Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff_free69 on April 22, 2019, 07:31:06 pm
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Greetings,
A few months ago I came across a 1972 Twin Reverb, kicked to the curb along with some old furniture. Skeptical me says it must be completely shot, but how could I leave it there to get tossed into a garbage truck? If nothing else, its a cool speaker cabinet. FYI I'm familiar with the basic safety considerations when working inside hi-voltage tube amps. Just haven't done any real repairs here...
After a bit of research (checking the SN# on the chassis and transformer) I'm fairly certain it was built in early 1972. Its a Silver Face with a Master volume (no push /pull), Utah speakers and casters. I found several schematics online, the closest being one labeled "CBS... 100 WATTS RMS AMP W/REVERB AND VIBRATO" . drawing 010182 RevC (shows Master Volume / NO pushpull)
(http://)
You can find the schematic here:
https://el34world.com/charts/Schematics/files/Fender/Fender_twinreverb_sf_100_mv_noboost.pdf
The overall condition is ok, just VERY dirty and dusty. I'm guessing its been sitting buried in the basement for a decade or two. I've done some cleanup and exploratory surgery.
After a couple months of video watching and forum researching I am encouraged to see the rich abundance of information and, more importantly, availabilty of parts. I've become fascinated by the point to point wiring and could definitely see myself doing new tweed style build later. For this puppy I originally thought I'd just plug it in and see what came out - music or smoke. But now I realize there are some basic things that could make a big difference.
Tubes:
The preamp tubes are a mishmash of rca, and (mostly) unbranded. All (4) 6L6 power tubes are Ruby "matched" (interesting that 2 are slightly larger !), so obviously also replaced.
Pulled the chassis, also looking mostly OK
Doghouse' Electrolytic filter caps:
The (3) big blue Sprague Atoms 20uF have dates that I think indicate '96.
The (2) 70uF/350V have already been replaced, also Ruby parts. So I assume we're looking at components already 20+ years old and the rule of thumb is to replace them , correct?
Heres where I found a problem: one of the original big blue Sprague Atom 20uF caps had a broken connection - the leg pulled right out of the solder joint. Note that when I pulled off the lid, there was some unexpectedly sticky foam between the lid and the caps. I was gentle, but I had to pry it off, so its not impossible that this pulled the cap up.
Being that I know just enough to be dangerous, for this first phase I'm leaning towards sticking to the original design, and consider Blackfacing mods at a later date (unless there are some no-brainer best practices you can recommend here)
part I
a. Rewire the power cord / remove death cap and ground switch etc
b. Doghouse electrolytic filter caps
20uF 500V F&T's (The Atoms are too hard to track down and expensive)
70uF 350V Ruby's
Bias / recitfier section : since this is a critical section it seems prudent to replace :
c. (2) 50uF 70V electrolytics in rectifier / bias section : Ruby 50uF 100V
d. 1k 1W resistor
d. (2) 68k balance resistors
e. (6) diodes - these seem pretty generic and cheap - anything special i should know?
part II
f. (5) .1uF 600V blue blob caps new part : Mallory 150S 5%
g. 2 of these caps are on Phase Inverter pins 1,6 plate, along with 2 resistors:
48k - replace these for better matching and low noise,
Question: metal oxide? metal film?
h. there's also a similar looking but smaller blue blob .1uF 200V (connected to pin 7 / grid of phase inverter) Can i use another . 1 600V here?
i. (3) .047 600V paktron : mallory 150s 5%
j. (5 )25uF 25V (white mallory's) replace with sprague atoms
Question: my V5 12AX7 tube also has one of these hanging off pin 3 - what is that needed for?
k. Replace Resistors 470 and 1.5k soldered to 6L6 sockets (in case they're cooked)
part III see what happens next
Any pointers greatly appreciated.
I have plenty of photos, but they are too large at the moment. attaching some layout/ markups I'm working on
Thanks,
Jeff
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circuit board layout ( crudely modified from a standard black face design) most of the difference is around the phase inverter and bias circuit . The bias circuit (not shown) is the balance version (but I imagine most around here are well aware of that)
This also shows where the Master Volume is spliced in : "MV2" "MV3"
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Electrolytics, probably.
Most of the rest: you can do more harm than good by wholesale replacement of random parts. Few of us can make solder joints as good as the workers at Fender did. Especially working inside the box (they had the boards out in the open). You can run the amp without the power tubes and verify smoke and no-load voltages. Many bad parts will be apparent as wrong voltages. 48k where should be 47k is NOT! a problem. (These amps would work with all 20% tolerance, 38k-56k; but good-quality parts in 10%-5% had become too cheap to ignore.)
And yes, do NOT! mod until you fix what's broke and play it a lot. It has been a fine amp for most of 50 years, it is new-to-you, learn what it is as-stock.
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I'm no expert but I would blow out the dust and cobwebs, fix any obvious things like unsafe power cord, and that broken cap lead. Then from that point remove all the tubes and treat it like it was a new build by running the input ac cord thru a current limit lightbulb circuit you can build, and before powering up without the tubes, ask on here what to do and check for. You may be getting rid of stuff that might possibly make that a sweet sounding amp even though it is mismatched like tubes and certain parts are original. Original can still be good. Get it working and if it sounds good, play it till it breaks.
Whoops just seen PRR basicly saying some of the same too.
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The '72 Twin Reverb is the best amplifier Fender ever made. No, really. I bought one 36 years ago that had a real rough first 11 years. I didn't take it seriously and I altered it and I've been kicking myself ever since. I picked up a '73 in Flagstaff, AZ more recently and this time I left it all stock except the 'lytics and the bias current limiting resistor. I had a directive drawn up to have the amp buried with me when I die. Not really.
You're kind of new to this forum and I really don't know how familiar you are with these things. Anybody that posts a question on these forums gets a whole lot of noise. A lot of that noise is actually useful, but it is such a random dispersion of knowledge that it just confuses people. Some of it is just plain wrong, adding to the confusion. So how are you to know? The number of posts doesn't mean squat. sluckey is one of the most knowledgable guys when it comes to guitar amps, but his post count on an alternate forum is low and some folks there don't pay the proper attention to him. pdf64 has a low post count here, but that cat has a vast knowledge. PRR has a real high post count here and he also has a vast knowledge. So why should you pay any attention to me? Frankly, I can't think of any reason to.
If you want to start with the power cord, that's fine. Note that Fender habitually wired the convenience outlet backwards, so pay attention to the neutral and to the hot. No need to remove the ground switch, just snip the .047uF capacitor out and leave the switch for cosmetic purposes.
The doghouse is the next thing that requires attention. It has already been altered from your description. Hopefully they only replaced the capacitors because sometimes they feel the need to alter the board. Some pictures would be really helpful. You're on Doug's forum and he sells the parts you need, so it would behove you to order them from him. Besides, he's a really cool guy and his service is excellent.
There is no need for designer products when it comes to filter capacitors. Use the 100uF/350V Illinois cap's for the two 70/350V cap's. Use the 22uF/500V Illinois cap's for the three 20uf/500V cap's. Use the 100uF/100V Nichicon cap's for the two 50uF/70V bias cap's.
Do those things first and report back. Do not, under any circumstances, alter the main board at this juncture.
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Frankly, I can't think of any reason to.
ah come on, you know there's a perverse enjoyment in being ignored :icon_biggrin:
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So why should you pay any attention to me? Frankly, I can't think of any reason to.
I would certainly, without hesitation, add 2deaf to the list he guys he mentioned, and off the top of my head, dummyload, Ed C. and tubeswell. There are several others too, that I can't think of right now. (2deaf is being modest/humble.)
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The '72 Twin Reverb is the best amplifier Fender ever made. No, really.
jeff_free69....that's a pretty big statement from a guy whose opinions really do matter.
So, 2deaf, does it look like he's got the right schematic for the '72 of which you speak? (I don't see a date on there) I have a special folder on my desktop for "the best" schematics.
I remember when I read PRR referring to the 5F6-A as a perfect amplifier. I immediately started gathering parts.
The beauty of this for you, jeff_free69, is that you stumbled upon 95% of what you need to recreate the "best amplifier Fender ever made". I'm happy for you, 'cause it wouldn't be cheap to try this from scratch. Most guys will never have the chance to hear it for themselves.
Get those pics sized down and let us have a look-see.
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Both Willabe and Silvergun are great help too Jeff. Listen to all these guy's and end up with a really great amp.
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So, 2deaf, does it look like he's got the right schematic for the '72 of which you speak?
010182 Rev. C is very similar to how my '72 was originally. Mine has a 1K bias resistor and the schematic shows 470r. I have seen others with the 470 and my '73 had a 470 originally. The cathode of V4 had an 820r resistor on my '72 and the schematic shows 1.2K. My '73 has a 1.2K resistor just like the schematic.
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If you want to start with the power cord, that's fine. Note that Fender habitually wired the convenience outlet backwards, so pay attention to the neutral and to the hot. No need to remove the ground switch, just snip the .047uF capacitor out and leave the switch for cosmetic purposes.
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@jeff
I dunno how the electric installation in the buildings are where you live, but down here in more than 95% of the places there is no ground in the wall where we plug the our amps. I did got a shock last gig just about a month ago, from the mic at the amp, was correcting its position in relation to the speaker, I did touch the mic and strings at the same time... TTZZZZZ didn't hurt just spook the hell out of me(I got that on video, not published), .. gladly I do have the ground switch on the amp, and the 3 wires cord with ground, some places have sockets on the walls with 3 pins connection but no ground wire.
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I recently restored a '72 Twin that didn't have the pull M switch on it. I replaced all the electrolytic caps in it. I changed the bias circuit to AB763 specs since we get quads these days that are fairly matched. The info on how to do it is here https://el34world.com/charts/bias_conversions.htm (https://el34world.com/charts/bias_conversions.htm) The cabinet on the one I worked on had water damage and was warped. I was lucky to find a Super Twin cab for sale to replace it.
