Bare Knuckle Pickups Forum
Forum Ringside => Tech => Topic started by: Transcend on October 28, 2010, 06:44:34 PM
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It was working absolutely fine the last time i used it.
It powers up but the valves do not heat up at all.
My first thing was to check the fuses both are fine
i then replaced all the valves and alas still no heat to them.
Does anybody have any idea what it could be and how much work it is to fix it.
I'm obviously not going to go inside the amp myself but is it going to be a lot of work and a pricey fix?
I have been toying with the idea of getting a JVM for a month or so now so i may just sell this on as spares or repairs and get the JVM if its going to be an expensive job.
Thanks
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I'm no expert but maybe the transformer supplying the heater voltage has died? or the diode bridge converting the 9v ish ac to 6.3v DC died. have a tech look there first? i know that's where i would go next if the valves were new and i still got no heater current.
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there might be no diode. a standard heater arrangement (i guess most common anyway) is an additional winding on the power transformer at about 6.3v AC rated for enough current given the valve arrangement. that will be two wires, and you might have a centre tap attached to ground. or a 'faux' centre tap which is a resistor on each wire meeting at a ground point. you might even have a pot. like a fender heater hum balance pot.
if you have DC heat anywhere, or anything fancy associated with that like diode and capacitors and regulators then you could have a problem there.
It should be a very simple circuit compared to the rest of the amp though. It also shouldn't have that many parts to go wrong unless marshall do something wild with it. repair costs might be a bit because you'll probably have to dismantle the amp to get to the underside of the PCB to swap any parts on it.
EDIT.
I had a look at the TSL100 schematic, it has DC heater on v1 and v2. the power valves and v3 v4 are AC heated.
The AC part has a faux centre tap. The DC is fed in series from the end of that chain of AC filaments (if you will). It does have a bridge rectifier and some diodes in place for that. But if that BR failed I might be inclined to think you'd loose heat to only 2 valves. rather than all of them...
...maybe someone can enlighten me.
http://www.drtube.com/schematics/marshall/tl10-60-02.pdf
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Thanks guys for all your help. It's been much more helpful than the marshall forum which was pretty much oh well its a cr@p amp buy a better one.
I have the combo version which is just 4 12ax7 and 2 el34. I am getting no heat to any of them at all.
I'll get James healey to have a quick look at it this weekend as he's up anyway now that i have more of an idea what it could be.
If anyone can spot it he should be able to with him building amps himself.
Thanks
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i had one of these with the same problem. It was a a blown internal fuse, something to do with the valve heaters. It went a few times, my local tech then did a mod which stopped it happening again
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i had one of these with the same problem. It was a a blown internal fuse, something to do with the valve heaters. It went a few times, my local tech then did a mod which stopped it happening again
I hope the mod wasn't along the lines of swapping the fuse for some scrunched up tin foil... lol
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It's quite rare to see an amp with fuses in the heater circuit, especially one fuse per tube as in that schematic. Don't really see the point of it, what's the point in cutting off the heater supply and leaving the HT supply on to the tube? And I'm surprised they're specified as 6.3 Amp when EL34s are rated at 1.5 Amp for the heater - if it's blowing a 6.3 Amp slo-blow fuse then something very weird is happening in that tube, maybe one of the resident amp gurus can shed a little light on this?
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oh yeah!
how odd. can't believe i didn't notice that. although it isn't really something i would have thought to look for.
only thing i can think of is it protect against some kind of short in the filament. The will be drawing over 6.3A in total, but a fuse on each EL34 is new to me.
still. there are 4 fuses. you'd need a combination of fuses going to take out all the heaters right?
I can't spot one fuse that would kill them all.
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I'm absolutely lost in all of this. I'll post an update tomorrow after James' has had a look at it and if he can't sort it after i find a tech that will actually touch it.
The Marshall service centre said they no longer service TSL/DSL's yet they said replacing the main board would probably fix any issue and i could order one through a marshall dealer :S
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can the OP take one of those fuses out the amp and confirm their rating - no way it could be 6.3A, even the mains fuse is only 2A.
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can the OP take one of those fuses out the amp and confirm their rating - no way it could be 6.3A, even the mains fuse is only 2A.
ill find out for you tomorrow when James is hear i dont want to mess around with it internally not knowing anything about them
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you'd need a combination of fuses going to take out all the heaters right?
I can't spot one fuse that would kill them all.
