212Mavguy Posted October 13, 2011 Posted October 13, 2011 This little fire breathing monster died in a tube rolling session, was trying out an old RCA 6bg6ga w/adapter in the power tube position, but the plates sit right up next to the plate cap on this unusually designed 6bg6ga, so the combination of slightly taller bottle plus height added by the adapter put a significant part of the plate about a quarter inch away from the speaker magnet. this tube is similar to a 6l6, but in a different pinout and more affordable than vintage 6l6's as well as being better sounding than any modern production 6l6. Some are rated at 22 watts, with a couple designs rated for 30 and 35 watts. The stock 8 inch Jensen alnico POS speaker had long since been used as a frisbee right into the trash can, replacement was a CTS snowflake basket squaremag ceramic unit, Well, after firing up, a steadily louder hum that only could be made by a tube going into runaway was coming out of the speaker, I looked at the plate and sure enough, power tube redplating unhappily. Just then I heard a soft "ping" inside the chassis and things started getting really stinky. At the same time of the little noise, the hum dropped back to nearly nothing coming out of the speaker. I looked for smoke coming out of the trannys and when I saw nothing I let it cook for a while longer. My philosophy is that when something poops the bed in an eyelet or turret board tube amp, let it cook, so that the burned part is easier to spot. I had built this amp circuit anyway, it had a blackface-y circuit on a turret board with PIO caps and tantalum resistors in the signal path, and top end HIFI resistors in other places. Both stock trannys in this amp had been swapped out for much bigger iron, especially the OT, a 5-7 pound unit that had to be mounted with t-bolts to the bottom of the cab (!) After a day or so I took the chassis out, and sure enough, the cathode bypass resistor had fried. But the only place I could go to get that part was in Salt Lake city, about 50 miles away from home. Due to the non stock power tranny running a little more voltage on the b+ and also using a 5r4 rec tube instead of the wimpy 5y3, my plate voltage was considerably higher than stock, 420 instead of 375-ish. That combined with the stock bypass resistor value of 470 ohms, made things a bit too hot for 6v6's. That amp was happier running 6l6's or el34's anyway. (Yes, I had connected pins 1 and 8 on the power tube socket at time of build). Yesterday I had taken the day off work due to a chest cold, so saw the opportunity to get out of the house. Went to visit my amp tech, spoke with him about the desire for a 5 watt rated resistor one step up from a 470 ohm rating. I was planning to go the a local electronics shop to get one anyway, but he reached behind his head into a box and pulled out a Clarostat 500 ohm 5 watter with printing instead of bands, it looked like it had a glass cover on it too. It measured out at 506 ohms, perfect. Since the time I first built the circuit, I purchased a book titled "designing tube preamps for guitar and bass" by Merlin Blencowe, the bible for the guys at the ampgarage forum. In it I learned that my deviation from the classic blackface circuit would induce too much hum, and I had been having that problem with this amp. The cathode bypass cap I had in for the first 12ax7 section was 2 mfd, 10 mfd for the 2nd section, and 50mfd for the power tube. These were the mythical Rubycon Black Gate Uber top-end HIFI units no longer made. The stock config was 22, 2, and 22. I went to my box of parts and got out brand new 22mfd and 33 mfd Black Gates, moved the 2 mfd unit to 2nd position, the 22 where it belonged, and the 33 to the power tube position, from experience with this amp I knew that higher capacitance for the power tube cathode bypass would both tighten and fatten the bottom end of the tone, with a corresponding loss of harmonic content in the upper mids and top end as value increases. Stuck the chassis back in, fired it up, everything worked fine, slightly more gain than before along with a noticeable sag at pick attack at high gain settings, which I attributed to the new caps not being broken in and completely formed. Sure enough, an hour later the sag was gone, and the tone was the most musical that this amp has ever had at lower settings, a very ah, inspiring at higher ones. The reason for the insane amount of gain before and after the surgery was that previously I had lifted the ground from the bass pot, effectively taking the tone controls out of the circuit, gain was MUUUCH more than stock. But set up this way, the amp has the scream of the earliest tweed designs, but much more than those even. A setting of 3 on the volume knob equaled a setting of 8 in the stock configuration. Because this amp has the worst case of short man's disease I have ever seen in a guitar amp, (it thinks it's a Marshall stack) it is one of those wolf in sheep's clothing thangs that has made a lot of jaws drop at the local jams. It keeps up with the band and drummer without any help from the soundman. Volume, and girth, and violin-y sustain/feedback in the tone are stunning, it seems incomprehensible that something like that could come out of what looks to be a stock silverface Champ with an 8 inch speaker. And the best part that happened besides a slight improvement in the tone was also the reduction in hum at idle with the volume knob turned down. YAY! ...how satisfying it is to buiild your very own amp and own your very own tone set as well as being able to self-diagnose and repair your toys. Boutique amps for boutique guitars!
