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SF Champ blown Output Transformer causes?

Discussion in 'Amp Tech Center' started by goldenhound, Sep 27, 2016.

  1. goldenhound

    goldenhound TDPRI Member

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    Hit a power chord on my '72 Champ, and there was a fading of the chord about a second after I hit the chord, and flashes of light from the vent holes at the bottom of the output transformer. I was sick. I have purchased a Classic Tone output transformer, but hesitate to put it in until I find out was caused the failure.

    Volume was on 6, bass on 1, treble on 10.
    JJ 6V6-S power tube.
    5Y3 rectifier and 12AX7 - all lit.
    No breaks in the speaker cable.
    I have 2 10" Oxfords wired in parallel to 5.7 ohms.

    About 3 months ago, I lifted the 15k resistor from the bass pot to prevent signal bleed off, and feed a stronger signal into the amp. I was under the impression that Leo did this to help keep the amp producing clean tones at higher volume back in the day. Today, removing that signal bleed path produces a much stronger signal into the tone stack/output transformer, which produces a *great* driven sound, sag/compression, rich sound. I was under the impression that this was a harmless mod.

    Could this have been the problem?
     
  2. Wally

    Wally Telefied Ad Free Member

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    Things occur. The power tube is suspect. I would want to know if the tubes were good before firing the amp up after repairs. The electrolytics might be suspect. If I were going to replace the OT, I would replace all electrolytics in the circuit...filters, bias cap, bypass caps. In other words, make everything correct.
    I never run a Champ with anything other than a true 4 ohm situation. Others'MMV.
     
  3. muchxs

    muchxs Doctor of Teleocity

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    That's part of it.

    It's difficult to find a 6V6 that won't arc in a SF Champ with the tone stack lift. The root cause is the elevated supply voltage approximately 100v higher than recommended by any 6V6 tube data sheet.

    What Leo did was balance an off the charts setup against grid excitation. Keep the signal low, it keeps carnage to a minimum. Stock Champs will eat a 6V6 tube, the cathode network and maybe part of the dropping string every decade or so.

    Unleash the beast, the beast is gonna eat.

    Stick an Allen TO8C output transformer in there. You'll need to move one of the mounting holes to get it to clear the cap can.

    I'd prefer the TO11 for its increased power handling except it's too large for a nice fit on the cramped SF Champ chassis.
     
  4. goldenhound

    goldenhound TDPRI Member

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    Well, I guess I got a wrong number on that one. Not a harmless mod at all.

    I'll have to see where I can get tubes tested locally. Maybe pay a tech to test them? I'll consult with Eurotubes.com, in Oregon, which is where I bought the 6V6-S. The rectifier and 12AX7 are from the mid-80's, though they all "look" good. This is an amp that has not had a lot of hours on it until recently.

    I've done everything one can do without actually testing the tubes electronically. Of course, I know that some tubes can look good, and test bad.

    In my search for causes of a blown OT, I ran across a review of a number of output tubes. JJ tubes were rated pretty well as far as durability, but there were others that were better in that regard, but unfortunately, not as sweet. I'm bummed. I really enjoyed how that amp broke up. (eh, figuratively speaking). Going back to the way it sounded before, well, it's anti-climactic.

    I appreciate the advice about changing the caps, Wally. I had replaced the filter caps about a year ago. But, it sounds like I have quite a few more to go.

    MuchXs - I like the advice about using an Allen TO8C OT. I'll look into it. I enjoyed the back story on the Champ, and it makes sense given the time that the Champ came about. He needed a low-cost student/starter amp, and a kluged circuit with fewer and lesser expensive parts would have been necessary. Hmmm.

    Gentlemen - I've definitely been enriched by the advice given.
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Something to consider to help protect your high strung Champ:

    Voltage spike suppressing 20mm MOV's (metal oxide varistor) installed between the power tube sockets plate pins (pin3) and the output transformer center tap. I used 625v 1.65kv clamping TDK MOV's from Mouser.com.

    [​IMG]
     
  6. muchxs

    muchxs Doctor of Teleocity

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    I wish someone would cut me a paycheck for actual objective analysis of parts such as tubes. There have been a zillion goofy subjective tube comparos and shoot- outs. The guys who review guitar amps and parts often don't have a clue and / or are clearly biased while the audiophiles speak a language musicians usually don't understand.

    For example a scientific test of tube reliability would be to burn a population of tubes... say 100 of a particular type and brand... burn 'em 24 / 7 for about three months. Count the dead ones. If it were me I'd continue to beat on the survivors for another three months.

    But... there aren't enough hours in the day as it is. It's not science but I judge reliability by what shows up dead, here.


    There are a couple caveman tests for Champs, Vibro Champs and Broncos that require no complicated test equipment but are still practically mandatory. They are:

    The snap, crackle, pop test.

    You have a choice of playing, not playing and just listening or mix 'n' match. Max your volume and tone controls. Listen for bacon frying, "hairballs" and especially for pop and crackle.

