Amplifier Modifications A. OK/Recommended amp modifications Read the SAFETY WARNING first, before you put your hands - or other personal parts - into a tube amp. Put a fuse in the B+ line after the rectifier(s) and before the first capacitor filter. This can save burning out your power transformer and maybe your output transformer if you get a shorted filter cap, shorted output tube, or lose bias on an output tube. It -* might *- save an output tube that has lost bias even though it also might not. The fuse current rating should be slightly larger than the max current rating for your output tubes, generally much less than 1A. Put a 130 or 150 Volt MOV surge protector across the AC line at the power transformer primary to absorb spikes from air conditioners and motors turning on and punching through the primary insulation. Recent articles say that 130V MOV's will eventually short, recommending only 150V MOV's. Get rid of all two wire line cords and line switching arrangements. Refit with three wire cords, tieing the safety ground to the chassis. You'll love this the next time you touch a mike or stand while holding a guitar. No shocks. Oh, yeah. Do it to ALL your equipment to be safe. Consider putting a small fan in your amp to cool it. Try a 240vac fan running from the 120 vac line supply, which will run much slower and quieter than a 120vac one. Install small cathode resistors and independent bias adjustment for each output tube to make biasing easy. Open the feedback from the power amp output to it's input for more power amp gain, more and earlier distortion. Or better yet, put in a spst switch and you can pick the characteristics on the fly... For the adventurous, add a separate filament transformer/rectifier/ filter capacitor to make 9-12VDC at several amps and then use a three terminal rectifier to regulate this down to 6.3VDC, and feed this to your preamp tube filaments. Hum from filaments will drop right through the floor. Lotsa work, though. Put 1500Volt, 1A silicon diodes in series with the two sections of your rectifier tube (if you have a rectifier tube) so that if the rectifier tube shorts, the silicon will save the output tubes, and power and output transformer. Gerald Weber advocates using a 270K/27K resistor divider from B+ to raise the filament windings in a DC sense above ground. This keeps electrons from the filament from hitting the plate, another source of hum. Put 1500Volt, 1A silicon diodes in series with the two sections of your rectifier tube (if you have a rectifier tube) so that if the rectifier tube shorts, the silicon will save the output tubes, and power and output transformer. The B+ will go up about 50V when (if!) the rectifier tube shorts, so the amp will have a little more power and run hotter. This can still hurt modern manufactured power tubes if it goes on too long, so check the rectifier tube frequently. Bill Webb's favorite tone mods for Fender amps at the Vibrato channel's second gain stage, change the ceramic 0.02uF coupling cap to polypropylene or polystyrene replace the coupling cap at the input of the phase inverter with a better cap (polypropylene - polystyrene - mylar in order of preference); change its value to 0.001 to make the amp "sparklier" and to 0.01 to make the amp sound bigger and more midrangy The 3.3M resistor which mixes the dry and reverb at the output of the 3rd gain stage, vibrato preamp, is paralleled by a 10pF ceramic cap. Change this to silver-mica to make the amp sparklier The power amp feedback loop resistor is usually 820 ohms; insert another 820 ohm resistor. This reduces the feedback, increases the power amp's gain, and softens the onset of distortion. Remove the single bias adjust pot in your amp and put in two, connecting one to each output tube. You can now set the bias voltage on each tube to be different, which can match the DC currents for un-matched tubes, or un-match matched ones for more even harmonic distortion. Tinker the driver circuit to let you adjust the relative amount of AC drive to each output