100 watt super lead lost sound

SilkWilk

Member
Joined
Oct 1, 2019
Messages
53
Reaction score
19
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
After reading my post I just realized I prob need to take it off standby lol for correct measurements - correct?
 

neikeel

Well-Known Member
Joined
Feb 12, 2014
Messages
7,216
Reaction score
6,758
There is no point in testing it live until you have replaced the dodgy socket - the arcing forms a carbon track, regardless as to whether there is a valve in and that socket will not be grounded due to no bias resistor.
There is not any true continuity between the B+ tie point of the OT as it is on the primary side of the OT and the impedance tap and speaker out jacks are on the secondary - these are electronically isolated from the primary.
If you are getting a resistance reading that is through the filter cap to ground.
I would wait until the socket, resistor and filter cap have been replaced, but static testing is fine.
 

SilkWilk

Member
Joined
Oct 1, 2019
Messages
53
Reaction score
19
There is no point in testing it live until you have replaced the dodgy socket - the arcing forms a carbon track, regardless as to whether there is a valve in and that socket will not be grounded due to no bias resistor.
There is not any true continuity between the B+ tie point of the OT as it is on the primary side of the OT and the impedance tap and speaker out jacks are on the secondary - these are electronically isolated from the primary.
If you are getting a resistance reading that is through the filter cap to ground.
I would wait until the socket, resistor and filter cap have been replaced, but static testing is fine.
Cool I will get the parts replaced asap
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
2,645
Reaction score
5,236
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.

Uh, that's *one* way of doing it, yeah. How are you measuring this? Why not just check for continuity (low resistance) between the OT and a jack in the socket of the relevant tap? I can assure you that you do not want your B+ going anywhere near your speakers. :)

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?

You're measuring between chassis and the relevant pin, correct? No harm in checking that the meter is working correctly, just in case of a fluke problem [pun not intended]. You get about 6.3V DC on the heaters, right?
 

SilkWilk

Member
Joined
Oct 1, 2019
Messages
53
Reaction score
19
Cool I will get the parts replaced asap
Uh, that's *one* way of doing it, yeah. How are you measuring this? Why not just check for continuity (low resistance) between the OT and a jack in the socket of the relevant tap? I can assure you that you do not want your B+ going anywhere near your speakers. :)



You're measuring between chassis and the relevant pin, correct? No harm in checking that the meter is working correctly, just in case of a fluke problem [pun not intended]. You get about 6.3V DC on the heaters, right?
Yeah heater voltage is good - I was in stand by when I took those readings so that would explain the strange readings. My meter should be ok it hasn’t failed me before but I agree you never can be to sure. I’ll look into it.

I know I’m making assumptions and I probably haven’t tested everything properly. I’m just starting to feel more and more like I popped the OT. It just makes sense even with a bad tube and what not. I have no sound and can’t find an open anywhere else.

I’ve had issues on the v6 before with a tube that shorted that time I blew the screen grid resistor and bias sense resistor. I had sparks that time but never lost sound.

New resistors, new tubes and was working good for nearly two years but knowing now I prob had arcing on the socket due to the leftover carbon trace has potentially been putting stress on the OT for along time.

I feel like ai should just get it to a tech who can confirm this and get on with it so I can get back to jamming.
 

SilkWilk

Member
Joined
Oct 1, 2019
Messages
53
Reaction score
19
here are shots of the board the first pic shows the burnt feed resistor note the black char on the side - the 2nd pic is the carbon tracing around 5&6 and then a full board view
 

SilkWilk

Member
Joined
Oct 1, 2019
Messages
53
Reaction score
19
Just when you think about throwing in the towel things seem to work out. I decided to replace the socket and burnt bias feed resistor.

In doing so I unscrewed the impedance selector switch to clear some room and get at V6 and the main ground wire from the selector switch to the output Jack all but fell off with one slight movement.

I mentioned before years ago I changed from variable to 4 ohm tap on the negative feedback. I think when I did that I left a weak solder joint that became cracked/cold overtime and finally let go.

I got the new parts in and she’s humming right along now.

I will check voltages and bias in the morning but so far great news and my OT seems fine assuming it was just an open ground.

Guys I swear I checked for continuity I even tugged on the thing in all my testing. But I’m thankful I think the problem finally presented itself to my novice eyes.

Thanks again for all your help this forum really is amazing!
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
2,645
Reaction score
5,236
Just when you think about throwing in the towel things seem to work out. I decided to replace the socket and burnt bias feed resistor.

In doing so I unscrewed the impedance selector switch to clear some room and get at V6 and the main ground wire from the selector switch to the output Jack all but fell off with one slight movement.

I mentioned before years ago I changed from variable to 4 ohm tap on the negative feedback. I think when I did that I left a weak solder joint that became cracked/cold overtime and finally let go.

I got the new parts in and she’s humming right along now.

I will check voltages and bias in the morning but so far great news and my OT seems fine assuming it was just an open ground.

Guys I swear I checked for continuity I even tugged on the thing in all my testing. But I’m thankful I think the problem finally presented itself to my novice eyes.

Thanks again for all your help this forum really is amazing!

Excellent! Always nice to have an easy fix, albeit with a bit of a hunt. :thumb:

Be sure to check and/or reflow any other solder joints that may be in poor shape.
 

SilkWilk

Member
Joined
Oct 1, 2019
Messages
53
Reaction score
19
Excellent! Always nice to have an easy fix, albeit with a bit of a hunt. :thumb:

Be sure to check and/or reflow any other solder joints that may be in poor shape.
It’s funny how everything worked out.

I got the amp back in action and the same day I got an email back from a guy that is well known in my area for amp repair but recently retired.

He actually got back to me and said he’s still doing side jobs - so I think im going to get it over to him to have a good look and make sure everything is doing what it should.

My V6 and v7 run hotter than the other pair and I worry about bias issues there that are probably beyond me.
 


Top