I also replaced the voltage dropping resistors under the cap can along with the caps. I didn't replace the 220k leveling resistors and Doug mentions that they see little stress. Good luck with your restore project.
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Earth to Jeff :dontknow: Hope you follow these guy's excellent advice.
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Wow! thanks for all the replies - I knew I came to the right place!
Just to address some of the many good comments:
Already built the current limiter
and blew out the cobwebs (original condition of cabinet shown - its pretty shabby) and surface scuzz on all the components (main circuit board shown after cleaning, but the rectifier shot still shows it)
Mine has a 1K bias resistor ( thats one of very few deviations from the Rev C schematic; the others are trivial 470 vs 460, 47k vs 48k)
I'm pretty experienced with soldering, so not concerned about that when the time comes. Probably not evident with the downsized photos, but on Mar 6, "Richard Hand" must have had one biga$$ed soldering iron - lots of burnt of wire and there's a brown resin residue on every joint. Dunno - maybe thats normal here?
Powercord - will bypass the ground switch, but leave it in for cosmetics. My days of lugging this 70 lb beast are over - this baby is staying well grounded in the studio! (Back in the day, my guitarist had this same amp - maybe even the same year; my back still aches thinking about it :)
BTW These images turned out to be around 250K , so I can make them larger if need be - lemme know
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more small-ified photos
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I've got some barn mice that would love that cab :icon_biggrin:
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Resin would not be considered normal, at least from the couple hundred of this era I have been into. I know 2deaf and PRR said keep it stock to start. I would too, but I would consider building a new board after you get the amp running.
Here is why. If I plan to play the amp a lot, which a twin if you have the space is my favorite combo amp. :laugh:
I had not that weighed 87 lbs combo and I was you get too. No casters either baby.
The early 70s were the worst I have run across for the boards to become conductive by taking on humidity. I have head the tone on my 73 Super Reverb change as it warmed up and also the floor noise drop. I made a G10 eyelet boart to match the fish paper board. I normally just use od225 caps and mf resistors leaving the stock board alone. I prefer heaters below, like tweeds but it doesn't matter. Just looks neater.
So eventually I will have replaced most parts, but I also have the opportunity to renew the chassis while it is bare. Any time I have ever tried to keep a player amp with older components I get noise sooner or later and it seems as if I am inside it more than I meet to, which a twin is a pain to handle even just that huge chassis.
I am sure you will have a fine amp. Just Sharing some information.
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in my collection i have a '67 super reverb that has pops and hiss at random in the reverb circuit for about 10-15 when fired up cold. after it warms up it's fine. when i measured DCV from the board to ground on the amp, i was seeing 40-50VDC. at some point in time in the future, i'm probably going to have to rebuild it. for now, i just let it warm-up before playing through it.
i also have a '75 bassman 100 that had a very rough life and judging from the amount of mildew and rust, seems it was stored long term in a high humidity environment. it wouldn't stop popping, hissing, and crackling at random. measuring from some areas of the board to ground, i was seeing over 100VDC. i gave up and i gutted it. i laid out a modified AB165 circuit on G-10/FR4 board for it. i punched another hole in the chassis for the 4th preamp tube needed. i need to finish assembling it.
the point is, as ed states, is that some fender amps of that era that have had a hard life, may need to be rebuilt to salvage. when you get yours up and running chopstick the board to rule out any bad connections, then measure DCV on random parts board to ground to get an idea of the condition of the board in yours. that said, i have have some fender amps with a conductive board on my bench, that had no noise issues at all.
good luck with yours. twin reverbs are very, very nice sounding amps.
--pete
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in my collection i have a '67 super reverb that has pops and hiss at random in the reverb circuit for about 10-15 when fired up cold. after it warms up it's fine. when i measured DCV from the board to ground on the amp, i was seeing 40-50VDC. at some point in time in the future, i'm probably going to have to rebuild it. for now, i just let it warm-up before playing through it.
i also have a '75 bassman 100 that had a very rough life and judging from the amount of mildew and rust, seems it was stored long term in a high humidity environment. it wouldn't stop popping, hissing, and crackling at random. measuring from some areas of the board to ground, i was seeing over 100VDC. i gave up and i gutted it. i laid out a modified AB165 circuit on G-10/FR4 board for it. i punched another hole in the chassis for the 4th preamp tube needed. i need to finish assembling it.
the point is, as ed states, is that some fender amps of that era that have had a hard life, may need to be rebuilt to salvage. when you get yours up and running chopstick the board to rule out any bad connections, then measure DCV on random parts board to ground to get an idea of the condition of the board in yours. that said, i have have some fender amps with a conductive board on my bench, that had no noise issues at all.
good luck with yours. twin reverbs are very, very nice sounding amps.
--pete
Yes DL, After trying to read what I wrote early this morning on a damn tablet (PRR I did get a nice USB Keyboard), but when outside at daybreak with my dogs I make a lot of typos. Yes, I could use spell check, but if I get too perfect I am impossible to live with.
So let me say, yea, what he said. I will say I had a 65 Deluxe with a conductive board, but this jewel was all stock. I am not sure any of the larger Silverface amps amps will increase a lot in value since, but I will not drill a hole in one. It lived it's life in a very humid environment and created a delimma for me. Well, the amp has to work well.
I did have end up having to remove and replace it. I did sell it and informed the guy I had replaced the board with one I made, but still had the original board and he was welcome to it, however the one it it had the same parts, brand and year. I had some of the blue molded caps and Carbon Comp. The guy actually preferred the amp this way. He plays at his church and the amp sits there and looks so nice most other players think it is a reissue.
The key on you twin is the master volume and how you like it. Like I said, I have played a lot of twins, but they do seem strange with preamp distortion.
The first one I ever played was at my local music store. Found the master, got it distorting an playing Train Kept a Rolling just as you would think a 14 year old would. I remember it sounding great right before Mr. Attina, the music store owner said turn it off and go home for a week. I was banned, but it was worth it. :icon_biggrin:
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The cabs from the 70's era were not finger jointed, instead they are rabbet jointed and then glued and stapled. I supposed it was a cost saving measure. What I've seen in many of the larger amps like Supers and Twins is that the glue has let loose and the joints have separated which has required peeling back the tolex to reglue and clamp the joints. The ones with the castors which have the reinforcements in the bottom have shown to be less prone to bottom joints coming loose. In some cases it seemed only the tolex was keeping the joint together. Might want to take a look at your cab joints. Also I tried once to cut out the speaker baffle to build in a "removeable" baffle like the earlier cabs.. it was a disaster. If the cabinet is not so hot, mojo has replacement that is nice.
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I think it's great that this amp was found before hitting a landfill somewhere. Everyone here has their opinion on how to proceed, me too.
Based on those photos, I would gut all the components and wiring. Total rebuild.
I'm NOT recommending that YOU do that. That's what I would do.
And that's probably why serendipity dropped the amp into your hands, not mine
LOL
:l2:
Seriously though. Good luck, take your time, use patience. This forum is definitely gonna help you through the process.
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I think mechanically the cabinet is fine, so not worried about that. but the tolex and hardware are grungy. Oh yeah - the front of the chassis is dented / bent. won't be surprised if the pots in that area have issues. yeah this pup has had a hard life....
I can see a complete rebuild may be inevitable. But at least that's possible (something i never realized). just hoping the big ticket transformer is good.
I'm going to start with the basics (Electrolytics, etc) - putting together my first Hoffman amps order now!
but I'm not seeing anything close to the 70uF 350V filter cap. Its suggesting a 100uF , since thats 50% out of tolerance , how will that affect things? ( I may have read that can thin bass somehow - dont quote me)
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It's very common to replace those old 70µF/350V caps with modern 100µF/350V caps. Consider it an upgrade.
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It's very common to replace those old 70µF/350V caps with modern 100µF/350V caps. Consider it an upgrade.
Close enough for rock n roll, I always say!
Order is in - hopefully will have some good news to report by this time next week
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:worthy1: :worthy1: I've got to congratulate You on your most serendipitous find. All I ever spot are pennies.
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While I patiently await the arrival of the parts and a fresh roll of Kester, I did a little more cleanup.
I didn't find any actual mice, but it sure did look like it came from a barn.
Its hard to imagine how something could get this greasy and grimey - what were they doing, cooking french fries in there?
Look at that gunk! On the right is where I already took several passes with windex and a toothbrush etc (there's still some gumminess there, compare to the shiney, pristine part under the doghouse; only a razor blade got it down to bare metal, but that started to scratch it).
I have no reasonable explanation where it came from...
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Maybe try a Scotch-Brite pad with some VM&P Naptha (lighter fluid). Best cleaner ever and doesn't harm parts.
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The pad helped a lot, thanks (no sense babying it like it was a silkscreened faceplate). And a little elbow grease - no need for the naptha ( I don't light that many things on fire anymore)
Hey - the parts order came! right on time and perfectly filled.
Time to get serious !
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windex contains ammonia & ammonia is VERY corrosive.
use mild soap & water & lots of elbow grease. use alcohol for the tuff stuff.
--pete
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Mineral spirits cleans a lot of grease and grime. I usually follow with denatured alcohol.
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Thanks - good to know - especially when i get around to the front panel.
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What do you think of my plan of attack for phase 1* ?
(out of an abundance of caution, basically just make sure the power section is good for just a few bucks )
Do the Deathcap bypass
Replace Electrolytics
5 in doghouse
2 on bias / rectifier board
Replace the (2) "dreaded blue" .1uF power tube Grid coupling caps
Replace the diodes
Inspect and resolder anything in need
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power up WITHOUT tubes (note I don't have a variac)
check voltages etc
(Where?)
Have a glass of wine, assuming there was no "BIG surprise"
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*taking the lead from this D-lab video (there is also a part 2).
NOTE I don't think he'e working from the correct schematic. This twin looks nearly identical to mine but appears to have the push-pull piggybacked on the Master Volume (so its later than my '72). He checks the bias pot to make sure it works but doesn't seem to take into account that on these later models it was a balance, correct?
