Help with Studio Preamp issue

[rmarshall]

Hi - new here and wanted some assistance with a c.1990 Studio Preamp. It generally sounds excellent but I get a slightly strange grainy distortion from the lead channel with single notes (generally higher pitch ones). Instead of a very pure tone I get some other harmonics/interference in the sound. Maybe this is normal but it is a little irritating to me as I like a pure singing tone in my leads. Note that i have changed and checked every tube (except the reverb driver) and the PI tube on the power amp.

Any ideas? Do I Have a failing cap somewhere?

I run standard settings - bass around zero, treble around 10 and a V on the GEQ. Volume in 5, Lead Drive around 8 etc.

Help!!

Richard.

 

[OryCheyne]

Hi,
Not sure if this will help, but I have my .22 '87 in for what I was expecting to be cap replacement. The sound would be great for around 15 minutes, then start to pop and fade before basically having no sound at all. I've since been advised that there was nothing wrong with either the caps, or valves and that it was likely due to dirty pots.
I am hoping to pick it up today, will see how it goes and let you know. Makes some sense, but I'm curious how the sound starts off fine then fads, if it is dirt in the pots, this would mean that the dirt is breaking down after a period of time after having the signal through it and either creates a short or open circuit. I'm not sure if the 0 positions are high resistance (open), or low resistance (short), nor do I think I will find out which pot it may have been. Hopefully someone who has some working knowledge with these circuits may be able to advise how the pots work and whether your issue may be related. Also if anyone has any experience with which products work best for cleaning pots that would be appreciated.

Will let you know how my 22 goes.
All the best with your issue.

 

[rmarshall]

My sound is strong and doesn't fade or pop and pots aren't 'scratchy' - i just have a hard grainy interference type of noise in my high gain sound only noticable with single notes. In reality I can live with it as the overall sound is still awesome but I am a bit of a perfectionist and none of my other amps do this ... I was wondering whether the absence of valve sheilds might be the cause.

All ideas or similar experiences most welcome!

Richard.

 

[OryCheyne]

Yes you are right, it won't be your pots. Just got mine back pots are no longer dirty but the problem is still there. It sounds delicious for about 15 minutes then it fades to a super quiet signal. The sound is still controllable just no volume. I've replaced the power valves, still the same. It must have a dry joint or a component that goes open when the circuit gets hot. I am still inclined to think it's a cap issue. There are many different ways components can fail.

My 22 doesn't have valve shields and it doesn't appear to present the problem yours is having. Not sure if any 22 had shields.

When you say 'Interference', does it sound like an external signal is getting in, or does it sound like your signal is breaking down, or modulating on it's own? (I'm not sure if it is possible to differentiate, but could help describe the sound you are experiencing)
Also, does it present the same issue with different pickups selected/ on different strings, other guitars/ other leads etc. Are you plugging straight amp? Last thought, is the problem still present, even slightly, when you drive the clean channel?

Sorry I can't tell you straight up what the problem is but I hope these questions help narrow down the field of possibilities. It really could be anything, is most likely a preamp valve or out of tolerance component in a part of one of the preamp vale circuits.

Best luck.

 

[rmarshall]

The sound issue is intermittent and sounds like a extra modulation/interference/distortion in the signal creating strange extra harmonics that come and go rather than being continuous. Hard to explain but it is kinda like the pure tone is being distorted by something that clips in a harsh way - maybe a bit like solid state clipping. I will do some more trouble shooting using different outputs, the effects send as an output, playing into a different power amp etc. to see if I can pin it down. Tried it this morning and didn't get the 'interference' sound - just a clean pure saturated tone - was just seeing if this type of thing was a) normal for a StuPre and/or b) linked to a known issue such as failing caps or other components.

Richard.

 

[rmarshall]

So I think I just have 'older amp syndrome' and will need to go over every component and solder joint. Maybe the tube socket solder has cracked - don't like these PCB mounted sockets. Just spent some time with it now and no issues at all ... just sounded Godly! Clearly an intermittent fault rather than a 'known issue' as when it happens, the sound isn't good ... but when it sounds good, it sounds perfect!!

R.

 

[OryCheyne]

Likewise, I'm resigned to resoldering all the joints also. Had noticed something I thought was strange last time I had the guts out of the cab. On the solder joints where the reverb circuit joined the circuit board and terminals I noticed short lengths of wire left sticking up. These are a bit less than 1/4 inch. I'm not sure what wavelength that may relate to, but having experience with non- valve related circuits, I am pretty sure these can act like antenna. In a high gain circuit even low level interference could end up in the output.
Have a look at yours and let me know if it is the same.

