Hi, anyone want to figure out what happened to this Fender PT July 31st 1970?

[mikeymike]

Hi folks, I just got this 1965 Fender Pro Reverb a few months ago now and I figured I would see what you guys think about this note a tech left inside it on the power transformer. This amp came from Asheville North Carolina by way of Guitar Center to southwest Louisiana where it currently resides now

To give a rundown, the tube chart is stamped OL (as seen in my avatar) and all the transformers date to 1965 except the power transformer which has the newer F022798 code (not sure about the F prefix? was this a factory repair?) and is dated 1969 from the EIA coding: 606935. It was basically all original save for the power transformer (which was replaced 5 years after it was purchased seemingly) 2 filter caps and the bias caps (you can see the bias caps in the photos below). The pot codes and the original mallory cathode bypass caps all are dated 1965.

This amp most definitely had some kind of failure inside with the hottest area being near the pilot lamp because the chassis shows heavy yellowing and blackening from smoke that seems to be thickest on that end of the amp and gradiates to a much lighter nearly nonexistant shade of yellow towards the preamp section. It does have a faint smell when warmed up that seems to be the residue's odor. This failure caused the emi screen shield on the top of the cabinet to melt a little as well burn the wood but the amp works just fine and sounds great. It also lead to the power transformer being swapped (obviously)

(thumbnails lead to full sized images)





So as you can see the person who worked on this amp on that fateful day in July of 1970 was not keen on 5881's for some particular reason as the note reads: "Warr 90 days only if 5881 changed to 6L6GC"

I know very little about 5881's and am curious what this clue might mean as far as what happend to this amp. If anyone here wants to figure it out (the mystery of this is part of the fun, the other part of the fun is trying to figure out or speculate what might have happened--it is for me, anyhow) please be my guest. As far as I have read, the 5881's are interchangeable with 6l6GC's but use less current at a given plate dissipation percentage. I might be wrong so I kindly ask any of you very smart folks what your take on this amp is. I guess we know one thing's for sure... some days before July 31st 1970 this amp was pouring smoke out of every single hole. Guaranteed.

I have since cleaned the brass plate btw, these pics were taken when I first got the amp. I fear this residue is causing my vibrato channel bass pot to act up as well. Isopropyl alcohol was not enough to solve it off the brass plate easily. I had to use some polishing sponge to fully remove it.

For the more curious folks, it does have the original oxford 12L5-1's and every component on the board was original until I put new F&T cathode bypass caps in (the big honkin dual ones, they fit.... barely).

The power supply filter caps have all been changed but I need to go back in and change the node resistors later as one of them was measuring a bit off (higher) than spec. Cant say it's bad though.... its keeping the voltage down on modern wall socket power. It came with 3 of the original alumalytics (the copper colored ones) but the main filter caps (the two 70uF 350v ones) were already replaced at some point with two 100uF 350v capacitors. I replaced those replacements (hah) with F&T's that are closer to the factory specification of 70uF. They are 80uF 450v F&T's. The other 3 are now 22uF 500v F&T capacitors and the copper colored GE alumalytics are hanging out in a plastic bag.


As a side question, does this '69 spec power transformer mean I can run a 5u4gb in place of the gz34 as the filament winding is rated to support the 5u4gb?

Here are a few pics of the amp itself, it's in great condition despite whatever it was that happened to it 49 years ago



If you made it this far, thanks for sharing in my interest of this amp. It's my first vintage Fender and I'm pretty stoked to be able to own it.

[Jennings]

Might be to do with 5881s being lower plate voltage and power dissipation than a 6L6GC?  So they might be more prone to failure in that amp circuit perhaps.

[sluckey]

We can all offer scenarios that caused the PT to burn, but it's all speculation. Even the guy that replaced the PT probably doesn't know why. The only thing you will ever know for sure is the PT burned up. It was replaced in 1970. What more do you need to know?

BTW, those 50V bias caps are in danger. I would replace them with a single 47µF rated for 100V.

[Ed_Chambley]

Just a guess, I assume one of the winding shorted.  I run 5881's in Bassmans and I like 6L6GB.  Old tubes, but I have used the New Production Tung-Sol 5881 in repairs with no comebacks at the voltages of a PRO.  Even 6v6S JJ's work well.  Tubes just break up sooner, but I will say if you have the place to play it, this amp with 6L6GC RCA running full tilt with a White Letter RCA 12AX7 Blackplate sounds like Gary Moore playing Hendrix's Red House.


Check it out

[mikeymike]

We can all offer scenarios that caused the PT to burn, but it's all speculation. Even the guy that replaced the PT probably doesn't know why. The only thing you will ever know for sure is the PT burned up. It was replaced in 1970. What more do you need to know?

