Hello! First post, and trouble.. (5F8-A tweed twin build)

[MikeW86]

I've lurked here a bit, but am having a problem with a new amp build that I'd like to see if I can get some guidance with. It's a 5F8-A high-power tweed twin assembled from a Mojotone kit. I checked filament and bias voltage before applying B+, and B+ straight off the rectifier tube (with no power tubes plugged in) was 450VDC.  Bias voltage at control grid pins was -49VDC at all power tubes.

Doing a full power up for the first time with a lightbulb current limiter and 4ohm speaker connected. Amp powered up, waited 30 seconds, then flipped the standby switch to 'on', fuse held, filter caps held, but after about 5-6 seconds I started getting a very loud low-frequency oscillation and immediatly unplugged the amp. I removed the preamp tubes, powered on again and got the same oscillation but much quieter. The pilot light and limiter bulb are flickering at the same frequency as the oscillation. It's steady, and about as fast as a drumroll, or a machine gun. I'm also getting very high voltage at the preamp tube plates, over 400VDC. I'm suspecting a short, since the limiter is glowing.

I unplugged everything and decided to leave it alone for the night. After I get home this afternoon I'll remove all tubes but the rectifier and check voltages according to the layout.

[sluckey]

Disconnect the NFB wire from the speaker jack (wire goes to the 56K on the board). Any better?

[MikeW86]

I'll give it a try when I get home. If it is the NFB causing the problem, then what? Change resistor value?

[sluckey]

Swap the OT primary plate leads to the sockets. They're blue and brown on the layout.

[MikeW86]

Swap the OT primary plate leads to the sockets. They're blue and brown on the layout.

I tried that last night, it made no difference.

[PRR]

> with a lightbulb current limiter

What exact type and size of lightbulb?

[MikeW86]

> with a lightbulb current limiter

What exact type and size of lightbulb?

200W incandescent, no LED or fluorescent. I tested it with a known working Fender HRD and it worked as it should (slight glow on startup, then fades as tubes warm up)

[MikeW86]

So removing the NFB wire from the speaker jack didn't help, and neither did reversing the OT primary wires.

I found that I accidently wired a 1M resistor to pin 5 on left two power tube sockets, when it should have been 1.5K, so I replaced it with the correct value resistor. I also rewired the filter cap board according to Rob Robinette's 5F6-A layout instead of Mojo's layout, and the oscillation is gone. My preamp voltages were the result of having no tubes in the amp when I measured, so nothing was dropping voltage. I measure again with just preamp tubes in and the voltages were in the ballpark of where they should be. I took the amp off the current limiter and powered it on- fuse held, no smoking, transformers stayed cool, and power tubes aren't redplating. I still haven't checked bias.

I can now play through the amp but the normal channel volume pot isn't working- it passes signal and is fixed at fairly low volume. The bright channel volume pot does adjust volume, but still passes signal at 0. The prescence, bass, middle and treble pots work as they should. I'm also getting some louder-than-normal 60 cycle hum, but that could just be a simple lead dress issue that I can correct with some chopsticking.

[MikeW86]

Well, I found my volume pot problem. I'm sure y'all can spot it faster than I did

[Willabe]

No ground wire. 

Happens to the best of them. There's a LOT of connections in that amp to solder up. 

Everybody misses things from time to time or makes a mistake, no big thing as long you don't blow something up ($$) or hurt yourself.

What's important is you found it. 

[sluckey]

May I suggest that rather than soldering to the back of the pots, run a short wire from the pot lugs to that ground buss on the board. IMO this is a much more reliable ground connection. Do this for both VOL pots and the MID pot. Presence pot should ground to the power ground.

[MikeW86]

That's what I'm going to do when I get home. The middle control ground is already on the bus, I'm not sure why I decided to solder the bright vol. ground to the pot.. But I'll fix it later.

[Willabe]

Here's a great way to check a new build for missing and/or wrong connections, from our host Doug;

https://el34world.com/Forum/index.php?topic=5.0

[MikeW86]

Update; amp works well, is quiet, and sounds great! All Voltages measure what they should.

