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Old 14th Apr 2017, 6:38 am   #1
candletears7
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Default Vintage WEM Dominator guitar amplifier?

Hi from Australia - I thought I'd try this forum as it's UK based!

I picked this up recently for a fair price - it's a WEM Dominator that about 20 years was rehoused into a rack unit for studio use, with a tube driven FX Loop and some kind of direct line out situation added.
Whoever did the work obviously put a lot of time and effort into it and knows what they were doing.
The amp doesn't work, apart from very low and crackly/distorted output.
I've researched and had a go at fixing this - I know what I'm doing with pedal repairs and mods, but amps are something I'm yet to really get into.

The capacitor feeding the EL84's had exploded. It's a 22uf/63v. I've replaced it with two 10uf caps in parallel to give about 20uf.
I took it to a tech friend of mine who said it could be a grounding issue, so I've reflowed all the joints under the circuit board and anywhere I can see star grounding.
Still no joy.

I know that the old Mullards were wired up with the connection to pin 1 on the EL84's, and to make these amps work for newer EL84's, the connection needs to go to pin 2. That's already been done.

Any help or pointers would be really helpful. So far I've paid about $300 USD for this which included a decent rack unit, so if I can't get it working, I can take it to a tech.
The goal is to have this put back into a WEM style headshell with a replica faceplate.
I'm dead keen to get this thing alive again.

Any pointers or initial components to look at? Voltages I can measure?
Any help very much appreciated!
Thanks,
Jesse in Melbourne, Australia.
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Old 14th Apr 2017, 11:33 pm   #2
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Default Re: Vintage WEM Dominator guitar amplifier?

I would replace the capacitors in the C12 and C13 positions in the example circuit I have shown. Your capacitor will most likely be connected across R17 in my example. Yours looks like one of those white square resistors.
Also you will need between 250 and 420 volts on pins 1,2 and 3 on your output transformer.
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Old 14th Apr 2017, 11:48 pm   #3
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Default Re: Vintage WEM Dominator guitar amplifier?

There are 2 versions of the dominator in the link below. We need to know which version you have and where the capacitor is in the circuit.

http://wem-owners.com/schematics/dominators/

Al
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Old 15th Apr 2017, 3:22 am   #4
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Default Re: Vintage WEM Dominator guitar amplifier?

Quote:
Originally Posted by Refugee View Post
I would replace the capacitors in the C12 and C13 positions in the example circuit I have shown. Your capacitor will most likely be connected across R17 in my example. Yours looks like one of those white square resistors.
Also you will need between 250 and 420 volts on pins 1,2 and 3 on your output transformer.
Hi, thanks for replying

Ok, on your schematic, those capacitors are 100n, I believe the capacitor I've circled on my pcb needs to be a 22uf/63v... is that schematic from a WEM Dominator?
So that little white square component is a capacitor? I'll replace both.

http://i304.photobucket.com/albums/n...psu2uvmzif.jpg

I'll check voltages tonight when I'm home, thanks for that.

Quote:
Originally Posted by Alistair D View Post
There are 2 versions of the dominator in the link below. We need to know which version you have and where the capacitor is in the circuit.
Hi Al, thanks for the reply.
I believe mine is the Dominator 15, as it doesn't have Tremolo.
I have absolutely no idea which capacitor it is, sorry.
I can't see a 22uf cap in that schematic anywhere...
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Old 15th Apr 2017, 6:51 am   #5
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Default Re: Vintage WEM Dominator guitar amplifier?

You've got a rare amplifier there, I've never seen a rack mount Wem Dominator and from the pictures you posted it doesn't look like a standard Dominator. I reckon it was specially built by Wem.


Firstly you need to make sure you power supply is ok. Start by building a lamp limiter - http://www.vintage-radio.com/projects/lamp-limiter.html so you don't do any damage.

