Tube amps and loads.

Corazon, I'm no expert on tube amps, but AFAIK you should not operate it without load. The fact that you don't apply signal doesn't change anything since practically anything can cause output transient that, with open transformer, will generate high voltage (called flyback voltage) that can fry output transformer (breaking insulation). Even switching power on or off can do it.

As for using Variac - 1.5 years is not enough to bother IMHO. I would be concerned with something that was unpowered over three years. Without voltage electrolyte eats up aluminum oxide layer reducing capacitor's breakdown voltage. When voltage is present aluminum oxide is being rebuild.

I agree with Kijanki. Also, the load needs to be within range of the spec'd load for the amp. That is if the output tap is suppose to be 8 ohms, you should connect speakers that are close to 8 ohms (+/- 50% is probably fine depending on the amp). If you want to connect a load resistor - it too needs to be the proper resistance - and should be adequate wattage. a 1/2 ohm resistor isn't enough - you need a 50 watt or so . . . depending on your amp.

I have a Scott 299B and like it a lot. If your restoration is about 1 1/2 years old, I would not worry about using a variac, however, I never power up any tube amp without a load connected. You are going to like that 299B. BTW: Do not use EL-84's in the 299B, use 7189's. These can be expensive, however Jim McShane has the Russian 6P14P-EB which is a direct replacement for the 7189, they are affordable and sound great. I replaced my quad set of Telefunken 7189's with them!

Hi Dave,

I too agree with Kijanki. While the chances of damage occurring as a result of running unloaded under no signal conditions are small, they are definitely not zero.

One example: Suppose one of the small signal tubes suddenly develops a problem such as a short during the process. That could put a "signal" through the amp, and chances are that with no speaker connected you would not have any indication that was occurring. That "signal" might persist for hours, until you concluded the variac procedure. Chances are that one or both output transformers would be damaged as a result.

The reason that particular example comes to mind is that when I purchased my present amplifier, together with a set of brand new tubes, after listening for about 2 hours, from one second to the next the amp suddenly went from producing beautiful music to producing a loud eruption of continuous static-type noise from one of the speakers. It turned out that a small signal tube had abruptly developed a short. There was no damage, of course, since a speaker was connected and because I shut the amp off within a few seconds. I subsequently found that the loud noise occurred even when no music signal was being sent into the amp.

If you want to use a resistor, this 8 ohm 20 watt non-inductive resistor from Radio Shack should be suitable, given that the intent is to use it under no signal conditions, and that your amps are rated at around 20 watts. Or you could order something comparable online from Digikey.com. One for each channel, of course. It would be prudent to touch the body of the resistor from time to time during the procedure, to make sure it isn't getting hot. Any sign of warming, beyond what might be caused by heat from nearby tubes, would indicate that the amp has developed a problem.

Best,
-- Al

Normally Scott amplifier design was pretty forgiving and no load was required. I would not go with that now. If the controls are clean there are probably no worries, but if there is a problem and your output transformer pays the price, then a load to prevent arcing would have seemed like a good idea.

However if its got new caps I would just plug it in with speakers and see how it goes. You might need to run it a day or so before you take it seriously!

I've worked on Scott amps and they're the easiest tube amps to use. Even with no restoration if the main filter caps do not show the signs of leakage, you may b OK to just turn on. As previously mentioned, use the load resistor at least if you're affraid to damage your speakers, but will less-likely be the case.

I usually do not load the output of the amp unless there's an issue with the amp i.e. coupling caps leaking etc.. What I do however is to place shorting plugs at the input.

Thank you gentlemen for your responses.

All you guys are among my favorite reads.

As suggested, will forego the variac and hook up the AR 7s and give it a whirl! And if the 7s develope a problem, there are always the Boston's!!!

All kidding aside, I'll give it a try this weekend. Will report back afterwards.

Thanks again,

Best

Dave

7189 Telefunkens are great choice for this unit. They bring a few more watts to the output power vs. EL84's.

Agree with Marakanetz. I have used both telefunken 7189 and russian NOS 7189 and there is no comparison. The Telefunken's made the Scott sing!