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phase 2
replace the (4) 470 ohm screen resistors on the power tubes ( 3 are measuring just under 500 , but one is over 700)
add 1 ohm shunts
Put in tubes
measure bias voltages
?
Any other tips?
Thanks in advance.
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Looks like a good plan except I'd have a beer instead of glass of wine. :icon_biggrin:
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check voltages etc (Where?)
B4 I do this, I OHM the PS, last TAP, should NOT be anything close to zero or low kohms, typical is infinity or really big.
then I ohm EVERY plate to it's tap, (1 lead at tube socket)
then I ohm EVERY cathode to ground
then I scribble the volts I expect from schematics, math, google
then I do the volts
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oh, forgot then I smoke my wine :icon_biggrin:
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Check out on youtube uncle doug (https://www.youtube.com/user/Stratosaurus1/videos?view=0&sort=p&shelf_id=2) he have lots of very good videos about tube amps!
Several about Fender amps, how tube amps work and so on.
Hint, Use some old teeth brush to clean the new solders, make sure the solder melts and looks shine after done, always clean the iron solder tip for every new solder.
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note I don't think he'e working from the correct schematic. This twin looks nearly identical to mine but appears to have the push-pull piggybacked on the Master Volume (so its later than my '72). He checks the bias pot to make sure it works but doesn't seem to take into account that on these later models it was a balance, correct?
So you have a pretty good indication that he doesn't know what he is talking about. The amp he is working on clearly has a balance control, but he thinks that it is a level control. He only has one 1-ohm cathode resistor, so he doesn't realize that the other pair of output tubes is getting colder while he increases the the current through the tube with the cathode resistor. May have something to do with the hum.
My personal favorite is the idea that a shorted OT doesn't put a load in series with the tube and the tube will run wild with no load. He may have topped Uncle Doug with that one.
I figure one of those ugly blue caps must have insulted his mother at some point because he has a completely irrational grudge against them. He replaced those 0.1uF caps for no reason. There is a demand for those blue caps, but I doubt he is removing them for resale.
My advice is to delete those videos. He seems like a pretty good guy, but he has no business posting a how-to video with such a serious lack of knowledge. Some guy on another forum said that Uncle Doug needs to be teamed up with a fact checker. Pegged it.
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What do you think of my plan of attack for phase 1* ?
Do the Deathcap bypass
Replace Electrolytics
5 in doghouse
2 on bias / rectifier board
Replace the (2) "dreaded blue" .1uF power tube Grid coupling caps
Replace the diodes
Inspect and resolder anything in need
----------------------
power up WITHOUT tubes (note I don't have a variac)
check voltages etc
Once you get the power supply working, you can determine if the 0.1uF PI coupling caps are leaking. I see no reason to replace them if they aren't leaking DC. I also see no reason to replace the diodes unless something is found that would indicate replacement. If you do replace the diodes, make sure to heat-sink them while soldering.
I see in your pictures that the convenience outlet is wired backwards. Not that big of a deal in light of the incorrect wall outlet wiring found everywhere, but if you replace the cord you may as well wire it correctly.
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Good score!
I recently (year before last) scored a 73 Quad Reverb complete chassis that had been dogified by previous owners and put into a shell-of-a head cab. It was badly mauled in fact - took about 40 hours to get it running okay (including output tube socket replacement etc). Your TR looks much more original - good foundation for restoration.
I know one guy who does painstaking restorations of silver face amps, he meticulously dismantles the board completely and scrapes all the excess wax off them (on both sides!) before reassembly. Probably no need to go to those lengths - I didn't with mine.
The filter caps aren't original, so yes - replace all of them, including the caps in the bias supply. As to whether you need to replace the diodes, your call. Test them and if they're all okay, keep them. Test the coupling caps (with no tubes in first) to see if they're leaking DC. If they're okay, leave them alone. Also test the LTP slave triode decoupling cap for DC leakage - a leaky cap here can skew the LTP (and output tube) voltages. Might also pay to disconnect the OT and test that it isn't shorted. Inspect the output tube sockets carefully for signs of fire/arcing, including the screen grid resistors (although from the photo, its hard to see whether there is intact any problem in this regard). Clean the mould off the boards.
Otherwise do the usual new build startup procedure (including running a dimmer bulb when you first power it up to check there is no PT short or other short in the power supply).
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Wow lotsa food for thought!
First and foremost..
Looks like a good plan except I'd have a beer instead of glass of wine. :icon_biggrin:
Yeah, my first choice too! Just mimicking Mr D-labs and his beverage of choice (not that there's anything wrong with that).
B4 I do this, I OHM the PS, last TAP, should NOT be anything close to zero or low kohms, typical is infinity or really big.
.... then I ohm EVERY plate to it's tap, (1 lead at tube socket)
OK so check the windings in the PS , but not following exactly (still learning the lingo) -
" last TAP ..."
and "ohm EVERY plate to it's tap ..."
Uncle Doug ... has lots of very good videos about tube amps! Several about Fender amps, how tube amps work and so on.
.... Some guy on another forum said that Uncle Doug needs to be teamed up with a fact checker. Pegged it.
Well, if I read it on the internet , I know it must be true! :)
But point taken, like everything else, with a grain of salt. I've watched at least a hundred videos by Uncle Doug, D-Labs, Guitologist, Mr Carlson etc. Some of them more than once. None may be perfect, but they gave hope that there's enough common knowledge out there to do this. I just think this stuff is so cool. After about 6 weeks I'm just starting to get a feel about whats right, like that bias-balance thing.
Once you get the power supply working, you can determine if the 0.1uF PI coupling caps are leaking. I see no reason to replace them if they aren't leaking DC.
Test the coupling caps (with no tubes in first) to see if they're leaking DC. If they're okay, leave them alone.
Also test the LTP slave triode decoupling cap for DC leakage - a leaky cap here can skew the LTP (and output tube) voltages.
As to whether you need to replace the diodes, your call. Test them and if they're all okay, keep them.
How do you test caps for Leaking DC ? I thought you need a special meter for ESR. Mine is V-O-C-L .
How to test for diodes? (why not just change out for something that cost 15 cents?)
What is "LTP slave triode " ?
probably something obvious - I'm just not getting it at the moment .
Time for a beer - Thanks!
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pick ANYTHING, learn "the language" you're golden :icon_biggrin:
TAP is each "node" for B+, typically you'll see A B C D in a circle or square on a schematic. preamps have 2-3, so use the one "associated" with that tube
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How do you test caps for Leaking DC ? I thought you need a special meter for ESR. Mine is V-O-C-L .
Once you get the power supply going, there should be a high voltage on one side of the 0.1uF caps and zero volts on the other side relative to ground.
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How do you test caps for Leaking DC ? I thought you need a special meter for ESR. Mine is V-O-C-L .
see 2deaf's response
How to test for diodes? (why not just change out for something that cost 15 cents?)
Use an R-meter with one meter probe at each diode lead (with the amp switched off, discharged and unplugged). Good diodes will measure a resistance one way, by when you switch the probes over to the opposite leads, they will read open connection. Another way is to (carefully) use a scope to check they are rectifying when the amp is powered up.
What is "LTP slave triode " ?
Non-inverting triode
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Well, if I read it on the internet , I know it must be true! :)
But point taken, like everything else, with a grain of salt. I've watched at least a hundred videos by Uncle Doug, D-Labs, Guitologist, Mr Carlson etc.
I don't usually watch those videos and this is the first time I've watched D-Labs videos. Man they were long. I must have fallen asleep because I never did see what he did about the hum (60Hz? 120Hz?). There is a difference between taking a video with a grain of salt and taking what I say with a grain of salt. There are a whole pack of fact checkers on this forum and they will immediately jump down my throat if I say something that is incorrect. I ain't complaining, that's the way it should be. They will frequently say exactly what I said right after I said it like as though I had never said it, so when you see that you can be more confident that what I said is indeed correct.
Enjoy your beer.
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Sorry, not trying to break etiquette or overlook your valid suggestion.
Out of all the videos I watched, the reason I highlighted that one is simple. Most amp repairs start off with a working amp with some common, known issues (noise, hum , wonky tremolo etc). So the tech just plugs it in and does his thing. This one had a major red-plating problem and started with the tubes pulled, which was new to me. With mine being a giant question mark, it seemed prudent to try that approach, which is why I came here for validation.
While I certainly don't want to spread false or incomplete information, does it not serve a purpose to provide clear commentary and critique to said videos? I hope anyone seeing the video link also sees the question I raised, and more importantly, the highly informed answers given here.
BTW I didn't realize the link would pop up as a huge picture here. No need for both of them - I'll at least delete pt2 for now.
FWIW - D-labs strikes me as an excellent military electronics tech and a very entertaining guy (reminds me of a cross between Walter White and Ned Flanders). But he's not a musician and his videos don't end with a money shot (until the latest where he invites a friend over to play).
Uncle Doug goes the extra mile and tries to explain the theory in relatively simple terms, which is helpful. I'm not smart enough to know where he gets off-course, but its not that crucial.
itSall Good,man!
The beer was good, real good :laugh:
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Tomorrow's looking like a nice rainy day, so perfect to get this thing going...
In the meantime I just want want to give a shout out to an amazing performer and amp I just saw - Amongst the most gorgeous amps I've ever heard, and never really heard of before - a Brownface Vibroverb.
The venue was the Greenwhich House School of Music and the artist is Celisse Henderson. Her current "regular" job is playing with Trey Anastasio in his new project "Ghosts of the Forest". She's an amazing singer, performer and talented multi-instrumentalist, in a solo show.
Its an intimate room, holds about 80 people, minimal miking and I sat front and center.
The amp was perfectly punchy, dynamic and expressive. Seems to be a relatively rare model - Even the reissue is no longer issued.
Under the glitter I think the guitar was a Jazzmaster. For this gig most of that hefty pedal board was unused (good call!)
