My pv10 has had a "moment"

[goonybird]

Two weeks and my PV10A has failed already.
Bear in mind it was recently serviced by a high end Tube service company in Leicester UK. before I took ownership
So turn it on yesterday and faced with Hum and Motorboating.
This caused one of the Permier 12 Kt88's to Red Plate. very quick turning off saved that and has been fine since.
So checking valves the line stage are unknown east german Ecc82s
When turning on one arcs brightly.
So plugged in a pair of ECC83's I had around and results in bad hum and low audio.
So diagnosed power supply HT+ line stage as both channels.
Both the Q2 and Q3 are short circuit. all the other diodes resistors "cold" check ok. as do all other Power supply components.
Problem is I can get the MJE 340, but can't find a MPSU10.
Internet suggests replace with another MJE340.
Any help again gratefully received

[Big Dog RJ]

Mmm... sounds like you've managed to pick out the problem. Since this is a fairly old unit, getting the original parts is going to be an issue, there's no doubt. If they (CJ or any other party advising) suggest to use the required alternative component then I guess you've got no choice, unless that particular part was available.

Just to be sure, check with CJ as well if they're willing to respond, it may be useful info. Or perhaps Bill Thalman.

Hope it goes well.
Best, RJ

[admin]

Agree, if it's ok to substitute the components then just go with that. You can pull the datasheets for both and compare:

https://www.onsemi.com/pdf/datasheet/mje340-d.pdf

https://datasheetspdf.com/datasheet/MPSU10.html

[goonybird]

Ok Sorted a case of RTFM -me being dumb
Actually read the transistor. Its an MPSA42 - ordered
BTW its is old but there isn't much that can't be replaced in a Tube amp. Have restored old Leaks Rogers and Radfords.

[goonybird]

Happy to report all working now the two transistors replaced.
Went on after testing to fit, as per PV10b, fast switching diodes RHR460, and bypassed larger capacitors with 0.15 polys. also replaced the 3 electrolytics but notes it still takes ages for relays to click on. will have to look at whether the NE555 can be speeded up?

[goonybird]

So another issue arisen.
When on phono with MM or MC (via SUT) I only need to turn volume control less than a quarter turn for good sound levels. Seems to me the phono section gain is excessive with the 2x 12Ax7, would the RIAA equalisation be affected if I change to 12AU7 or even 12AT7 for less gain? the following stage is a 12AT7 so cant do anything there

[AnotherJohnson]

The official position, with which I would agree, is that using tubes other than what’s called out, will lead to a less correct result.

You can try it and see what you think. It’s yours to play with.

What cartridges have you tried? Are your speakers particularly efficient? Is your listening position near the speakers? Is the room small and “hard?”

Most people are happy when they have sufficient gain to run at 9 O’Clock.

Obviously, since the unit has to work in a variety of installations, each one will require a different volume setting.

Try the MC w/o the SUT. Maybe you’ve got enough gain for that to run at 12 O’Clock or so.

[admin]

Changing the tubes could lower the gain. I am not intimately familiar with the circuit.

You may want to read this article: https://producerhive.com/guitar-tech/12 ... r%20sound.

In summary:

12AX7 VS 12AT7 VS 12AU7: DIFFERENCES EXPLAINED
The biggest difference between these power tubes is the amount of gain they will provide. The 12AX7 has a gain factor of 100, while the 12AT7 has 60. The 12AU7 has the smallest, at just 20.

I suppose the alternative would be to put a resistor in to lower the output level.

[goonybird]

Popped in a couple of 50year old+ well used 12AU7.
Brilliant - Volume now 10-12 oclock and the tube rush has disappeared = near silence
Sound balance seems ok, so as long as it doesn't upset the RIAA then its seems a good move.
Other inpust levels ar fine as go through just the line stage.

[AnotherJohnson]

That’s great (if you like the new sound). It does not have to be accurate ... it just has to please the listener.

[goonybird]

OK so fitted new EH 7025s and fine and quiet.
But tried CD input with CD player and volume gradation is still poor. Fairly loud is 9 o'clock.
So looking at my options.
One in Rothwell Attenuators between PV10A pre and Premier 12 power amps
or fit a alps pot of 50k in place of the 100k which should allow a smoother gradation.

[admin]

I think both would work. The real question is which one will produce higher quality sound. If the cost is not prohibitive, maybe try both and compare.

[goonybird]

Put in a spare 50K Log pot and twas a dismal failure, My thinking is it would give better gradation, but in fact its the same.
Made up an l-pad 10db attenuator according to vaules of the CJ pre and power and didn't give enough reduction.
So will play more at the weekend, having found out the values in the Rothwell Audio 10db attenuator
The PV10A is rated at 20V output which does seem a bit excessive.

[roberto]

Hola,

Also, you can try a pi attenuator. The truth is that I did use this attenuator long time ago, when I was an electronic student, and I remember it worked...

[goonybird]

Made up a proper pair of L pad 10db attenuators using values of the Rothwell attenuators. All good now.
Normal Listening levels now about 10-11 o'clock and loud is 1oclock.

[roberto]

Hola Goonybird,

Great to know...well done, good job!

Happy listening!