Mid 1960s Kent amp question

[AmberB]

Hello!
I just got a mid 1960s Kent amp to repair and test.  This amp has a 2 wire cord, as usual, and I'm supposed to convert it to 3 wire.  Normally this is pretty straight forward, but this amp has a quirk that I haven't seen before that prompts me to ask a question. 
When the amp came to me, the wire to the standby switch was broken from the switch.  The first thing that I noticed was that that wire goes to the power transformer.  The second thing I noticed is that the other side of the standby switch goes to ground.  Upon further investigation, I discovered that the standby switch on this amp works by lifting the center tap for the B+ winding from ground.  The amp uses an EZ-81 rectifier tube.  This is the first amp that I've seen with a standby switch set up like this.  The configuration is correct according to the schematic I found glued inside the amp's case.
So, my question is, do I just remove the "death cap" and connect the ground wire of the 3 wire cord to the chassis as usual, or is there something else I need to do because of the way the standby switch works?

[sluckey]

So, my question is, do I just remove the "death cap" and connect the ground wire of the 3 wire cord to the chassis as usual?

yes

[Latole]

 "I discovered that the standby switch on this amp works by lifting the center tap for the B+ winding from ground.  The amp uses an EZ-81 rectifier tube.  This is the first amp that I've seen with a standby switch set up like this.  "
-AmberB

That is very common with SS rectifiers ( Traynor), With tube rectifier ? I don't know

[AmberB]

Here's a few pictures of the amp.  This a Kent model DR-57.  There's a small schematic glued inside the cabinet.  Since this is just a head and not a combo, the schematic is a bit hard to read, but at least it helped to confirm the circuits in the amp.  Someone did a rebuild on this amp not that long ago, judging by the nice tube sockets and the new tubes.

[AmberB]

The rebuild was done correctly, but sloppy.  I cleaned up some sloppy soldering, and removed a few cutoff ends where it looked like the old component had been snipped off and the new one added to the connection.  It works, but I tend to consider that to be sloppy work.  The rebuilder had used ceramic capacitors for almost all of the replacements.  I replaced 2 of them with film caps, they were .02uf caps.  I'm almost done with the 3 wire power cord conversion now.

I found a schematic on line that's pretty much identical to the DR-57, I'll attach that so you can see how weird this amp is compared to amps like Fender, Marshall, and even Ampeg, which has some fairly weird circuits in a few of their amps.
You'll notice that the DR-45 does not have a standby switch.

[Latole]

Tubes rectifier are often slow to produce full DC voltage. No need of a stanby switch.

[AmberB]

Well, this thing has a hell of a buzz, so now I have to figure that out...

[AmberB]

The grounds on this amp were a real mess, the parts were just grounded wherever was convenient.  The person who rebuilt it gave no thought to routing the grounding.  It reminds me of me when I first started working on amps, when I didn't understand ground loops...
Another thing with this amp, one side of the heaters was just routed top ground.  That's how the schematic shows it, and I know that other amps of the period were also done that way.  I'm going to add a 100 ohm resistor to each side of the heater circuit to ground, at the far end of the heater string, at the preamp tube.  All of the preamp grounds are now routed to one spot at the furthest spot away from the power supply.

[Latole]

You are right about the heater wiring; isolate them from the ground and put artificial center tap to ground with two 100 ohms resistor.
Amp will be more quiet.

[Willabe]

The grounds on this amp were a real mess, the parts were just grounded wherever was convenient.  The person who rebuilt it gave no thought to routing the grounding. 

Ever read this? Very good grounding info and grounding scheme;

http://www.valvewizard.co.uk/Grounding.pdf

[AmberB]

I seem to have fixed the amp.  I tested it today, and the nasty buzz is gone.  There's a slight background hum with a guitar plugged in, but it's quiet, and doesn't seem to get louder when you turn up the amp.  To make the artificial center tap for the heater string, I used a grounding lug that I held on the chassis with a #6 screw and nut, and then soldered to the chassis.  There was already solder on the chassis at the location that I used, so that worked out well.  I soldered both of the 100 ohm resistor tails to that lug.
I'm pretty happy with the results of this repair.

[AmberB]

Here's a few more pictures of this old 1966 Kent guitar amp head.
I wired an artificial center tap for the heater circuit that goes from both sides of the heater circuit to the cathodes of the EL-84s to lift the heaters above ground.  It seems to work well.  I had to change the artificial center tap setup from the previous one because it didn't work to get rid of the buzz.
  The EL-84s are cathode biased by a 135 ohm-7 watt resistor and a 100uf bypass cap to ground.  I moved that resistor and cap to better locations on the chassis.
I also put tube shields on the preamp tube and the PI tube.  Since the tube sockets are mounted from under the chassis, it was easy enough to install the tube shields.

[AmberB]

Here's a close up of how I wired the artificial center tap for the heaters.  I used a pair of 100 ohm flameproof resistors and covered one leg of each with shrink tubing where they were close to other components.

[AmberB]

There's a schematic inside the case of this amp.  I copied it onto a piece of paper as best I could, to have a schematic that I could read easier.  Upon comparing the schematic for this amp, the DR-57, with the schematic for the DR-45, I found that they are almost identical, with only a couple of differences.  The DR-45 shows a 10K resistor between the hot and ground wires of the speaker side of the output transformer, that's not on the DR-57, and the DR-57 has a standby switch where the DR-45 does not.