-----------------------
back when i was a student in germany, i was bemoaning the sale of my first tube amp, a WEM/Watkins control ER15. i found this beaut on eBay really cheap - some €90 or so - and snapped it up.
what can i say, it played like a dream. it smelt kind of weird though, and at some point, the tinkerer in me simply had to open it up to see if i could mod it. it was gunked up inside, i posted this thread on "that" forum (i didn't know of TAG's existence back then, or i'd have posted it here).
http://www.thegearpage.net/board/archiv ... 99944.html
here are the pictures: http://www.flickr.com/photos/31937352@N ... 155220192/
when i saw the shape of the "dog turd" caps, i automatically assumed they were going to die on me soon due to the deformed shape. didn't think they
were just made that way. i don't think it's a great loss, though. they aren't supposed to be great caps, from what i've been told.
i found the schematic a lot later over here: http://www.igoramp.it/photogallery/ampl ... elphon.jpg
and decided to fill in the values from the boards. also, i noticed they had missed a junction at R15/C9/R45/C10, this has been rectified in my schematic. there was a 10n cap from the junction to ground on the solder side.
some questions:
1. was it a bad idea to get rid of the brown "dog turd" caps with respect to tone? particularly the large ones after the PI look deformed, i'm assuming they aren't manufactured that way? i replaced the 470nF PI coupling caps with 330nF, i doubt that's going to make a big difference in the bass area...
2. what the hey are R19 and R20 doing? is that some sort of balancing feedback or something? can i just get rid of them? one of them is way off spec, reads 2M, the other one is close to 500k. both are carbon comp, look to be 1/2W types. i have 1/2 W 1% metal film resistors, but the leads are falling short. could i just omit them without it being too big a problem, or is there some crucial function that they're fulfilling? i've never seen anything like that before in an amp schematic.
3. can i omit the 4n7 1.5kV cap across the OPT primaries? this amp is fairly dark, wonder if this would open it up a bit.
4. i'm thinking about getting the 500 ohm bias balance pot off the board and panel mount a 1k 1W pot in its place. 500 ohm seems ridiculously small.
5. i'm assuming it won't be a problem to replace C5 with a cap rated for 250VDC? Also, doesn'T 470n seem rather high? will stick to that value, because the bass boost is quite awesome. just asking.
---------------- end of edit-----------
6. had the following other mods in mind:
a. thinking about adding a 1M master volume pot after the treble pot. the problem is that there is a criminal lack of space on the front, i'm not sure where i could place that pot even if i wanted to... maybe i'll put it at the back, near the input to the PI. what say?
b. the simplicity of the power supply wiring, along with the fact that there IS a bit of space inside, makes this amp look perfect for a VVR mod... except for the fact that it's fixed bias, 30W and the power section PCB just can't be modified (to affix the 1 ohm resistors) without repercussions. for this reason, i'd rather do a simple master volume mod rather than VVR.
please chime in with your recommendations. started work on this again, i'm thinking about having it up and running in about a week. there isn't all that much to do, although it could use a good cleaning; already got rid of a lot of dust and the creatures that had started to make a residence in the cabinet. i may need to replace the B+ filter caps as well. although they look extremely solid. will measure their capacitance and decide.
thanks for reading and any information!
can't wait to have this beast up and running again.