Biasing a Traynor YBA-1A

[Leader]

I got some new tubes (JJ E34L) for a YBA-1A and I'm wondering if I should just pop em in or should I bias the amp? I know I SHOULD bias it but did all you old guys bias you amps back in the 70's when you changed the power tubes?

[Sparky6string]

I'm pretty sure that's a fixed bias amp and it needs no biasing. What year is the amp?

[Mark Kane]

That thing will have a b+ voltage of 540v to 560v. You need to bias it for sure and make sure those JJ's will run at those voltages. Back in the early '70's the tubes available were generally more robust.

[Leader]

It's from 72 or 71 and I got the JJ E34L because they handle a higher plate voltage.

[Fuchsaudio]

Easiest way to do it is to put a 1-ohm/3-watt resistor on each power tube cathode (pin 8, it would replace the jumper wire currently soldered there). Measure on each reisistor with an accurate digital voltmeter. Radio Shack sells some surprisingly good ones for 20 to 30 bucks. Set the bias based on the b+ voltage (measure at Pin 3 of either power tube), and use this calculator for the proper operating point: http://www.webervst.com/tubes/calcbias.htm

You should have a voltage (in millivolts) equal to the actual current i.e.: .035 volts (35 millivolts) - 35 ma of current. With matched tubes, these two voltages should be about the same or within one or two millivolts of one another.

[Sparky6string]

Here's a schematic that should fit yours to work from. I only took a quick look but I didn't see variable resistors for biasing. I know my 71 YBA-1 has none also.

http://www.lynx.net/~jc/700912_YBA1_A_Mk2.gif

[Scottone]

No Traynor ever had a bias pot installed, but you can certainly install one. I don't have a schematic handy for the 1A, so I can't tell you the value of the resistor that needs to be replaced, but you should be able to find a schematic on the web. For Traynors, I normally create a little assembly with a trimpot and a fixed resistor that is 10-20Kohms larger than the current fixed resistor. The resistor is usually a 47K or 56K value.


As Andy suggested, the bias resistors are a great idea as well. I just checked the bias on my friends Fuchs modded amp, and it has the resistors, and seperate bias adjustments for each tube...very user friendly:D

Just checked the link posted above...the resistor is R31 and it is a 47K. You could use a 25K trimmer and 33K resistor in series.

[vibroverbus]

No Traynor ever had a bias pot installed, but you can certainly install one.
Right on, unless there's some early Traynor weirdo setup I've never seen.

That kind of circuit design is so common it underlines how mild the importance of exact bias setting was to tube circuit builders/designers. They designed the circuit so that with an average tube idle current has a decent safety margin being reasonably sure that even the hottest tubes wouldn't push the envelope too far, and left it at that.

A lot of Fenders just have a 'balance' pot to even out matching issues, but the emperical value is assumed to be close enough to keep the amp in AB and at the same time not fry tubes (and usually on the cold-side of the range on a Fender). Not to mention how many HiFi / Radio circuits had no bias controls. I've seen old-time-guru-types roll their eyes with 'yeah whatever if you want me to charge you another bench charge for nothing' looks when asked about biasing for replacement tubes...

Dungetmewrong of course I always check and tweak the bias and recommend it, just sayin... it wasn't a big deal at all back in the day and most circuits were designed with an assumption of 'plug and play' maintenence replacement.

Anyway, on my Traynors I like to get a Fender-like 'full size' trim pot with screwdriver access from either side and mount it in the old ground switch hole if I haven't already used (or plan to use) that for some mod control. That hole is in a nice proximity to the bias voltage divider circuit most of the time. Then I run the bias to the pot wiper, bridge one side of the pot to ground using the aforementioned new resistor value, and good to go.

[Leader]

I emailed Yorkville and they said that back in the day no one ever heard of the term biasing an amp. It wouldn't hurt of course but popping in a matched pair should sound fine.

[Swarty]

The amp will probably sound fine, but you risk greatly shortened tube life.

[fuzz+strat]

I never biased mine...in fact, I was using the same stock tubes in my '71 YBA-1A and it sounded great. I installed a matched pair of EHX EL34s and it was even better.

[donnyjaguar]

My YBA has no adjustment either. I tested the bias current and it was perfect so I just left well enough alone.

[Leader]

My multi tester only goes to 500 volts so screw it. Were just gonna pop in the new tubes and see what happens.

[Plague Dog]

So how did it sound?

Mine came with RCA 6CA7's and sounds pretty incredible. It is frikken loud though, so I need to install a master volume I guess, or buy a hotplate or equivelent.

[daveski]

No Traynor ever had a bias pot installed, but you can certainly install one. I don't have a schematic handy for the 1A, so I can't tell you the value of the resistor that needs to be replaced, but you should be able to find a schematic on the web. For Traynors, I normally create a little assembly with a trimpot and a fixed resistor that is 10-20Kohms larger than the current fixed resistor. The resistor is usually a 47K or 56K value.


As Andy suggested, the bias resistors are a great idea as well. I just checked the bias on my friends Fuchs modded amp, and it has the resistors, and seperate bias adjustments for each tube...very user friendly:D

Just checked the link posted above...the resistor is R31 and it is a 47K. You could use a 25K trimmer and 33K resistor in series.

My MK III twin had a bias pot installed and the suggested voltages were written on the schematic glued to the inside of the top panel.

[Fuchsaudio]

I've seen more than a few with internal trim pots on the tag board. Usually a little screwdriver adjustable thingie. Maybe not all of them got it. I never really noticed. Not tough to add one though.