Question about 60R4 "balancing" resistors

[Deke609]

I recently noticed that I had toasted the 60R4 resistors running from the +ve speaker outs to ground (the -ve to ground looked fine). I suspect I toasted them in one my first attempts last summer to do some frequency sweeps of the amps - not sure what i did wrong, but I remember the 25W dummy load resistors got pretty hot.  So I'm guessing the 1/8W resistors got smoked at the same time.

That's all by way of preface. What I'm wondering is whether the two 60R4 resistors per side add up to a 120R8 resistor in parallel with the load (speakers or, in my case, headphones).  If so, I guess they would have only a small effect when the amp is used with, say, 8ohm speakers. But in my case, I'm using 200ohm headphones, so would have a bigger impact on reflected loads, making all my previous estimates of the latter way off.

Today I replaced the stock "balancing" resistors with 1.2Kohm 0.1% 1/8W resistors just in case. But I'd appreciate any insight into the whether they are in fact acting as parallel loads on the secondary.

cheers and many thanks, Derek

 

[Paul Birkeland]

You have to remember that having a 200 ohm load on a 3K:8 transformer doesn't mean you're reflecting a 75K load back to the tube.  Those resistors provide a ground reference for the secondary of the output transformer and do it in a balanced way.  If you are providing your own ground reference for the speaker outputs, you can connect the black binding post to earth and leave the resistors off.  If you are testing the amp and you are using gear that provides the black binding post with its own earth reference, but you leave the resistors in place, you will stress out the resistor between the red binding post and the chassis reference, as it is now handling the whole load.  If that resistor fails, you could also just leave it out of the circuit and leave the single resistor from the negative binding post to the chassis.

 

[Deke609]

Many thanks PB.

And apologies for being dense, but just to be clear: are the balancing resistors a load in parallel with the speakers/headphones?

You have to remember that having a 200 ohm load on a 3K:8 transformer doesn't mean you're reflecting a 75K load back to the tube.

I think I understand this - at least insofar as the load on the OPT is comprised of two parallel impedance paths to ground: (1) choke/psu; and (2) parafeed-cap/OPT primary. 

Those resistors provide a ground reference for the secondary of the output transformer and do it in a balanced way.  If you are providing your own ground reference for the speaker outputs, you can connect the black binding post to earth and leave the resistors off. 

I added something similar to my SII's. I thought they made a positive difference. So I'll keep them.  I don't believe I have any other ground reference for the speaker outputs.

If you are testing the amp and you are using gear that provides the black binding post with its own earth reference, but you leave the resistors in place, you will stress out the resistor between the red binding post and the chassis reference, as it is now handling the whole load.

And here is where my struggles with the o-scope continue.  I believe that both the scope and signal generator are grounded to house ground -- as are all my amps.  Is that a problem?  Or more to the point: when passing test signals through the amp and testing response (frequency response, THD), I should disconnect the balancing resistors?

many thanks, Derek

 

[Paul Birkeland]

The ground reference isn't an issue, just make your tests short. 

The ground reference resistors are in parallel with whatever load is on the amp.  This is unlikely to be meaningful in terms of the load the 300B sees.

 

[Deke609]

Many thanks PB.