Dynaco MKIII high B+

S

script56

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Is it normal for B+ to be around 560? Chart I think says 490. I replaced all caps and selenium rectifier with diode. The tubes do red plate a little, but have heard it’s common on mkiii. I also set bias 1.56.

If not normal, what should I do to lower?
 
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Based on your excessive HV readings,and the fact that your power tubes are red-plating at 1v56 of bias,I going to have to call your meters accuracy into question.

If the voltage were,in fact,running at 560v,you would have played ''pop goes the filter cap'' by now. The original is rated at what,525v?
 
Maybe check the meter's battery as a low one will sure screw you up.

Run the bias as low as you can where it still sounds good for decent tube life.
 
Shadowdog said:
Maybe check the meter's battery as a low one will sure screw you up.

Run the bias as low as you can where it still sounds good for decent tube life.
Click to expand...

Good call on the battery.

You do have to be careful when reducing bias. Often,the power supplies in amplifiers ''sag'' when the amp is properly biased,but can rise to excessive levels when the specified bias is lowered in order to increase tube life or reduce heating by lowering quiescent current. You can safely reduce bias,but you do have to monitor your HV levels while doing so.
 
If not the battery perhaps the meter itself? Some of the import, ahem, DVMs may not be too great above line AC voltages.

I trust my Fluke 87 and an ancient analog voltmeter I have but not too sure on the new ones.
 
is the resistor on pin 8 of the output tubes the correct value? 1.56 volts only works if that is the 11.2 ohm value.

and the meter is set to DC volts, not AC volts? 5AR4 instead of a silicon rectifier?
 
Line voltage does run higher than when these were designed. B+ should be about 510 with today's line voltage. Suggest you try it direct without variac which has the wiper on the secondary and designed with head room to increase the voltage.

The problem typically seen is B+ is high until the outputs start conducting, the solution is inrush current limiters (thermistors) if you do a search you should be able to find suggested part number.
 
I will check other MKIII.

Resistor is read at 11.2.

I’m using original GZ34. Tested good.

When I did test 6550, they all tested good on line test but all six I tested had some “noise” (noise bulb lit up on TV7) when I tested shorts on TV7, but the sound is good from amp so not sure if that’s normal for 6550 tubes to test like that on TV7 tester.
 
Could one come to a reasonable understanding of the B+ load/current draw by measuring DC voltage drop across the choke?

Aren't the chokes supposed to be about 55 ohms DCR?

Or, the voltage on the cap before and after?
 
Maybe? Use Kt120's instead of 6650s and get the higher voltage (550v) filter cap off of Ebay. The amps were made to run a preamp so extra heater voltage is availible. Common problem...
 
Here is other amp readings. Also see the red plate on one of the tubes. I did measure resistance of one of the chokes when still connected to amp. It read 61 ohms.
 

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Best to turn the bias down till the tubes don't red plate (even when testing) as you are ruining them!

As mentioned though the tubes could be the problem, but trying to get them not to redplate may show this.
 
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