2. amps-preamps
  3. 139131-120014

Randall RG50TC tube amp issue

Hi there. My tube amp - Randall RG50TC (I have 2 of them) started to:
1. sound very thin even on high gain - it's like very low distortion all the time
2. if I palm mute and strum a chord the sound coming out of the amp "vibrates". Like an echo
I have changed pre-amp tubes, I have a sticker that says position (1, 2, 4) 12AX7. Position 3 sticker is missing. I found on the web it is the same so I hope even if I have an "older" model it should be the same.
I put everywhere 12AX7 various combos (1xEX7, 3xShuageng) - all new. I tried combination with different old tubes I have, keeping the new one I just bought in V1...
The distortion was there but after a while it is weak again. And the vibration sound is there all the time. Any ides? Power tubes - do they influence anything? I mean the amp is loud as it should be. I haven't changed the power tubes for 10 years. Both amps behaving the same.
Thanks!
pumukly
roberjerman
4,319 posts
 
Those poor guitar amps! 10 years on those output tubes! Get NEW sets and bias them correctly.
roberjerman
4,319 posts
 
Another case of "rock musician" = NOT SMART!
pumukly OP
1 posts
 

Thanks, all worked, didn't want to touch it...I'm a little more knowledgebase now...

I was browsing a little, I found my amp came with EL34 however I have 5881 (=6L6) inside. I bought the amp used and I have had it this way for 10 years or so...and I was wondering whether I can put EL34 back and bias them properly and that is it? Or does it require some mod to do so, which would mean my amp has been either modded by previous user or it can simply accommodate EL34 and 6L6 and biasing is all that is required...?

2nd amp has output tubes with no name and there is only a number

?2718(guess)7 so no idea what type of tube that is...

2nd question: biasing is required only once, when changing tubes or is it good to do it on a regular basis (would this make tubes 'healthier')?

roberjerman
4,319 posts
 
Find a competent technician nearby that can examine your amps and compare them to an original schematic (to see if any mods have been done). EL34's will put out a bit more power than 6L6's (35 watts/pair - 25 watts/pair). They also take a higher bias voltage (55 volts) than 6L6's (45 volts). The higher the bias voltage the less current going through the output tubes. Too little and the plates will get too hot and glow red. THIS IS BAD! Never let this happen as it will kill your output tubes! Bias should be checked depending upon how much usage the amps have had. Stick with the 6L6's. Their heaters draw about half the current of EL34's - so less stress on the power transformer - it will run cooler.
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