BTW, those 50V bias caps are in danger. I would replace them with a single 47µF rated for 100V.

I figured it wouldn't hurt to see if there was something about 5881's that I didn't know about. Are there considerations that one must take if they want to run 5881's in an amp like mine? Should I steer clear? The heater filament supply seems to be related to the failure and I don't know if there is a common type of failure with regards to this circuit so I want to defer to your guys' knowledge on the topic. Is it possible the owner of the amp at the time mismatched a speaker load on 5881's and then the plate arc'd to filament supply since the pins are right next to each other? I noticed that the tech installed an artificial center tap right at the pilot light either because this was the standard design for filament supply by the early 70's or possibly because of the previous failure taking out the whole PT he figured two 100 ohm resistors would be a lot cheaper for the next go-around haha. This area also seems to be where the most damage occured (the blackened chassis and the accompanying scorch on the ceiling of the cabinet).

The bias caps along with the node resistors are definitely on my list of things to change. When I ordered the parts I had no idea what a cap job required so I missed a few things here and there (I'm very new to this). I appreciate you letting me know, thank you. Just to further take advantage of your knowledge, what happens to those caps if they're underrated in that position? Will they short? Basically what should I look out for if you wouldn't mind sharing?

[sluckey]

Measure the voltage across the bias caps. What have you?

[mikeymike]

Measure the voltage across the bias caps. What have you?

This is what I'm getting, but not quite sure if the way I measured it is correct




It is definitely above the 50v specification of the two caps in there. When they're parallel like that the voltage rating doesn't change? I'm pretty sure the capacitance doubles when they're parallel?

[sluckey]

Lot of expensive stuff riding on that little cap!

[mikeymike]

Lot of expensive stuff riding on that little cap!

Certainly and I'm definitely not arguing it. Just curious about what the common failure modes are and what are the consequences of such a failure? I assume a redplating situation would occur if the bias voltage was lost? Come on don't be so tight with the knowledge! Im a sponge here... trying to learn! In all seriousness though, I really appreciate your advice (and patience) :)

Thanks btw, any insight on the voltage rating when multiple capacitors are used? Or does it not matter in parallel?

 

Also to touch base on my original question ish. Would putting 5881's in here against the advice of the original repairman be a bad idea? I initially wanted to get a Pro Reverb and do exactly that.... until I opened this one up and read that little note. Now I'm worried I'll blow it up! :(

[yorgle]

I was taught that capacitor voltage rating increases in parallel, capacitance stays the same.  In series, voltage rating stays the same, but capacitance increases (perhaps doubles?)

[sluckey]

I was taught that capacitor voltage rating increases in parallel, capacitance stays the same.  In series, voltage rating stays the same, but capacitance increases (perhaps doubles?)

Really! Hope you didn't have to pay much for that lesson.   

[SILVERGUN]

Here you go:

[mikeymike]

I was taught that capacitor voltage rating increases in parallel, capacitance stays the same.  In series, voltage rating stays the same, but capacitance increases (perhaps doubles?)

Really! Hope you didn't have to pay much for that lesson.   

haha 

I learned that capacitors are the opposite of resistors basically in this regard. resistors in series increase and resistors in parallel decrease and capacitors in series decrease and capacitors in parallel increase. but beyond that i know nothing as far as voltages go.

[sluckey]

That kind of basic info is just a couple mouse clicks away. Try it. It's very rewarding learning something on your own. Just don't go to the same school that yorgle did. 

[mikeymike]

Here you go:

That kind of basic info is just a couple mouse clicks away. Try it. It's very rewarding learning something on your own. Just don't go to the same school that yorgle did.  :wink"

Fair enough, what questions would actually allow for a discussion instead of a default to "go look it up on google"? If speculating what happened to the amp is a moot point and discussing the type of failures that might happen with the bias caps being too low of a voltage is something of a google-able offense what type of inquiry would actually get the conversational juices flowing? Any tips on where to take this if not here? I don't mean to say this in any kind of offending manner, I just don't see an in on where to start a discussion. I've tried 

[shooter]

what questions would

I like stories about amps that self destruct on stage, how all the pretty girls left and you're now having to fix the smoldering mess, why it exploded is always easy, too much current flowing through inadequately equipped parts 

[sluckey]

"Hi, anyone want to figure out what happened to this Fender PT July 31st 1970?"
Perhaps I should have just answered the subject matter. In that case my answer is "not me."

[mikeymike]

"Hi, anyone want to figure out what happened to this Fender PT July 31st 1970?"
Perhaps I should have just answered the subject matter. In that case my answer is "not me."