yoslate Posted October 13, 2011 Posted October 13, 2011 I actually...ah...read all of this. I have the tech savvy of your basic thirteenth century feudal serf. What I can relate to is carrying that little lunch box out to a jam or gig and basically pithing the room! I'm in envy of your experience and expertise, particularly because of what you do with it, Mavguy. I watch in astonishment as the guy who builds amps for me listens to my "arty" subjective descriptions of what I want to hear, while he fumbles through boxes of caps and resistors, and, finding one, alligators it into an amp's circuit, giving me exactly what I want. Glad you guys are around!
fxdx99 Posted October 13, 2011 Posted October 13, 2011 Whoa - that's some post! Good read - even tho much of it is beyond my limited knowledge of circuits. Congrats on getting that puppy back to better than new shape.
Gitfiddler Posted October 13, 2011 Posted October 13, 2011 Geez, I really envy you amp techno geeks. (And I mean that in a good way!). Your Champ upgrade sounds killer!! One of my Champ II amps sounds wonderful and the other one (the cleanest looking one, of couse) just sounds like...meh. No bass control and very poor tone all around. I took it to an amp tech and he says that at some time in its past, moisture got inside of the chassis and now I need to replace all or most of the guts. I've suggested he give me a price with Weber replacement parts to bring it back to life. It's been a month now. Nada!! Wish one of you amp gurus lived close to the SF Bay Area. I am not happy with my amp dude. He takes too long to diagnose or repair stuff.
H Posted October 13, 2011 Posted October 13, 2011 Love the detail in your post, 212mavguy It's early days for me in the build side but you've given me lots of things to research! One thing I can understand is the satisfaction of a beautiful noise emanating from your own amp project. More please
DetroitBlues Posted October 13, 2011 Posted October 13, 2011 This is a story only KBP would fully appreciate. Seems to be all Greek to me! As long as it ROCKS it's all good to me!
FredZepp Posted October 13, 2011 Posted October 13, 2011 I finally had the time to give this post a thorough read....a very enjoyable read it is. An amp with just the right tweaking is a magical thing.
smurph1 Posted October 13, 2011 Posted October 13, 2011 I actually...ah...read all of this. I have the tech savvy of your basic thirteenth century feudal serf. What I can relate to is carrying that little lunch box out to a jam or gig and basically pithing the room! I'm in envy of your experience and expertise, particularly because of what you do with it, Mavguy. I watch in astonishment as the guy who builds amps for me listens to my "arty" subjective descriptions of what I want to hear, while he fumbles through boxes of caps and resistors, and, finding one, alligators it into an amp's circuit, giving me exactly what I want. Glad you guys are around! Yeah that's cool..Wish I could do that kinda stuff..guess I'll just have to stick to playing through what you guys do..
Trouble Posted October 14, 2011 Posted October 14, 2011 Yeah that's cool..Wish I could do that kinda stuff..guess I'll just have to stick to playing through what you guys do.. Me too, I can solder, If you tell me what part to change I can change it, but at this point in my life I just don't have the drive to learn a new skill set, but I'm really glad to know a few good techs and soon I p;an on trying a KBP build.
Trouble Posted October 14, 2011 Posted October 14, 2011 Oh, I did replace the Peavey speaker in my Valveking with a Celestion this week, is that impressive?
212Mavguy Posted October 14, 2011 Author Posted October 14, 2011 What got me started on this wierd little obsession was a half ass amp tech screwing me...I took in an amp, 4 1/2 months later after several phone calls I picked it up, the problem wasn't fixed, and the chassis came loose and dropped down into the cab while driving on a dirt road with the amp in the front seat right next to me...and electronics are my weakest science area. It's just that every thing I have gotten good at in my entire life I failed miserably, often repeatedly at the first. Was the last picked kid at recess in elementary school, now I'm in my 37th year as a professional ski instructor. Doing it all over again to be an electric guitar player. I'm positive that every single person that posted in this thread has more skills with their fingers than I possess. So my hat is actually off to you, brothers! Thanks for your positive comments.
