Repair suggestions for PV12 please

The PV-1 to now...
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pedalhead
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Repair suggestions for PV12 please

Post by pedalhead »

Hi all. I've just borrowed a PV12AL from a friend & it's intermittently going silent (more silent than working actually). I've rules out tubes and everything else in the chain & that's as far as I can go from a troubleshooting point of view.

Can anyone recommend where I could get this unit repaired in the UK please? I'm located in Oxfordshire/Berkshire, but could post it if I have to.

Thanks!
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Re: Repair suggestions for PV12 please

Post by admin »

Also owner of a PV12. Can the silence vs sound be broken via the volume control manipulation? It might be a bad connection. I would open up the unit and with a non-conductive instrument (like a pencil), push on all the components and the different areas of the circuit board to see if physically stressing the components has any effect. Obviously this would need to be done with the unit on and playing music. Be careful as there are high voltages and there is risk of electrocution if not done properly. It could be as simple as a bad solder joint. If nothing works, I would recommend pulling the schematic and testing each component individually (or at least all the components that are in line with both channels as I presume this effects both channels). Let us know how it turns out.
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Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
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Re: Repair suggestions for PV12 please

Post by pedalhead »

Thanks very much for the advice. I'm not very clever on such things, but an amp builder mate of mine is going to take a look at it. Cheers.
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Re: Repair suggestions for PV12 please

Post by pedalhead »

By way of an update, I took the PV12 to an electronics geek friend of mine today and he identified the power switch at the problem. We replaced it and this has cured the problem I mentioned in my original post on this thread.

I'm curious about another issue with the preamp, namely that it seems to have an awful lot of gain. I'm using it with a CJ Premier 11A power amp into Audio Physic Tempo 25 speakers (pretty middle of the road sensitivity-wise). I can only move the volume knob a short distance before it's starting to get pretty loud....9 o'clock is loud...much beyond that and I fear for the speakers. I wonder if this is normal for this preamp? My sources are a Rega Aria phono amp and a DiDiT DAC212SE dac. Cheers.
Last edited by pedalhead on Sun Apr 23, 2017 11:07 am, edited 3 times in total.
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Re: Repair suggestions for PV12 please

Post by ronenash »

CJ preamps do use any negative feedback and thus have fairly high gain, typically 20 or above db. Make sure you are using the right tubes in the PV12. High volume at 9 o'clock position seems a bit extreme.
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Re: Repair suggestions for PV12 please

Post by admin »

I have the PV12 and agree, that seems extreme. I usually play between the 11 and 1 o'clock positions depending on input and desired listening volume level.
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Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
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Re: Repair suggestions for PV12 please

Post by jeffreybehr »

On too-much preamp gain

GENERALLY...where one runs his preamp-volume control depends on many things:
1. How much gain the preamp has;
2. The taper of the gain control;
3. How loud you want the music to be;
4. How much Voltage gain your poweramp(s) has, and
5. How sensitive your speakers are.

One can't do much with items 1 and 2 except replace the preamp. See below for possible tune-based solution for this.

Item 4 is one that I think most of us have wrong, in that amp manufacturers build most poweramps with (IMO) too much Voltage gain. Ten deciBels would be plenty; some have as much as 30. For every dB of gain you don't need, one must turn the preamp down a dB. Complicating the issue is that many of us think there's something wrong with a system that requires the preamp's volume control to be operated beyond, say, 'noon'. This is not true, in that, generally, preamps don't have too much distortion until they're clipping.

In fact, having 'too much' gain in the backend (= poweramps and speakers) amplifies the fixed noise present in 'all' preamps. If one has a noisier-than-average preamp and speakers with too-much sensitivity, your system will indeed have too much noise. One solution is to turn the gain contols on your poweramp(s) down to about half or less and turn the preamp volume up. What? Your poweramp doesn't have a gain control? Too bad. My poweramps, Atma-Sphere M60s, don't either, but A-S sells a tube-replacement plug that reduces the amp's gain about 10dB. I use one in each of mine.
Image

If your preamp uses a member of the 12An7 family, one could try substituting a lower-gain version. For instance, people often use a 5751 in a position spec'd for a 12AX7 (the highest-gain member of the family, AFAIK), and there are 'U', 'T', and 'V' members, too, all of which have lower gain. AS FAR AS I KNOW, and I'm not thoroughly knowledgeable in this area, these tubes are, safety-wise, interchangeable, but that doesn't mean the rest of the circuit is designed for optimum sound quality. See http://www.thetubestore.com/Resources/G ... ain-Factor
for more info. If I had your situation, I'd look for an inexpensive 'Y' or 'V' version to experiment with IF IF IF the original tube is a 12AX7 or a 5751.

Good luck; please let us know if you try replacing a tube.
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Re: Repair suggestions for PV12 please

Post by jeffreybehr »

admin wrote: Mon Apr 03, 2017 6:30 pm ...
It might be a bad connection. I would open up the unit and with a non-conductive (sic) instrument (like a pencil) ...
YIKES!!!!!!!!!!!!!! Pencil lead is CONDUCTIVE. DO NOT DO THIS!
Last edited by jeffreybehr on Tue Apr 25, 2017 8:53 pm, edited 1 time in total.
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Re: Repair suggestions for PV12 please

Post by admin »

jeffreybehr wrote: Sat Apr 22, 2017 6:22 pm
admin wrote: Mon Apr 03, 2017 6:30 pm ...
It might be a bad connection. I would open up the unit and with a non-conductive (sic) instrument (like a pencil) ...
YIKES!!!!!!!!!!!!!! Pencil lead is CONDUCTIVE. DO NO DO THIS!
You are correct about lead being conductive,... I should have specified how I have done this in the past. The eraser is non-conductive as is the wood housing. It is also soft and springy so it should not physically damage any components. I have used an unsharpened (so the lead is not exposed) pencil with an eraser cap but I only use the eraser for actually touching the components. Also, if the eraser is held with a metal piece, I would not use it either as that could make a contact between components (or put the eraser cap on the other side so there is only wood and eraser being put inside the equipment). Eraser caps are nice, and you can put them on plastic pens as well.

On the rare occasions I worked on any electrical equipment with the power on, I always wear a non-conductive glove (made of rubber). You can buy these for under $20 (just search for "electrician gloves").

Safety first, and if you don't feel comfortable working with live electricity then leave it to a professional.
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Main stereo: ART Amplifier and ET7s2. 2nd stereo: PV-14L and MV-55. Previously Owned: PF2 preamp, Evolution 2000 Amp, PV-12AL preamp, D/A-2b Vacuum-Tube Digital Processor.
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Re: Repair suggestions for PV12 please

Post by pedalhead »

Thanks for the responses. My power amp is a CJ Premier 11A so I don't think that should be the cause of the issues. Speakers are 4 ohm 89dB sensitivity. The PV12 is using 2 x ECC82 tubes, a lower gain version of the ecc83. They look like stock tubes actually. They do need replacing as I'm hearing background noise when music isn't playing, but I don't believe tired tubes would cause this high gain.

I keep coming back to the pot as the possible culprit. I've known faulty pots to mess with gain in the past, most notably a headphone amp that wanted to blast my inefficient planars into little pieces with a few mm of volume travel.
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