Looking for Hard to find PT

[dude]

Hello all, hope everyone's doing well.

I took my 92 USA made Fender Champ 25 SE out of the closet, what a beautiful sparkling clean channel, awesome. The distortion channel is not very good, very grainy, buzzy, just awful. I read a post on you tube about a few changes to make the OD channel better. Well, that opened up a can of worms.

This amp is a hybrid, 6L6 PP tube output, 12Ax7 phase inverter and "solid state preamp". I'm sure you guys know about this amp.

The amp played great (except the OD channel) before I opened the chassis and did the - one cap change and shorted out a few of diodes in the gain stage of the second channel.  I'm sure these two changes didn't cause my problem, as I put it back to original specs and still the problem.

I'm not too knowledgeable with SS but I think somehow, either by the age of the amp or I did something as the filament PT windings are bad. The AC on the two windings are way off, one is .54 vac and the other is 4.86 vac with respect to ground.   

The schematic reads 6.45 vac, the amp plays but with very, very low volume and just unusable. I disconnect the two filaments windings from the Bd and got these voltages above but wired still way off at the tubes. I put the speaker jack, 8 ohms on the chassis instead of direct to the speakers where it was, maybe this cause the filament windings to go bad as this amp has a headphone jack. Just not up to date on SS.

I tried to find a replacement PT  (Fender part number #039044)  but with the hybrid PT there are four sets of windings, B+, bias, SS preamp, and heater filaments. I checked all the other windings and the voltages are perfect according to the schematic. I have no idea why the filament blew but thinking mounting that headphone jack on the chassis might have had something to do do with it....?

So, I can't find this PT anywhere, it's the SS state windings that are the problem and I think the PT is no longer available...? If you know where I can get one, please let me know.

A second thought was since the filament is the only problem can I get a small transformer that puts out just 6.45 vac ? And if so where?

I've never done this before, I guess I would just tap off the 120 ac (from the stand-by) for the primary, and put the CT to ground, then each winding to where the originals where?

Any guidance would be greatly appreciated.   

With the SS preamp, will the CT to ground be a problem?

I see Doug has a 5vdc transformer, I need a 6.45 vac. if this would work...?

Thanks in advance,
al

[AZJimC]

You can get a 6.3vac transformer with center tap at radio shack's normally. Doug may have one on his store pages here as well. That may not be the best route, but it can restore the heater supply. I think it better to troubleshoot the existing problem, and murphy says it is highly probable that it's related to something you did. Check the schematic and locate the whole path of the heater circuit. check back until you find where it stops.

[dude]

Thanks for the advice AZJimC.

There's nothing in the path from the PT filaments to the heater connection on the BD, PT green wires direct to tube filaments, no stripped green wire in the filament supply or no separate CT for the heaters, just two green wires.  The connection goes direct to tube heaters. So only two green wires from PT, and every voltage off all other windings are correct except the heater filaments. They are low as above.

I can't think of anything to look for.  I assume the filament winding just went sound. The amp is from 1992, made in USA.

Speaking of Radio Shack, I have a transformer I used to use to break in speakers. It's Sec. 6.3-0-6.3 Vac at 3A, 120 primary. But when I measure the voltage it's 7.4 vac on each winding. Can I use that transformer? I assume the voltage is a little high as I have no load on it...?

If anyone can think of anything to check, I'd appreciate it, the amp plays but very low volume and no playable as I have no heater voltage.

Thanks, al  (Added a schematic)

[2deaf]

Mouser P/N 546-166L6.  Do what AZJimC said before buying anything.

[dude]

Thanks for the part number.

Here the full schematic, all I did was short out R28 with a jumper and changed R27 to 33K, look in the diode section of the second channel.

That's it, except I put the speaker output jack mounted on the grounded chassis which it was not grounded on the chassis before, just two speaker wires to the speaker below. Could this jack change have caused the filament windings to short out, I don't know. Can't see how but I'm not up on SS. I took it off the chassis and still no correct voltage on the filaments.

al

[shooter]

What you did "shouldn't" have caused the problem BUT, we ALL have caused things to not work that way!  That aside, you could try pulling the 6l6 tubes, ohming the filaments and see if one is different by say 20% or more - then ohm either fil pin to grid then cathode they should be open to the filament.

[dude]

What you did "shouldn't" have caused the problem BUT, we ALL have caused things to not work that way!  That aside, you could try pulling the 6l6 tubes, ohming the filaments and see if one is different by say 20% or more - then ohm either fil pin to grid then cathode they should be open to the filament.

