Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jatchley on November 05, 2020, 02:16:23 pm
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so i've never seen this before. my friend brought in an ab165 it had a cloud like SHHHH all the time independent from all controls and had a busted bass switch. i opened it up fixed the switch and saw some usual suspects. ancient electrolytics and a sketchy looking joint in the phase inverter section. but then i saw the p-light and was looking at the heaters and theres no CT winding from the trafo and no 100ohm resistors to ground. instead is a single wire from one side of the lamp to ground. and the power ground scheme seems slightly altered. it is linked with jumpers from 3 PT mounting bolts. looks like due to lead length? anyway the owner said this thing has been maybe a little hummy but fine for 10 years no tube changes no service til now. its hard to believe a 50w amp with a sketchy heater CT hasn't made this thing hum city. i put the 100ohm resistors on. a bit of 60hz hum disappeared but the shhhh cloud remained. then i was like maybe its these ancient carbon plate resistors. tried 3 sets of tubes in this thing because the entire chassis is microphonic as well. i'm currently awaiting on more 100k resistors because i guess i only had a couple left. i did v1 and half of v2 the shhh is less but still very there. so im just gonna replace all of the plate resistors. it just seems strange to me this amp was sounding great a few months ago and then all the plate resistors go to hiss town? my suspicion is theres 1 thats real bad and the rest are just a little hissy. theres no change in microphonics poking around. its loud anywhere around the preamp with any tubes. after replacing v1 resistors some of the microphonics vanished from that area of the board and lines.
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Wow! Where to start trying to make sense of that dense block of text?
Maybe begin with explaining exactly what is meant by 'a cloud like SHHHH'?
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Wow! Where to start trying to make sense of that dense block of text?
Maybe begin with explaining exactly what is meant by 'a cloud like SHHHH'?
ha sorry my brain goes faster than my fingers. so it's just like someones going SHHHHHH all the time,couple crackles here and there, sometimes it'll soften up or go shhhhhHHHHHHHHHHH like its ramping. all very subtle and also is at the same volume regardless of controls. it gets washed out if i play above 3 on the vol knob. but it's definitely louder than an idle hiss on vintage amps usually are. the heater thing is a different thing all together. but the SHHH sound and the chassis being microphonic seem to be linked. as i said changing the plate resistors on v1 fixed like 20% of the hiss and made v1 and all components tied to v1 on the board and chassis not microphonic anymore. but v2,v3, from the tubes to knobs are still microphonic. v4 seems just like residual microphonics from v2&3. and by the time you reach the end of the board by the power tubes the microphonics stop. i'm leaning towards just some real hissy carbons on the plates since that fixed v1.
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Ok, replacing the other plate resistors seems a good next step.