Too much Stand by

[Middleman]

Last night I flipped off my Vibrolux but missed the off switch and left it on standby. I was in a hurry and didn't notice that the light was still on. This morning I discovered that the amp had been left on for about 10 hours. When I flipped off standby a loud deep hum was coming through the speakers with nothing even plugged in. Not a subtle hum either but very loud.

Looks like something broke down internally while on standby for 10 hours. I checked all the tubes and they are working, reseated them but no change.

Any thoughts on what may have occurred? Amp was working fine with no problems prior. Another piece of data, I switched the bright switch on and the hum stopped then the amp crackled and it started up again. Seems like it may be components prior to or around the 7025 tube on this channel. Any thoughts would be appreciated.

[jefrs]

Thread re- standby switch http://www.tdpri.com/forum/amp-centr...by-switch.html

re- Hartley Peavey's article, see 'sleeping sickness' on p.7 of http://www.peavey.com/support/techno.../chapter_6.pdf

You could have a valve failure rather than another component, maybe you woke the valve up. A bright switch usually just cuts in one cap around the vol pot.

[Middleman]

Really good data. I have an extra rectifier tube and will try that before pulling out the meter and soldering iron. Much appreciated.

[Middleman]

Ok, wild beyond imagination in routing out the culprit.

I changed rectifier tubes. No change
I swapped channel 1 tubes with channel 2 tubes. No change.

At one point it suddenly went a little less loud so I plugged in a guitar cable. Wiggling the cable while in the jack suddenly sent it through the roof in volume. Hmm maybe bad resistors on the jack.

I unsoldered the wire going from the jack pair on channel 2 to the channel 2 7025 tube. Then I unsoldered the same assembly from Channel 1 and soldered it to the channel 2 7025 tube. The amp was suddenly fine. Quiet.

So it comes down to either one or both of the 68k ohm resistors or the 1M resistor attached to the channel 2 input jacks. That's an easy replacement.

Weird science. Ordering the parts now.

[redstringuitar]

Originally Posted by jefrs

re- Hartley Peavey's article, see 'sleeping sickness' on p.7 of http://www.peavey.com/support/techno.../chapter_6.pdf

Everyone who uses a valve amp should read this article.

[Wally]

Regarding cathode stripping. I have always been an advocate of using the stand by to warm up the tube elements before pplying full voltage. IN the recent months, the internet has opened up this 'can o worms' for me. IT seems that when one gets to studying the effect of cathode stripping one finds that it occurs only at very high voltages....10K or more. I have read more than one article that reveals this. Her is one...
http://yarchive.net/electr/tube_time_delay.html Read a few paragraphs into the article.
You can google more in depth if you want. I have done 3 searches at different times and found different articles that point to the same reality...it takes 10K volts to strip the precious metal from those cathodes.
I still use the stand by, but I don't worry if a guitar amp doesn't have one.

[jefrs]

Indeed, the standby is there to warm the valves up before applying HT.

If you have a GZ34/5AR4, it seems they have a slow start (by design) and don't need no standby switch anyway. The same may not be true of solid-state rectifiers.

It is not there to silence the amp whilst it is idle, apparently prolonged use for that will damage the valves.

Today's lesson:
how to pull your jack plug so the input is shorted and shuts the amp up,
or,
how to turn the volume knob down to zero when you are not using it.

[jefrs]

Originally Posted by Middleman

Ok, wild beyond imagination in routing out the culprit.

I changed rectifier tubes. No change
I swapped channel 1 tubes with channel 2 tubes. No change.

At one point it suddenly went a little less loud so I plugged in a guitar cable. Wiggling the cable while in the jack suddenly sent it through the roof in volume. Hmm maybe bad resistors on the jack.

I unsoldered the wire going from the jack pair on channel 2 to the channel 2 7025 tube. Then I unsoldered the same assembly from Channel 1 and soldered it to the channel 2 7025 tube. The amp was suddenly fine. Quiet.

So it comes down to either one or both of the 68k ohm resistors or the 1M resistor attached to the channel 2 input jacks. That's an easy replacement.

Weird science. Ordering the parts now.

None of those resistors a likely to blow, because they don't carry any big current. The cable and/or the jack socket are likely to develop faults because they are "moving parts". Check the lead out (try another). Check the jack socket is making full contact. Test the resistors with meter, re-solder their joints.

[morroben]

Interesting stuff. I must have some serious "cathode poisoning" in my tubes. My 5E3 is on standby for hours at a time sometimes.

[Middleman]

Originally Posted by jefrs

None of those resistors a likely to blow, because they don't carry any big current. The cable and/or the jack socket are likely to develop faults because they are "moving parts". Check the lead out (try another). Check the jack socket is making full contact. Test the resistors with meter, re-solder their joints.

