Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on August 07, 2010, 11:50:20 pm
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Hello, I am new to the forum & to amp building. Let me start at the beginning.
I got a 5E3 "on steroids" kit (40W) from torres engineering & built a Tremolux head out of it. Since I had the room in the chassis. After a few initial problems I got it to work fine but didn't really like the tone, too much mids. Being compulsive I slapped together a B/M/T tone stack & installed it between the volumes & driver. It really wipes out the gain but interestingly the volume became much more responsive (I don't need a lot of volume, for now) & I like the tone much better. After tightening things up, I must have been distracted because I MAY have forgotten to plug in the speakers :angel.
After a few hard strums wondering why there was no sound, my cathode bias cap exploded. Smoke issued forth. :cry: In my rush to power down & disconnect everything I don't remember whether the speakers were hooked up or not. Probably not. And I have a new bias cap on order.
Several questions: Is it possible I have blown my OT? The primaries read @200 ohms & speaker connections are similarly appropriate. If it is bad, do you think I could use the 50W (2 EL34s) OT out of my old Laney AOR? My amp has 2- 6L6s. I'm tired of spending $$$ on this thing.
Also, at some point I will probably try to get that gain back. Any ideas on stacking the 2 separate preamp stages, i.e. resistor/cap values I should use? I tried doing it w/ the existing circuit but it was way too much, even with a 12AY7.
Anyway, thanks for reading all the way to the end, I know I may have bitten off more than I can chew.
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Cut-out cap carcass.
CONNECT SPEAKER.
Plug in and play. Does it play? The cathode cap may improve gain, but is NOT essential to make it "work".
I don't see how a missing speaker would explode a bias cap.
Are you using the tubes in the kit, or did you replace them? I do not know what Torres ships, but some other kit companies toss in lowest-bid "starter" tubes which may bust a gut (or a cap) for no reason.
> B/M/T tone stack & installed it between the volumes & driver .... stacking the 2 separate preamp stages
If you have to ask (are not already a self-confident Designer).... steal a known-good plan. Fender made a ton of models with BMT tone and push-pull output. Deluxe AB763 (http://www.schematicheaven.com/fenderamps/deluxe_ab763_schem.pdf) is an example. Build one channel, omit the 220K mix resistors, skip the tremolo. (You can add that stuff later; get a Basic Amp working first.)
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Also, make sure you had the cap installed in the correct "orientation".....with cathode bias, positive "+" goes to the tube cathode, & negative "-" goes to ground/earth.
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Yeah, it was installed right. I guess these things just happen sometimes. Maybe the power resistor was too close, right up against it? I went ahead & put in the Laney 50W OT- still nothing. Then I replaced the cathode bias resistor since the 470 I had read 24M ohms. Now we're cookin'. But it's late & all I know is that the amp works, can't test it & stay married. As far as I can tell my 6L6s (JJs) are still good. I will post again when I know more, probably won't get the bias cap for a few more days. If it sounds all right without the cap, I may set up a switch for it. That is all.
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It will sound o-kay w/out the cap but it will sound much better with it. It appears you found the problem? Mount your cathode resistor up off the board a bit (for cooling) and away from any caps if possible to keep them from this heat source.
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So I put the old (041318) OT back in & it's fine. Everything is back up & running. The tone stack seems good too. I wonder what caused the big failure in the bias circuit though? Also piggybacked the 2 preamp stages ala AB763 & it works fine. any advice regarding proper bias current for 2 6L6s on a cathode bias?
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Glad you got it figured out. Use this for your biasing assistance.
http://www.webervst.com/tubes1/calcbias.htm
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Great site, thanks a lot. It's late, plus I left my guitar at the station. I'll test drive it tomorrow but so far it sounds good with the radio running thru it.
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WOW!!!!
I played it this AM for a little while and oh my god! Lots of power too. Thanks again for the bias link, I think that put me over the top.