Demystifying Tube Amps---The Standby Switch

[catalin gramada]

I've designed DC coupled cathode followers, and there's no difference in sound from the diode, because the cathode is swinging up and down along with the grid from the input signal. As you say, the diode is there to prevent the tube's grid from momentarily going positive on power-up, and degrading tube life.

The problem in dc coupling is at startup the grid will be at huge B+ rail potential, whilst the cathode will be at 0 potential as time the tubes are not heated. Such of high voltage diference (450 -500v) between grid and cathode can provide arcing, special in low u triode as 12at7, 12au7 where the grid are physically close spaced to cathode. When tubes are heated and start to conduct the plate voltage of driver tube will go down whilst the cathode voltage of the drived tube will go up resulting few volt difference between grid and cathode quite enough to bias the tube. A diode between grid and cathode will be quite enough to keep the voltage between g and k to a value low enough to be safe when the tubes not conduct (meant not warmed).The same author recommend a neon lamp to be used instead a diode which work as good as well. When the tubes will start to conduct will be out of the circuit. Extremely smart protection.

 

[JimmyM]

Why have a standby switch if it’s useless?

Nobody said it was useless. It's just not as important in the grand scheme of things to the safe operation of your amp or tube life.

 

[JimmyM]

One thing not mention is the Tubes that are made today. Most are Chinese or Russian, which have had problems
with quality control over the years. Switch or no Switch, a bad tube will flash over at any given time. I have used many a 572B with problems out of China. Good news, they seem to be fixing the problems. It has been argued for years, its better to run a tube constant then on /off stressing the filaments with inrush voltage and current.
Every thing has a life span. Electro-Harmonix anyone ?

By and large, QC control on new tubes is a million times better than it used to be.

 

[JimmyM]

Some years back I had a switch nameplate made which eliminated all confusion for two amps I’d built: in one position it read Play and in the other it read Standby. I probably got the idea from a commercial product.

The Standby switch controls the plate voltage supply (and usually, where there is one, the bias supply) and doesn’t affect the heater (filament) circuit which is, in all consumer designs I’ve seen, energized by the Power switch. The concern for stressing filaments with inrush current is misplaced because that is suitably addressed in the tube’s design. Just don’t slam the thing repeatedly with excess line voltage.

Now, if you were to address transmitting tubes we’re in an entirely different ballpark. Guitar, bass, PA and hi-fi amps do not use transmitting tubes so applying those guidelines is inappropriate and misguided.

Unless you have an early SVT that hasn't been converted to 6550's They use 6146b's, which are transmitting tubes.

 

[okcrum]

The problem in dc coupling is at startup the grid will be at huge B+ rail potential, whilst the cathode will be at 0 potential as time the tubes are not heated. Such of high voltage diference (450 -500v) between grid and cathode can provide arcing, special in low u triode as 12at7, 12au7 where the grid are physically close spaced to cathode. When tubes are heated and start to conduct the plate voltage of driver tube will go down whilst the cathode voltage of the drived tube will go up resulting few volt difference between grid and cathode quite enough to bias the tube. A diode between grid and cathode will be quite enough to keep the voltage between g and k to a value low enough to be safe when the tubes not conduct (meant not warmed).The same author recommend a neon lamp to be used instead a diode which work as good as well. When the tubes will start to conduct will be out of the circuit. Extremely smart protection.

Yep. In non-fault, non-cold start conditions, that few volts of bias will be negative, so the diode doesn't conduct, and the neon lamp won't light. The other factor to manage is not exceeding the heater-to-cathode voltage rating of the tube, which is almost always lower than the B+/HT voltage.

This approach is unique to common anode (cathode follower) configuration.

 

[Wasnex]

You can't strip a cathode with or without B+ applied in a receiving tube.

Cathode poisoning is a possibility though.

 

[Bboopbennie]

hey use 6146b's, which are transmitting tubes.

Jimmy - Yes 6146 /6Lq6 and the rest were horizontal sweep tubes in TV's when we were kids!! They later became power tubes for RF and audio amplifiers. Getting harder to find some of those older / odd tubes.
8950's were $ 15.00 years back. Now $ 120.00 used. Be kind to your Tubes !!$ !!

 

[SactoBass]

I've been playing bass though tube amps since 1972. Here is what I have always done regarding usage of the power switch and standby switch:

• Turn power switch on
• Wait a minute or two
• Turn standby switch from standby to on
• Play music
• (If we take a break, I temporarily flip the standby switch to standby)
• When totally done playing, I turn off both switches at the same time

For that last bullet, it's actually fine to just turn off the power switch, but turning off both puts the standby switch in the standby position so that it's already in that position the next time around.

 

[Wasnex]

Unless you have an early SVT that hasn't been converted to 6550's They use 6146b's, which are transmitting tubes.

True, but the 6146 really wouldn't meet the criteria he is talking about. He means transmitting tubes like klystrons that have several thousand volts on the plates. I believe such extreme voltage is what is required for cathode stripping. At least that's what this article says: The Valve Wizard

 

[okcrum]

Cathode poisoning is a possibility though.