Somewhere (probably one of those two valves on the metal stand off or are they the two EL84's ? ) , there will be a EZ80 or EZ81. Take it out ,( in fact take all the valves out) and see if you have an AC voltage of around 300v on two of the pins, pin 1 and 7 I think. Or see if the same voltage is present at the transformer on the left. The wires disappear through that hole, where to? See that green wiring? That's the heater wiring, that should read 6.3v.

When you know you have HT and heater voltage, you'll need to check all the electrolytics with an ESR meter. But make sure they are discharged by making a cap discharger, something like this -
https://www.youtube.com/watch?v=gLDCV-n1BDw&t=8s A 100 ohm resistor on a bit of wire will do.

Replace the rectifier valve (EZ8/1) and you'll need to put a signal in using a signal generator (available online) and trace it to the anode of that valve in the can, (pic 1 top middle), then trace all the way through at some point.

A good tool if you intend doing a lot of audio work is one of these - https://www.youtube.com/watch?v=dFj_fa78y7c&t=1133s if you have the cash.

There's more to do, but that should do for now.

Andy.
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Old 15th Apr 2017, 7:21 am   #6
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Default Re: Vintage WEM Dominator guitar amplifier?

Amazingly Charlie Watkins only died in 2014 at the ripe old age of 91. He was certainly still active about ten years ago, because I emailed him about a WEM Dominator Bass that I was fixing up for my son (he still uses it as a practice cab). He was a really nice, easy to approach guy. Used to share bags of chips with Jimi Hendrix at gigs that he was doing the sound reinforcement for, apparently.

Craig
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Old 15th Apr 2017, 11:41 am   #7
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Default Re: Vintage WEM Dominator guitar amplifier?

Charlie I think was the last one alive of the old boys who started the British amp companies. Whilst WEM amps aren't in the same league as Marshall, theyre still decent amps. I have a Dominator I use as my practice amp and for doing studio stuff. Whilst only 15w it can be heard over the drums and feedsback nicely with plenty of sustain.

Andy.
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Old 15th Apr 2017, 1:42 pm   #8
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Default Re: Vintage WEM Dominator guitar amplifier?

On the WEM diagram C6 and C7 will need to be replaced as they will most likely have caused the cathode resistor to go open circuit and the capacitor to blow. Your diagram shows two capacitors and resistors in the cathodes. They look like 50uf. R16 and R17. C11 and C12. Your amp looks like a mixture of my sample diagram and the diagram we have now got. There might be just one shared resistor and capacitor.
I would first measure all the transformer coils and make sure they are all right.
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Old 15th Apr 2017, 9:40 pm   #9
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Default Re: Vintage WEM Dominator guitar amplifier?

Jesse,

There was also a "Dominator 35 Accordion Amp" which used two BY127 diodes as HT rectifiers, so no EZ81.

I would have been inclined to think that a 22uF, 63V capacitor would be a cathode bypass capacitor and certainly electrolytic. That kind of capacitor is rarely (if ever) used in a valve amplifier to feed the grids of the output valves. It would not only extend the frequency response down to sub-sonic, but also have a poor high-frequency ability. It could have died from many causes, but if I'm right and it is the cathode bypass for both output valves, then there will be a resistor (probably hefty and wire-wound, because it has to cope with the current in both output valves) in parallel with it. Check it out - maybe 100R or so? The aforementioned bypass capacitor should be spaced away from this resistor, as it gets pretty warm at times and could damage the capacitor. Perhaps this is how it died?

You say that the amp doesn't work, apart from a "very low and crackly/distorted output". Can you give a bit more detail, like if the volume control(s) affect the sound or if they are scratchy? Have you any gear which you can use to inject a signal to find out where the fault(s) may lie? You need to be careful around any of the valve anodes, as there will be significant voltages there - not your usual 9V from a stomp-box battery. A bit of care in injecting a signal from an isolated source onto the grids of the output valves first, then the phase-splitter, then the pre-amp valves could focus in on the problem(s).

