100 watt super lead lost sound

SilkWilk

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I’ll be honest guys so far my readings on OT are fairly consistent. I’m measuring from the b+ tap where the OT primary comes in and connects to the the first mains cap. Then I’m measuring at the output tube sockets and getting consistent resistance across all of them. Later tonight I will post my readings.

I was considering impedance selector as well seemed like a reasonable thought. The wires are all solid but I’ll have to check resistance there as I have not.

I did try both output jacks to no avail. I still need to try a new preamp tube I did switch them around with no change but my thoughts are an open preamp tube is an open circuit no matter where you put it the signal is dropping. I’m still hopeful that’s all it is but won’t be able to grab a new tube until tomorrow. I’ll recheck OT tonight as well as the output section.

I did change from a variable feedback - to the 4 ohm tap (100k nfb) a couple years ago but it never caused any issues.

I typically run the amp on 16 ohms to a 16 ohm 1960a 4x12
 

SilkWilk

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Can I check voltage somewhere in the circuit to determine if I’m losing signal in the preamp? Like at phase inverter?
 

XTRXTR

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So take out all tubes, leave them out.
BTW make sure your fuses are correct ratings

no power to amp, test resistance from pin 3 of each power tube socket to OT CT (usually a post on the board) It should read some resistance to each. If there is no resistance its open circuit, if its only .2 or so its a short and in either case you have a dead OT. On the other hand if you read a decent resistance you are probably good with the OT.

If you have a bulb limiter this would be a good time to use it.

EDIT: I just realized while this section is a good thing to do it won't help your problem. If no negative bias then your tubes would be redplating, and if too much negative bias they would be faint to not conducting and would barely hear anything.
Power on the amp leave standby open (not in play mode). Test that the Bias circuit is providing negative voltage to all four power tube pin 5 sockets. If not you found your problem section. If there is neg voltage then turn standby to play mode. If a fuse blows there are other issues.


Since all tubes are out of the amp you don't need to plug in a cabinet. Further if there was some unfortunate short from primary to secondary of your OT it could feed DC current to the cabinet and blow the speaker coils. A dummy load is good for testing.

Test that all power tube pin 3 are getting high voltage, then test the rest of the B+ to each preamp tube pins 1 and 6, as you go toward the first tube it will be less voltage.
 
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SilkWilk

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Great info thank you so much!
I might have finally made some headway.

I swapped a new preamp tube in all positions and nothing.

I checked OT resistance fromt CT to pin 3 all readings were around 16 ohms. I checked secondary from CT to pin 6 all sockets were around 120 ohms.

I also went back through the board rechecking values and it occurred to me I hadn’t checked caps at all. They are all ohming out between 380-400 ohms which appear to be in spec 50uf/f&t’s throughout)

Except one - one of the main caps is open I can’t get a reading on it to save my life.

I’ve got power to the amp all tubes glowing? Just no sound.

Could a blown mains cap cause this? I would think I wouldn’t even have power.
 

PelliX

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I might have finally made some headway.

I swapped a new preamp tube in all positions and nothing.

I checked OT resistance fromt CT to pin 3 all readings were around 16 ohms. I checked secondary from CT to pin 6 all sockets were around 120 ohms.

I also went back through the board rechecking values and it occurred to me I hadn’t checked caps at all. They are all ohming out between 380-400 ohms which appear to be in spec 50uf/f&t’s throughout)

Except one - one of the main caps is open I can’t get a reading on it to save my life.

I’ve got power to the amp all tubes glowing? Just no sound.

Could a blown mains cap cause this? I would think I wouldn’t even have power.

To check caps, you will need an ESR meter or similar. You can pick them up for a tenner or two (and up) on fleebay and the usual places. If you ever find yourself doubting a cap, it will have been worth the purchase.

Back to the problem at hand, no it's quite unlikely a cap suddenly failed in an instant without emptying its guts all over the show, which you would have noticed by now when inspecting the innards. Considering your OT seems OK, how about the output jack/selectors and so on? Have you checked these as suggested?
 

SilkWilk

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To check caps, you will need an ESR meter or similar. You can pick them up for a tenner or two (and up) on fleebay and the usual places. If you ever find yourself doubting a cap, it will have been worth the purchase.

Back to the problem at hand, no it's quite unlikely a cap suddenly failed in an instant without emptying its guts all over the show, which you would have noticed by now when inspecting the innards. Considering your OT seems OK, how about the output jack/selectors and so on? Have you checked these as suggested?
Good info thanks -

I checked the impedance selector 4,8,16 taps are all at or around .5 to ground - is there a different way to check impedance selector? What’s the best way to confirm if an input Jack is bad?
 

