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PostPosted: 12 Mar 2017, 10:04 
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Joined: 16 Dec 2015, 03:05
Posts: 12
Hi all,

I have Spark 550 with 300b push pull.

Power tubes is 300b with 2 pcs 12AX7 and 2pcs 12AU7 for the driver.

Today, my amp is dead. Both channel are not working.

Some of the trouble shooting I made:

1. Filaments are working. All power tube and driver tube are glowing.

2. High voltage rail for power tube is working. Tested between plate and ground is around 360 Volt on both channel.

3. Driver tube voltage rail, no problem.

4. Tube replacement on both left and right.

Anybody has the schematic for this beast?

I also thinking to rewiring this old amp into new one.

Anybody has a schematic for 300B pushpull?

Thank you in advance.


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PostPosted: 12 Mar 2017, 15:19 
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Joined: 05 Aug 2016, 14:35
Posts: 220
[quote="aatjitra"]Hi all,

I have Spark 550 with 300b push pull.

power tubes is 300b with 2 pcs 12AX7 and 2pcs 12AU7 for the driver.


Please note the interpolations:

Today, my amp is dead. Both channel are not working.

Define "dead".
Is there any Hiss, Hum, Crackle or other noise at either speaker?
Have you verified that the speaker are connected properly? Any cats/dogs/kids in the house?
Have you verified that the inputs are connected properly?
Are there any internal fuses? And have you checked them with a VOM? Fuses are wearing parts, by the way.
Are you getting anything at all into the primary side of the output transformers?
Have you checked the secondary windings for the OPTs? This is a common failure.
Have you checked the delay relay?
Have you checked inside for anything loose, or broken?
If there area any boards, have you checked for burnt traces? This is also very common with these beasts.


These are not well-supported amps, but as I remember, Yaqin put their schematics somewhere directly on their units, so it may be inside, on the bottom plate. But, you might try here: https://www.shenzhenaudio.com/customer-service They also have a habit of doing exactly as you describe - going dead for no discernible reason.

Best of luck with it!


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PostPosted: 23 Mar 2017, 19:31 
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Peter W. wrote:

Please note the interpolations:

Define "dead".
Is there any Hiss, Hum, Crackle or other noise at either speaker?


No sound at all on both channel.

Quote:

Have you verified that the speaker are connected properly? Any cats/dogs/kids in the house?


Yes of course. Both speakers are connected properly.

Quote:

Have you verified that the inputs are connected properly?


Yes. I traced from the RCA connector at the other end of the cables until the grid of driver tube.

Quote:

Are there any internal fuses? And have you checked them with a VOM? Fuses are wearing parts, by the way.


Yes. Fuse for power is okay. No fuse for OT.

Quote:

Are you getting anything at all into the primary side of the output transformers?


Yes. B+ voltage still can be detected at both tubes anode.

Quote:

Have you checked the secondary windings for the OPTs? This is a common failure.


Yes. Looks like both windings are okay.

Quote:

Have you checked the delay relay?


No relay at all despite it is using solid state rectifier. I noticed some circuits has been altered.

Quote:

Have you checked inside for anything loose, or broken?
If there area any boards, have you checked for burnt traces? This is also very common with these beasts.


These are not well-supported amps, but as I remember, Yaqin put their schematics somewhere directly on their units, so it may be inside, on the bottom plate. But, you might try here: https://www.shenzhenaudio.com/customer-service They also have a habit of doing exactly as you describe - going dead for no discernible reason.

Best of luck with it!


Yes. I couldn't find any schematic of it in the internet.

This could be my homework to rebuild it from scratch. I may discard the whole circuits and build the new one with the same 300B tubes for power tubes.

Only the driver tubes will be changed or using pentode such as EF86 for the driver.

Thank you for your reply. Really appreciate it.


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PostPosted: 23 Mar 2017, 19:38 
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Joined: 13 Feb 2013, 11:02
Posts: 38
Location: Canada
Quote:
. Driver tube voltage rail, no problem.

But are they drawing current. What's the voltage on the cathodes?

_________________
Dan Santoni
DTS Audio Electronics
www.dtsaudioelectronics.com


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PostPosted: 24 Mar 2017, 00:53 
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blackdog wrote:
Quote:
. Driver tube voltage rail, no problem.

But are they drawing current. What's the voltage on the cathodes?


If I remember correctly, it is about 400Volt...


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PostPosted: 24 Mar 2017, 03:28 
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Location: Chilliwack, BC
400V... on the cathode?

I suspect an open cathode resistor or bad ground connection.

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-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 24 Mar 2017, 12:29 
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Geek wrote:
400V... on the cathode?

I suspect an open cathode resistor or bad ground connection.



Keep in mind that the 300B is a directly heated triode. Properly, no cathode as a separate element.


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PostPosted: 24 Mar 2017, 18:15 
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Location: Canada
First all 400v on the cathodes is wrong, either on the input tubes or 300b. If the power supply voltage is 400v then the plate voltage on the input tubes should be around 200v and the cathodes should be a few volts on the first and could be higher on the second stage. The 300b shouldnt have more than 20 to 30 volts depending on cathode resistors. I'm generalizing here, because i don't have a schematic, but this is simple measuerments and certain things are just wrong.
If you want to find out what's happening accurate meaurements need to be made.
If you don't know how to do this then it's time to find a good aervice tech.

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Dan Santoni
DTS Audio Electronics
www.dtsaudioelectronics.com


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