Hi,
I just built a really cute clone of a fender champ #AA764.
I built this one so I would have a bit more tone control with the extra bass/treble pots instead of just a single tone control.
Champ AA764
Now, I used one of Triode Electronics OPTs because it appears to an easy low cost drop in replacement PLUS it allowed for a 4/8 ohm switch which I like because I have several different speaker cabs.
The PT was a salvage from an old (60's) Kay Guitar amp that drove a pair of 6V6's and 3-12AX7's. & 5Y3 Rect. So I highly doubt my symptom is a result of lack of verb in the PT.
1st instance.
Im playing along about 15 Minutes, Volume set about 75% and I got to a heavily corded portion of the song, Suddenly, I hear a bit of distortion and The amp drops severly in volume like a built in compressor circuit?.
My First thought was Oh Well, 1 NOS Motorola 6V6GT Tube just bit the dust.
I proceed to install a NOS 1963 RCA 6V6GT. I put another maybe 1 hour on the amp without incident Though I had not run it as hard nor corded it heavily. GREAT!
or so I thought.
When running the amp In the 80% or more range on volume, The symptom returns. If I stop playing a minute, It returns to normal. If I continue blasting out cords, It Fades and distorts to less than a 9V transistor radio. Interestingly, This only occurs during Corded portions, Never During Lead or single note play.
Now, I have tested and re-tested several sets of nos tubes, RCA, Tung-Sol, Sylvania, GE's outputs, And RCA,Tung-Sol, Amperex,Mullard,GE preamps. All tubes test new or better.
Im Stumped as everthing in this baby checks out great, And I Love the sound, But I really need to beat this issue.
The only thing I am not very sure about is If possibly I am pushing to much current at the 6V6GT, The PT is a tad over rated for the job. BTW, It has enough verb to drive a 5881 tube. I have not tested it long enough to see If the problem would occur with the 5881.
Gene
I just built a really cute clone of a fender champ #AA764.
I built this one so I would have a bit more tone control with the extra bass/treble pots instead of just a single tone control.
Champ AA764
Now, I used one of Triode Electronics OPTs because it appears to an easy low cost drop in replacement PLUS it allowed for a 4/8 ohm switch which I like because I have several different speaker cabs.
The PT was a salvage from an old (60's) Kay Guitar amp that drove a pair of 6V6's and 3-12AX7's. & 5Y3 Rect. So I highly doubt my symptom is a result of lack of verb in the PT.
1st instance.
Im playing along about 15 Minutes, Volume set about 75% and I got to a heavily corded portion of the song, Suddenly, I hear a bit of distortion and The amp drops severly in volume like a built in compressor circuit?.
My First thought was Oh Well, 1 NOS Motorola 6V6GT Tube just bit the dust.
I proceed to install a NOS 1963 RCA 6V6GT. I put another maybe 1 hour on the amp without incident Though I had not run it as hard nor corded it heavily. GREAT!
or so I thought.
When running the amp In the 80% or more range on volume, The symptom returns. If I stop playing a minute, It returns to normal. If I continue blasting out cords, It Fades and distorts to less than a 9V transistor radio. Interestingly, This only occurs during Corded portions, Never During Lead or single note play.
Now, I have tested and re-tested several sets of nos tubes, RCA, Tung-Sol, Sylvania, GE's outputs, And RCA,Tung-Sol, Amperex,Mullard,GE preamps. All tubes test new or better.
Im Stumped as everthing in this baby checks out great, And I Love the sound, But I really need to beat this issue.
The only thing I am not very sure about is If possibly I am pushing to much current at the 6V6GT, The PT is a tad over rated for the job. BTW, It has enough verb to drive a 5881 tube. I have not tested it long enough to see If the problem would occur with the 5881.
Gene
There is another possibility. If you increased the value of the coupling capacitor between the plate of the input tube and the grid of the 6V6 you could be experiencing a bad case of "farting out" or blocking distortion. Try reducing the value of the cap, or reducing the value of the grid leak resistor (grid to ground on the 6V6). Most 6V6's are tough and can take a bit of extra voltage.
