GOLDENTONE: Reverbmaster 40watt guitar amp

I saw this one pop up recently and decided to take a punt on it (thanks Damian).  For the money, I figured it was worth a look even if it only ended up being a decent chassis and set of transformers.

This amp is a testament to the quality work that Goldetone produced - despite what has clearly been a very hard life, she works just fine (apart from the heavily corroded reverb tank). 

image from the ebay listing

Cosmetically, it has had a bit of a rough life (perhaps an understatement).  Where to begin? The original tolex has been removed, the cabinet was painted yellow, then black, and then at some point it was covered in some kind of 1970s or 1980s woodgrain vinyl, to cover up the paint.  The head still feels solid, so there’s something to work with.

I didn’t think the original faceplate was on the amp, so I was pleasantly surprised when the silver paint started wiping away with isopropyl alcohol, revealing the original panel underneath. It's still in good shape too - maybe the dodgy paint work protected it?  

The reverb tank, on the other hand, is absolutely cooked.  It is in terrible condition, so that will be going straight in the bin.

Inside the chassis was a much nicer.  It looks to be all original, with no obvious signs of previous work or failed parts.  It was dusty as hell, but I’ll take dust over a butchered circuit any day.

GOLDENTONE - REVERBMASTER 40 / SLIMLINE

There are a couple of possibilities as to exactly what model this is - but I'm just calling it a Reverbmaster for now.  It may be the earlier slimline.   It's a combo that has been cut down to a head.

Preamps

12AX7.  Bass and treble controls using a James style tone controls

REVERB / TREM

First gain stage

Ra 220k

Rk 3k3, 25u bypass

Second gain stage

Ra 220k

Rk 1k8, 25u bypass

NORMAL

First gain stage

Ra 100k

Rk 3k3, 25u bypass

Second gain stage

Ra 100k

Rk 3k3, 25u bypass

All pots 1MC (Volume, bass and treble)

B+ 240v from filter node

Reverb

Valve driven, 6GW8

Rola LRH3 transformer

Pioneer EAV-201 reverb tank

Trem

12AX7 oscillator, modulating the cathode of a preamp triode.  No depth control, just speed.

Phase inverter

12AX7, longtail pair

100n coupling caps

22k negative feedback from OT secondaries

B+ 340v from filter node

Power stage

2 x 6DQ6B fitted - although schematics often show 6DQ6A (6GW6)

~350v on plates

~180v on the screen

47 ohm resistor to screen

10k grid stopper resistors

470k grid leak resistors

Fixed bias -30 to -32v.  Bias not adjustable - no pot

Output transformer (OT)

A&R 2700

Primary 3k3

Secondaries 16, 8 & 4 ohms

OT Measurements

84 ohms DCR primary

Turns ratio 18.1:1  (10.5 VAC, 440hz sine wave applied to primary: result, 0.58 VAC on secondary)

~2.6kΩ at 8 ohms, 5.2kΩ at 16 ohms

This is different to the specs above - possibly the result of construction variations and a low-voltage test.

Power supply

A&R 5892 (which is different to schematics)

Silicon bridge rectifier

Rola choke TV301, May 1965

B+ expected to be about 360v, currently untested

My little moment of joy, finding the original faceplate under the silver paint.  They may be original knobs, but I'm not sure. 

The 6DQ6 valves have their anode on a top cap - hence the warnings. 

A&R 5892 power transformer &  a big Ducon filter cap for the power supply.

Choke & reverb transformer& 6GW8 valve for the reverb

Prior to dust removal - all the mustard caps look fine, no cracks.  Can't see any resistors that have been heat stressed.

This was a reverb tank at some stage in life.  

This is after most of the horrific vinyl has been removed, showing the remants of varius paint jobs underneath.

SCHEMATIC

There's also this version by Peter Hutch, on Aussie Guitar Gear Heads - which has a few variations by the look of it.  Mine appears to be laid out the same way, but there are differences.