Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SHaD0ST31N on May 21, 2019, 09:13:37 am
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hi all, im new here :D
i need some input from people who are in the know about grid leak biased 6SJ7 preamps.
i built a moded 5C1 and its working fine but i bin doing some research about grid leak bias cuz i never built an amp with it before this one. and i bin hearing peeps say all kinds of things about it. one of the concerning things is the fact that grid leak bias apparently has low headroom and hits distortion pretty quickly and thats one of the reasons why it was phased out in favor for cathode bias.
now heres where my concern comes in, my grid leak 6sj7 has PLENTY of headroom, in fact, to get the thing to overdrive i have to use a boost pedal along with a compressor that has alot of available volume gain, and if i max out the boost and the comps output vol, i get a decent bit of gain from the preamp.
but its completely opposite of other peoples experience with this type of preamp and bias method. so my first thought was maybe the bias is way too hot and its allowing all this unusual headroom, but when i checked the voltages, everything was normal.
plate 128v (130 normal) plate resister around 280k
screen 17v (21 norm) screen resistor around 2.1meg
grid leak resistor -0.77v (-0.5v norm) (note i realize this reading could be off as a result of the meter screwing with the grid leak resisters resistance) grid leak resistor around 5.1 meg
now heres the thing, the first 6sj7 stoped working. i turned the amp on for maybe the 3rd time and it was dead, the 6sj7 had very little output. it did not die in the middle of playing through the amp it worked fine, turned the amp off, then turned it on later and minimal sound. tracked the problem to the 6sj7 replaced it and it worked fine again. checked voltages again and everything still looked fine. played for awhile periodically touching the tube to see if it felt way hotter then normal but nope it was normal temp both the new and the old 6sj7's are used old tubes so the first one dying could just be a random fluke. but that doesn't explain the unusual high headroom.
definitely would love to get input from people who are well experienced with this preamp type. thanks!
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I'm not an expert with grid leak bias but I do understand how it works. Sounds like everything is working as it should. What voltage do you read on each of the three filter caps?
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the cap nearest to the rect. is 369V
middle cap that feeds power plate is 343V
and the 3rd that feeds power screen & preamp is 275V
thanks a bunch for the reply!
another question i have is, is it possible to make the bias go dangerously hot by injecting it with too big of an input signal? the way some guys where talking about it made it sound like that was possible.
i strongly feel like either they are wrong or im misinterpreting what there saying cuz to me that doesn't make any sense.
i never heard of driving a tube too hard with a boosted signal to the point it damages the tube :\
at least not by normal signal boosting ways lol
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ok after doing some reading, i decided to convert it to cathode bias.
i dropped the grid leak resistor down to 1M, and added a 1.2k cathode resistor.
the voltages moved very little,
plate 130V,
screen 19V.
the tone changed a little but not in a bad way and im happy with the way it sounds.
i decided to make the change to cath bias because it will be more consistent from tube to tube and have less drawbacks (according to what others with experience are saying)
im not 100% sure where the bias is in this arrangement but with a 280k plate resistor and 130 plate volts with a 1.2k cathode resistor, i shouldnt be near the dangerzone :icon_biggrin:
and its sounding good and drives the power tube plenty so i dont think i have a need to change the bias from here