How to fix Repair Fender Twin Reverb Tube Guitar amp Bias issue Hot topic

D-Lab electronics on the path. Fixing hot flaming tubes. This one came to the shop mail order. Complaint, warp drive burning up 6L6 tubes. Well, thats a fun thing to troubleshoot. I took on the challenge. Watch the video for details. More work is required on this amp, however the out of control bias issue has been remedied. I love doing this type work. Solving problems, helping you guys out. Hope this info is good for you. If so, Please tell your friends, tune into D-lab for more cool tips. See ya, TD

[@veerchasm1]

“Three months ago” ...quit drinking wine, filming and send me my amp back! 😂😄

[@robertstrickland2121]

The tweeker reminded me of a guy we were stationed with at Sunnyvale AFS. He was what we called a “knob dicker”. My boss had a special tweeker made for for him, had a ball bearing in it so you could turn the handle, but the blade wouldn’t turn the pots. Wish I could find the guy that made it, would like to get a few for presents😁

[@jjiacobucci]

Wow! This man really knows his business! I am a recipient of his expertise and I am totally thrilled with the transformation of my Fender Deluxe Reverb. He fixed all of the noise and it sings like an angel! It is now as quiet as a church mouse! Thank you, Terry.

[@emerybryant2698]

i know im asking randomly but does someone know of a trick to get back into an Instagram account? I stupidly lost my password. I would love any help you can give me!

[@camdenmiguel225]

@Emery Bryant Instablaster :)

[@emerybryant2698]

@Camden Miguel Thanks for your reply. I found the site on google and im in the hacking process atm. I see it takes quite some time so I will get back to you later when my account password hopefully is recovered.

[@emerybryant2698]

@Camden Miguel it worked and I actually got access to my account again. I am so happy:D Thanks so much, you saved my account!

[@camdenmiguel225]

@Emery Bryant Glad I could help xD

[@JesusIsLordofMybeing]

You're the man Terry. So good at explaining all steps to a successful repair!! Thanks again

[@d-labelectronics]

Thank you my friend

[@rickcostin]

I like the way they say the green goo is non toxic but then advise wearing gloves when dealing with it ! Love the videos by the way...

[@electrolytics]

Another great video. Always informative and entertaining. Thanks D-Lab!

[@d-labelectronics]

Thank you my friend

[@charlesshaw9090]

Another fascinating video. That looks like the Fender AA270 Twin Reverb circuit. The bias pot is a balance not an adjustable bias. When you set the one side to the maximum negative bias you were actually also setting the other side to the minimum. It’s a silly design but very different.

[@kirkp_nextguitar]

Yes, he should have checked the bias circuit to determine which type he was working on. Other than that his method is excellent. I have the same amp. I added a trim pot in series with the balance pot to allow both bias voltage and balance to be adjusted. The new trim pot is only accessible by removing the chassis, which is very inconvenient. I’m considering either drilling a new hole to mount it next to the balance pot or rewiring the bias circuit to AB763.

[@rciancia]

Excellent Video Terry.... I always admire your approach to troubleshooting... straight and right to the point. Well done my friend. Ron

[@joemcgraw5529]

really enjoy all the very usefull info thats comes with all your projects ,and also comments from people that are good amp techs as well

[@mikedavis1429]

I watched a lot of your videos and i got to say your a very smart man when it comes to electronics i sure would like to be your student and learn what you know keep up the great work

[@barrychristiansen4579]

Really appreciating your simple straight forward aproch in discourse Terry. Know just enough to follow your lodgical thinking and No dought your experiance working on these ol beauties! Thx. again for show of great work! t

[@ptwamps3682]

Thanks for the video! As I'm sure you're aware (but others might not know), those same crappy IC caps are found in tons of other amps, including the entire Fender Hot Rod series, the 65/68 Fender DRRI, PRRI, etc, and all the Classic 30 offshoots, like the Peavey Delta Blues. In my shop, it's gotten so the customer doesn't even have to finish the sentence, "my amp has hum" before I tell them what is likely the problem.

