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41 minutes ago, chinsettawong said:

I need a little help with my GG.  I just can't adjust the offset of both channels below 50V for some reasons.  The balance adjusts to zero just fine though.

My PS is measured at 465V.  The bias is set at 0.8V or 16mA.

lower the tail resistor by some ~50ohms and re-adjust

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Just like Carbon we built. Same box, same PS. Relay=120s. Separated filament transformer.      

Another Grounded Grid lives.  Have been listening to it for about a week. Powered by the same GRHV/GRLV PSU I use to power my KGSSHV, Carbon and Blue Hawaii. 400VDC rails and 20mA plate current.

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14 minutes ago, sorenb said:

lower the tail resistor by some ~50ohms and re-adjust

Agree if the offset is measuring positive voltage. Chinsettawong might consider engaging the opto-servo.

If the offset is measuring negative (like my GG when running at +/-400 @ 15mA per tube), I went to a 200ohm tail resistor and was able  to adjust to ~0V. 

 

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No, I think it’s the 120R resistor below, right of the  50K resistor and  left  of the 12V zener. Need to confirm with the schematic - it’s the resistor between the offset trim pot and B- in series.

Edited by mwl168
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It sings!  Thanks for all your advices.  For some reasons, the silkscreen on my PCB asks for 250 ohm for tail resistors.  I replace it with 120 ohm and now everything can be adjusted correctly.   

IMG_1373_copy.jpg

Now I need to work on the top cover.  The amp sounds awesome.  

 

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Does your PCB also have 18V zener instead of the 12V zener? There appears to be different versions of GG boards with different value parts marked. I don’t recall the differences and their reasons.

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2 hours ago, mwl168 said:

Does your PCB also have 18V zener instead of the 12V zener? There appears to be different versions of GG boards with different value parts marked. I don’t recall the differences and their reasons.

The zener provides over voltage protection for the 4N25 optocoupler when adjusting the offset. A 12V zener is used on the Carbon and works there OK. That value was carried over on the GG PCB, but was found close to being too low when setting the offset in that circuit. So Kevin advised using 18V as a replacement to give a little more headroom. 

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The voltage was about 465V without load.  Took out the 2n3904 and connected the amp board.  Fired the amp up and the B+ became around -12V.  :(  Switched it off and the voltage gradually came up to around 45V then went down to 0.

 

 

Can I try to change some R to increase the current limit a bit more instead of taking the 2n3904 out?

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3 hours ago, chinsettawong said:

The voltage was about 465V without load.  Took out the 2n3904 and connected the amp board.  Fired the amp up and the B+ became around -12V.  :(  Switched it off and the voltage gradually came up to around 45V then went down to 0.

Can I try to change some R to increase the current limit a bit more instead of taking the 2n3904 out?

I had this issue with my Blue Hawaii.  It was drawing more power than the GRHV was set to output.  As I recall, I changed the 5R1 resistor (the large 3W one) to 2R5 or thereabouts and that solved the issue.  I just soldered another 5R1 on top of the existing one and it worked fine.   The 5R1 sets the amount of current that the PSU will supply.  

Edited by Blueman2
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3 hours ago, chinsettawong said:

The voltage was about 465V without load.  Took out the 2n3904 and connected the amp board.  Fired the amp up and the B+ became around -12V.  :(  Switched it off and the voltage gradually came up to around 45V then went down to 0.

 

 

Can I try to change some R to increase the current limit a bit more instead of taking the 2n3904 out?

you might check the current sources:

 

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Thank you so much for all your advices.  I've finally fixed the problem.  The C2M1000170 and 10M90S on my board have gone bad for some reasons.  I replaced C2M1000170 with a SCT3080KLGC11 from a local store, and luckily it worked. 

I parallel the 5R with one more 5R, and now the power supply is working perfectly.  The amp has been running for at least 2 hours with no problem.

I couldn't have fixed it without your help, Kevin, Blueman2, and Soren.  Thanks again.  

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  • 5 months later...
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Some quartet tubes wanted to say "Hello! We are here."

DSC_0001.thumb.JPG.c5e7fc4e483e88cf2780ecfb033bce03.JPGDSC_0005.thumb.JPG.ac502e4ac59b72a725740f33903ff72c.JPGDSC_0006.thumb.JPG.29df9158e5e00117fe2b2daea4a8ee19.JPG

Neotech UP-OCC wires in teflon tubes, power delay board.

Simply plugged power and tested both boards that they are working. Not heard sound yet.

Edited by bbest
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19 hours ago, Blueman2 said:

Nice work!  Very small amp case, which is pretty neat.  What holds the amp boards in place?  They seem to be just attached by the silicon right now.   

Thanks! Right now boards holds by transistors only.

 

17 hours ago, JoaMat said:

Nice work indeed. I like the two box approach. 

The transformers – are they DIY made?

factory plus cosmetic DIY.

Edited by bbest
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