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Hi,
I'm troubleshooting this amp for an aquaintance. This is not the schematic of the amp. The front end is similar but the output tubes are dual triode 6N5s. Left channel was working and no sound from the right. I found that the 100K plate resistor on the input tube 6N2 was open. I measured the working channel's plate resistor and it measured 210K ohms. The color code on the resistor says 100k but it measured 210K. I replace the bad resistor with a 210k resistor and the amp seems to work fine. So my question is should I change both the resistors to 100k as shown on the schematic and as indicated on the resistor color code or leave it? What would I gain or lose with the change, if any? Any help would be appreciated. Also, this amp has two switches. One says power and the other says protective switch. What's the purpose of the protective switch?
One more question, the amp's 2A fast acting fuse would blow on turn on every once in a while . Would a slow-blow 2A or 3A fuse work instead?
Regards,
David
Edits: 07/03/15 07/03/15Follow Ups:
Since the first and second stages are direct coupled you need to know the design voltage for the input tube's plate; because, this voltage is also the grid voltage of the second stage phase splitter/driver. Changing the load resistor will change this voltage.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Thanks for the reply. All I have is the above schematic and voltages. Given the info what would happen if the 100k ohm load was increased to 210k ohms? Will this harm the amp in the long term?
" Given the info what would happen if the 100k ohm load was increased to 210k ohms? Will this harm the amp in the long term?"
Problem here is you need more info. The two stages are interconnected. Changes to one affect the other. To start with: measure the plate voltages of each stage and measure the voltage drop across the cathode resistors to determine the bias currents of each stage. Then plot out the operating points on the two sets of plate curves. Doubling the load on the first stage will change the gain and the plate voltage with no changes to the cathode resistor. This plate voltage needs to by compatible with the operation of the driver/phase splitter stage.
If it was up to me I would replace the cathode resistor of the 2nd stage with an active CCS. That way the 2nd stage would be properly biased for whatever plate voltage the 1st stage presents to it (as long as the CCS isn't starved for voltage).
Another approach is very empirical. Try the 100K resistors and listen to familiar music and measure voltages and bias currents. Compare to results with 210K resistors. Go with what sounds best.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Thanks for the information and advice. Since this is not my amp, I'd not spend the time to play with it. The amp works, sounds good and is stable. My job is done and I'll inform the owner what I've discovered about the resistor values. Thanks again
David
One thing to ponder. One plate load resistor was open, the other way off value. That may indicate they were under rated for power dissipation.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
mt
No, I didn't. The bad resistor was open, but after installing the new 210k resistor, both channel's plate resistor measure the same-about 210k ohms. I've been playing music through this amp for about 3 hours now and everything seems to be fine. So, should I leave it as is or change the resistors to 100k as indicated in the color code and schematic?
David
I'd leave the resistance value at whatever was in the amp --- as long as the amp was working well. Value of this resistor effects the bias of the tube. And the overall quality and balance of the music.
Edits: 07/04/15
Thanks. I think I'll take your advice and leave it as is. It's been running for about 10 hours now and it still sounds good. Any thoughts on the use of a slow-blow fuse instead of the fast acting one?
Regards,
David
I was kinda puzzled at the manufacturer's use of fast-blow fuse. Tube amps are notorious for blowing out fast-blows, esp on cold startups.
You should email them, to be sure that it is safe to go to Slo-blo.
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