tube. This lets you match/unmatch output tubes in an AC sense just like the bias mod lets you change the relative DC points. C. NOT Recommended amp modifications These are likely to be just plain bad, either grossly (it dies soon) or subtly (it dies slowly, eats tubes, or other sicknesses). Don't do these or let a tech do them to... er... for you. Using a variac to run it at a higher or lower line voltage. This might be OK except that running it higher can overdissipate parts and burn them up or overvoltage things like filter caps, which can short and burn out your -* expensive *- output transformers, as well as burning out your tube filaments by putting too much current through them; and running it lower starves the filaments for current, so they can't put out enough electrons, and any remaining gas in the tube bombards the cathodes, poisoning the electron emitting materials on the cathode surface, and wearing the tubes out early. Adding massive amounts of capacitance to the power supply filters to reduce hum. Probably OK with solid state rectifiers, but in amps with tube rectifiers this can cause current spikes in the rectifiers that exceed the instantaneous current rating of the rectifier and wear it out quickly. Nathan points out "I seem to recall one of my Tube Amp Mentors telling me that this is pretty much only the case with the first filter cap after the rectifier, and that the impedance of the power supply was high enough that you could dump hundreds of uf worth of filtering on latter stages (though the only place it's of much benefit is at the power tube plate supply point.) replacing your rectifier tube with a solid-state plug in module replacement. This effectively just puts in a pair of silicon diodes which take the place of the tube. But it also lets the B+ come up about 50V. This won't kill the amp immediately, but it runs the outputs hotter. Fender often put more than the rated maximum voltage on the output tubes to get moe power out of them; old US and Euro manufactured tubes would usually handle it just fine. Some lower cost modern manufacture tubes CAN'T stand the extra volts as a steady diet, and can succumb to the Dark Side of the Force - soon. -------------------------------------------------------------------------------- Tube Characteristics and substitutions Some quick and dirty subs and some tube data such as recommended bias current and appx voltages. These subs are all taken from the Tube Substitution Handbook sold by Antique Electronics Supply. or provided from the net. A (short) catalog of tubes you are likely to see in a guitar amp: 12AX7[A, WA] and substitutes - preamp and driver tubes 12AT7, 12AU7 and subs, preamp and driver tubes 12AY7 - driver tubes 6EU7 - dual triode used in some older amps for preamp tube 6L6 types - power output tubes, up to 50 watts/pair, a mainstay of Fender EL34 - Euro power pentodes, up to 50 watts/pair, many Marshalls 6V6 - smaller, lower power cousin of the 6L6, 10-14 watts per pair; used in smaller Fenders EL84 - fits a 9 pin socket like a 12AX7 but twice as tall; miniature power pentode, good for 12-18 watts per pair; used in smaller Vox amps, and a quad of these drives the Vox AC-30 for 30 watts. Substitutions: * means appropriate for parallel filament circuits # means may not work in all circuits Preamp and driver tube substitutions: 12AX7 (high gain dual triodes with pinout 9A) 12AD7* 12DT7 7729 12AU7# 5751* B339 12AU7A# 5751WA* B759 12AX7 6057 CV4004 12AX7A 6681 E83CC 12AX7WA 6L13 ECC803 12BZ7* 7025 ECC83 12DF7 7025A M8137 12DM7* 7494 12AU7 (moderately high gain dual triodes with pinout 9A) 12AU7[A,AW,] 6189 7730 12AX7* and subs 6670 ECC186 5814[A,AW]* 6680 ECC802 5963 7316 ECC82 6067 7489 M8136 12AT7 (medium gain dual triodes with pinout 9A) 12AT7[many suffixes] 7492 E81CC 6201 7728 ECC801 6679 A2900 M8162 ECC81 B152 QA2406 12AZ7[A]* B309 QB309 6060 B739 6671 CV4024 12AY7 (low gain dual triodes with pinout 9A) 12AY7(and suffixes) 6072 2082 Power tube substitutions: 6BQ5/EL84 (miniature pentode with pinout 9CV) 6267 