True, given the circumstances of present day tube manufacturing. I've gotten cartons of a dozen 811s where only one or two even lit up.

 

[Ric Vice]

........He means transmitting tubes like klystrons that have several thousand volts on the plates.

Yes, like the Klystron monitor in United Planets Cruiser C-57D

https://getyarn.io/yarn-clip/3370aa31-efa2-451f-8265-f70ef58ac2ed#rk4eqByRYCB.copy

 

[xk49w]

By and large, QC control on new tubes is a million times better than it used to be.

I don't doubt there is better QC, what with automated production and so forth. However, I have worked on amps with new tubes that don't function in circuits that worked with old tubes. Specifically, 6550s. Grid leak current that exceeds the original specification to where the tube bias increases and, in a circuit that used to work fine, the tube thermally runs away and glows red - at idle. Or 400 kHz oscillation that could not be fixed with the usual pushing around of coupling caps, and relocating wires. It was only after the "OMG, this old amp doesn't have grid stoppers!" moment that I realized the problem. So now I add them to all my Leslie amp rebuilds. This happened with new tubes.

Maybe its a spot thing, but I'm just not confident that the new-ish, generic power tubes are up to what the old ones were. Now, this depends greatly on the circuit design into which the tube is expected to operate and there are, assuredly better designed amplifiers than the Leslie. However, my experience does give one or two data points showing that IMO some things have changed along the way.

 

[DavidEdenAria]

Ask The Amp Tech: Will I damage my tube amp by not using the standby switch? - Guitar.com | All Things Guitar

Folks who get tube amps for the first time usually have a whole lot of questions. Next to "How often should you change tubes," the most common question I see about tube amps is "How to use the standby switch." This article popped up on my news feed today, and I think it says it all.

It really doesn't matter. It's not brain surgery. Most audio devices with tubes have a relay installed that prevents you from being able to make sounds until the tubes are sufficiently warmed up, as playing through tubes that aren't warmed up sufficiently can damage them. However, occasionally you will run into one that doesn't. For example, I have an old Danelectro-made Airline guitar amp from 1962 that doesn't have a standby or a relay. So I have to remember to give it 30 seconds before it warms up before I play through it, and in most cases, that's enough time. They may not be warmed up fully, and you might notice an improvement in tone after 3 or 4 minutes, but it's enough to prevent damage.

After that, game on. I almost never use the standby, and if I do use it, it's for short durations when I'm plugging in or unplugging something in my signal chain. I also use it sometimes if I'm doing AB tests with another amp into the same cab. This prevents signal going through the output section of your amp while it's turned on without a cab, which we all know is a big no-no in the vast majority of tube amps as it can take out your output section in short order.

I've done shows where someone on the crew sees my amp with standby off when I'm not up there and they switch it on. Then I go onstage for the show and panic for 3 seconds till I realize someone turned the standby on. If you're on a crew, please just leave the amps alone so you don't give the musicians heart or panic attacks!

Anyway, read the article, feel free to comment or ask questions, but just know that there is no great mystery to the standby switch. You won't save tube life using it past the first 30 seconds of warmup, and while I think the risk of what they call "cathode poisoning" is greatly overstated if you leave standby on for hours at a time, it has been known to happen on occasion, so IMHO, you're just as well off to ignore it for the most part.

These old amps at todays wall volts present a challenge.....caps must be replaced if they are original and rebuild the bias circuit as well....still i use a variac set to 110 volts to be safe with say pre early 1960s amps.

 

[Downunderwonder]

Well that's just silly. Camp commonsense...pfft! Don't make me bust out Bernie Mac again...RIP.

Mesa and Ampeg are camped together in camp common sense. Invoking the ghost of Bernie Mac is unlikely to change their minds.

 

[CryingBass]

Horse - it’s killing me you have not chimed in on this yet.

Tubes are good for popcorn too...

I paid list price for an 8:88. Call me a woos, but I am sticking to the manual.

 

[agedhorse]

When using tube amps, I've always used the standby switch when going on break. It prevents the instrument from spontaneously beginning to feed back. And, as one who is a little compulsive about turning off lights, it saves a little energy.

Which is very much appreciated by the sound guys

I just got what you guys meant...doesn't the relay usually prevent this from happening in most amps, though?

Few tube amps have a relay, though there are a few that have a time delay warm-up before the B+ (HT) is applied to the plates/screens. At one time, I had designed an amp that allowed the B+ to ramp up after 15 seconds, which was elegant but overkill. It did eliminate issues with high voltage being applied as an impulse to the plate circuit.

I guess but I'm really curious what kind of savings there is, now that you brought it up! Have to say I never even thought about it, and I'm a guy who shuts off lights constantly, too

The savings is probably about 50% when in standby. For a larger amp like an SVT, that's significant.

Standby doesn't turn off the heaters - that's where a bunch of the power you feed into a tube amp is going.