I apologize, it was really just something I thought people may find interesting

Nevertheless thank you for your insight

[SILVERGUN]

I'll throw a dart at the initial questions for you.
The label was possibly worded that way because the amp was originally spec'd out for 6L6GC and, since the PT blew, the repair guy at the music store was covering his ass and extending the warranty only if the amp was used as shipped, with 6L6GC.

The position of the fire damage in the cab suggests that maybe the bias circuit failed, which in turn put the output tubes into runaway at which point they will draw whatever current they can get. That is unfortunate for the PT and it wouldn't take very long for the light show to end.

IF the amp had 5881s in it when it failed and IF the (possibly not very knowledgeable)repairman thought that's what caused the issue, then there is one possible explanation.

[sluckey]

I'll throw a dart at the initial questions for you.
The label was possibly worded that way because the amp was originally spec'd out for 6L6GC and, since the PT blew, the repair guy at the music store was covering his ass and extending the warranty only if the amp was used as shipped, with 6L6GC.

The position of the fire damage in the cab suggests that maybe the bias circuit failed, which in turn put the output tubes into runaway at which point they will draw whatever current they can get. That is unfortunate for the PT and it wouldn't take very long for the light show to end.

IF the amp had 5881s in it when it failed and IF the (possibly not very knowledgeable)repairman thought that's what caused the issue, then there is one possible explanation.

I object your Honor! This is pure speculation. Without the dead body of the original transformer, we cannot even say a crime has been committed. For all we know, some pot head was burning incense in his Buddha idol on top of the amp. The amp burned up while he was making a run to the 7/11 for some more Boone's Farm Strawberry Hill.

[SILVERGUN]

Well now, you know you could have just given us all that perfectly logical explanation about 10 posts ago and saved us all a lot of aggravation.


I used the word maybe as a highly elastic alibi.

[sluckey]

Just wait til the real jibber-jabbers ring in. We'll get some real live specks them.   

[mikeymike]

I'll throw a dart at the initial questions for you.
The label was possibly worded that way because the amp was originally spec'd out for 6L6GC and, since the PT blew, the repair guy at the music store was covering his ass and extending the warranty only if the amp was used as shipped, with 6L6GC.

The position of the fire damage in the cab suggests that maybe the bias circuit failed, which in turn put the output tubes into runaway at which point they will draw whatever current they can get. That is unfortunate for the PT and it wouldn't take very long for the light show to end.

IF the amp had 5881s in it when it failed and IF the (possibly not very knowledgeable)repairman thought that's what caused the issue, then there is one possible explanation.

Thank you for offering a speculation! Seriously, I genuinely thank you . The bias circuit is covered in yellow-golden colored residue from whatever the smoke failure was (I suspect it was the power transformer's potting material vaporizing and inundating everything inside the chassis), thus implying the parts were there when the failure happened. If they (the diode and the resistor) failed they would have been replaced and not be colored yellow-golden by the smoke residue. One exception being the 2 capacitors of course. Those are very clearly modern replacements, but they couldn't be from 1970 as they seem too new (but admittedly I have NO idea what an electrolytic capacitor from 1970 is supposed to look like... judging by what i've seen in silverface amps they weren't this modernized yet). Of course the replacements could have been replaced and that might be what I'm looking at today 

The fun with trying to figure out what happened to this amp is the fact that whatever was in it at the time of failure got stained by this yellow-golden residue. It kinda helps suss out what was there (sort of like a tamper evident coating if you will...), what was replaced etc. The bias board looks fairly intact and not scorched at all (I've had it out of the amp) the screws that hold it down have the yellow residue on them. The bias pot, resistor, lugs on the pot itself all have yellow residue so the pot wasn't touched during the repair. The wires left a "shadow" of sorts under them where the yellow staining did not leave a mark on the chassis.

Maybe my idea of fun is not fun to most folks.... lol.

Just hoping someone else here shares my enthusiasm

I object your Honor! This is pure speculation. Without the dead body of the original transformer, we cannot even say a crime has been committed. For all we know, some pot head was burning incense in his Buddha idol on top of the amp. The amp burned up while he was making a run to the 7/11 for some more Boone's Farm Strawberry Hill.

I get it, there's no way to know. I don't expect to get a concrete explanation! There's usually no answer at the end of any speculatory discussion like this and I'm okay with that. It's just enlightening to see what other people's experience have been with working on broken amps and what it might lead them to believe happened. I figure there's someone out there who has run into this exact same scenario at some point and hell there might even have been the guy who repaired it back in 1970. 

