Slightly disappointing NAD

drwiddly

It's probably divine retribution but last week I succumbed to impulse GAS and ordered a Peavey 6505 from GAK because a) I've fancied one of these for a while and b) it was very cheap in their sale.

They were out of stock for a few days but the amp has arrived in perfect condition and sounds great. The only problem is when I hook up my Digitech GSP1101 either in the FX loop or via 4CM, then there's a fairly loud hum which sounds very much like an earth loop. The level is constant and is not affected by having a guitar plugged in or not or by changing any gain or volume levels on either the amp or the GSP. I've not had much time to play around with it but I'm certain it's the FX loop on the amp.

I've used the GSP with 3 other amp heads and a combo (all valve) and never had any noise issues with any of them. I've checked all the cables, cleaned contacts, etc. but there's no change. A bit of interweb research reveals this to be a common problem with this amp and the 6505+ but there's a lack of sensible solutions. It seems to me that there's likely to be a design fault with the loop that can probably be fairly easily remedied by someone who knows what they're doing but I'd value some advice from the experts on here @ICBM, @ecc83 and @jpfamps, if you have any experience of this problem. Thanks in anticipation ....

gearaddict

I had the same problem with my Orange TH-30 and the TC Electronic G Sharp. Works fine with other amps but on the TH-30 was so noisy as to be unusable.

drwiddly

After I posted, I played around some more with it and it may just be an inherently noisy amp. With nothing connected it's quiet until you start turning up the pre and post gain controls, then there's pretty loud hiss but no hum.

With the FX loop off but either send or return cables connected, there's a mixture of low level hum and crackling. With the loop on and cables connected there's a steady hum which is louder than most amps make (even the noisy ones) but still (just) useable. The noise gate on the GSP takes care of the hiss and any other noise but doesn't affect the hum at all.

ICBM

drwiddly said:

With the FX loop off but either send or return cables connected, there's a mixture of low level hum and crackling. With the loop on and cables connected there's a steady hum which is louder than most amps make (even the noisy ones) but still (just) useable. The noise gate on the GSP takes care of the hiss and any other noise but doesn't affect the hum at all.

That's interesting and suggests it's definitely the amp at fault, since the gate doesn't stop it it's probably not in the signal path via the processor. It could be a ground loop from the processor if it has a grounded power supply. If it doesn't, it's not that…

Try connecting a single cable from send to return in the loop and see if the hum happens. If it doesn't, connect the processor but leave it disconnected from the mains. Then connect it to the mains but leave the power off. Then finally turn it on. When does the hum start?

I must admit I haven't come across any 5150/6505 users who use the loop before, so I was not aware of this problem.

drwiddly

Thanks @ICBM. I'll try that tomorrow and report back.

Dave_Mc

Nice score

Sorry to hear about the hum, interested to hear if you get it sorted.

drwiddly

Thanks Dave. It's a strange one really. The amp sounds great and has shed loads of gain + that huge bottom end thump that these amps are renowned for. There's so much gain that at any stage volume, the hiss might be a problem. The noise gate gets rid of all the hiss but then there's hum. I'm sure there's a fix for it, then it'll be killer!

Dave_Mc

No problem. They're definitely considered to be a classic, and when I tried them I liked them a lot. I had a hard job talking myself out of one at that price ) I think they do have a little bit of a rep for being a little bit noisy, though.

fretmeister

I had one for a year.

They just are noisy. You don't notice at stage volume, and I did really like the amp - but it's a shit choice for a home amp.

drwiddly

That seems to be justified! I was hoping the GSP1101 would solve any noise issues :-S

Had some time to test a bit more in  accordance with @ICBM 's advice above:

I should explain to begin with that I hadn't used the amp's footswitch as I need to be able to control the channel switching and the GSP's patches via midi. I'm therefore switching channels using a Voodoo Lab Control Switcher (VLCS). I've just tried the footswitch to see if it made any difference and there's no hum with the loop off and nothing connected to the loop BUT the footswitch doesn't switch the loop on and off for some reason :-?

Test 1: VLCS connected with loop on and a jump lead connecting the send and return sockets and both gain controls on 4: On the rhythm channel, increasing the post gain (master volume) for that channel there is a loud hum which increases with volume. Switching to the lead channel and increasing that channel's post gain there's loud hiss but no hum at all!

Test 2: Connected the GSP1101 to the FX loop with GSP disconnected from the mains: Same result as above.

Test 3: Connected the GSP1101 to the FX loop with GSP connected to the mains but powered off: Same result as above.