[@oldscoolamps1049]

Very useful info beyond just biasing .Thanks Terry

[@mobicus1]

Just the info I needed! Very timely too, as I'm enjoying a glass of home made wine atm. Thanks for this!

[@chungaleta1234]

Looking forward to a further diagnosis on this amp. Cheers mate!

[@nine-graves]

What a great and funny teacher you are Terry. Gotta love the good ol' glass of wine references. Your couple chemistry is an inspiration too! Such a good channel you have, hope you reach the 100K subs soon. God Bless!

[@d-labelectronics]

Thank you my friend. Wife and I are striving to make the channel more popular. Probably get more looks if I were a 22 year old blonde doing this work!

[@Tonetwisters]

Amen, on that "God bless."

[@roccoarellanes8126]

Thank you so much for sharing your knowledge.

[@ballsdead1]

Thank you soo much for your effort and all the Info you give us with your videos !!! You helped me big time !

[@jarrydee2799]

Love watching these videos. Comforting for some reason

[@rhmayer1]

Isn't that weird? 99% is over my head, but I still love watching these and other repair and project videos. I've seen this comment many times, about how comforting it is, like watching them emits pleasure hormones or something. There's some interesting human psychology and biochemistry going on here, something instinctive I think. I'd love to hear from a psychologist about it.

[@mhs0034]

Great video. About to work on the exact board (Fender Silverface Twin Reverb). When last used many years ago, it all works, however I´ll be measuring plate voltages and current through 6L6s, measuring neg bias. I´ll need the amp more useful for studio work so will lower the output. Removing 2 of the output tubes, 2 outers or 2 inners, THEN unsolder one of the speakers which is set in parallel as 4 ohm impedance to single speaker 8 ohm impedance. Remeasure, check. Check tone. BTW, great gag with the wine!!! Best wishes.....

[@thome1547]

Hi Terry, good video, as usual, very well explained. When I bias my amps I do it with the Output Transformer Shunt method, just plug your Amp-Meter parallel to the Output Transformer windings. Thus you do not need to solder in an 1 Ohm resistor. Greetings from Germany

[@d-labelectronics]

Thanks, I will give it a try. Maybe make a video showing how the 2 methods compare. Glad yo have you on the D-lab team!

[@CJ-rf9jm]

Good as always n that intro music stuck me with bladerunner flashbacks.

[@BlueberryStinkFinger62]

Great video Terry.. always learn something New..

[@johnschoppmusic]

Thank you for showing us this...very interesting!

[@dhpbear2]

3:41 - A shorted output transformer will have NO effect on the output bias current. The tubes will simply deliver the same (admittedly high) current to a shorted load.

[@paulkielt9301]

Those germanium rectifier diodes look very sexy! Old capacitors in vintage amps need nothing but a good replacement.

[@mitovlz]

I learn so much of you, quite a person you are than you very much.

[@edwardmonsariste4050]

I really enjoy watching your videos about guitar amp repair. I do cringe watching you use those alligator clips in some of these older videos. More bravery juice! I just recently purchased the long hook clip type leads to keep my hand out and above the chassis. Bravery juice still needed!

[@martinreid1740]

Thanks for your great video, top class work. But I had to laugh when you pulled that fuse!

[@d-labelectronics]

I was thankful it was just the fuse. If that winding on the PT was open it would have been curtains for D-lab!

[@dhpbear2]

21:48 - 60 Hz hum is NOT a sign of failed power-supply filter caps. Those exhibit 120 Hz ripple, not the smoothsine wave you showed on the oscilattin' TV) It's probably bad shielding of the preamp stage

[@jonathanbelzley2877]

That’s a good looking twin!

[@BlueberryStinkFinger62]

very interesting for sure love watching this

[@skycarl]

Good vid Terry,,, Thanks bud.

[@mikesamra9126]

Do you change the diodes out to 1N4007s? I usually do because of the age and they are cheap.

