bias pot on traynor yrm-1 and mexican voltage

[musicamex]

hi from west coast mexico

i just joined this forum and am impressed with the level of knowledge and willingness to help found here.

i am a 62 yo musician. here in mexico that often must include keeping your own gear working too. that is, unless you want someone with a wallmart soldering iron and old television parts trying to bring life back to a dead piece of gear. i seem to have the best "bench" around here so i wind up fixing lots of my friends gear too. i have built a couple of simple tube amps and except for smt modern delicate boards i manage to fix lots of broken gear.

i am currently working on a friends traynor YRM-1 head.

http://www.lynx.bc.ca/~jc/traynor_YRM1.gif

it was designed around the 115 VAC of the 70's. the owner had problems with short power tube (EL34) life.

i replaced 4 old leaking filter caps with sprague atom blue 40 mf 500v and brought the amp up slowly with a variac to form them. the voltages on pin 3 are 430 with the variac at 115, the current (using a cathode resistor adaptor) is around 46 ma. static dissipation wattage of about 20. does that seem high for current production EL34 tubes? there seems to be a range of opinion of between 20w and 25 w max static dissipation for EL34 depending on if nos or new production. the owner supplied some matched groove tubes he had and the cathodes aren't getting red at idle at 115v input. at mexican voltage (128) the sdw is around 26.5.

any opinion on best current EL34?

the owner has a line conditioner that he says puts out a constant 122v.

the owner would like longer tube life and more clean headroom and be able to tweak his sound so i would like to put a resistor / pot network in place of using the resistor substitution (decade box) method of tweaking the bias. am i correct i replacing R41 (39k) with a 22 ohm resistor and a 50k 0.5w trim pot in series? the 50k pot:

http://www.steren.com.mx/catalogo/in...p?pdto=135-50K

is all i can find locally that seems a close fit. next closest is 10k or 100k. mexico is a tough place to get parts. any help with double checking me on this or offering suggestions will be a great help.

the main problem here is that mexican voltage is 128-130. i have been able to mod a couple of old CVR units to bring the voltage into the 115-120 range for my own gigs, but all CVR units that are currently available here are designed to to kick in at plus or minus 10% of 128. effictively 115-142 before they kick in the transformer. the ones i have modified had rare multiple output taps on the tranny so i just rewired them.

i always carry a vom with me to gigs and have read 105 to 136 line voltage during the course of a single evening. wiring sucks here and there are extension cord chains everywhere. it is easy to see why there are a multitude of fried amps and gear in mexico as most came from the land of 120v. i actually rewired the bar i play at most so there was a dedicated line from the main to the stage with 7 outlets and its own ground. physical grounding here is 99% non existant and is still a mystery to electricians. i actually caught a "professional" electrician installing a fused ground in a main fuse box because it had an extra place for a fuse (240 box).


you guys are light years ahead of me so please, if you have any ideas on taming mexican voltage with something i can build or modify on my bench it will help keep allot of my friends from spending the little they make playing music on fixing their gear.

3 gigs this weekend--gotta get ready.



thanks amigos musicamex

[Bruce / Mission Amps]

I know money is very tight all around the Americas but have you considered buying an auto transformer such as a VARIAC?
I think you can get a great one for under $100.00 usd
Try ebay...

http://shop.ebay.com/items/variac?_dmd=1&_sop=2

[musicamex]

I know money is very tight all around the Americas but have you considered buying an auto transformer such as a VARIAC?
I think you can get a great one for under $100.00 usd
Try ebay...

http://shop.ebay.com/items/variac?_dmd=1&_sop=2

thanks for the suggestion. i have a variac on my bench and have a couple of modified CVR units that work for my stuff at gigs, but to be honest most of the musicians here don't have the bucks for a variac, especially with freight and customs duty to mexico. if it is made in china duty is 100%. the peso has gone from 10 to 14.5 to 1 us dollar in just a few months. many musicians have 2-3 gigs a week and most club gigs pay from 300-500 pesos plus beer and sometimes food. private gigs pay more. if it weren't for the food value in beer, starving musician sure fits the m.o. with so little pay.

they also don't have the knowledge to not use a variac wrong. like they often say, my amp sounded great until it started smoking.

musicamex

[dyermaker8]

Lightbulbs?

[musicamex]

wouldn't the lightbulbs only help limit the current at startup and in case of overloador act like a fuse in the case of a short and not change the voltage much during normal use? that is unless i had a really big series of them? i guess i am after something really foolproof and simple. which of course a lightbulb is, but maybe a string of bulbs in a gig bag frobably wont be adopted by the average musico here.

the cvr units in the usa designed for 120v might work. has anyone ever tested them with a variac to see what they actually put out at various voltages?

[tubeswell]

i replaced 4 old leaking filter caps with sprague atom blue 40 mf 500v and brought the amp up slowly with a variac to form them. the voltages on pin 3 are 430 with the variac at 115, the current (using a cathode resistor adaptor) is around 46 ma. static dissipation wattage of about 20. does that seem high for current production EL34 tubes? there seems to be a range of opinion of between 20w and 25 w max static dissipation for EL34 depending on if nos or new production. the owner supplied some matched groove tubes he had and the cathodes aren't getting red at idle at 115v input. at mexican voltage (128) the sdw is around 26.5.

any opinion on best current EL34?

