WEM Custom 15

[Bushbaby]

Hi Trevor
Good idea. How do I then reattach it though as the grill was stuck on to the baffle board when I got it and it's never had any bolts or screw at the front and I did the same when I reattached it. I cant say I'm keen on metal grills but as indicated I do want to keep it looking original.

Best regards

Rob

[Murphyv310]

They are either held by contact adhesive or double sided tape. Have a look though for discreet pins as that's a common way of attaching the grill. Solvent or a little heat will also do the trick on glued or taped fixing.

[Bushbaby]

Looks like I am back to square one!
Thanks to help of Trevor and Andy, the amp is working reasonably well. I decided to replace the switch on the volume control to try and overcome the mains hum that is still there. Initially I thought it was an old valve amp with a natural level of hum but in using it I really do think that its very much annoying and needs to be rectified. The switch is in and working BUT the mains hum is still there and also the speaker 'pop' when turning off has returned (I have removed the mains cap as I thought I wouldn't need it with the switch separated from the volume control).
As it was indicated before, and I big learning curve for me, is using my oscilloscope to find the source of the hum. I am assuming that this must be caused by a ground loop(?).
I must admit I am no expert at all in using and oscilloscope or where on the circuit to take readings. I am concerned that about connecting a probe to ground just in case I short the circuit due to the earth design as it appears to run to earth at the opposite end of the circuit board from where it initially starts which I don't believe is a great approach (certainly from videos etc I have seen).
Best Regards
Rob

[Murphyv310]

Hi.
The pop is due to removing the mains filter cap. Try bypassing the switch altogether just wire the mains cable directly to the Mains TX, don't forget to disconnect the existing wiring so there is no chance of mains hum leaking from there.
If it remains you have an unrelated issue, if it goes or reduces then you know where it's coming from.

[Bushbaby]

Just as a thought, could it be the pilot light or wiring around it?

[Murphyv310]

Disconnect and see. Anything is possible.
Any wiring that carries AC should be twisted to cancel out leakage.

[Diabolical Artificer]

As mentioned in an email you want to find out what frequency your hum is, that'll point you in the right direction

When scoping a bit of gear clip the black scope lead to a ground. Finding a ground can sometimes be tricky but the chassis will usually do. I extended one of my scope probes with a longer ground lead, the 4" jobbie that is usual fitted is a tad limiting.

Next, find out what your scopes max IP voltage is before going to probe HV gear. Your amp with an HT of 300v ish should be ok, use your Tek scope. Leave the vertical IP set to AC, I hardly ever used DC coupling but I mostly work on HV AF gear. Probe on x1, scope set to as low as it'll go.

Start by clipping your probe onto the grid of V1 or whichever is the IP valve, with no IP into the amp, power it up,once everythings warmed up you should see nothing, if you see 50hz then look at the heater wiring or any other nearby AC wiring to see if it's an issue. If there is something does turning the volume control do anything? Does shorting the IP stop the hum?

Next move the probe to the anode of V1, again you shouldn't see anything major. Also as mentioned in my last email measure the AC ripple on the HT with your DMM. You can check it with your scope, if your worried about exceeding your scopes Vmax just tack a big 630v cap onto the point you want to measure, whatever you have to hand, EG 100-470n. This is not the perfect way to take reading but it will do.

From reading your posts and what you've said in emails I know your worried about "blowing things up" and wary of doing the wrong thing. With a good DMM like a Fluke and a scope like your Tek, it's highly unlikely you'll do any damage to either by doing something daft. Also making things go bang, letting out the magic smoke etc is fun and part of the learning process, try to be a bit more relaxed and not to worry so much if you can, your unlikely to do much damage to valves as unlike transistors they'll take loads of abuse.

Andy.

[Bushbaby]

Thanks Andy
Much appreciated. Its funny in way, lately I've been more concerned that my 'working' 'scopes are exactly that - working! If they weren't its pointless using them.

Best Regards

Rob

[Murphyv310]

Good sound (pardon the pun) info from Andy.
We have ascertained that the hum is most likely to be from the first stage. We discussed the possibility of a hum loop inside the amp, possibly caused by one of the input Jacks earthed and the screen lead also earthed at the other end causing a loop. I've explained how to check this. Also how valve bases can leak or even heater cathode leaks can cause this.
As the hum increases with volume control without an input, I'd still think its down to a hum loop, possible inadequate screening, bad earth at the volume control or poor routing of cables. Single point earthing though usually cures many hum issues.
It's virtually impossible to trace hum loops at source on a scope due to the virtual earth effect.

[Bushbaby]

I would like to take this opportunity to thank Trevor (Murphy V310) for all his help, guidance and advice in reviewing the 'hum' issues. Under his mentoring, today we reviewed with a scope (Tektronix T935a with x10 probe) the smoothing cap, phase splitter valve (ECC83) and finally across the speaker itself. Its apparent that its a ground loop issue and its due to the circuit design so it does seem that I've reached the end of the road on this one. On a positive note, the amplifier is working well and to be honest the 'hum' is only exacerbated when the volume is control is quite high with nothing connected.

Perhaps I can understand now why old WEM amps were not as popular as other UK brands such as Marshall etc but I am also aware that they had some issues too.

Best Regards

Rob