Re: rating of 15.6 ohm bias resistor in ST-70

On Sep 13, 8:54 am, Patrick Turner <i...@xxxxxxxxxxxxxxxxxx> wrote:
Leigh Orf wrote:

Hi,

Does anyone know what the current rating is for the 15.6 ohm bias
resistor in the Dynaco ST-70 (from pin 8 of the EL34's to ground)?

Among my upgrades was to replace these resistors, which had drifted,
with 1/4 watt 10 ohm resistors, which I keep frying at loud volumes. I
suspect 1/2 watt would be OK but am not taking my chances.

Leigh

--
Leigh Orfhttp://orf.cx
SDF Public Access UNIX System -http://sdf.lonestar.org

I would ONLY use 5 watt wire wound resistors, 15 ohms will be close
enough,
or 18 ohms 5W plus a trimmer of 120 ohms 1 watt to bring R close to the
non standard
value of 15.6 ohms.

But I also would not use the existing bias adjust methods, and
would have 4 adjust pots, one for each output tube, and
a 10ohm x 5W ww from each cathode to 0V. Then 0.5V = 50mA of cathode
bias current.

A 1/2 watt 15.6 ohm resistor needs only 179mA for the R to reach 1/2 a
watt dissipation,
and depending on speaker load the resistor sure will smoke because
average class AB current
exceeds 179mA. I have seen 1 watt 10 ohm R blow when used with EL34.
But that was partly due to the guy using bad speakers.

200mA slo fuses are a good idea in cathode to ground of each output tube
where you don't have active excess output current protection circuits
fitted.

If you blow such fuses, your speaker load is too low, and should be
moved from 8 ohm to 4 ohm outlet.
If they still blow, speakers could be faulty. Or you need to turn the
volume down, you must be well into clipping.

Patrick Turner.- Hide quoted text -

- Show quoted text -

Thank you, Patrick, for a rational response to the basic question.
Leigh's OP on the ST-70 wondered why a 1/4 watt resistor smoked after
a mod that he did. The similar mods with which I am directly familiar
call for a 10-ohm, 3-watt resistor (despite Bret's blathering). I also
agree with your suggestion of a fuse as a protective measure.

I will be mildly hunting for a 70 at Kutztown in a few days, there are
usually half-a-dozen in evidence. If I find one, I will do the
separate-resistor mod together with the separate pots and such... I
might just make it a "let's try this" test-bed. I write "mildly" as I
kinda-sorta would like to get along with my own home-brew, and have no
need for additional diversions.

Peter Wieck
Wyncote, PA
Kutztown Space 338

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