I would like to post my evaluation of the following new production
EL84 tubes as installed in my ST-35 "original" dynaco amplifier.
Unit is completely stock (recently restored) with the exception of
a 100 ohm 5 watt fixed bias resistor.
All tubes tested a new "matched quads"
(a) EI Gold series total current 153 ma (dc) @ 15.3 vdc
![Beer (b)](./images/smilies/beer.png)
![Coffee (c)](./images/smilies/coffee.png)
![Martini (d)](./images/smilies/martini.png)
Please understand I am just a beginner with vacuum tubes with
minimal electrical background. I do respect the lethal voltages.
Based on the above readings, it appears to me that the JJ brand
tubes are the nearest to the recommended 140 ma total current
measurement, although a bit high at (+7 ma).
As suggested by Ed B (NH) the bias resistor could be increased to
obtain the desired 140 ma total current or alternatively a 15 ohm
5 watt potentiomter could be added in series with the existing 100 ohm
resistor.
Based on the Sovtek readings it appears that I would need to go the
other way reducing the value of the bias resistor. Correct ?
Obviously, the potentiometer option would allow for use of almost any
brand tubes and this is under consideration although I am not 100%
sure how to wire this correctly. I have wired simple volume controls
before and assume that it is basically the same routine using a linear
pot with the (3rd) leg to ground ? Is this connection needed as I believe
the 100 ohm resistor is already grounded ?
A prior post stated that new production EL84 tubes tend to run hotter as
compared to earlier production tubes such as "Mullard".
Does this imply that the newer production tubes tend to draw a higher
current and the 140 ma recommended current spec is not that critical ?
As an an example, the JJ band tubes were running at a total current of
147 ma. Would this be considered to be in the prefered current range.
On the "flip side" side of this would running the tubes below the 140 ma
current as is the situation with the Sovtek brand tubes be adverse to
the operation of the amplifier and/or sound quality.
Sorry for all these questions; I look forward to contributing to this
Forum in the future. Thanks to all Charley Rose