Valve amplifier service

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Greg
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#31 Re: Valve amplifier service

Post by Greg »

My experience with PIO’s in several amps is they are likely to go leaky after several years use. Considering the age of the amp, I would replace them as a matter of course. Interestingly, when I have serviced amps for other people, replacing the PIO coupling caps has often been the fix where hum and high frequency distortion has been the problem. Going back to the original orange drop polyester caps would be cheap and they sound pretty good.
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#32 Re: Valve amplifier service

Post by simon »

Thanks both. I had similar thoughts about replacing caps with cheap to test/prove. With any luck I might even have appropriate in stock. The amp's a pig to work on though - the big AN PIOs cover a fair bit of the tag strip. I might be able to remove them without snipping their leads - I'd be less concerned about doing this if I knew for sure the caps were knackered.

I did notice that when I put the meter probe on pin 7/grid of V2B on one channel the hum stopped.

It sure is hot inside that chassis though so I tend to agree with you Greg that the PIOs, and even the lytics might be heat and time damaged. Whilst there looks to have been a lot of upgrades at some point in time (over 15 years ago judging by the Jensen style of the AN caps) the LCR 50uF+50uF reservoir cap has what looks like a date stamp of 1993 so some of the lytics could be 30 years old possibly.

Let's see what Dave wants to do next. I fancy a recap with AN silver PIO at over £4k... :-D
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#33 Re: Valve amplifier service

Post by pre65 »

When Nick had my Audionote (copy) pre-amp in to repair the culprit was an Audionote PIO capacitor.

From memory it was replaced by a mid range Mundof.
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#34 Re: Valve amplifier service

Post by bostod »

Looking to replace the AN capacitors for a reasonable price.
Looking up on Google is a minefield. No one agrees and it appears the amp design matters alot.
I like the look of the Arizona Blue pio. Mylar content which some suggest will ladt longer.
Would this be a good (albeit much cheaper) alternative or is it suck it and see as most online suggest.
Failing that I like the mid priced Claritycaps or Mundorf.
Am I in the right ballpark.
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Nick
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#35 Re: Valve amplifier service

Post by Nick »

I would find out for certain they need replacing. When I have seen them (AN PIO) causing a problem in the past is when the following resistor is 1M or so, as in Phil's preamp.
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#36 Re: Valve amplifier service

Post by simon »

I agree with Nick, best to find out where the problem(s) is first to be sure. It might pay to recap anyway, but better to make informed decisions.
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#37 Re: Valve amplifier service

Post by simon »

What's the likelihood that a resistor is dodgy? That would be a bit of a job removing them one by one working through them.
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#38 Re: Valve amplifier service

Post by Nick »

Small, but in general there will be two in a series connection so you can compare voltage drop across the pair. Measuring voltages will normally show where the problem is if its a resistor.
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#39 Re: Valve amplifier service

Post by simon »

Do you mean by comparing channels rather than working out voltages around the network?
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#40 Re: Valve amplifier service

Post by Nick »

Both
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#41 Re: Valve amplifier service

Post by Mike H »

Just a thought - if possible I would disconnect C4, 5, & 6, then test all the Voltage points again. If Voltages become more sensible..... :?:
 
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#42 Re: Valve amplifier service

Post by simon »

Thanks Mike. Just to be sure I understand I think you mean remove the connection rather than bypass each with a wire?

I've had a bit of a tidy and found all my caps - looks like I've enough to start replacing the suspect ones. (Found a few Blackgates I'd forgotten all about.)
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#43 Re: Valve amplifier service

Post by simon »

I spent some time earlier trying replacement caps, starting with those easiest to access.

First out were the 6no. AN PIOs

Next were the 4no. PS decoupling caps.

Then the first stage Blackgate cathode bypass caps.

Then I unhooked the 4no. 5881 BG cathode bypass caps.

None of which made any difference to the hum on the cheap nasty speakers I use for testing.

That just left the LCR filter cap which I really was hoping not to have to attack. It's the cap I most suspected though. What a pig to remove. Anyway, I checked the capacitance with my cheap DMM and both sections measured around 0.1uF. The other lytics all measured jack too (except one 5881 bypass cap which measured 47uF rather than 100uF).

The AN PIOs measured okay.

I'll have to bodge a replacement for the LCR filter cap as I don't have an equivalent. I just hope that does solve the hum...
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#44 Re: Valve amplifier service

Post by simon »

I found a WKZ to replace the LCR but it didn't make any difference to the hum sadly. For reference there's now 100uF + 100uF in the initial filter rather than 50uF + 50uF, and C3 and C7 decouplers are now 47uF rather than 22uF. I feel that the HT PS can be discounted now? Here's the circuit for reference, though the diodes have been replaced with HR4120 hyperfast alternatives.
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20230629_182815.jpg
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.
There are also a pair of ceramic disk type bypass caps on the two halves of the initial cap, C12/13. I haven't investigated these but as the hum only affects one channel it seems reasonable to ignore them?

All the voltages are pretty similar to previous following today's re-cap, nothing jumped out at me.

But I noticed again that the hum goes when I put the meter probe on the grid of V2B (top of the 0.22uF cap). This didn't follow the valve when I switched channels. In fact, after switching valves both channels hummed more when the probe was applied.

I plugged the amp into my main speakers (just the 285GMF) and the hum behaviour reverted to disappearing when the probe touched V2Bg.

Odd. The amp now has new caps and new valves so there appears to be an issue somewhere else. Wish I understood how a LTP works, need to have a read up when I'm fresher. I perhaps need to follow the circuit with the scope to see what that might tell me (I love using a scope!).
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#45 Re: Valve amplifier service

Post by simon »

For completeness I remeasured the caps properly out of circuit and I take it back - I was obviously in a bit of a rush for lunch earlier and it took a long time for the meter to read but:

C12/13 LCR 50uF+50uF - 61uF and 59uF
C2 100uF BG - 99uF and 100u
C3 22uF lytic - 22.5uF L, 21.9uF
C7 22uF lytic - 22.2uF L, 21.8uF
C4 0.22uF PIO - 0.22uF and 0.23uF
C5 and C6 0.68uF PIO - 0.71uF, 0.72uF, 0.71uF, 0.74uF
C8 and C9 47uF BG - 44.2uF V3 L, 43.5uF V4 R, 43.8uF V3 R, 43.3uF V4 R

Which would explain why replacing the caps made little difference...
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