    Crackle can be a funky contact, a funky tube or maybe something more sinister.

    You really, really don't want to hear loud pops from a Champ.


    Next,

    The "lights out!" test.

    If your Champ pops especially when you smack a big chord... remove the upper back panel. Turn on the amp, max the controls, turn out the lights. Bang a couple big chords on your guitar. Watch the 6V6 tube. If you see a nice blue arc ("lightning") inside that accompanies the pop, swap your 6V6 with one that doesn't pop and arc.


    It goes without saying but I rarely hear anyone mention it. Everyone offers dire electrocution warnings but they rarely tell you...

    Do not operate your amp without a speaker connected. The speaker load is part of your power amp circuit. The quick way to cause nasty damage to a tube amp is to run it without its speaker connected.
     
  7. peteb

    peteb Friend of Leo's

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    How long does it take cause a problem, having the champ on with no speaker? What if you play the guitar thru the amp with no speaker, versus just having it on with no speaker? the guitar signal, AC, is what causes the problem isn't it?


    Thanks
     
  8. Thin69

    Thin69 Friend of Leo's

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    I think this will vary considerably. If there is no signal you can generally get away with it for a while but from personal experience I had a speaker jack fall out on an amp I turned on and very quickly had smoke billowing from the OT even though there was no signal going through the amp. If your playing your guitar with the speaker disconnected I would expect the OT to fail fairly quickly.

    The failure mode is fly-back or arcing within the OT.
     
  9. Wally

    Wally Telefied Ad Free Member

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    The problem with a stock Champ like this that has an RCA jack is that the OT's output is not shorted to ground when there is no speaker connection. IN these amps with 1/4" jacks, a shorting jack is used. When there is no plug in the jack, the OT sees a short. That 'short' is better than the OT seeing an open...which is what it will see with the RCA jack. I am probably going to be changing my '71's jack to a 1/4" shorting jack output....for this reason in addition to facilitate using it with other speakers.
     
  10. FenderLover

    FenderLover Poster Extraordinaire

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    This is your opportunity to make two simple improvements to your champ: bigger, better OT and a shorted 1/4" jack. The 1/4" jack fits in the same hole, so that's a no-brainer. It also makes it easy to plug into extension cabinets with confidence.
    While you're replacing the OT, I'd look for one with 8 ohms out and change the speaker to 8 ohms too, especially if you want to use the new 1/4" jack with 8 ohms cabinets.
     
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  11. goldenhound

    goldenhound TDPRI Member

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    The snap, crackle, pop test.

    You have a choice of playing, not playing and just listening or mix 'n' match. Max your volume and tone controls. Listen for bacon frying, "hairballs" and especially for pop and crackle.

    Crackle can be a funky contact, a funky tube or maybe something more sinister.

    You really, really don't want to hear loud pops from a Champ.


    Next,

    The "lights out!" test.

    If your Champ pops especially when you smack a big chord... remove the upper back panel. Turn on the amp, max the controls, turn out the lights. Bang a couple big chords on your guitar. Watch the 6V6 tube. If you see a nice blue arc ("lightning") inside that accompanies the pop, swap your 6V6 with one that doesn't pop and arc.


    It goes without saying but I rarely hear anyone mention it. Everyone offers dire electrocution warnings but they rarely tell you...

    Do not operate your amp without a speaker connected. The speaker load is part of your power amp circuit. The quick way to cause nasty damage to a tube amp is to run it without its speaker connected.[/QUOTE]


    This is excellent advice. Especially, the lights out test. Common sense, and no-cost!

    Regarding operating a tube amp without a load -> I second the need for more broadcasting of that possible tragic trainwreck! Better, I welcome articles that show DIY'rs like myself, layouts and good explanations on how to add these safety circuits to unprotected classics. I'd love to bulletproof my gear, especially when its simple and/or easily reversible to preserve the value upon resale.
     
    Last edited: Sep 29, 2016
  12. goldenhound

    goldenhound TDPRI Member

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    Thanks for the layout RobRobinette. I'm a little unsure how to apply it given that the schematic is for a different amp. I see you've given me a tip there regarding where they should go. I think I'd feel a little bit more at ease if this were an image of the layout of an actual Champ. But, again, thank you for your best intention here.
     
  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    On a single ended amp like the Champ the MOV would be connected from the power tube plate (pin 3) to the + terminal of the first filter cap (B+1) which is the output transformer's power source. In other words the MOV goes between the output transformer's primary input and output.

    See this for more amp protection mods: https://robrobinette.com/5F6A_Modifications.htm#Protection_Mods
     
  14. muchxs

    muchxs Doctor of Teleocity

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    I'm working on a little amp that was built around 1960. It has a switch to mute its output. The way they did it was to lift the speaker load. They disconnect the speaker to mute it.

    Why doesn't it blow?