I can see that this is definitely not something you guys want to talk about so I'll just stop, please accept my apologies. Thank you for the ideas though. I'll definitely be changing those bias caps this week.

[sluckey]

Don't apologize! We're just having fun. SG's scenario is a very logical one. Mine was just a poke at him. But really, he has no more hard evidence to support his idea than I have to support my silly idea. I made that point in my first reply, which was an honest opinion.

Another serious speculation is that the first two filter caps shorted, which caused the rectifier tube to short, which caused the PT to draw excessive current, melting the tar in the PT, and starting the fire. Now you have three speculations, two of which are very believable. Nonetheless, you still don't have enough evidence to prove any of those scenarios. Nada!

As entertaining as this thread is, the only thing worth any value is that now you are aware that bias cap is woefully underrated. So replace it. Otherwise, SG's scenario may become a reality.   

[SILVERGUN]

Don't let sluckey kill your dreams.
MAYBE he got into the Boone's a little early this year, and MAYBE he's still mad that he sold the old ceramic Buddha at a yard sale, and MAYBE he's still mad that the 7-11 closed down, and MAYBE he's mad that he's not allowed to burn incense anymore, or smoke pot, and MAYBE he's getting a little nervous that we're gonna figure out that it was his handywork back in 1970?
I think he would've been about 30 years old then, so it all makes perfect sense.

We all started somewhere.

[sluckey]

All pure conjecture! And the Buddha was genuine lava rock! 

[DummyLoad]

https://www.youtube.com/watch?v=YYeWdukr3Ik

[DummyLoad]

All pure conjecture! And the Buddha was genuine lava rock! 

ha! you were too baked to know it was cheap nippon plastic!


--pete

[sluckey]

Oh no man. It's the real stuff. The guy in the head shop in Ft. Walton Bch. swears! 

[yorgle]

That kind of basic info is just a couple mouse clicks away. Try it. It's very rewarding learning something on your own. Just don't go to the same school that yorgle did. 

As I recall, there was a lot of smoke in my school.

[mikeymike]

Don't apologize! We're just having fun. SG's scenario is a very logical one. Mine was just a poke at him. But really, he has no more hard evidence to support his idea than I have to support my silly idea. I made that point in my first reply, which was an honest opinion.

Another serious speculation is that the first two filter caps shorted, which caused the rectifier tube to short, which caused the PT to draw excessive current, melting the tar in the PT, and starting the fire. Now you have three speculations, two of which are very believable. Nonetheless, you still don't have enough evidence to prove any of those scenarios. Nada!

As entertaining as this thread is, the only thing worth any value is that now you are aware that bias cap is woefully underrated. So replace it. Otherwise, SG's scenario may become a reality.   

I appreciate the accomodating, thank you lol I wasn't sure how people would receive my post so I assumed the worst like a good tech in training would  .

Very interesting theory about the filter caps, did they have electrolytics like that back in the 70's? Even if they didn't as always it's possible that the replacements may have been replaced. But the originals are nowhere to be seen which could very well be because of whatever happened leading up to July 31 1970.

Also, I knew it was underrated.... just didn't fully realize what would happen if they ever failed. Now I'm a lot more worried and anxious to replace them. hah. Are there any considerations when choosing a cap for this job regarding the voltage? I have found a few that are pretty high voltage (350, 450 etc). Other than size and cost are these suitable replacements? Thanks again!

[tubenit]

Are there any considerations when choosing a cap for this job regarding the voltage? I have found a few that are pretty high voltage (350, 450 etc). Other than size and cost are these suitable replacements?

Look at the voltages in your reply #6.

BTW, those 50V bias caps are in danger. I would replace them with a single 47µF rated for 100V.

With respect, Tubenit

[sluckey]

Also, I knew it was underrated.... just didn't fully realize what would happen if they ever failed. Now I'm a lot more worried and anxious to replace them. hah. Are there any considerations when choosing a cap for this job regarding the voltage? I have found a few that are pretty high voltage (350, 450 etc). Other than size and cost are these suitable replacements? Thanks again!

You don't need a 350 or 450 volt cap. Sure they'll work just fine but may be too big to mount on the bias board.

Click on the capacitors link at the bottom of this page. Buy this one...


Surprisingly, Fender used a 25µF @ 50V cap in that amp when it was built!   

[Greg]

As for the note recommending 6L6GC's... Probably for reliability due to the lower SCREEN dissipation/voltage recommended for original 5881's or 6L6G more than the plate I would say. Fender where running them as high as the plate. Gibson always had some sort of tube regulator (0C3, 6V6 etc...) to bring this down 50 volts or more but not Fender. They use a big resistor on some early tweeds but not generally on blackfaces/silverfaces. So they just probably went GC instead for reliability. With modern 5881's this might not be a problem though.

