Safety First. You need a bleeder resistor in your power supply that will bring the b+ down to a safe level quickly. Since your circuit is failing to conduct the b+ it is staying in the caps, you mentioned it takes a long time to come down. Should be under 30v 10 seconds after you turn off the amp. So you need a ~470K 1w from one of the nodes in your power supply direct to ground. Yes, it eats a few mA from your circuit. it will also save your life if you keep having to go back in there.
a 10 ohm 10w resistor soldered to a couple alligator clips so you can manually drain the caps is a must, as well. Please don't put any of this above your safety.
a 10 ohm 10w resistor soldered to a couple alligator clips so you can manually drain the caps is a must, as well. Please don't put any of this above your safety.
I've replaced the output tubes, the power resistors (and changed values to 560R for the KT66's), replaced that resistors bypass cap (220uf panasonic FC). I've also changed the coupling cap to the Jantzen one as in the photo..
So, the parts that are still in the amp from the catastrophic failure.
- the 5U4G rectifier and 6n1p tubes,
- the power supply caps, 10uf, 33uf plus the 100uf + 100uf.,
- the 6n1p cathode led bias and resistor to ground and plate resistors.
- the 200k feedback resistor, the 220R resistor to ground after the 0.22uf cap and the 100R and 1.2k resistor from output tube pin 4.
I measure the tube anodes, so referring to the schematic, for the KT66 I've got my + probe on the pin 3/200k feedback resistor (above point 3 exactly where it says 450v) and the black probe at my star earth point.
For the 6n1p, I've got my + probe at the junction from pin 1 (or 6) and the 200k feedback resistor (at the point of 229v on the schematic)
These all have the same measurement, which is also the same as B+. I.e. very high and rising.
I can't measure screen grid I'm sorry as its tucked underneath the board.
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I'm just trying to zoom out til we can see the whole picture. It's as if the ground circuit isn't connecting. no power thru the tubes. have you measured the resistance across your PT's secondary, where the HV comes out 300-0-300? I'm worried you're going to have to do a tear down and meter every component.
Well, I've got B+.
I dont know about the filter caps if they've been damaged in the failure but I'm getting B+ readings of very hi dc.
I dont know about the OPT, could have been damaged?
I've got a ,(very high) anode voltage, cathode resistors and bypass cap are grounded.
So caps and OPT, not sure about those if they have been damaged. Tubes should be PK (from very reputable dealer and sold tested and matched.)
I'm not educated enough to spot what's missing. The first time I build this it was straight to music time no gremlins, no rodeo.
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That OK you know, I don't mind doing that as long as it gets up and running.
All I know, as of now, is that every resistor measures at specified/schematic. Albeit, the led and 180r have a combined resistance of 8.7k or so, so I assume they are OK.
Could I ask, how would I measure the resistance across the PT? Secoundary? Take the rectifier out and measure resistance from the centre tap 0 and one of the 375v secondaries?
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The power has no where to go. You need one of these:
I got a bunch of those alligator clips cables already made up on Amazon, I think. Cut one in half and put a nice beefy resistor in there, under 100ohms and over 5w. Then you clip one end to a ground point (GROUND FIRST!) and then clip the other end to a node on the power supply.
Then install a 470k resistor across those same points so you'll be safe. That's a bleeder resistor. Gives that HV a path to ground when there's something wrong with the amp. Protects you.
You can't measure components when they're in a circuit, so you've got to extract them as much as necessary to test. For caps, that's de-soldering one leg so it's isolated and you can test it. It's a chore, but you've got to find which component is misbehaving.
Now that I think of it, after draining your PS caps, set your DMM to Ohms and measure between ground and pins 4&6 on the rectifier socket. Should be 1/2 the resistance of the whole secondary, but that's not important now. They should be equal, maybe around 100ohms each. As long as it's not infinite...
I got a bunch of those alligator clips cables already made up on Amazon, I think. Cut one in half and put a nice beefy resistor in there, under 100ohms and over 5w. Then you clip one end to a ground point (GROUND FIRST!) and then clip the other end to a node on the power supply.
Then install a 470k resistor across those same points so you'll be safe. That's a bleeder resistor. Gives that HV a path to ground when there's something wrong with the amp. Protects you.