[@Jennifer_Prentice]

Hi love to just watch your your videos , Its amazing . I was wondering I have a new guitar amp that has this auto bias system that is suppose to bias each sep power tube. You can change out a couple 6L6 and couple EL 34 or 4 of each what ever and the amp is suppose to bias them for you . I am wondering if this system of there is actually working properly. I do have a multi meter but I am not very good at tube stuff lol. The amp has this strange kinda thin tin canish kinda sound in the mid range when playing threw the high gain stages. I replaced all of the tubes with very high quality matched sets that went threw 24 hours of testing. The amp seems to still have that strange mid range tone that I just cant seem to get rid of unless I really scoop out the mids. Could you explain to me what happens when the tubes are UNDER bias. The tubes do not red plate so I do not think they are running hot on bias but wondering if maybe they are low. Can you tell me if this could be the issue and what is a good basic bias current for a quad matched set of 6CA7 (EL34) tubes. Thanks for any info you may give :)

[@calvintodd1274]

I'd like to ask, I see that many people say that internal pot adjustment on the Silver Front Fenders is only a balance adjustment not a bias adjustment... Is this Twin already modded to have that internal pot act as the bias? Are you "biasing" by just making sure at least one duo of the quad is at the right current and then assume that the other tubes are also in the safe range? I'm about to retube a 73 Twin I just got from Junior Brown and I'm a little scared so I have a lot of questions! Thanks for your videos, you explain things great and are very helpful. Cheers!

[@umajunkcollector]

The Twin is my fave Fender, I wish that I had a good one. They don't come cheap. My other fave is the VOX Super Beatle.

[@bradconklin2878]

Wine is good. Love your videos. Your wife seems cool.

[@moodyga40]

hot subject terry

[@rickcostin]

Now and then on a job you may come across twin and earth cable that is oozing a strange green goo-like substance, almost like washing up liquid. The wet green liquid that you are seeing is the product of a reaction between decomposing plasticiser (a plasticiser is an oily liquid used in the manufacture of PVC compounds) and the copper conductors of PVC cables. It’s better known as cable ‘greening’. The decomposition of the plasticiser is caused by overheating the PVC compound. It is usually a slow process, but can be accelerated when high temperatures are involved. High temperatures can be caused by external reasons, but overloading and loose connections are two common examples. Although all cable grades of PVC may look the same, the cheaper grades can be more prone to producing the green goo. The main instances of ‘greening’ cable arose during the 1960’s and 1970’s, although some instances are still seen today. Inferior grades of plasticiser were prone to greening even if the cables were not overheated. The plasticiser of better grades of cable, such as those produced by Prysmian contain an antioxidant which inhibits the decomposition rate of the plasticiser when heated. Even these plasticisers can be broken down eventually under adverse conditions of heat, but are much better protected than those which do not contain an antioxidant. Although fumes may be given off when PVC is decomposing (this statement is applied to house wiring) it would not be considered to be a health hazard, but there is a danger that the breakdown of a cable could cause an electrical fire. The plasticiser itself is a clear oily liquid that is non-conductive. The green substance is a combination of copper oxide and plasticiser which is conductive. Therefore, whenever this green substance is found at socket outlets etc. it should be removed and the terminations cleaned (gloves should be worn) otherwise it is possible that tracking/overheating may occur. It is normally considered that if greening is found that rewiring should strongly be considered. If, however, it is considered that rewiring is not required and the effect on decoration and equipment is acceptable then the following actions are advised: Periodic inspection and testing is carried out Visual inspection of cable ends, equipment and cleaning/replacement is carried out as required That the resistance of conductors is monitored to ascertain that conductors are not being over oxidised (in parts of the cable that cannot be seen) and their resistance thereby increasing The question of how frequent inspection and testing should be carried out is a good one, but no established guidance exists. Therefore, only by experience of the particular installation concerned can the frequency of periodic inspection and testing be determined.

[@zummliller]

EEEK!! Terry, my '68 Bassman has a bunch of similar looking blue .1uf caps. You suggest that ANY of that style should be removed or EXACTLY like the ones in the video? Mine are like a big capsule, white label printed: ''MOLDED....1mfd 400vdc...Made in U.S....OUTSIDE FOIL'' Iknow you've seen 'em! Thanks.

