Standby or Not

[jaffro]

Has anyone ever seen a standby on a hi-fi amp?
This has always dumbfounded me because I had learned, like others apparently, how necessary standbys are. Hi-fi people are the guys and gals who really freak out about tubes spending thousands and thousands on their gear obsessing on every little detail. They make guitar amp lovers look like the poor relations in this tube world and yet, I'll ask it again, has anyone ever seen a standby on a hi-fi amp? Why not?
On the Valve wizard site he has an interesting and definitive take--
http://www.freewebs.com/valvewizard1/standby.html
If it is stated directly in the tube manufacturer's manual that it's unnecessary what else is there left to say?

[Ears]

You beat me to providing that link jaffro, , here's a teaser as to the Wizard's opinion: "[cathode stripping is] an urban myth borrowed from transmitter and cathode-ray tube technology NOT ordinary 'receiving' valves"

[tonestack]

Yep. And yet the tweed Deluxe amps never had a standby..

Cathode stripping is not an issue at the voltages at which cathode-biased amps operate. Cathode stripping is a problem in tube-type RF amplifiers; however, tube-type RF amplifiers tend to operate at B+ voltages that make guitar amplifiers seem harmless.

[Chappy]

But that depends on what type of tube rectifier it is.

With directly heated cathodes (where the heater is the cathode) you will get high voltage rather quickly where as an indirectly heated cathode will take longer and will delay the high voltage to the tubes until it has warmed up sufficiently.

I have an old Bogen Challenger PA amp that I converted into a guitar amp. It did not have a standby switch and it used a 5U4 rectifier and 6LGG tubes in the output (6L6G's are rated at 400V max). I put my digital voltmeter on it to check the voltages. When I flipped it on the B+ shot up to 530V. OUCH. After about 10sec the rectifier tube would warm up and the voltage would settle back down to about 420V.

The amp now has a standby switch to warm up the filaments prior to putting B+ on my power tubes. Regardless of cathode stripping, I don't think that it is a great idea to put 530V on the plates of my vintage Tungsol 5881's every time I start up the amp.

Chappy

Fella's

Cathode stripping aside for the moment. What about my point above. 530 volts on the plates at startup until the rectifier warms up. Is this causing any extra wear and tear on the tubes?

Chappy

[Ears]

Fella's

Cathode stripping aside for the moment. What about my point above. 530 volts on the plates at startup until the rectifier warms up. Is this causing any extra wear and tear on the tubes?

Chappy

Hang on a minute, you made the statement, "Regardless of cathode stripping, I don't think that it is a great idea to put 530V on the plates of my vintage Tungsol 5881's every time I start up the amp."
So you tell us why you don't think it's a good idea?

[Ears]

[Chappy]

I don't think it is a good idea because the tube data sheets for my 6L6G's and my 5881's state that the maximum plate voltage is 400V. So every time that amp was powered up the max plate voltage was being exceeded by a fair bit. I'm not sure if this is damaging to the tubes over many startup cycles or not. I was hoping that someone with more knowledge/experience would chime in and clarify whether this is an issue or not.

Chappy

[Ears]

My guess is, cathode stripping aside, the only major issue would be the possibility of internal arcing, and that much the same considerations would then apply whether the tube is at operating temperature or if it's cold.

[tubeswell]

... if the amp has a tube rectifier, since they take some time to fire up anyway, a standby is less critical.

Yup.

A SS rectifier does tend to slam the filter caps with a big in-rush surge of power too.

[Merlinb]

I don't think it is a good idea because the tube data sheets for my 6L6G's and my 5881's state that the maximum plate voltage is 400V. So every time that amp was powered up the max plate voltage was being exceeded by a fair bit.

The max rating is for full temperature operation. If you look at tube uprating curves (which are not widely published I admit) you'll see that the max voltage can be doubled if the dissipation is zero, because voltage breakdown of insulating components is a function of temperature, for most receiving valves anyway. This is why many 6V6s are widely known to take higher voltages than specified, because we usually use them in Class AB at 60-70% of max dissipation. Your 5881s are at no risk (but underrated smoothing caps might be...)

[Chappy]

I don't think it is a good idea because the tube data sheets for my 6L6G's and my 5881's state that the maximum plate voltage is 400V. So every time that amp was powered up the max plate voltage was being exceeded by a fair bit.

The max rating is for full temperature operation. If you look at tube uprating curves (which are not widely published I admit) you'll see that the max voltage can be doubled if the dissipation is zero, because voltage breakdown of insulating components is a function of temperature, for most receiving valves anyway. This is why many 6V6s are widely known to take higher voltages than specified, because we usually use them in Class AB at 60-70% of max dissipation. Your 5881s are at no risk (but underrated smoothing caps might be...)

I've never heard of "tube uprating" curves. Thanks, I'll have to check them out.

Chappy


Chappy

[Structo]

Is it ok if I use a standby switch on all of my amps?
Thanks

[Merlinb]

Just to provide a reference, The Radio Designer's Handbook 4th ed. mentions stand-by operation on page 84.

[C Moore]

I'll cast an unpopular vote.
I always use the SB switch...... as a soldering post. I'll join ranks with the "I have never seen any proof" camp (small though we are). I would bet that if you were to buy two Fender DR's from GC, played and turned them on/off several times a day for a year, (while using the SB on one and only the power switch on the other), there would be no "cat stripping" in either amp.
I will further stick my neck out by saying I equate the SB switch with:
1. Black cat crossed your path.
2. Never walk under a ladder.
3. Broken mirror brings seven years of bad luck.
4. Lee Harvey Oswald acted alone, or even at all.
5. Preheating an oven???????

[Andy Le Blanc]

If you gonna invest $1500 into a set hand made 300b current production
tubes from KR or euro audio team....
your gonna have an amp with an automated power cycle and take no risks.
The operation for Hi Fi doesn't pound the tar out of tubes like a guitar.
I know guys who get 4 to 6 months out of a set of tubes in a guitar amp
and are happy,
and I ve seen cathode biased amps assembled in 52' running like new on
the original tubes. If your happy with a few months to a year out of a set
of tubes, the market is already flooded with crap tubes just for you,
but if your expectation is on the scale of years to decades use the stand by and
give the amp a good long warm up, every time.

Go to the gig and turn the amp on once, have a beer ( two is better), before
you throw the SB, and even if it don't matter you'll a least have a couple beers.