I say a prayer: "Please God, help me make my amp sound as good as this one!"
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OK - some decent progress today. Some good news, some not so good (but maybe its very good news!)
1) Power Cord
I see in your pictures that the convenience outlet is wired backwards. Not that big of a deal in light of the incorrect wall outlet wiring found everywhere, but if you replace the cord you may as well wire it correctly.
Good catch! I wasn't paying much attention to this area, but your are correct. PLUS while tracing this out I discovered an intermittent fault in the power cord! The hot had no continuity at all when the cord was twisted a certain way. Isolated that down to the area where the cord restraint screws into the inside of the cabinet about 10" down (more abuse - somebody must've yanked it good!). So looks like another part to order. But I'm thinking that could've been sufficient cause for someone losing interest to just give up.
What do you tink about this diagram to rewire the new plug? (with outlet and bypassing ground switch/death cap)
2) Doghouse
moved on and started replacing the filter caps.
I'm including a shot of the starting point showing all the resin/ mystery substance slop. Trying to clean that up.
It quickly became apparent to me that the best way to do this is to completely remove all the leads in any given eyelet. Clean them all up and start fresh (as can be).
3) finally , just looking to get a sanity check (its getting late).
The last (2) 80uF (now 100 uF) caps on the left have unfamiliar markings. Before I tack them in - is that the correct orientation?
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If the original two were correct then you have those two on the left correct also. Those two just show which end is - instead of showing which end is + like the originals. I would check the polarity probably against the schematic instead of relying on those supposedly originals to be correct. Just me though.
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illinois 100uF 350V caps - orientation is correct in the last pic.
about the polarity marks on the illinois caps: the markings on the cap body "> - > - >" the arrows point to the negative lead.
--pete
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Thanks! - For sure the left most one is supposed to be opposite of the rest (they need to be in series , to double the voltage).
Was trying to remember that arrow thing ("it points how electrons flow from positive to negative"). gimme a break whats so hard about printing a couple of "+++"?
Funny thing, now that I look at my photo, I can see its got a dimple at the positive end like the rest. Totally overlooked that with all the confusing and distracting graphics! Its easy, once you know...
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yeah really. I hate those markings.
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Thanks! - For sure the left most one is supposed to be opposite of the rest.
Was trying to remember that arrow thing ("it points how electrons flow from positive to negative"). gimme a break whats so hard about printing a couple of "+++"?
There is a negative symbol in the white bubble and arrows pointing to the lead that is negative. Better check that electron flow thing. Ben's conventional current flows from positive to negative.
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ok finished the dog house and also the 2 Ecaps on the recitifier before ( man those new ones look so puny!)
Now, about those diodes...
How to test for diodes?
Use an R-meter with one meter probe at each diode lead (with the amp switched off, discharged and unplugged). Good diodes will measure a resistance one way, by when you switch the probes over to the opposite leads, they will read open
Thats kinda what I thought , but can I do this in-circuit? I had tried that but got bizarre readings doing so. Went back again and gave it another shot - still seeing things i don't understand.
For the lone reverse guy it measured about 20 k in both directions (very suspicious).
For the string, each measured 380K as shown, and Open when reversed ( weird thing is the reading changes as you change the scale - it seems to read about 3/4 of whatever the scale is ????)
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I would eliminate the possibility that the rest of the circuit is affecting your readings by temporarily lifting those three red wires on the left side while testing. Does your meter have a diode test setting on it? The link shows how to test with or without a diode test setting.
https://www.fluke.com/en-us/learn/best-practices/test-tools-basics/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter
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You cannot accurately check those diodes in circuit. Do what Papa says. I really doubt anything is wrong with that board. The bias caps have been replaced and are probably good too. In fact, I bet all those caps in the doghouse were good too. Even the one with the broken lead could have been repaired.
At some point you need to just turn the amp on with no tubes and check the B+ and negative bias voltage.
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One of those ugly blue caps must have insulted his mother at some point because he has a completely irrational grudge against them. He replaced those 0.1uF caps for no reason. There is a demand for those blue caps, but I doubt he is removing them for resale.
I am not a fan of the man's videos, and his dealings with the bias balance pot are shockingly stupid. However, as far as i can tell he only replaced the two coupling caps in the power amp. In my experience, those blue caps in that position fail frequently, and when they do, the results are arguably more serious than in other positions, so that move isn't completely irrational...
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One of those ugly blue caps must have insulted his mother at some point because he has a completely irrational grudge against them. He replaced those 0.1uF caps for no reason. There is a demand for those blue caps, but I doubt he is removing them for resale.
I am not a fan of the man's videos, and his dealings with the bias balance pot are shockingly stupid. However, as far as i can tell he only replaced the two coupling caps in the power amp. In my experience, those blue caps in that position fail frequently, and when they do, the results are arguably more serious than in other positions, so that move isn't completely irrational...
I would agree with you if I had ever experienced a failed blue cap, but I haven't. I've never seen a terrible blue cap cause 60 cycle hum, either.
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How do you test caps for Leaking DC ? I thought you need a special meter for ESR. Mine is V-O-C-L .
Once you get the power supply going, there should be a high voltage on one side of the 0.1uF caps and zero volts on the other side relative to ground.
Where's the peer review? There would be HT on one side and negative bias voltage on the other for the 0.1uF PI coupling caps.
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How do you test caps for Leaking DC ? I thought you need a special meter for ESR. Mine is V-O-C-L .
Once you get the power supply going, there should be a high voltage on one side of the 0.1uF caps and zero volts on the other side relative to ground.
Where's the peer review?
Now that you've reached the prestigious Level 3 status :wav:
and obviously always know what you're talking about you will be subject to way less review. We will, of course, expect you to continue to review yourself.
Don't just go around spewing alternative facts.
Anyone with over 1000 posts must be filling the world with facts, so please, be more careful.
Don't make us catch you. We all have better things to do.
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Level 3?! I had no idea a guy on the no-fly list could get a security clearance that high. I better clean up my act.
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no-fly list
:l2:
You piss of a TSA agent tooo?? they really didn't like a long-hair telling them, just a little left with that hand, ahhhhhh, that's the spot
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I would eliminate the possibility that the rest of the circuit is affecting your readings by temporarily lifting those three red wires on the left side while testing. Does your meter have a diode test setting on it? The link shows how to test with or without a diode test setting.
https://www.fluke.com/en-us/learn/best-practices/test-tools-basics/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter
My little Radioshack meter does not have. But my other cheap meter does! (fancier , but I have shied away from using it since it seems to consistently measure a little on the low side) Imagine that, after all these years i never measured a diode - most of my work has been passive speaker crossovers.
So I think there's good news - in open position it reads about 3V. shorted its 0V.
On one side of the diode its reads the full 3V (open). Reversed, it reads about .5V This is consistent for diodes in circuit and a few other new ones I have out of circuit.
The bias caps have been replaced and are probably good too. In fact, I bet all those caps in the doghouse were good too. Even the one with the broken lead could have been repaired.
possibly an overabundance of caution - but the big blue atoms start with "96.." , I think making them 20+ years old. I'm under the impression that the rule thumb for Electrolytics makes them over the hill. The bias caps look original to me, making the almost 50. What is the life expectancy?
Nothing has been tossed - and can be un-replaced.
At some point you need to just turn the amp on with no tubes and check the B+ and negative bias voltage.
New power cord is in the house now.
Any comments on that wiring diagram from a couple of days ago? basically bypassing the polarity switch and going to fuse then power switch
Thanks for your patience. I realize this is all an afternoon's work for most of you.
Same thing happened when I tiled my bathroom. Learned a lot and the final result was excellent . Even if it did take a ridiculous amount of time. But now I'm all set to do the the upstairs bathroom and the kitchen, and won't need to ask so many pesty questions :)
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regarding power cord wiring. see this link and scroll down to page 16 http://sluckeyamps.com/misc/Amp_Scrapbook.pdf (http://sluckeyamps.com/misc/Amp_Scrapbook.pdf) you will see the best way to wire it. I have been using x1/y2 safety caps in place of the .047 cap in the stock amp.. and keeping the ground switch function. There are many opinions on this but I see nothing wrong about it.
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Got the nice new Husky 14 ga cord wired in (for now polarity/deathcap bypassed).
One last check before the smoke test - measuring the resistance on the PT windings. I have no idea what to expect, other than the Primary (Black-White) to Secondary (Reds) should be open.
I unsoldered the red secondary leads and here's what I measured
Black - White : 4.5 ohms
- Red1, : Open
- Red2, : Open
- Red /BLue : Open
Red1 - Red2 : 21
- Red/Blue: 14
Red2 - Red/Blue: 15
Ground - Red1: 10 (shown in photo)
- Red2 11
-Red/Blue 4
Seem reasonable?
Anything else to check at this point?
(just for the record , that huge blob of solder on the ground was there already - i dont think my soldering iron can melt it all at once)
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Do you have a current limiter for the first power up to use. It always settles my nerves before switching a new amp one. :icon_biggrin:
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dont think my soldering iron can melt it all at once
the Fender folk got paid by the miles of solder they used!
I took out 3 pots, 2 caps, clogged my solder sucker twice and used 3' of wick :BangHead:
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Got the nice new Husky 14 ga cord wired in (for now polarity/deathcap bypassed).
saywhat? 14AWG for a 100W amp? (yes, i know it pulls 3x that), but jeeez. 18AWG is overkill @ 10A or 1200W @ 120V.
respectfully,
--pete
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Do you have a current limiter for the first power up to use. It always settles my nerves before switching a new amp one. :icon_biggrin:
yup - thats was the first thing I did. And found a couple of NOS 100W incandescent bulbs in Grannies apartment.
1 bulb or 2? I guess this Y adapter puts them in parallel, so not sure if it helps...
say what? 14AWG for a 100W amp? ... but jeeez. 18AWG is overkill
respectfully,
--pete
Ha - I felt bad because I wanted to use the original and just cut off the bad 10" segment. But punky 47 year old rubber and rope didn't inspire confidence.