However 5881 instead of 6L6GC would not provoke such a commotion inside your amp I think, just a premature failure of the tubes. Maybe the guy inserted a tube with a broken key? So you can imagine the heater being some sort of a jumper between the heater center tap (ground) and many of the socket pins including the negative supply and HV ones including pin 6 used as a terminal. An artificial center tap (100 ohms resistors) might save a transformer assuming a no-blow fuse. Anyway's the note on the transformer has no logical link to the disaster that happened in your amp.

[Greg]

The owner was afraid of blowing the fuse again by flipping the standby switch after his break for another set, so he came back with his half pack cigarette foil to stuff the fuse compartiment with.

[PRR]

> what happened ... July 31st 1970?

https://www.onthisday.com/date/1970/july/31
American saxophonist (Charles Mingus), dies of kidney disease at 39
Chet Huntley retires from NBC, ends "Huntley-Brinkley Report"
https://takemeback.to/31-July-1970
Top songs in the USA
Carpenters - (They Long To Be) Close To You
Bread - Make It With You
Three Dog Night - Mama Told Me (Not To Come)
https://en.wikipedia.org/wiki/July_1970#July_31,_1970_(Friday)
The traditional daily drink of rum for British sailors was ended as the Royal Navy ended the rum ration.

The ELEPHANT here is 63V on 50V caps. *Don't talk about it, fix it!!* When these caps fail they may go SHORT. Then the power tubes have NO bias. Their current will go up about *ten times.* It is July 31st 1970 all over again. NObody wants that. 63V caps are OK for a weekend. 100V caps are the better plan.

5881 was a heavy-duty 6L6G. 6L6GC has even higher ratings than 5881. While I suspect the 5881 made by Tung-Sol in the 60s were at least equal of G.E.'s 6L6GC, this amp was MADE for 6L6GC and worked them hard. I hesitate to agree with the original tech, but can't say he was wrong. After a major blow-up it would be quite right to use the tubes *Fender* specified. The Tung-Sol 5881 is an upgrade for the OLDER 6L6G in older amps, and was stock in the 5F6a, but Fender moved to 6L6GC for some reason. Yes, maybe price! But 48 years on this is all moot. We can't go back to the prelims of Jim Morrison's trial (10 Aug 1970).

Replace those caps!!

IMHO this was the original failure. Fender probably used the right-Volt caps but had a random failure (caps or solder joint). Like my Bogen with a bad joint, things finally go wrong with spectacular smoke. At this date there is NO way to discover the original problem. And no real use, because other problems will happen.

Add 1 Ohm cathode resistors and measure idle current.

The real Tung-Sol quit the tube racket a long time ago and true Golden-Age 5881 must be scarce. The "5881" and "6L6G" and "6L6GC" we can get today are all variants of Russian copies of 6L6/6L6GC. The better ones today will all be on a "6L6GC" frame, with glass-shape appropriate to the number stamped on the glass. It is possible that *some* "5881" have been dinked for a different "sound", or typical current, but I doubt all makers take the trouble to know what we think we want. It plays for 90 days, their job is done.

[Greg]

Just wait til the real jibber-jabbers ring in. We'll get some real live specks them.   

I decided to search for the meaning of Jibber Jabber being not exactly sure...  I might feel that I could be one of them? I learned english mostly from one of my early girlfriends with her family and the fact that we were surrounded by english/american TV shows.

I noted that sometimes, some gibberich can be produced in the text box doing cross translation verification in Google Translate and comparing it with Bing and often stunned in a bad bay at the results.

This situation won't better sooner so my english is improving looking through this forum so this is a win win for me. Great builders and advisors here, I like it when it goes funny. There is  a good bank of proven schematics suggestions and all!

Regards,
JP

[mikeymike]

You don't need a 350 or 450 volt cap. Sure they'll work just fine but may be too big to mount on the bias board.

Click on the capacitors link at the bottom of this page. Buy this one...

Thank you, it had slipped my mind entirely to check the store here haha

Surprisingly, Fender used a 25µF @ 50V cap in that amp when it was built!   

Yeah it's a mystery to me as to why that was in there from the factory.... doesn't the layout notate -51 volts even back in the 60's with their lower wall voltages that's still above the rating  ?

