Saturday, 13 August 2022
This poor Amplifier gave itself a hard time..
Too many components faulty. Mainly electrolytics.
The main resistor supplying the BIAS voltage to the Output Tubes (6L6GC)
changed value under load conditions.
It was replaced with a Bourns 20 turn potentiometer.
The 6.3V AC from the mains transformer burnt out (don't ask...)
and a standby transformer was used to drive the filaments of the phase inverter and output tubes during these tests.
The Amplifier has now been in full operation for > 4 hours. There is hope.
That allows for the adjustment of the voltage drop across the resistor (current) that
Glued the Bourns Pot with Pratley Clear quickset.|
(Tested before it got dry. NO conductivity.)
Wednesday, 10 August 2022
NAD Service 9 Aug 2022
Approximately 7 hours work.
I don't count.
Full cleanup, contact cleaner, blow dry.
Replacing 13 electrolytic capacitors.
6 800uF with 10 000uF on split power supply. (Had to drill some holes.)
Replaced germanium bridge diodes with 1N5408 3A 1000V
Replaced 2N3055 output transistors with RCA (to match 2N2955 and due to corrosion.)
Re-calibrated power meters.
Glued mains broken socket. (Did not try to get a new one).
To do BIAS and DC offset adjustments require soldering resistors on the back of the board.
I will check the current values. If they are OK then I won't delve into that as well.
It's quite a laborious task. Let's not split hairs.
Full check... Especially decoupling resistors for differential input stage.
The Amplifier is done.
Tuesday, 9 August 2022
Standard response when someone sees this Amp for the first time.
Hand make Dutch engineering.
Three quality assurance signatures.
See for yourself:
Couldn't find any significant component drift.
The cabinet is not appealing I'd say.
Here's an ad. for one:
Koch TwinTone 50
Features All-tube circuits, matched Ruby EL34 power tubes, built-in power soak, Accutronics reverb tank, 11-ply birch plywood cabinet, two-button footswitch, independent clean and overdrive channels, independently voiced inputs for single-coil and humbucker pickups, rhythm/solo boost circuit.
Changed BIAS to 35mV
Monday, 8 August 2022
Sunday, 7 August 2022
Options are ash; black ash and cherry.