[@DOUBLEWIDEVEGAS]

Hi! I've got two classic fender amps that could use some work. Twin and a Quad.

[@corbinrandall6897]

Any way I could grab the link/type of Orange Sprague Drop? // just looking to buy the same ones and need to get pointed in the right direction.

[@user-so6wx8mr8f]

I have a 74 with jj6l6gc do you recommend 30ma for my bias

[@vonzigle]

I worked at Vandenberg AFB also-1966 to 1970...😀

[@d-labelectronics]

I was there 1984-92

[@lightshadows1997]

Hello. I have the same amp I think. It’s a 1972 twin Reverb silver face. I know the inside looks exactly the same. Do you take on projects still? If so, how can I get in contact with you?

[@stevelark9840]

Hi Terry. Concerning the 1 ohm shunt resistors, do you have a recommended wattage for these? .5 w, 1w?

[@d-labelectronics]

Hi Steve, I use a 1 watt for each tube

[@donstewart368]

I wish I could do this kind of work. My older brother could do it but he got most of the brains in the family. LOL

[@anthonymazzola767]

Great work and video. I have a question, Where did you get that big spool of braid next to your solder spools? I need that in my life.

[@d-labelectronics]

I purchased it at a Ham Radio swapmeet

[@anthonymazzola767]

Thanks so much for your reply! Love your channel!

[@rocanrola]

I have changed the same output coupling caps in my 73 Fender Deluxe Reverb...

[@roberthurless3295]

My dad was stationed at VAFB from 67 to 68 or so. I went first to Lompoc Jr. High, and then to Vandenberg Jr. High. I am wondering why you did not use axial caps? Was it just because you did not have any at the time? I have worked on those awful MV models and they can be a pain. I will bet the blue gunk leaked from the old caps. I wonder why someone used three good Atom caps and then two of the crummy IC caps. All in all another good video. Thank you Terry, keep 'em comin'

[@d-labelectronics]

Yep, just some radial stock caps I had in stock. Roger that on the crummy IC caps, I thought about mentioning that.

[@grindingmorre]

Hi! My name is Ignacio im from Argentina. i have a fender twin reverb 75/77 its benn working preety good all these years... but sudenly no explanation it doesn't turns on... it's been always at the same plase, conected to the same socket.... can you help me with same tip o checking way with a tester?

[@MrStubat]

The bias pot on these is just a balance pot. The bias is fixed

[@roryoconnor31]

Brilliant , great post thanks Terry

[@kenhancock8931]

First :-) exellent video terry!! ;-)

[@murfbass]

What range of resistor value would work loading the high voltage to dissipate it to ground? I can't find a 20W 40K but have a couple of 50W 10K.

[@lwilton]

The amount of resistance you pick determines the load on the power supply. It also determines how many watts your resistor will dissipate. You can either calculate V squared over R or I squared divided by R. Assuming 400V for B+ and 10K, you get 400 * 400 / 10000 = 16 watts. That would be OK with a 50W resistor, but it is going to get HOT. But also calculate the load on the power supply: I = E / R. 400v / 10000 = 40ma. Since this amp would draw at least 120ma at idle you are OK, but that might not be a good load for a preamp. In general, a 40K-50K power resistor makes a good safe dummy load for the B+ in most power amps, and you don't have to think much about whether the load will be safe for the supply or the resistor will get smokey hot. Much lower than that and you have to start thinking (or maybe worrying) about load and heat, and much over 100K and it can take a LONG time to discharge big filter caps.

[@RobertKohut]

Nice!!

[@gregorypinkowski7809]

You're about the only one who doesn't like the "dark blue drop" caps. Most techs (like myself) regard them more like an Orange drop with a different color epoxy. OK that's not a great description and there are different varieties of orange drop too..but they're usually the least-suspect when I dive into a Fender of that age. Not pretty, but IMHO they hold up pretty well; but I don't service anywhere near the quantity you do. But...techs with much more experience tell me they have very little trouble with those.