I think I have a bad GZ34 rectifier though, it blew a fuse switching off standby, then did a little bit of internal arcing when I hit an open E chord with the amp pushed a bit. So I'm leaving it be until I get a new rectifier.

[sluckey]

I think I have a bad GZ34 rectifier though, it blew a fuse switching off standby, then did a little bit of internal arcing when I hit an open E chord with the amp pushed a bit. So I'm leaving it be until I get a new rectifier.

Move the two 22µF reservoir caps to the other side of the STBY switch, ie, so they will be connected directly to the 5AR4.

Or, maybe use a 5U4 or 5R4. They're a bit tougher than the 5AR4, but you lose the slow startup. Whatever tube you use, it will be happier if those caps are connected directly to the tube socket.

[MikeW86]

Move the two 22µF reservoir caps to the other side of the STBY switch, ie, so they will be connected directly to the 5AR4.

Or, maybe use a 5U4 or 5R4. They're a bit tougher than the 5AR4, but you lose the slow startup. Whatever tube you use, it will be happier if those caps are connected directly to the tube socket.
[/quote]

Okay. I was only blowing fuses when I would use the procedure of flipping power on, waiting 15 seconds, then flipping standby. When I ignored the standby switch it stopped, but I don't know if the periodic arcing was due to the tube already being damaged from the first time or not.

[tubeswell]

... I don't know if the periodic arcing was due to the tube already being damaged from the first time or not.

Do you have (minimum 1000V) back-up rectifier (protection) diodes installed in series between each HT winding end and each tube rectifier plate? https://www.premierguitar.com/the-immortal-amplifier-mod

[MikeW86]

... I don't know if the periodic arcing was due to the tube already being damaged from the first time or not.

Do you have (minimum 1000V) back-up rectifier (protection) diodes installed in series between each HT winding end and each tube rectifier plate? https://www.premierguitar.com/the-immortal-amplifier-mod

I do not, but I will soon. I just followed the schematic and wiring diagram. This is actually my first amp build, lol.

[pdf64]

Standby is nuts, best not to use it.

[MikeW86]

So to put the reservoir caps before the standby switch, I'm thinking I'll leave the wire that runs between the output lug of the switch to the board, and cut and splice the wire from the reservoir caps and run that wire to the other (AC side) leg of the switch?

[sluckey]

There are a hell of a lot more amps out there with standby switches than there are naysayers preaching don't use it!

To connect the reservoir caps directly to the rectifier tube look at this pic. Oh, and bust the glass on that 5AR4 that had the fireworks. It's no longer trustworthy.

[MikeW86]

That's what I figured. I haven't powered it on since. I have a couple old 5U4GBs, but I wasn't sure if they'd be safe in this amp in the meantime.

[MikeW86]

Other than this small issue that is easily fixed, I'm really happy with how the amp turned out, especially being my first build. It's very quiet, and I'm surprised at how close the voltages are to where they're supposed to be. There are a few more small workmanship details I'd like to clean up before it's done.

Too bad it's not mine:(

[pdf64]

...
Too bad it's not mine:(

Well done, but now you’ve got the bug and the confidence, you can remedy that by starting to build them for yourself 

There are a hell of a lot more amps out there with standby switches than there are naysayers preaching don't use it!

Wow, sorry for the derail but this seems like a trip into topsy turvey land.
Following a valve being apparently wrecked by a stupid and damaging standby circuit, a mod has been put forward to make a into one that makes it merely stupid.
Yet the weight of numbers argument is being used to slap me down as a nay-sayer, whose position is obviously incorrect by virtue of it not being the mode, by querying the point of the stupid circuit?
Words fail ...

[sluckey]

I value your opinion.

[MikeW86]

Another solution I read was soldering a resistor (Rob Robinette used 100K 2W) across the standby switch to allow some current to flow and charge the filter caps. Would this work in lieu of moving wiring around and splicing?

[pdf64]

It would help, but why not just leave the switch alone (closed) and let the GZ34 do the job properly?

