[ColinTheAmpMan1]

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.

Originally Posted by Dr Wobble

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.

Originally Posted by Dr Wobble

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.

Originally Posted by Dr Wobble

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.

Originally Posted by Dr Wobble

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.

Originally Posted by Dr Wobble

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...

Originally Posted by candletears7

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).

Originally Posted by candletears7

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.

Originally Posted by ColinTheAmpMan1

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.

Originally Posted by candletears7

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.

Originally Posted by candletears7

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.