As for the note recommending 6L6GC's... Probably for reliability due to the lower SCREEN dissipation/voltage recommended for original 5881's or 6L6G more than the plate I would say. Fender where running them as high as the plate. Gibson always had some sort of tube regulator (0C3, 6V6 etc...) to bring this down 50 volts or more but not Fender. They use a big resistor on some early tweeds but not generally on blackfaces/silverfaces. So they just probably went GC instead for reliability. With modern 5881's this might not be a problem though.

However 5881 instead of 6L6GC would not provoke such a commotion inside your amp I think, just a premature failure of the tubes. Maybe the guy inserted a tube with a broken key? So you can imagine the heater being some sort of a jumper between the heater center tap (ground) and many of the socket pins including the negative supply and HV ones including pin 6 used as a terminal. An artificial center tap (100 ohms resistors) might save a transformer assuming a no-blow fuse. Anyway's the note on the transformer has no logical link to the disaster that happened in your amp.

Thanks for the explanation, I'm now interested in looking up those tube-regulated screen circuits (I am completely unaware of them).  A couple days ago I had a chance to buy some coin-based Mesa branded 5881's (likely Sovteks) that seem to have been sitting in a store for a long while but I had my reservations because of the note in the amp. The allure for me is being able to turn the amp up higher and have less output (mostly to protect the speakers but also for the ability to reach overdrive at slightly less volumes) however, from what I'm gathering modern 5881's are basically just 6l6GC's and there's no benefit to doing this?

RE: the heater filament circuit, I had speculated that maybe at the time it blew up the owner had been running too high of a speaker impedance (possibly an ext. cabinet) and the plate maybe arc'd over to the filament circuit but that's just a wild guess in the dark as well. It does seem to have taken place very close to the pilot though. The tech installed an artifical center tap on the lamp as you can see in the photo.

And thank you for your input, it is greatly appreciated. I am grateful.

[sluckey]

Bias cap again... Just for shits and grins I flipped through all the Fender AB763 schematics this morning. Guess what? Fender used a 50V bias cap in every one, even the mighty Twin Reverb! What were they thinking? Oh well, CBS fixed that. So now I can say something good about the CBS era amps.   

Look at this pic...

[mikeymike]

> what happened ... July 31st 1970?

https://www.onthisday.com/date/1970/july/31
American saxophonist (Charles Mingus), dies of kidney disease at 39
Chet Huntley retires from NBC, ends "Huntley-Brinkley Report"
https://takemeback.to/31-July-1970
Top songs in the USA
Carpenters - (They Long To Be) Close To You
Bread - Make It With You
Three Dog Night - Mama Told Me (Not To Come)
https://en.wikipedia.org/wiki/July_1970#July_31,_1970_(Friday)
The traditional daily drink of rum for British sailors was ended as the Royal Navy ended the rum ration.

I love the history links, When I got the amp home I actually searched youtube for that date and found this

This being one of Jimi's last two shows in the United States before his untimely death in September.

It is July 31st 1970 all over again. NObody wants that.

This actually caused me to laugh out loud haha!

It's getting fixed I swear! :)

The real Tung-Sol quit the tube racket a long time ago

Yeah that's sad to be honest. Ah well.

I have measured idle current via shunt method and it's within 50 to right over 60% (33.9 and 40.89 ma) at 453.5 volts on the plate (measured yesterday when I took the bias voltage measurement). Out of curiosity is the current supposed to vary that much on the power tubes? My thoughts are that it isn't, so I've got some precision grid stoppers, grid leaks, and screen grids on order because of the variance between the two tubes. They are new production JJ 6l6gc's that came with the amp. No telling how old they are but they're not vintage. The rectifier tube is the same story, a modern GZ34 by JJ.

The entire preamp section is chock full of vintage tubes that I'll share in my next post if you're curious. There are some amperex bugle boy marked ones (but they're not holland, they're great britain so I presume they're mullards) a ge or two (one with the real GE dot matrix code and one marked made in gt britain), an RCA, a Philco (yes that philco ) that I'm not quite sure on who made it but it is marked made in the USA. Forgive my enthusiasm, I've never even seen vintage tubes before much less own 6 of them. Imagine my surprise when I first pulled them out and saw amperex and googled it only to find it's highly sought after if it was made in Holland. hah!

[sluckey]

Out of curiosity is the current supposed to vary that much on the power tubes? My thoughts are that it isn't, so I've got some precision grid stoppers, grid leaks, and screen grids on order because of the variance between the two tubes.

I would not be concerned about that difference. Nobody was matching tubes when it left the factory. None of the resistors you mentioned will make any difference. Well maybe the screen resistors if you have one that is 470Ω and the other is 4700Ω!

But if you really want exactly matched tubes you'll need to buy a matched pair and cross your fingers and hope that they are really matched. Or modify the amp to have two bias pots. I don't recommend this. I recommend leaving the amp as original as you can. If it sounds good, don't replace anything except that bias cap.