For a technical explanation of this type of distortion, see:
http://www.aikenamps.com/
Then click Tech Info
Then click Advanced
Then click what is "blocking distortion"
It is interesting to note that although this is primarily a guitar amp issue, it does occur on Hi-Fi amps, especially SE amps.
For a technical explanation of this type of distortion, see:
http://www.aikenamps.com/
Then click Tech Info
Then click Advanced
Then click what is "blocking distortion"
It is interesting to note that although this is primarily a guitar amp issue, it does occur on Hi-Fi amps, especially SE amps.
tubelab.com said:
Hmmmm This is a very interesting article. I used a coupling cap(brand) I have never used before on the Plate to grid location. Some tubular yellow looking thing with no brand name. .02 600V.
Looking at the article I think perhaps I will replace that first. I have several other choices in that value, Orange Drop Sprague, Black Beauties, Mallory ect. That was the only componet I had not used previously in any projects. I have about 30 of them and was hoping to use them up.
Secondly, If the cap does not work, I will try lowering the Value. Now, Lowering to .01? Thats where I get confused.
I have a .02 cap and 220K resistor in these locations. So If I am understanding this correctly, I should Reduce the resistor to a value closer to 100K? Maybe 150K for starters?
Since there Is no grid stopper resistor feeding the 6V6 that part of the article doesnt apply I guess.
I checked the cap and resistors against the other similar champ and princeton models and they seem to match up to mine, I am actually surprised this issue has exposed iteslf as on a simple Amp like this.
I will check back in later on this, and let you know the outcome.
THANKS!!
Gene
I have built a similar amp, although it was about 10 years ago. I don't remember any issues and I tend to severely abuse guitar amps.
Before digging deeply into the amp, try hooking up a voltmeter to the cathode of the 6V6 and driving the amp into silent mode. If the above mentioned problem is happening, the voltage will go down. If that doesn't show any problem, probe different points in the amp until you find something that is changing. Then probe the adjoining circuitry. There aren't many things to go wrong in this design. You might have a resistor that is changing value when it gets hot. If there are no voltages that change when the amp fails you have a cap that is going open with heat.
By probing a voltage, I mean connecting a voltmeter up with clip leads, one lead to ground, the other on the point to be probed. Then set the amp up so it can be played in the normal fashion. You can then read the meter without touching anything, while you are playing. I like to use multiple cheap digital meters to watch multiple points when searching for an intermittent problem like this. I use the $5 ones from Harbor Freight.
You didn't say, but is the amp installed in a cabinet? If so, does it act up when it is removed from the cabinet? If not try heating each part individually to induce the failure. I use my soldering iron for this, but be careful, hold it with one hand and don't touch ANYTHING with the other hand. Feed the amp with a signal generator, or a CD player. A portable CD player with a test CD makes a good cheap signal generator.
If a heat sensitive component turns out to be the problem, I would bet on a resistor, especially if you used vintage carbon composition resistors. A cap could go open when it gets hot, this doesn't happen often with modern parts, but it used to be common.
Before digging deeply into the amp, try hooking up a voltmeter to the cathode of the 6V6 and driving the amp into silent mode. If the above mentioned problem is happening, the voltage will go down. If that doesn't show any problem, probe different points in the amp until you find something that is changing. Then probe the adjoining circuitry. There aren't many things to go wrong in this design. You might have a resistor that is changing value when it gets hot. If there are no voltages that change when the amp fails you have a cap that is going open with heat.
By probing a voltage, I mean connecting a voltmeter up with clip leads, one lead to ground, the other on the point to be probed. Then set the amp up so it can be played in the normal fashion. You can then read the meter without touching anything, while you are playing. I like to use multiple cheap digital meters to watch multiple points when searching for an intermittent problem like this. I use the $5 ones from Harbor Freight.
You didn't say, but is the amp installed in a cabinet? If so, does it act up when it is removed from the cabinet? If not try heating each part individually to induce the failure. I use my soldering iron for this, but be careful, hold it with one hand and don't touch ANYTHING with the other hand. Feed the amp with a signal generator, or a CD player. A portable CD player with a test CD makes a good cheap signal generator.
