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Old 21st Apr 2017, 7:17 am   #21
PJL
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Default Re: Vintage WEM Dominator guitar amplifier?

Quote:
Originally Posted by Dr Wobble View Post
Attached below is the Dominator Custom schematic.Andy.
Post #16 pictures confirm this is a Dominator Mk III and includes a link to the correct circuit diagram.

The circuit has been modified and now includes an additional valve and various switches located on the back panel. There are all kinds of possibilities including a scenario where it is intended to work with an external effects unit. It needs to be wired back to the original circuit before any progress can be made on fault finding.

Last edited by PJL; 21st Apr 2017 at 7:26 am.
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Old 21st Apr 2017, 10:28 am   #22
candletears7
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Default Re: Vintage WEM Dominator guitar amplifier?

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Originally Posted by Alistair D View Post
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.
Hi Al, sorry this has me confused.
Are you asking me to measure resistance across the paralleled capacitor that I replaced?
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Old 21st Apr 2017, 11:07 am   #23
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Default Re: Vintage WEM Dominator guitar amplifier?

Yes. If you look at the schematic there is a resistor in parallel with those caps. It is in fact the cathode resistor for the pair of EL84s. The actual resistor is the white square block that is beside the caps on the PCB. The reason for doing this is that for a 63Volt capacitor to explode the voltage across the resistor must have risen to a level that is likely to damaged it as well as the original capacitor.

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Old 21st Apr 2017, 11:27 am   #24
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Default Re: Vintage WEM Dominator guitar amplifier?

Ok, there seems to be a mountain of advice and tips here for me to work through, for which I am hugely grateful!
Please bear in mind you’re all dealing with someone who is pretty inexperienced with amps. I can build and repair most effect pedals, can read schematics, identify power and ground through a circuit, so I’m confident (fingers crossed I won’t shock myself), but I am unaware of terminology relating to valve amps such as terminals on transformers, commonly understood resistors, capacitors and their functions within valve amps.
I actually wish this was a turretboard/point to point amp as I could trace the signal much easier than on a pcb, but then again, I am lucky to find a real WEM circuit in Australia and am keen as mustard to get her firing.
I think, approaching from a logical point of view, I need to develop a step by step approach with this amp if I’m going to trouble shoot it effectively.
I’ll go through each post and extract what I believe I should do.
(Once again, my sincere thanks to everyone who has chimed in with your experience!)

1. I will check the pin 3 voltages on the EL84’s again. Yes, I have the amp wired up to a 16 ohm Celestion G12-50 to listen to.

2. I’m sorry Colin, by this:
“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 literally have no idea what you mean!  Do you mean other pins on the FX Loop valve?

3. I think you’re right. That smaller transformer must be the output transformer. I don’t believe this amp uses a choke, in fact I know the 18w Marshall (which the WEM Dominator is, as Marshall basically took the circuit and stuck it in a Marshall badged box) doesn’t use a choke.

Quote:
Originally Posted by ColinTheAmpMan1 View Post
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 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.
4. I really wish I knew what most of this meant. I think I need to learn some basic amp theory before attempting to understand this, I’m sorry.

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Originally Posted by Alistair D View Post
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.
5. Yes – I’ve removed all the valves, measurements are with valves removed. Is there an easy way to identify from any of the pics I’ve posted which one is the cathode resistor that I need to measure?

Quote:
Originally Posted by PJL View Post
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.
6. Should I just replace those components first, or wait until I’ve checked voltages and the cathode resistor mentioned above?

Quote:
Originally Posted by ColinTheAmpMan1 View Post
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.
This I can do!

Quote:
Originally Posted by Dr Wobble View Post
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.
Andy,
If you can read that schematic then you Sir, have the eyes of a hawk. I can’t even find the 56k resistor off the first ECC83!
7. Ok, so I need to find 6.3v somewhere either on the power transformer that also connects to the heaters of the EL84’s?

Last edited by candletears7; 21st Apr 2017 at 11:33 am.
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Old 21st Apr 2017, 11:32 am   #25
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Default Re: Vintage WEM Dominator guitar amplifier?

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Originally Posted by Alistair D View Post
Yes. If you look at the schematic there is a resistor in parallel with those caps. It is in fact the cathode resistor for the pair of EL84s. The actual resistor is the white square block that is beside the caps on the PCB. The reason for doing this is that for a 63Volt capacitor to explode the voltage across the resistor must have risen to a level that is likely to damaged it as well as the original capacitor.
Right, gotcha. I'll post back.
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Old 21st Apr 2017, 4:51 pm   #26
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Default Re: Vintage WEM Dominator guitar amplifier?

