I tried it out, after about 5 minutes or so, it doesn't matter anymore how much of additional time you may spend for "warm up"
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6550 Tubes for Ampeg SVT
- Thread starter Slpbass81
- Start date
But not at once, it takes a couple of minutes (give or take) till the bias is down again.
240V is the same as 120V here, instead of the the two primary coils being run in parallel for 120, they're run in series for 240. All you have to do is move a wire, problem is, I forgot which ones.
Techs sometimes did it that way to save time. They’d do it quickly to verify the bias to ensure that it is within the safe operating region. Typically this was with low wattage amps. I wouldn’t recommend biasing with a shorted load with a 300W amp.
...for about 2..3mV ???Stuck in my ways.
I think, it doesn't hurt anything, and was out of concern.
I would not be surprise to see a little bit of drift if you are really cranking the amp for a long time. The only amps I push to their limits are my small Ashdown and Ampeg tube amps. They all seem to hold the bias pretty stable even when pushed into drive territory. So while I won't argue, I am not really observing a lot of drift that is not attributed to variation in the line voltage.
As long as there is no signal running through the amp, the amplifier can't distinguish between a "short" and a 4 Ohm load.
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No, it's almost instantaneously. Again I am not saying that drift does not occur, but I am not observing it in my amps. Maybe I am not pushing them hard enough for long enough.
Spikes can happen that can be induced within the amp. For instance, if a nearby motor starts up or an impulse in the mains power due to an external event. It makes sense to not risk going without a load. If a tech breaks it, he has to pay to fix it.
IMHO, letting the tubes stabilize before setting the bias is a good practice, but 15-20 minutes is usually long enough for my amps.They'll drift a little for at least a half an hour, I have the time, I'm retired.
One observation, some tubes pull more current as the warm up, and some pull less. JJ KT88s go one way fairly consistently and Shuguang KT88 go the other way.
Geez, with all these numbers and measurements floating around here, I’m just glad that “loud as begeezuz” works for me….
All the SVTs I run into is the occasional backline rig. So far, except for varying degrees of hum that wasn’t show-stopping nor in my REDDI DI line to FOH, they all seemed to work quite decently enough. And fun to come across after not owning one for 3 years now….
All the SVTs I run into is the occasional backline rig. So far, except for varying degrees of hum that wasn’t show-stopping nor in my REDDI DI line to FOH, they all seemed to work quite decently enough. And fun to come across after not owning one for 3 years now….
Those SVT's which are DIY modded with speakon connectors for the output jack, the amplifier sees then open load if nothing was plugged in.I wouldn't do it. I blew up a head one night, forgot to plug the cab in (two heads, shows I didn't need both
Right, you might damage the tubes rather quickly once you start playing while open load was presented to the amplifiers output.
But, the situation is different for the amplifier as long as the amplifier does not amplify any signal.
A couple of years ago I did some service to the preamp of my SVT, unfortunately I did forget to plug a shorting bridge plug into the return/pwr-amp jack to mute the power amp on its input.
Nothing was plugged into the pwr-amp-out jack! Thus pwr-out was shortened by internal connectors.
It may have taken about 20 minutes or so till I noticed that the power amp was not muted during I did service on the preamp with signal running throughout preamp and(!) power amp (cause power amp was not muted).
Nothing happened to the amplifier, no damage nothing at all, neither to tubes nor to anything else.
I'm pretty sure, if there would have been open load presented to the power amp instead of a "short" I would have easily ended up at least with damaged tubes, or even more of damage to other components as well.
The thing is, as long as NO signal was amplified then even an open load might(!) be still save for the amplifier.
The big problem is, it may happen that the amplifier starts to oscillate if the load becomes to too high for stable operation, respectively for sufficient phase margin on its negative global feedback loop. If phase margin becomes too weak, the amplifier starts to oscillate, and in worst case may destroy itself very quick.
It could be checked out if the SVT would stay stable/robust even with open load while its power amp was muted. Yet I never did.
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Mine are stock late 70's heads. I don't take shortcuts, prevents stupid mistakes. No offense intended.
Passinwind
I know nothing.
- Dec 3, 2003
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Umless they used a shorting Speakon panel connector, which did still exist the last time I checked.
I've never seen one in the flesh though.
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Thank's Charlie, and that's very good news to know.Umless they used a shorting Speakon panel connector, which did still exist the last time I checked.
Yet I don't think that I'll ever do an connector upgrade to my vintage SVT.
The stock phone jack is of very good quality, with pretty good contact pressure. Thus I see no need for any modding.
In real practice the average current draw is anyway way smaller than it was at 300 Watts with a sinusoidal waveform.
