Missing Tube In Gibson Skylark GA-5

[Jmoog]

I was given a Gibson Skylark GA-5 about twenty years ago but have never really used it because it's tone was always a bit too bright for my tastes. I recently got it out again and while poking around in the back of it I noticed that it is not only missing a tube but the port for the tube has been taped over with a piece of electrical tape. I did have the amp serviced when I first got it years ago but I cannot remember what the problem was and I don't remember ever being told about a modification that involved doing away with one of the tubes. The wiring to the taped over tube port looks intact.

I guess my question is does the "taping over" of the tube port look like a common/legit modification to this amp? I can't figure out why a mod would be made that would involve removing a tube from the circuitry but admittedly I am not the most well versed guy when it comes to electronics. The amp works as well as it always has but I'm not sure if I should buy new tubes including one for the abandoned port.

I've included some pics of the amp and the taped over port.

[JHow]

Was the rectifier replaced with solid state? Is it 6X4 that is missing? If it works it should be fine and is no trouble for a tech to reverse/repair, if ever needed.

I found this schematic which may be yours?:

http://www.harpamps.com/gibson/pg_0089.jpg

[Jmoog]

Yes it is the 6x4 that is missing. What would be the advantage of doing this modification? What would be the tonal changes in the amp if any? Thanks for the schematic as well.

[Enzo]

The advantage is that a pair of 2 cent diodes will replace the tube. If someone wanted to get the amp running and didn;t have a 6X4, it gets it back in operation. The B+ voltage will rise some as a result.


Actually, unless they dewired the socket, sticking a 6X4 in there now would do pretty much nothing. The diodes would still conduct and the 6X4 could only watch.

[Jmoog]

Ok. Thanks for the help. I have a couple more questions. I'm thinking about buying all new tubes for the amp. I'll need 1-6EU7, 1-6C4, and 2-6AQ5's. Are these easily available? Where would be a good place to find these tubes? Are there other tubes I could buy for the amp to cut down on the brightness and make it more "mellower" in sound. It's a good sounding amp but I have to play it with an EQ pedal in my signal chain to cut some of the highs.

[Bruce / Mission Amps]

Antique Electronic Supply
search vacuum tubes in the contents bar on the left side.
6AQ5s are cheap... $6 to $7 range.
6EU7s and 6C4s are still plentiful too.

[Jmoog]

Thanks for the info everyone.