shaunf wrote:What do you reckon, did I misunderstand this diagram? I must admit, I just saw the Standby switchand just went from there, but now looking at it more closely now that you've opened my eyes, it would seem that the standby switch in the diagram is in a different circuit position to where it is in the Express. Is that right? and is that where I went wrong?
The standby switch is in the same location for the way Dana has drawn it in the installation document you posted as well as on the Express schematic. My guess, but I can't say for sure without studying your photos further is that you had either the VVR upstream of the 80uF cap (attempting to source current directly off the rectifier) or somehow otherwise incorrectly installed. In the way shown in Dana's document, the reservoir cap will fully charge upon powering the amp and then the cap will be immediately available to the VVR and the rest of the amp when the standby is thrown.
If that's the case shouldn't the VVR B+ in and out be between the standby switch and the 25w resistor on the PS Board, instead of how it is between the multicap can and standby?
No, because (if I am understanding your question correctly) that will mean that you would regulating the screens and the preamp, not the plate voltage. I think you've got a spatial block in your mind's eye on this. You want to regulate the WHOLE amp. To do this, you must have the VVR upstream of all B+ points. The OT centertap connection is B+1. If you take out the standby just to simplify the drawing (I think that is somehow throwing you), just make the amp look like this:
If you put the Standby back in to the above, it should be inserted just after VVR B+OUT. This will then go to the standby. From the Standby you will have TWO leads going out. The first will go to the OT's centertap (B+1) and the second will go to the 1k 25W resistor (on it's way to B+2). I think this branching off from the standby switch is what is messing with your head.
Colossal wrote:
The standby switch is in the same location for the way Dana has drawn it in the installation document you posted as well as on the Express schematic. My guess, but I can't say for sure without studying your photos further is that you had either the VVR upstream of the 80uF cap (attempting to source current directly off the rectifier) or somehow otherwise incorrectly installed. In the way shown in Dana's document, the reservoir cap will fully charge upon powering the amp and then the cap will be immediately available to the VVR and the rest of the amp when the standby is thrown.
If that's the case shouldn't the VVR B+ in and out be between the standby switch and the 25w resistor on the PS Board, instead of how it is between the multicap can and standby?
No, because (if I am understanding your question correctly) that will mean that you would regulating the screens and the preamp, not the plate voltage. I think you've got a spatial block in your mind's eye on this. You want to regulate the WHOLE amp. To do this, you must have the VVR upstream of all B+ points. The OT centertap connection is B+1. If you take out the standby just to simplify the drawing (I think that is somehow throwing you), just make the amp look like this:
If you put the Standby back in to the above, it should be inserted just after VVR B+OUT. This will then go to the standby. From the Standby you will have TWO leads going out. The first will go to the OT's centertap (B+1) and the second will go to the 1k 25W resistor (on it's way to B+2). I think this branching off from the standby switch is what is messing with your head.
Thanks man! That makes perfect sense to me. But then look again at the diagram in those installation instructions I uploaded on the previous page, I think. If I take what you have described above then, in the case of the Express amp, the Standby switch is actually on the B+ Out connection on the VVR then, not B+ In as in that diagram. That is what was confusing me.
shaunf wrote:But then look again at the diagram in those installation instructions I uploaded on the previous page, I think. If I take what you have described above then, in the case of the Express amp, the Standby switch is actually on the B+ Out connection on the VVR then, not B+ In as in that diagram. That is what was confusing me.
You can do it either way, they are electronically equivalent so long as anything with B+ on it is getting fed from the VVR B+OUT. The standby is really irrelevant and even unnecessary (only a relic of convenience, not necessity). The only difference is the location of the standby switch NOT what is under the influence of the VVR. The way Dana is showing it 1) allows the reservoir cap to be fully charged when the amp is powered up and before any current is demanded 2) the whole amp is being regulated.
My guess is that you had the VVR installed prior to the reservoir cap which was shutting the amp down.
I would do it the way Dana has shown it in the drawing. This is a proven concept with thousands of installs in everything from the simplest SE amp to 50W push-pull amps. First however, I would verify that your amp functions perfectly and that voltages at every point in the amplifier (every plate, grid, and cathode for every stage) are where they should be, then add the VVR into the equation.
shaunf wrote:But then look again at the diagram in those installation instructions I uploaded on the previous page, I think. If I take what you have described above then, in the case of the Express amp, the Standby switch is actually on the B+ Out connection on the VVR then, not B+ In as in that diagram. That is what was confusing me.
You can do it either way, they are electronically equivalent so long as anything with B+ on it is getting fed from the VVR B+OUT. The standby is really irrelevant and even unnecessary (only a relic of convenience, not necessity). The only difference is the location of the standby switch NOT what is under the influence of the VVR. The way Dana is showing it 1) allows the reservoir cap to be fully charged when the amp is powered up and before any current is demanded 2) the whole amp is being regulated.
My guess is that you had the VVR installed prior to the reservoir cap which was shutting the amp down.
I would do it the way Dana has shown it in the drawing. This is a proven concept with thousands of installs in everything from the simplest SE amp to 50W push-pull amps. First however, I would verify that your amp functions perfectly and that voltages at every point in the amplifier (every plate, grid, and cathode for every stage) are where they should be, then add the VVR into the equation.
Thats good advice, and I will certainly do that.