I know but I was careful with just those two changes. The big problem is I didn't play the amp "right before" I did the changes. I played the amp a few weeks ago and it played fine. i should have play it right before.  I have a small recording studio and some other player may have been playing it between the two weeks, I don't know and he may have did something or it just was it's time...?

Anyway, the only ohm reading I get on the filaments with out tubes is .4 ohms between each winding, nothing at the pins. The voltages are .56 vac and 4.86 vac, should be around 6.3v at each winding, right?

It wouldn't take long to hook up the 12.6 transformer, 6.3 to each filament. If it plays then it looks like the the heater winding went bad...?

 I did have a hard time getting the bd. back in the chassis, pushing hard against wires. i checked the bd. for cracks, loose solder joints with ha magnifying glass, looks good and continuity checked out on the traces.

I just wanted to know if I put this transformer in and ground the CT, hook the primary to the power on switch and each winding to the two filament points on the schematic is correct. 

The only other thing could be a loose solder joint pushing on the Bd, to get it in... I did push hard. I'll check the bd. one more time but is this the correct way to add a separate filament transformer?

al

[shooter]

look at your schematic, one side of the filament winding is ground, hence the  voltage difference.  It also says that the ground is B+ ground, not chassis ground, so that "might" be were your volts difference lies.  I'd verify that b4 swapping things.

[dude]

So to check the voltage on the filament windings I put the meter to B+ instead of ground and the other lead to the filament connection point?

I never really understood heater supply voltage....

I appreciate your patience with me, it is possible that the filament windings are good in this PT as I really can't see why just that portion of a PT would go bad.

Sorry if my ignorance is annoying but I have no electronic education just hands on from the internet or forums, no teacher explaining anything to me ever.

Let me know how to check the voltage for the filaments correctly. What I did was put the meter to ground and the other lead to the each of the windings and got these different voltages.

Thanks,
al
al
 

[dude]

You guys were right, there is nothing wrong with my filament voltage.

Amp turned on, meter on pins 2 and 7 of power tubes, ac voltage reads 6.6 on each tube, preamp tube - pins 4 and 5 to pin 9 same voltage 6.6 ac.

I must have done something taking this bd out, it's not the two resistor changes I made as "shooter" mentioned. I must have loosen a solder joint pushing on the bd. or wire or something.

The amp plays but very low volume, just a trace of amplification coming from the speaker. Something came loose.  I'll just have to keep looking where I was pushing  hard on the bd to get it back in.

I realize that it's usually 90% or better I did something when problems happen after changes, I'll find it.

Any suggestions what area of the Bd. I might put the most attention.

I know I only ask questions on this forum when I need help (but I read the posts of others all the time to learn), I'm sorry I don't have the knowledge to help others. But If I could help anyone, I would  in heart beat. Hope I'm not considered a troll, whatever that is.

al

[shooter]

I believe a troll is someone that posts just to irritate, you are just a newbie.  Troubleshooting is a logical process, mostly, but you need a good understanding of what you're troubleshooting, electronics.  You might get by reading forums, watching youtubes but without that understanding it won't stick.

You misunderstood when I said use B+ ground, ground being the key word. your amp has 2 grounds, chassis or earth ground and electrical/electronic ground, not knowing the difference could get you killed.

we might get you through this problem but at the end of the day it probably won't be much good the next time something goes wrong.

You made a statement, something like" the board was hard to get back in, I had to push hard etc".  That alone will cause you more problems than all the amps you'll ever own.  I used to joke, "if it don't fit force it, if it breaks it was bad to begin with"  Anyway, your problem is like you suspect, bad connection, solder, wire, crack in the foil, so spend your time with power OFF, pull the board, inspect EVERY part, wire, wiggle them gently, a bad solder connection on say a cap will sometimes "feel" like a loose tooth, wiggle to hard and you have another problem. Use a magnifying glass even, It might take a week, a month, fixing broke things isn't about how long it takes, it's about doing it right.  It should be fun, if it isn't, find an amp tech and pay him/her.  hang in there, if you fix it AND find it's fun, you will probably become an addict!

[6G6]

Besides looking very caefully at every connection to the board,
you can use your meter to check continuity from every component
 lead to a point maybe a 1/4 inch from it on the trace.
Sometimes they can look way better than the real are.
Also, CC resistors can crack, yet look good.
Check across each of them.
The value you read, in circuit may not be the same as the resistor value,
but it should not be open or very much higher.

[dude]

All great advice, thanks.

And thanks for telling me what a troll is, I was thinking it was someone who takes info and doesn't contribute to help others. Unfortunately, I don't have the knowledge to help anyone on "this" forum but I have on other forums.

