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Randall RGT 100 Amp Problem
Randall RGT 100 Amp Problem
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Old 10th May 2008, 09:41 AM   #1
GF33 is offline GF33  Canada
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Default Randall RGT 100 Amp Problem

Hello all,

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

I'm still learning and I'm really hoping that I am overlooking something obvious.

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. As far as I can tell, attempting to follow some guides I've found on the internet, it is okay. Proper power and resistance readings off all the leads that should have a reading. The Output Tranny...well I thought it has tested okay but the more I read, the more uncertain I am as to whether I am testing correctly.

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.

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!
Attached Files
File Type: pdf randall rgt-100 - schematic v2.x (clearer).pdf (35.0 KB, 199 views)
 
Old 11th May 2008, 09:26 AM   #2
korey is offline korey  United States
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Sounds like your not getting any B+ (high) voltage. Do you own a volt meter? With one of these, you can trace down your problem within minutes. I suggest testing the voltage coming off your stand-by switch. If nothing there, then your rectified DC is down. I looked at your schematic and would suggest looking at all of your power resistors associated with the DC power supply, even re-check the one you replaced after it fried. If there is no voltage reading on pin# 3 of the 6L6 tubes, then check your voltage at the 1st, 2nd, and 3rd stages of your power filter capacitors. I didn't want to go into too much technical detail incase you were an ametuer but the problem is somewhere within your AC to DC power supply. Check your resistors at points A, B, C, and D on your schematic. Your speaker transformer is most likely OK, but you can check the resistance between the BLUE and BROWN wires with respect to the center tap (red wire).
 
Old 11th May 2008, 01:33 PM   #3
SY is offline SY  United States
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Randall RGT 100 Amp Problem
Yes, do carefully check to see if you have high voltage. If the B+ looks OK, then check the screen voltages on the output tubes.

When you say you replaced the diodes, was it the diodes that are strapped across the output transformer?
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Old 11th May 2008, 03:02 PM   #4
kevinkr is offline kevinkr  United States
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Randall RGT 100 Amp Problem
The fact that a 1.5K resistor tied to the control grid cooked is really weird, seems like there must have been a short somewhere to cause that to happen - this is normally a high impedance circuit and there is no place for large currents to flow. I certainly would not reinstall any of the old power tubes as the only plausible paths I can think of involve the two output tubes on that side of the transformer.

Are you sure those diodes on the primary of the opt were actually bad - if they are rated for 5KV piv as the schematic seems to indicate, they would frequently have a forward voltage beyond the capability of a normal dvm - something on the order of 2 - 3V is possible depending on how they are made. Incidentally most regular diodes won't live too long doing this job, you need to replace them with similar types. What did you use to replace them with?

Check all of the resistors in the pi-filter circuit, it would not surprise me in the least if there were not at least a few fusible resistors in there, and one or more of them may have opened up.

Also check the bias supply once you have identified the other failed components and make sure it is making the proper voltage and that it is present at the grids of all 4 power tubes before applying B+.
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Old 12th May 2008, 02:26 AM   #5
GF33 is offline GF33  Canada
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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.
 
Old 4th May 2010, 02:47 AM   #6
valdio is offline valdio
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I have one of this randalls rgt100es heads, I was wondering if someone out there have an schematic or instructions to modify this baby so that can use EL34's instead of 6L6's, please help, I have an idea of how to it but if there is a tested method to upgrade this babies to the 34 tone share that with me, thanks

Jose Valdiosera
 

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