Wem Dominator rebuild

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fenderbass39uk
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Wem Dominator rebuild

Post by fenderbass39uk »

Hi all. This is my first post. I'm rebuilding a 1970 WEM Dominator. Not far off powering up and I'm assuming I may need a little guidance. This would be the correct forum?

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geoff 1965
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Re: Wem Dominator rebuild

Post by geoff 1965 »

hello,
seeing as the marshall 18 watt has it's roots in the Wem amp i for one would like to see your progress with this,it would be interesting if you could post some pics and the schematic you used.
good luck
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Re: Wem Dominator rebuild

Post by crgfrench »

I would love to follow your progress.
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fenderbass39uk
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Re: Wem Dominator rebuild

Post by fenderbass39uk »

Thanks for the welcome all. I acquired this combo in 1976 and when it finally failed in 1985 I put it up the loft till 12 months ago. The mains transformer melted and I must have removed it years ago and lost it. The O/P tranny was replaced by someone in the 70's with a massive second hand one (looks like an ac30 one) so I have removed that also.
I have a reasonable background in electronics due to my 40 odd years at BT (I'm retired now) After successfully building a 5watt amp kit from Ampmaker I thought it was now time to tackle the mighty Dom. I've attached a couple of pre build pics and the original WEM circuit (which they sent to me by post in 1978)
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Last edited by fenderbass39uk on Wed 05/06/20 5:52 pm, edited 1 time in total.
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Re: Wem Dominator rebuild

Post by geoff 1965 »

thank's for posting the pics and that cool "original schematic",
what are your plans for the transformers,and doe's it have a celestian greenback speaker?
stay with us!
note; here's the earliest marshall schematic "1966" so you can do some comparisons.
18wr.gif
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Re: Wem Dominator rebuild

Post by fenderbass39uk »

It has a Greenback G12M that I put into it in 1978. I sourced my trannies from Ampmaker as his Carnhill o/p Tx had good reviews and was reasonably priced. I have since realised that his Primary Windings Power Tx is designed for a solid state rectifier not a valve EZ81. I decided to try it anyway and the amp does work.
Albeit the power is reduced quite a bit due to the 190v-0-275v design. I really need to source a proper lay down 275v-0-275v transformer. Can anyone recommend a supplier?
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Re: Wem Dominator rebuild

Post by fenderbass39uk »

Thanks for the Marshall diagram it's interesting to compare as they're both very similar. Not surprising really given the history. I've also been using this diagram which someone had redrawn and posted inline.
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Re: Wem Dominator rebuild

Post by geoff 1965 »

i have the ampmaker danbury PT in my lite2 with a bridge rectifier,i liked the option of the 190V tap so my initial B+ is on the high side but i like the headroom/tone and i can switch to the 190 tap for earlier breakup.
290-0-290 is the norm for the ez81 and if you look at the first stage B+ on yours and the 1966 marshall they are within 5 volts of each other,they must have had lower wall voltage then note the marshall has a HT of 300-0-300.
the reissue marshall's use 275-0-275 with the ez81 with adjustment to the el84 biasing.
steve at valvepower has good transformers made and usually sell's them on ebay also there's "modulus amplification" here in the U.K. you can check out. i've just been looking into this myself,i like the heyboer and classic tone transformers but when you add on the overseas shipping they are quite expensive, "allparts u.k." have the mojotone transformers so check them out as well.
good luck
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Re: Wem Dominator rebuild

Post by fenderbass39uk »

Useful info thanks. yes the B+ are very similar I'll check out your recommendations much appreciated.
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Re: Wem Dominator rebuild

Post by geoff 1965 »

apart from that PT being designed for a bridge rectifier it has'nt got sufficient heater supply,you need at least a 4 amp supply for an ez81,2 el84's and 3 12ax7's so be careful with that.
struggling to find a 275-0-275 PT,ampmaker has one for the 36W which is probably overkill.
you could contact marshall and see how much a replacement PT for the reissue 1974X is but make sure you have someone with you who knows how to perform artificial resuscitation!
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Re: Wem Dominator rebuild

Post by geoff 1965 »

have you checked components in your amp to see if they match the original 1965 custom 15 schematic or the model20/T schematic?
i.e. the custom 15 has the el84's individually biased whereas the 20/T has them shared.
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Re: Wem Dominator rebuild

Post by geoff 1965 »

there is some valvepower PT's on ebay uk item no;264572034419, i think these are good value and well made,they have the solder lugs like the heyboers Josh uses.
there is also a used 275-0-275 with sufficient heater supply for your amp if you decide to go for the 275 HT, item no;163916236480
both are drop through.
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fenderbass39uk
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Re: Wem Dominator rebuild

Post by fenderbass39uk »

My Dom has the custom 15 arrangement for the EL84 biasing.
Good point about the heater taps not being suitable for my valve arrangement. I'd thought of that and have been using a separate 6.3v tranny for the ecc83/el84s as a workaround till I get a proper one. That Valvepower one looks perfect I'll probably go for it..thanks again for your help. Here are the latest pics. you'll notice I have flipped all the components on the circuit board. This is how WEM did it on the earlier Doms. All electrolytics have been changed and any other components which were out of spec.
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Re: Wem Dominator rebuild

Post by geoff 1965 »

cool! yeah the custom 15 is closest in bias and B+ to the early marshall so the valvepower would be ideal. i notice with the 20/T version they cool the biasing on the el84's and i wonder if the 3K3/.005's across the plates is an early experiment of "snubbing" the high frequencies.
you can see from both schematics where marshall got their ideas for the 18W.
keep us posted with your progress
note; just pointing out you have the secondary HT wired incorrect, it is'nt centre tapped and you have the 0 going to ground.it's for bridge rectifiers as mentioned earlier so you wire it 0-275 for full 360VDC or 0-190 for 260VDC.
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Re: Wem Dominator rebuild

Post by fenderbass39uk »

I tried 0-275 but that only gave 6 volts on the b+ through the EZ81. So as I said this is a temporary workaround. I've ordered the Valvepower 290-0-290 today so I'll leave it down powered until then. I need to change some of the valve seats as the contacts are breaking down.
Interesting you noticed the snubber 3K3/.005's on the circuit diagram output.
Apparently only WEM used these, they present a 15khz load at all times. I think it was to help the EL84's and prolong the life of the rather weedy(but toneful) transformers that they used. Barry at Ampmaker recons that modern spec trannies don't need them. I intend to add them anyway when everything up and running. BTW the amp sounds great. The tremelo channel works fine, the normal (mic) channel is very quiet so I'll investigate that later maybe with your help.
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