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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 7th December 2016, 08:21 AM   #21
CalTurk is online now CalTurk  Germany
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Thanks Win. Yes, just like you describe.
I'm familiar with how to set the bias but since the pcb is tightly in place there's nowhere to clip the testers. I'm considering soldering little wire nubs in place to clip on to so I can test from underneath.
The repair man had said that he had done the check out and biasing but that now seems really doubtful.
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Old 7th December 2016, 02:35 PM   #22
w5jag is offline w5jag  United States
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OK, red plating is the problem, then?

Just to further clarify, it is one of the 300B red plating, and when you swap the left and right tubes, the problem does not follow the tube?

If it is the 300B, you need to check, and then attempt, to set the plate current to a safe value. I don't own any 300B's, so I don't know what the correct value is for your amp. The good side measurement may or may not be correct. Perhaps someone running 300B's can give some useful information here.

Unfortunately, a blown MOSFET on that side, depending on the mode of failure, can also cause this problem. The way your luck is going ......

Check and attempt to set the current first.

If it is a 5842 orange / red plating, then the 10M45 on that side has likely failed.

Win W5JAG
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Old 8th December 2016, 08:47 AM   #23
CalTurk is online now CalTurk  Germany
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I hope to get time to check this all out this weekend.
As I've ordered another pcb to build a 2nd amp anyway, I went ahead and ordered spare MOSFET. So I'll have them on hand either way.

Thanks again Win.
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Old 8th December 2016, 03:56 PM   #24
w5jag is offline w5jag  United States
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I looked back at the pictures and notice the board has oversized audiophile style coupling capacitors.

It's probably also worth checking to make sure the coupling cap has not failed or become leaky.

Win W5JAG
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Old 8th December 2016, 08:27 PM   #25
CalTurk is online now CalTurk  Germany
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Quote:
Originally Posted by w5jag View Post
I looked back at the pictures and notice the board has oversized audiophile style coupling capacitors.

It's probably also worth checking to make sure the coupling cap has not failed or become leaky.

Win W5JAG
I'll add that to the list then thanks.
(Good thing my lady is out of town this weekend.)
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Old 7th February 2017, 01:18 AM   #26
CalTurk is online now CalTurk  Germany
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Default There we go!

Win et al,

Thanks for all the help.

I realized I was way out of my depths having never built anything like this so I just dove in and ordered a board from George and built out that out from scratch. For a hands on guy like me it was a great learning experience and seems to have gotten everything working (check out and biasing went well) and sounding good so far (touch wood). 3am now so listening quietly.

Will get a better idea how everything really sounds later tomorrow and how everything holds up in the coming days.

Thanks again.

Deniz
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Old 7th February 2017, 07:43 AM   #27
CalTurk is online now CalTurk  Germany
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Meters are reading right. 385 volts at B+ (R30) and 50 ma at R18 and R29.
The bass however distorts on really bass heavy tracks (Tosca "Pearl In" https://www.youtube.com/watch?v=mNk9cbJqtjs ((first track)) ).

Also there's a very intermittent static sound that happens every few tracks but that's just around for 1 to 3 seconds.
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Old 7th February 2017, 03:19 PM   #28
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Quote:
Originally Posted by CalTurk View Post
Meters are reading right. 385 volts at B+ (R30) and 50 ma at R18 and R29.
50 mA may be too low for 300B. Try 60 - 65 mA and see how it sounds.
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Old 7th February 2017, 04:17 PM   #29
CalTurk is online now CalTurk  Germany
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Well things didn't last long.

The intermittent static was coming from the right channel but on the last track of a Dexter Gordon CD the left channel went erratic. First some static / crackling and popping. I watched the meters closely and the meter at R29 overloaded then read numbers as high as 167. I quickly shut it down.

2 hours later I swapped in a known good rectifier (a Mullard GZ34) and same thing happened. This time with a steady and audible 50hz hum that wasn't affected by changing the volume.

I'm a bit short on patience at this point so I'll wait before doing any more test but what should I be looking for? Oscillation? Grounding issue?

The board is new as are all it's components and I've triple checked all of the connections. 6.3 volt secondary is soldered in and is secure, sockets are new and securely soldered in. Voltages all read right during check out and biasing. 5842's are new as are the 2 different GZ34's I've tried. Other than some bass distortion, things ran well for about an hour.

Gavin had this working for quite a while before I bought it. Is there something that might have broken or loosened in shipping it from England (besides the mosfets) that could be causing these problems??

The one and only thing I've reused from Gavin's board are the risers / extensions for the 5842's that bring them up high enough to clear the enclosure. I guess I'll pull those just to see but that seems too simple and they're in there very firmly.
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Old 7th February 2017, 05:00 PM   #30
w5jag is offline w5jag  United States
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Quote:
Originally Posted by CalTurk View Post
Well things didn't last long.

The intermittent static was coming from the right channel but on the last track of a Dexter Gordon CD the left channel went erratic. First some static / crackling and popping. I watched the meters closely and the meter at R29 overloaded then read numbers as high as 167. I quickly shut it down.

2 hours later I swapped in a known good rectifier (a Mullard GZ34) and same thing happened. This time with a steady and audible 50hz hum that wasn't affected by changing the volume.

I'm a bit short on patience at this point so I'll wait before doing any more test but what should I be looking for? Oscillation? Grounding issue?

The board is new as are all it's components and I've triple checked all of the connections. 6.3 volt secondary is soldered in and is secure, sockets are new and securely soldered in. Voltages all read right during check out and biasing. 5842's are new as are the 2 different GZ34's I've tried. Other than some bass distortion, things ran well for about an hour.

Gavin had this working for quite a while before I bought it. Is there something that might have broken or loosened in shipping it from England (besides the mosfets) that could be causing these problems??

The one and only thing I've reused from Gavin's board are the risers / extensions for the 5842's that bring them up high enough to clear the enclosure. I guess I'll pull those just to see but that seems too simple and they're in there very firmly.
My first WAG is you have lost a MOSFET on the side that wants to pull 167 ma. I would first pull the tube on that side so you don't toast it, and then check the voltage at the grid to make sure you have the right bias voltage, and that the coupling cap is not leaking.

FYI only: intermittent static and the persistent hum could also be a filter cap that was failing ( intermittent static; bass distortion) and has now failed completely ( steady 100 Hz hum unaffected by volume ).

I had similar symptoms when I was troubleshooting bad filter caps in my TSE modified to use 801A/SV-811A.

Win W5JAG
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