[Leigh Orf]

Hi,

I have a Dynaco ST-70 which I have posted about here before. I have
done a few upgrades (SDS Labs capacitor upgrade, Triod driver board,
replaced all tube sockets), one of which involved replacing the 15.6 ohm
bias resistors which go from pin 8 of the pairs EL34s to ground on both
channels with a 10 ohm resistor. I have fried this resistor twice (once
on each channel) when playing loud music. I suspect it may be due to the
fact that I have put 1/4 watt resistors in, when I should be putting
in a 1/2 watt resistor. The original schematic/manual does on give the
power rating on the original 15.6 ohm resistor. Would I be safe going
to 1/2 watt, or would I run the risk of frying something more expensive
upstream when playing loud?

Leigh

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

[Sal Brisindi]

Leigh Orf wrote:
Hi,

I have a Dynaco ST-70 which I have posted about here before. I have
done a few upgrades (SDS Labs capacitor upgrade, Triod driver board,
replaced all tube sockets), one of which involved replacing the 15.6 ohm
bias resistors which go from pin 8 of the pairs EL34s to ground on both
channels with a 10 ohm resistor. I have fried this resistor twice (once
on each channel) when playing loud music. I suspect it may be due to the
fact that I have put 1/4 watt resistors in, when I should be putting
in a 1/2 watt resistor. The original schematic/manual does on give the
power rating on the original 15.6 ohm resistor. Would I be safe going
to 1/2 watt, or would I run the risk of frying something more expensive
upstream when playing loud?

Leigh


Leigh,
I am sorry I don't have an answer for you but why would you change that
15.6 ohm resistor with a 10 ohm resistor? Why not put the original
resistors in place and see what happens.

Sal Brisindi

[Peter Wieck]

On Sep 12, 8:18 am, Sal Brisindi wrote:

I am sorry I don't have an answer for you but why would you change that
15.6 ohm resistor with a 10 ohm resistor? Why not put the original
resistors in place and see what happens.

Sal:

This has to do with the related upgrades and changing the bias supply
wiring.

Leigh:

As I remember, the precision-cut 15.6 ohm resistors in the original
design were 1 watt units and pretty much stayed cold when the bias was
correct. But that is not relevant to this. I am vaguely familiar with
the modification you have done, and as I remember, the OEM paperwork
calls for replacing the 15.6 ohm resistor with a 10-ohm *THREE* (3)
watt resistor from Pin 8 to Ground.

So, frying a 1/4-watt resistor is no surprise at all.

For the record, please make a note of all modifications you do and
attach them to the unit, either underneath or in some location away
from heat. It future owners when faced with an unfamiliar unit for the
first time.

Hope this helps.

Peter Wieck
Wyncote, PA
Kutztown Space 338

[Arny Krueger]

"Leigh Orf" wrote in message
...
Hi,

I have a Dynaco ST-70 which I have posted about here before. I have
done a few upgrades (SDS Labs capacitor upgrade, Triod driver board,
replaced all tube sockets), one of which involved replacing the 15.6 ohm
bias resistors which go from pin 8 of the pairs EL34s to ground on both
channels with a 10 ohm resistor. I have fried this resistor twice (once
on each channel) when playing loud music. I suspect it may be due to the
fact that I have put 1/4 watt resistors in, when I should be putting
in a 1/2 watt resistor. The original schematic/manual does on give the
power rating on the original 15.6 ohm resistor. Would I be safe going
to 1/2 watt, or would I run the risk of frying something more expensive
upstream when playing loud?

On a really bad day, this part may see, what 200 milliamps?

The voltage drop across the 15.6 ohm part would then be 3.12 volts and the
dissipation would be .624 watts.

Because resistor power ratings usually are for a sizzling operating
temperature, a 2 watt resistor would not be overkill at all, if wirewound or
metal oxide.

If carbon comp, 5 watts would be mandatory. 5 watt wirewound resistors being
small and cheap, would still be reasonable.

The 10 ohm part would dissipate about 2/3 the power, but a 2 watt wirewound
or metal oxide resistor would still be very reasonable.

Your 1/4 watt resistors were accidents waiting to happen. :-(

[Leigh Orf]

Peter Wieck wrote:
On Sep 12, 8:18 am, Sal Brisindi wrote:

Leigh:

As I remember, the precision-cut 15.6 ohm resistors in the original
design were 1 watt units and pretty much stayed cold when the bias was
correct. But that is not relevant to this. I am vaguely familiar with
the modification you have done, and as I remember, the OEM paperwork
calls for replacing the 15.6 ohm resistor with a 10-ohm *THREE* (3)
watt resistor from Pin 8 to Ground.

So, frying a 1/4-watt resistor is no surprise at all.

Well... wow. I'm surprised this doesn't happen more often, then.
I do like to turn up the volume with this amp!

I'm not 100% sure changing the 15.6 jobber was an official part of any
of the upgrades I did; but I do recall both of them being far from 15.6
ohms, and knew that its value could be changed so long as the bias
voltage was set to match the change.

For the record, please make a note of all modifications you do and
attach them to the unit, either underneath or in some location away
from heat. It future owners when faced with an unfamiliar unit for the
first time.

Good advice, although I never intend on selling it. But you never know.

There isn't a whole lot to this amp, and I haven't done anything
terribly weird. Both major upgrades were kits.

Hope this helps.

Quite!

And apologies if another post with my query comes up, having newsreader
weirdness on this end.

Leigh

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

[Chris Hornbeck]

On Thu, 13 Sep 2007 02:06:32 +0000 (UTC), Leigh Orf
wrote:

I'm not 100% sure changing the 15.6 jobber was an official part of any
of the upgrades I did; but I do recall both of them being far from 15.6
ohms, and knew that its value could be changed so long as the bias
voltage was set to match the change.

The value of this special resistor "seems" important, but its history
is now often lost. Dynakits are from an era long before three-
digit-accurate DVMs in every toolbox. But one accurate voltage
reference was available to everybody, a fresh battery.

No, there was only one kind. Arf.

This value of cathode resistor was chosen so that, at rated bias
of 100mA, the same voltage would appear across it as across a fresh
battery. Your meter didn't need to be accurate, only repeatable.
Cool, not?

Nowadays, separate resistors for each cathode are highly
recommended, along with separate bias trims. 10 ohms at 1/4
watt is idling dissipation (50mA) in this use, so a power
rating somewhat larger, but still small enough to provide
a fusing effect is needed. Call it half a watt or a watt.
Your work is done.

All good fortune,

Chris Hornbeck