This is the best forum out there, there are people here that would likely fix my amp in a heart beat, but I'm sure they put in their dues in. My problem is I usually get my amps running but not completely understanding the theory, you can get that info by reading but nothing better than a hands-on-teacher explaining things.

I know a lot of you guys don't have an electromechanical background but have learn on their own, like me but absorbing and understanding much more. I'm trying, I just love music and it makes me feel so good when I built an amp, sometimes changing the circuit from what I did learn. Friends think I'm Mr. amp fixer and I'm really not. It's fun, I love it. I get more pleasure from fixing someones amp and looking at their face as they play it, of course I like fixing it too. I own a small kitchen cabinet shop, so I'm a woodworker that likes to tinker. Maybe someday I can make some "kit cabinets",  all finished pieces ready to glue up and cover, for cheap.

I was looking last night and found a few suspicious spots on the bd. I'll find the problem and post what it was. I learned one thing with this post, "don't jump to conclusions too fast" go over that you did last that made the amp stop working and look for the most likely problem, and not like a bad OT or measuring filament voltage "wrong". 

Thanks again very much,
al

[HotBluePlates]

I know a lot of you guys don't have an electromechanical background but have learn on their own, like me but absorbing and understanding much more.

Some of us (like me) don't absorb it any faster than you... we've just been at it for 10-20 years, and have made more mistakes to learn from.

[shooter]

I'll 2nd HBP, 30+ years ago I spent 3 years eating & drinking electronics, then the next 28 years I spent fixing high-end systems.  Now I'm not much more than a newbie, but that experience keeps me outta the deep end....mostly!  I'm still not past the dumb question stage...yet!

[PRR]

> considered a troll, whatever that is.

http://en.wikipedia.org/wiki/Troll_%28Internet%29

a troll is a person who sows discord on the Internet by starting arguments or upsetting people, by posting inflammatory, extraneous, or off-topic messages in an online community (such as a newsgroup, forum, chat room, or blog) with the deliberate intent of provoking readers into an emotional response or of otherwise disrupting normal on-topic discussion.

http://curezone.com/forums/troll.asp

You have not begun to troll.
__________________________________

> someone who takes info and doesn't contribute to help others.

That's something else, I don't know a name for it, and it isn't "bad".

Without "dumb" (ignorant) questions, where would smart answers come from?

And-- Einstein said "If you can't explain it to a six year old, you don't understand it yourself." Many of us have a fuzzy internal grasp of a subject, but EXPLAINING it forces us to focus and refine our understanding. (Makes no difference if "child" is 6 or 36 or 66 years old-- nobody is born knowing this stuff, we are all "children" at something.)
___________________________________

And then there are "lurkers", people who never post (ask or answer). In one sense they are leeches. But as a wize old wizop once explained: "Bless the lurkers, they pay the bills." In most forums lurkers out-number posters by a wide margin. In the old days we'd pay $6/hour to read a forum (and that wizop was living on forum income). The internet seemed to lower costs, but there's still real cost. Most forum have ads, and lurkers see those same as posters.

[dude]

Well, you're right PRR, I'm not a Troll. Interesting read, thanks for posting.

I'll answer a question only if I'm 98% sure I have a correct answer although I try not to "butt in" on a discussion two might be having till the time seems right if I want to post. I feel if I'm helped on this forum and don't have the knowledge to payback by helping others, I can contribute in other ways, like buying parts from the guy who took all the time to post all this "free information", thanks Doug. I admire people who help others from the goodness of their character and there are many here.

For a bit I felt I was not wanted on this forum, I guess because I didn't get an answer to my question (shallow thinking on my part) but now I understand that question was needless and something else was my problem. How can you solve a problem when you actually don't know what the problem is...? 

Anyway, I keep "learning" as I grow older and I don't mean about amplifiers only. 

Thanks,
al

[PRR]

> How can you solve a problem when you actually don't know what the problem is...?

That IS a problem. A common problem.

Right now my furnace makes too much too-cool "hot" air. I dunno if the problem is the blower or the ducts. I thought they put the 64K blower in the 40K furnace, but today I metered the blower and the current is low for 40K, too low to be the 64K blower. The ducts are not over-ample, and I've loaded them with A-coil and thick filters.

I don't even know how to propose the problem to a heating forum.

I'm gonna have to stare at this problem a while.

Sometimes the polite answer to such a request is to ignore it. Sometimes someone will get-in-face and say "So what's your problem??!" Sometimes someone will propose a complex solution to a different problem. There may be no good answer.

> I don't have the knowledge to help anyone on "this" forum but I have on other forums.

What goes around comes around. It averages out.

> "don't jump to conclusions too fast"

Great wisdom in that.