[@jacobocolmenaressuarez7156]

please Sr. can you help me...i have same amp (twin reverb 1971) my amp sound so low (have meters in 10), and de bright switch no working...please can you help me.

[@dwightjt]

I love it Terry your lovely wife always has a comeback for you When you make those silly comments You guys are a riot Haha live long together Love the videos Again sorry about your Mom inlaw. Hail mary Full of Grace. Sounds like your Mom inlaw had a lot of Grace Dwight.

[@dancraver3815]

Terry where do you find caps at how do you know if you can use electro or orange drops need a little clarification on caps

[@lwilton]

Look at Digikey or Mouser or Allied for current stock electronic parts, which is sure what I'd use for cap replacements. Avoid eBay like the plague for this, unless you *really* know what you are doing and can tell if what you are getting is useful or trash. Most old stock caps (especially electrolytics) are likely to be trash. As to poly or electrolytic caps, you should generally be replacing with the same kind that the original was. Electrolytics have large capacitance values (generally 1 uF or larger) and poly caps have small capacitance values, generally 1 uF or less, often much less. In general, filter caps (power supply filters, large cathode bypass caps, large screen bypass caps) will be electrolytics. In general, small coupling or tone filtering caps will be paper/mica/poly, all replaceable with poly caps.

[@PecanPie1102]

Do you know where i can get a schematics for the KLH 100 table top radio? Scratchy pots and maybe a recap.

[@d-labelectronics]

Hello Julia, sorry nothing here for that vintage radio, to new. For control cleaning, standard deoxit with lube will work.

[@eugeneeaton7262]

Terry what do you think about the brown Mallory caps?

[@d-labelectronics]

Hello, if they are grid caps, I say change them. If they are the dual electrolytics going to the pre-amp tube cathodes, also change them. Normally I simply change caps in those areas as preventative measures.

[@raoulduke806]

God bless PTP amps

[@glassblower1438]

You are super ❤great guy greetings from Germany my wife is French and we like Merlot

[@ferraridinoman]

Good man Young Tel! (For Gods sake get him his glass of wine!! phew! ha ha!)

[@danielnunez2660]

Thanks a lot! great video i was working on my twin reverb and it had this problem and i was able to solve it

[@dhpbear2]

18:27 - Remember to screw the cap back on when your done! Don't wanna mess up that new, fresh wine :)

[@d-labelectronics]

Only corks here my friend

[@Tonetwisters]

@@d-labelectronics And I think Marsha already took care of that problem!

[@AC2237]

Great job, you should have mentioned the dummy load at the beginning otherwise people mimicking your actions would have blown up their system...

[@wadereynoldsgm]

Can't wait for part goo, err, part two about the goo. BTW, I take it the candles symbolize that you quenched the red plating?

[@d-labelectronics]

Yes Sir, that was the intent of the candles, good catch

[@wadereynoldsgm]

D-lab Electronics Great video as always! Keep it up!

[@AALONSO1962]

Thanks In Advance...!!! ;)

[@MrStubat]

And the yellow alligator clip from the 480-500v ht is sitting on your workbench on the other end...500v right there.

[@user-tb9rw7dh2t]

👍

[@elwrongo]

a bottle drinking woman too, man this guy has everything

[@joelaughlin5532]

I was gonna suggest a double glass of wine!

[@GregWolodkin]

At the shop where I worked in college the fuse played a role in the repair estimate. 20A auto fuse in your amp? Might be bad news!

[@bernhardnizynski4403]

Are the Blue Caps Fender originals - are they US/European or Asian manufactured?

[@kwybus]

Fender originals

[@conjering]

shouldn't your max negative voltage be -36 for 6L6?

[@lwilton]

You can take the grid bias for almost any tube down to about -100V. The farther negative it is the less current the tube will draw. Eventually it will "cut off" or stop drawing current at all. Then it isn't an amplifier, it is just a filament sitting there drawing power. If you take tubes down below their design bias point you can start introducing distortion, because the tube will cut off during part of the input wave cycle. So you could end up clipping the top or bottom of the output waveform. Doing that also introduces a bunch of harmonics the output transformer might not like a whole lot, so it is best to keep the bias near the design center. But in general you won't hurt the tube by taking the bias too far negative. You sure can hurt the tube if it isn't negative enough (red plates, anyone?).

