Demystifying Tube Amps---The Standby Switch

[Wasnex]

Running the screen voltage as high as possible gets you also the highest possible power gain from the output tubes. Some amp designers will trade tube life for power by keeping the average screen and plate dissipation within limits, and allowing it to exceed them on peaks like note attack.

As the screen voltage goes higher I think you have to be more conservative. The Matamp is running the screens over 500V with 1K screen resistors. I am running VA EL34Bs which I believe have a 500V screen rating. FYI JJs are rated for 450V max on the screens, so probably not a good choice of tube for this amp.

Consider my Hiwatt DR201 with 711V on the plates. I don't think the best EL34 would last long with 700V on the screens, so the amp has a dual rail power supply and runs the screens under 500V. The schematic say 410V, but I suspect my amp is running closer to 450V. I think it is wise to run a conservative setup with this amp, but the screens are probably not the biggest problem here. 711V Yowsa!

 

[catalin gramada]

As the screen voltage goes higher I think you have to be more conservative. The Matamp is running the screens over 500V with 1K screen resistors. I am running VA EL34Bs which I believe have a 500V screen rating. FYI JJs are rated for 450V max on the screens, so probably not a good choice of tube for this amp.

Consider my Hiwatt DR201 with 711V on the plates. I don't think the best EL34 would last long with 700V on the screens, so the amp has a dual rail power supply and runs the screens under 500V. The schematic say 410V, but I suspect my amp is running closer to 450V. I think it is wise to run a conservative setup with this amp, but the screens are probably not the biggest problem here. 711V Yowsa!

DR201 have the screens supplied at 400v as I remember. This arrangement put the amp straight into B class operation, different story than you Matamp are you talking about
I.ll bet yo have no more than 380v on the screens of you Matamp at nominal power. You have 500v at iddling when the screens draw an insignificant amount of current which mean nothing ,3-5mA or so...

 

[Wasnex]

DR201 have the screens supplied at 400v as I remember. This arrangement put the amp straight into B class operation, different story than you Matamp are you talking about
I.ll bet yo have no more than 380v on the screens of you Matamp at nominal power. You have 500v at iddling when the screens draw an insignificant amount of current which mean nothing ,2-3mA or so...

I believe the Matamp would be more problematic if it was really pushed into hard clipping for a long time. My understanding is with this plate/screen setup the plate voltage can dip below the screens, and when this happens the screen dissipation goes through the roof. I have tried pushing the amp hard with bass and it is among the best sounding amps in my collection for this. It transitions very gracefully into OD, the distortion has a nice dark quality, and the low end doesn't wash out until insane levels of saturation have been reached.

I don't have any idea how long it can cope with the overload. I only pushed it that hard to compare the OD characteristics with my DR103. Obviously the Matamp won that battle. But people seem to think the Matamp is pretty hard on tubes. Then again people seem to say that about a lot of amps.

 

[catalin gramada]

I believe the Matamp would be more problematic if it was really pushed into hard clipping for a long time. My understanding is with this plate/screen setup the plate voltage can dip below the screens, and when this happens the screen dissipation goes through the roof. I have tried pushing the amp hard with bass and it is among the best sounding amps in my collection for this. It transitions very gracefully into OD, the distortion has a nice dark quality, and the low end doesn't wash out until insane levels of saturation have been reached.

I don't have any idea how long it can cope with the overload. I only pushed it that hard to compare the OD characteristics with my DR103. Obviously the Matamp won that battle. But people seem to think the Matamp is pretty hard on tubes. Then again people seem to say that about a lot of amps.

I never saw one but from recordings I have to agree. Congrats!

 

[CryingBass]

Great post guys. Thanks

 

[Camaro69]

Great post guys. Thanks

I’ll echo that sentiment, by far one of the more fascinating conversations on the site!

 

[okcrum]

As the screen voltage goes higher I think you have to be more conservative. The Matamp is running the screens over 500V with 1K screen resistors. I am running VA EL34Bs which I believe have a 500V screen rating. FYI JJs are rated for 450V max on the screens, so probably not a good choice of tube for this amp.

Consider my Hiwatt DR201 with 711V on the plates. I don't think the best EL34 would last long with 700V on the screens, so the amp has a dual rail power supply and runs the screens under 500V. The schematic say 410V, but I suspect my amp is running closer to 450V. I think it is wise to run a conservative setup with this amp, but the screens are probably not the biggest problem here. 711V Yowsa!

That's what you call running 'em hot! EL34s have one of the higher zero current voltage ratings of common audio power tubes. KT88s too, both are somewhere in the neighborhood of 2 kv. You can operate good tubes of either type at 700-800V as long as you keep an eye on the power dissipation.

