PV-12 help needed

[lutzn]

Hello,

I just inherited my first piece of CJ gear, a PV-12 line preamp. This pieces was used a lot up until around 10 years ago but has stood quiet since then.

When I hooked it up to my system I could tell that the balance was left of centre so I switched the tubes around and the balance was still left of centre. I opened up the top cover and could tell that some of the parts around the tubes look a bit burned out and some of the capacitors look a bit stained. I am not very familiar with tube gear so I can't diagnose the state of the piece from what I am seeing so I thought I would ask in this forum.

Can someone help me diagnose the state of the amp? I don't mind putting a bit of effort into making this preamp sound its best again. See attached photos.

Thank you!

JB

[admin]

First, welcome to CJO. Nice to have you with us.

I personally owned a PV12 for a good number of years. It's really a great preamp. I think you would really enjoy it if you can get it working well.

As for your unit specifically. It's hard to tell exactly what is going on. You can get single channel issues with bad tubes. It's low hanging fruit but if you have an extra set of tubes, it would be worthwhile to just swap the tubes for a new set to see if that fixes the issue. Beyond that, I would replace any component that overtly looks damaged. Any bulging or leaking caps. Any components with burn marks. BTW, it's hard to tell if the discoloration (first picture, just right of tubes) you have there is just color change due to heat changes over decades or they are bad. But if you are going to start replacing components, I would probably replace those.

Of course you can always send it in to CJ and I'm sure they would be able to fix it for a price.

[lutzn]

Thank you for the reply and your kind words.

I will order another pair of tubes just to rule that out. Do you have any idea what those discoloured blue components on either side of the tubes are called? I tried googling the visible numbers without any luck.

I live in Europe so sending to CJ for repair would be a bit expensive I think. I might look for someone who can help me locally if I fail myself.

JB

[admin]

Pretty sure those are resistors (the ones labeled NA65).

[goonybird]

Check the Tape output to see if imbalance is there, to isolate probability of the ALPS Volume control bad tracking through age? and isolate problem to input stage or output stage

[roberto]

Hola,

You can measure the value of the resistors top left vs the value of the ones that are odd burned look at the right. The value of those resistors are mirror and same for left channel vs right channel. If you have any value difference, they must be replaced. It looks too that the right channel is the lower volume signal. Using the tape output will tell of course that you have one channel with less volume, because it is a direct connection from the source. This signal is a pass through without entering into the preamp section. Only the selector input switches.

[lutzn]

Thank you for your good tips! I will do some testing today and rapport back.

[RJKflyer]

At the risk of stating the obvious, have you checked the Balance control? I recall it has limited range on the PV12 - not much more than 3dB but might be the source of an issue. Given it is a switch it may have contact issues if it’s been sitting about.

However, the very different condition of the cathode resistors either sides of the tubes suggests an issue there.

Looking at the schematic, I would be fairly surprised if a capacitor were at fault to give channel imbalance, plus the PSU feeds both channels so an issue there would seem unlikely to cause one channel to be off.

I wonder if it's been fed with a DC offset on one channel which has done the damage? Not that this matters now...

[lutzn]

At the risk of stating the obvious, have you checked the Balance control? I recall it has limited range on the PV12 - not much more than 3dB but might be the source of an issue. Given it is a switch it may have contact issues if it’s been sitting about.

However, the very different condition of the cathode resistors either sides of the tubes suggests an issue there.

Looking at the schematic, I would be fairly surprised if a capacitor were at fault to give channel imbalance, plus the PSU feeds both channels so an issue there would seem unlikely to cause one channel to be off.

I wonder if it's been fed with a DC offset on one channel which has done the damage? Not that this matters now...

The balance control is set to neutral position and there does not seem to be an issue there.

I cleaned the tube sockets and got a pair of matched tubes and I must say that I don't hear the imbalance anymore. Maybe the amp needed some running in to get going again. I also turned the knobs back and forth a lot and they feel smoother as well. I guess these devices are happiest when being played regularly.

I measured the cathode resistors and the left side reads 40.5 on all three and the right side reads 40.7 on all three with my multimeter set to 20k. What kind of damage could I expect if the right channel has indeed seen some DC offset?

If I do replace them, with what value should I replace the cathode resistors?

[admin]

All's well that ends well. Not sure if I would do anything further at this point unless another issue presented itself.

[RJKflyer]

I measured the cathode resistors and the left side reads 40.5 on all three and the right side reads 40.7 on all three with my multimeter set to 20k. What kind of damage could I expect if the right channel has indeed seen some DC offset?

If I do replace them, with what value should I replace the cathode resistors?

_____
EDIT: just read that you're getting "40.5" on a 20k setting? You should be getting 40.5kΩ and therefore not likely to be displayed on a 20kΩ setting?
_____

I'm not an expert, but here goes. Apologies if you know all of this!

A triode works by having -ve applied to the cathode and +ve to the anode (or 'plate'). The grid acts as the 'controller' - sitting between the -ve and the +ve. Thus with zero volts on the grid electrons flow unimpeded from cathode to anode.

The 'control' is effected by applying a (small) negative voltage to the grid - this tries to repel electrons from the cathode being emitted from the heated and -ve cathode.

Equally if a +ve voltage is inadvertently applied to the grid this ENCOURAGES flow and causes the tube to be turned hard on. If this is DC then it is 'permanently' on so to speak and thus drives the tube too hard. In output tubes this is often called 'red plating' as the 'plate' (anode) does indeed glow red hot. It usually happens when the grid capacitor leaks and thus allows DC through. A phenomenon well know to those of us who restore old radios with wax caps which tend to fail leaky (electrically that is, although sometimes physically too...).

