i've just finished making a 100w valve amp for bass/guitar. i have 2 200w tube monsters so i wanted something a bit smaller to be able to push into overdrive when i feel like it. the amp can run on 4 6l6's or 4 el34's. im running it on 2 6550's atm, but i'd be hesitant to run 4 as they draw alot of heater current and dont want to blow the power transformer which i dont have specs for.
the transformers are second hand transformers i bought from a guy i met through ebay. the power transformer is apparently from an old tv (40's or 50's he said, but it works perfectly, and havent felt the slightest heat on it yet). the preamp tubes are 6bl8's, which have a pentode and triode inside. they were designed for tuning of tv's and radio's, but they sound great in this amp. their frequency response probably isnt flat, but thats the last thing that matters with instrument amps.
the amp is quite simple, it has an input into a pentodegain stage, to the volume, to the 1st triode as a cathode follower, to the tone stack (like a marshall tone stack, but with a mid shift knob to change the mid band from 250hz to 1khz). then to the second pentode as a gain stage/input from the negative feedback and then to the split cathode phase inverter being the triode on the second tube. the output can run on a fair few tubes. the voltage is about 390V, so its probably going to put out 80-90w with el34's or 6l6's.
theres also a switch to disengage the tone stack, as well as a switch the change the negative feedback strength.
thus far im not sure wether i'd prefer 6l6's or el34's, but the amp will take both. i may adjust the grid no. 2 resistor to 820ohms if i use el34's.
here are some pics. i havent made a shell for the amp yet, but im thinking of using some type of timber for the front and back, and possibly the whole shell. i can inlay symbols for the controls and some type of logo.
the front, i will cut the pot shafts to more manageable lengths once i have a shell for the amp made.
the back.
the gear i was testing the amp with. the guitar cab is an old australian 4x12 with some old alnico speakers. very good sounding with subdued highs. it is very inefficient (my 2x10 is louder, and the drivers in that are only 96db @1w/1m) which is good when you want to crank a tube amp. the 1x15 i made myself. its loaded with a warehouseguitarspeaker 15". the cab sounds great with quite a wide frequency range. it goes nice and low and still has plenty of high's on its own. the guitar and bass i made myself.
heres the inside. there are some wires that cross the chassis which make it look pretty messy. they are just the high voltage, the feedback and the connections to the transformers, so there isn't any long signal wires adding hum. the preamp was wired between tagstrips and the sockets, making the length of leads a minimum. i like it better than a tagboard, but it does take longer to do. i put the tubes connection to ground via a spade connection making bias adjustments easy as you dont have to desolder anything.
im really happy with the amp so far. i designed the preamp myself almost completely as i was using very uncommon tubes for MI amps. the amp has virtually no hum. because of this i thought that it wasnt working when i fired it up. i touched the input expecting nothing but got a buzz showing me that it worked. after debuggine a few bits and pieces i'm very happy with it.
i wont be able to give any samples atm, but may be able to at some stage when i get some things recorded by a friend.
if anyone has any advice on which output tubes i should get please chime in. im leaning towards jj el34's atm.
) too many old radios and TV's without salvagin the trannies and tubes. I'm kicking myself for doing that.
