Fender Bassman 100 red plating issue

[SoundmasterG]

Hi guys,

I am wondering if any of you have any ideas on this one. I'm out of ideas at the moment as to what is causing this problem.

Amp is from a working band's bass player and is a Silverface Bassman 100. Customer hadn't noticed anything wrong with the amp other than the fuse holder was cracked. So I replaced that and fired it up, only to notice that power tube #3 was glowing red. So I shut it down and tested that tube on my mutual conductance tester and it was almost completely dead. All the tubes were Tung-Sol reissue 5881's and had all been replaced at the same time. Power tube #4 had no cathode current and I replaced a blown screen grid resistor. Ordered a new Tung-Sol reissue 5881 for power tube #3 and replaced it. Power tube #1, #2, and #4 all tested out fine, so after retensioning and cleaning the sockets, I fired everything up again and monitered the amp. Set the bias to around 64% of the max and everything sounded good and worked well. Let it cook for 20 minutes with no problems. Called up the bass player and told him to pick up his amp. So he calls me two days later after a show and amp made it through 5 songs and then the sound got funky and he looked and power tubes #3 and #4 were red plating. He shut it off and I have it again on my bench now. Power tubes #3 and #4 are dead and burned, and power tubes #1 and #2 are in perfect health. With no tubes each socket has bias voltage. The screen voltage makes it across the screen resistors. The plate voltage is on all sockets. The coupling caps from the phase inverter to the power tubes don't leak on either side. The amp had been completely rebuilt several years ago and has mostly new carbon comp resistors, Sprague Atom electrolytics, and Mallory 150 coupling caps. Everything in the bias supply is new too. Previously before the second failure all tubes showed proper current draw and were biased and matched well. No evidence of carbon traces on the sockets. It is only power tube sockets #3 and #4 that have the problem, and they both are on the same side of the output transformer. It measures fine as far as I can tell though. The amp will run happily with no problems for hours with no power tubes in there. Somehow when the tubes are added, the two on that side go out of control. It seems like they are losing bias somehow but I haven't checked that since I got the amp back....sort of reluctant to blow any more tubes until I have an idea of what could be going on.....

I'm stumped as to what to look at next. Anyone have any ideas?

Greg

[stratele52]

Did this bass player use the right impedance speaker for this Bassman  100 head ?

[stratele52]

By the way tube tester ( most off them if not all we can buy ) don't have high voltage to test amp's tube, specially power tube. So don't trus it. Just good if your tube are dead short or open heater.  You can check tubes only when they are working in the amp.

About your amp, I don't know your skill to fix amp. I take for granted  you really know how to mesure and adjust bias and and socket tensionning properly or clean socket contacts.
_______________________________________________________________________________
The problem could be the negative bias voltage going to the tubes. You think the same as I read.

Bad solders ? Reheat them . Or socket have a crack on their base and arcing to chassis ? Or too much solder on tubes pins = arcing on chassis , or short ?  Or contact on socket are not good even you clean them. They rust few days after clean it ?
_______________________________________________________________________________
OR ;  Phase Inverter could be suspect, too much signal to these tubes.


Have a good day,

[gldtp99]

Read this: http://thetubestore.com/tungsol5881.html

Pull those RI TS 5881's and use a true 6L6GC-type tube-- bias to @60% of 30 watt dissipation----the studio across the hall from my workshop has a SF Bassman 100 that i've serviced up a couple of times--- i've put in two quads of Sovtek 6L6WXT+ tubes over the last several yrs---- this amp gets massive hrs of use for both bass and guitar and it just keeps going, except for one time the heater circuit burned up from coming into contact with plate lead--- PT survived that-- i re-wired the heater circuit and it's still going strong yrs later.
I firmly believe that the problem in your amp is the use of underated output tubes--- It's a miracle that any of them have lasted this long--- the only 5881 that would hold up under those conditions is the Sovtek 5881, and it's not a true 5881 anyway--- but it is a known tough tube that will take a great deal of abuse.
Again, i suggest the Sovtek 6L6WXT+ tube for that amp----- rugged, inexpensive, and good sounding--- i use a lot of them in builds, re-tubes, etc---- It's not a glamorous tube but it is a good, honest performer.
Doug sells the EH 6L6 tube which is similar, if not identical, to the Sovtek 6L6WXT+--- buy a quad from Doug, bias them, and you're in business.
It seems that most other possibilities involving the amp failures have been ruled out---new bias circuit parts, you've checked bias voltage and checked for coupling caps leaking voltage to output tube grids.
But still the underated tubes fail----- Why ? Because they are not rated for the conditions to which they're being subjected--- they overheat and die.....................................gldtp99