 

[OryCheyne]

Ok I'm rather new to this forum too, I'd mostly been looking up MkV and IIC posts. Didn't realize how much great info is posted on the 22s. Only a few pages in on the Vintage amp section and I have learned a truck load. Some really useful detail dating back to 2008 which solved my problem.

After getting the .22 back from service yesterday and it cutting out after 15min, today I played until it cut out, then followed forum advice and plugged a short jack between Send and Receive sockets. All fixed thanks guys. Will clean it up and see if OK with jack out.

I couldn't find the actual author who supplied that detail but thanks all the same. (time to get financial with the forum given it just CV saved me a whole lotta effort).

I'm not sure about my previous attempt to assist with the grainy single notes issue. Given all the posts on the.22, I'm sure that your issue has already been discussed. Some earlier posts in there mention the reverb tank wires (green black from memory), and shielding options for noise cancelling. It may reduce stray noise if you remove the short lengths of wire from those reverb circuit leads. Mine does pick up the typical noise when gain is set high. I should get some time to open it up this week and I'll take some photos of what I'm talking about. I'll trim them and see if it reduces noise any. Will record before and after to compare.

This is a great forum. Thanks again to all those who have contributed.

 

[rmarshall]

Thanks for helping.

I am just struggling to pin down the issue to be honest. It seems to be connected to the preamp getting hot (never happens cold) but I ran it open for a long time last night and no issues. I also wiggled everthing I could find to test joints and nothing - no noise from any tube socket or caps etc. The only think I noticed was that the 680R resistor to the PSU gets mega hot and I wonder if this is going out of spec and creating noise on the supply. I have read that this can cause issues - if anyone could verify that would be helpful. A pretty easy job to switch the resistor for a higher wattage one if necessary. I also wonder if the extra cooling from running the unit open prevented the issue.

Hmmm ... I hate intermittent issues!

Richard.

 

[OryCheyne]

Haven't had a chance to open it up or do anything this week. Likely to get a crack at it this weekend.
Of interest, the frequency range that could be picked up by a 1/4 inch piece of wire is around 50 to 60Ghz (satellite telecommunications type frequency), I don't think that could affect the sound or appear as audio through the amp. That said, if one of those reverb tank wires gets broken that could pick up a much lower frequency (say in the AM band), because those wires are actually opposite ends of the same wire, just that they become a super fine wire wrapping at the inductors, one each end of the spring tank. One end breaks, the other still connected = antenna. Reverb models only that is, also only likely to be a problem if damaged by some extreme event, or by accident if you're working on or disconnecting the reverb circuit. The wires can actually break at the induction coil which is where one of mine was. My 22 has had a greatly traumatic life, has fallen off stages, received less than loving treatment whilst in transport etc etc. Reverb stopped working about some time 1992 after it was smashed so hard by a courier that the cab cracked. Reverb hadn't worked since without introducing really bad interference. So reverb wasn't used. Simple.
Before it was sent in for this last repair (I don't mean to make it sound like this amp is unreliable, it's actually been actually been very little trouble), I had it open and noticed the busted wire at the coil. Because the wire is too fine to solder so I just pushed it back into the point where it belonged in the winding. But it still didn't work. Since getting it back the reverb works great, but I wasn't charged for having it fixed. I'll need to chat with the Wizard that fixed it and find out what he did.

By the way the power supply resistor went on mine within a year of buying it (around 1988), there is also a weird size cap in there.
Cheers Ory
Anyway I'll get some photos this weekend. Hopefully Saturday.

 

[rmarshall]

Well my issues seems to have gone ... I've opened up, fiddling with everything and done a bit of valve moving vs other kit and haven't had the issue in 10+ hrs of playing time. The only other thing I'm doing differently is never using the stanby switch as it is pointless anyway....

Hmmm ... good news I guess!
R.

 

[OryCheyne]

Just an update and to apologize.
It's great to hear Richards amp is running fine.
I'd stated in earlier post that I would open the 22 and take some pictures but that's not going to happen. The 22 is working and I don't have the time to play around with opening it up. Far too busy playing.
So, Im sorry if anyone was wanting to see those reverb issues I'd mentioned. Im sure it's no biggie! When opened in future I'll put up photos as new post.

Sweet Jams'

Ory