WEM guitar valve amp problem resitor value - UK Vintage Radio Repair and Restoration Discussion Forum

shugtech · 25th Sep 2022, 3:57 pm

Hi, I'm looking for a bit of advice, I picked up this old Wem dominator mkiii, one of the ecc83 valves (12ax7?) was cracked, so I've replaced that, but found this broken resistor, it's soldered to the big capacitor right at the start of the power circuit, it says on the schematics online that it should be a 1.5k ohm, I've ordered a couple of different ones online, but wasn't sure about the power rating, I've ordered a 5w and a ceramic 10w one, any advice on which would be best would be appreciated. 👍

ms660 · 25th Sep 2022, 4:17 pm

2 Watt would be ok so the 5 Watt will be plenty.

Lawrence.

stevehertz · 25th Sep 2022, 5:36 pm

Check that PCB on the print side (obviously) for dry joints. Just resolder without further ado any joints that look less than perfect ie dry or dull grey.

Robert Gribnau · 25th Sep 2022, 8:17 pm

If you have the means, also check if the 50 uF capacitor just 'downstream' this 1K5 resistor is not too leaky. If it's too leaky, the current through the 1K5 resistor could get much higher than designed. But just measuring the voltage drop over it will at least give you some indication already.

shugtech · 25th Sep 2022, 9:10 pm

thanks folks,

ill have a tinker with it once the parts arrive and let you know how I get on.

Diabolical Artificer · 26th Sep 2022, 6:07 am

It'd be worth cleaning the PCB whilst your waiting with warm soapy water, guitar amp gunk can be conductive at the high voltages present.

Andy.

Craig Sawyers · 26th Sep 2022, 4:04 pm

I restored a WEM dominator bass for my son when Charlie Watkins was still alive. He was both gracious and helpful. A legend of big outdoor festivals, like the legendary IOW, where in spite of Watkins best efforts and a prodigious amount of power the wind from the sea meant the sound went to the back of the stage rather than to the audience!

The dominator was in better shape than the one you have, which seems to have been through the mill.

Although my son has a huge Trace Eliot RAH1000 and a massive three way loudspeaker unit I built to handle the output of that beast, that tends to not get used much. In a terraced house that is a bit OTT!

So the good old valved WEM gets used!

Craig

joebog1 · 26th Sep 2022, 10:40 pm

Just an observation, That resistor is for the screen grid and is scaled to suit the power required by the output. Putting too big a wattage in that position is perhaps not such a good idea. If the amp is driven very hard, that resistor will let go and save the output stage. Fender have a bunch of 470 ohm 1 watters protecting their 6L6's, and I have repaired quite a few where these resistors had been replaced with 5 watt wirewounds. The result? a dead output transfomer and dead 6L6's.

Joe

25th Sep 2022, 3:57 pm	#1
shugtech Diode Join Date: Sep 2022 Location: Glasgow, Lanarkshire, Scotland, UK. Posts: 2	WEM guitar valve amp problem resitor value Hi, I'm looking for a bit of advice, I picked up this old Wem dominator mkiii, one of the ecc83 valves (12ax7?) was cracked, so I've replaced that, but found this broken resistor, it's soldered to the big capacitor right at the start of the power circuit, it says on the schematics online that it should be a 1.5k ohm, I've ordered a couple of different ones online, but wasn't sure about the power rating, I've ordered a 5w and a ceramic 10w one, any advice on which would be best would be appreciated. 👍 Attached Thumbnails

25th Sep 2022, 4:17 pm	#2
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: WEM guitar valve amp problem resitor value 2 Watt would be ok so the 5 Watt will be plenty. Lawrence.

25th Sep 2022, 5:36 pm	#3
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,834	Re: WEM guitar valve amp problem resitor value Check that PCB on the print side (obviously) for dry joints. Just resolder without further ado any joints that look less than perfect ie dry or dull grey. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

25th Sep 2022, 8:17 pm	#4
Robert Gribnau Heptode Join Date: May 2017 Location: Konongo, Ghana Posts: 516	Re: WEM guitar valve amp problem resitor value If you have the means, also check if the 50 uF capacitor just 'downstream' this 1K5 resistor is not too leaky. If it's too leaky, the current through the 1K5 resistor could get much higher than designed. But just measuring the voltage drop over it will at least give you some indication already. __________________ Robert

25th Sep 2022, 9:10 pm	#5
shugtech Diode Join Date: Sep 2022 Location: Glasgow, Lanarkshire, Scotland, UK. Posts: 2	Re: WEM guitar valve amp problem resitor value thanks folks, ill have a tinker with it once the parts arrive and let you know how I get on.

26th Sep 2022, 6:07 am	#6
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,669	Re: WEM guitar valve amp problem resitor value It'd be worth cleaning the PCB whilst your waiting with warm soapy water, guitar amp gunk can be conductive at the high voltages present. Andy. __________________ Curiosity hasn't killed this cat...so far.

26th Sep 2022, 4:04 pm	#7
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,993	Re: WEM guitar valve amp problem resitor value I restored a WEM dominator bass for my son when Charlie Watkins was still alive. He was both gracious and helpful. A legend of big outdoor festivals, like the legendary IOW, where in spite of Watkins best efforts and a prodigious amount of power the wind from the sea meant the sound went to the back of the stage rather than to the audience! The dominator was in better shape than the one you have, which seems to have been through the mill. Although my son has a huge Trace Eliot RAH1000 and a massive three way loudspeaker unit I built to handle the output of that beast, that tends to not get used much. In a terraced house that is a bit OTT! So the good old valved WEM gets used! Craig __________________ Doomed for a certain term to walk the night

26th Sep 2022, 10:40 pm	#8
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: WEM guitar valve amp problem resitor value Just an observation, That resistor is for the screen grid and is scaled to suit the power required by the output. Putting too big a wattage in that position is perhaps not such a good idea. If the amp is driven very hard, that resistor will let go and save the output stage. Fender have a bunch of 470 ohm 1 watters protecting their 6L6's, and I have repaired quite a few where these resistors had been replaced with 5 watt wirewounds. The result? a dead output transfomer and dead 6L6's. Joe