Trouble Posted October 14, 2011 Posted October 14, 2011 What got me started on this wierd little obsession was a half ass amp tech screwing me...I took in an amp, 4 1/2 months later after several phone calls I picked it up, the problem wasn't fixed, and the chassis came loose and dropped down into the cab while driving on a dirt road with the amp in the front seat right next to me...and electronics are my weakest science area. It's just that every thing I have gotten good at in my entire life I failed miserably, often repeatedly at the first. Was the last picked kid at recess in elementary school, now I'm in my 37th year as a professional ski instructor. Doing it all over again to be an electric guitar player. I'm positive that every single person that posted in this thread has more skills with their fingers than I possess. So my hat is actually off to you, brothers! Thanks for your positive comments. Don't think for a second I meant to belittle your accomplishment, I know that I am cabable of doing things like that with someone to tell me how, but I am impressed as hell by people who have an interest in something later on in life and just learn how to do it. I'm a pretty good auto mechanic because I had to fix my own bikes/cars growing up. As a teenager I got into hot rodding and racing and if you've got the money I can build you a winning engine. But I wouldn't want to learn how to do that all over again either, old and tired I guess. Again, my hat's off to you for taking the time out to garner the knowledge you posses, and utilizing it in something you love.
212Mavguy Posted October 24, 2011 Author Posted October 24, 2011 Put it back on the operating table and fired up the soldering iron again...tone tweakage. When I put that 33mfd cathode bypass cap in for the power tube, I noticed just a bit of fartiness on the low end when using the bridge or combined PU's on my humbucker equipped guitars. Was hoping for that to go away as the cap broke in from playing, but not the case. I also noted that changeing the amount of bypass cap values in the preamp section caused what I thought was a bit too much gain. when I dimed the amp it got too saggy for my tastes. Since all that extra gain came from disconnecting the ground on the bass pot, which in stock form is actually a 10k resistor soldered between that pot and ground, I mused that perhaps a higher gain resistor might restore use of the tone controls and also allow more gain than stock. I had some old mil stock 1w 1% tolerance resistors lying around, so found a 97k and stuck it in place of the long gone 10k. Gain was still quite strong, but the sag was gone at dime-age. But the tone controls did not have enough effect on the tone to be useful. So I stuck in a 67.5k and that was pretty darn good! I had a bit less gain, but still way more than stock, enough for huge sustain and some feedback through the guitar as well. The tones were adjustable now through the tone stack, but that fartiness was baresly (and still) there... so I yanked out that 33 and put in a 47mfd Black Gate cap in, a huge improvement, but I wanted a bit more bottom end, so added a 10mfd to the 47 in parallell, that was a bit better, then since the iron was hot and I had it, I yanked that pair and stuck in a 100mfd/50v Black gate cap in and woooooeeeeee! Big, huge, full bottom end that could be bigger than huge, but still able to be taken down to where ever needed via the bass pot. All fartiness, was long gone, but now also had a big fat and tight bottom end that Now the amp sounded just plain lovely, started tube rolling again for power tubes, stuck in a 40's vintage coke bottle shaped Sylvania made but etched labeled on the base "Admiral" 6f6g, with the round black plate, it was smooth as butter, and would sustain insanely at a loud small venue volume. That tube is close to a 6v6, same pinout, and 6v6 is stock. With a "slow to warm up" 5v4 rec tube, the 6f6g was running as hot as dared, just the faintest bit of red on the plate, so that tube was running at 100% of max current at idle, red plate would go away when playing through the amp, I yanked out the RFT 12ax7 and stuck in a Siemens e83cc, more upper mids and top end detail, which now was just plain wonderful. Took it out to the local jam last night with that amp driving an open back D-style cab with a pair of 60's vintage Jensen Vibranto 12's in it for 16 ohms. Had a TC electronic Nova fx processor and a Hilton volume pedal in front of the amp. Was a huge joy to play through, fat, full, huge sustain, and buttery smooth overdrive, bass knob 10:00, treble 2:00. Could lay back in the pocket and then spring out for solos like a trap door spider with that little thang. Just like in KBP810's very recent thread about sticking a reverb in front of the amp I used my fx processor, and things sounded great with a bit of delay or reverb. For a couple of solos I stuck it on the bridge pickup and selected an octave effect, eyebrows went up. Tone sounds like a very, very expensive boutique amp. Easy to find a vintage silverface Champ, rip out the guts, swap in some beefy iron, and rebuild it into a baby tone monster! Much less expensive than a 1500-2500 dollar hand wired commercially produced unit... which this sounds like now. Layout for a blackface champ is available online... Boutique amps for boutique guitars...
NoNameBand Posted October 24, 2011 Posted October 24, 2011 Great read. Thanks for sharing. It sounds like fun.
FredZepp Posted October 24, 2011 Posted October 24, 2011 a great read. I'm hoping that it inspires me to check out a couple of my tube amps,including a silverface champ, that have been acting sick lately. I don't have the time/energy to do an overhaul .. but they probably just need some tubes. I've switched to other amps that are laying around to delay looking into what they need done. But your tone quest is inspiring , for sure. good stuff.
Hfan Posted October 24, 2011 Posted October 24, 2011 Ill reread this one as time allows. Hope to do more modding and some amp builds one day when leisure time actually presents itself. Good to have amp gurus around.
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