So where does that DC come from? In this circuit, nowhere. However, if the driving equipment has a fault then it can supply DC to the PV12. Most if not all decent separates will have a "DC blocking" capacitor on the output, but if it fails... (The PV-12 has a 2µ on the output to the main map).

The cathode current, in the second stage of the PV12 circuit, will flow through the three parallel 121k cathode resistors. And thus scorch them if it's sufficient.

They are, FWIW, 121k RN65 Vishay resistors. Try searching DigiKey or Mouser for something like RN65D1213FB14, although I cannot confirm that these are exactly the correct ratings.

TO STATE THE OBVIOUS: this unit has high voltages present - so undertake any checks at your own risk.

To answer: "should I change them?": one channel are definitely past their use by - I personally would as they have been subject to excess dissipation. I would do both channels ideally.

With the UNIT OFF, and DISCONNECTED, and given some while to cool down/discharge:

You might want to re-check the resistance: three 121k in parallel = 40.3kΩ - ideally pull the tube and then you can measure across them without anything untoward influencing the reading. It should be within about 1% of this. (Give your meter time to settle as the output blocking cap and 1MΩ to ground will cause a small initial misreading).

Report back perhaps?

[Big Dog RJ]

Now that's a proper analysis of what needs to be done.
See how you go and definitely report back.

Probe carefully... lethal voltages exist!
Cheers, RJ

[lutzn]

Thank you! That is a very thorough explanation and not at all stating the obvious to me. I love learning more about this preamp and tube amps in general.

Should I then also replace the dc blocking capacitors if that is where the leak originates. My only source so far has been a fairy new Cambridge streamer so unlikely that is faulty.

I take your safety warnings seriously and will make sure to be cautious if I start changing parts.

Is this the correct resistor?

https://nl.mouser.com/ProductDetail/Vis ... uk6A%3D%3D

[RJKflyer]

Apologies for the slow reply - I had forgotten to mark the thread for notification.

Yes, the Mouser link would appear to be correct.

The 'DC blocking capacitors' are not in your PV12AL - it's line only and thus any DC must be injected from upstream, and thus likely from the previous owner. (The phono version DOES have a 2µF DC blocking cap on the output of the phono section before it is routed to the line amp).

You could turn it all on, with your usual source connected and on, and examine the three resistors with an IR camera to see if one set is hotter than the other.

If not then it is not your equipment and confirms damage done previously to your ownership.

A less ideal alternative would be to use a thin but insulating glove, AND with your other hand firmly in your pocket and feel the resistors to see if the two triads are equally warm. Not sure I should be recommending that and you'd need to be very careful!

[admin]

A less ideal alternative would be to use a thin but insulating glove, AND with your other hand firmly in your pocket and feel the resistors to see if the two triads are equally warm. Not sure I should be recommending that and you'd need to be very careful!

I would be cautious with this one. You can burn your finger if very hot, a small rip in the glove and the electricity is now passing through you, and our sensation of heat is not very good unless one resistor is cool and the other very hot.

I like the IR camera option the best, but of course that is the most expensive. You can also try a regular cooking temperature probe. Be careful as the tip is metal so conductive but you can hold the body which is plastic. Just make sure you don't short anything. These are under $10 with free shipping from Amazon.

You can also try an IR gun. The only issue is that they are not very good at small target areas. They are also pretty cheap with an ok one about $20 and some very cheap models under $10.

[lutzn]

Hello hello,

Thank you for your good advice! It took me a while, but I finally bought a laser thermometer so I could measure the resistors. I let the preamp play for around 4 hours connected to my Cambridge CXN network streamer and my amplifier before I took the measurements. The right side with the "burned" resistors measures 58°c (136°f) and the left side measures 51°c (123°f), so there is indeed a imbalance. The resistors around the tube sockets already looked burned when I received the amplifier so I don't think it is my source that is causing it.

If we can rule out the source being the problem, do you think the problem is with the resistors or somewhere else along the chain? Maybe next step is to replace the resistors and measure the temperature again.

Thanks!

JB

[admin]

If you think the resistors are not functioning properly, really the only way is to pull them and measure their resistance. My general rule is that if I see any visual abnormality (whether be it resistor, cap, etc...) such as burn marks, deformation, discoloration, I tend to replace them. These kids of abnormalities signal that the component is either faulty or has been exposed to some significant heat source that may have caused damage.

As for diagnosing the ultimate problem, each potentially faulty components needs to be ruled out one at a time.

[RJKflyer]

Subjectively: those resistors are life-expired. Replace.

Objectively: you've done well measuring the temps. Assuming correct, the burnt pair (which you've confirmed are approximately the same value to the lesser damaged ones) are some 7˚C hotter, which assuming an ambient of about 20˚C is about 20% more dissipation (38˚ vs. 31˚)

This needs investigating. It *might* be that their hot resistance falls due to the heat damage - try turning off and measuring the resistance hot of both triplets to see if the damaged set are lower when hot. If so, this explains why they're running hotter.

If the hot resistance is the same between triplets, then there must be more voltage present to drive more current. This can only be true if the voltages around the tubes vary between channels.

If you feel confident and competent to measure these, measure the voltages at plate/anode and cathode of EACH section in EACH channel. You'll need a DMM and voltages at the plates will be in the hundreds of volts DC range. Cathodes will be low single digits I would expect.

If you look at the schematic you will see the first SECTION of each triode has a plate resistor, and at the second does not - it goes direct to the HT. The direct to HT voltages must be the same, unless there's a very odd fault. The voltage at the first section plate may vary, but only slightly as the plate resistors are of same value each channel, obviously. Any discrepancy (beyond a very few %) indicates a faulty plate resistor.

Let us know how you get on.