Tremelo problem on a 60's WEM

[old tube]

Hoping someone can help.

Got a friends mid '60's Watkins Scout to have a look at. Problem is that the tremolo is always on, can't switch it off, even with a known working footswitch. Also the tremolo is always on, on both channels, it should only operate on one channel. Otherwise the amp operates just fine. I'm OK with simple to moderate tasks on tube amps but this has me stumped. However, I am only beginning to learn diagnosis, so it's probably something obvious.

There is no available schematic for the Scout that I can find, but this WEM seems to be identical from matching up the resistor and cap values also the valve complement and connection.

The circuit is mounted on an early "PCB" with thick solder tracks. I can't see any thing that is obviously gone, broken or shorting. The thought of methodically removing components from such an old board with fragile tracks to test them, fills me with dread.

Hoping some you experienced guys will point me straight at the obvious problem

Thanks in advance.

[Blackburn]

Very cool amp! Same as the Dominator, just with a single 10", as I understand. Trem shouldn't be on both channels, obviously, as indicated on the schem. I would be very cautious removing anything from an amp as old as that, especially being a Watkins. They are valuable. Maybe post some pics?

David

[jjman]

Sounds like it was once owned by a real Trem Whore. Not that there's anything wrong with that.

[old tube]

It is a pretty cool little amp, I'll post some pictures tomorrow.

Should have said that both the trem speed and depth controls work as they should, if that is any help.

[JazzGuitarGimp]

I have a feeling that you are dealing with two separate issues:

1. Something is causing the tremolo to be active regardless of the footswitch position.

2. Something else is causing the tremolo to affect both input channels.

For #1, I'd look at the FS Jack. Has it possibly rotated and allowed the tip terminal to short to the chassis, or some other ground?

For #2, the first thing I'd suspect is C20 may be leaky.

HTH,
Lou

[old tube]

Some photos as requested. It's about the same size as a Tweed Princeton. It would originally have had a metal speaker grille same as the vent grille on the back. Someone has replaced it with what looks like black curtain material! However it does kind of work, gives it a very "art deco" look. The Speaker is not original. It's a early '69 10" Celestion greenback. Original would have mist likely been an Elac. Its got the original transformers and most components seem original other than the RS cap in the preamp. Which is the wrong value at 100uF! Also that weird re-wiring of the tremolo input sockets.

Many thanks Jazz guitar. Got the tremolo on / off sorted, it was just accumulate crud I think, gave it a good clean and it seems to operate OK with a pedal now.

After having the amp on for a while and well warmed up, the problem with the tremolo being present on both channels becomes intermittent. It seems the "normal" condition is for it to be affecting the normal channel however, every so often there is a soft "whump" and it stops coming through the normal channel. It may stay like that just for a few seconds or for a few minutes. So I'm pretty sure you are right and it is that cap. I've ringed it in one of the photos and being a red "Hunt" that gives me confidence that it is indeed the cap, most 60's Hunt caps seem to have failed by now. I just hate that "PCB" so I'll start by trying to reflow the solder on the cap junctions ... hopefully that will work! I really don't want to have to replace anything as the tracks are likely to lift.

There has been a problem with a track in the past and one of the Mustards has been "re-seated" as it looks like the track had broken or lifted.

I've ringed what looks like a mod around the tremolo input. It looks like a 1M resistor has been removed and two 100k resistors added in series and the connection to V2 been re-routed .. no idea what that is all about!

The other real problem with this amp, now I've had tine to use it, is that its output is low. Like 2-3 watts max I'd say. I've changed all the valves to known good (Mullards and a Telefunken, as in the photos) the original valves were probably exactly that; "Mazda" badged RFT's by the looks of them. I'll get them on the valve tester at the weekend. I've checked the HT voltages and the Transformer is putting out 320V, Rectifier (EZ81) 313V, plate voltages on the EL34's are at 293V, Phase inverter at 180V, pre-amp valves are at about 160V, so nothing seems too amiss there?

Again my limited diagnosis skills let me down and I've no idea what might be causing the low output. The amp is very quiet, no background noise or intermittent crackles or the like.

Any ideas as to where to look for a cure to the low volume would be much appreciated.

[Blackburn]

What a fantastic little machine loaded with wonderful tubes and caps! I want a Watkins...

[old tube]

Ha! just looked at the circuit diagram again and it looks like the resistor "bodge" on the tremolo channel is in fact how it should be!

[old tube]

What a fantastic little machine loaded with wonderful tubes and caps! I want a Watkins...

These are going up in value here. But you can still pick up a lot of great WEM amps ... quite often with all their original components and Mullard valves for around the $400 mark. In the UK in the 60's and early 70's they were the beginner amps, so most of them were not gigged but lived in bedrooms and have therefore survived. Nice simple circuits, reasonably well built, same components as Marshall and from the mid '60s, Celestion / Goodmans speakers.

I've got 2 ER-15s which are fantastic amps, dominator circuit without the tremolo, in a head form. One trick pony but what a trick! Stick it through a decent 20watt Celestion greenback or a blue and you're straight in Marshall 18 Watt territory. (one of them is in the background of one of the photos .. looks like a fan heater on top of a Marshall cab)

[Blackburn]

Man, the V-front Dominator is way up there on my just-won-a-jackpot list. Here in the states they are pretty valuable. I've seen them around the $4K mark and higher. Probably like Voxes, they're more expensive here because of the scarcity. I can't believe they are that cheap over there! Bring me one. I'll buy you an ale.

Here's a favorite clip of mine.

http://www.youtube.com/watch?v=3AoKNwxCJm0

And another.

http://www.youtube.com/watch?v=9tZm9j2hR00