I have a hi-fi background and always use 12 GA on my speakers, so why not beef up the power cord? (there are idiots who pay hundreds and thousands to upgrade their hi-fi power cords - this was only $2 more). Although the hard plastic grommet did have to be whitled down on its internal "clamp". Next time 16ga.
So nothing alarming about those measurements, I take it?
I just measured everything in sight. I was a little suprised at the Ground - secondary measuring so low (not that I know what I'm yapping about)
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You really need to take this to someone that has a clue. Or take it to the curb!
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Quit doing this :w2: :w2: :w2: :w2: and just power the sucker up before all the help just ceases dude.
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> I have a hi-fi background and always use 12 GA on my speakers, so why not beef up the power cord?
Speaker is 8 Ohms and the wire IS the signal path.
The amplifier is like 120V/2.4A or 50 Ohms, far higher than 8; and isn't directly in the signal path.
Simple proportion suggests that you could use 8/50 as much copper. Scaling from #12@8r, you could use #20.
Actually the Code suggests using a wire which will pretty-sure pop the fuse in the cellar before it sets the carpet on fire. This will be lamp-cord, formerly #20 but IIRC now #18.
However for a lousy 6 feet on a 40 pound appliance, around a bunch of drunks, I have never objected to OVER-kill. Hell, I use #14 for instrumentation carrying 0.050V 0.010A, just because it is readily available (mowed extension cords) RUGGED.
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oh, forgot then I smoke my wine :icon_biggrin:
I find that combustable wine to go well with Cream, even their reunion DVD. :laugh:
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combustable wine to go well with Cream
:laugh:
thanks for the suggestion
I picked up topographic oceans to test drive my latest build, works well!
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While most, if not all, the posts have been on track to get this amp back up and working, the suggestion to just gut it and but new board and such in it has a lot of merit. This is a not a collector amp, and to restore it to that condition with the proper type parts is an extremely expensive proposition. The great news is that the "Blackface/Silverface" Twin Reverb is one of the great amp circuits ever designed and the various "upgrades" done to the classic AB763 circuit are easily reversed.
Hoffman, as well as other vendors, make brand new eyelet or turretboards, that are a drop in fit. You could also purchase a "small parts" kit that has all of the parts to rebuild it using only the chassis and transformers. There are a couple of really good "how to build a tube amp" instructions that can walk you through how to get it built - and get it built to work.
I rebuilt my first Fender almost 50 years ago and unless someone walks in with "Grandad's Tweed Twin that was in a closet" old amps are just that - OLD! When I do a rebuild I am perfectly happy to spend someone else's money if they just have to have some particular "magic component" but many of the $10 caps out there are rebranded $3.00 ones. But usually they want an amp that works, works reliably, and can handle gigs, etc. It's like owning a 1965 Vette and want to drive it - some stuff needs to be rebuilt/replaced.
If you want to use this for any real playing get the old stuff out of there - it will still sound better than 90% of the newest amps. Metal Oxide Plate resistors, Metal film Cathode resistors, new film coupling caps make for a solid & quiet amp. I've even started using large film caps for the Power Supply caps. They are about the same as the better electrolytics.
Nothing electronic that is over 20 years old is worth saving save the transformers.
I have a 1970 Bassman that I rebuilt with a new board and sockets. It is dead quiet and should last another 20 years with only an occasional tube replacement.
Good luck
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That being said by Saransk then power the thing up because you have nothing to lose really.
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AS Alan Shepard said "Light this candle."
I've never seen a Fender catch fire - some smoke but no fire like older Marshalls are sometimes prone to do when abused.
Use a cheap speaker for the initial test - even massive Electro-Voice speakers don't like pure square waves and such.
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Thread is going on like 26 days and still no power testing. :dontknow:
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You really need to take this to someone that has a clue. Or take it to the curb!
Done!
For anyone interested, it is by a dumpster behind the old Sears at the CrossRoads Mall in Omaha. I figure you have a few days before the truck comes...
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Just kiddin (jeez - where 's your sense of humor?)
Clues, I have aplenty. Experience, which varies directly with equipment ruined, not so much.
So moving forward ( 1 agonizingly slow step at a time. sorry my day job sux and I can only get an hour here or there for this)...
Since there were no unexpected shorts or opens seen on the PT, I then tested its voltages, while still disconnected. As far as I can tell things are good :
The receptacle tester is happy with the rewired outlet.
The VOM seems happy too: The red secondary reads at just over 340VAC.
The RevC schematic (no push-pull, like mine) is pretty fuzzy, but it looks like it could be saying 340VAC. RevF/G are clearer and also shows 340VAC.
The bias voltage is a little amibiguous. Mine reads 62V on the red-blue wire.
RevC shows what looks like -69V, but on the other side of the resistor and diode. However the resistor value there is 470, whereas mine is actually 1K.
The RevF drawing (has push-pull) has the 1K and shows -63V.
The truth probably lies somewhere in the mythical Rev D-E (any sightings?). But I'm hoping it means its all in the ballpark and the PT is OK. .
Also took another look at the diodes out of circuit and they measured OK
The next step is to reconnect the PT and get the B+, Bias Voltage and readings at other points of interest.
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jf69 hope you read the notes on the schematic..
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jf69 hope you read the notes on the schematic..
If you're referring to the vibe pedal ground, then yes; and thanks for the reminder. I've been meaning to make a shorted RCA jack, which I think is whats needed, correct? ( I do not trust the actual pedal). So thats in place now.
If something else - then please elucidate.
With the PT reconnected I powered up and took some measurements (No tubes).
Some good news and a clueless question, of course.
1) The B+ came in pretty much spot on at 478VDC.
2) The bias right in there at -64VDC
3) no smoke or sparks
I took a couple of other measurements, but my notes are based on having the tubes in. What I'm seeing does not match those:
In the doghouse (see image) the red wire, which is connected to the diodes reads correctly as expected. But the others are way off. Is this simply because of the lack of tubes, or indicative of a problem?
On the 6l6 sockets my notes show that pin 6 should read 446V it reads 270 . So, as above.
What else is relevant / meaningful without the tubes?
Thanks
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I took a couple of other measurements, but my notes are based on having the tubes in. What I'm seeing does not match those:
In the doghouse (see image) the red wire, which is connected to the diodes reads correctly as expected. But the others are way off. Is this simply because of the lack of tubes, or indicative of a problem?
You said you took measurements with the tubes in. Then you asked if the voltages are wrong because the tubes are not in.
You got a problem, alright. Take the power tubes out and leave the little tubes in. Post the voltages without the light bulb limiter. If the choke/screen node is still 175V, shut it down immediately.
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> You said you took measurements with the tubes in. Then you asked if the voltages are wrong because the tubes are not in.
Sorry for the confusion, but what I was trying to say was that i took notes from studying the schematics. Then traced out where those points were in the layout and measured there.
But my measurements are way different, and I'm hoping thats because I still have not put the tubes in, and that the schematics wouldn't account for that.
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What about the standby switch? With no tubes and the standby switch closed, all of the voltages should be the same after all the cap's charge up.
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If you really want to isolate the problem then just disconnect the wire after the standby switch that goes to the center tap of the output tranny, and each individual wire that goes from the different caps B+ nodes to the amps different circuits or stages. Measure and they should all be pretty high. Connect one wire up at a time and determine which one is causing a large drop down to 170vdc that you see. Troubleshoot the component and wiring in that stage of the amp. That would just be my own personal method. Maybe not everyone's but it is a start to doing something. Post the voltages before and after connecting back each B+ node and output tranny center tap.
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Sorry it appears that cbs in their wisdom didn't include the note on this schematic that voltage readings are + or - 20% - also today's wall voltages sometimes cause many volt readings to be higher than indicated on older Fender schematics.
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What about the standby switch? With no tubes and the standby switch closed, all of the voltages should be the same after all the cap's charge up.
Excellent question!
I was playing with the Standby while testing directly at the diodes ( there's one place on the schematic that shows that : 478V "off" , 450V "on". )
But by the time I flipped her over to get into the doghouse it may have been off (can't be sure) .
Redoing everything today, I get very different results.
Not sure I have great explanation, but yes i think I see what you suggest now
1) Powering On with the Standby OPEN (opposite the power switch)
those 3 test points on the caps towards right start off essentially 0 V ( actually 6)
The red wire towards left measures 476
2) Closing the standby (same direction as power):
all test points in doghouse measure 473V ( after a quick charge up)
3) Opening the standby again a discharge begins - counting down at a noticeable rate, pretty much 1V sec.
With this being the case is that looking better?
Possible explanations of the discrepancy:
1) Last night I had standby open by the time i went to the doghouse. When testing I did not notice the slow but consistent discharge. possible but tonight it was quite evident.
2) Today I reset the the probes and ground point. Did flipping it over last night disturb ground point? possible , but the red wire still measured normally last night, just not the other 3.
I guess #1 is more likely a better explanation .
We'll see what happens tomorrow ( 2 outa 3 aint bad)
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I knew what you did when I looked at the numbers you posted. I say you put the tubes in and hook the power cord up to that wang-dang limiter with one 100W bulb.
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yep - a common rookie mistake, no doubt
Time to get tubular. First I had to build one of those stands and tacked on a dummy 8 ohm load.
With the wangdang special 100 W limiter and tubes in :worthy1:
I noted the bulb glowed a noticeably brighter than without the tubes, even after it settled down after the initial flash.
is that normal?
B+ revbias
standby open 362 -50
closed 262 -40
No smoke , sparks or me going :cussing:
so taking off the training wheels...
Without limiter
standby open 470 -64
closed 450 -64
I'm expecting either 478 (revc) or 470 (revF) open
and either 450 (revc) or 445(revF) closed
and either -69 (revc) or -63 (revF) rev bias
So mine's a little of each
close enough for Rock n Roll?
What next?
I have the 1 ohm resistors but didn't install yet - are we good to go ?