[@charlespatrick8650]

what was your AFSC? when did you go through motel hell at Lackland??

[@d-labelectronics]

I was a 31650G Missiles, Went thru basic in 1980

[@islanderwinder]

Good ol' box wine .

[@gregorypinkowski7809]

Did you mean to say "output transformer open circuit" because that's what you describe, not "shorted" as you said.

[@Banjoandguns]

Can you permanently leave the 1 ohm shunt on the ground?

[@d-labelectronics]

Yes, I plan to add them to all 4 tubes.

[@DeadKoby]

It's always the dang caps....

[@Bleats_Sinodai]

They used a 20mm fuse on a 30mm fuse holder. I feel like I should be surprised, but I'm not

[@EmbelekoBand]

Hi. I have the same amp. It's working fine. Nothing is wrong with it. But you gave me something to think about with the parts you changed. I assume mine has all the original parts. This amp belonged to one of my uncles and I believe he was the first owner. He is no longer with us so I can't ask. Can you please name the parts you changed and tell me where can I purchased them. I know you said the name of them on the video but you could've said it in Chinese and I still wouldn't understand. 😂😂😂 I just know how to soldier. So I'm a complete idiot when it comes these electrical parts on an amp. Unfortunately another uncle of mine who could've helped isn't with us either. Loss 3 of them in the pass 3 years. I would appreciate the help and any other advice or info you would like to share or guide me with. Thanks.

[@d-labelectronics]

I would highly recommend changing the 2 bias & grid coupling caps as a minimum to protect your amp

[@Tonetwisters]

See. Fender amps are tough! 14 amp fuse!!?

[@Tonetwisters]

"TweakadeeDooDah." The last word in funny screwdrivers!!

[@flintdavis2]

or "Tweakadeeday"?

[@jorgemellooliveira9611]

ótimo

[@johnsimms3957]

I wonder what that Invasion of the body snatchers goop was?

[@d-labelectronics]

I believe it was produced by the old caps as they failed. When replaced it was not cleaned up. The residue on solder joints is not a good thing.

[@dougcombs71]

It is plasticizer leaching out of the wire insulation.

[@dopeyjake]

Get some radial caps.

[@michaelhall9513]

I believe the GREEN stuff is acid flux from acid core solder it can be blue too.

[@brownmonkeybananayellow]

I have some green in an all original '64 Champ

[@TheGalaxyhopper]

hi

[@RobertKohut]

Those caps are "blew", "orange" you smart to replace them...LOL

[@cgavin1]

Ugly caps man. Maintaining the aesthetic is part of the charm of these old amps. :P

[@RobertKohut]

Well since you don't like "negative" comments I won't say a thing....LOL (negative bias repair)

[@rupertmaxwell7044]

the green goo is a reaction between cheap flux solder and the metal over a period of time. even happens with soldered water pipes

[@russellesimonetta3835]

That goo! Fender was using really junky wire that exuded thst goo all over the insides

[@luthiervandros]

Always find that green crap on silverface amps.

[@greggaieck4808]

D_lAB ELECTRONICS the owners Fender Twin REVERB Guitar amplifier is cool my hobbys are painting pictures and lisining to shortwave and ssb iam thinking about getting my ham license I have 4 shortwave receivers me and my cousin are going to a Swap meet June 5th 2022 Sunday morning at 8 am in Milwaukee my other hobbys are lining to music records and CDs and fmstreo on my Yamaha reciver

[@greggaieck4808]

D_lAB ELECTRONICS your utube videos are awesome I give you 👍👍👍👍👍👍👍👍👍😀😀😀😀😎👍👍

[@greggaieck4808]

D_lab Electronics when Tony play the Guitar it is cool I like hear the Guitar

[@greggaieck4808]

D_lab Electronics I like wine to and beer

[@flintdavis2]

D-Lab, If your right about this test, then this is just another case of manufactures deliberately shortening the life span of the product. smh