RGT 100 Problem...FIXED!!!

[GF3364]

Hello all,

I figured I'd try posting here to see if anyone may be able to shed some light on this problem.

About 2 weeks ago, I warmed up the amp on standby for about 7 minutes, and then, when I switched to full power, one of the 1.5k resistors on one of th power tubes cooked.

I quickly powered the amp off, examined the whole amp, which was visibly okay. I replaced the resistor.

The amp powers on okay, and all tubes glow orange as the heaters are getting power and it appears to be in standby mode.

When I power on, no sound! The tubes stay orange like they are in standby but DO NOT switch to the familiar pale blue like a tube should.

Since then, I've checked the the Power Tranny. It's okay. Power readings off all the leads that should have power. The Output Tranny has readable signal on the leads that connect to the tubes. I've tested the whole power board, and did replace two diodes on this board. They weren't bad but they seemed like the wire leads were weak.

I also replaced the 3 22uF/450V (which appear to be listed as 20/450 on the schematic) caps on the board though the originals seem alright. Infinite resistance so it doesn't seem like they are leaky.

The symptoms are the same.

I found two dead diodes on the 2 outer power tubes as can be seen in the schematic. Replaced those. Still nothing.

I've tried new preamp tubes and new power tubes.

Nothing! Still no sound/output.

The amp responds okay in all other respects. The channels switch, and from what I have been able to find following the schematic, all signal flow and components seem okay.

Anyone have suggestions. I'm sure someone besides me would have this figured out in minutes. I know I'm missing something simple or obvious!

Any help would be greatly appreciated!

Thank you very much all!

http://ca.geocities.com/[email protected]/docs/randallrgt100schematic.pdf

 

[kc2eeb]

Check the Standby switch. If you have Plate voltage on one side and 0 volts on the other side in Play position, the switch has failed. Hope this helps! Pat L.

 

[GF3364]

Hey Pat!

Thank for the reply!

Funny you mentioned that because about 30 minutes ago, I was actually comparing the Randall schematic which is a bit sketchy as far as signal path in some areas...at least for me. The way it's drawn, they often use a lettered legend method so one has to kind of find where "A" in a circle meets "A" in a circle elsewhere in the schematic.

I happened to be looking at a couple other schematics for other 6L6 based amps for some various tube pin reference and confirmation of a few other things. I just checked the switch which on this amp is a single not 2 like most other tube amps so, one direction is Standby, the other is On and middle is off.

Long winded I know but anyhow, the switch is okay.

I've rechecked the Power Tranny and the Output tranny based on some technical guides at GeoFX.com. Again, from what I can tell, both transformers seem okay.

I know my inexperience is a large part of not finding the problem.

Hope some others chime in with some info.

Thanks again.

Take care!

Check the Standby switch. If you have Plate voltage on one side and 0 volts on the other side in Play position, the switch has failed. Hope this helps! Pat L.

 

[GF3364]

SUCCESS!!

I got the amp working!

It was, as I had suspected all along, my "NEWBNESS"!

A $0.25 5W 470ohm resistor on the power board! ARRRGGH!

Simply my mistake.

As I studied, re-studied, re-checked, followed the circuit path, re-read the instructions on testing transformers making sure I was understanding everything and testing correctly, referring to my Electronics for Dumbasses book and various other literature, etc., it finally dawned on me!

I missed a continuity check on the power board circuit, and had my head up my *** when checking the voltages at the standby switch. Flip the switch voltage passes through...of course. Flip back to standby, no volatage of course. So, voltage going to the Output Transformer, when on...okay. Voltage on one side of the resistor but incorrectly followed the lead going to the other side of the 470ohm. Duh!

Popped in the resistor and bang zoom! The "blue arc" when I flipped out of standby!

And, sound!

This thing kicks ***! It's loud! Has a very AC/DC ish type sound. Very cool. I'm sure if one were to run something in front to bang on the preamp section a bit or run a fairly hot pickup into it, it'll liven up even more! I was using my Tele to test it.

THANKS VERY MUCH TO ALL WHO REPLIED AND MADE SUGGESTIONS!

Kept me thinking and re-analyzing!

Cheers.