7189 EF86 6BQ5 7189A EL84 6BQ5WA 7320 N709 6P15 E84L Z729 6L6 (beam power tube with pinout 7AC) 6L6(many suffixes) 7581(A) 5881 WT6 5932 EL37 EL34/6CA7 (power pentode with pinout 8ET) EL34 12E13 6CA7 KT77 7D11 KT88 6550 (power pentode with pinout 7S) 6550[A] 7027A# 7D11 KT88 12E13 Cautionary Tubes - these are very hard to find 7591/7591A - legend has it that these otherwise excellent tubes were all bought up by an oriental buyer who toured the USA paying cash for all of them he could find, then disappearing. You are likely to only find used ones or the odd pair in some out of the way place. Dealers will in general not have them. I have personally seen trays full of NOS 7591A's for sale in the Akihabra electronics district in Tokyo, lending some credence to the rumor. These were used a lot in old Ampegs. They are very small and high gain for their physical size, so there may not be a lot of room in a chassis for a larger replacement. The 5881 will work in some circuits, but has significantly lower transconductance. Rumor Update: The rumor mill on the net says that the Russians will soon be making 7591's soon. Cross your fingers... 7199 - combination pentode/triode used as a one-tube voltage amp/phase inverter/driver for a pair of output tubes in some Ampeg amplifiers Note: These were once popular, but are now getting rarer and more expensive. There are a number of other pentode/triodes that can be substituted, but the pinouts are different and this will require require rewiring the socket for the tube. Examples are the 6AN8 and the 6U8. There is a Russian tube that is labeled 7199 which may work, although this is new. 7027/7027A - this is a high power tube similar to a 6550. The supply of these is very poor. 7189/7189A - a higher power/voltage version of the 6BQ5/EL84. Hard to find. A stock 6BQ5/EL84 may work if the power and voltage conditions in the amp are not right out at the limits of the tube design. -------------------------------------------------------------------------------- Maintenance Issues Cap Jobs - Do I need one? How often? Why? What's a cap job? A technician may recommend you have a "cap job". This means that he will replace every single electrolytic capactor in the amp, from the power supply right down to the cathode bypass caps. This is because electrolytic (polarized) capacitors have an inherent wear-out mechanism and will eventually die even if you don't play death/metal/country/barbershop through them every day - in fact they may wear out sooner if you leave it sitting in the attic. Here's why. A capacitor is essentially two conductive plates separated by an insulator. The bigger the plate area and the thinner the insulator, the higher the "capacitance" is. Electrolytic capacitors get a very thin insulator by "growing" an insulating layer of aluminum oxide on the outside of a rolled up piece of aluminum foil. The oxide layer is "formed" at manufacture by feeding the aluminum foil a very small and carefully controlled amount of current. The current causes a chemical reaction between the foil and the water solution (electrolyte! ... hey... is that where they got the name?? yep.) which makes an oxide layer grow. As the layer grows, they use higher and higher voltages to force the same small current through the layer, which gets thicker and more resistive with time. When they have to use the full rated voltage to get the forming current through, the cap is fully "formed" and ready to ship. If the capacitor is used regularly, has voltage applied to it, and does not get too hot, the oxide film lasts up to a few decades. If the capacitor is not used much, or gets too hot, the oxide film slowly un-forms, the leakage current goes up, and it will eventually short. Electrolytic caps are designed to last ten years. It is a tribute to the quality of manufacture that they often last three, sometimes four times that. Old amps, particularly if they have not been used regularly need to have every electrolytic cap replaced. This cap job may be needed every ten or so