[stratele52]

+100 gldtp99

This what we can read on Tube Store.com   


There are some important things to know about the reissue Tungsol 5881 tube before you decide to use it. The Tungsol 5881 is a 23 watt tube and is rated for a maximum plate voltage of 400Vdc. The many amplifiers that have higher plate voltage but use a tube rectifier can use the Tungsol 5881 tubes without issue. A prime example is the ’59 Bassman LTD. However it is also critical to rebias the amp heeding the 5881’s 23W rating. This will provide a vintage tone and less clean headroom. Leaving the bias set to a 6L6GC setting and/or using the Tungsol 5881 in a high voltage amp with a solid-state rectifier, will likely cause problems and are not covered by warranty. For these cases consider the Tung-sol 6L6GC-STR tubes instead. They are rated for 30 watts and 500V so they will hold up better. Their tone is quite similar to the 5881 too.

[stratele52]

On the Fender Bassman 100 schematic , plate voltage is 447 volts , it exceed tungsol spect.  I don't know how many volts soundmasterG read.

[SoundmasterG]

Yes the bass player used the right impedance. He is a professional working musician and has played this particular amp with the same rig for the last 20 years with no problems.

I know the tube testers don't test at the same voltage as what is in an amp, but this is at least a mutual conductance tester and can test for shorts, and overall function without being completely unreliable like an emission tester. I've never had a tube that tests ok in this tester not work in an amp. The tester isn't the problem here.

I know what I'm doing on tube amps....have no fear there. I've been doing repairs and making my own amps for 15 years and I am a senior at the moment for a Bachelor's Degree in electrical engineering and work at Intel in a test lab. I have a fair bit of knowledge for both the technical end and from the hands on end, for tubes and solid state, and have a reputation in the area as someone who can fix the hard cases. This particular problem has also stumped several engineers I know, and a good friend who ran one of the big amp companies in the 60's.

I have a feeling the tubes are somehow losing bias, but why is the question. These particular tubes have been in use for 6 months for no problems in this amp...well at least until the recent problems. Whatever is going on, it seems to be sockets 3 and 4 as 1 and 2 have no issues and those two tubes are still working just fine. The sockets are fine...no evidence of arcing....pins are not shorted to the chassis....the amp has about -48v bias at the sockets with no tubes in them the way it is adjusted at the moment. The bias control has been changed by past technicians to a coarse bias adjust and it has enough range and works fine. All of the parts are relatively new in the amp as it was overhauled within the last 5 years.

I read the stuff on the Tube Store...but he goes on to say that he tested the tubes in a Bassman, which has more than 400v on the plates. The other thing is what are the real specs...New Sensor doesn't really share. I may have to call them and ask...but most tubes made these days can handle 400v+ as long as you don't exceed the max dissipation. I doubt New Sensor would be making a tube with such a voltage restriction as that would limit their sales. I would guess the 400v max is someone going off the old datasheets, but I suppose a call to new Sensor wouldn't hurt. Incidentally I did set the bias on this based on the 23 watts max and not the 30 watts max of a 6L6GC.

Plste voltage on this amp is about 448v. I can try some other power tubes in it, but the fact that the problem seems localized to one side of the output transformer and not the two tubes on the other, and the fact that whatever is going on killed a brand new tube quickly also makes me think it is something else.