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Shut it down. Plug in speakers. Plug in guitar. Power up, no limiter. Hit it.
Attached are the actual voltages from a '72 Twin Reverb.
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Finally hit it!
Plugged in the Strat and the results weren't half bad (not counting my rusty playing).
Both channels worked. No exceptional buzz or hum.
The Normal channel was normally clean and bright
The Vibrato channel had a quite a bit more bite and drive.
But the vibrato wasnt vibrato-ing. The opto thingie was flashing and you could hear a slight ticking, but no change in volume whatsoever.
Note: the reverb tank is not connected that this point
Tubes
Giving things the chopstick test, some preamp tubes are slightly microphonic.
But V4 is highly microphonic (this is for the Reverb i believe). And the .003 ceramic cap connected to it is even more so. if you gently rub it, it sounds like the chopstick is on your eardrum
I have to double check which are which, but some tubes are RCA and others had faint markings that vanished when I wiped off the dust. not sure what to make of that - very old or very cheap replacements?.
Power Tubes
3 out 4 make a weird grunting sound when tapped. Like a distorted, thuddy reverb. the guts rattling around I presume?
Bias check
I took a crack at calculating the Plate Dissipation using the Output Transformer measurement .
Please feel free comment if I got any of this wrong:
Measuring from (diode string /OT center tap) to Plate Pin 3
Voltage drop 2.2 V 2.3 V
R 36.4 ohms 38.5 ohms
calculated current (V/R) = .060 mA .059 mA
B+ voltage x 456 V
power dissipatation (VxI) 27.3W 27.2W
While these numbers look impressively close, isn't that too high for a grid-biased 6L6GC tube?
My understanding is the recomemended max power is usually considered to be 30 W x .7 = 21 W
These tubes are matched Ruby's - are they made to handle this?
Your thoughts ?
Thanks
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power dissipatation (VxI) 27.3W 27.2W
Remember there are two tubes on each side so you need to divide those numbers by two. So, 27W for two tubes means 13.5W for each tube. That's a bit low.
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power dissipatation (VxI) 27.3W 27.2W
Remember there are two tubes on each side so you need to divide those numbers by two. So, 27W for two tubes means 13.5W for each tube. That's a bit low.
Ah! Of course, I completely forgot about that (paying too much attention to the math - where's that forehead-slap smiley at?)
Most of the vids / tutorials have a single tube per side and aren't geared towards the twin.
I'm traveling this week, so no time to do anything. But when I come back it looks like I have some biasin' to do.
I've reviewed the Blackface bias conversion links (change from balance to level adjust). But again, the examples I've seen so far are geared towards schematics with a single tube per side. Is this also recommended for a TR?
Is it important to balance the tubes within each side somehow (how do you balance 4 tubes)?
BTW - I did make a change before I started: For the (2) 68k resistors coming from the bias balance pot, one measured 66k and the other 75k ( so way more than 10%). I didn't know how much range the balance pot was going to have , so i replaced the 75k with a new ~67k. Haven't touched the balance pot yet
Thanks!
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I've reviewed the Blackface bias conversion links (change from balance to level adjust). But again, the examples I've seen so far are geared towards schematics with a single tube per side. Is this also recommended for a TR?
Is it important to balance the tubes within each side somehow (how do you balance 4 tubes)?
The number of tubes makes no difference to the bias circuit because there is no grid current, so circuits geared toward single tube per side are valid for a TR.
That balance thing is for idle so that hum is minimized. It is not necessary to balance the tubes within each side (although some folks do), only the balance between sides.
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Fancy-folk try to get all four bottles gushing the same at idle.
A TWIN is not made to idle but to ROAR. Anything over a whisper, the idle bias is overwhelmed by signal, the tubes conduct as needed. Anything under a whisper, you should have brought a smaller amplifier.
With four tubes, two each side, "balance" is not real important, differences average-out. Try not to have one tube under 50% Pdiss and another at 90% Pdiss, but anything half to two-third Pdiss is plenty "balanced" for any Twin use.
(And yes, I knew a guy played jazz cello through a Twin, and I would not change my advice for him.)
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OK - back in action. Only had a little time last night, so before I started making any real modifications, I wanted to check with another measurement. In the interest of Socratic endeavor I added the 1 ohm shunt resistors and took some measurements there.
All 4 resistors checked out very close to 1 ohm, as far as I could tell. The shorted leads on my meter measure .2 ohm. Each R measures 1.2 I even tacked them all together and they measured 4.2 (close enough to use mr ohm's law and pretend the meter is reading milliamps instead of millivolts)
V10 V9 V8 V7
32mv 29mv 31mv 29mv
So in terms of A-B balance , the sides are within 1 mv
today my B+ was 479V, VxI 's are
15.3W 13.9W 14.9W 13.9W
Slightly higher than the OT calculation (as expected) but still too low.
Also, wanted to double check all my doghouse voltages:
red yel2 brn yel4 (per 2deaf's image)
mine 454 453 416 343
Expected 450 446 420 345
2deaf's 450 448 418 348
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There are lots of things that can be done to increase the dissipation. You could change the 1K resistor to 1.5K in the bias circuit and see what happens. Robrob has an adjustable bias mod for silverface Fenders. Or you could change the bias balance control to a bias adjustment control.
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I don't think 14W-15W is "too low", especially on a Twin. It's not a whisperer.
I'd leave it be.
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Looks like your done and it was a sucess. :icon_biggrin:
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Or you could change the bias balance control to a bias adjustment control.
I mentioned this to him in reply #11 and one of the best discussions with graphics for changing over to adjustable bias is right here https://el34world.com/charts/bias_conversions.htm (https://el34world.com/charts/bias_conversions.htm)
I think its one of those things CBS did to Fenders circuits that doesn't make much sense to keep in todays environment and I'm not a fan of the bias balance circuit. Can be done easily and doesn't take long to do it.
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Also remember the '72 Twin is really based on older plans which assumed the original 19W Pdiss of a 6L6. Yes, in 1972 they were using the "30W" 6L6GC, but they may have had no reason to re-bias since it worked fine. And nobody had invented any "70%" rule yet. (70% of 19W is 13W.)
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Or you could change the bias balance control to a bias adjustment control.
I mentioned this to him in reply #11 and one of the best discussions with graphics for changing over to adjustable bias is right here https://el34world.com/charts/bias_conversions.htm (https://el34world.com/charts/bias_conversions.htm)
I think its one of those things CBS did to Fenders circuits that doesn't make much sense to keep in todays environment and I'm not a fan of the bias balance circuit. Can be done easily and doesn't take long to do it.
Why's that? Unmatched tubes--especially good NOS tubes--are cheaper than matched tubes, still.
And PRR is right. Twins we're never designed to be biased hot... A balancer pot (biased somewhat cold) is way more useful than an adjustable bias pot.
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Or you could change the bias balance control to a bias adjustment control.
I mentioned this to him in reply #11 and one of the best discussions with graphics for changing over to adjustable bias is right here https://el34world.com/charts/bias_conversions.htm (https://el34world.com/charts/bias_conversions.htm)
I apologize if I stepped on your toes.
Now that I have looked at Doug's bias mod's, I notice that none of them are the same as the mod's I posted.
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With an almost complete disregard for the Socratic method and critical thinking in general, I submit yet more bias mod's. Unlike Robrob's and Rob Hull's mod's, the bias level pot may be mounted anywhere.
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an almost complete disregard
in this new century, humor and intelligence doesn't compute well with them :icon_biggrin:
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. . . doesn't compute well with them
Giant ants?
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more like pacifist zombies with upgraded tri-corders :icon_biggrin:
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Excellent - thanks for all the tips!
There are lots of things that can be done to increase the dissipation. You could change the 1K resistor to 1.5K in the bias circuit and see what happens. Robrob has an adjustable bias mod for silverface Fenders. Or you could change the bias balance control to a bias adjustment control.
To start, I'll follow the path of least resistance ( get it, resistance ? :laugh:)
I was wondering which way to adjust that (up or down) -
if there's a 50 -50 chance of guessing, then there's a 90% chance I'll get it wrong. (like when you drop a piece of toast - does it land butter-side up or down?)
Looking back to the older schematics, they had the 470 ohm there, so I was curious to try that first (easy to clip in another 1k in parallel). Sure enough, the mV's went the wrong direction: down.
I don't have a lot of parts in stock, but i did happen to have a 1.5k (planning to replace those Rs cooking on the bottom of the sockets).
Put that in and all the mV's went up by 1/3 , eg 30 -> 40
So my VI's are now about
20.3W 18.5W 19.8W 18.5W
... just below that magical 70% level.
With these tubes there doesn't seem to be much need to balance. I think I'll try the level adjust conversion next.
But I really like the idea of both, because I may eventually change out the tubes. But I'd have to order another pot etc.
Did a quick test play on the normal channel (prevol= 8, mastervol = 8) and there might have been a bit more crunch and less brittleness. Unfortunately i forgot to play right before the change, so I can't be sure. And you know, there may be some implicit perception bias here (get it, bias :l2:? awright nevermind). But with the adjustment mod it will be easy to compare.
More unfortunately, not only does the vibrato channel not vibrato, it no longer even channels any more. Shame , I was looking forward to hearing that cranked, now its completely dead. I'll have to look into that too ( could it be some of Them dang ants got in there ??? )
Also there's been a buzz coming from the cabinet on bass notes. Could be one of those nifty Utah speakers. Not a big deal I guess, those things don't seem to get much love - do they? (I've got an old JBL and a recent Greenback I'm interested to compare once things are in order)
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Decided I'm going to add the extra bias level pot and keep the balance. Just hate to put in a tiny order when I know there's more coming soon. Very soon.
I investigated the tremolo issue. Good news is that the channel works - when I did a quick check last time I couldn't see the knobs on the other side of the chassis. So i turned up the volume, but all the tone pots were off. I didn't realize that, or did I even suspect that would essentially kill the sound so much; so live and learn.