years. Non-electrolytic caps do not have this wear out mechanism, and do not need replaced for this reason. Modern capacitors can in some circumstances be much better than old ones, and you can sometimes get a clearer, more sparkly tone by changing the non-electrolytic caps - assuming that is something you want to do. Do new caps need to be formed? There's a lot of controvery on "reforming" replacement caps. Here are a few answers. Manufacturers of caps design their caps for a ten year working life, and a five year shelf life. That means that the stresses and heat of working in equipment will leave the vast majority of caps functioning OK after ten years of normal operation. After that, it's gravy to the buyer. They also design them to work OK after sitting on a shelf unused for five years, meaning that the cap should not fail if it's put into operation at rated voltage after sitting unused for five years. As noted above, the caps do slowly un-form without regular use. If the electrolytic caps you use to fix your amp are over five years old as determined by the date code on them, you ought to at least worry about forming them, and if they're over ten years old (like NOS multisection cans), definitely re-form them. Other than that, put them in and turn it on. How do I "re-form" electrolytic caps? You'll hear folks talk about "bringing an amp up slowly on a variac"; this can work but is not particularly good for your tubes. A better way is this: Pull out all the tubes. if your amp has a tube rectifier, solder in temporarily some high voltage silicon diodes across the tube lugs to be a rectifier that does not depend on the filament voltages. If your amp has silicon diodes, you can skip this. open up the wire that goes from the rectifier tube (or solid state diodes) to the first power supply filter stage and solder in series with the wire a temporary 100K 2- 5W resistor. This resistor will limit the current that can flow into the caps and the amount of voltage that is applied to them to safe values that will cause the insulating layer to re-form. clip your voltmeter across the resistor button it up. Turn it on (no tubes in it, remember). Watch the voltmeter. when the voltmeter reading drops to less than 20-30VDC, your caps are formed. open it back up and pull out those diodes and resistor, putting it back in original shape. The forming could take hours to days. Sockets Sockets get dirty, corroded, broken, and "arced" To recondition them, get a can of spray contact cleaner, the kind that says "no residue". Squirt some in each socket hole, then insert that tube in the socket, wiggle it around, and remove it several times to get the crud off. Take a thin tool like a jeweler's screwdriver or ice pick and gently bend the contacts inside each hole so they hold the pins better. If the socket is cracked, or has blackened lines from pin to pin (where an electrical arc has actually burned the socket into a carbon material that conducts electricity), replace the socket. Q: Are plastic or ceramic sockets better, or is there any difference? A: The material is significant. Thermoset plastics are what are usually used for sockets. The black-brown stuff most are made from is "bakelite" a trade name for a kind of clay-reinforced phenolic. Maybe there's a variety that is purer or more sturdy phenolic that is more resistant - I'm fuzzy on that one. All plastic sockets are vulnerable to arcing. When you get enough voltage from pin to pin on an output tube to cause a spark to jump from pin to pin (like when you run the output transformer unloaded) the spark runs along the surface of the socket material and burns a trail on the surface. Since the plastics contain carbon, there is often a carbon residue left on the surface. This residue is partially conductive, and makes that path susceptible to arcing over at lower voltages next time; this can be so bad that it interferes with normal operation. Ceramics are not carbon based, do not burn in the normal sense, and don't soften or melt at temperatures achieved in an arc