Normally I fix stuff like this myself without any issues, but this one has been perplexing for some reason and I was hoping someone might have a magic answer for it here...haha.

Thanks,

Greg

[DummyLoad]

i'm betting it's a bad OT. had a similar problem with a SG 8417 powered bass amp last year. pulled and replaced every component, some twice. drove richard and I batty - BTW, our OT "tested" good as well. we suspected a thermal issue with the winding on that 1/2 of the primary. i saved the OT to dissect at later date...never got around to it.

--DL

[SoundmasterG]

i'm betting it's a bad OT. had a similar problem with a SG 8417 powered bass amp last year. pulled and replaced every component, some twice. drove richard and I batty - BTW, our OT "tested" good as well. we suspected a thermal issue with the winding on that 1/2 of the primary. i saved the OT to dissect at later date...never got around to it.

--DL

When you checked that OT before pulling it, were there any readings from it to suggest it was bad? This one tests fine for resistance from side to side and there is no physical evidence of any issues.

I have a couple Sovtek used but still good 5881's that I can stick in it and see how it will work. I'll put them in, bias them up, and run a signal through and see if I can see anything on the scope. If all looks good and they don't red plate, then I'll run some tubes through it and let it cook for several hours and see if anything odd or bad happens. Since power tube #4 did previously blow the screen grid resistor, I suppose it could have been bad and pulled down #3 also causing it to lose its bias and red plate.

Thanks for the suggestions!

greg

[DummyLoad]

BTW, our OT "tested" good as well.

it tested with balanced DCR readings. nothing that would indicate shorted windings. a real-world test perhaps would be to apply some AC to each 1/2 primary and take secondary AC measurements.

--DL

[SoundmasterG]

BTW, our OT "tested" good as well.

it tested with balanced DCR readings. nothing that would indicate shorted windings. a real-world test perhaps would be to apply some AC to each 1/2 primary and take secondary AC measurements.

--DL

Interesting that yours tested fine yet was bad. The AC test is easy enough, so I may try that too....can't get to it until wednesday though, so I'll update after then.

Thanks!

Greg

[HotBluePlates]

And figure out the health of the bias circuit.

[stratele52]

To gldtp99 and SoundmasterG.

IMO the tungsoll 5881 with proper bias to not exceed 23 watts can work without red plating no ? What do you think about that ?

SounmasterG ,do you have the books written by Gerald Weber ?  Inside one of them , he explain how to test a transformer with AC voltage. And many many useful informations. these are my bible with other . I have them all .

[PRR]

> I have a feeling the tubes are somehow losing bias, ... about -48v bias at the sockets with no tubes

Get several voltmeters and clip-leads. Monitor the bias voltageS at the G1s while _cooking_. Both hot-idle and hard playing.

(Set up meters on the far side from the preamps.... that many grid-signal clip-leads could bleed enough signal to cause oscillation if they get anywhere near preamps.)

Agree that the Bassman 100 is brutal even on genuine 6L6GC. Original or "G" 6L6 or even original 6L6-upgrade 5881 are going to die later or sooner. For a working amp, I'd really consider fitting 6550 (Sovtek was good to me). They cost more but won't be stressed in a 100W UL rig. There is the extra 18W of heater power: idle it for some hours and see if the PT runs hotter than with 6L6-class tubes.

[DummyLoad]

They cost more but won't be stressed in a 100W UL rig.

respectfully, bassman 100 model is not UL. i used 6L6GC mfg. by sylvania they are the STR387/415 and seem to hold up in this beast. bias should be approx. -57V measured at the wiper of the bias balance pot. speaking of which, make sure that outer fixed leads of the bias pot doesn't have a cold joint or opens up internally during a thermal cycle.

--DL

[SoundmasterG]

It looks like the problem with this amp was a bad tube socket on #4 tube. Once the screen ressitor blew, the socket must have been susceptible to arcing. I replaced the socket and everything is working great now. I plan to let it cook for hours and see if it is still keeping its bias, but assuming it is ok with that, then a bad socket was the problem.

Greg