "To overcome Hammond's patent we made our own units, using piezoelectric transducers instead of magnets, and this required 100V plus drivers, at the time not easy to have with transistors."

http://www.combo-organ.com/Farfisa/Compact/#Compact

https://www.amplifiedparts.com/tech-corner/spring-reverb-tanks-explained-and-compared

Clever buggers..'splains it!

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Re: valves in a Farfisa pre amp

Postby Hugh Robjohns » Mon Feb 18, 2019 5:25 pm

Thinking about it... I was going to ask if you were sure the current reverb tank is the original unit? I initially wondered if it might have been replaced with a more generic low-Z type, which wouldn't work well if at all in that circuit configuration.

...but delving around the web it would appear that those piezo-driven spring tanks are known to fail with age, so it may well just be dead.

The problem then is that no one seems to make suitable direct replacements, which means you are going to have to re-construct the driver circuit in some way to cope with a more modern style of low or moderate impedance tank.

You could install a transformer to convert the drive impedances seen by the existing driver and receiver circuitry, but I think it would actually be better to reconfigure the existing valve stages along the lines of a typical Fender amp or similar. There are plenty of suggestions kicking around on the web such as this one:

https://mytubeaudio.com/2010/01/15/echo ... b-project/

And this one: https://groupdiy.com/index.php?topic=65960.0

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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Feb 18, 2019 5:41 pm

I think Hugh is right, the tank is probably tanked!

I'll have a look at those links but if you are not "snobby" about Silicon you could just hang a MOSFET source follower on that drive triode and use a 1k or so reverb unit.

https://www.google.co.uk/search?q=IRF83 ... vAKCQX09ZM:

I dare say I have that MOSFET somewhere that I could jiffy to you.

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Re: valves in a Farfisa pre amp

Postby Folderol » Mon Feb 18, 2019 5:44 pm

I was rather curious about the small size of the couple capacitor, but a piezo device certainly explains it.
Personally, I'd be inclined to replace the tank with the highest impedance one you can get hold of, and use a transformer in place of the anode load resistor, then swap that valve with an ECC82. It's a fair bit of work though :(
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Re: valves in a Farfisa pre amp

Postby Hugh Robjohns » Mon Feb 18, 2019 5:57 pm

As well as the piezo' high-impedance, that small cap may also be due to wanting to remove most of the low end from the reverb drive -- this is an organ, after all, potentially with a lot of low end, and that's quite a normal arrangement!

But it seems the consensus is to replace the tank and rebuild the drive (and possibly recovery) circuitry to deal with the revised impedances -- either by re-wiring the existing valves, or by using a coupling transformer, or a solid-state driver...

Personally, I favour the second option as sticking most closely to the original design, but the first has a certain attraction and may be physically more practical (although more design and rebuild work) easier than trying to find somewhere for an extra transformer...

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Re: valves in a Farfisa pre amp

Postby ef37a » Mon Feb 18, 2019 9:07 pm

Folderol wrote:I was rather curious about the small size of the couple capacitor, but a piezo device certainly explains it.
Personally, I'd be inclined to replace the tank with the highest impedance one you can get hold of, and use a transformer in place of the anode load resistor, then swap that valve with an ECC82. It's a fair bit of work though :(

Rather more than you think Will, look at that g'awful HT supply? 1/2 wave rectifier and 22k smoothing resistor and not huge caps. Fine for the 4 mA or so those 83s will draw but an 82 as a mini PA will pull some 5mA per section. I think he would have to beef up the supply to a bridge and fit 50+50mfd of smoothing and I am not sure the mains transformer is up to that?

In fact a MOSFET buffer would pull 3-5 mills so it might be as well to load up that HT supply and see what it can cope with?

You can drive a tank with the right class B chip LM386 or there is one for headphone amps I don't have the number of at the moment but power is still a problem.

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Re: valves in a Farfisa pre amp

Postby Folderol » Mon Feb 18, 2019 9:50 pm

ef37a wrote:
Folderol wrote:I was rather curious about the small size of the couple capacitor, but a piezo device certainly explains it.
Personally, I'd be inclined to replace the tank with the highest impedance one you can get hold of, and use a transformer in place of the anode load resistor, then swap that valve with an ECC82. It's a fair bit of work though :(

Rather more than you think Will, look at that g'awful HT supply? 1/2 wave rectifier and 22k smoothing resistor and not huge caps.
Dave.

I did notice! Actually even if you did nothing else but change to a bridge rectifier, you'd probably hear the the mains TX give a sigh of relief. Also, on cogitating a little it occurs to me that with the existing valve, running it as a cathode follower might work. You can get tanks with a drive coil impedance in the 1k to 2k range. Leave the resistor in place, take off the 100n decoupler, and couple to the coil with about 1u.
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Re: valves in a Farfisa pre amp

Postby blinddrew » Tue Feb 19, 2019 12:47 am

It's like wandering into your own episode of Star Trek round here sometimes! :)
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