over, so they are essentially immune to arcing unless covered with dirt and gook that can burn and leave stuff on the surface. I would class them as poorest - thermoplastic sockets; medium (and most common) - thermoset, which included phenolic; best - ceramic. Ceramics are the premium solution, IMHO. Dirt and Dust The dusty, hairy, oily layer that collects on the chassis can conduct electricity as it absorbs humidity from the air. Vacuum it away periodically. Blue Glow in tubes The blue glow in power tubes is a flourescence from the few ionized molecules of gas that still exist in the non-perfect vacuum achievable in tubes, driven to fluorescence by the high voltages in the tubes. Unless it is excessive, it is not harmful. Tubes with softer vacuums glow more. Other Issues Lots of good info is contained in Jack Darr's "Electric Guitar Repair Book", if you can find a copy (it's now out of print) and in Pittman's "The Tube Amp Book" and Webers "Desktop Reference...". Look for: - checking for capacitor leakage From watching a tech work on Fenders, I picked up a nice tidbit. The eyelet boards in Fenders have most components mounted across the eyelet board. A very few parts run along the length of the eyelet board. Because the eyelet board flexes, there is a lot of stress on the solder joints at the end of these lengthwise components and the joints often crack. Every time you open up a Fender, take a look and maybe a soldering iron to these joints. If it's your personal amp, you might want to get a new part for these positions with long leads and bend a loop in the leads so that the leads can flex and not put stress on the solder joints. -------------------------------------------------------------------------------- Appendix A. Tube Stuff Suppliers The following article appeared in rec.radio.swap. Some time ago, and is now badly out of date. My apologies, I'm working on an update. -------------------------------------------------------------------------------- Gang: The enclosed is a bit long but it's the most complete list of tube sellers I've seen presented on the Internet. You'll want to send this list to your printer! I didn't edit it down like I usually do since that would have made it quit difficult to read. - Jeff NH6IL Article: 18193 of rec.radio.swap From: brian.carling@acenet.com (Brian Carling) List of suppliers and sources for finding vacuum tubes: Adkins:Charles P. N8QXP (313) 382-0272 1821 La Blanc. Lincoln Park,MI 48146 Tubes Allied Electronics: 800-433-5700 Located in 36 states, 83 branches. 7410 Pebble Drive Call for nearest branch. Call for Catalog. Fort Worth, Tx. 76118: Electronic components and tubes. Min.Prepaid order $25. Min. Credit Card order $50 as well as COD is $50. Min. Antique Audio 512-467-0304 5555 N. Lamar, Bldg. H-105 Austin, TX 78751 Tubes, parts, books, kits Antique Electronic Supply Co. 602-820-5411 6221 S. Maple Avenue Tempe, AZ 85283 (Tubes & other components) Antique Radio Classified 508-371-0512 Write for free sample. P.O. Box 802 Magazine. You'll find almost ANYTHING here Carlisle, MA 01741 for older radios, obscure parts, tubes etc. Arlen Supply Company (610) 352-9311 / -9388 = FAX 7409 W. Chester Pike Upper Darby, PA 19082 Tubes. 1 million stocked. Minimum $150.00! A.R.S. 602-820-5411 Arizona Need address Tubes David Ask No phone number given RR 2 Houston, MN 55943 Tubes 4, 5 and 6 pin tubes. Send $1 for list Mel Brooks, K5DJB No phone number given 932 Macklyn Lane Bartlesville, OK 74006 Tubes, parts, schematics (incl. antique) Bauman:Jeff, WB5KZW. 313-435-9922: 313-661-0202 Jeff had 1500 Tubes 6647 Stonebridge East, for sale as of December,1993 West Bloomfield, MI 48322 Cable:John, 619-258-7931 Address needed. Tubes from 1941 to 1960, new. CeCo Communications. 