If a heat sensitive component turns out to be the problem, I would bet on a resistor, especially if you used vintage carbon composition resistors. A cap could go open when it gets hot, this doesn't happen often with modern parts, but it used to be common.
OK Heres A Bit More Info.
The Amp is not In A Cabinet. BUT, I tried flipping the unit upside down so I could Look underneath. Oh My, Arcing across the socket occurs when at near full volume and Hit an A cord!
This acring is from solder terminal to solder terminal on the socket itself, No place near the tube pins.
Oddly, It only seems to happen at that cord? arcing is between pins 2 & 3. pin 2 being a filament and pin 3 being plate.
It was first seen as a momentary acr. We then discovered that it can actually start acring constant and of course this is not a good thing.
The gap is over an 1/8th inch !! I have heard of acring sockets before, But never had actually witnessed it.
But It appears that I might have actually uncovered it though accidently.
The Giveaway was the pilot light going out at the moment it occurs. No Pilot=Arcing, Pilot On=Not acring Interesting.
I am going to try A: heatshrink insulation over pin 3 solder terminal.
hopefully this is all thats actually wrong.
Gene
The Amp is not In A Cabinet. BUT, I tried flipping the unit upside down so I could Look underneath. Oh My, Arcing across the socket occurs when at near full volume and Hit an A cord!
This acring is from solder terminal to solder terminal on the socket itself, No place near the tube pins.
Oddly, It only seems to happen at that cord? arcing is between pins 2 & 3. pin 2 being a filament and pin 3 being plate.
It was first seen as a momentary acr. We then discovered that it can actually start acring constant and of course this is not a good thing.
The gap is over an 1/8th inch !! I have heard of acring sockets before, But never had actually witnessed it.
But It appears that I might have actually uncovered it though accidently.
The Giveaway was the pilot light going out at the moment it occurs. No Pilot=Arcing, Pilot On=Not acring Interesting.
I am going to try A: heatshrink insulation over pin 3 solder terminal.
hopefully this is all thats actually wrong.
Gene
Oh,, I forgot the #1 commandment, haha
Thou shall post pics of all builds!
Top Front View
Top Rear View
The Messy Side
If I had to do it over again, And I am sure I will, I could have simplified The point to point a bit more. But Hey, after being away from this hobby 30yrs, I am off to an ok start.
Gene
Thou shall post pics of all builds!
Top Front View
Top Rear View
The Messy Side
If I had to do it over again, And I am sure I will, I could have simplified The point to point a bit more. But Hey, after being away from this hobby 30yrs, I am off to an ok start.
Gene
I just thought I'd add that the problem sounds like inductive kick back from the output transformer and that is the one part you substituted. It is a proven design after all, with the original transformer. Also the secondary is not well damped since it sees the voice coil inductance of the speaker. You might try a snubber on the output, just to add some damping at HF - say 25 ohms in series with .5 uF. You might try driving it with a square wave into the speaker and you'll probably see some ringing on the edges without the damping. Nice work!
Pete B.
Pete B.
Giaime said:Are you sure that you're ALWAYS using a load on the secondary of the output transformer? That also to me sounds like inductive flyback. Always connect a suitable speaker to the amp when it's on.
100% sure the speakers connected. It has done this on 2 seperate cabs. 1- 4X12 Hartke, And 1 Single 12" seymour duncan cab. 2 Different speaker cords were used.
This happens "while" playing.
It has also happened 1 time seconds after turning it on. In every case, It has been in the upper volume settings.
If played at lower (under 60%) settings on volume. It runs fine, No arcs, No weirdness of any sort.
I had run the amp over an hour without incident.
I am pretty sure the OPT is good, I am sure if they were having problems with this particular model, Triode Electronics would have not sold them. There must be hundreds of them out there. I have several myself and never had a bad one or problems with them.
I know alot of guys do use these as they are a direct drop in replacement on the champ/princetons.
I am certain I observed the proper wiring (polarity) color codes as supplied with the transformer also.
