Fender Twin Reverb "Snap Crackle Pop"

admin · Post by **admin** » Fri Apr 25, 2003 5:05 am

Don: Over the past year or so my Fender Twin Reverb (1970) has developed a a "crackly" sound which eventually stops after about a 10 minute warmup. The noise ceases sooner if I play through the amplifier right away. The tubes are all relatively knew. I replaced a 12AT7 (rectifier?) tube most recently but this problem has been gradually coming on for a while. Do you have any suggestions?

toneman · Post by **toneman** » Fri Apr 25, 2003 7:50 am

Peter; Try replacing the 100K grid resistors in the preamp. those will develop noise, snap, crackle, pop after a number of years. they drop alot of voltage across them and work pretty hard. the old carbon comp one's get this way after 30+ years sometimes. Replace them with 100K/ 1 Watt metal films'. Carbon comps are excellent in the signal path but in voltage circuits can develope these problems with age and use.

ric480 · Post by **ric480** » Mon Apr 28, 2003 3:05 am

Peter: the 12AT7 you replaced is probably the phase inverter (right next to the 4 power tubes). The Twin Reverb has a solid state rectifier. This link has a wealth of information about vintage Fender tube amps.

http://www.ampwares.com/ffg/

admin · Post by **admin** » Tue Apr 29, 2003 3:55 am

Thanks John. This is indeed a great resource.

philco · Post by **philco** » Sat May 17, 2003 4:03 am

My Heathkit W-5M amps used to crackle on startup then quit after about a minute. The sound was not coming through the speakers, but from the tubes themselves! It did this with Russian 6L6 tubes or with new Sovtek KT66 tubes.

I decided to study the schematic, which you can get online at places like Triode Electronics. The schematic lists 5R4GY rectifier as the correct tube, where the previous owner had substituted a 5U4G rectifier. This let the amp B+ run much higher, especially at startup before the KT66 cathodes heated up and began emitting electrons. 5AR4 rectifiers solved that problem at a cost of $30 + S&H. Also, the voltage runs 123V around here most days, and when these old vintage amps were designed, the voltage ran closer to 110V because that is what is listed, with 115V being about tops you would expect. Consider 10 extra volts multiplied by the winding ratio of the HV transformer windings. At 455V, that gives a 4:1 ratio which means 10 more volts at input raises the voltage 40V at output. You also add the boosted B+ caused by the heftier rectifier. I was getting over 625V at startup, and the KT66 is rated at 550V max! The "crackling" was leakage voltage across the plates! The W-5M runs the output tubes very close to the design max to begin with. Many guitar amps run at max design voltage as well. They cannot tolerate the extra voltage given by 1) the higher household voltages of today, and 2) the substitution of tube rectifiers with solid state or higher current tube rectifiers in the interest of increassed current supply and less "sag" when playing loud. If you substitute solid state rectifiers for a tube, you add a power resistor after the rectifier to emulate the output resistance of the rectifier tube. OR, you can do what I did and buy new power transformers that have a lower B+ rating and enjoy the benefits of a heftier rectifier tube. That cost about $140 for two transformers (W-5M is a monoblock amp). Also, if the B+ is running high in your vintage amp, the heater voltage may be high by as much as 10%. Output tubes need their heater voltage to stay within plus or minus 5%, or drastic reduction in cathode life will result. That is 12.0V-13.2V, and do NOT ignore this or you WILL be paying for new output tubes a lot sooner than necessary. It is a good idea to change out power transformers in really old vintage amps, or power them through a variac so you can reduce the input down to the necessary 110V.