Do you see any sign of a blue or purple glow in the output valves when turned on? EL84s often do this, especially if they are a bit old and have had a tough life. It doesn't mean that they need replacing yet, but the glow will indicate that the valves are drawing current. If, on the other hand, the anodes are glowing red, then they are drawing far too much current and you need to find out why before they die.

A few people have suggested that you should replace the two capacitors connecting from the anodes of the phase-splitter to the output valve grids, on the logic that they are likely to be leaky, given their age and likelihood of being "waxies". You can check if they are leaking by measuring the voltage on the output valve grids. They should be pretty much at zero volts with respect to ground. If there's more than a few millivolts there, the capacitor is leaky and should be replaced.

I am surprised what you say about the base connections on EL84s. I have a "Wireless World" Radio Valve Data Book that dates back to 1965 and it shows pin 1 as "internal connection" and pin 2 as "g1". Other "ic" pins are 6 & 8. I have never come across any mention of g1 connecting to pin 1, but I suppose you could be right.

These amps are usually pretty simple, but it's easy for me to say that from the other side of the planet. I'm sure that with the help you will get on this forum, you will get it working.

Good Luck, Colin.
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Old 16th Apr 2017, 12:23 am   #10
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Default Re: Vintage WEM Dominator guitar amplifier?

Looking more closely at your pictures it seems that both EL84s share a common cathode resistor and capacitor. The rectangular white block is the the resistor.

With the amp unpowered measure the resistance across your replacement capacitors. According to the schematic this should be 220 ohms. Much higher and the resistor has failed.

Before proceeding any further, are you happy working at voltages of 300+V?

Unplug both EL84s and power up the amp. Measure the voltages on pins 2,7 and 9 of each valve socket. Remember that looking at the top of the valve base the numbers count anti-clockwise. Let us know your results.

Al
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Old 16th Apr 2017, 1:13 am   #11
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Default Re: Vintage WEM Dominator guitar amplifier?

Quote:
Originally Posted by Alistair D View Post
Before proceeding any further, are you happy working at voltages of 300+V?

Unplug both EL84s and power up the amp. Measure the voltages on pins 2,7 and 9 of each valve socket. Remember that looking at the top of the valve base the numbers count anti-clockwise. Let us know your results.
I agree. Be careful with these voltages. Alistair is quite right, but also bear in mind that the connections that you will be able to access will be on the connections to the valve-base below chassis and will be clockwise.

If you have any doubts, DON'T. It can be lethal. Colin.
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Old 16th Apr 2017, 8:34 pm   #12
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Default Re: Vintage WEM Dominator guitar amplifier?

Quote:
Originally Posted by candletears7 View Post
The goal is to have this put back into a WEM style headshell with a replica faceplate.
Jesse,

I was just looking at my copy of Aspen Pittman's useful (if pricy) book, "The Tube Amp Book" and my eyes landed on some pictures of a late 1950s Dominator and a 2002 reissue. They both have V-front cabinets with two speakers, one on each side of the V-front. While not impossible to reproduce, it wouldn't be a trivial job. There is also two-colour covering material with piping between them and plastic-looking details on the speaker-panels. I don't want to sound like a killjoy, but if that's what you want to reproduce, you're taking on a tricky job and I for one would like to see the result - it should be impressive!

My father was a cabinet-maker and I'm no slouch on making guitar-amp cabinets, but I would think several times about making one of these.



Colin.

Last edited by ColinTheAmpMan1; 16th Apr 2017 at 8:37 pm. Reason: Clarification
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Old 19th Apr 2017, 7:03 am   #13
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Quote:
Originally Posted by Dr Wobble View Post
You've got a rare amplifier there, I've never seen a rack mount Wem Dominator and from the pictures you posted it doesn't look like a standard Dominator. I reckon it was specially built by Wem.


Firstly you need to make sure you power supply is ok. Start by building a lamp limiter - http://www.vintage-radio.com/projects/lamp-limiter.html so you don't do any damage.