800-221-0860: 212-646-6300 2115 Avenue X Vacuum tubes Brooklyn, NY 11235 C & N Electronics 800-421-9397: 612-429-9397 6104 Egg Lake Road FAX 612-429-0292 Hugo, MN 55038 Buy & sell tubes Daily Electronics 800-346-6667: 206-896-8856: FAX 206-896-5476 10914 N.E. 39th Street Vancouver, WA 98682 Tubes, all types. Davilyn Corp. 800-235-6222 xct.CA: 818-787-3334 CA: FAX 818-787-4732 13406 Saticoy St. Electronic Tubes. Good Prices, Call for Catalog North Hollywood, CA 91605-3475 Also Surplus Electronic Gear. DH Distributors 316-684-0050 P.O. Box 48623 Wichita, KS 67201 Tubes, radio, TV, industrial Electron Tube Enterprises 802-879-0611 Box 311 Essex, VT 05451 Tubes Electronic Bits 'N Pieces 303-361-6530 P.O. Box 31654 Aurora, CO 80041 Tubes, transistors, diodes, chips William Erickson, W4UIL No phone number given 3905 Cherrywood Lane Annandale, VA 22003-1901 Tubes, older radios etc. E.S.R.C. (407) 735-3397 P.O. Box 1192 Delray Beach, FL 33447-1192 Buy, sell, swap tubes of all kinds Fair Radio Sales. 419-227-6573:419-223-2196: FAX 419-227-1313 1016 E. Eureka Box 1105 Parts, transformers, power supplies, Lima, OH 45802 Surplus and gov't surplus radios etc. Fala Electronics (No number listed) send S.A.S.E. P.O. Box 1376-1 Milwaukee, WI 53201 Vacuum tubes Melvin Heineken, K5MNJ No phone number given 2204 Spruce Needle Rd. N.E. Rio Rancho, NM 87124-6308 Tubes. New unboxed. Other parts. Henry Radio Co. (310) 820-1234 2050 S. Bundy Drive Los Angeles, CA 90025 New tubes International Components Corp.800-325-0101: FAX 503-336-4400 1803 NW Lincoln Way Cabinets, components & vacuum tubes Toledo, OR 97391 Jolida Tube Factory 800-783-2555 10820 Guilford Road Vacuum tubes Annapolis Junction, MD 20701 KB5QOH (No name given) No phone number given 667 Nine Mile Hill Road Fairbanks, AK 99712 Tubes, parts, used amateur gear Kirby No number listed 298 W. Carmel Drive Tubes, new up to 90% off Carmel, IN 46032 Cliff Kurtz, N6ZU No phone number given 6727 N. Pershing Avenue Stockton, CA 95207-2522 Tubes. Minimum order $10.00 Robert Lang AA2EO (212) 877-0980 120 W. 70th Street Apt. 7-A New York, NY 10023 Tubes, vacuum variables, xfmrs etc. Madison Electronics (800) 231-3057 12310 Zavalla Houston, TX 77085 Tubes, meters etc. Rex Mason (704) 392-0359 100 Honeywood Avenue Charlotte, NC 28216 Tubes, antique parts, amateur, TV, VCR New Sensor Corp. 800-633-5477: 212-529-0466: FAX 212-529-0486 133 Fifth Avenue. Vacuum tubes galore! Call for list.Min.Order $50. New York, NY 10003 Std. test= $0.75/Tube. Premium Match $2/Tube No name (SHY?!) No phone number given 5150 Merritt Road Black Hawk, SD 57718 Tubes. S.A.S.E. for list P.E.M. Tubes (916) 383-9107 7392 French Road Tubes, radio, TV, transmitting, CRT Sacramento, CA 95828 Pride Tubes 800-638-3925: 205-650-5522: FAX 205-880-8077 8200 South Memorial Parkway (800) 456-5642 100% RF Tested Tubes Huntsville, AL. 35802 Rauchwerger, Lawrence 217-352-6195 1610 1/2, W. Union St Champaign, IL 61821 Tubes. S.A.S.E. list R.F Parts. To Order 800-737-2787:619-744-0700 or 0750 for Tech info 1320 Grand Avenue FAX 619-744-1943 San Marcos, CA 92069 Diamond Antennas, RF Power Transistors & Tubes. Richardson Electronics (708) 208-2200 / (800) 235-2143 40 W. 267 Keflinger Road La Fox, IL 60147 Tubes, RF parts Steinmetz Electronics 219-931-9316 7519 Maplewood Avenue Tubes Hammond, IN 46324 Svetlana Electron Devices Co. (415) 233-0429 / - 0439 = FAX 3000 Alpine Road Portola Valley, CA 94028 Tubes, RF power etc. Turner Electronics No number listed 16701 Main Street Suite 121 Hesperia, CA 92345 Tubes, capacitors, S.A.S.E. list Unity Electronics No number listed P.O. Box 213 Vacuum tubes Elizabeth, NJ 07206 C. Verderber No phone number given 2266, Route 9G Rhinebeck, NY 12572 Radios & tubes Carl R. Warren, W0KWS (417) 869-4738 MPO Box 567 Springfield, MO 65801 Tubes & parts. Also repair service Wayne (no last name given) (301) 963-4619 No address given Gaithersburg, MD Tubes, equipment, parts, books Westgate Co. (800) 213-4563 Need address! Tubes & transistors This list was prepared by AF4K, Brian Carling. AF4K @ W3INK brian.carling@acenet.com -------------------------------------------------------------------------------- Appendix B. Tube Makers Producing Today (Eric Barbour news posting) (The following is the text of a note posted to the alt.guitar news group by Eric Barbour