Wednesday, October 21, 2009

Epiphone Valve Junior - the Gain Matrix Mod Part II

I'm going to start describing the Valve Junior Gain Matrix Mod with the physical side of the job. There are three gain stages in the Valve Junior. I'm installing one switch for each of these stages. Here's the chassis marked out for drilling the switch holes:

Valve Junior Gain Reduction Mod - chassis marked for drilling

The switches will change the gain and frequency response by selecting and disconnecting the cathode bypass capacitors. Here is a front of chassis view the three switches installed:

Valve Junior Gain Reduction Mod - switches installed

And here they are from inside the chassis:

Valve Junior Gain Reduction Mod - switches from inside chassis

In the following post you'll see that the wires are color coded the wires to make it a bit easier to follow the wiring.
The yellow wires are for the "stock" configuration for each stage.
They connect to the left hand terminal of their respective switch.
The blue wires are for the "mod" value caps.
They connect to the right hand terminals.
The black wires are for ground and they connect to the center terminals:

Valve Junior Gain Reduction Mod - wire color coding

Next up I'll show the actual circuit wiring and in the fourth I'll draw up a schematic for the mod.

Tuesday, October 20, 2009

Champion 600 Speaker Upgrade Comparison

I installed four different speakers in my Champion 600 and ran some frequency response sweeps for the sake of comparison.*

What I saw in the graphs pretty much confirmed what I heard in each of them.

First the stock speaker compared to a Jensen Mod:

Champion 600 Stock Speaker vs Jensen Mod

Just as my ear was telling me - the Jensen is a touch more efficient and a touch smoother but they really didn't sound all that different. I found the difference to be hardly worth the upgrade cost.

Here is the Weber alnico with a late breakup cone compared to the stock Champion 600 speaker:

Champion 600 stock speaker vs Weber alnico late break up ribbed cone

Notice that the Weber late breakup alnico is a good bit more efficient than the stock speaker up to about 3K, where it drops off dramatically. This probably accounts for some people experiencing the Weber as dark sounding. What isn't evident from the graph is that in addition to the increased bass response the Weber is more able to handle the bass so it doesn't fart out the way the stock speaker does.

The next graph is interesting. It compares the Weber late break up ribbed cone to the Weber early break up smooth cone:

Weber alnico speaker comparison - late break up ribbed cone vs early break up smooth cone

They're pretty similar in the bass but in the upper mids and treble they're very different. So in addition to the breakup characteristics they have quite different upper range response. Personally I prefer the smooth cone sound - it sounds a bit more vintage to my ear. The ribbed cone sounds tighter and more modern. They both sound great though and are both worth the upgrade.

Here is the smooth cone Weber compared to the stock speaker:

Weber smooth cone early break up alnico compared to the Fender Champion 600 stock speaker

It look much more like the original in the treble response. Kind of like the stock speaker with increased bass and mids (and better bass handling).

Here's plots for all four speakers on the same graph - just for ease of comparison:

Frequency response of all four Champion 600 speakers

Incidentally, I found that the stock grill cloth really does flap around at higher volumes making some nasty farting and flapping along with the low notes. I cut mine out when I cut down the baffleboard for the Weber speaker installation and am looking for something to replace it with.

* These graphs are for the sake of comparison between the speakers. Since they aren't anechoic chamber tests the graphs include the room effects so don't pay to much attention to each tiny peak and dip. They don't show isolated speaker response but work fine to illustrate how the speakers differ from each other under the same room conditions.