Citronic PPX900 bias problem. Please help!

[vinyvamos]

Hi,
I recently bought a Citronic PPX900 power amp and all seemed fine when I tested it at the guy's place. However, when I got it home and ran it for a while with the lid off I noticed some strange behaviour. Regardless of signal going in or loads connected both heatsinks (top & bottom) seem to heat up fairly quickly. Within two minutes the fan kicks in which means the heatsinks are at approx. 60 degrees C. I then checked all the usual faults like DC on the outputs or bad supply rails etc and found nothing a miss. I then thought maybe the bias might be set way too high but the bias pots still have the red factory seal on them COMPLETELY untouched.

With a signal and speakers connected the amp seems to run ok with no distortion but I haven't had a chance to run it at full tilt yet (I share a house) to see if the shut-off thermal switches trip out. It still gets just as hot though. A thing that really surprised me with this amp when I first connected speakers to it though was the extreme lack of background noise from the speakers when idle. I see that there is a muting circuit in the design but I didn't think it could be this effective! Maybe this could explain some of my biasing problem???

After getting the schematic (from one of Jan Dupont's posts. Thanks Jan!) I was hoping to see some info on biasing guidelines but there was none. I also got the user manual but as you'd expect there's no info in that either on biasing.

The first question would be DO these amps generally run fairly hot when idle???

The second question would be HOW do you set the bias???

I really don't wanna rely on this amp for a gig and then see it blow up while I'm playing!



If any of you guys have ANY info that you think would be of help it would be REALLY appreciated! I'd bring it straight back to the guy and get my 120 euro back but he's the other side of the country and it is technically working fine in his eyes.

Thanks in advance!!

Vinny

[megajocke]

You will need to lift one of the drain fuses and insert an ammeter there to measure bias. Bias is adjusted with the pot marked bias in the schematic. Remember the power supply caps, they might need to be discharged manually - with a resistor, a screwdriver isn't a very good discharging tool.

What kind of heatsink design is this amp? Is it a tunnel cooled amp or are the heatsinks on the outside?

Does the fan have a high and a low speed? Is it a DC or AC fan? If it is tunnel cooled and the fan is totally off when cold then the idle dissipation will heat the sinks up pretty hot. If the sinks are on the outside or if the fan has a low speed then it probably shouldn't get to the 60 degrees for fan full speed.

[vinyvamos]

Thanks for your reply. This amp is tunnel-cooled. The fan is 110VAC and is switched on by two paralleled 60 degree C thermal switches. My main reason for concern is that this amp is fairly battered and I don't know it's history. The bias pots have never been re-adjusted though so they should be set correctly. As you said though the idle dissipation will be quite high so maybe I have nothing to worry about.

If I do as you said by replacing a drain fuse with an ammeter what current should I be adjusting the bias to? This is a 450 W/Channel (into 4 ohms) MOSFET amp with six MOSFETs per channel. Each MOSFET has it's own drain fuse. The supply rails are +-80V.

[megajocke]

You could try comparing the bias of the two channels if you want. If all is well it should be similar for left and right. Those mosfets used are often biased pretty hot so you might be ok. If it is biased at 100mA/fet for example you will have 100W of idle dissipation - this will heat the sink up and require the fan to run. Luckily idle dissipation doesn't add to the dissipation when playing - as signal level is increased the time during which current flows through the output stage without going to the speaker decreases.

If TH1, ZD3 or VR2 are open or have a bad connection the amp could get very high bias though so it would probably be a good idea to measure it.

Is the fan run from the secondary of the transformer then? You could add a suitable power resistor in parallell with the fan switches and get two speed

[vinyvamos]

Right that clears a lot of things up thanks! I'll check them three components and the bias and if they're fine I'll leave it as is. Thanks for your help!

[John Audio]

The PPX range of amplifiers are very well designed and built - and possibly one of the most stable MOSFET power amps ever made. (That means that many other amplifiers are not!).

There are separate fuses for each of the MOSFET's - so in the event of power device failure the individual fuse blows - and the amplifier carries on working. I have had amplifiers working on just one pair of output MOSFET's (with reduced power of course!). This won't affect temperatures though.

These amplifiers can get very dusty, which obviously prevents effective cooling - and require some dismantling to gain access to clean properly.

To check the bias, just measure the DC voltage on the gates of the output devices (each device is fed via a resistor).

What do you get?

[megajocke]

Do these mosfets have good enough threshold and transconductance spread in your experience to make that accurate enough for setting bias?

OTOH, if there is a circuit problem affecting bias it will probably be more than just a little bit wrong. These mosfets are very robust so they will almost certainly survive an open bias pot for example with that protection circuit used. The power dissipation if that were to happen will be very high though...


John, do you know anything about newer "Citronic" amps like this one? Is it still good stuff or is it just a trade name used by someone else nowadays? That toroid isn't small though
http://www.billebro.se/show_prod.php?prod=2187

[microsim444]

That amplifier sound great, specially they are using the 2SJ, 2SK mosfets.


I love it

It you need the service manual , I have it

[pengpeng]

microsim444.
I need the ppx900 sercive manual.

One channel of my ppx900 output afloat DC voltage. if I can have sercive manual, it will be great helpful for me.

Thank a lot.
My email: py@chainzone.com

[cazmac1]

Originally Posted by microsim444

That amplifier sound great, specially they are using the 2SJ, 2SK mosfets.


I love it

It you need the service manual, I have it

Hi Microsim, I have a PPX 900 that I've owned for around 20 years trouble free service. I loaned it out to a friend and it has come back with four of the drain fuses blown. I’m sure that all I need do is replace these fuses, but just in case could you send me the service manual. If you can't do that could you or another member be so kind as to tell me what kind of replacement fuses I should use as I'm having trouble getting hold of genuine replacements