I run pretty hot screens (~450 volts under load, ~500V quiescent) on my home brew amp with JJ 6CA7s, but I've got the screen dissipation limited so it doesn't eat tubes, and my plate voltage under load is lower than your Hiwatt (~560). I didn't want to go so close to plate voltage with the screens that I lowered the plate resistance (and the damping factor and bandwidth) significantly. The amp still compresses nicely and has a good punch. What you definitely want to avoid is for the screen voltage to go higher than the plates.

 

[bassdude51]

Poor choice of language. In most instances, putting an amp in standby involves putting the switch to the off/open position, and that is the case with the 400+. One caveat, you have to turn an amp on for it to be in standby mode. Otherwise the amp is simply off. Standby means the amp is ready to operate, but it is not operating. After the tubes have warmed up, move the standby switch to the operate position.

Setting the amp to operate mode is the same as taking the amp out of standby.

I have the schematic for the 400+ power supply. It looks like the tube heaters and negative supplies are energized when the amp is in standby. The standby switch breaks continuity after the bridge rectifier and main filter caps, so none of the positive voltages are supplied to the circuits. When you close the standby switch (or take the amp out of standby), the voltage is provided to a voltage dropping ladder formed by a choke and a 10K resistor to supply 480V and 340V to the circuity.

WOW! And I think I am a "tech head"! Sorry for my "poor choice of language". I've only been speaking English for 67 years.

OK..............hand typed right from the Mesa 400+ manual. (Trying to redeem my poor language choices.)...........................

Before POWER is switched on make sure the STANDBY switch is in the STANDBY position. Wait at least 30 seconds and then push the STANDBY to the ON position. This prevents tube problems and increases their toneful life substantially. For more information regarding tube maintenance, see the specified section further back in this manual.

 

[agedhorse]

Well, you are the authority on amps! Thing is, my wall volts tend to vary and usually above 120 not below...as well, slightly lower wall volts tends to tame the sharp top end on some of my amps.

Tonight, the wall volts are around 122-123 range measured with a Fieldpiece SC66 and a few other meters.

I might know a thing or two since that's my job.

Amps are (supposed to be) designed to handle the rated nominal AC voltage plus/minus roughly 10%.

Because many products (especially in that era) were designed by the seat of the pants without full recognition of the safety issues associated with such decisions, regulations and safety agencies came into being to test and verify that unsafe conditions are not designed into products like amps.

The good old days were full of poor designs (from a safety and reliability perspective), things are indeed different today.

That said, tolerances on parts like caps and tubes may have been more liberal to accommodate such design sloppiness, so some of this way average out.

 

[Mark Reccord]

I think another potential problem, especially with high powered amps like the SVT is that the plate voltage for the power amp is 600V+ and the HT supplies for the preamp tubes are dropped to the 300V range with a resistor. If you slam the HT on when the tubes are not able to conduct, the plates of the preamp tubes see the full plate supply until the tubes warm up a bit because without current there's no voltage drop across the HT dropping resistor or the plate resistors. Can't get around V=IR. And this is made even worse as with no load the HT voltage will be higher than it is under operation because of the series resistance of the transformer. Not as big a deal if the preamp tubes have their own winding on the power transformer or if there are no power tubes. Am I thinking about this correctly?

 

[iiipopes]

Wow! With six pages to this thread already, it appears not much has been "de-mystified."

 

[StatesideRambler]

Wow! With six pages to this thread already, it appears not much has been "de-mystified."

Go back and read.

 

[catalin gramada]

That's what you call running 'em hot! EL34s have one of the higher zero current voltage ratings of common audio power tubes. KT88s too, both are somewhere in the neighborhood of 2 kv. You can operate good tubes of either type at 700-800V as long as you keep an eye on the power dissipation.

I run pretty hot screens (~450 volts under load, ~500V quiescent) on my home brew amp with JJ 6CA7s, but I've got the screen dissipation limited so it doesn't eat tubes, and my plate voltage under load is lower than your Hiwatt (~560). I didn't want to go so close to plate voltage with the screens that I lowered the plate resistance (and the damping factor and bandwidth) significantly. The amp still compresses nicely and has a good punch. What you definitely want to avoid is for the screen voltage to go higher than the plates.