In retrospect the hum sound was similar to plugging in a guitar cable and then touching the tip of the plug at the other end. I don't know why standby would have revealed that issue unless, when I pulled the cable out, something broke in the jack set up or (this amp is 28 years old) a resistor finally degraded to the point that being on all night just took it down. Since things are working fine with the other harness wired to the same pin on V2, I am just going to order all new parts and rebuild the jack setup, including the wire that leads to the tube.

[Middleman]

Circling back to report that replacing all the resistors above and also the jacks for the vibrato channel solved the problem. Amp is quiet as a mouse.

[Wally]

Middleman, I am going to think that it was coincidental that the shorting jack went 'open', and the standby situation just clouded the issue. IF the noise went away when a cord and a guitar were plugged in, the shorting jack was 'open'. Rebending the shorting tab or replacement of the jack cures this.

[Middleman]

Originally Posted by Wally

Middleman, I am going to think that it was coincidental that the shorting jack went 'open', and the standby situation just clouded the issue. IF the noise went away when a cord and a guitar were plugged in, the shorting jack was 'open'. Rebending the shorting tab or replacement of the jack cures this.

Yeah, that's a close summation except that the noise did not go away with the chord inserted. It just got louder. The amp had not been on this long possibly ever since 1981. I used to use it every night for years 5 hours a night in clubs. I do agree that it may have been coincidental. Possibly the resistor just degraded and the long hours it was running brought the problem to the surface.

On another note, I took a suggestion from the tube amp book and removed the channel 1 preamp tube. I never use that channel and the book indicated you get a little more responsive playing with it out. That is indeed the case and I am going to save the other tube for a future burnout.

[jefrs]

Middleman, glad you sorted the problem. I suggest Wally was right that the socket had been bent open, that will make it hum and it will hum worse with a lead in, and quiet down with a guitar on the end. Sockets do break, a good sideways tug on the lead will do it. If those components are near a valve, cooking them overnight doesn't help.

Re the valve, by removing it, I suggest you are increasing the rail voltage on the second channel by doing so, by lessening the current load (that drawn by channel 1), channel 2 will get a few more volts. A similar effect may be had by altering the anode or cathode resistors - compare a few schematics.

One should however be cautious about pulling random bottles out of an amp, and be very leery about lifting them from the output stage unless you are very sure about what you are doing.

[telemania]

I have a 5U4 rectifier. Been playing for many years - but this discussion on the standby switch is a first for me. Thanks Jefrs for the Peavy post, very enlightening.

So, I have a slow to warm up rectifier - what do I do about it on:
1. Powering up for the first time
2. Taking a 15-20 minute break
3. Powering down

Second question - and I hope it's not a hijack... Can I just switch to a 5AR4 rectifier? I do know from this thread (http://www.guitaramplifierblueprinti...ectifiers.html) that aside from a solid state rectifier the 5AR4 has the quickest response time...

[Wally]

telemania, regarding the switch to the 5AR4/GZ34 form the 5U4. The 5AR4 draws less current so that will lighten the load on the PT. The voltages will go up, so that could be a consideration depending on what amp you are talking about and what tubes you run in it. You would want to rebias or at least check the bias if the amp is fixed biased.
REagarding the 'when to use the stand by'.....
As I noted above (anybody read the link? OR....do a search on the subject?), if your amp has 10K volts on the B+, you might want to use the standby to stop cathode stripping. LOL What I have found in reading is that these guitar amps don't deal in voltages that are high enough to warrant much concern on this subject....in other words, it is not necessary to warm the amp up. I still do it out of habit since I was taught that the standby warm up would deter cathode stripping, but now I don't worry if an amp doesn't have a standby or if I don't use it.
Cooling down? I think that shutting the amp off...power and standby... is good.
Break? I am of the opinion that keeping the tubes at full operating voltage probably keeps things stable and consistent.

[telemania]

Thanks Wally. I have a Torres Super Texan. Switched from EL34s to KT88s w/out rebiasing (I have a fixed bias)...so it would not be out of order to check the biasing when switching to a 5AR4 (should have done it on the KT88 mod). As for the P1/P2 tubes I think they can take the increase in voltage (P1 is a JAN 5751 and P2 is a JAN 12ax7).

So, in the end Middleman - the standby issue was a red herring. Glad you found the solution, but thank you for the post as I learned quite a bit today.

[limbe]

I have never had a standby switch on any of the tube amplifiers I have bought(Fender,Vox)or on the ones I´ve built myself.I admit that I never go beyond the tubes maximum rating when I build an amplifier .The transformers I use are also big enough so they can handle the current without getting hot.I use silicon diodes because A:If you don´t buy transformers specially designed for guitar amps who are more expensive than"normal" transformers you usually don´t get a 5 Volt winding and B:You can add more filtering if you wan´t to.If you want "sag" there are other ways of getting it.I can honestly say that I never have had any tube problems that I can attribute to the lack of a standby switch.Sure,I change tubes when the amp starts to sound a bit "tired",but that´s about it.Limbe