Quote:
Post #16 pictures confirm this is a Dominator Mk III and includes a link to the correct circuit diagram.
I don't think this is quite confirmed. The info came from PJL and not Jesse (the OP). The circuit diagram also shows a tremolo in one channel with speed and depth controls. The OP hasn't mentioned that his amp has a tremolo or not.

Quote:
Originally Posted by Dr Wobble View Post
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.
I would agree that it is a simple guitar amp, but those shared anode and cathode resistors are not particularly "bog standard". Fender used to use common cathode resistors, but shared anode resistors? Even then, the shared cathode resistors were in different channels (except in reverb circuits), rather than the same one.

Quote:
Originally Posted by Dr Wobble View Post
HT at the transformer seems too high too at nearly 400v. the pink wire which I suspect is the centre tap should read 0v
As I said, I think the OP was looking at the output transformer, not the mains transformer. If it was indeed the mains transformer, then nearly 400V on the centre tap would be startling, but if the pink wire is the centre tap of the output transformer, the voltage would be credible.

Quote:
Originally Posted by Dr Wobble View Post
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.
Agreed. I was assuming that the OP hadn't read his multimeter too carefully - it happens to the best of us. The OP didn't give a voltage for the cathode at pin 3.

Quote:
Originally Posted by Dr Wobble View Post
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.
Again, I agree.

Quote:
Originally Posted by Dr Wobble View Post
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.
It is certainly non-standard, but that doesn't imply that it was originally made that way. Perhaps some capable engineer re-jigged it at some time. I am inclined to think that a 12" Celestion is more likely - 15" speakers were generally used either in much more powerful amps, or in amps intended for bass guitar use. Now if I knew a source of 12" vintage Celestion speakers at 20 apiece, I would be a happy chappy...

Quote:
Originally Posted by candletears7 View Post
1. I will check the pin 3 voltages on the EL84’s again. Yes, I have the amp wired up to a 16 ohm Celestion G12-50 to listen to.
Pin 3 on an EL84 is the cathode, which is also internally connected to the suppressor grid (g3).

Quote:
Originally Posted by candletears7 View Post
2. I’m sorry Colin, by this:
“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 literally have no idea what you mean!  Do you mean other pins on the FX Loop valve?
Yes. The voltages that you gave us were on just one of the triodes of the 12AX7, which is a double triode. If the second triode in the envelope is not being used for anything, then all of its electrodes should be connected to chassis ground. If the other triode is being used for something, then there will be voltages present that you can measure. Since this is a part of the circuit that we don't have a diagram for, we are guessing as to what is there. I have a few ideas from a knowledge of FX loops, but I can't assume anything yet. A cathode-follower in an FX loop can be expected, that's for sure. I'm guessing that maybe you don't know what a "cathode follower" is?

FYI and at the risk of stating what almost everyone else on this thread knows, the pin-out for a 12AX7 (or any of that family of double triodes or its equivalent ECC8x series) is:
P1=anode of triode 1, P2=control grid of triode 1, P3=cathode of triode 1, P4= heater, P5=heater, P6=anode of triode 2, P7=control grid of triode 2, P8=cathode of triode 2, P9= centre-tap of heater.
If you notice, there are three pins which connect to the heater; if P4 and P5 are connected together, then the other heater wiring will go to P9 and the valve will be operating with 6.3V. This is the most common connection for British-made gear. It is possible to ignore P9 completely and connect the heaters to P4 and P5, but the heater voltage needed would then be 12.6V and more unusual.

If you don't know this, then I think I should probably advise you to pass the amp over to someone who does, before you give yourself a serious shock and/or destroy something.

Quote:
Originally Posted by ColinTheAmpMan1 View Post
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 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.
Quote:
Originally Posted by candletears7 View Post
4. I really wish I knew what most of this meant. I think I need to learn some basic amp theory before attempting to understand this, I’m sorry.
If you really don't know what I am talking about, then I think I must advise you that you need to consult someone that does. These voltages and currents can be lethal.

Quote:
Originally Posted by candletears7 View Post
Andy,
If you can read that schematic then you Sir, have the eyes of a hawk. I can’t even find the 56k resistor off the first ECC83!
7. Ok, so I need to find 6.3v somewhere either on the power transformer that also connects to the heaters of the EL84’s?
The 56k resistor is R4 on the circuit diagram and goes between the anodes of the V1a and V1b triodes and the HT rail marked as "A" and 250V. The circuit diagram is a bit small and blurred, but to those of us who have been reading circuit diagrams of valve equipment for decades, it isn't too difficult.
As I said before, the 6.3V heater wiring should be a twisted pair and so fairly obvious.