Somewhere (probably one of those two valves on the metal stand off or are they the two EL84's ? ) , there will be a EZ80 or EZ81. Take it out ,( in fact take all the valves out) and see if you have an AC voltage of around 300v on two of the pins, pin 1 and 7 I think. Or see if the same voltage is present at the transformer on the left. The wires disappear through that hole, where to? See that green wiring? That's the heater wiring, that should read 6.3v.

When you know you have HT and heater voltage, you'll need to check all the electrolytics with an ESR meter. But make sure they are discharged by making a cap discharger, something like this -
https://www.youtube.com/watch?v=gLDCV-n1BDw&t=8s A 100 ohm resistor on a bit of wire will do.

Replace the rectifier valve (EZ8/1) and you'll need to put a signal in using a signal generator (available online) and trace it to the anode of that valve in the can, (pic 1 top middle), then trace all the way through at some point.

A good tool if you intend doing a lot of audio work is one of these - https://www.youtube.com/watch?v=dFj_fa78y7c&t=1133s if you have the cash.

There's more to do, but that should do for now.

Andy.
Good advice mate, Ive been flat out and hope to finally have some time to get into it tonight! Thanks!

Quote:
Originally Posted by Refugee View Post
On the WEM diagram C6 and C7 will need to be replaced as they will most likely have caused the cathode resistor to go open circuit and the capacitor to blow. Your diagram shows two capacitors and resistors in the cathodes. They look like 50uf. R16 and R17. C11 and C12. Your amp looks like a mixture of my sample diagram and the diagram we have now got. There might be just one shared resistor and capacitor.
I would first measure all the transformer coils and make sure they are all right.
Something else I'll try, thanks also!

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Jesse,

There was also a "Dominator 35 Accordion Amp" which used two BY127 diodes as HT rectifiers, so no EZ81.

I would have been inclined to think that a 22uF, 63V capacitor would be a cathode bypass capacitor and certainly electrolytic. That kind of capacitor is rarely (if ever) used in a valve amplifier to feed the grids of the output valves. It would not only extend the frequency response down to sub-sonic, but also have a poor high-frequency ability. It could have died from many causes, but if I'm right and it is the cathode bypass for both output valves, then there will be a resistor (probably hefty and wire-wound, because it has to cope with the current in both output valves) in parallel with it. Check it out - maybe 100R or so? The aforementioned bypass capacitor should be spaced away from this resistor, as it gets pretty warm at times and could damage the capacitor. Perhaps this is how it died?

You say that the amp doesn't work, apart from a "very low and crackly/distorted output". Can you give a bit more detail, like if the volume control(s) affect the sound or if they are scratchy? Have you any gear which you can use to inject a signal to find out where the fault(s) may lie? You need to be careful around any of the valve anodes, as there will be significant voltages there - not your usual 9V from a stomp-box battery. A bit of care in injecting a signal from an isolated source onto the grids of the output valves first, then the phase-splitter, then the pre-amp valves could focus in on the problem(s).

Do you see any sign of a blue or purple glow in the output valves when turned on? EL84s often do this, especially if they are a bit old and have had a tough life. It doesn't mean that they need replacing yet, but the glow will indicate that the valves are drawing current. If, on the other hand, the anodes are glowing red, then they are drawing far too much current and you need to find out why before they die.

A few people have suggested that you should replace the two capacitors connecting from the anodes of the phase-splitter to the output valve grids, on the logic that they are likely to be leaky, given their age and likelihood of being "waxies". You can check if they are leaking by measuring the voltage on the output valve grids. They should be pretty much at zero volts with respect to ground. If there's more than a few millivolts there, the capacitor is leaky and should be replaced.

I am surprised what you say about the base connections on EL84s. I have a "Wireless World" Radio Valve Data Book that dates back to 1965 and it shows pin 1 as "internal connection" and pin 2 as "g1". Other "ic" pins are 6 & 8. I have never come across any mention of g1 connecting to pin 1, but I suppose you could be right.

