Fender Prosonic Amp gone "funny"

[gsl]

Hi everyone!

I have a problem on my bench (floor actually) and I don't have a clue where to start looking.

This Prosonic (here's a schem: http://www.blueguitar.org/new/schem/fender/prosonic.zip ) acts weired. I have narrowed down the behaviour to the following:

On the normal channel with all pots turned down to 0 and no signal the amp is quiet. But when I turn up any of the tone pots (with all others still at 0) it starts growling and crackling proportional in volme to how far up the pot is and in tone depending on if it's the treble, mid or bass pot. The drive channel doesn't do this athough the tone stack is shared???

I have made the visual and chopstick tests without results and the voltages look reasonable.

Any suggestions? Please?


As an afterthought, although I doubt this has anything to do with this: The power tubes sockets are not evenly tight. One is nice and tight as can be expected, but in the other the tube can be "rocked" with about 5mm deflection at the top of the tube. The tube rocks together with the connectors though which are sitting tightly on the pins. And otherwise it seems all right. Should this socket be replaced?

Thanks, Michael.

[Amp Kat]

I couldn't open that that link up but found it on the Mr. GH CD and I am assuming you have allready changed the preamp tubes. Being that the send out actually is a post MV preamp out or after the master volume rules out the master volume pot since only one channel does it so it's back in that section from the first tube to the tone stack maybe the volume pot would be my guess or one of the 12ax7's.

[Don Symes]

Blueguitar.org seems to be down. Anybody heard from Mr. Ahola lately?

The FFG has it, too.

The tone stack is shared, so the problem is a ways back up the chain. I'd take a close look at the relay (gummed up contact on the Normal side), and then the Normal channel volume pot and its coupling cap. This is a PCB amp (?), so be on the lookout for iffy solder joints and broken PC tracks, too.

Hope this helps!

[gsl]

The tone stack is shared, so the problem is a ways back up the chain. I'd take a close look at the relay (gummed up contact on the Normal side), and then the Normal channel volume pot and its coupling cap. This is a PCB amp (?), so be on the lookout for iffy solder joints and broken PC tracks, too.

Thanks Don! There are 2 difficulties I encounter here:
1) How to go about "checking" the relay?
2) I'm inexperienced with PCBs for I usually tinker with P2P equipment. How do I get to the underside of the PCB without taking the whole bloody amp apart, pots, jacks switches and the works? Is there a technique, or are PCBs the nightmare they seem to be (to me)?

BTW: It is not quite correct, that the growling and crackling is in the normal channel only. It's also in the drive channel, but there the master volume needs to be up for it to be audible. In the normal channel it's there regardless of the the volume pot and only the tone pots up. But it's definitely gone in both channels when the tone pots are all zeroed.

Another BTW: Here is an audio sample of the growling/crackling the amp does with tone stack full open and all else zeroed http://lendian.bauchladen.at/VOC001.mp3

[Satamax]

Have you checked C11, C12 and C13?

[Amp Kat]

BTW: It is not quite correct, that the growling and crackling is in the normal channel only. It's also in the drive channel, but there the master volume needs to be up for it to be audible. In the normal channel it's there regardless of the the volume pot and only the tone pots up. But it's definitely gone in both channels when the tone pots are all zeroed.

We were under the impression that the growling was not present in the drive channel also and that changes everything because as I mentioned before in my post that the master volume is NOT a post peak inverter MV but more of a signal attenuator in the front end after the preamp tone stack. Neither channel will be heard unless you have the master volume cranked and the tone pots will also attentuate the signal if zeroed which explains why you don't hear it then either so in actuallity the growling is in both channels. You can check the relay by measuring at R-14 330k for AC movement when you switch channels and at the wiper of the 500k wiper pot. The other relay can be checked at R-22 1 meg for right after the master volume. I would now have to believe that your problem is either bad power tubes or the footswitch jack. Try the send out to another amp and isolate the problem from preamp to poweramp and also patch a cord to send to return to make sure the loop is making good contact. The footswitch jack is tied to the peak inverter and could be causing the problem. As for PCB amps you are correct that taking the board off is sometimes the only way to go and yes that's why they are more difficult to work on. Your lucky ! Others are waaaaay worse than this one.

[Don Symes]

Thanks Don! There are 2 difficulties I encounter here:
1) How to go about "checking" the relay?
2) I'm inexperienced with PCBs for I usually tinker with P2P equipment. How do I get to the underside of the PCB without taking the whole bloody amp apart, pots, jacks switches and the works? Is there a technique, or are PCBs the nightmare they seem to be (to me)?

PCBs are the nightmare they seem to be - great for mass production, hell for service access.

"Check" the relay by measuring the DC resistance of the contacts in each position - there are 2 positions NC-C (normally closed to common -- relay 'relaxed') and NO-C (normally open to common -- relay energized). You want those two readings to be low, stable and pretty much equal.

That the tone and volume controls control the symptom says they're either _the_ problem or they're after where the problem starts.

Since the problem is present in both channels, you now need to look at the input jacks, the first stage or the tone stack drive stage V2a.

If you zero the GAIN1 control and the problem disappears from the OD channel, it's probably the input or first stage. Otherwise, you need to concentrate on the section around V2a - bad socket, bad traces, bad solder, etc.

My suggestion is that you start with the input jacks - are the ground connections securely soldered and do they may solid contact with the plug? Bad grounds can do some weird stuff.

Hope this helps!