The Picture I posted shows a 5881 installed which BTW sounds fantastic in this baby just plug and play. Both tubes experience the same acring issue.
gene
If you are using their "TF103-48 Champ Transformer", I have used them with out any issues. There is another possibility. The amp could be oscillating at a frequency well above the audio band. Since the inductance of the transformer is high enough to look like an open circuit at these frequencies, the voltage can rise up to the arcing levels. This can be confirmed with a scope. If you don't have a scope, try adding a grid stopper resistor to the 6V6, and maybe the driver tube. Try about 1K ohms right at the grid pin of the socket. No scope, try playing the amp next to an AM radio, or TV, if there is any interference, you have oscillation.
Beware, if you insulate the plate lead good enough to stop the arc, the rising voltage will find another path, inside the transformer! If the socket is clean and in good shape the normal plate (with signal) voltage should not arc.
Beware, if you insulate the plate lead good enough to stop the arc, the rising voltage will find another path, inside the transformer! If the socket is clean and in good shape the normal plate (with signal) voltage should not arc.
Hi Gene,
I'm glad you found it. Lucky or what?
You must have some really high HV spikes to break down the air across 1/8". I have respect for the socket!
No spikes on the solder? You might want to use some HV dope as well, but finding the root of this spike is best. Something else will eventually break down.
-Chris
I'm glad you found it. Lucky or what?
You must have some really high HV spikes to break down the air across 1/8". I have respect for the socket!
No spikes on the solder? You might want to use some HV dope as well, but finding the root of this spike is best. Something else will eventually break down.
-Chris
tubelab.com said:
Yep TF104-48 Champ Tranny.
A Great Buy at 15 bucks.
I have an old scope(No Real Idea on how to use it though) Also have a vintage Eico Signal generator, Same as above. And several other vintage test pieces, Even a couple tube volt meters haha.
I have gone back and looked at every possible difference in this amp, and the one thing I overlooked I really doubt is the cause was the cap across the 470 ohm 5 watt resistor value is not at original schematics value. I had Subbed a 47uf/50V for the 25uf/50V shown in the schematic.
In Fact, The 2- 25uf/25V caps on the 12AX7 have also been upped to 47uf/50V.
Raising the caps value on the 12AX7 seems to give the amp a bit more breakup and gainyness. Maybe this is where I have messed this up.
I am very suspicious of the wire used to run the pilot and the way I routed it. It may actually be arcing through the insulation iteslf at pin 8 also (not visibly)
So, tonite I am going to re-wire the Filament leads, Move the Pilot leads off the socket, Replace the sockets for both the tube and the pilot. Change the 47/50V cap on the 6V6, Also Replace that Yellow Corniel Dublier .02 cap on the grid.
Then I'll give her a whirl on Full Volume a bit, If its still acring,Then I will start adding stoppers if needed.
Gene
Arrg,
I moved rewired the filament replaced the lamp socket and re-routed the wires so as not to come in close proximity to anything .
Tinkered with a couple other after thoughts, No acrs now, BUT,
I start getting a motorboating type of oscillation once the thing hits the critical point where trouble occurs.
The Amps set very loud, I start cording out some old journey tune and suddenly It starts fading. this time no arcing. Then I start to hear a bit of pulsating thumping noise as it fades.
Kill the power, wait 10 seconds and everythings normal until I repeat the cording.
Now, Another thing I have noticed. I put a different 6V6(clear top) in there so as not to further risk damage to my NOS stuff.
Should the Internal glow(not Filaments) change as I Cord notes out? There is a significate change in brightness. It changes level of brightness on each heavy cord action.
gene
I moved rewired the filament replaced the lamp socket and re-routed the wires so as not to come in close proximity to anything .
Tinkered with a couple other after thoughts, No acrs now, BUT,
I start getting a motorboating type of oscillation once the thing hits the critical point where trouble occurs.
The Amps set very loud, I start cording out some old journey tune and suddenly It starts fading. this time no arcing. Then I start to hear a bit of pulsating thumping noise as it fades.
Kill the power, wait 10 seconds and everythings normal until I repeat the cording.
