Fading or Frying Fender Clone Issue

[Trout]

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

[anatech]

Hi Gene,
Monitor the voltage on the cathode of the 6V6 at different levels. Not much else to go wrong, really. Anyhow, a zener across the cathode resistor to limit the voltage climb might help a bit.

-Chris

[tubelab.com]

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.

[Trout]

Originally posted by tubelab.com
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.

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.

Fender amps are particularly susceptible to this because of the large values of coupling capacitors on the grids of the power tubes (0.1uF). Blackfacing your Super Reverb can actually make the problem worse, because you change the grid bias feed resistors from 100K to 220K, which increases the time constant of the AC coupling to the output tube grids. You will note that most Marshalls use 0.022uF coupling capacitors and 100K resistors, which gives a much faster time constant. In addition, the preamp stages have a much more rolled off low frequency response. This is why they sound tighter when played wide open.

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

[tubelab.com]

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.

[Trout]

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

[Trout]

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

[PB2]

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.

[Giaime]

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.

[Trout]

Originally posted by Giaime
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