These amps are usually pretty simple, but it's easy for me to say that from the other side of the planet. I'm sure that with the help you will get on this forum, you will get it working.

Good Luck, Colin.

This is brilliant Colin, thanks so much.
Can't wait to get the kids off to bed and get stuck in to this amp!

Quote:
Originally Posted by Alistair D View Post
Looking more closely at your pictures it seems that both EL84s share a common cathode resistor and capacitor. The rectangular white block is the the resistor.

With the amp unpowered measure the resistance across your replacement capacitors. According to the schematic this should be 220 ohms. Much higher and the resistor has failed.

Before proceeding any further, are you happy working at voltages of 300+V?

Unplug both EL84s and power up the amp. Measure the voltages on pins 2,7 and 9 of each valve socket. Remember that looking at the top of the valve base the numbers count anti-clockwise. Let us know your results.

Al
Al,
I've not worked on amps too much, but as long as I know where the high voltage is and don't touch ground I'm confident.
Oh and keep one hand in my pocket!

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Quote:
Originally Posted by candletears7 View Post
The goal is to have this put back into a WEM style headshell with a replica faceplate.
Jesse,

I was just looking at my copy of Aspen Pittman's useful (if pricy) book, "The Tube Amp Book" and my eyes landed on some pictures of a late 1950s Dominator and a 2002 reissue. They both have V-front cabinets with two speakers, one on each side of the V-front. While not impossible to reproduce, it wouldn't be a trivial job. There is also two-colour covering material with piping between them and plastic-looking details on the speaker-panels. I don't want to sound like a killjoy, but if that's what you want to reproduce, you're taking on a tricky job and I for one would like to see the result - it should be impressive!

My father was a cabinet-maker and I'm no slouch on making guitar-amp cabinets, but I would think several times about making one of these.

Colin.
Ah I'm thinking more of a WEM style head, not the V Front cab. That's definitely too hard!
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Old 19th Apr 2017, 2:32 pm   #14
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Ok, I've got some voltages here for reference:

Looking at the two EL84's , left and right, counting pins anti clockwise:

left EL84, pin 2 - 0v right EL84, pin 2 - 0V
left EL84, pin 7 - 383v right EL84, pin 7 - 379v
left EL84, pin 9 - 379v right EL84, pin 9 - 383v

http://i304.photobucket.com/albums/n...psk1yrfc0h.jpg

Looking at the three ECC83/12AX7's in the foreground

left 12AX7, pin 1 - 334v middle 12AX7, pin 1 - 351v right 12AX7, pin 1 - 335v
left 12AX7, pin 6 - 336v middle 12AX7, pin 6 - 353v right 12AX7, pin 6 - 336v



FX Loop 12AX7
Pin 6 - 337v
Pin 7 - 80v

http://i304.photobucket.com/albums/n...psgcb5ots5.jpg

Looking at what I assume (never assume!) is the power transformer (?)

light green wire - 385v
pink wire - 385v
Dark green wire - 384v

(is there a pinout for transformers? I have no clue!)

Thanks so much for the previous pointers guys - If these valve voltages check out then I guess I'm going to have to start looking at tracing the signal.
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Old 19th Apr 2017, 4:39 pm   #15
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Hi Jesse,

The voltages you have on the output valves seem ok and the 0V on the control grids suggests that the coupling capacitors between those control grids and the phase-splitter are not leaking too much, if at all. It might be an idea to check at a lower range on your multi-meter to see if there are a few (or more) millivolts on those control grids. The voltages on the cathodes of the output valves (pin 3; g3, the suppressor grid goes here too) would be interesting to know, too. Just as a point of interest, do you hear anything from the loudspeaker when you touch the output valve control grids (pin 2) with your probe? You should hear something. I hope you aren't checking without a load on the output transformer!

