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Old 21st Apr 2017, 12:27 pm   #24
candletears7
Diode
 
Join Date: Apr 2017
Location: Melbourne, Victoria, Australia.
Posts: 8
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.

Quote:
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 12:33 pm.
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