At that plate voltage 41mA idle current on the plate is about right I'd say for a longer tube life. But where your wall voltages measure higher than 120V, maybe bias the the tubes to ~65% (as measured when you are using the variac at 115V) - which would be about 37mA (or a little bit higher if that sounds too sterile)

Svetlana EL34 (or JJ KT66) would be pretty reliable.

the owner would like longer tube life and more clean headroom and be able to tweak his sound so i would like to put a resistor / pot network in place of using the resistor substitution (decade box) method of tweaking the bias. am i correct i replacing R41 (39k) with a 22 ohm resistor and a 50k 0.5w trim pot in series?

I did a pretty picture (see attachment - .pdf). If the range of the bias sweep isn't low enough, just go to a 5k fixed resistor in series with the 50k pot. Maybe 22R might not provide you with enough high range (on the sweep of the pot) to get it sounding good. Wire the pot wiper to the ground/return side of the pot, so that if the wiper contact fails, you've still got some connection to the ground/return (which will save the power tubes from self-destructing). On second thoughts - it might be better with a 20k trimpot in series with a 22k - 27k resistor - that way if the wiper fails, you've still got about 40k-ish there

mexico is a tough place to get parts. any help with double checking me on this or offering suggestions will be a great help.

I know what you mean about getting parts. Its the same here in NZ (Cut-off from the rest of the world by thousands of miles of ocean) Fortunately the postal service is still running and that I have some spare assorted 50k trimpots lying around (they are cheap as chips). If you want to PM me with your address I'll flick some of them to you gratis - if they make it through the mail at your end, it'll be a bonus. (Who knows? one day you might be able to help me :-)

if you have any ideas on taming mexican voltage with something i can build or modify on my bench it will help keep allot of my friends from spending the little they make playing music on fixing their gear.

Either find or make a variac, as the others have said (or mod all your amps to adjustable bias and reset them individually at each gig)

[dyermaker8]

Well,. i can attest that they reduce available juice the whole time they're plugged in,. not just at startup,. I happen to use a series of lightbulbs and an electric space heater to reduce the line voltage in my home-made arc welder to get it low enough to weld sheet metal,. don't laugh. FYI,. would probably take a pretty big lightbuld to make this work though,. check out the chart listed on this thread,. i'm guessing something like a 500 watt bulb,. maybe double as the guitarist personal spotlight!!!


http://audiokarma.org/forums/showthr...im+bulb+tester

[loudthud]

One solution might be an Uninteruptable Power Supply intended for a computer. Some run off the inverter all the time so you get relatively constant voltage although not a pretty sine wave.

[musicamex]

[QUOTE=dyermaker8;91613]Well,. i can attest that they reduce available juice the whole time they're plugged in,. not just at startup,. I happen to use a series of lightbulbs and an electric space heater to reduce the line voltage in my home-made arc welder to get it low enough to weld sheet metal,. don't laugh. FYI,. would probably take a pretty big lightbuld to make this work though,. check out the chart listed on this thread,. i'm guessing something like a 500 watt bulb,. maybe double as the guitarist personal spotlight!!!


you have really peaked my interest in doing a little experimentation. mexico is still big on bare incandescent bulbs hanging from a wire, so there are plenty available. i hope in the long run the bulbs don't eat up so much power that at the end of the extension cord chains so common here, that there is actually something left for the amps. thanks for making me what is probably a very "bright" simple solution, albeit something the al gore police might come south of the border to cite me for. ;~}

[musicamex]

what an awesome and helpful reply tubeswell!! thanks for the time it took to post the schematic amigo. i plod my way through allot of my repairs and through my "paint by numbers" projects and hope a little of what i read here on this site to rub off.

do you have a program to do the schematic drawings? i still use a pencil with a fat eraser to do mine and would like to find a simple program to make cleaner schematic drawings.

i will post a more complete reply tomorrow, but i just got home from an out of town gig and am really fried.

i will also pm you tomorrow. thank you for the generous offer of the trim pots. seriously, parts are hard to come by here, and i wasn't joking about many repairs being made from a pile of old tv's etc. mexicans are very innovative and when something finally goes to the dump here it really belongs there. picked clean to the bone so to say. i have a good basic supply of resistors and caps and am a dealer for new sensor. probably have the best cache of tubes on the west coast of mexico, but most are current production.

a really great friend of mine told me NZ was the most beautiful country he'd ever seen. we get a number of kiwi's here in the bay of banderas. this is quite a sailing destination and one of the reasons i wound up here too.

thanks again, more tomorrow

[musicamex]

one guy here uses one he brought from the states. pretty big, but i think it is about 600w with a lead acid battery in it. i never checked the output voltage.

quite a few moons ago i lived near liberty hill near austin. loved the hill country and trips to gruene hall on sundays for a few cold long necks and some great music.

[tubeswell]

do you have a program to do the schematic drawings? i still use a pencil with a fat eraser to do mine and would like to find a simple program to make cleaner schematic drawings.

I use JSchem - free here:

http://dhost.info/jschem/