But then the bugs came out to bite me...
It turns out the tremolo does work. The light flashes and it looks like there's a disco party going on inside the roach house.
And you can hear it - but only for a small range of notes. I would say its most pronounced playing the octave on the A string so roughly 220 Hz. And it gradually diminishes as you go up or down. On the hi E and B there is no effect. On the low E , very little. In way its kinda cool - but how bizarre is that?
What would cause it?
Also, turning both preamp volumes off elicits a ticking thump of the tremolo. (not a big deal , but maybe that helps indicate where the bugs are...)
Poking around :
V2 is pretty microphonic. Haven't tried any jockeying yet, but I'm feeling a new set of preamp tubes coming in the next order.
I noticed that there's a lot of microphonics and popping going on when I tap any of the caps, or even the board itself, on the right side (by V3,2,1). 3 screws hold the board down and theres a big warp between the middle screw and the right screw.
Tapping almost anywhere in between makes a lot of noise (of course everything is dimed for this part).
But the popping is not 100% consistent. If I tap a couple of times in a row it pops, but then might not the 3rd time. Wait a few seconds and that repeats.
Could this be bad caps (discharging / charging) ?
Something with the board itself?
Reverb - hooked the tank up and seems pretty much OK. only issue is that turning the reverb level all the way down still passes a very slight, hi-pass sounding reverb (like a tambourine in an airplane hangar). Not a big deal, since turning off the switch kills it. it might actually be useful percussive effect.
Just wondering if thats normal?
Speaker Looks like that buzzer went to meet its maker back in Utah. It actually crapped out during the tap test (told you it was noisy! ) I hadn't paid it much mind, but it turns out there was also a weak solder joint on the other speaker and that one wasn't even playing. Fixed that and now its the other speaker's turn.
I am ASSuming that running at 8 ohms with one speaker is acceptable (for the amp - not intending to give 1 speaker the full monty / 100W).
Is that OK ?
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If I tap a couple of times in a row it pops, but then might not the 3rd time.
pretty common for bad solder joint, tap - conduct - pop
3 screws hold the board down
Old fender boards suck, one of the 1st things I do, remove all the hold-down screws, make sure It's securely "elevated" and see what "effect" that has.
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st says ;
Why's that? Unmatched tubes--especially good NOS tubes--are cheaper than matched tubes, still.
And PRR is right. Twins we're never designed to be biased hot... A balancer pot (biased somewhat cold) is way more useful than an adjustable bias pot.
You and PRR are right about that.. the silverface twins sure weren't designed to be run with a hot bias. I'm sure the bias balancer has usefulness and I can respect that. In my experience I have come to like the simpler adjustable bias of the AB763 - and so have many others. The implementation of both would be ideal then, if you have tubes that need to be balanced in circuit and wanted to be able to adjust the Pdiss where you want it. Running vintage tubes is a good point.
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3 screws hold the board down
Old fender boards suck, one of the 1st things I do, remove all the hold-down screws, make sure It's securely "elevated" and see what "effect" that has.
Yes, it can sound very different.
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ST it should still be noted that even with both bias balance and adj bias with a Twin there is still two tubes per side, and each duet should still be fairly matched..
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ST it should still be noted that even with both bias balance and adj bias with a Twin there is still two tubes per side, and each duet should still be fairly matched..
Agreed, but only because you said "fairly" matched. For all practical purposes, in a guitar amp (!) such as a twin, you can use unmatched tubes without problem unless they are wildly out of spec. Take 4 random tubes, put the two coldest on one side and the two hottest on the other, that way the tubes on each side will be *fairly* matched. Then use the balancer (!) to *fairly* match the two sides.
In my personal view, perfect bias adjustment makes little sense in a tube guitar amp, which is about as crude and lofi a piece of electronic technology you may ever come across... Furthermore, the way bias is set does not influence tone. Much much more importantly, changing the way it is set is almost never the solution to a real problem. In almost all instances I see it discussed, the better advice would to get the amp working properly. (Inspecting the board and its screws themselves is, in that respect, sound advice!)
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the way bias is set does not influence tone.
There is a tonal change, enough to bother you, subjective.
the last amp I set bias, you could definitely "hear" the amp come alive going from fridged to warm bias
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. Much much more importantly, changing the way it is set is almost never the solution to a real problem. In almost all instances I see it discussed, the better advice would to get the amp working properly. (Inspecting the board and its screws themselves is, in that respect, sound advice!)
Strange you would say this. Who said converting bias adjustment circuits from one type to another in a SF Twin was a prelude/substitute to troubleshooting or problem discovery? It was a simple recommendation for when the amp is up and running, not a suggestion to help find something wrong. And as shooter elucidates above... being able to take a Twin to that sweet spot in bias - in consideration of the output tubes characteristics is in many ways a good thing. Tell me sir.. why would Doug Hoffman discuss changing bias circuits to AB763 from SF bias balance if it wasn't considered a good thing to do? And why would Shooter and myself find it better to be able to have that adjustment capability based on what we have heard out of the Twin amplifier. I have had 0 complaints.
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I didn't mean to suggest converting the SF to the BF bias arrangement is a bad idea; I think instead it depends on personal preference and your tubes (matched or not). The truth is, the thread starter, who is a self-declared newbie (all due), asked for advice on restoring this amp, indicating he wanted to stick to the original schematic, and immediately someone mentions changing the bias arrangement. Same thing happened in a different recent thread (which I now can't seem to find). Anyway, don't take it the wrong way. I don't think it's bad advice, it's just that I prefer a "first things first" approach and I don't consider the bias arrangement a "first thing" (unless of course there is no neg bias voltage).
And apologies to the thread starter as well. I didn't mean to hijack the thread.
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That's alright, I enjoy the banter and appreciate the info, even when it spins into a world of its own. Sometimes those are the most entertaining parts! Fortunately I work so slowly it fills my dead-air nicely :)
To me adding the bias level adjust just increases the versatility without committing to any SF vs BF conversions.
Plus, I don't see any harm as if it were defacing a vintage museum-quality collectors item.
I see some people like to "bias by ear", so I'm interested to hear the difference for myself, once I know what the reasonable limits are.
Only a bit progress this week. Traveling for the job and had a high school band reunion this weekend. Aint saying what year, but there was plenty of Led Zep, Bowie, Allman Bros and Cocaine (the song) on the set list.
Took a look at that buzzing speaker. wow - never seen anything self destruct so completely (pretty much started the reconing process for me but i don't think I'll be going there). I tacked on 50W worth of 8 ohm resistor in parallel to keep the amp happy and make sure the other speaker doesn't follow suit just yet.
Took another look at the board and tried removing the screws.
Didn't really reveal anything new ; if anything that made it worse. Ch1 ( all the way by the right screw) even got a bit microphonic when tapped (but no popping like the other channel). Seems worst at the high point of that bulge (kinda wish I could simply screw it down there).
Also noted on Ch2:
Sticking the chopstick in between the terminals on the Volume pot or bright switch induces a slight hum (doesn't happen on any other pots)
Swapped V1 and V2 and the tube in the v2 socket was still more microphonic than the tube in v1 socket.
Tapping the wire from input jack to pin 7 also produces the popping noise
The terminals on socket 2 are "wobbly", from the underside.
Not sure if this explains all the noises, but that socket is trouble. I'd swear there was a contact mic on that board - it comes from anywhere.
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That's alright, I enjoy the banter and appreciate the info, even when it spins into a world of its own. Sometimes those are the most entertaining parts! Fortunately I work so slowly it fills my dead-air nicely :)
To me adding the bias level adjust just increases the versatility without committing to any SF vs BF conversions.
Plus, I don't see any harm as if it were defacing a vintage museum-quality collectors item.
I see some people like to "bias by ear", so I'm interested to hear the difference for myself, once I know what the reasonable limits are.
Only a bit progress this week. Traveling for the job and had a high school band reunion this weekend. Aint saying what year, but there was plenty of Led Zep, Bowie, Allman Bros and Cocaine (the song) on the set list.
Took a look at that buzzing speaker. wow - never seen anything self destruct so completely (pretty much started the reconing process for me but i don't think I'll be going there). I tacked on 50W worth of 8 ohm resistor in parallel to keep the amp happy and make sure the other speaker doesn't follow suit just yet.
Took another look at the board and tried removing the screws.
Didn't really reveal anything new ; if anything that made it worse. Ch1 ( all the way by the right screw) even got a bit microphonic when tapped (but no popping like the other channel). Seems worst at the high point of that bulge (kinda wish I could simply screw it down there).
Also noted on Ch2:
Sticking the chopstick in between the terminals on the Volume pot or bright switch induces a slight hum (doesn't happen on any other pots)
Swapped V1 and V2 and the tube in the v2 socket was still more microphonic than the tube in v1 socket.
Tapping the wire from input jack to pin 7 also produces the popping noise
The terminals on socket 2 are "wobbly", from the underside.
Not sure if this explains all the noises, but that socket is trouble. I'd swear there was a contact mic on that board - it comes from anywhere.
Both the input wire and the volume/bright switch are sensitieve areas. There is no reason to worry if they only cause noise when agitated. If they also pick up noise from vibration, you'd have to do something about it. Is the input wire shielded?
Re: the tube socket, you could try tightening the pin sockets... Sometimes replacing the socket is the best thing to do, but keep in mind the socket itself is not microphonic. It may however, introduce vibration to something on it that may be microphonic (like a tube or a ceramic Cap)
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Both the input wire and the volume/bright switch are sensitieve areas. There is no reason to worry if they only cause noise when agitated. If they also pick up noise from vibration, you'd have to do something about it. Is the input wire shielded?
Re: the tube socket, you could try tightening the pin sockets... Sometimes replacing the socket is the best thing to do, but keep in mind the socket itself is not microphonic. It may however, introduce vibration to something on it that may be microphonic (like a tube or a ceramic Cap)
Yes - its that shielded wire.