SilkWilk

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Good info thanks -

I checked the impedance selector 4,8,16 taps are all at or around .5 to ground - is there a different way to check impedance selector? What’s the best way to confirm if an input Jack is bad?
Also I wondered - this amp has a zero loss effects loop could I try sending the preamp out to another amp to test for signal there - could I plug into return of power amp to check for sound? Just trying to see if I can figure out what side I’m losing signal on.
 

neikeel

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There was an iffy batch of Marshall type selectors (coin operated type). I had this issue about 4 years ago. Might be worth spraying some contact cleaner lube inside and spinning the switch round a few times.
You have checked OT primary and 16-17ohms is normal between anodes and supply.
If you remove tubes and measure voltages on pins what do you get?
Common place to get flash over with carbon tracking is between pin 3 (HT 480v) and pin 2 (3.15v). I guess it is possible to get flash over between pin 6 (screen voltage) and pin 5 (-ve bias volts) but I’ve not seen it yet.
Odd that one of your mains can elements is open circuit too. But as they are parallel /series wired the other half is making ground and providing some capacitance!
So I presume you have new socket, bias resistor (1R - how many watts rating?) and a new F&T can on the way?
I’d go through a full check of the power stage with these bits before installing new tubes.
There is clearly something else not quite right as I would normally expect sound on 3 cylinders unless the HT fuse blew (checked it with a meter of course!).
 

Pete Farrington

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Common place to get flash over with carbon tracking is between pin 3 (HT 480v) and pin 2 (3.15v). I guess it is possible to get flash over between pin 6 (screen voltage) and pin 5 (-ve bias volts)
Just to note that arcing between terminals doesn’t start at idle, so the idle DC voltages don’t really paint the full picture of the mechanism.
At full clean power output, the anode will be swinging down to about 50V and up to nearly 900V.
Overdriven into an inductive load, the peaks of the voltage spikes are likely to be well over 2kV.
It’s such extremes that ‘stress test’ the insulation between the anode and the adjacent heater terminal, which is traditionally referenced to 0V common.

Hence DC elevation of the heater circuit will reduce the (momentary) differential between anode and heater, which may act to decrease the likelihood of it occurring.
 

neikeel

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I was keeping the reply short. I have experienced 3->2 arcing myself a couple of times and the crackle/internal firework display inside the affected valves was sufficient warning to shut down. Each time using the amps hard into attenuator.
 

PelliX

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Good info thanks -

I checked the impedance selector 4,8,16 taps are all at or around .5 to ground - is there a different way to check impedance selector? What’s the best way to confirm if an input Jack is bad?

Just to make sure we're on the same page here; do you get any sound out of the speaker at all? A pop when turning standby or the amp itself on/off? Any hiss or hum? If there's truly zero output, this has nothing to do with the preamp or input in any way.

What you're looking for in the taps and selector is continuity; between (f.e.) the selector and/or OT and the speaker jack. Measuring .5 Ohms is probably just the resistance of the (suboptimal) contact between probe and what you're probing or even the probe leads themselves. You shouldn't however have that little resistance between say the positive of the speaker output and ground.

@neikeel 's suggestion about giving the switch a shot of contact cleaner is not a bad idea, but let's start with clearly documented measurements. Where exactly are you getting half an Ohm from?

There is clearly something else not quite right as I would normally expect sound on 3 cylinders unless the HT fuse blew (checked it with a meter of course!).

That made me chuckle... though, I suppose a valve is a cylinder in a way... :applause:
 

Matthews Guitars

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Just something to consider: If plate, grid, or cathode become disconnected, that tube section does nothing. (Filament, too, of course.) Check for opens. An open plate resistor, open grid resistor, or open cathode resistor will shut down that tube or section.

I fixed a DSL100H that had gone fully muted and it was the fusible bias resistors that had gone open.

If not sure about the impedance selector, you can bypass it. If this 1959 has four speaker jacks, wire up the transformer taps direcdt to the speaker jacks. 16 ohm tap to one jack, 8 ohm tap to 2 jacks, 4 ohm tap to 1 jack. This can be a permanent solution if you mark the
jacks appropriately and remember to never use more than one tap at a time.
 

