1967 Ampeg B15NF Run Away

[Gumps]

Paul, I'm new at this...what does PM stand for and how do I do that? Thanks so much for your offer.
Incidentally, I received my 100uF/100V Sprague Atom bias cap. I installed that and measured -79V on bias and 492V of B+. This is with tubes in speaker connected. Still concerned about the high voltage so more to examine.

 

[beans-on-toast]

Always run a tube amp with the speaker connected. Otherwise you can seriously damage it. Something that you're probably aware of. Well worth mentioning.

 

[agedhorse]

With the bias more negative with the new cap indicates the old cap was likely almost open. 50 x 1.414 ~ 79

 

[agedhorse]

One thing about the sensitivity to plate voltage from line voltage...

In general, as the plate voltage rises positive with increasing line voltage, the bias will go more negative with rising line voltage helping to cancel out any runaway potential. The increased line voltage requires less cathode current for a constant plate dissipation. As the grid voltage goes more negative, the cathode current decreases, which is what we want.

 

[beans-on-toast]

That's true. As line voltage changes everything goes in the right direction so things work. But the change in bias is not necessarily linear with the change in line voltage. It can leave an amp sounding inconsistent from one venue to the next. Which is why some touring bands carry their own regulated power supplies. They lock in the line voltage and frequency.

Interesting to note, people that sell regulated bias servo circuits such as this one (Tube bias control) argue that a more stable bias helps in the low end and with distortion. Many would argue that this is not necessary in a musical instrument amp. It's interesting.

 

[Gumps]

I'm simply going to have to get plate voltage/ current to meet around 70% max dissipation and have a circuit that is stable.

 

[Gumps]

I enjoy and appreciate the insights you have brought out here.

 

[beans-on-toast]

I'm simply going to have to get plate voltage/ current to meet around 70% max dissipation and have a circuit that is stable.

You never mentioned what your line voltage is. If it is high, some people lower it with a bucking transformer: Vintage Voltage Adapter. Others use a variac, good as long as you don't accidently move the dial that sets the voltage.

A good way to deal with higher voltages inside the amp.

 

[Gumps]

It is 122.5V....when I checked yesterday.

 

[agedhorse]

Check with your new more negative grid bias voltage and see if you are closer to where you need to be.

 

[beans-on-toast]

It is 122.5V....when I checked yesterday.

The reference voltages on the schematic are with a 117VAC line voltage, so higher power supply voltages are expected. This will stress your power supply caps, especially the first node cap when power is turned on and the caps are drained. Until they charge, there is a surge that is higher than when the amp is normally operating. Often this first cap has a higher voltage rating because of this. This is where a bucking transformer that lowers the line voltage can help ansure a longer cap life.

 

[agedhorse]

There is a current surge, not a voltage surge. If the voltage rating of the power supply input stage filter cap is 10% above the nominal voltage of the power supply, you are fine.

 

[beans-on-toast]

Yes, inrush current, thanks.

 

[BassmanPaul]

My line voltage is typically about 126V. My amp works flawlessly at this voltage. Don’t sweat the small stuff. The high B+ voltage is to be expected.

To PM anyone click on their avatar and you’ll see a link to “start a conversation”

 

[Gumps]

I'm looking forward to getting the new 100K 2W resistor in the circuit to see the effect on voltage on stability. The two resistors, both the 100K and then the 56K divider resistor seem instrumental in reducing voltage following the rectifier. This is before the 47K bias resistor.

 

[beans-on-toast]

I'm looking forward to getting the new 100K 2W resistor in the circuit to see the effect on voltage on stability. The two resistors, both the 100K and then the 56K divider resistor seem instrumental in reducing voltage following the rectifier. This is before the 47K bias resistor.

You can pull one leg and test them with an ohm meter. The measured value can change when it is hot but if it is off when cold, change it for sure.

The 100k runs hot and can drift out of spec with time.

Remember to discharge the e-caps before poking around.

 

[Gumps]

Thanks for the safety warning beans....lest we forget! You know, this resistor measured high at 130K ohms but I think it is more disguised than this and it goes to your point about what happens when the circuit flows. I believe this resistor has arced and changes over time of flow and heat. We will know more on Thursday when the new resistors hit the door!

 

[beans-on-toast]

Thanks for the safety warning beans....lest we forget! You know, this resistor measured high at 130K ohms but I think it is more disguised than this and it goes to your point about what happens when the circuit flows. I believe this resistor has arced and changes over time of flow and heat. We will know more on Thursday when the new resistors hit the door!

So it was out of spec. What type of 100k resistor did you order?

 

[Gumps]

I ordered a metal oxide power resistor, 100K 2W

 

[Gumps]

But it is quite likely worse than just out of spec....my hypothesis is that it has arced and measures just out of spec at start and then goes way low after heating.