Bias setup

[justinsweber]

I have DIY's MKIII board... and I was trying to figure out what the bais should be set for? I am using the SDS cap baord...

Will use of a SS rec make a diff? WHat would it be for either?


Thanx... just trying to set it up correctly.

[Ty_Bower]

The Mk3 ought to be biased about 70 mA per tube, and should have 500 volts at the plate. That's 35 watts overall dissipation. You can tell when you have 70 mA through the tube by measuring the voltage drop across the 10 ohm resistor connected between the output tube's cathode and ground. 700 millivolts indicate 70 mA idle current. Keep a set of speakers or a dummy load attached to the outputs, and mute your preamp or install shorting plugs at the Mk3's input jacks.

If you intend to use solid state rectification, you will find the plate voltage is slightly higher than the usual 500 volts. In this case, you may wish to adjust the bias to keep the idle current slightly lower - maybe 65 mA or so. This should keep the overall dissipation under the 35 watt mark.

Take a look at page ten of the TubeZone version of the Poseidon manual. It describes the biasing technique fairly well.

[justinsweber]

The amp was setup by a freind, but I wanted to go to a new tube rectifer and try out the SS one I have... The current bias is 1.10Volts DC... The orig. bias was 1.56... I guess Im asking the basic bias question of what to set the bias voltage to... with a 5ar4 and with a SS... I will need his to demo. how to do it from scratch again.

[justinsweber]

The MKIII is runing... this is just an asjustment. I looked at step 5... 700mv... .7V?

[TomMcNally]

Yes - with a 10 ohm cathode resistor, you want to adjust
for .7 volts (700 mv) The 1.56 reading from the old
MK-III's was based on a 15.6 ohm cathode resistor.
The story behind it was back in the days of less than
accurate meters, you could measure the voltage of
a fresh dry cell, then compare the bias to that.

[justinsweber]

and if I use SS rectification should I set 50mv less? so .65v?

As a seperate question. I have triode Electronics MKIV board... seems louder/ more powerful.... is it a product of 2 12ax7 for more gain , where DIY board uses 1 12ax7 and 1 12ux7?

Im supper new to all of this.... just trying to learn while lost.


Than you for the consise answer before!

[justinsweber]

Tried to get bias down to 700mv... can only bring down to ~750mv... any thoughts on why?

[Wiesiek Lipowski]

Yes - with a 10 ohm cathode resistor, you want to adjust
for .7 volts (700 mv) The 1.56 reading from the old
MK-III's was based on a 15.6 ohm cathode resistor.
The story behind it was back in the days of less than
accurate meters, you could measure the voltage of
a fresh dry cell, then compare the bias to that.

The old MK-III's reading was based on a 11.2 cathode resistor, common for both the tubes. It gave the 0.78 mA reading for a single tube at 11.2R = 70 mA.

15.6R was on ST-70 as I recall it.

[Ty_Bower]

Tried to get bias down to 700mv... can only bring down to ~750mv... any thoughts on why?

There is a 4.7k resistor in the bias supply circuit. It is shown on the top of page four in the manual. Some people have needed to replace that resistor with a 1k resistor in order to get the idle currents in spec. Go read this thread, where another user describes nearly the same problem: http://www.diytube.com/phpBB2/viewtopic.php?t=2296

You do have one of Shannon's red DIYTube Poseidon boards installed in your amplifiers, correct? One of the posts above suggested it might still be running with the stock circuit.

The current bias is 1.10Volts DC.

[Wiesiek Lipowski]

... It gave the 0.78 mA reading for a single tube at 11.2R = 70 mA.

Sorry, if course it gave 78 mA, and not 0.78 mA. My mistake

[justinsweber]

Im running one of shannon's MKIII boards, bought through Triode electronics... their newer kits... with a single test point.

[EWBrown]

Those "oddball" 15.6 and 11.2 ohm resistors in the original Dynacos were chosen, because the correct bias settings would produce a reading of 1.56 volts, which was exactly what a fresh carbon-zinc D cell would produce.

In both the ST70 and the MKIII, the output tubes were connected in pairs, to the cathode measuring resistor.

So, the ST70, with its EL34s, when set to 50 mA each, would read through the resistor as 1.56VDC, which equated to 100 mA total. This mandated having a matched pair, which was supplied with the kit.

Similarly, the MKIII, had the single 11.2 ohm resistor, once again the bias was set for a 1.56V reading, and that equated to 70 mA per output tube.

This eliminated the need for an expensive precision voltmeter, just measure the battery, make a temporary mark on the meter, and then set the bias to match the mark.

This was about 2-3 decades before digital voltmeters were available (except as very expensive laboratory equipment).

Today, when decent DVMs are common and cheap, it is easier to replace these "weird" resistors with 10 ohms 1% W resistors, and then read the idle current directly, 100 mA would read as 1.00 VDC, and 140 mA would then read as 1.40 VDC .

/ed B

[justinsweber]

I went reading through the DIYtube MKIII pdf... It shows R10
68K, 2W and R11 68K, 2W. So Im thinking of setting bias at 1.36volts.

I asked a frined and he replied: "We measure the current from the resistor 100 ohm which connect from the two power tubes pin 1 & 8 to ground. There has a wire connect from above 1,8 pin to front socket (should be pin 5 or 8, either one, I forgot). The volt is around 1.36v (68ma x 2).



1.36 sound right for this setup?

[Ty_Bower]

R10 and R11 are the plate loads on the phase splitter. Their values do not determine where you should bias the output tubes.

I'm hoping you made a typo when you referred to the "100 ohm" resistor between pin 8 (cathode) and ground. That should be a 10 ohm resistor.

Am I correct in understanding that your amp has only a single 10 ohm resistor, and the cathodes of the two power tubes are wired together? Why was it done that way? You should be careful to make sure the ten turn trim pot R15 is set so that equal bias voltage is delivered to both output tubes - unless other changes have been made to eliminate the bias balance feature of the Poseidon board.

[justinsweber]

I will have to sepnd some time and check all voltages.