02-06-2021, 07:56 AM
As mentioned in an email you want to find out what frequency your hum is, that'll point you in the right direction
When scoping a bit of gear clip the black scope lead to a ground. Finding a ground can sometimes be tricky but the chassis will usually do. I extended one of my scope probes with a longer ground lead, the 4" jobbie that is usual fitted is a tad limiting.
Next, find out what your scopes max IP voltage is before going to probe HV gear. Your amp with an HT of 300v ish should be ok, use your Tek scope. Leave the vertical IP set to AC, I hardly ever used DC coupling but I mostly work on HV AF gear. Probe on x1, scope set to as low as it'll go.
Start by clipping your probe onto the grid of V1 or whichever is the IP valve, with no IP into the amp, power it up,once everythings warmed up you should see nothing, if you see 50hz then look at the heater wiring or any other nearby AC wiring to see if it's an issue. If there is something does turning the volume control do anything? Does shorting the IP stop the hum?
Next move the probe to the anode of V1, again you shouldn't see anything major. Also as mentioned in my last email measure the AC ripple on the HT with your DMM. You can check it with your scope, if your worried about exceeding your scopes Vmax just tack a big 630v cap onto the point you want to measure, whatever you have to hand, EG 100-470n. This is not the perfect way to take reading but it will do.
From reading your posts and what you've said in emails I know your worried about "blowing things up" and wary of doing the wrong thing. With a good DMM like a Fluke and a scope like your Tek, it's highly unlikely you'll do any damage to either by doing something daft. Also making things go bang, letting out the magic smoke etc is fun and part of the learning process, try to be a bit more relaxed and not to worry so much if you can, your unlikely to do much damage to valves as unlike transistors they'll take loads of abuse.
Andy.
When scoping a bit of gear clip the black scope lead to a ground. Finding a ground can sometimes be tricky but the chassis will usually do. I extended one of my scope probes with a longer ground lead, the 4" jobbie that is usual fitted is a tad limiting.
Next, find out what your scopes max IP voltage is before going to probe HV gear. Your amp with an HT of 300v ish should be ok, use your Tek scope. Leave the vertical IP set to AC, I hardly ever used DC coupling but I mostly work on HV AF gear. Probe on x1, scope set to as low as it'll go.
Start by clipping your probe onto the grid of V1 or whichever is the IP valve, with no IP into the amp, power it up,once everythings warmed up you should see nothing, if you see 50hz then look at the heater wiring or any other nearby AC wiring to see if it's an issue. If there is something does turning the volume control do anything? Does shorting the IP stop the hum?
Next move the probe to the anode of V1, again you shouldn't see anything major. Also as mentioned in my last email measure the AC ripple on the HT with your DMM. You can check it with your scope, if your worried about exceeding your scopes Vmax just tack a big 630v cap onto the point you want to measure, whatever you have to hand, EG 100-470n. This is not the perfect way to take reading but it will do.
From reading your posts and what you've said in emails I know your worried about "blowing things up" and wary of doing the wrong thing. With a good DMM like a Fluke and a scope like your Tek, it's highly unlikely you'll do any damage to either by doing something daft. Also making things go bang, letting out the magic smoke etc is fun and part of the learning process, try to be a bit more relaxed and not to worry so much if you can, your unlikely to do much damage to valves as unlike transistors they'll take loads of abuse.
Andy.