I really don't trust the shunt method of measuring plate current. I much prefer using 1Ω resistors on the cathodes.

[mikeymike]

I would not be concerned about that difference. Nobody was matching tubes when it left the factory. None of the resistors you mentioned will make any difference. Well maybe the screen resistors if you have one that is 470Ω and the other is 4700Ω!

But if you really want exactly matched tubes you'll need to buy a matched pair and cross your fingers and hope that they are really matched. Or modify the amp to have two bias pots. I don't recommend this. I recommend leaving the amp as original as you can. If it sounds good, don't replace anything except that bias cap.

Fair enough, I am definitely in the camp of conserving originality as much as possible. Though I have an insatiable itch to modify as well. I think I'll just have to build a blackface at some point in the near future for that.

I really don't trust the shunt method of measuring plate current. I much prefer using 1Ω resistors on the cathodes.

If you wouldn't mind sharing your thoughts on this I'd be grateful. I know very little about how that method works

I am vaguely aware of a method of measuring current that includes more than just the plate draw, is that the method you're suggesting? I seem to recall it being that the methodology in question includes the current being drawn by the screens as well as the plates. If that is indeed the case, how do I accurately thresh that figure out of my measurement to get a precise plate current draw measurement?

Another quick note/question, I've measured the resistance of the OT primaries from CT to each end of the winding and while I'm aware that each half of the primary is not going to be exactly the same as one another I'm left a little curious about the results I'm getting of each. I know the measurement of each varies on whether or not the winding is warm or cold but I've noticed (or seemingly noticed) the measurements also vary with regards to whether the primary winding is connected to the tube socket (with tubes installed) or not. Do I need to completely isolate the OT (both primary and secondaries) to get an accurate measurement?

I suppose in a roundabout way what I am asking is: which way yields the most accurate figures (actual representation of idling current draw). I know it's a guitar amp and it's not rocket science with regards to precision but I prefer less vague to more vague in terms of figures (within reason of course, i don't need precision down to the ten millionth or anything absurd like that  ). This is why I chose to do the shunt method because it's happening as the amp is running. Not that I'm aware of the consequences of doing it this way in regards to accuracy. Hence I ask your advice on why you don't trust it.

I hope my questions aren't too convoluted 

[mresistor]

I was using the shunt method of checking my amp bias..   

[sluckey]

the methodology in question includes the current being drawn by the screens as well as the plates. If that is indeed the case, how do I accurately thresh that figure out of my measurement to get a precise plate current draw measurement?

Yes, it does include the screen current. My point is... it's good enough. There's no need to be precise. But if you insist, measure the voltage drop across the screen resistor and do some simple math to determine screen current. Then subtract that from the cathode current to get the "precise" plate current. I'm pretty sure that Fender never did this at the factory. Carlita simply tweaked the bias pot to give the bias voltage shown on the schematic. That method worked just fine until the new millennium.

I know the measurement of each varies on whether or not the winding is warm or cold but I've noticed (or seemingly noticed) the measurements also vary with regards to whether the primary winding is connected to the tube socket (with tubes installed) or not.

No it doesn't. The end of the winding connected to the socket is an open circuit so the resistance can be measured accurately regardless of whether or not a tube is plugged in. Same as accurately measuring any resistor with one end dangling in the air. The accuracy depends on making a good connection with the probes. Needle probes work really well.

The difference in the resistance readings from each plate to CT is due to the length of wire required to wind each half of the primary. The start of the winding to the CT is laid down first and is wound with a certain number of turns. Then the winding is continued out to the other end and is wound for the same number of turns. The fact that the second half of the winding is laid down on top of the first half means the wire will have to be longer to get the same number of turns. Resistance is directly proportional to the length of the wire, therefore the second half will have a higher dc resistance. There is a method of winding that will produce exactly the same DCR, but it's usually considered too expensive for a LO-FI guitar amp.

This is why I chose to do the shunt method because it's happening as the amp is running.

The cathode resistor method requires the amp to be running also. How could you measure any current if the amp is off? Either method works just fine, but the shunt method requires a good probe connection for reliable/repeatable results. The cathode resistor methode is much safer just because you are dealing with millivolts. The shunt method can result in a damaged DMM if you don't pay close attention to what you are doing. But anyone with a Fluke 87V should be able to check current with either method. I got one too, but I choose to use the cathode resistor method because I sometimes don't remember to rotate the dial or switch probe jacks. 

[sluckey]

I was using the shunt method of checking my amp bias..