[MikeW86]

If it were my amp it wouldn't even have a standby switch, but it's for someone who's religious about using it, so I want to make it as painless (for the amp) as possible.

Old myths die hard, especially in the guitar world.

[pdf64]

Oh yeah, sorry I forgot 
Take the DC output from the rectifier pin8 to the reservoir cap and to the standby switch.
Take the output of the standby to the choke and OT CT. (Then the rest of the amp is fed from the choke).
That shouldn’t require too much rejigging things around?

[sluckey]

Oh yeah, sorry I forgot 
Take the DC output from the rectifier pin8 to the reservoir cap and to the standby switch.
Take the output of the standby to the choke and OT CT. (Then the rest of the amp is fed from the choke).
That shouldn’t require too much rejigging things around?

Yes. Just like the pic I posted above. Only need to move one wire.

[Willabe]

Old myths die hard, especially in the guitar world.

Oh bunk. 

You guys just don't get it. It's not about saving tubes.

A standby switch is a good thing to have. There for working, gigging musicians. As long as you have the 1st filter cap(s) before the switch.

The biggest reason is when your playing gigs. If your a sideman in a band you need to be ready to go when the band leader hits the stage. If your not ready when the boss man want's to start, you can get fired. (Because if he/she doesn't start on time, they can get fired from the gig, they won't get hired back.) Plenty of other players/head liners/groups standing in line for your gig. Or if your in a group/band, it looks bad to hit the stage and wait for your amp to warm up before you can start to play.

And club/bar owners don't like it. Neither does the audience. They want you to start your sets on time, they watch the clock, to the minute, when you start and when you stop. Their paying you to play and they have customers paying to hear the band and buying drinks. You can lose your gig over it, they wont hire you back again. They won't and don't have to deal with a band cutting their sets short. Plenty of bands standing in line for that gig.   

[brewdude]

Why can’t you just turn on the amp and mute it with the guitar’s volume pot?


Maybe just get a tuner pedal with a mute function.


I just think there are ways to be ready that don’t require a stand by switch.


The tradition of having a stand by switch on classic tube amps seems to be the only (best?) reason to include one.

[JB]

I reckon you can get on the stage and the amp can be warming up while you put the guitar on, plug in and check your tuning!  Personally I'd leave it powered on from soundcheck.  If you leave the guitar plugged in then just turn down the volume.  If you unplug the guitar then pop the jack a little way out the amp to silence it - or use a guitar lead with a Neutrik Silentplug. 

[Willabe]

Why can’t you just turn on the amp and mute it with the guitar’s volume pot? Maybe just get a tuner pedal with a mute function. I just think there are ways to be ready that don’t require a stand by switch. The tradition of having a stand by switch on classic tube amps seems to be the only (best?) reason to include one.

Yes you ~can~ do those things and no, that's not why.     (See below.)

I reckon you can get on the stage and the amp can be warming up while you put the guitar on, plug in and check your tuning!  Personally I'd leave it powered on from soundcheck.  If you leave the guitar plugged in then just turn down the volume.  If you unplug the guitar then pop the jack a little way out the amp to silence it - or use a guitar lead with a Neutrik Silentplug.

You can do whatever you want, but I think your trying to reinvent the wheel.

There's another angle to look at this from;

It's a matter of; respect for the audience, the club/bar owner, the band leader and yourself.

For working musicians, it's a job, like any other job. Your getting paid for working. So you show up on time and are ready to go to work, simple as that. I think Leo Fender knew this and put in a standby switch for professional working musicians. If your not playing out/gigging, then maybe you don't need a standby?     

When you hit the stage, your supposed to be ready to play, not go up there and start tuning up. People didn't pay to get in to watch you tune up. That's just sloppy, unprofessional and wasting the audiences time. People can tell/feel it if you don't care about them being there and entertaining them. The best musicians conduct themselves the same at a show whether there's 1 person there or 40K. When you do that, that's a part of growing you fan base and keeping it. These things really do matter.

