UTC LS-65

[archeus68]

I have a chance to buy a couple UTC LS-65 transformers at a pretty reasonable price.
I was looking at the Williamson UL schematic that the UTC guys show in their 1963 catalog.

http://bit.ly/2pfdBH5

For what ever reason I've never been a big fan of UL amps. They sound a little soft/slugish

I guess the 6550s could be triode strapped.
But if there are some other LS-65 circuits out there, please share your favorites.

As H. Ross Perot use to say...."Larry I'm All Ears!"

 

[zackthedog]

I know what you mean. But I think that's more often than not a function of mediocre design and/or components. I'd be willing to bet that that circuit won't sound soft and sluggish, if you need 60 wpc. And triode-strapping the 6550's will screw up the feedback and tuning.

 

[gadget73]

nothing says you have to use those UL taps. Triode or pentode is an option. The original Williamson design was triode-strapped.

 

[zackthedog]

nothing says you have to use those UL taps. Triode or pentode is an option. The original Williamson design was triode-strapped.

You don't think the feedback ratio will change?

 

[gadget73]

It will, but not possibly enough to really matter. It will have less feedback in triode since the open loop gain is reduced, and more in pentode. Increasing the feedback *might* produce stability problems, but not all Williamson designs were extremely stable to begin with. Any amp build should be checked for stability before it gets turned loose on a set of speakers anyway.

 

[archeus68]

The schematics I was looking at.

 

[Retrovert]

Any amp build should be checked for stability before it gets turned loose on a set of speakers anyway.

+1. Ignore this advice and wander into the lands of peril.

Many amplifiers oscillate, and this can only be seen with a good analog scope. The oscillation is often in the hundreds of kHz or MHz—tubes are high bandwidth—and many design choices inadvertently create oscillators. An amplifier is not much different from an oscillator whose Q was ruined. This is one of the many purposes of grid and screen stopper resistors; ruining the Q of the tank circuit formed from parasitics, including those internal to the tube, which increases high-frequency losses to the point that oscillation either cannot begin or is damped out.

This is one of the reasons why replacing transistors or opamps with faster parts results in destabilizing a formerly stable amplifier. The old devices were bandwidth limited and tended to not oscillate; increase the speed and it's all over.

 

[archeus68]

Also for those who enjoy looking at schematics check this site out http://bit.ly/2KGQZan
There are four chapters.

 

[maxhifi]

You don't think the feedback ratio will change?

The ratio will stay the same, but since the open loop gain will be reduced, so will the closed loop gain, making the amp less sensitive. This may or may not matter. Also consider that the driver should be checked to make sure it can swing enough voltage to drive triodes.