2 channel H combo problem

[BlueSky]

I've had the H combo for a few weeks now and it always sounded great. However, today when I switched it on I got very low volume (on both channels). I hardly reacts to the Master volume. Not only is the volume extremely low but it also sounds bad like played through a cardboard box. It sounds as if the send/return cables in the loop had been reversed. The problem is also there when I play just with one cable straight into the amp with nothing in the loop.
There is obviously something wrong. Strange, I've handled the amp (like all my gear) with the greatest care.
What could that be? Tubes? Power or preamp? I'm comfortable with changing tubes and rebiasing - no problem. It's just that I haven't got any spares at the moment.
Or is the problem something else?

 

[BlueSky]

The volume / sound is best described as if you heard the amp from somebody's headphones sitting next to you.

 

[BlueSky]

So I did some more tests in the meantime. I plugged the guitar straight into the loop return. The volume is back to normal, albeit only clean sound. Plugged into the front again the very low volume. So that seems to rule out power tubes and transformers, I guess. Must be a preamp tube then. What do you think? As I have the problem on both channels it could be the PI tube maybe?

 

[alantig]

Is there a loop volume control? Is it possible that got bumped?

 

[BlueSky]

Both loop volume controls are at 2 o'clock.

 

[Egads]

Probably preamp tube. Easy enough to swap. Let us know what happens.

 

[BlueSky]

Probably preamp tube. Easy enough to swap. Let us know what happens.

Will do the tube swap but I only have some spare regular 12AX7s. I don't have a 12DW7. Is that the PI tube? Which tube drives the FX loop? I'll have to get some replacement tubes first.

 

[BlueSky]

Waiting for the tubes to arrive. Another thought: could it be a cold or broken solder joint? I noticed the amp gets very hot after a while and mine does not have ventilation. I wondered about that earlier. Why does the combo version not have the two little propeller thingies that the top has? Is it possible that the heat of the amp could affect the solder joints?

 

[László]

It's doubtful that the amp could get hot enough to melt solder, unless something is drastically wrong. So unless you're getting a burning smell, I'd suspect s a preamp tube. Have you used the extra 12 AX7s you have on hand to test and see if it's one of the 12 AX7s that came with the amp?

Most 50 watt amps don't need fans, unless the tubes are biased to run hot. My DG30 head has fans because the tubes are biased much hotter than similar amps, however the fans are a bit noisy if you're standing/sitting near the amp. Fortunately, I keep my amp heads separate from the cab; when I'm recording, the mics don't pick up fan noise.

I'd suspect that the 112 combos don't come with fans because the chassis sits in a larger space, which means a greater volume of air to circulate around the tubes, plus the piston movement of the speaker cone pushes air in and out, but that's just a guess.

Combos definitely subject tubes and components to a bit more torture just because of the amount of vibration inside the cabinet.

 

[BlueSky]

This is getting weirder and weirder. Now the amp works with both only a patch cable and also with FX pedals in the loop. But still the extremely low volume (as if heard through headphones next to you) if nothing is in the loop. Surely, the amp should work with just a guitar and cable in front. Weird.
That seems to rule out the tubes. It would be useful to have the schematics for this amp, especially the part with the loop, to take to the tech. Anyone?

 

[László]

This is getting weirder and weirder. Now the amp works with both only a patch cable and also with FX pedals in the loop. But still the extremely low volume (as if heard through headphones next to you) if nothing is in the loop. Surely, the amp should work with just a guitar and cable in front. Weird.
That seems to rule out the tubes. It would be useful to have the schematics for this amp, especially the part with the loop, to take to the tech. Anyone?

If what you say is so, then it isn't likely to be a difficult problem, it's fairly obvious that sticking something in the loop is completing the circuit, and removing it is breaking the circuit, and that usually means the jacks. What could be a simpler issue? Nothing, if I'm right.

I wonder if there's some shmootz or corrosion on one of the loop jacks that's screwing up the connection...or maybe it's just not making good contact when nothing's connected.

I'll venture a guess that's the problem. It's happened to me with various pieces of gear over the years in my studio, and even with guitars.

Quarter inch jacks are notorious for this kind of thing happening. It's why most non-guitar professional studio connections are XLR or D-Sub these days. Both make a better, gas-tight contact.

Before taking it to a tech, if it's a dirt or corrosion issue, you could clean the contacts on the jack easily by running a small wire brush into the jack, or spraying in a little bit of Deoxit; if it's simply a matter of not making a good tight connection, a gentle squeeze on the jack's metal contacts might make the connection tighter and solve it.

Disclaimer - I could be way wrong, but these "fixes" are unlikely to hurt anything. Be sure to unplug the amp and follow all the standard safety procedures first!

 

[rocguitar]

My brand new 2 channel H Amp had a weird "noise issue" that was strangely complex, and it seemed to be related in some way to the effects loop. Shawn sent me a new effects loop tube and, after it was installed, the problem remained exactly the same.
Shawn then suggested that I send the amp back to the factory for a warranty-covered repair.
The source of all of the strange noise problems was completely eliminated after they replaced the amp's defective effects loop "send" pot.
The PRS amp guys will no doubt get to source of your amp's problem. If it's not still covered by the warranty, it may cost you a few bucks, but the factory fix will be well worth it!

 

[BlueSky]

Isn't troubleshooting tube amps fun? I think Les is right, so I will take the amp to a tech to check the loop jacks and possibly the soldering on those.
I hope this thread does not put people off this amp. It is awesome with a fantastic range of tones (clean and OD) plus the FX loop has zero tone suck (at least to my ears). Highly recommended.

 

[BlueSky]

The loop problem has been fixed. There was a bad connection in the return jack.

 

[rocguitar]

The loop problem has been fixed. There was a bad connection in the return jack.

Glad to hear the problem has been fixed!

I totally agree with you......the 2 Channel "H" is a great amp! :top:

 

[László]

Super! Now you can enjoy it again!