So, you tune up when you 1st get there and set up your amp. Or in the bigger venues that have a dressing room, you tune up back stage. If you want to keep a following you quickly learn these things. When you watch the better/best bands you see them doing this. It looks professional and is. The whole band hits the stage at the same time, not straggle up 1 at a time, players with amps hit the standby and off they go.

You ~can~ just leave the amp on and turn down the volume on the amp and/or guitar, BUT, now you come to the secondary reason for a standby. When you have the B+ on the tubes your burning up tube hours needlessly. Most working musicians have a very tight budget to live on. They try to save $$ where they can. Again, I think Leo knew this and Leo being frugal himself, elected for the standby as the best option for these reasons combined.

Playing clubs, it's 45 minutes/20 minutes off or 1 hour on/30 minutes off. The last break/set times get adjusted a little shorter. Gig starts at 9:00PM, club/bar closes at 2:00AM, you play to 1:45AM/1:50AM. There are also late hour clubs/bars with a 4:00AM closing time, and both the 2:00AM and 4:00AM clubs/bars get an extra hour on Sat night, 3:00AM/5:00AM, Chicago Il. So you show up at the latest 8:30, set up/tune, leave the amp on in stand by. So you save ~ 1/4+ of the total time the amp is on each night from burning up power tubes. Studio recording work is the same, they use a standby switch. It all adds up.

Most working musicians, current or past, will tell you pretty much the same as I've said.

[JB]

That's a shame, means professional musicians aren't allowed to play Tweed Deluxes or old AC30's! Someone tell Bonamassa or The Edge!   

[Willabe]

That's a shame, means professional musicians aren't allowed to play Tweed Deluxes or old AC30's! Someone tell Bonamassa or The Edge!   

That's not the point and you know it.

1st off, they weren't playing sets, they played 1 show. Still most bands playing single show concerts used their standby. The pilot on their amp was on and as they announced the band/artiest to the crowd, either a roadie or the player flipped the standby on into play.     

2nd, Tweed Deluxes were student amps. Old AC30's are just that, an older model. Amps developed over time and the standby was added as a convenience as time went forward. Leo and other companies didn't see every need all at once. As time went on and as they talked with working musicians, the need for the standby switch became apparent. There's no way Leo would have added anything to an amp that he didn't think was needed. He was a pretty sharp business man.   

And Vox AC30's without standby's are far the minority of stage amps used compared to Fender/Marshal. Yes, very early on in England they were top dog, but once Marshall came along that was all but the end for Vox.   

I stand by what I said, most working musicians used their standby switch, if the amp had one, for the reasons I listed. If you played enough live gigs you'd know.

You left out Rory Gallagher, Brian May (Vox AC30) and Neil Young (Tweed Deluxe), but they weren't playing sets.

From your response, seems like you may have a slight problem with the meaning and concept of respect? You don't have to agree with me, but you don't need to be a smart guy either to disagree. 

You can lead a horse to water, ....... 

[JB]

It was tongue in cheek, hence the  !
I've got amps with and without standby switches.  Mostly I gig with an old Boogie that has a Standby which I use as intended.  Occasionally I'll gig with a homebrew 5E3 Deluxe that doesn't.  You use the amp that best suits the band, music and venue and make use of whatever features it has. 

The Standby debate goes round and round on many forums with it's lovers and haters and will continue to do so.  I'm not bothered either way.

[pdf64]

...Amps developed over time and the standby was added as a convenience as time went forward. Leo and other companies didn't see every need all at once. As time went on and as they talked with working musicians, the need for the standby switch became apparent. There's no way Leo would have added anything to an amp that he didn't think was needed. He was a pretty sharp business man...

That’s a plausible hypothesis, but really, it’s supposition, we don’t have any documented rationale as to why standby was originally introduced and then retained (the reasons may have been different).
All that’s revealed is what the manufacturers put in user manuals, ie the hypothesis (stated as fact) that powering up and down in standby extends valve life; which seems almost certainly fallacious.
My suspicion is that as much as standby being a convenience feature, musicians seem to revel in the ritual, lore and, for some, the oneupmanship of their ritual being the only correct version, of how to use it.

