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i've just finished my latest amp project. its a 100w amp designed for guitar or bass. theres a fair bit of writing so i put a couple questions in bold for anyone skimming who would like to help.
i got some old transformers from a guy with a bit of a collection. he gave me some 6bl8's and told me to try them instead of the ef86's i was thinking of using. 6bl8's are a pentode/triode tube that were used in the tuning of tv's/radio's, but they sound great in MI amps. the amp is quite simple, with an input into a pentode gain stage> volume > triode cathode follower> marshallish tone stack> pentode gain stage+ -ve feedback > split cathode phase inverter.
the output will run 4 6l6 or el34's, but i have a pair of 6550's in it at the moment as i havent bought the tubes that i will run it with yet.
i had a bit of trouble getting it running well as the bias pot i bought was faulty (wiper connection was intermittent) and the 6bl8's i tried first were bad. when i got a good one i didnt think it was working again as there was no audiable hum. when i touched the input it gave that buzz i wasnt expecting. the amp has virtually no hum!!!!
the amp sounds great with the 6550's in it (16 ohm cab into the 8 ohm tap to match the impedance as well as possible).
the tone stack is very similar to a marshall, but has a mid shift control (a variable resistance in series with a cap, in paralell with the cap for the mids, which is a higher value than usual. the mid frequency should be adjustable from 250-1khz. this is pretty useful. there is a switch to lift the ground from the tone stack as well.
the transformers and choke were pulled from old equipment by the guy i bought them off. the pt is apparently from a tv from the 40's or 50's. the ot and choke seem old as well.
i elevated the heaters 60v so the max cathode to heater voltage of 100v wasnt exceeded in the cathode follower. this probably helped eliminate some hum as well. i used some copper tape to shield signal cables early in the amp by taping them to the chassis (the sticky side conducts). worked well, but looks a bit messy. i used tag strips either side of the tube sockets for the preamp circuitry. this means there arent any long cable runs. in the preamp.
the -ve feedback has a spdt switch to change the resistance from ~7k to 11k to 18k. its a nice feature, but it created a bit of a mess with wires going from the jacks on the back, to the front and then to the preamp circuit. the bias circuit and the high voltage supplies seem to be a bit awkwardly placed as well, and meant there are some annoying wires in the amp that could look nicer, but they arent signal so they dont effect the sound/hum levels.
the plates are on 384v with the 2 6550's in, but for some reason the grid#2 is at the same potential. thats after going through the choke (40 ohms out of circuit) and a 470 ohm resistor. the ot windings only dropped around 5v. is this unusual? the circuit is designed for 6l6's or el34's, as opposed to the 6550's in there.
the 6550's are quite different in terms of their current. there is a difference of ~10mA. switching sides means the current follows the tube, so its not a circuit or ot issue. this strikes me as strange though as there is no/very little hum.
the amp sounds great. i've plugged it into an inefficient 4x12 (my 2x10 with warehouse guitar speakers in it is noticeable louder, those speakers are supposedly 96db/1w 1m). the 4x12 has some old australian made alnico speakers that sound great and mellow in it. the amp sounds great clean, and really lets the guitars tone shine through. cranking the vol with the 2 6550's gives a great chunky sound. i could get some nice feedback from my selfmade walnut guitar when i put the 2 singlecoils in series. the tonestack works very well to vary the sound. lifting to ground from the tonestack works well to get a nice middy sound, and sounded nice when turned up.
does anyone have any reccomendations as to what tubes i should put in it? im thinking jj el34's as i can get them quite reasonably. im also thinking maybe some 6l6's, but not sure which. a guy i tend to get my tubes from seems to have jj's at good prices, are their 6l6's good? im not really sure if i want 6l6's or el34's. i dont really know wether i want to use it for guitar or bass more, but i play bass in a trio, so maybe i should try to suit that. any advice/tips?
i'll post some pics in the next day or so. if anyone wants to see a schematic im happy to scan it and post it up as well.
i got some old transformers from a guy with a bit of a collection. he gave me some 6bl8's and told me to try them instead of the ef86's i was thinking of using. 6bl8's are a pentode/triode tube that were used in the tuning of tv's/radio's, but they sound great in MI amps. the amp is quite simple, with an input into a pentode gain stage> volume > triode cathode follower> marshallish tone stack> pentode gain stage+ -ve feedback > split cathode phase inverter.
the output will run 4 6l6 or el34's, but i have a pair of 6550's in it at the moment as i havent bought the tubes that i will run it with yet.
i had a bit of trouble getting it running well as the bias pot i bought was faulty (wiper connection was intermittent) and the 6bl8's i tried first were bad. when i got a good one i didnt think it was working again as there was no audiable hum. when i touched the input it gave that buzz i wasnt expecting. the amp has virtually no hum!!!!
the amp sounds great with the 6550's in it (16 ohm cab into the 8 ohm tap to match the impedance as well as possible).
the tone stack is very similar to a marshall, but has a mid shift control (a variable resistance in series with a cap, in paralell with the cap for the mids, which is a higher value than usual. the mid frequency should be adjustable from 250-1khz. this is pretty useful. there is a switch to lift the ground from the tone stack as well.
the transformers and choke were pulled from old equipment by the guy i bought them off. the pt is apparently from a tv from the 40's or 50's. the ot and choke seem old as well.
i elevated the heaters 60v so the max cathode to heater voltage of 100v wasnt exceeded in the cathode follower. this probably helped eliminate some hum as well. i used some copper tape to shield signal cables early in the amp by taping them to the chassis (the sticky side conducts). worked well, but looks a bit messy. i used tag strips either side of the tube sockets for the preamp circuitry. this means there arent any long cable runs. in the preamp.
the -ve feedback has a spdt switch to change the resistance from ~7k to 11k to 18k. its a nice feature, but it created a bit of a mess with wires going from the jacks on the back, to the front and then to the preamp circuit. the bias circuit and the high voltage supplies seem to be a bit awkwardly placed as well, and meant there are some annoying wires in the amp that could look nicer, but they arent signal so they dont effect the sound/hum levels.
the plates are on 384v with the 2 6550's in, but for some reason the grid#2 is at the same potential. thats after going through the choke (40 ohms out of circuit) and a 470 ohm resistor. the ot windings only dropped around 5v. is this unusual? the circuit is designed for 6l6's or el34's, as opposed to the 6550's in there.
the 6550's are quite different in terms of their current. there is a difference of ~10mA. switching sides means the current follows the tube, so its not a circuit or ot issue. this strikes me as strange though as there is no/very little hum.
the amp sounds great. i've plugged it into an inefficient 4x12 (my 2x10 with warehouse guitar speakers in it is noticeable louder, those speakers are supposedly 96db/1w 1m). the 4x12 has some old australian made alnico speakers that sound great and mellow in it. the amp sounds great clean, and really lets the guitars tone shine through. cranking the vol with the 2 6550's gives a great chunky sound. i could get some nice feedback from my selfmade walnut guitar when i put the 2 singlecoils in series. the tonestack works very well to vary the sound. lifting to ground from the tonestack works well to get a nice middy sound, and sounded nice when turned up.
does anyone have any reccomendations as to what tubes i should put in it? im thinking jj el34's as i can get them quite reasonably. im also thinking maybe some 6l6's, but not sure which. a guy i tend to get my tubes from seems to have jj's at good prices, are their 6l6's good? im not really sure if i want 6l6's or el34's. i dont really know wether i want to use it for guitar or bass more, but i play bass in a trio, so maybe i should try to suit that. any advice/tips?
i'll post some pics in the next day or so. if anyone wants to see a schematic im happy to scan it and post it up as well.
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