Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ALBATROS1234 on March 30, 2018, 03:11:57 pm
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I got my latest experiment mounted up the other day and it was seeming quite nice. I mostly played it on low to mid volume so last night when I cranked her up to see what she's got I was get a small oscillation/squeaky rumble when I got the master volume up past 3/4 but it would disappear when it got close to max. Experimentation showed it only did it when the tweed style one knob tone was all the way up or almost all the way up.
My first thought was maybe signal was too hot hitting the power tube grid so I put a grid stopper which helped slightly but when I added a larger resistor it did nothing more to reduce the noise. Next I padded the input leg of the tone. Figuring when it was wide open and virtually no resistance was when the noise was occurring a larger resistor made it better and I ended up with a 220k to make the noise go away. The tone seems to have suffered no I'll effect nor the volume. So problem solved?
My question is did I just sweep a pile of dirt under the carpet or is this an appropriate fix for such an issue? Will this be a problem later or is it truly problem solved?
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Show us a schematic.
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as requested the resistors i added that kill the noise are circled with dots.
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I believe the resistor added to the tone control effectively placed a limit to the maximum treble and the grid stopper added to the PA is probably a good idea anyway.
You might try a grid stopper on the second triode(?).
It looks like you do not have a screen grid resistor on the PA... you might try one (or not) to help with the lifespan of the 6K6(?).
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Ok perhaps I will try out a screen resistor. I am feeding it from a different lug on the power supply after a 12kohm and it usually puts the voltage a little lower than the plate.usually about 10vdc.i noticed on another build when I had a larger resistor in that slot and it made my screen like 50vdc less than plate it made the volume go down substantially than when I had a smaller resistor and the voltages were closer .
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it made the volume go down
this isn't very engineering speak but works for me
G1 is like the gas pedal, G2 is like the brakes, when you move G2 closer to 0, a couple things happen, you need somewhat less drive signal, but at a certain point you choke off all the electrons you set free with G1. Most PA tubes running less than 500vdc plate just need G2 limited to it's max I minus a fudge factor, so ohms law, tube datasheet and lots of schematics will give you a good idea of where to start with R value
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> master volume up past 3/4 but it would disappear when it got close to max.
Crap is coming in the wiper. Full down it is grounded. Full up it sees the somewhat low impedance of the stage before it. Somewhere "mid way" the wiper impedance rises, in this case to 250K (1Meg/4), and small crap causes significant signal.
I'm not seeing where the crap is from, before (tone section you mentioned) or after (the huge signal on power tube plate). However re-routing or shielding the wiper wire is something to try.
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thanks guys, i was trying to think of ways to supress the noise it seemed to work using these 2 resistors i just didnt know if i was putting a bandaid on a bullet wound so to speak.still learning to balance my circuits
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with the 220k resistor in that side of the tone pot, you are limiting tha max treble avalaible.
If you put it in the other side (betwen pot and 0.047uf to ground), you will be limiting the effect in that cap.
With a dark guitar you will need that extra highs..
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the amp sounds quite good and there are enough highs for my humbucker guitar and i have to back off the treble a touch with my strat . it seems as though these 2 resistors eliminated the oscillation(at least audible result of the oscillation) but i am not sure if the problem is still there and buried or if this could cause trouble later with tubes wearing out or whatnot. thats kinda what i was asking about, is my fix ok or do i need to figure something else out. the only thing i might change is the kathode resistor on the power tube is 330ohm 5 watt. i usually use 500ohm or 750ohm both of which give more overdrive. this amp has a tad less grind than my last amp which gets a nice overdrive even at lower volumes which seems to be the result of the larger pa kath resistor/ bypass cap sizes. i have tryed 6f6,6k6,6v6 and 5881 in the pa slot and they all work well albeit tonal differences and harmonic overtone differences. the 6k6 will likely live in this amp as i am going to build a 12ax7/5881/6x5 and a 6sl7/6v6/6x5 both of which i havent decided on schematics. i am propbably going to build a supro clone using this pair of 6em5 to sub for the 6973 which i have read sound close to the same and only have a lower max plate current. thanks guys
if anyone can answer my initial question about if these 2 resistor are a sufficient fix to the minor oscillation pproblem i had or if theres something else i must try i would love to hear suggestions.