[sluckey]

for some, the oneupmanship of their ritual being the only correct version, of how to use it.

That holds true for some tech people too!

[Willabe]

....for some, the oneupmanship of their ritual being the only correct version, of how to use it.

How else are you/can you use it?

It's either in standby or in play. 

[MikeW86]

All issues are fixed, the amp is done, my buddy (who bought the kit) is really happy with it. Thanks for all the help!

[Willabe]

All issues are fixed, the amp is done, ........

And the fixes were????? 

[MikeW86]

Moved reservoir caps to the other side of the standby switch, added 1N4007 protection diodes to the rectifier socket, 100K 2W resistor across standby switch. Caps charge slowly when amp is powered on, no popping when standby is closed. The amp sounds great. Plate voltage is a couple volts higher than before (417.6V vs. 414V), but I'm guessing that's just the new rectifier tube being a different brand than the old one and not putting out exactly the same voltage.

The only thing I don't like is the volume pot taper is kinda weird- it has 1Meg audio taper pots for the volume controls and there's no volume until about 3 on the knob, and the volume doesn't increase dramatically until about 6 or 7, then it jumps very quickly. Just nitpicking I guess, It's owner is happy with it.

[pdf64]

....for some, the oneupmanship of their ritual being the only correct version, of how to use it.

How else are you/can you use it?

It's either in standby or in play. 

I was thinking of the timings between the switching of power, HT and actually using the amp.
And then the reverse for shut down.
It's unsurprising I suppose given the lack of a technically valid rationale, but people have made up all sorts of very specific variations on the above. eg from the Marshall Forum

Turn it off. Don't put it on standby, just turn off the power switch and DO NOT run any signal through it while it's powering down. To power up, have it on standby when you turn it on, leave it there for two minutes. Turn the standby switch on and DO NOT run any signal through it for at least twenty seconds, preferably at least 45. Your power tubes and filter caps will love you for this

I also do a little trick with my Marshall that a tech once told me about, when powering off I put the standby on first, let the amp sit for about 10-20 seconds or so, then flip the standby off before shutting off the power switch..Never had an issue doing that.

Fenders:
I turn on my amp with standby on for about 30 seconds or so to warm up. When I'm done playing, I just turn the power off and then switch the standby on for the next time I power it up.

Marshalls:
I turn on my amp with standby on for about 30 seconds or so to warm up. When I'm done playing, I switch on standby, then turn it off.

Warm up the amp on standby for a minimum of 45 seconds, in my opinion a minute and a half. If not, the plates try to draw electrons faster than they can be generated and will oxidize prematurely. when you take it off standby, don't run any signal through it for about ten seconds at least. To power off, just flip the power switch and don't generate any signal until you're sure it's all the way powered down. Then put it back on standby before powering up again.

STANDBY is used in conjunction with the MAINS power switch (Front Panel Function #1) to ‘warm up’ the amplifier before use and to prolong the life of the output valves. When powering up, always switch MAINS power on first, leaving the STANDBY switch in the ‘standby’ position. This allows the application of the voltage required to heat the valves to their correct operating temperature. After approximately two minutes the valves will have reached the correct operating temperature and the amplifier can be engaged by switching MAINS power ON.
Note: In order to prolong valve life, always set the STANDBY switch to the ‘standby’ position when turning the amplifier sound on and off during breaks in performance. Also, upon full power down, always put the amplifier in ‘standby’ prior to switching off the MAINS power.

This last one is a Marshall user manual 

...The only thing I don't like is the volume pot taper is kinda weird- it has 1Meg audio taper pots for the volume controls and there's no volume until about 3 on the knob, and the volume doesn't increase dramatically until about 6 or 7, then it jumps very quickly...

CTS pots?

[MikeW86]

They're Mojotone-branded pots, which I assume are made by CTS.

[pdf64]

CTS have a 15% end stop metallised dead zone at the beginning of their track, where nothing happens.