Regards, Colin.
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Old 22nd Apr 2017, 5:42 am   #27
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I think we are overwhelming the OP here, with different advice in terms he can't understand, I'll drop down a gear.

Right Mr Candle... when you have a dodgy pedal, whats the first thing you do? Check the battery right? So in this case we want to check out your amplifier's power supply or PSU for short. So we whip out all of the valves to make sure your mains transformer (tfmr short hand) is ok. Make sure you know where each valve goes, draw a simple diagram.

So, pad of paper and pen, valves out, power on, left hand on top of your head or in your pocket, probe in your right hand, DMM/meter on AC volts, black probe clipped to chassis.. See that tfmr on the left pic 1, what do get? You should get something like 230/40v ( mains in Oz same as UK) on the brown wire, CAREFUL, THIS IS LIVE MAINS, then you should have around 230v 0v 230v or maybe just one 230v, then you should have either 3v 0v 3v or 6v ish. If so mains tfmr is ok.

I'm trying to walk the OP through here step by step, I know it could be a dodgy cap or resistor, but he's never worked on a valve amp before.

Next power off, make sure those big caps are discharged with the discharger I told you about. If your going to work on this amp, lets be safe. So discharge big cap, meter on DC volts, black lead clipped to chassis, red probe on cap +, you should have a few volts, about 3 or 4v, discharge again. See that other tfmr on the top right corner pic one? Meter on ohm's, pad of paper and pen, black lead in one hand, red in the other. See that red and white stripped wire and the yellow one? What do you get there? Take some other readings, you should get readings of about 5 - 100 ohms as in the the DCR table in the attachment below. Post your readings here.

Were mostly in agreement Colin, the G15 was in the Dominator bass model but sounds well on the guitar version. My Dominator has a 12" Goodmans. I managed to pick up 4 NOS Greenback G12's for less than a fiver, the Gods were smiling on the casa del Wobble that day. Ah, so he was reading the OPT, those readings are still high.

Anyhoo, back to the amp, your right using one anode R is unusual, I guess WEM were being cheapskates. My WEM Dominator has two normal inputs with a tremelo that can be switched in by a FS. I put a little SW on it for home use. It also has two mic input's. Looking at the pics in post one, it looks like we have an input stage, top valve in the screened can, whats the betting it's an EF86? I think that will be a mic input. then we may have a normal/guitar in, phase splitter and maybe a rectifier.

Hope that helps, Andy.
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Old 17th Jun 2017, 10:24 pm   #28
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Hey guys - apologies for my absence on this one! Lots of work outside of music and lots of travel :/

I have a good friend who is more knowledgeable than me who is back in the country in a week or so. I'm actually off overseas again for a couple of weeks, so the plan is in a month or so we'll sit down together and get this little amp sorted. I'm very keen to get it up and running.
I'll absolutely use the information in this thread to help guide us, but for now I'll have to put this on "Pause" as life takes over!
Will report back when I'm able - thanks again everyone for your input and advice.
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Old 18th Jun 2017, 2:37 pm   #29
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Hi Candletears7,
It's good to hear from you again; I was beginning to think you had fried yourself! I'm also glad that you are getting assistance from someone who has more experience in high voltage circuitry.
I have taken a few belts in the past, mainly from our 250VAc mains, but I did once get a 2kV one from a leaky Z-modulation input capacitor on a home-made oscilloscope. We all get shocks, but no-one wants to get the one that kills you. Caveat emptor.
Good Luck and keep us posted, Colin.
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Old 19th Jun 2017, 1:28 pm   #30
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Default Re: Vintage WEM Dominator guitar amplifier?

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Originally Posted by colintheampman1 View Post
Quote:
post #16 pictures confirm this is a dominator mk iii and includes a link to the correct circuit diagram.
i don't think this is quite confirmed. The info came from pjl and not jesse (the op). The circuit diagram also shows a tremolo in one channel with speed and depth controls. The op hasn't mentioned that his amp has a tremolo or not.
The link to the correct circuit is in post #16 itself and does not include a tremolo. The circuit board, components and controls look identical but the amp has been heavily modified when it was installed in the chassis. It either needs to be changed back to the original Mk III design or the circuit modifications need to be documented.
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