You can't measure components when they're in a circuit, so you've got to extract them as much as necessary to test. For caps, that's de-soldering one leg so it's isolated and you can test it. It's a chore, but you've got to find which component is misbehaving.
Now that I think of it, after draining your PS caps, set your DMM to Ohms and measure between ground and pins 4&6 on the rectifier socket. Should be 1/2 the resistance of the whole secondary, but that's not important now. They should be equal, maybe around 100ohms each. As long as it's not infinite...
So that's two resistor in parallel with ground and + of the power supply, after the filter caps?
I do have a 100R 50w resistor, would need to get 470k one, would 5w be ok or would you recomment higher?
I've put a bid on a variac see how that goes, it would be good to have, they are quite expensive though.
Could i also check caps (electro's) with DMM using resistance?, infinity = good cap. I dont have a meter that can measure cap values so thought this would be quick alternative.
I will take out all components individually and measure them. it will take a few days and I won't be able to start until Saturday due to work commitments.
I do hope you (all) will keep an eye on posts and updates I really want to get this up and running again, even if it means total rebuild. Learning though as we go along so would like to see if we can get to the bottom of this.
I do have a 100R 50w resistor, would need to get 470k one, would 5w be ok or would you recomment higher?
I've put a bid on a variac see how that goes, it would be good to have, they are quite expensive though.
Could i also check caps (electro's) with DMM using resistance?, infinity = good cap. I dont have a meter that can measure cap values so thought this would be quick alternative.
I will take out all components individually and measure them. it will take a few days and I won't be able to start until Saturday due to work commitments.
I do hope you (all) will keep an eye on posts and updates I really want to get this up and running again, even if it means total rebuild. Learning though as we go along so would like to see if we can get to the bottom of this.
The 100R 10w resistor with clips in the picture is a tool you should have to safely discharge caps.
Your amp should have a bleeder resistor to drain the caps in the power supply. Any resistor between 470k and 1M is fine, and it only consumes a few mA so 1 watt is fine.
Before you go cutting wires, drain those caps and measure the resistance in ohms between your ground and pins 4 and 6 in the rectifier socket. We need to make sure you have that connection, that the center tap in your transformer wasn't damaged.
Your amp should have a bleeder resistor to drain the caps in the power supply. Any resistor between 470k and 1M is fine, and it only consumes a few mA so 1 watt is fine.
Before you go cutting wires, drain those caps and measure the resistance in ohms between your ground and pins 4 and 6 in the rectifier socket. We need to make sure you have that connection, that the center tap in your transformer wasn't damaged.
OK, so the just the 470k one for my amp and I also have a seperate 100R one for cap discharging generally. Sorry to be a pain (!)
I will have to order the 470k ones, at the moment I'm down to 23v using just my DMM as a load.
As soon as the caps are discharged, I'll measure and post rectifier pins 4 and 6 resistance measurements.
I will have to order the 470k ones, at the moment I'm down to 23v using just my DMM as a load.
As soon as the caps are discharged, I'll measure and post rectifier pins 4 and 6 resistance measurements.
This is not a very smart way to proceed.
Look, it's just a tube! Measure all voltages on all pins and you will find the issue. You're jumping around too much.
Go methodologically. Did you check that there is continuity between that star ground and the bottom of Rk and the bottom of the grid leak?
You really need to go for it step by step, there's no magic involved!
Jan
Yes, there is continuity between star ground, bottom of Rk (both 6n1p & kt66 cathodes) and the grid leak resistor (220k). Plus, continuity with the input ground too and also the 100k grid leak resistor at the input (1M in circuit). All joined together.
OK so Caps discharged.
Black probe to ground.
Pin 4 of the rectifier - 26.2R
PIn 6 of the rectifier - 24.6R
Ah, well, that will be difficult, the Mains Tx, OPT's and the Choke were all custom made by Sowter Transformers in the UK back in 2008.
The mains transformer was a size P, 0442 type but, other than invoice and wiring diagram, I have no other details and there is nothing on their website either, presumably as they are bespoke.
I'll follow the path from the Mains, so I'll isolate the filter caps and test those next...
The mains transformer was a size P, 0442 type but, other than invoice and wiring diagram, I have no other details and there is nothing on their website either, presumably as they are bespoke.
I'll follow the path from the Mains, so I'll isolate the filter caps and test those next...
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