Now, Another thing I have noticed. I put a different 6V6(clear top) in there so as not to further risk damage to my NOS stuff.
Should the Internal glow(not Filaments) change as I Cord notes out? There is a significate change in brightness. It changes level of brightness on each heavy cord action.
gene
Hi Gene,
The heater brightness change is due to the heater voltage dropping. This must be due to a heavy load on the power transformer.
Is it possible that you are exceeding the maximum DC current through the output transformer, causing the core to saturate? This would drop the inductance and draw even more current.
-Chris
The heater brightness change is due to the heater voltage dropping. This must be due to a heavy load on the power transformer.
Is it possible that you are exceeding the maximum DC current through the output transformer, causing the core to saturate? This would drop the inductance and draw even more current.
-Chris
anatech said:
The Heater Brightness isnt changing, They are staying constant. I beleive its the screen glow? At Idle, The tube seems to be glowing normally like my other amp. But When I hit a cord, It Gets much brighter. Not the Filaments though, They stay correct.
I have been suspect of the DC current issue as this PT was a Pull from an old amp with a Pair of 6V6's and 3- 12ax7's. I figure it had plenty of verb for the original setup, so it has to be over rated in this one.
I am thinking you might have just nailed it down. But Now, What can I do to correct this?
gene
Soon as mama gets home I will try to get voltages off the PS and Plates Ect. I never ever putz with this stuff when nobodies home.
It is a good idea not to play with electricity when you are the only one at home.
If the screen grid (or any of the grid wires) are glowing red, that is a bad thing. Glowing grid wires (red hot kind of glow) are the vacuum tube equivalent of screaming in pain. Glowing grid wires will kill a tube quicker than a glowing plate. A bluish glow that changes with sound is normal.
Glowing screen grid wires are caused by too much screen dissipation. Measure the plate and screen voltages. If they are much above 350 you are in trouble. The specified maximum screen voltage for a 6V6 is 315, Fender did not always follow the rules as is evident from this schematic. 350 volts on the screen, maybe 15 on the cathode, that gives 335 across the 6V6, above the recommended max. Fender found out what we now know 6V6's sound best right before they blow up! Measure the cathode voltage, divide by .47 (cathode resistor in K ohms), that gives the tube current in milliamps. You should not go much beyond 40 to 45 milliamps for good tube life. Put a meter on the cathode, and watch the voltage. See what happens to the cathode voltage when the amp starts to act up. If it rises abruptly, I would suspect an oscillation problem. An oscillation can cause high screen or plate current without any other symptoms. Sometimes it causes mild distortion, sometimes it causes total trash through the speaker.
You mentioned motorboating. Try disconnecting the feedback path. The easiest way to do this is to take a clip lead and jump the 47 ohm resistor in the cathode circuit of the 12AX7. Disconnecting the 2700 ohm resistor is another possibility, whichever is easiest. If that solves your problem, you need to swap the primary, OR the secondary leads (not both) on the output transformer.
If the plate or screen voltage is too high, there are a few things to try. Try raising the 470 ohm cathode resistor to 560 or 680 ohm. This will reduce the tube current and tube dissipation. The 1k resistor in the B+ line could be raised to lower the screen voltage. Another way to lower the B+ voltage without swapping the power transformer, would be to add a choke between the 5Y3 and the first filter cap.
You mentioned that you had the same problem with a 5881. Do you get the same glow effects? If so voltage may not be your problem since they can take 400 volts, plate and screen. I would suspect oscillation. Try the test with an AM radio, and a TV. Try the TV on all of the low (2 to 13) numbered channels. I have found two oscillating amplifiers because I happen to have a TV in my work room. One amp totally blanked out channel 4 (even with cable) and the other caused faint lines on channel 6 when I played it loud. Grid stoppers fixed both.
If the screen grid (or any of the grid wires) are glowing red, that is a bad thing. Glowing grid wires (red hot kind of glow) are the vacuum tube equivalent of screaming in pain. Glowing grid wires will kill a tube quicker than a glowing plate. A bluish glow that changes with sound is normal.