[gsl]

We were under the impression that the growling was not present in the drive channel also and that changes everything because as I mentioned before in my post that the master volume is NOT a post peak inverter MV but more of a signal attenuator in the front end after the preamp tone stack. Neither channel will be heard unless you have the master volume cranked and the tone pots will also attentuate the signal if zeroed which explains why you don't hear it then either so in actuallity the growling is in both channels.

Yep, I know Amp Kat. I was under the same impression. Sorry to have misled you in the first post. This is because your second statement is not correct. The "Master Volume" is not a real Master in this amp but works in the drive channel only. The normal channel is governed only by the Volume Pot and is completely independent from the "Master" (like in the Hotrod De Ville if I remember correctly). And yes, the fender/vox/marshall tone stack attenuates the signal and shunts it to ground if all 3 pots are zeroed. But in the normal chanel the growl was present even if the Volume Pot was zeroed so the problem has to be in the last preamp stage and/or tone stack.

Thanks for the tips to check the relays. They will sure come handy in the future.

I would now have to believe that your problem is either bad power tubes or the footswitch jack

Speaking of which. The friend that gave me the amp to try and fix also thought that there was a problem with a power tube (socket). He said that the amp was "frying" the power tubes. Maybe that was a coincidence because the tubes glow nicely to me apart from a faint blue-violet glow, noticable only in the dark. Is this normal? Should I check the bias? The voltage looks ok to me at 34V for AB2 operation. What do you think? Should I take it apart again and shunt the middle OT tap with an ammeter?

Have you checked C11, C12 and C13?

Hi Satamax!
Yeah, I was just about figuring out how many screws I had to open to get to c11-c13 when I read your post. Now I've replaced c11 with a new one and the noise is gone.
c11 seems to have leaked DC into the tone stack and caused that ugly growl.
I have played the amp for 1/2 an hour now and it seems quite OK to me.

Thanks everyone for all the support.

[Mark Black]

Noisy caps - gotta love 'em...

[Amp Kat]

Hey the main thing gsl is that you got the growl to stop so good job on that fellows. Blue glow is perfectly normal but deep purple is a sign they are ready to go. If you have to use shunt method to bias then do what you have to do but yes I would check it or at least monitor the bias voltage at idle for awhile and see if it changes when the tubes warm up. If it sounds good with no crossover distortion then I wouldn't worry about it to much. Yes you are correct that the MV only gets the drive channel. I think your good now

[gsl]

Thanks Amp Kat.

This community is an invaluable resource and amazes me every time ever since I discovered it in 1998 or so. Thank you all!

Since I read from your post ("If you have to use shunt method to bias then do what you have to do") that you don't quite approve of the shunt method (I thought it was the most reliable) I was content to check all the voltages again and make a calculation in class A operation. That yielded a bias of 47V with 470V plate voltage in class AB2 operation (which looks reasonable to me) and 33V across a 224Ohm cathode resistor in class A operation. That makes for 148mA of current divided by 2 makes it 74mA per tube at (439-33) = 406V plate voltage gives about 30W plate dissipation per 6L6-GC (a little less if you subtract the g2 current) which is about the limit of the tube's specs. Too hot for my taste but it's within specs and it's the factory settings and the amp is not mine so I left it there.

Thanks again for all the help.
Michael Einem

[Mark Black]

Hi Michael - (response to your PM)

I actually edited the post twice: the first time I realized the plate resistor I was blaming couldn't have been the culprit and that the following stage one was more likely, and after that I realized you had solved the problem. So I was feeling kind of embarrassed & stupid. What can I say - I was sneaking this stuff in at work...

Speaking of everything happening so fast - I'm surprised you had time to read (and copy) my initial post. I must have changed it (twice) within less than 5 minutes!

Anyway I'm glad you found the real problem.

BTW - posting the audio clip was great! Now if we only had a way to post smells... I find myself using all my senses to diagnose problems.

Best of luck!

[Amp Kat]

You know it's a lot harder to troubleshoot these things on a forum when you dont' have the amp right there in front of you. In this case that coupling cap is right off the plate and can take some pretty high dc especially if it gets an unwanted surge. I've seen output tubes give the same symtoms and connections. GSL I'm not against shunt method it's just dangerous when you can get a bias rite or even use a current sense resistor which on pcb amps is difficult. I do use test clips but have even forgot my discharge rig once and fired it up by mistake or try to make a high voltage check still in amperage mode which gets a nice pop. I really didn't even notice the fixed biased/Cathode bias switch until I went back and looked the schemo over so yeah you have a nice option there with that cathode resistor at idle so I can relate to Marks post a bit but it's all good.

[caesparza]

Prosonic noise on clean channel after half an hour.

Have you checked C11, C12 and C13?

Hi guys, this is an old thread but I have a Prosonic with the following behavior, maybe you can give me a hand . I turn the amplifier on and everything goes ok with it in both channels, great sound. After around 20 minutes - half an hour the clean channel stars to add some horrible noise, like a plane taking off. Is just in the clean channe, the gain channel works fine all the time.

The C11 cap is mentioned here, could it be the reason for that noise? Is not the same problem that gls is having.

Amp Kat said "We were under the impression that the growling was not present in the drive channel" but I can't find any other information related (anywhere), but that is the problem that I'm having with the amp right now.

Any help would be appreciated. I'll make some measurements for BIAS and different spots on the amp, to see if I can find something abnormal.

All the best.
Carlos.

[Tone Meister]

Carlos, you should start a new thread for your problem. Some of the best techs, engineers, and designers in the world provide help on this forum.