You've opened my eyes to a whole comedy of errors when it comes to the VVR. When I look back, I had B+ IN connected to the standby switch, and B+ OUT to the multicap which is the wrong way around, although I'm not sure what effect this would have had in terms of the short or popping the MOSFET. Another concerning thing is that I've since seen people referring to some plastic insert or something that goes into the hole of the MOSFET to insulate it from the bolt. Now not one of the unopened spare mosfets I have has any such insert. They only have mica insulating pad in the packet and that's it. Do you think me simply mounting it with a metal bolt and the mica pad could have been the cause of the short?
Just to be safe, I've ordered some nylon bolts and nuts to mount it when I get my replacement Zener diode early next week.
Hi Shaun - I've installed quite a few of these & have used a steel nut / screw
to fix the mosfet to the chassis. If you look at the back face of the mosfet you'll see that the mounting hole is fully insulated.
As long as you don't over-tighten the screw (crack the plastic housing), all you need is the insulator pad that comes with it.
Good Luck - you'll like the VVR control.
Hey guys, I I have just finished my first express build. And my amp is also failing the light bulb limiter test. In the same way as the original poster.
I have checked all my wiring and everything seems fine.
My amp also has the vvr3 wired in, set to regulate the whole amp. It's wired after the standby switch.
I'm testing with no tubes installed, and standby switch off.
I haven't tested any voltages yet, as I haven't been keen to keep it turned on. (the on light does go on to when the amp is powered up.
Thanks in advance guys. I'm looking forward to getting this baby running and sharing some pics etc.
Miket wrote:I'm testing with no tubes installed, and standby switch off.
I haven't tested any voltages yet, as I haven't been keen to keep it turned on. (the on light does go on to when the amp is powered up.
For what you have so far, favourite causes would be wiring errors on the PT or mains switch; shorting out un-used secondaries on the Power transformer by not capping them off properly; shorting the heater supply by misunderstanding the connections on the pilot light (is this what you mean by the on light? in which case, it's less likely); rectifier diodes backwards / failed or 1st reservior cap backwards
I guess the next step (if another visual check doesn't find anything) is to disconnect and cap off all the secondaries on the power transformer and see if the light bulb behaves itself.
You could do a lot worse than re-trace your steps a little and follow Paul Ruby's start-up guide (along with keeping the light bulb limiter in circuit):
I have been doing some more testing, but no changes so far. The light dims for a second, but then glows fairly bright, it's a 70w globe if that's relevant at this point.
I have checked the unused pt wires, and they are all insulated separately.
I then unwired the high voltage secondaries from the ps board, unwired the varistor and pilot light. Then removed the heater wiring after the first power tube socket ( so the heater filaments terminate at the first socket).
I have measured the following voltages ;
Wall voltage at the switch - 238v (then drops to 194v when switched on).
Hv secondaries - 480v
Heater filaments 5.4v (2.75v between each one and it's ct) this seems low doesn't it? I'm using marstran express transformers.
The only other thing I can think to mention is that the output transformer secondaries all seem to be shorted together, when I probe the black common with the 4/8/16 ohm taps, they all read 0.00 resistance on the meter, I'm fairly sure the selector switch is wired correctly, is there some discrepancies with the shorting jacks I may have overlooked?
I'll upload some photos for you guys to have a look at.
Thanks again guys.
Mike, Check your power switch.. What is the extra white line doing, going from the left middle to the top right? Are you not creating a short? And what's the extra red doing?
Output secondaries are indeed shorted by measurement... same as the PT primaries.. well actually they measure 4 ohms or so...
also is your Mosfet properly insulated? I see you use a normal metal screw.. measure continuity between the mosfets plate and the chassis.. pretty good chance it's blown. I would advise you to build the amp first without the VVR and any other modifications...
Edit: those primary wires... you have insulated them nicely... Can I conclude that you insulated everything seperately? Can I also conclude that you didn't connect two wires together? You have 2 wires correctly soldered to the power switch. 120 and 120 I presume.. but you must also solder the two 0-wires together that belong to the two 120 wires.
Edit: heat grease and an insulator is really not necessary for the 1k chokeresistor. Not that it really matters anyway.
Miket wrote:I have been doing some more testing, but no changes so far. The light dims for a second, but then glows fairly bright
Is that before or after you did the following?
I then unwired the high voltage secondaries from the ps board, unwired the varistor and pilot light. Then removed the heater wiring after the first power tube socket ( so the heater filaments terminate at the first socket).
Wall voltage at the switch - 238v (then drops to 194v when switched on).
Hv secondaries - 480v
Heater filaments 5.4v (2.75v between each one and it's ct) this seems low doesn't it?
The heater voltage is about right, taking into account the reduced input voltage. Is the lightbulb still coming on brightly?
(I think 70W is plenty on 240V, btw.)
I don't think there's necessarily anything wrong with the OT - the secondaries should measure a few ohms, however the OT resistances (and VVR) are irrelevant at the moment, as you said that you had the problem even with the standby switch open.
Hey guys, great news!
It's all working now. It turns out my interpretation of the light bulb being bright wasn't quite up to scratch. There was never any short, just an over cautious builder!
Lesson learned.
Thanks so much for your replies and help!
The lower heater voltage was indeed caused by the light bulb.
The heat grease and insulator is indeed not necessary for the choke resistor, but I wanted to go all out! The tube of grease cost me 12$!
The mosfet is happy as Larry, the vvr3 is working well.
Kassie, the extra wires you speak of are the varistor, the red you can see is its body glued to the side of the switch.
I will start a new thread tomorrow for my build, I have been itching to share it will like minded folk! Sadly my wife doesn't appreciate my tonal endeavours!