Also, the previous post about carbon comp resistors going noisy is very true as well. The more I mess with old vintage amps, the better I like the new amps. Transformers and resistors are just two of the issues you have to address. It is NOT cost effective to restore/upgrade an old amp unless you get the parts for next to nothing and count your time as lost to a hobby. I talk from a player's, not collector's, perspective. I work on electronic devices for a living, and run like hell from anybody needing work done on old consumer electronics.

toneman · Post by **toneman** » Sat May 17, 2003 6:45 am

Phillip; now are you talking about old guitar amps or other consumer electronics?
In guitar amps, I personally feel the older transformers and use of carbon comp resistors & good caps is part of the tonal superiority of older amps(like Vox, Marshall, Fender) especially those made from 1959-1970. Orignal Vox was gone by `68 & Fender started to suck from `67-`68 and Marshall changed most of their line and parts around late `69 but kept the turrent board constuction until late `73. These days what I find is usually amps made from parts supplied by lowest bidder. Transformers have laminations that are the exact same material as Campbell's Soup cans. Carbon film and metal film resistors do not offer the same harmonic content & warmth as carbon comp's do. Now in voltage paths I prefer the metal or carbon films for their low noise factor and stability. Coupling & tone caps I'd rather have polyester, mylar and silver mica. I never heard a polypropelyne cap I liked as they sound harsh and brittle to my ears. All those little green and brick coloured caps from the far east sound like ****.
Anyway, I'd say 75% of the guys I do work for prefer old amps over any standard production new one's. We're not talking about guys like Dr.Z, Top Hat, Mojave or Roccoforte, Blockhead, etc. though as those are a small production, handmade and use good parts like Mercury Magnetics' Axiom tranny's and CC resistors, etc.

philco · Post by **philco** » Sat May 17, 2003 10:27 am

I'm talking about "low price" (usually, but not necessarily) electronics built down to a price level rather than up to a quality level. They were made to sell to Joe Average Consumer that can't tell a jewel from a turd, and if he could, would prefer the turd if it was cheap enough. He wants the repair to be just as cheap, and if it's so cheap as to cost ME money in the process, so much the better for him.

Even some of the cheap stuff can sound OK if a real hotshot engineer that knows his stuff can make the most of the cheap parts he has to work with. The Marshall AVT20 and AVT50 are two amps that are far from stellar in parts quality, but manage to sound good, at least for a hybrid design. I just wouldn't want to make a living fixing that stuff!!! The AVT50 sells for the same price as a YCV40, I own both, and the difference in the quality of materials is quite noticeable. Glass epoxy is better than resin impregnated fiberboard, and baltic birch plywood is better than particle board. The speakers are both Celestion, but the Seventy 80 in the Traynor is a grade better than the Bottom Range Celestion in the AVT series. Needless to say, the YCV40 wins the tone war, but the AVT50 is not junk in that regard. It sounds really good for a hybrid design. The YCV40 may become a classic. The AVT50 will NOT, except maybe due to number of sales, much like a Model T Ford. The quality of materials is not there for long term durability. It's sort of like a television, designed to be used up and discarded.

You mention the BETTER guitar amps from the old days, but Joe Average Consumer did not buy them. He bought something like a Silvertone, at best, if he was in a spendy mood. The boutique guitar amps of today are like audiophile gear. They will always be worth something. If I ran across an old Conrad-Johnson Premier 1 amp that needed a little work, I wouldn't junk it. Just a transformer from it would be worth more than most other amps. If someone had an old Adcom amp that needed work, I would have no interest whatsoever. I already have an old B&K that would blow it away in the sound department. B&K amps sound wonderful, and rarely break down anyway. Almost every one you see is still running. The Old Guitar Amps that your customers prefer are the High End of their day, and far from the average amp that went out a music store's door in the old days. An AC-30 is not to be compared with a Pathfinder. They both say Vox on the front.

I actually own a Kustom Tube 12, it is consumer grade junk if it breaks down, and would not be worth your repair bill. It was actually bought as a cheap tube tester, because it only uses one tube, and leaves no doubt as to what that tube sounds like. I value the Celestion Vintage 8 speaker I have in it as much or more than the rest of the amp. It plays Rock Guitar MUCH better than a Fender Champ, largely because of the Vintage 8 speaker and more power to control it. People who play quietly might disagree.

