Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Improving Wiring on Valco/Supro Spectator Amp

I'm back to working on the Supro amp.  I believe it's a Supro Spectator, model 510.  You may recall I started it a while ago.  Then I got interrupted by other projects - two mandolins, a tenor guitar, you name it.

The amp works well after replacing the filter caps, and now I'm going to replace the 2 coupling caps and the tone cap.  And I'm going to change some of the old wiring to make it a bit quieter.

In the picture on the right, you can see the needle nose pliers pointing to the old filament wiring.  It's not even twisted!  So I'll replace that.

It also has one side going to ground.  I'm not sure why, but I will lift that and replace it with an artificial center tap.

Here's that ground I mentioned.  It's at pin 8 of the 6SL7 preamp tube.  Pins 7 and 8 are the filaments - you can see the green and blue leads there.

The second stage cathode resistor - 3.9k - also grounds there.  So I'll ground the resistor to that tab on the socket, but connect the new filament lead just to the connection on pin 8.

We have a saying in The Dungeon.  "It's not a project if there's no minor injury."

I cut my finger while running the new filament leads through the amp.  Ouch.

You can see the new (green) twisted lead in place.

Couple of things going on in the picture on the left.

Note the red cap in the left center of the picture.  That's the coupling cap from the preamp to the volume control.  In addition to replacing it, I'm going to move it to a better location.

You can also see the new green cloth insulated filament wiring in place.

And most importantly, the old wiring for the coupling cap to the output tube.  That's labelled in the picture.

This doesn't really follow the best wiring practice.  To keep noise to a minimum, and for best tone, there shouldn't be a long run of wire from the coupling cap to the grid (input) of the next stage.  What we have here is a long (yellow) lead running from that cap across the chassis to the input of the 6V6 output tube.

Best practice dictates that plate leads can/should be long to increase inductance (natural filtering).  And grid (input) leads should be short to resist noise.  And if they can't be shortened, we can use shielded cable.

Because this amp is so small and the chassis could have been laid out better, the lead has to pass near the filament wiring and the wiring to the transformers.  Which means it can pick up hum from the AC lines there.

I'm going to run a shielded cable from the plate of the preamp tube across the chassis, to the (new) cap which will be right near the 6V6 grid.  The shield on the cable will be grounded on just one end so there won't be a ground loop.

Also, note the grid leak resistor.  It's for the input of the 6V6.  Electronically, it's connected where it should be, from the grid input to ground, but I think it's better off located over at the 6V6 grid along with the cap that will go there.  So I'll change that.

A lot of these amps were built to a price and there are some layout goofs you'll see.  This layout would have been better if the output tube was located further away from the power supply - maybe near where I have the label "grid leak" on the chassis.  But it was not, so we're improving the wiring layout to cut down noise and hum at least.

Here's one change I mentioned now implemented.  The coupling cap from the preamp tube has been moved over to its 'grid' (input to volume control).  I ran a shielded lead from the plate of the tube to the cap.

The stock value of this cap was .01 uF, and I'm upping it a bit to .02 to increase bass and mids.

Closeup of the input to the preamp tube.

I got a bit crazy and did the short lead to the first stage of the tube from the input jack(s) with shielded cable.

You can see the output to the volume pot is also shielded.  The shields on both of these cables run together and are soldered to the ground side of the first input jack.  We only run one end of the cable's shield to ground.  If you connect both ends, you have a ground loop which will cause hum - just what we are trying to avoid!

That ground connects to the ground buss for all three input jacks which I had wired previously.

All of the input grounds, including the cathode bypass cap and resistor, run to one common ground point.  Again, just basic good wiring practice.

Before I change out that long wire I spoke of earlier, I did a 'taste test' of some different coupling caps.  I just tack-soldered them to the input of the 6V6 and connected a lead to each of them in turn.

On this test, I have an Angela/Jensen PIO, an Orange Drop, and a Mallory 150.

I loved the tone of the Jensen, but I only have one and it's an .05 cap, and I want to try a .1 - again, for a bit more bass and mids.

So I dipped into my stash of Soviet K40Y-9 PIO caps to try a .1 PIO.

Stay tuned.

 
 
 
 

Post a Comment 0 comments:

Post a Comment