Some power is dissipated in the heaters, but about 2/3 is dissipated in the plate circuit (talking about the power amp portion of the amp). On a 6550 (per tube basis), the heater power is about 10 watts, the quiescent dissipation in the plate circuit is about 20+ watts, so for 6 tubes you will save 20 x 6 - 100 watts. The hidden savings is in the reduced operating temperature of the output transformer primary and the extension of tube life due to lower operating temperature while in standby.

The energy saving is so miniscule that you nedd nice lab equipment to measure it.
I.E. moot point.

I don't think so... see the above calculation.

OK, so whatever the thing that causes amps with tubes to not come on unless the tubes are heated up is what I'm talking about. I thought it was called a relay because of old SVT's that have a relay tube that delays startup for 30 seconds, and are usually replaced with a shorting plug in the socket.

Funnily enough, they still delay coming on for about 30 seconds on startup even without the relay tube.

It's the time required for the cathode to heat up enough to begin boiling off electrons.

One thing that irks me is the inconsistent standby labelling on amps. Some manufacturers label standby “on” as the amp is in playing mode. Others label “on” as the amp is in standby. It’s confusing.

Agreed.

These old amps at todays wall volts present a challenge.....caps must be replaced if they are original and rebuild the bias circuit as well....still i use a variac set to 110 volts to be safe with say pre early 1960s amps.

This is not necessarily true either. Many older amps were designed for 117V and 120V nominal operation and pose no problems whatsoever. The 1969ish schematic set I have shows the Ampeg SVT and B-15 were designed around 120V, The Oliver amps were 115/117V, and most of the Fender amps were 117V which later became 120V in the CBA and post-CBS eras, Musicman was all 120V, Rivera was 120V, Sound City was 115V, the old Trace V series was 120V. Even my Ampex service binder shows all amps built in 1956 were set up for 117V. None of these amps will have a problem with nominal 120V.

In fact, I'm having trouble finding ANY in the mainstream that are less than 115V.

If you go back to the 1930's and 1940's, you may find more, but there weren't many amps designed for bass back then.

There are certainly some, but in general they represent the solid minority.

 

[beans-on-toast]

Unless you have an early SVT that hasn't been converted to 6550's They use 6146b's, which are transmitting tubes.

It’s interesting in that the GE 6146 data sheet specifically talks about using standby if the amp is idle for over a certain period. They also discuss lowering the heater voltage when in standby. All this is for longer tube life.

Some transmitters have backup units that are powered up and left on standby so that they can be switched over in the event of a failure of the primary transmitter.

 

[JimmyM]

Mesa and Ampeg are camped together in camp common sense. Invoking the ghost of Bernie Mac is unlikely to change their minds.

LOL...well on the bright side, I still manage to figure it out.

 

[Downunderwonder]

LOL...well on the bright side, I still manage to figure it out.

Of course you figured it out, it's common sense!

 

[Wasnex]

This is not necessarily true either. Many older amps were designed for 117V and 120V nominal operation and pose no problems whatsoever. The 1969ish schematic set I have shows the Ampeg SVT and B-15 were designed around 120V, The Oliver amps were 115/117V, and most of the Fender amps were 117V which later became 120V in the CBA and post-CBS eras, Musicman was all 120V, Rivera was 120V, Sound City was 115V, the old Trace V series was 120V. Even my Ampex service binder shows all amps built in 1956 were set up for 117V. None of these amps will have a problem with nominal 120V.

In fact, I'm having trouble finding ANY in the mainstream that are less than 115V.

If you go back to the 1930's and 1940's, you may find more, but there weren't many amps designed for bass back then.

There are certainly some, but in general they represent the solid minority.

Some of the old British amps have problems on US main voltage. Early 60s Vox AC30s were tapped for 115V, but a lot of people recommend running them on 110V. I have a 62 or 63 and it biases perfectly at 90% on 110V with an 82 ohm cathode resistor. The original cathode resistor was 50 ohms. The DC operating voltages are still higher than the schematic lists. Heater voltage is 6.25V, which is slightly low but within spec. I run this amp on a variac, but the rest of my collection is setup for 120V. Some of them sound better at 115 though.

My 76 Matamp GT120 PT also runs really agro on US mains although the tap is labeled 118V. I bought it in Germany and it was still running all the original filter caps. It started having problems after a couple years here in the US. When I opened it up, I noticed many of the caps were erupting. I checked the voltages and they were over the cap ratings. Plate voltage at idle 516V on 120V line. I did a full cap job, upscaled some of the caps to a higher voltage rating, and also increased the size of the resistor feed power to the preamp voltage ladder from 10K to 16K. I believe old Orange amps have the same problem as Matamp on US mains.

I also have a couple of Hiwatts. I haven't personally worked on the DR103, but I did check the bias on the DR201. The PT tap is labeled 117V and the plate voltage is 711V at 120V line, which I believe is way above spec . The tubes are biase a little cold though and there is no adjustment.

I have a 70s 100W JMP and an 80s 100W JCM800, and they are both fine on 120V 60hz.