+1   Picture = 1000 words   

[mikeymike]

I was using the shunt method of checking my amp bias..   

oh there's no doubt it's dangerous but he was speaking to the accuracy of the method and i was curious about that

I have a procedure or method for testing bias by shunt because I've blown the fuse in 2 multimeters 3 times doing it without a procedure in place lol.

I'll pre-position the alligator on the center tap and get the lead laid out ready to plug into it but not near it yet. Then I'll either manually probe the pin on the socket but using the hole in the tab as a resting point or I'll do the same thing I do for the center tap; preposition the alligator clip on the socket tab and then I plug the center tap in first while fully mindful of the other lead's position. Once the center tap lead is plugged into the alligator clip I'll plug the other lead into the plate alligator clip (or touch it to the hole on the socket tab if i'm doing it manually for a quick check).
By keeping the multimeter in sight along with the socket at no point in time do I take my eyes off the probe tip.

I'm still not sure whether or not I should connect to the center tap first or the plate first when doing this but I'm very careful either way.

[sluckey]

I'm still not sure whether or not I should connect to the center tap first or the plate first when doing this but I'm very careful either way.

Well, if you're not sure then it might be a good idea to switch the amp to standby to make connections and switch to operate to do the readings. You see, you're getting more and more into the reasons I don't do this!

[mikeymike]

Yes, it does include the screen current. My point is... it's good enough. There's no need to be precise. But if you insist, measure the voltage drop across the screen resistor and do some simple math to determine screen current. Then subtract that from the cathode current to get the "precise" plate current. I'm pretty sure that Fender never did this at the factory. Carlita simply tweaked the bias pot to give the bias voltage shown on the schematic. That method worked just fine until the new millennium.

Thank you for explaining, also the extra info about how bias was set at the factory is super well received. That's pretty cool and again, I appreciate you sharing.

No it doesn't. The end of the winding connected to the socket is an open circuit so the resistance can be measured accurately regardless of whether or not a tube is plugged in. Same as accurately measuring any resistor with one end dangling in the air. The accuracy depends on making a good connection with the probes. Needle probes work really well.

Right on, I'll re-measure with some new found confidence then! thanks

The difference in the resistance readings from each plate to CT is due to the length of wire required to wind each half of the primary. The start of the winding to the CT is laid down first and is wound with a certain number of turns. Then the winding is continued out to the other end and is wound for the same number of turns. The fact that the second half of the winding is laid down on top of the first half means the wire will have to be longer to get the same number of turns. Resistance is directly proportional to the length of the wire, therefore the second half will have a higher dc resistance. There is a method of winding that will produce exactly the same DCR, but it's usually considered too expensive for a LO-FI guitar amp.

Not that you haven't shared a wealth of f'ing awesome information here, but this might be the coolest thing I've learned yet. I didn't think about that at all and now it makes sense as to why each side of the primary has a different measurement. The first time I ran into this I thought the OT was blown. I looked up other results and even found an ad showing the same results so I felt pretty confident that it was normal for one side to be higher than the other. Now I know WHY and that is the coolest damn thing in my opinion. 

The cathode resistor method requires the amp to be running also. How could you measure any current if the amp is off? Either method works just fine, but the shunt method requires a good probe connection for reliable/repeatable results. The cathode resistor methode is much safer just because you are dealing with millivolts. The shunt method can result in a damaged DMM if you don't pay close attention to what you are doing. But anyone with a Fluke 87V should be able to check current with either method. I got one too, but I choose to use the cathode resistor method because I sometimes don't remember to rotate the dial or switch probe jacks. 

Well what I meant by running is, it's basically in playing condition. You're able to check the bias quickly so to speak. The transformer is up to temp etc. With the winding resistance measurement/voltage drop method you have to turn the amp off (if even for a few moments) to get readings.

And yes forgetting to switch the leads is something I did once, never again. Hah. It barks at you if you try to measure voltage and there's lead in the current jack though... so there's that.

Well, if you're not sure then it might be a good idea to switch the amp to standby to make connections and switch to operate to do the readings. You see, you're getting more and more into the reasons I don't do this!

Fair enough, but! in regards to accuracy is it prone to error? Not that it will justify anything, just curious I swear

I'll probably use the resistor method from now on it just seems a lot less anxiety inducing and I can definitely appreciate that :)

[sluckey]

I think we've covered this pretty good.

[shooter]

The bias circuit is covered in yellow-golden colored residue

did you smell it?
having cleaned up suicidal caps many times, they have a unique smell and golden yellow is a typical EDIT color, but I have started and it is good and my ace hardware brass isn't melting and I didn't catch any fish today