Glowing screen grid wires are caused by too much screen dissipation. Measure the plate and screen voltages. If they are much above 350 you are in trouble. The specified maximum screen voltage for a 6V6 is 315, Fender did not always follow the rules as is evident from this schematic. 350 volts on the screen, maybe 15 on the cathode, that gives 335 across the 6V6, above the recommended max. Fender found out what we now know 6V6's sound best right before they blow up! Measure the cathode voltage, divide by .47 (cathode resistor in K ohms), that gives the tube current in milliamps. You should not go much beyond 40 to 45 milliamps for good tube life. Put a meter on the cathode, and watch the voltage. See what happens to the cathode voltage when the amp starts to act up. If it rises abruptly, I would suspect an oscillation problem. An oscillation can cause high screen or plate current without any other symptoms. Sometimes it causes mild distortion, sometimes it causes total trash through the speaker.
You mentioned motorboating. Try disconnecting the feedback path. The easiest way to do this is to take a clip lead and jump the 47 ohm resistor in the cathode circuit of the 12AX7. Disconnecting the 2700 ohm resistor is another possibility, whichever is easiest. If that solves your problem, you need to swap the primary, OR the secondary leads (not both) on the output transformer.
If the plate or screen voltage is too high, there are a few things to try. Try raising the 470 ohm cathode resistor to 560 or 680 ohm. This will reduce the tube current and tube dissipation. The 1k resistor in the B+ line could be raised to lower the screen voltage. Another way to lower the B+ voltage without swapping the power transformer, would be to add a choke between the 5Y3 and the first filter cap.
You mentioned that you had the same problem with a 5881. Do you get the same glow effects? If so voltage may not be your problem since they can take 400 volts, plate and screen. I would suspect oscillation. Try the test with an AM radio, and a TV. Try the TV on all of the low (2 to 13) numbered channels. I have found two oscillating amplifiers because I happen to have a TV in my work room. One amp totally blanked out channel 4 (even with cable) and the other caused faint lines on channel 6 when I played it loud. Grid stoppers fixed both.
Hi Gene,
You just nailed the problem. You are using a P-P transformer in a single ended application. There is no air gap in a P-P transformer since the plate currents oppose each other and cancel (or very close to cancel) each other out. You need a single ended transformer.
The other option is to keep building and make this one push pull.
-Chris
You just nailed the problem. You are using a P-P transformer in a single ended application. There is no air gap in a P-P transformer since the plate currents oppose each other and cancel (or very close to cancel) each other out. You need a single ended transformer.
The other option is to keep building and make this one push pull.
-Chris
Gene states that the PT (power transformer) was removed from a push pull amp. The OPT that he refers to is a SE specific transformer intended (and marketed) as a direct replacement for the transformer in a Fender Champ. It is a tiny little thing but it was cheap ($15USD) so I got a pair and put them through the usual tests. For their intended use (a 5 watt guitar amp with limited frequency response) they are excellent.
I tried a pair of these in a transformer sound off in a 300B SE amp with 60 mA of current flowing through them. They obviously didn't stand a chance against Electra Prints and One Electrons, and came in dead last, BUT half of the listeners could not tell them from the Hammond 125CSE ($30USD). Everyone was shocked when I revealed the size and cost of the transformers. The results will be on my web site (along with measured data) when I do the next major update. I am doing a Hi-Fi amp (6AQ5 SE) with these.
I have used these in SE 6V6 and SE 6L6 guitar amps, and they ROCK!
I can't believe that the transformer is the source of the problem.
I tried a pair of these in a transformer sound off in a 300B SE amp with 60 mA of current flowing through them. They obviously didn't stand a chance against Electra Prints and One Electrons, and came in dead last, BUT half of the listeners could not tell them from the Hammond 125CSE ($30USD). Everyone was shocked when I revealed the size and cost of the transformers. The results will be on my web site (along with measured data) when I do the next major update. I am doing a Hi-Fi amp (6AQ5 SE) with these.
I have used these in SE 6V6 and SE 6L6 guitar amps, and they ROCK!
I can't believe that the transformer is the source of the problem.
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