And hopping we talk the same language (as I.m not a native language speaker btw.) The screen voltage is not what is measured in the top of its supply decoupling cap (this is the screen supply point), but the voltage present after the limiter resistor right on the tube screen. To say the screen voltage is xxx V it have no absolute sense in actual context because in fact the screen voltage is anywhere between its iddle voltage and the voltage resulting after substract the voltage dropped on screen limiter resistor. Eg: if the screen at iddle is 450v and at full throtle 350v (meant 100v will be dropped over the limiter resistor) meant the screen voltage is anywhere between those values during tube operation, and is a very dynamical characteristic. You can mention the screen voltage just for a point of its operation because the screen voltage is not regulated. Add the dropped voltage over the B+ supply rail during the operation. This do the tube operation very non linear but good enough for geetar aplication. On the other hand hi-fi amps have voltage regulators in this position which keep the screens voltage stable. In this case we can talk about a screen voltage cause those voltage will be consistent over the whole operation of the tube.
Keeping the screen supply voltage to a low value if will help to keep the tube more linear cause the voltage will not swing so much over the limiter resistor. An other way is to use not individual screen resistors but a half value common one. As time it works in a push-pull circuit one tube will draw more the other less keeping balanced more consistent the dropped voltage over the limiter and in consequence over the screens. This was used as Mullard patent in Hiwatt 100 circuit.
So it have to do a difference between supply screen voltage and screen voltage. What is determinant for unregulated screen voltage it is screen voltage as it is base of calculation for the power dissipated by the screen in respect with current draw. Of course voltage will be not affected at small amplification level the serious current will be drawn at high level of amplification
Nothing related with stb sw.

 

[logdrum]

Go over to thegearpage.com and you can start a 200 page thread easily with this debate. I use it although I have amps and amps I made myself which does not use standby switch.

 

[catalin gramada]

Go over to thegearpage.com and you can start a 200 page thread easily with this debate. I use it although I have amps and amps I made myself which does not use standby switch.

I did it also when have no potential danger for the circuit.
Right now I have a Dynacord Bassking on my bench which I converted to dc coupled cathodyne inverter. The amp have no stb sw. I put a diode protection and elevated the heaters and have not concerns about.

 

[logdrum]

I haven't seen any relay. The tubes just have to be warm enough for it to amplify. Randall Smith (Mesa Boogie) knows a thing or two about bass tube amps (Ampeg too but the original founders are long gone) and he recommends it. In fact his latest and probably last tube bass amps, he recommends a longer warm period, instead of the 30 seconds or so, he has extended it to 5 minutes if I recall. I have the Prodigy 4:88. Usually single ended class A do not have it but my THDs have it.

The Mesa hifi tube amp I have (BAron) has some sort of soft start which could probably mean a relay

 

[easyj]

Wow. That article completely contradicts everything I’ve been taught and everything I’ve read. I’ve been carefully using standby mode for years to warm up my amp. Are we sure this information is correct? If it is, what a colossal waste of time for all of us to be concerned about nothing. I’d really like to read more about this point of view.

 

[catalin gramada]

I haven't seen any relay. The tubes just have to be warm enough for it to amplify. Randall Smith (Mesa Boogie) knows a thing or two about bass tube amps (Ampeg too but the original founders are long gone) and he recommends it. In fact his latest and probably last tube bass amps, he recommends a longer warm period, instead of the 30 seconds or so, he has extended it to 5 minutes if I recall. I have the Prodigy 4:88. Usually single ended class A do not have it but my THDs have it.

The Mesa hifi tube amp I have (BAron) has some sort of soft start which could probably mean a relay

Just talking to you thinking to another reason for delayed start.up not quite a stb sw but more relay assisted as follow:. Supposed the mains will break down by fault and is restarted in seconds again .The warmed power tubes will conduct instantly whilst a well decoupled bias circuit will start to charge. (Not good)

 

[JimmyM]

I haven't seen any relay. The tubes just have to be warm enough for it to amplify. Randall Smith (Mesa Boogie) knows a thing or two about bass tube amps (Ampeg too but the original founders are long gone) and he recommends it. In fact his latest and probably last tube bass amps, he recommends a longer warm period, instead of the 30 seconds or so, he has extended it to 5 minutes if I recall. I have the Prodigy 4:88. Usually single ended class A do not have it but my THDs have it.

The Mesa hifi tube amp I have (BAron) has some sort of soft start which could probably mean a relay

It's not so much that you SHOULDN'T use the standby, just that there's no great mystery about it and no great harm will come to your amp or tubes if you use it or don't, unless you leave it in standby for a very long time.

And the 5 minute thing is more about getting them warmed up as much as they can possibly get warmed up so they sound good, not the minimum for tube safety.

 

[catalin gramada]

Wow. That article completely contradicts everything I’ve been taught and everything I’ve read. I’ve been carefully using standby mode for years to warm up my amp. Are we sure this information is correct? If it is, what a colossal waste of time for all of us to be concerned about nothing. I’d really like to read more about this point of view.

I think it was covered all the reasons for potential concerns but also for imaginary concerns and some off topic in between, sorry man.