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Many years ago, I built a pair of the JE Labs 300b monoblock amps. In the power supply, I omitted the first capacitor after the rectifier, so that the choke is the first component. It results in voltages at the 300b plate of about 340V, and about 82V and 230V at the 6SN7 plate stages.
One of the amps has exhibited, more recently, a noticeable drop in midrange and bass output. The other one sounds fine.
Measurements show that the voltages are nearly similar between the two amps, the resistors all measure what they should (no irregular drift from their original values). There are no crackling sounds.
Where should I look next to diagnose this problem? Do the capacitors (coupling and cathode) cause problems like this?
Does it use 'boutique' PIO coupling caps ? Some of these 'boutique' types sound great at first but have short lifespans and go out of spec within a few years
The coupling cap is an Angela Instruments/Jensen copper in oil.
Given the responses I've seen, I think it is prudent to replace the coupling and bypass caps in both amps.
After reading through a lot of posts last night, electrolytic bypass caps look to have fallen out of favor. Are there caps out there that are well regarded for the bypass position, and hopefully not as physically large as the P/S caps? Current bypass caps are a 47uf/160V and a 100uf/100V, Black Gates.
Replace the couplers first . If possible measure the value of the caps .
There a few film bypass caps to try . The smallest are polyesters and these are also available in lower voltage ratings which helps keep the size down . Kemet R60 series are pretty decent and fairly small
According to the only schematic I could find, it looks like there are two cathode bypass caps that are electrolytic and yes they could fail and change the frequency response/gain in some undetermined way.
Did this occur all at once, quickly, or gradually? Have you checked the tubes? Easy enough to switch he tube from one am to the other, one at a time, to see if the problem remains with the amp, or switches to the other. Any other symptoms, buzzing, humming, crackling?
It has been a gradual fading. I did not mention that I have tried virtually every combination in my system - like swapping tubes from the good amp to the troubled one, swapping speakers between sides, swapping cables between sides, and the bass and midrange always sound diminished on the one amp. There is no humming or crackling; I have not had to adjust the variable resistor between the 300b heater legs.
Possibly a PS electrolytic cap that has leaked over time.
Would a degrading capacitor in the PS necessarily cause voltage to fall? I saw 4V difference between amps at the PS capacitor-output transformer junction, and the same 4V difference (or very close to that) at the 300B plate. This seems pretty consistent from when I built the amps (circa 2002).
The PS caps are paper-in-oil, and I have not seen any physical indications of leaks.
The 6SN7 cathode cap is a Black Gate. The coupling cap is an oil filled cap (I believe .22uf). All the iron is Hammond (300BX power, 1627SE output, 193J choke)
Besides the .22 uF 300B grid cap replacement, I would replace the electrolytic cathode resistor bypass caps with DC Link poly caps. Panasonic, Wima, Vishay brands.
I have a Kemet and a Vishay cap ordered for each amp. I'm replacing the coupling cap with a V-cap.
I should be soldering in a week or two.
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