Saturday, 13 August 2022

Peavy 6505 Guitar Amplifier On The Bench

 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.

Most of the electrolytics have been replaced. Some resistors burnt out and was replaced.
Added a Bourns 20 turn POT for R108 (5.76K)
That allows for the adjustment of the voltage drop across the resistor (current) that
supplies the BIAS voltage.
Glued the Bourns Pot with Pratley Clear quickset.|
(Tested before it got dry. NO conductivity.)

Adding another transformer (6.3V~ 20VA), to supply power to the relevant tubes should do the job.

Some pics of the inside...

Wednesday, 10 August 2022

Steve Vai Live (2008)


Sorry about the poor quality.

I just got reminded about Guthrie Govan. Great guitarist.


NAD Stereophonic Amplifier Restoration (1973)

 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

Koch Twintone II - The Guitar Tube Amplifier that no one's heard of..


"I've never heard of it..."
Standard response when someone sees this Amp for the first time.

Hand make Dutch engineering.
Three quality assurance signatures.
See for yourself:

The chassis as it came out of the Cabinet.

Those HT Caps still look good.

Not bad for an 18 year old Amp.
Couldn't find any significant component drift.

Populated with TAD Tubes except for the two JJ EL34's

Transformers are 18 years old.

Pretty sturdy.

Feeling a bit more confident seeing the chassis.

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.

Price $2,150

Bias 'out-of-the-box'

Changed generic Chinese Phase Inverter Tube for a 12AX7EH
Changed BIAS to 35mV

Sunday, 7 August 2022

B&W CDM7 Speakers Service: Disassembly; Quality Assurance...

Not a one man job

MDF Side panels; Poplar bracing and real wood veneer
Options are ash; black ash and cherry.


Tweeter checks

Spikes check

Seal rejuvenation 

Crossover checks

Bass and Bass/Mid Driver checks

Assembly hardware.

The crossover design:
Aircore coils and BENNIC Capacitors