Amplifier service
- Thread starter RVA
- Start date
OP
RVA
Ambassador
Thank you all for encouraging me to take the simple route. A power tube swap and bias did the trick. One of the old tubes was cracked at the plastic base! Now it is warm and sweet and not emitting any unusual feedback. It handles pedals like a champ (th winner, not the amp). I am reminded why this is my all-time favorite amp.
While I was in there, I took some gut shots for those interested
While I was in there, I took some gut shots for those interested
chilipeppermaniac
Ambassador of Decibels
Ray, I see it was a tube.
My fix for my buddy's amp was 2 new KT66's here is why.
Well, based on the suggestion from Ivan to put the ohm meter on the valves themselves on pin 2 and 7. Both valves read OL
2 bad tubes.
My fix for my buddy's amp was 2 new KT66's here is why.
Well, based on the suggestion from Ivan to put the ohm meter on the valves themselves on pin 2 and 7. Both valves read OL
2 bad tubes.
chilipeppermaniac
Ambassador of Decibels
Ivan----Ok, I'd say that's it. Should be only a couple or few ohms. I don't have any KT66's but an EL34 reads 2.2 ohm. Check the lamp, I imagine its blown too. Well done Chilli. Cheers
Definitely!
OP
RVA
Ambassador
I spoke too soon. Now I am getting periodic crackle and cut out after the amp is on for a minute. Tried another set of power tubes, chdcked the cables, cleaned the fx loop and input jacks without luck. Next step...new preamp tubes
OP
RVA
Ambassador
Ok, I just solved the prior problem and this one. When I swapped tubes about a year and a half ago, it seems I used the bias settings for the 410/100w, not the 205/50w. This would account for the excessive feedback. I keep a log, so I looked at it tonight and initially followed this mistake again. In researching my cut out issue, I realized my mistake and set it right. No more cut out!
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OP
RVA
Ambassador
Well, the biasing mistake was my fault, but at least it is undone!
Catmandue
Well-Known Member
Poop happens bro! You cherryed it out right now! Ray, what do you use to bias. I have one of those double type probes by VHT. I use that with my Klein auto ranging meter to set bias. I like to set it on my DSL15C because, there are two different trimmers to adjust each tube. On my SR there is only one deal to adjust both 6L6 tubes. It is best to have them as closely matched as possible. Some of these tube places do a less than fine job on matching power tubes, I'm afraid.
syscokid
Ambassador of War & Peace
First of all... Great news on fixing the amp. You went about it the right way...
Thanks for posting the gut shots. Scary, yet interesting in there!
Questions...
Was the glass cracked at the base?
What brand and type of tube?
Do you recall any physical trauma or mishandling of the tube?
How old was the tube?
How much difference in the setting of the bias between the 410 and 205?
Where is the bias control?
What tools do you use to check and adjust the bias?
OP
RVA
Ambassador
Thanks!First of all... Great news on fixing the amp. You went about it the right way...
Thanks for posting the gut shots. Scary, yet interesting in there!
Questions...
Was the glass cracked at the base? [Nope. The plastic stem broke while I was removing it. It probably weakened due to heat]
What brand and type of tube? [Electro Harmonics EL84]
Do you recall any physical trauma or mishandling of the tube? [None. I am careful]
How old was the tube? [15 months]
How much difference in the setting of the bias between the 410 and 205? [BIG. Double actually. The 410 is 70-80mV per side, the 205 is 35-40mV]
Where is the bias control? [see below. The 3 test pins are circled in red, the trim pots are circled i blue. The middle and right pin is one tube, the middle and left in is the other. The trim pot is ultra responsive]
What tools do you use to check and adjust the bias? [just a multimeter set to mill-volts and a small screwdriver]