I am NOT against buying cheap electronics, I am just against repairing them as an economical undertaking, AND TRYING TO MAKE ME PART OF THE UNDERTAKING. They are good for kids (even old kids) to screw around with and learn a few things without wrecking something valuable.

toneman · Post by **toneman** » Tue May 20, 2003 6:09 am

Phillip; All great points! I guess I'm luckier than most as nearly everything I see here is the better than average stuff. I'd say that around 85% of the stuff I see here is in the $1K & up price range. I get the occasional Hot Rod Deluxe or Deville that are a pain to take apart with a zillion ty-wraps and screws. But even good resistors, caps and a Mercury hand made output tranny will make a huge sonic improvement in those.
when I was a kid we would count ourselves lucky if we could buy a Silvertone Twin 12 and really lucky if we could get a Fender Deluxe and if you could get a Vox or bigger Fender then your parents were really cool to spend that amount of money on you. Even the sonically awful Vox/Thomas Organ "Super Beatle" was $1,000. new. That was way more than most parents made in a month in the sixties..

toneman · Post by **toneman** » Tue May 20, 2003 6:11 am

BTW, my first amp was a Kalamzoo Model II made by Gibson. Point to point wired single ended amp with 1-EL-84 & 2-12AX7's and a EZ-80 rectifier and a 10" alnico speaker. I think it was $45.00 new..I moved up to a brown Princeton a year later...

philco · Post by **philco** » Tue May 20, 2003 1:03 pm

I'm wondering........is the new Epiphone Galaxie 10 based on the old Kalamazoo amp? It uses an EL84 and 12AX7. Probably not hand wired. It also uses a 10" speaker that I heard was a Celestion. The Harmony Central reviews are very positive. The cost is incredibly low for the reviews it got. I am expecting my 650C with vintage pickups in Fireglo to arrive in a month or so. I am sure it will sound great with the YCV40, but I really want a little 10 watt single ended tube amp to go with it for home practice and easy portability to outside jam sessions. The YCV40 is really a club amp, and too loud for me to crank up in the house to where it sounds its best. I can get a Galaxie 10 on sale in dark blue for $200. The 650C and Galaxie 10 would color co-ordinate: Red, Blonde, and Blue. I would use it in a fairly overdriven mode most of the time. I have a Korg AX-1G pedal that would probably go well with it, and provide some reverb that it lacks. The Galaxie 25 has a tube reverb, but it is push-pull with a 12" speaker which makes it very similar to my YCV40. It's out of the question, and I suspect the YCV40 is the better amp.

Yeah, most people forget how hard it was for the average working stiff to buy good music equipment in the 60's. Willie Nelson must still think it's hard, judging by his old guitar. Did anybody ever think of taking up a collection and buying him a Rickenbacker acoustic? I might be willing to donate $20 or so, if somebody wants to start a SERIOUS collection. Think about it.......it would probably be on display some day in a museum and you could have your name on a plaque as one of the contributors to give Willie a guitar without holes worn in it.

admin · Post by **admin** » Thu Jun 05, 2003 5:41 am

I thought I would let you know what my Amplifier Tech had to say about the "snap, crackle, pop" in my Fender Twin Reverb. His comments were that he

Repaired a number of bad connections in the wiring. Replaced noisy 100K resistors in the preamp. Also found and replaced one bad tube(6L6). Adjusted the bias which was out by a considerable degree. Tightened and cleaned output sockets.

The amplifier is a good as new with not a snap, crackle or pop at this point. He added that the amplifier has served its owners well seeing as it is about 30 years old.

Thanks for all the comments and suggestions here which motivated me to send it along to a technician after my attempts at cleaning did not solve the problem. In the end, the problem was multi-factorial involving a tube, resistors and bad connections. The enjoyment of listening to the golden tone of a Fender Reverb, priceless. For all other things there is Mastercard $160Cdn or $119US.

ric480 · Post by **ric480** » Fri Jun 06, 2003 2:59 am

Great news, Peter. That amp will probably last you another 30 years. I intend on using my '74 Super Reverb that long as well. Enjoy !

admin · Post by **admin** » Fri Jun 06, 2003 5:57 am

Thanks John. It really is a very good amplifier and I love the sound of a Rickenbacker 12 string through it. I must say, however, that moving the Fender Twin Reverb or Vox AC-30 for that matter is not as easy as it once was. Must be that amp cover that I added.

toneman · Post by **toneman** » Sat Jun 07, 2003 7:38 am

Ah.. the old 100K screen grid resistors! I knew it!glad it's working again Peter!!