[dude]

Right now my furnace makes too much too-cool "hot" air. I dunno if the problem is the blower or the ducts. I thought they put the 64K blower in the 40K furnace, but today I metered the blower and the current is low for 40K, too low to be the 64K blower. The ducts are not over-ample, and I've loaded them with A-coil and thick filters.

Off topic but I can answer this one, you need to turn the temperature regulator of a hot air furnace to the proper settings. Sounds like the regulator is not shutting off soon enough and you're blowing too low of an air temperature to the ducts. Depending on the regulator which has a sensor into the chamber (near the burner), could be digital, knobs or manual dial with sets that need to be moved to change fan settings regarding temperature. Not the high limit but the range of the lower limit, turn it up five degrees in it's range, it's the fan shut off that needs to be raised. Try five degrees first. The more you raise that low temp fan shut off, the higher you might have to rasie the fan high temp start. I believe the differential setting should be about 20/25 degrees. Just move the range up in small amounts. Also putting you hand near a blowing duct to "feel" the air temp can be misconceiving, we tend to feel the air cooler. Should feel hot, progressively getting cooler, if it gets too cool might need to be set to shut off sooner. They have gauges to do this stuff but usually 5 degrees raise in both directions is all you need. There is a heating forum that's pretty good, email me for the link.

Thanks for your wisdom PRR,
al

[PRR]

> turn the temperature regulator of a hot air furnace to the proper settings.

Hah! Yes, all very true... for my old furnace.

Actually that thermo in the plenum "should" only set the start-blow and stop-blow points. Ideally, when stable, the fire-rate and the blow-rate are balanced to give a reasonable temperature. The blow-rate was adjustable on my old furnace. The manual covered different models with different pulley settings for each fire-nozzle used.

If under-blown it will get too hot. The thermo also has a flame cut-out. And I found that when marginally under-blown, it would hover near the high limit until demand stopped, fire stopped, chamber surface would cool and blow stopped for a minute, *then* the stored heat inside the chamber would raise the temp again and the fan would after-run for another minute. Took a while to see what was happening. (What had happened is the previous owner had run it many years with way inadequate return air, cooked its guts out.)

But this NEW furnace is different. It has no routine temp sensor (there is a rollout in case it gets way over-hot). The fire-rate and the 4-speed fan are supposed to balance each other so the temp is OK. If I believe the manual, and assume low flow resistance (I got ample duct), it should work out well on the MED-LOW speed, 700CFM, 50 degree rise (70-120) at 38mBTUh. I'm getting 30 deg rise (105 at the "hot" plenum). And today's numbers suggest very near 750CFM (averaged over the return area), but on the LOW setting.

I'm wondering if this 38mBTHu (net) furnace is really giving me 28mBTUh. While there is a propane conversion, and *sometimes* that changes the rating, there's nothing in this furnace's manual about propane derating. I'm wondering if they shipped the wrong propane nozzles.

[dude]

Well men, I found the problem.


Here I was getting ready to put in a small PT for the filaments because I was measuring the voltage wrong.


Spend hours "Easter Egg Hunting" the circuit board looking for loose joints, cracked traces, bad resistors and caps and of course nothing was wrong with the board at all, nothing. No bad traces, not even a loose wire where I was pushing to get the Bd. in the chassis.

Just guess what it was, probably the simplest thing that you'd almost never think about.

Somehow, when I moved the speaker jack to the chassis I had a small piece of solder between the shorting leg and tip. Hence the weak volume and static. I was about to give up, take the amp to someone who knows about SS. And the simplest thing was wrong, replaced the speaker jack bingo, perfect. Even the mod I initially started made the gain channel much better.

Man did I learn a ton. The more you know the faster you work and then more chances for a stupid mistake.

Thanks for helping me understand not to jump to conclusions and think about what I did right before the problem. I should have thought about checking the jack but I wired so many jacks, I could do it in my sleep, so I thought. Took me two days of re-soldering every joint, checking half the resistors, re-tentioning the tubes pins, you think of it I did it.   

I was feeling defeat, close to giving up, at the end of the rope and I thought of the jack, tested it and found it shorted.

What a great feeling, I feel like the first time I build an amp and it played, pure bliss.

al   





 

[DummyLoad]

 


good to read you found the issue(s).   


--pete

[shooter]

persistence is the better part of valor, or something like that!!!  there is a 12 step program just in case:)
well done.

[PRR]

An "expert" is someone who has made so many mistakes he can find them and fix them fast.

[Tone Junkie]

Good job dude. Wont be the first time you find the problem, slap your forehead and say. I cant believe I missed that.
 These guys have great patience. Hey they still let me post (LOL).
Bill