The voltages on your 12AX7 pre-amp valves (pins 1 & 6) are the two anodes and seem reasonable, too. What does have me bothered is the conditions of the FX loop 12AX7; while the second anode (pin 6) is about right, 80V on the corresponding control grid (pin 7) seems odd, unless this valve is in a cathode-follower configuration. This wouldn't be unusual, but if you could tell us the voltage on the corresponding cathode (pin 8), that might shed a little more light. Also, I do hope that is +80V on the control grid, because -80V would be cutting that valve off totally. What is happening on the anode, control grid and cathode of the other triode in that glass envelope? Do they have any voltages or are they all connected to ground and therefore unused?

I think the transformer voltages you quote must be the output transformer, not the power transformer. There aren't any common rules for the colours of wires on any transformer that I know of. The manufacturer usually supplies a piece of paper with that information. However, you should see something like your 380-odd volts on the centre-tap of the output transformer primary (it's the full HT, probably unsmoothed and straight off the cathode of the HT rectifier), with perhaps a few volts less on the wires going to the output valves (via a couple of relatively low value resistors, probably). You won't see much DC of significance on the secondary of the output transformer, because its all AC when its working. There may well be connections for different impedance loudspeakers on the secondary - 8 Ohm and 15 Ohm are common. The output transformer is likely to be the smaller of the two transformers, but you just may have an HT smoothing choke, which looks a bit like a small transformer, but only has two wires going to it. I'm assuming also that there is no built-in reverberation, so there isn't a reverb transformer.

The mains transformer is likely to have a multitude of connections - more than the typical five or six that an output transformer has. The primary mains winding will have a 0V and possibly a number of tappings for differing mains voltages. It might even have a split primary so that it can operate off the typically 240V of European mains and the 120V of US mains. There might even be some kind of selector for the right mains voltage. It will also have the HT winding - something like 300V-0-300V, which will go to the anodes of the HT rectifier (the 300V ones) and chassis (the 0V one). There will also be at least one heater winding at 6.3V, which will be connected by a (hopefully) twisted pair to all of the valve heater pins in parallel. The heater winding would also ideally have a centre-tap, but maybe not. The heater-winding wires will be heavy-gauge wire, (possibly solid, but maybe stranded) - all three if there are three. If there is no centre-tap, then one of them might connect to chassis (not a good idea), or there will be a couple of resistors (~100 Ohm, 1W or so) connecting each side to chassis. There may be a separate heater winding to feed the heater of the HT rectifier-valve. This one might be 6.3V, or maybe 4V, depending on the type of rectifier in use. It is usually necessary because (a) the rectifier operates off a different heater voltage than the 6.3V for the other valves, or (b) the heater-cathode voltage of the rectifier would be exceeded if there was a common heater winding. Of course, if you have a semiconductor HT rectifier, you don't need a heater winding for it.

Bear in mind that I am generalising here; the info I'm giving you doesn't apply to all valve amplifiers, but I'm trying to give typical conditions for a valve guitar amplifier.

Good Luck, Colin.
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Old 19th Apr 2017, 7:28 pm   #16
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Default Re: Vintage WEM Dominator guitar amplifier?

It looks to be a Dominator Mk III but not quite.
Pictures here http://www.vintagehofner.co.uk/brita...tor/dom24.html
Circuit diagram here http://www.vintagehofner.co.uk/brita...cs/dommk3.html.

There are some obvious modifications such as the yellow cap routed over other components.

The 12AX7 voltages also all look too high.

Seems to me the first task is to identify what it is then decide what you want it to be!

Last edited by PJL; 19th Apr 2017 at 7:34 pm.
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Old 19th Apr 2017, 11:34 pm   #17
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Default Re: Vintage WEM Dominator guitar amplifier?

The EL84 voltages look OK. The other voltages look a bit high though. The reason I had suggested removing the EL84s before measuring the voltages was simply for ease of access. The other voltages would be OK if you had also measured them with the valves removed. Is that the case?

Did you measure the resistance of the cathode resistor(post#10)? If the value is around 100/200R then power up the amp with the EL84s fitted and measure the voltage across it.

