DRRI - not a good day

Richardj

So the family are in a different room so I have a little 'me' time and I break out the DRRI.  I plug in the Ibanez I recently got from Timmysoft and all is well and nice. I then decide to plug in my modded Strat and it is less good, suddenly I get a shrill distortion when I play with the reverb on. Only when I play a note or chord. Turn it right down or off and all is well and quiet. I notice that (what I think is) the rectifier valve, first on the left, is glowing a little brighter than the power valves next to it, but not excessively. I give the cabinet a little tap and there is just a flicker of blue in the power valves. Not wanting it to go bang I turn it all off.  Any ideas where to look first?

ICBM

Sounds like distortion in the reverb circuit. Blue glow in the power valves and the rectifier glowing brighter are normal. (Or less bright - there's no reason it should glow the same as the power valves, because it's a different type.)

First, try swapping V1 (closest to the Normal channel end of the amp) with V4 (next to the reverb transformer on the power valve side), and if that doesn't fix it, swap V3 (other side of the transformer) with V6 (nearest the power valves).

If none of that fixes it, check that the reverb tank connections are in tightly - you can usually do this by feel through the bag, without taking it out. The plugs are roughly 1/4 of the way from the ends, on the long side of the tank facing the speaker.

If it's not any of that it may need a trip to a tech...

Richardj

Cheers @ICBM.  Swapped over the valves and it didn't make any difference.  The connectors on the tank were facing the back and I couldn't feel anything amiss so I really, really carefully removed the tank and again really carefully had a look at it and it all seemed connected and everything in place.  Is it worth the effort/expense of new valves or is it off to a tech?

ICBM

It's not the valves if swapping those ones round made no difference and the problem is definitely coming from the reverb circuit, since the valves you have now moved to those positions were not in the reverb circuit before (and vice versa) - they were the Normal channel gain valve and the phase inverter.

The connectors on the reverb tank should face the speaker if it's the right way up, which is with the open side of the metal tray facing down (the wider part of the assembly inside the bag at the bottom). Check this, even if you have to take the bag out again - the Output end of the tank should be at the opposite end of the amp from the power transformer (power valve and rectifier end) otherwise it can pick up interference. I'm note sure that would produce distortion, but it may be a possibility.

It's also worth checking the connections are the right way round at the tank - unplug the Output cable and touch the tip of it with the amp on and the reverb up a bit - it should make a loud buzz. If it doesn't, try the Input cable, and if that buzzes they're the wrong way round.

Richardj

Well, the tank is definitely the right way up, ie open spring side down, but it has the connectors facing the back, from the factory! Worth swapping round? It's only two screws. Funny thing is that has been fine since I got it, just mysteriously went bad whilst in use.

Richardj

How do you know which end of the tank is input and output?

ICBM

Richardj said:

How do you know which end of the tank is input and output?

It's marked on the outside of the casing.

But if the connectors are facing the back, it's the wrong way round! Unless the tank is the opposite of every one I've ever seen, which seems unlikely. I almost didn't trust myself there (getting old ) so I went and checked on one I have, and it's definitely that way round.

Richardj

Again many thanks for your time.  So, and apologies, is the red connector send and white return?  I want to make sure it is all connected properly back up into the chassis connectors.

ICBM

Richardj said:

Again many thanks for your time.  So, and apologies, is the red connector send and white return?  I want to make sure it is all connected properly back up into the chassis connectors.

Can't remember - just touch the tip of both of them with the amp and the reverb on, the one that buzzes is the return (tank output).

Richardj

Still not good.  I spend ages removing the back panels and very carefully undoing the reverb tank and sliding the bag off. I turn it round and miracle of miracles it sounds fine again. Out of bloody mindedness I spin it back to how it was and it's still fine. Result!

I very carefully slide it back into the bag, try it and it's squealing again, bugger!  I slide it out again, check the connections and all that looks OK.  Not sure if it was a good idea but I unplug the tank and try it again. It still squeals, but this time up around 7 instead of 3 on the dial. With it all plugged in I also get a big ground type hum when touching the tank.  There is a strange thick disc thing @1.5" across in shrink wrap on the reverb send/return cable, it is vaguely crackly when touched, what's this bit?  Is it knackered and can only be sorted by a tech poking around inside?

ICBM

Sounds like you have a bad ground connection on the tank, or (outside possibility) at the other end of the cables inside the amp - you should not get hum when touching the tank. It may be a partial or intermittent problem if the distortion is coming and going or in severity). Looking at the tank I have here, the rivetting between the jack and the tank (which makes the ground connection) is rather poor and the jacks can move about a bit - my guess would be that yours is either very loose or corroded. Tightening the rivetting or drilling one out and fitting a nut and bolt should fix it. But first, make really sure the plugs are tight in the jacks, especially the outer sleeves - squeeze them a bit tighter so they're a force fit, if necessary.

The disc thing is a ferrite core designed to stop RF interference - it's not knackered unless it's physically broken into pieces, so it's fine!

Although it is possible that the bad ground is a break in the cable itself, and next to the ferrite (where it's been stressed a bit more than elsewhere when it was wrapped) might be a possible place...

At least it does now seem almost certain that the problem is in the tank circuit somewhere.

Richardj

So it could be the tank itself?  It is all very new and shiny looking so it isn't corroded. I will have a look at the mechanical connections.

I also managed to briefly speak to someone at Fender France and their first response was to offer to send a new tank!

Richardj

An interesting twist, I get an email from Fender and they have offered to have the amp collected from the nearest dealer, send it to their service centre and fix it, allegedly all for free.  Handily I have the receipt etc so all could be well.

ICBM

Richardj said:

An interesting twist, I get an email from Fender and they have offered to have the amp collected from the nearest dealer, send it to their service centre and fix it, allegedly all for free.  Handily I have the receipt etc so all could be well.

Worth taking up if it's under warranty.