TOL50 el34 bias

[tacoma5050]

Hi,

I'd like to check the bias for the two el34's in this Egnater TOL 50. See schematicatic here: http://schems.com/manu/egnater/egnator_tol50_50w.pdf

I've done this many times on my Marshall ma50. The Marshall has 2 precision 1 ohm resisters near the test points and you just need to adjust the two trim pots until each test point measures 36mV across each of the 1 ohms resisters. Very convenivent.

In the TO50 schematic, I only see one trim pot (r95, on second page near bottom) for bias adj.

1. Why is there only 1 trim pot when my TOL50 has 2 el34's just like my ma50? The ma50 has two trim pots one for each el34.

2. Where do I measure the voltage on the TOL50 and what do I set it to?

[jim]

Looks like the Egnator also has a 1 ohm resistor to ground off of pin 8 of the output tubes.  I don't see a test point but you could possibly clip a lead onto pin 8 and the other to ground.  Alternatively, you could and should eventually build a bias probe you can use with your meter that actually measures cathode current at the tube base.   

http://www.hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=Tools&ORDER_ID=450802245

This amp has a single pot controlling bias for a pair of tubes so you use a new matched pair of tubes.  The cathode current will be the same in each tube.  35mV measured across the one ohm resistor is good to start.  Be careful with the probe tips--clip them on --do not poke.   Jim

[SILVERGUN]

Check this out:
http://www.webervst.com/tubes1/calcbias.htm

Use the calculator at the top and choose AB in the first box,,,then EL34 in the 2nd box,,,then enter your actual plate voltage (the 2nd page of your schematic says it should be around 495vdc).....then hit calculate, and it will show you approx. what your bias reading should be...
Down at the bottom of the page it shows you examples of acceptable ranges, but the hotter you go the more you run a risk of early tube failure

In fact I did it for you and it came out to 35mA (mV = mA thanks to those 1 ohm resistors) , just like Jim said

[PRR]

> Why is there only 1 trim

My 1961 (1937) Willys had four valve adjusters, and they were fussy.
My 1967 Ford was "self-adjusting"-- at 150,000 miles you had to scrunch-up the wear-slop, but it wasn't fussy.
My 2002 Honda's valves are not supposed to be adjusted. I forget if they even have a trim. The way they sound at 127,000 miles, I'll probably never care.

Two tubes "should" bias-up pretty near the same. If not, while dual-trims can make the idle currents the same, that won't fix the mis-match at full ROAR.

The "perfect sound" for a rig like this would be idling red-hot. Being cooler than that doesn't sound awful, but lasts a lot longer. And there's feedback. And it's only rock-and-roll. In fact a bit-cool seems to be part of the charm of the 5F6A (what this amp derives from, albeit with very different big-bottles).

Meter at pin 1-8. At 500V, 60mV is too hot, 40mV would be a safe guess, but as I squint a smudge near the bias pot, I think Egnater intended 28mV (28mA or ~~14 watts per plate). Trim while checking one tube then the other, compromise any unbalance. I'd accept 20mV on one and 40mV on the other. Unless I sold tubes at a profit, then I'd suggest a new matched-pair.

[SILVERGUN]

I'd accept 20mV on one and 40mV on the other. Unless I sold tubes at a profit, then I'd suggest a new matched-pair.

NICE!!.....now that's a "range of tolerance"......tube co.s would have you believe that your amp would catch fire if you mis-matched that far
Don't believe the hype!