    The trick is to use heavy duty parts everywhere they might fail. Advances in technology have brought us 2 watt, 3 watt and 5 watt resistors that are approximately the same size as the old 1/2 watt, 1 watt and 2 watt resistors. Flame resistant resistors get hot but they don't burn like old carbon comps. Since we're talking about Champs...

    If we use a 3 watt resistor everywhere we see a 2 watt resistor we're 50% overbuilt. If we use 5 watt resistors in place of the stock 2 watt resistors we're more than 100% overbuilt. We're practically bulletproof considering flame resistant resistors are indeed flame resistant.

    Next, we identify built- in failure modes and use best practice to all but eliminate them. The built- in failure mode in Champs and Vibro Champs is the 6V6 cathode bypass capacitor that's usually in physical contact with the adjacent 470 ohm resistor. Don't do that. The resistor bakes the capacitor. When the capacitor's dielectric burns away the failed capacitor acts as a resistor in parallel to the existing bias resistor. Plate dissipation rises to well over 100%. Usually the fuse blows. If you push the issue by feeding it fuse after fuse or worse, using a bigger fuse the transformers are going to go next.

    The easy fix is to leave some space between the bias resistor and its bypass cap. Stand the resistor off the board a little bit. Leave room for air circulation. I ought to re- think my layouts and provide separate tie points for the resistor and the cap so there is no way to install them in physical contact with each other.

    That's called "mistake proofing"...


    Way I figure it the world will eventually get hip to hot rodded amps. They're faster, they handle better and they're more reliable than stock amps. It wouldn't be the first time the world has come around to my way of thinking. :D
     
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  15. FenderLover

    FenderLover Poster Extraordinaire

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    Good points XS, but I think maybe some Hod-Rodded examples have a connotation of wreckless abandon, sometimes unintended. These examples that we strive for might be better called Blue-Printed. Just musing. Carry on.
     
  16. goldenhound

    goldenhound TDPRI Member

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    Received the Classic Tone OT and installed it. Without plugging in the guitar, or guitar cord, but with the tubes installed, I turned it on.

    There was a noise like a shorted guitar jack, and I had my eyes on the 6V6 when I noticed the 5Y3 rectifier arcing rapidly in all four corners at the top of the plate. I shut off the amp immediately. Maybe three seconds. No flash, no smoke from the OT so I think its still OK.

    I did NOT have any equipment to bring the OT up slowly, like a varistor. In hindsight, I think I should have turned the amp on first without the tubes installed, for a few seconds, and then installed the tubes, and turned it on again in order to slowly break in the OT. At least that's my understanding. I was too tired to be working on it, and should've taken my time.

    I didn't have a lot of time to look at the 6V6. I just noticed when the amp was turned on, there was an increased amount of light coming from the tube 6V6-S (not a flash, but a steadily increasing orange/yellow glow), that lasted a little over a second, and then began to reduce in intensity as I noticed the arcing in the 5Y3

    So now, a bad rectifier? Or might that have been the problem to begin with? I opened up the old OT, saw where the short is, but its unsalvageable as far as I can see. No way to get to the wire to reconnect it as the paper is wrapped tightly.

    I have not yet re-capped the whole amp. Just the filter caps.
     
  17. muchxs

    muchxs Doctor of Teleocity

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    Probably a shorted 6V6.

    Are you sure the bias network is o.k.?
     
  18. goldenhound

    goldenhound TDPRI Member

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    I'm afraid that diagnostic is beyond my current knowledge. But, I would be eager to do such a thing if you suggest it. I have only a MM. Is there a page that details how to do such a thing?
     
  19. Darkness

    Darkness Tele-Afflicted

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    Lower the voltage to the circuit. Champs do not eat tubes and components when they operate within the specified parameters of said components. This is a super easy fix that was recommended to me by David Allen of Allen amps. It not only gets the CBS SF champ voltages under control, it also opens up the tone.
     
  20. Ten Over

    Ten Over Tele-Holic

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    Your Champ is self-biased, so the amp would have to be working to check the bias. You can use your MM to check the bias resistor and capacitor for gross faults with the amp off and the 6V6 removed. Pin 8 on the 6V6 socket goes to a 470R resistor with a capacitor in parallel. The capacitor may cause the ohm meter to be a little erratic at first, but it should settle down to 470R plus or minus 10% or so. Even if the bias value is way off, the tube still shouldn't conduct enough to fry an OT.

    As long as you have the 6V6 out, check pin 3 for continuity to any other pin. Even if it passes this test, the 6V6 still may short when in actual operation in the amp. If it shows continuity to another pin, take the appropriate steps to make sure that the tube is never inserted in an amp again.

    Even though it may be for only one use, you should make either the neutral or hot conductor (not ground) go through an incandescent light bulb before the power cord on the Champ. If you don't have anything at home to cut up, you may have to hit the hardware store, but it will be well worth it. You may have destroyed or damaged your rectifier tube and your new OT. This could have been avoided with a light bulb.
     
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