Modifying An Epiphone Valve Junior (Legacy Valve Head)

The Epiphone valve junior amps were sold under various names around the world. In Europe a version was sold under the name Harley-Benton GA5 (combo) and a head-only version as the GA5H.
This combo was pretty much exactly the same as an Epi VJ mk2 (in the case of the head, it was the same as the Epi head version, which was a Mk 2 with 3 output jacks)
The only difference is that the Harley Bentons have a tone control. More on this later.

In Australia, this very same amp was sold in a black tolex cab ( the HBs are blonde), branded "Legacy".
The head version is called a "valve head". I dunno about the combo. I wouldn't be surprised if it is a "valve combo".

I own an Epi mk3 combo, and I also have a Legacy Valve Head.
Despite these amps being almost exactly the same I noticed the Legacy was much tamer. I wondered if something was wrong with it.
Turned out something was wrong with it, but that's not why it lacked grunt.
I changed the valves, thinking they were likely worn out. That made no difference.
I removed the chassis and looked closely for signs of damage. I noticed two wires leading to the tone control appeared to have some scorching and the board around the EL84 power tube appeared discoloured.
I decided this was the amp with which to begin learning about amps.
I'd been eying a mod kit on eBay for some time. This was the kit sold by "alnicomagnet" to modify the Harley Benton GA5 and GA5H. I bought the "blues harp" version.

The kit consists of a bunch of (clearly labelled) resistors, capacitors, switches, lengths of wire, a fuse holder and fuse, a phono jack, a couple of potentiometers, a pair of chicken head knobs, and a disc with the instructions.
First thing I did was run the disc and print the instructions. It's a hefty document; about 40 pages.

Next thing I did was read it. It's not really as daunting as it may sound. The instructions are well written and easy to follow. Topics are set out in logical order, but I'd recommend reading all the instructions before starting on the work, as things do become clearer when you have it all in context. Quite similiar to using a recipe.

I checked off the parts list to ensure I had everything, and got to work.

Nigel recommends drawing the amp which I expect would actually be a really good way to familiarise oneself with all the components and the layout. This is to help with reassembly. I used my phone camera and took several photos of the circuit board and all the connections inside the chassis.
Once I felt confident I'd completely documented how things looked, I disconnected all the leads from the circuit board and unscrewed it from the chassis. There were some connectors I wasn't familiar with, but I soon worked them out.
The thinking behind these mods is not simply to re voice the amp for harp, but also to improve the reliability and longevity.
The very first thing I noticed was the amp is wired to run from European continental power supply. Just like the Harley Benton amp. It's an easy fix, just swap the wire running the mains AC into the power transformer.

The next thing I noticed was just how discoloured the PCB was around the EL84 socket. That tube has been running really hot. It was much more noticeable when I could see the underside of the board, without the other components covering it up.
I guess for one thing, if the PT has been seeing higher voltage in the winding than it was designed for, it would have been also putting out higher voltages. And also I have been turning up the volume much higher than I would on any other amp, so I run it relatively hard in an electronic sense.
Anyway, down to business. I ran through the list and removed all the resistors and capacitors scheduled for replacement. On a PCB I actually find desoldering trickier than soldering. This job took me a while, using solder wick to suck up the old solder as much as I could, then removing the unwanted components. I put them all aside for possible future use.
I also removed the leads to the mini PCB for the EQ control.
Soldering in the new parts was straightforward. I'd done some practice leading up to this, putting together a low power solid state amp from a kit. I was happy with the result, all looked neat.

Fitting the standby switch, fuse and line-out all required me to drill holes in the chassis. I fitted these in the back and I just referred to the photos included with the instructions re placement. It makes sense to locate the fuse and standby close to the power transformer, and the line-out comes from the speaker jacks, so it simplifies things to put it next door.
Drilling the holes required some gear. The fuse in particular needs a 12mm hole. That's not something you can really do without using both hands on the drill and the chassis in a vice.
After cleaning up the edges of the holes it was pretty straightforward to install the standby switch and fuse its just a simple series circuit. If I did it again I'd probably fit some heat shrink over the connections, but I made very neat connections. I've seen much worse in amps straight from the shop.
The line out confused me a little. At first I got it wrong. The instructions said 'connect as shown in photo' but I was away from my photos at the time. That was ok, though, where I went wrong was in not realising that while the phono jack had two sets of lugs for tip and sleeve, when you insert a plug the connection breaks on one set. Id wired my tip to sleeve resistor on that side. Anyway, the instructions are fine, you just have to actually follow them rather than make up your own when you don't really understand!
Basically, there's a resistor between tip and sleeve of the line out jack, and then this jack is connected to the speaker out jack. A resistor between the tip connections of both jacks. Alnicomagnet mod also places a 20 ohm resistor between the two sleeves, but from what ive been able to gather this is not really necessary if both amps are properly grounded. It doesn't hurt though.
The trickiest job was changing the tone control. And it was only tricky because the wires are quite fine, the lugs fairly close together and I didn't want to strip more insulation than necessary and then have exposed wires. (Some suitable heat shrink would have helped perhaps.) really the work wasn't complicated, just a little fiddly.
I removed both tone and volume pot. The volume pot connects to the main pcb via a lead housing 3 wires, with a 3 pin connector at each end.. I removed the connector from the end which connects to the pot.
The tone pot has its own mini pcb, connected to the main board by two wires. These wires were completely removed.
To assemble the new tone and volume arrangement,I installed the pots in the chassis,but on the outside, and upside down. Using the chassis as a jig in this way allows you space to make connections between the two pots and then install them the right way around and they fit perfectly.
Its a straightforward concept; a condenser between volume and tone pot that allows you to dial out high frequencies without cutting any gain. As I said, some of the connections are fiddly but I coped.
Reassembly was easy with reference to my photos, but I'd learned so much about how the amp works in the process that I probably could have done it without reference. It was good to have the photos though, just for certainty.
Testing the amp with the multimeter as described was also cool. Everything checked out within spec. Putting the amp back together and playing through it I found everything worked, except line out, which I eventually sorted...my bad, as noted above.
The amp should now be much easier on tubes. Honestly, I think it would not have lasted much longer, it was clearly way too hot before. If a tube blows, my power supply is now protected.
It's much louder now, the tone is significantly better than my stock Epi VJ, as I can now cut any annoying treble ring, but its just a subtly smoother rounder sound, but still lively especially through my twin 8" weber cab.
I played it through a 412 cab and it drove that pretty well too. Not as much cut as with the twin 8"s, but a bigger sound. I reckon a pair of 10"s would be cool with it.
I used it at open mic and very pleased with it. Ok, it's my new amp of choice for now, its personal now. ;0)