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Old 14th February 2012, 06:02 AM   #1
trancy is offline trancy  Slovenia
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Default 300b problem

hello all

after few years my 300b amp, made from this schematic, stops to work correctly. I have a problem with one channel, it takes more than 10 minutes to warm up (other one takes 1-2 minutes). I have same PSU for both channels, also heaters for 300b are supplied from one source and for 6SN7 from another. I checked over the amp with multimeter, but all values compared from one to another channel seems to be the same. Swapping tubes did not help, problem stays on same channel. Resoldering some suspicius points also did not help...Actualy, when defected channel warming up, it plays very quiet, and then power rise up with some clipping sounding sound... after 10-15 minutes amp plays as should....all resistors are kiwame 2w, 300b cathode bypass resistor is dale 25w, capacitor 100uF 100v elna cerafine, 6SN7 cathode cap is F&T 47uF 160v, coupling cap is PIO. I think that one of capacitors may leak, but multimeter did not shows me any difference between channels....

please help

boian
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Old 14th February 2012, 05:39 PM   #2
tomchr is offline tomchr  United States
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Do you get roughly the correct DC voltages (as listed on the schematic) on both channels?

If the DC operating points are OK, my inclination would be to replace the electrolytic decoupling caps (across cathode resistors) and the coupling cap. Or at least swap them (one at a time) to see if you can move the problem from one channel to the other.

Also, when you say you swapped the tubes from one channel to the other, did you swap all the tubes or just the 300B?

~Tom
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Old 14th February 2012, 06:29 PM   #3
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You need to invest in an oscilloscope. A good secondhand one shouldn't set you back more than 50 to 100 GBP.
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Old 15th February 2012, 05:24 AM   #4
trancy is offline trancy  Slovenia
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hello

I didnt measure voltages, i suppose, if one channel works fine and PSU is one for both, resistors are good, it must be correct, but I will. I swapped all tubes....I intend to swap caps, I just need to find good ones...@toprepairman, I have an oscilloscope, i think tektronix something digital, but i dont know what to measure...
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Old 15th February 2012, 08:15 PM   #5
tomchr is offline tomchr  United States
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Instead of blindly swapping components, do yourself a favor and measure the DC voltages at the various nodes in the circuit. If they match the values on the schematic within reason (say +/-10 %) at least we know that the DC operating points are good. You should also have reasonably good matching between channels.

~Tom
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