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Repairing Hi-End Preamplifier
Repairing Hi-End Preamplifier
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Old 5th May 2018, 05:34 PM   #141
merlin el mago is offline merlin el mago
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Yups just missed. I used the attached schemati, only change -3.3V in place of -3V at grid.
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File Type: jpg SCHEMATIC WITH OPT EDCOR.jpg (609.1 KB, 146 views)

Last edited by merlin el mago; 5th May 2018 at 05:39 PM.
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Old 5th May 2018, 05:57 PM   #142
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
And indeed, this particular Tamura is ungapped and generally unremarkable (not permalloy).

Luckily, even if you short the cap out it will still not blow
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Old 5th May 2018, 06:49 PM   #143
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by Boydk View Post
Just thought it was pretty obvious, that connecting 91 volts DC to a coil that is directly coupled to ground would be as good as a short :-)
Quote:
Originally Posted by analog_sa View Post
And indeed, this particular Tamura is ungapped and generally unremarkable (not permalloy).

Luckily, even if you short the cap out it will still not blow
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Old 5th May 2018, 07:24 PM   #144
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
I meant the transformer will be safe, if connected to the anode without a cap because of the current source. Obviously the anode will not stay at 90v after such a connection.
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Old 5th May 2018, 07:37 PM   #145
merlin el mago is offline merlin el mago
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Draw you badly the schematic?

Quote:
Originally Posted by analog_sa View Post
To stop dc flowing through the transformer.
What's the purpose of OPT having the output coupling capacitor?
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Old 6th May 2018, 04:30 PM   #146
anatech is offline anatech  Canada
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Repairing Hi-End Preamplifier
Hi Merlin,
If the capacitor wasn't there, the plate voltage of the tube would be very low and performance wouldn't be good. Also, the core of the transformer would have a large DC component which would shove the operating point up into a very non-linear portion of the B-H curve.

analog_sa gave you the answer to this question.

-Chris
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Old 6th May 2018, 07:48 PM   #147
merlin el mago is offline merlin el mago
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Thanks Chris. I guess the OPT is to lower the output impedance.
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Old 6th May 2018, 08:05 PM   #148
anatech is offline anatech  Canada
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Repairing Hi-End Preamplifier
Hi Merlin,
Yes, it is a matching transformer from the load the tube wants to see down to the impedance of the speaker.

-Chris
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Old 7th May 2018, 06:20 AM   #149
jahsavage is offline jahsavage  United Kingdom
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Quote:
Originally Posted by anatech View Post
Hi merlin,
I would believe that if I could see a resistor in the ground return to generate that bias voltage. It's probably there and is simply missing as an error in the schematic.

A corrected schematic would be nice so we don't have to guess at things.

-Chris
Apart from a few resistors used for the LED supplies there are none in the audio path or inductor's (excluding transformers). The small transformers in the preamp are used for balanced input and output and do not feature if just rca input and outputs are used.

The amp is currently all back together but I have a NOS Tung-sol JTL5687 to fit some time soon so I'll remeasure any voltages people are interested in.
Attached Images
File Type: jpg ps_top.jpg (970.6 KB, 77 views)
File Type: jpg ps_rear.jpg (899.5 KB, 72 views)
File Type: jpg partialCircuitPS.jpg (117.0 KB, 79 views)
File Type: jpg 20180328_120852.jpg (958.1 KB, 76 views)
File Type: jpg 20180328_124054.jpg (748.2 KB, 52 views)

Last edited by jahsavage; 7th May 2018 at 06:38 AM.
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Old 7th May 2018, 10:46 AM   #150
merlin el mago is offline merlin el mago
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Now makes sense the input & output transformers, thank you.
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