The induced hum when prodding the pot terminals was pretty low compared to the other racket, so what you say about being expected makes sense.
I pulled the tubes, and using a jewelers screwdriver, squished the clips inside each socket hole to get a tighter grip on the pins. Also re-deoxied.
That seems to have helped a bit as far as the popping, for the most part. One area (eyelet that connects to V2 pin 1?) can still do it , but at least its not sounding like the 4th of july already.
The terminals still wiggle within their holes, so I'm thinking of replacing the whole socket anyway (the clips in V3 socket wiggle too)
But the board is still microphonic as heck, so maybe we're talking mainly about caps. I can play that first .1uF all the way to the right on chan 1 like a bongo. It sits well above the board and sounds like its twanging away.
While I put together this next order, I'm also looking at the tubes:
So far I have a pretty good feeling about the Ruby power tubes. Swapped a couple around so the match within each side is better, then rebalanced so they are now all biased to within ~ 1 mv of each other.
But the pre-tubes are a bit of a mystery, at least as to what they are.
Here's what they should be / and what I have
V1 7025 / RCA 12X7
V2 7025 / ? (markings obliterated - looks very similar to V1 )
V3 12AT7 / RCA 12AT7
V4 7025 / RCA 7025
V5 12AX7 / RCA 12AX7
V6 12AT7 / ? ? (markings obliterated, does not look like any other tube)
I gather 7025 is an obsolete reference to a "better" 12AX7 (please feel free to correct).
Note that I have (1) 7025 in V4, but V1 (and probably V2) are 12AX7.
I have no idea what the V6 PI is - doesn't resemble any others. (maybe they were getting fancy and popped in a new one).
Any recommendations here?
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For all practical purposes: 7025 = 12ax7. If you have to replace with new tubes, go with 12ax7 (there are no new production 7025s).
A little pin wiggle in the tube socket is normal as long no tube is installed. The socket pin should not wiggle on the tube pin when installed.
The "microphonic board" sounds worrying. Are you sure you can't narrow it down to a specific component (ceramic cap*)? Sure it's actually microphonics and not intermittent connections/shorts to ground? Have you tried lifting the board as shooter has suggested?
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The worst offender that I noted a while back (and still is, by far), was the little .003 ceramic cap connected to V4 pin1
> if you gently rub it, it sounds like the chopstick is on your eardrum
(Edit:) turning down the reverb level eliminates this completely.
And with the reverb off
... that big blue .1uF on ch1 still makes a nice bongo sound, maybe more like a conga drum.. , the blue .047 uF next to it is the bongo
I don't have many stock of parts on hand. So I don't see any reason not to order the all the parts at once and methodically replace them all.
TUBES:
Also thinking of trying some different AT and AX tubes to see what differences there might be. (nothing crazy exotic, but for under $20, why not)
The RCA's I have could be original (tired?) and there's at least one (the unmarked PI) "who knows?" (wish I paid more attention before i cleaned them, not realizing the paint might come off). It strikes me that the PI needs to be in tiptop shape.
TubeDepot has a tube comparison chart for many different brands (brightness, gain, noise, etc)
Whats in your PI slot (or would be if you had a 72 SF Twin)?
Favorite 12 AX7's here?
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Personally, i always track down the problem first, before doing something as drastic as rebuilding a whole board. To me, that's the fun part--and you learn a lot from it. But I can see why you'd prefer to do it all at once...
Anywho, the microphonic ceramic cap has to go. It will produce noise under vibration, which you always have in a combo amp. Replacing with a new ceramic will do, so will any other type of cap.
Trying some new tubes is also a good idea. Any new production preamp tube will work, which one is up to personal preference. I have a weak spot for the sovtek 12ax7wps, but many would disagree. The pi takes a 12at7.
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I hear ya, but buying parts one at a time is too inefficient ( and you're talking to the king of inefficiency! :laugh:
Gonna be some good practice soldering in tight spaces.
Aside from that one bad ceramic, those 2 blue blobs have some microphonics going on too.
And 2 other ceramics (feeding pin1 of each channel) should be 250p according to schematic, but are measuring 320 and 350. unless Fender built it like that on purpose I'd think they should go too.
And lets not forget the Tremolo issue - it only works on a range of notes. I haven't heard any theories on what might cause that weirdness, so what else can I do except change out those caps for a couple of bucks?
BTW I take it silver mica are acceptable instead of the ceramics here (upgrade or snake oil?)
Uploading some shots of the tubes.
V3 is AT
V6 (far right) is supposed to be also, but doesn't look anything like V3 or any other tube. Of course it could be something new n' fancy, but who knows...
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V6 (far right) is supposed to be also, but doesn't look anything like V3 or any other tube. Of course it could be something new n' fancy, but who knows...
Chinese. Better than Russian, usually.
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Chinese. Better than Russian, usually.
Looks like it could be a Ruby (like the power tubes) - it has those little horizontal metal sections that distinguish it from other AT7's.
How do you mean better? build quality ? sonically?
Not having had the chance to compare yet, I think I'd might like something on the warm (yet crunchy) side, and quiet (for recording)
What's the general rep on Ruby - your basic Chinese tube?
I have a weak spot for the sovtek 12ax7wps, but many would disagree. .
I'm not finding that one (wps)- is it an older version? finding LPS, WA,B,C etc
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Looks like it could be a Ruby (like the power tubes) - it has those little horizontal metal sections that distinguish it from other AT7's.
How do you mean better? build quality ? sonically?
The transconductance and amplification factor were more consistent for the two triodes in Chinese tubes than for Russian tubes for the ones that I tested. I've never had either one fail in an amplifier, so I have no reason to think that one has a better build quality than the other. There is no way I could tell any kind of subtle sonic difference between the two having been diagnosed with "profound hearing loss" clear back in the '80s.
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Sorry. Typo. Should be lps...
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And lets not forget the Tremolo issue - it only works on a range of notes. I haven't heard any theories on what might cause that weirdness, so what else can I do except change out those caps for a couple of bucks?
I don't think there is anything wrong with the oscillator capacitors since you can see the bulb flashing and you can hear the classic Fender tremolo thump. It seems more likely to me that the opto-isolator is malfunctioning.
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And lets not forget the Tremolo issue - it only works on a range of notes. I haven't heard any theories on what might cause that weirdness, so what else can I do except change out those caps for a couple of bucks?
I don't think there is anything wrong with the oscillator capacitors since you can see the bulb flashing and you can hear the classic Fender tremolo thump. It seems more likely to me that the opto-isolator is malfunctioning.
Yeah been thinking about that - I haven't spent a lot of time studying how the vibrato circuit works, so i will have to review.
I was originally thinking that caps in general are frequency dependent, and in this case dirt cheap to replace.
Here's were things are still not clear for me yet (feel free to , you know..) :
But now that I think about it, it doesn't seem to me the 3 caps that modulate the rising and falling voltage that ultimately controls the photo- part of the roach, are in the signal path at all. So then it would not help...
OTOH the -resistor part of the roach is a variable resistor, which in general I would not think to be frequency dependent.
And that its job is to bleed off signal going into the PI:
Light on = low resistance = more signal going to ground (and not into PI, so reduced volume)
So is that considered "in the signal path" (and could be frequency dependent) ?
Maybe i just should've ordered a new roach too, but cest la vie, for now.
Some different colored caps and a couple of tubes are on their way....
Also, don't think I mentioned it, but this channel is highly hissy. No wonder- it's pretty complicated!
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I debated starting a new thread but this one has a lot of good 72-75 Twin/Vibrosonic information and I feel my comments might fit better here instead of a new thread. I recently repaired a 74 Vibrosonic (Twin w/ one 15") and this thread was useful!
The amp had lots of noise problems and under the hood it was obvious that it had been worked on a good bit. all caps had been changed, and someone spent some money on Jupiter caps and nice Sprague 22/25Vs, but workmanship was shoddy for a few repairs.
Especially under the dog house:
(https://i.imgur.com/7hAte3r.jpg)
- left most: spliced axial lead. I suspect tech was in hurry and put it in backwards and later fixed the mistake.
- resistor values changed and completely wrong. maybe misguided plans to revert to pre-BF?? dunno.
- and the decision to abandon Leo's grounding scheme for ???
Fixing these things plus re-flowing all the ground solder joints on this board and the primary board resolved most of the noise issues.
I did have another noise find & fix and I thought it might help others:
with the amp on I went along each lead wire from the primary board to the sockets with (insulated) needle nose pliers and gave each wire a jiggle, when I got to the wire that goes from the reverb driver grids (V3) to the eyelet of the 500pf/1M union. amp noise dramatically increased as I neared it with the pliers. this is a high impedance wire and it is unshielded. the reverb knob was turned down, so I think the noise was possibly going the other way: from wire back through 500pf to V4B. It seemed like it was acting as an antenna! I replaced the wire with some shielded RG174 (from hoffman's store), grounding at the axial lead of nearby cathode bypass cap, and overall level of noise was reduced (and not just with proximity of needle noses...) here's a picture:
(https://i.imgur.com/L0ackvo.jpg)
Last here's a nuance of the Vibrosonic. the Twin schematic of this era shows a 100R under the Long tail pair PI which serves as the NFB circuit along with the 820R. This one has 2 100's in parallel and both look original to the board. One of Robinette's pages mentions this ( https://robrobinette.com/AB763_Modifications.htm (https://robrobinette.com/AB763_Modifications.htm) ) as 100ohm for two speaker amps and a 47ohm for 1 speaker amps. It seems that applies to this Vibrosonic. here are the two paralleled 100 ohm resistors at the top of the board:
(https://i.imgur.com/UEebkqA.png)
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Nice!
Although I've not updated this thread in the past year, I have not forgotten it. The 72 Twin is doing very nicely, using the clean channel and some new speakers as a pedal platform. Resolution for 2021 is a return to normalcy and to complete the other channel rehab. And then I also have a old VibroChamp sitting there waiting for some attention and TLC.
Happy Holidays!