Front end amp input query/issue

hubcap2000

Hi Y'all,

I believe this is my first post on here so, hi. I used to use MR quite a lot and then it disappeared one day which was sad but, life goes on.

ANYWAY. I was trying out a couple of new OD and distortion pedals with my amp the other day and was getting a problem I've not experienced before. I've always run my pedals pretty hot to push the front end of the amp a bit harder, usually setting the amp nice and loud so it's naturally on the verge of break up then smashing it over the edge with a hot front end signal. It's always worked a treat and has long given me a nice crunchy, aggressive but not overly fuzzy sound.

I noticed the other day, whilst trying a few pedals out that when the pedal output (Paul Cochrane Tim and Zvex Box of Rock) was set over half way, the amp seemed to respond in a weird way; it would take the signal fine and it sounded great but as soon as I'd stop playing or punch the pedal out, I'd get this dramatic drop in volume for a split second or so before it sort of 'breathed' and got back up to full volume again. I guess I could best describe it as saying that it was like the amp was in shock at the sudden change in level and had to catch up with itself a bit, or possibly sounded a little like there was a voltage drop in the chain somewhere. I then backed the pedal off to somewhere around half way and it seemed to be alright.

So, I suppose my query is, is that just me treating my amp poorly and just needing to run my pedals with a bit less level, or is it an indictment that there might be an issue with my amp somewhere?

For a bit of background, I have a Tipton Electronics Plexi-Clone thing, it's a 50w head built on a G-Electronics chassis. I don't know a lot about it as I bought it blindly on ebay about 4 years ago but I know it's sounds great and runs on two EL34s in the power section and 4 (I think) ECC83s in the pre.

If anyone has any possible suggestions then that would be super great. I'm relatively electronics savvy but by no means an uber tech (as I'm sure you can tell) but any help would be appreciate.

Ta x

ecc83

Do you have a schematic of the amp? 

Are you sure it is an all valve signal train? There is no problem in mixing op amps and valves but you have to know what you are about and make certain adjustments to the circuitry since one problem with the very common (and very useful!) TL0 series of op amps is that they can latchup if over driven. 

Then again, amplifiers can throw up problems well down the design road. Not until certain, rare conditions obtain does a fault or aberrant behaviour show up. A GOOD design team will move heaven and earth to fix the problem as soon as possible but lesser firms might say, "***k 'em!" "Only does it if so and so and so happens. Why worry?"

Dave.

hubcap2000

I'm afraid I don't and I'm fairly sure I'm unlikely to be able to get one. Interesting point though; and one of the gambles taken when not owning a tried and tested amp. I'm curious that it's only really started doing it recently, as in the the last few days. If it was an inherent design flaw in the amp, wouldn't it have shown up sooner?

ICBM

Try replacing the first preamp valve, nearest the input jack. If you don't have a spare, swap it with the second one.

It's possible that the valve is on the way out - they're usually direct-coupled to the input jack on traditional designs. They can also leak DC to the input jack, which can sometimes do odd things to true bypass pedals. The same valve will quite often work perfectly well elsewhere in the amp, but I'd still prefer to replace it. (Keep the dodgy one as an emergency spare though, if it is the cause.)

hubcap2000

Thanks chap. I'll give it a try; I've got some spares knocking about the place.

hubcap2000

Changed out the first pre-valve; works fine now, cheers for the help.