SilkWilk

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There was an iffy batch of Marshall type selectors (coin operated type). I had this issue about 4 years ago. Might be worth spraying some contact cleaner lube inside and spinning the switch round a few times.
You have checked OT primary and 16-17ohms is normal between anodes and supply.
If you remove tubes and measure voltages on pins what do you get?
Common place to get flash over with carbon tracking is between pin 3 (HT 480v) and pin 2 (3.15v). I guess it is possible to get flash over between pin 6 (screen voltage) and pin 5 (-ve bias volts) but I’ve not seen it yet.
Odd that one of your mains can elements is open circuit too. But as they are parallel /series wired the other half is making ground and providing some capacitance!
So I presume you have new socket, bias resistor (1R - how many watts rating?) and a new F&T can on the way?
I’d go through a full check of the power stage with these bits before installing new tubes.
There is clearly something else not quite right as I would normally expect sound on 3 cylinders unless the HT fuse blew (checked it with a meter of course!).
Thanks man - yes I have a new socket and bias feed resistor coming should be here Monday. I haven’t ordered a cap as was waiting to hear your guys thoughts.

Something that was mentioned was the resistance of my meter/leads I’ll have to recheck those. I’m almost considering if my HT fuse isn’t open and the reading I’m getting on it is actually my meters internal resistance - probe leads and it’s actually 0 (open). I will also try a new fuse just to see.

The impedance selector theory has me intrigued and I have read about hardwiring it I usually only run head on 16 ohms so I could prob try that.

I’m trying to keep it simple as possible I have no sound and I have carbon tracing on v6 with a burnt resistor and a faded tube that was new 2 years ago.

I’ve got to focus there I think in clearing that up and I’ll report back again.

Good to know everyone mostly seems to think OT is ok.

Once I have new parts I’ll def do plate/bias voltage read outs - I try to keep that testing to a minimum haha
 

Dr.Twang!

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Not to confuse the issue but you mentioned amp has an effects loop. Have you tried a short jumper cable in that? I have seen a problem a few times where the jack doesn’t close properly when plug removed, thereby restricting all signal to PI.
If it’s a Metro kit, check switch, etc., too.
 

PelliX

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Something that was mentioned was the resistance of my meter/leads I’ll have to recheck those. I’m almost considering if my HT fuse isn’t open and the reading I’m getting on it is actually my meters internal resistance - probe leads and it’s actually 0 (open). I will also try a new fuse just to see.

Uh, I was indicating that you may well find a very small resistance in your readings incurred by your probes, the contact surface and the probe leads. So what should be a dead short (like a copper wire) might read as 0.5 Ohms. A fuse would either have infinite resistance (blown, dead, gone, ex-fuse, gone to meet its manufacturer) or something very low (fuse is fine). The minor factor I was referring is neglible in that scenario.

The impedance selector theory has me intrigued and I have read about hardwiring it I usually only run head on 16 ohms so I could prob try that.

Did you check the continuity between OT and output jacks, selector, etc?

I’m trying to keep it simple as possible

Isolating the fault to a section of the circuit and checking for mechanical problems (dud switch, loose solder joint) fit that description in my book. Care to share some pics - the more eyes principle works well from time to time. :)
 

SilkWilk

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Uh, I was indicating that you may well find a very small resistance in your readings incurred by your probes, the contact surface and the probe leads. So what should be a dead short (like a copper wire) might read as 0.5 Ohms. A fuse would either have infinite resistance (blown, dead, gone, ex-fuse, gone to meet its manufacturer) or something very low (fuse is fine). The minor factor I was referring is neglible in that scenario.



Did you check the continuity between OT and output jacks, selector, etc?



Isolating the fault to a section of the circuit and checking for mechanical problems (dud switch, loose solder joint) fit that description in my book. Care to share some pics - the more eyes principle works well from time to time.
Uh, I was indicating that you may well find a very small resistance in your readings incurred by your probes, the contact surface and the probe leads. So what should be a dead short (like a copper wire) might read as 0.5 Ohms. A fuse would either have infinite resistance (blown, dead, gone, ex-fuse, gone to meet its manufacturer) or something very low (fuse is fine). The minor factor I was referring is neglible in that scenario.



Did you check the continuity between OT and output jacks, selector, etc?



Isolating the fault to a section of the circuit and checking for mechanical problems (dud switch, loose solder joint) fit that description in my book. Care to share some pics - the more eyes principle works well from time to
I can post pics here in a bit
 

SilkWilk

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I can post pics here in a bit
I was able to get a reading on the impedance selector and output jacks.

From B+ CT to all impedence taps I’m getting 3.80 k ohms. This also the measurement on the jacks.

I measured voltage with tubes out amp warmed on standby I’m getting the following:

Pin 5 all tubes = -45 vdc

Pin 3 v4 = 0
Pin 3 v5 = -285 vdc
Pin 3 V6 = -285 vdc
Pin 3 v7 = 0

Funky - what is this telling me?

I also tried jumping the effects loop no change.

It’s pretty evident I’ve got a plate voltage issue - mind you this is still with the potential bad socket at V6 and a burnt bias feed resistor on V6
 


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