Al
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Old 20th Apr 2017, 11:04 am   #18
PJL
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Default Re: Vintage WEM Dominator guitar amplifier?

As it doesn't work, I would recommend printing a copy of the circuit linked to above and marking up each difference you find in your amp and checking each resistor value with a DMM as you go. It looks like it has some modification and if you want it to be a standard Dominator III you will want to reverse out any changes you find. I would also recommend replacing the 4x 100K anode load resistors and the 3x 47K used in the phase splitter as the values change with time and they can cause crackle/noise.

Last edited by PJL; 20th Apr 2017 at 11:10 am.
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Old 20th Apr 2017, 3:41 pm   #19
ColinTheAmpMan1
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Default Re: Vintage WEM Dominator guitar amplifier?

Quote:
Originally Posted by Alistair D View Post
The other voltages look a bit high though.
Thinking about it again, you could be correct, Al. It rather looks as though all of the 12AX7s aren't drawing any current! Can you see if the heaters are lighting up, Jesse? There should be a visible orange glow seen inside all of the valves and the envelopes should be hot when warmed up - don't check this with your fingers, though, as they should be hot enough to give you a nasty burn.

If that circuit diagram that PJL located is correct, then it will help a lot.

The comment on replacing the 100k 12AX7 anode resistors and the three 47k phase-splitter resistors may be a bit unnecessary, however. If all of these resistors are carbon composition, they were probably a bit noisy when they were put in the amp. In that case, they will still be a bit noisy, but may well have drifted in value; it's worth checking. If they are carbon composition, they will in all likelihood have been 10% tolerance so there's no need to expect exact values (100k, 10% can be 90-110k). It they are metal film 1% resistors, then that is a different story. There are all sorts of opinions about using carbon composition, carbon film or metal film resistors in guitar amps. You shouldn't replace original carbon composition resistors in a valuable vintage amp with metal film resistors, for instance, unless you want to ruin the resale value.

As PJL has said, if you can check as many voltages as you can and check the resistor values (you might need to unsolder one leg to avoid errors introduced by parallel components, such as the 200pF capacitors paralleling the 100k anode resistors in what I assume is a "bright" channel), that will help a lot.

Regards, Colin.
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Old 21st Apr 2017, 6:46 am   #20
Diabolical Artificer
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Default Re: Vintage WEM Dominator guitar amplifier?

Attached below is the Dominator Custom schematic. As you see it's a pretty bog standard guitar amp. the first ECC83 shares a 56k anode R on the two triodes as well as a cathode R to give you two inputs. With HT @ 250v and a cathode R of 1k5 you should get something like this for both triodes pin 1 anode 165v pin 2 grid 0v pin 3 cathode 1.1 to 1.5v pins 4+5 3.3v pin6 165v, pin 7 0v pin 8 1.1v ish pin 9 3.3v.

Next comes a simple tone section followed by the phase splitter which is an ECC83 again, so anodes should be at about 200v grids 60v ish cathodes 1 or two volts higher than the grids.

The EL84 anodes and definately the screens you measured are too high but your HT at the transformer seems too high too at nearly 400v. the pink wire which I suspect is the centre tap should read 0v with your DMM black lead to ground. The EL84 in push pull for an output of around 12 - 15w needs about 320v - 350v tops HT. The cathodes should be at around 7 - 8 v and the screen should be at 300v tops! there is something wrong there.

As I suggested start with no valves in, we need to establish the HT first and make sure you have 6.3v on the heaters, take readings with your black meter to ground. It could be the mains tfmr primary is on the wrong tap and/or you have no heater supply.

As you have a non standard Dominator be cautious as it will probably differ from your average Dominator. I wouldn't put it in a different cab, you have a very rare amplifier there, maybe a one off, it's a piece of British amplifier manufacturing history. If you want to use you just need a speaker cab with a 15" Celestion G15 in it. These can be found for very little, I have five, got each for around £20.

Andy.
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