Test 4: Connected the GSP1101 to the FX loop with GSP connected to the mains and powered on: Constant low level hum coupled with an oscillating/beating sound which disappears after a few minutes leaving just the hum. Connecting the GSP in 4CM, rather than just to the loop, increases the hum slightly. Increasing the pre or post gain levels on either channel has no effect on the hum.

I use the VLCS with a 2 x mono to 1 x trs jack as it appears that the channel switching is on the tip and ring (very poor design - means you can't use a mono jack lead to switch channels in an emergency). If I disconnect to tip and sleeve jack with the loop left on and everything connected, turning up both channels leads to very loud hum increasing with volume.

I also tried running an extension cable from a different ring main to the amp but that made no difference.

Just to make sure that there was nothing else wrong, I connected up my Wharfedale TCT50 head in exactly the same way using same cables, etc and it was absolutely silent and worked perfectly (this may say something about Steve Grindrod's amp designing capabilities). 

Any ideas?

drwiddly

fretmeister;341082" said:

I had one for a year.

They just are noisy. You don't notice at stage volume, and I did really like the amp - but it's a shit choice for a home amp.

Thought as much but I'm wondering if a decent tech could sort it?

It's far too loud (and noisy!) to use at home so, if I can get the hum sorted, it will be gigged.

drwiddly

Further update:

I can set the switches on my Voodoo Lab Control Switcher to be either Normal (Off = Open On = Closed), Reverse (On = Open Off = Closed) or Momentary (Pulse).

I've had the switch set to Normal but, if I alter it to either of the other two settings, it reverts to massive hum increasing with volume on both channels but still switches the loop. 

I've also tried another two button footswitch with a trs cable but that doesn't work either. I'm beginning to think there's some problem with the FX loop switching? It's useless to me anyway as there's no scenario where I'd want to switch the FX loop off.

ICBM

It sounds like the hum is being generated in the FX unit, not in the amp - because even with the FX unit connected to the mains there is no hum, until it's turned on - but if it doesn't occur with another amp then the problem must be that the cause is still a ground loop being caused by the Peavey… I think!

If this is the case, you may need the amp modded so the 0V rail is separate from the chassis ground - some modern amps have this as stock (eg most newer Marshalls, and possibly the Wharfedale if Steve Grindrod had a hand in it), but if the amp is not designed like that it may be difficult to achieve if the chassis is used as the ground connection in several places. The 5150 does appear to be like this from the schematic.

You might be able to break the loop by undoing the ground connection at one end of one or both signal leads from the amp to the GSP, or inside the amp by lifting the grounds to the FX loop jacks. Sometimes it works better with a resistor in line, typically something like 10 ohms or 100 ohms.

drwiddly

Thanks @ICBM, that makes sense. Getting the amp modded (if possible) would seem like the best option so I'll have a word with my local tech and see where we go from there. If I get it sorted, I'll report back.

ICBM

See if you can get a couple of non-grounded signal leads first and try that, it should tell you whether that will solve it, even if you then want it done inside the amp.

drwiddly

I'll make a couple up and give it a go. Cheers!

drwiddly

Tried it with a couple of ground lifted cables. Tried it all ways - i.e. one to send with a grounded cable to return and vice versa and both ungrounded but it made no difference at all  :x

I've e-mailed my local tech and I'll see what, if anything, he suggests.

ICBM

Does the physical placement of the GSP relative to the amp make any difference?

The noise is definitely being generated in the GSP, but only seems to occur with the Peavey… and does not seem to be a ground loop… which doesn't leave a lot...

drwiddly

Doesn't seem to.

I wouldn't argue with your much greater expertise but I'm still thinking there's an issue with the FX loop switching in the amp that is part of the problem and it may be exacerbated by the the Voodoo Lab switcher but, as neither the Peavey foot switch or my other two button one seem to work properly, I can't really test it.

The Digitech GSP is one of the quietest and most reliable bits of kit I've ever owned. I've used it with several valve amps as well as three or four rack preamps with both valve and SS power amps and I've never had any noise problems with any of it.

ICBM

It's definitely the amp which is causing it, but somehow the noise is being generated in the GSP. Without having both bits of gear myself it's a bit difficult to diagnose it any further, sorry! It doesn't seem to be any of the usual issues.

drwiddly

I've had a word with my local tech and he says those Peaveys are difficult/impossible to mod but he will have a look. It's a strange fault which I'd like to get sorted out but the hum is probably tolerable for live work so, if nothing can be done, I'll learn to live with it.

Thanks for your help @ICBM