Two stroke voltages

[Paul]

My two stroke runs with about 375-380V on the output tube plate and 345-350V on the grid. The preamp tube runs about 145/150 each side.

I’m using a NOS Sylvania rectifier and the 125V primary tap on the PT. When I first used the 120V tap, I was getting about 400V on the plate and 370ish on the grid.

The PT is the weber as spec’d and it has 330V and 300V taps on the secondary, i’m using the 330 and capping off the 300.

1) Since I’m running high relative to DH specs from the book, does it make sense to try to lower these output tube voltages, or at least on the grid? If so, to do that would I increase the 10k/5w resistor? If so by how much?

2) What will be the effect of lower vs higher voltages on the output tube – less headroom vs more headroom? What about volume?

3) If there are going to be interesting differences between different voltages, is it worth putting the 120V/125V and 330V/300V taps on switches?

Thanks.

[Andy]

Those kind of changes can be expected depending on the tube. The Russian (Sovtek) rectifiers run a bit hotter than some of the others. It is still pretty well within spec for the tubes used.

Personally, I wouldn’t make changes to the biasing there. Biasing hotter (more voltage) gives you more headroom and more power, thus more volume (theoretically). But the tubes die quicker. Running these as you are describing is not really running “hot” as much as a but hotter than with the other rectifier so you are really in a good place with your measured levels.

Tube curves are a fun thing to get into. Here are a couple good sources:

http://www.webervst.com/tubes1/calcbias.htm

http://www.duncanamps.com/index.htm

If you havent read Blencowes book on Preamps, it is a great (best in my mind) resource. We still have some that we are offering with donations:

Quote:

I have been able to pick up a few copies of Merlin Blencowe’s books “Designing Power Supplies for Tube Amplifiers” and “Designing Tube Preamps for Guitar and Bass”. Both are excellent resources for the tube amp builder.

As a way of supporting both the site and builders, we will give away the books for a PayPal donation. We have limited numbers available, but we can get more if necessary. Right now, we will offer BOTH books for a $70 donation. If you check on Amazon, the price for both books is around $80 pre shipping. We will include shipping for the $70.

If you are interested, click the “Donate via PayPal” button on the sidebar of the site. Once our quantities run out, we will have to reorder so be patient if we have strong demand.

[Paul]

added the switch to go between the 300 and 330v taps.

the 300v setting gives me 345v on the plate and 316v on the grid.

exactly 30v less .

no change in volume or tone .

are the lower voltages better for the tube?

[Robin]

Yes….slightly. Lower voltage is kinder to the tubes but either voltage is acceptable, so go with what sounds best.

[Author]

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