• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Repairing Hi-End Preamplifier

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Member
Joined 2009
Paid Member
Yups just missed. I used the attached schemati, only change -3.3V in place of -3V at grid.
 

Attachments

  • SCHEMATIC WITH OPT EDCOR.jpg
    SCHEMATIC WITH OPT EDCOR.jpg
    609.1 KB · Views: 248
Last edited:
Administrator
Joined 2004
Paid Member
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
 
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.
 

Attachments

  • ps_top.jpg
    ps_top.jpg
    970.6 KB · Views: 176
  • ps_rear.jpg
    ps_rear.jpg
    899.5 KB · Views: 172
  • partialCircuitPS.jpg
    partialCircuitPS.jpg
    117 KB · Views: 186
  • 20180328_120852.jpg
    20180328_120852.jpg
    958.1 KB · Views: 179
  • 20180328_124054.jpg
    20180328_124054.jpg
    748.2 KB · Views: 139
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.