Looking at the schematic, all 4 fuses would have to blow for the all 4 EL34 heaters and the ECC83 heaters to stop working. So maybe there's a serious short in the heaters? Or the power transformer has packed in?
The neon light coming on means ... not a lot. If it's connected across the PT primary (not checked this on schematic) then all that means is there's mains voltage coming into the amp from the wall socket, I'd check the schematic then ignore that completely.
my local tech then did a mod which stopped it happening again
please tell me he didn't just push a steel nail into the fuse holder? WARNING - DO NOT DO THIS!
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can the OP take one of those fuses out the amp and confirm their rating - no way it could be 6.3A, even the mains fuse is only 2A.
Yes it can!
The 2A is the 240V supply, if it's stepped down to 6.3V then 6A is no problem. The EL34 heaters would add up to 6A for 4 tubes.
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Well mine is the combo version that only has the 2 power tubes.
Where are you guys getting these shchematics from?
The general consensus from the Marshall forum is the same as here
that its the 6.3a fuse but that its caused by a diode shorting due to heat and it being undersized for the job
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there are a few places you can find them
you just have to google it. I couldn't find a specific 601 schem but i found teh TSL100 and it probably translates quite well.
The BR is only supplying the first 2 preamp valves with filament voltage. I know with usual AC heat a whole 12AX7 draws 300mA. I'm not sure if that changes for DC heat. I think it just about doubles current consumption (correct me if im misinformed). Which would give you ~1.2A draw from V1 & V2. If that diode if the problem then there are probably loads you could fit in its place which are higher spec depending on PCB.
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Hey guys got it all sorted.
It was the Bridge rectifier(br102) that caused the fuse to blow as it was shot a quick replacement of that and the fuse and everythings back in fully working order.
And also yes that fuse was actually 6.3amp
thanks for all your help.
Tony
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just one fuse?
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yeah it was just the one fuse that had gone F3 on the schematic
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The DC heater bridge rec in the TSL601 and TSL401 is a bit on the shandy side, and can also suffer from dry joints. (I had a Bogner Alchemist in last week with exactly the same problem).
Additionally caps filtering the supply in the TSL401 are rated at only 6V!
You need to bear in mind that rectification and filtering with a large capacitor causes very large current pulses to be draw from the filamennt winding.
I've experimented with several methods of generating DC heaters, and to be honest trying to rectify the 6.3 VA winding is daft due to the high currents involved. You are far better running your preamp valves at 12.6 VDC, which will require half the current draw. OK, you will need another winding on your PT, but of a mass producer like Marshall this will be inconsequential.
Peavey us 24VDC to supply the first 2 preamp valves wired in series in some amps. This reduces current requirements even further.
Also if a power valve shorts plate or screen to cathode it is likely that it will also break the cathode heater insulation and impose the HT on the heater supply. If you are using the same heater for the power valves and DC pre-amp heater there is likely to be considerable collateral damage. Thus generating a DC filament supply for the pre-amp valves from another transformer winding makes the most sense all round.
Re the fuse in your amp. Well it did it's job. It saved the amp from any furthere damage, as a short in the fialment supply is unlikely to draw enough current on the tranformer primary (remember it is stepped down from 240V) to blow the mains fuse, so a short could easily burnout the filament winding and ruin the transformer.
Apart from very exacting applications (eg studio gear, v-v-v-v-v-ery high gain amps) I find that referencing the filament supply to around 30VDC and using well-dressed filament wiring renders filament hum innocuous.
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The BR is only supplying the first 2 preamp valves with filament voltage. I know with usual AC heat a whole 12AX7 draws 300mA. I'm not sure if that changes for DC heat. I think it just about doubles current consumption (correct me if im misinformed).
The total power consumption is increased by a factor of 1.4, although you will now get large current pulses draw by the DC filament filter caps. The larger the cap the worse the problem but the better the smoothing.
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can the OP take one of those fuses out the amp and confirm their rating - no way it could be 6.3A, even the mains fuse is only 2A.
Yes it can!
The 2A is the 240V supply, if it's stepped down to 6.3V then 6A is no problem. The EL34 heaters would add up to 6A for 4 tubes.
there is a 6.3A fuse on each of the EL34s, not one 6.3A fuse for all four - its on the schematic.
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true. seems weird.
I also don't understand how blowing just 1 fuse can kill all the heaters, unless breaking the bridge rectifier can somehow cause that kind of fault too.