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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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21st Apr 2017, 8:17 am | #21 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Vintage WEM Dominator guitar amplifier?
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 8:26 am. |
21st Apr 2017, 11:28 am | #22 | |
Diode
Join Date: Apr 2017
Location: Melbourne, Victoria, Australia.
Posts: 8
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Re: Vintage WEM Dominator guitar amplifier?
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Are you asking me to measure resistance across the paralleled capacitor that I replaced? |
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21st Apr 2017, 12:07 pm | #23 |
Nonode
Join Date: Jan 2010
Location: Edinburgh, UK.
Posts: 2,008
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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.
Al
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I won't tell you how I discovered that. |
21st Apr 2017, 12:27 pm | #24 | |||||
Diode
Join Date: Apr 2017
Location: Melbourne, Victoria, Australia.
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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:
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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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21st Apr 2017, 12:32 pm | #25 | |
Diode
Join Date: Apr 2017
Location: Melbourne, Victoria, Australia.
Posts: 8
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Re: Vintage WEM Dominator guitar amplifier?
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21st Apr 2017, 5:51 pm | #26 | |||||||||||
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Vintage WEM Dominator guitar amplifier?
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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:
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As I said before, the 6.3V heater wiring should be a twisted pair and so fairly obvious. Regards, Colin. |
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22nd Apr 2017, 6:42 am | #27 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,670
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Re: Vintage WEM Dominator guitar amplifier?
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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Curiosity hasn't killed this cat...so far. Last edited by AC/HL; 22nd Apr 2017 at 2:39 pm. Reason: As requested |
17th Jun 2017, 11:24 pm | #28 |
Diode
Join Date: Apr 2017
Location: Melbourne, Victoria, Australia.
Posts: 8
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Re: Vintage WEM Dominator guitar amplifier?
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. |
18th Jun 2017, 3:37 pm | #29 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Vintage WEM Dominator guitar amplifier?
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. |
19th Jun 2017, 2:28 pm | #30 | ||
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
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Re: Vintage WEM Dominator guitar amplifier?
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