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HELP! Problem with 5F1

11/1/2004 11:02 AM
HELP! Problem with 5F1
Hi All,  
I just finished up a 5F1 kit and am having an issue. It works, but not quite right. When I play hard, like a hard struck chord, the sound distorts heavily and then fizzles out to no sound at all. Sometimes it comes back, other times I have to shut it off and wait a while before it works again. It tends to do this more easily at higher volumes. It also gave me a wicked sqeal once while turning the volume up. I have double and triple checked all the connections and solder joints, and everything looks good.  
Any ideas as to what would cause this?
11/1/2004 1:34 PM

A few different things could cause this, but on a new build often it is a tube grid which is not properly referenced to ground.  
On the 5F1 the most obvious example is the 220K from the 6V6 grid to ground. Make sure its there and soldered in real good.  
(Remember if this was not a cathode bias amp, this 220K would be connected to a negative voltage supply instead of ground, so this is aimed primarily at the 5F1)  
But each grid of a 12AX7 needs a stable ground reference, usually 1megohm.  
I like grid references best when there is a dedicated resistor soldered from the socket to ground, and depending on your layout and grounding technique you can achieve as good performance as possible using a recycled 1Megohm pulled from a PCB, having about the shortest leads you can use, and solder that between the grid pin and one of the ground ears of the tube socket mounting ring.  
For this to work ideally, the socket and/or chassis ground must be part of a coherent grounding system, whether intentionally coherent or not :)  
But odds are in your favor, even if the rest of the amp is wired for anything but chassis ground, you can probably add a resistor directly to a socket that needs it, and improvement will be heard.  
If this is the problem.  
There is supposed to already be a direct DC path to ground for each grid in this amp. Something like a megohm seems like a lot of ohms, but this is a highly critical location, and if you don't have a good ground reference you will be sorry. For instance if your soldering is not ideal, you may end up with one joint which has almost an ohm of resistance where you normally get way way less than half an ohm usually, for all practical purposes the resistance you add because of solder is negligible. The resistance of a good solder joint is too small to measure anyway with the kind of DVM's we typically use.  
Even if one joint has an ohm, what difference does that make when there is a million there already in the form of a megohm resistor which you are soldering to?  
The grid is too critical a location, you will hear it and it will suck.  
Although . . . there is bound to be a performing style where someone will be able to build a cult around it's suckiness . . . ;)  
Anyway, assume your tube mounting rings are ideal grounds, and that short little second-hand megohm resistor is soldered there in the most physically ideal location from the grid pin to the ring ground. With a resistor having such short leads, there is virtually no additonal length being added to the grid terminal of the tube. Normally people do add lengths of wire from the tube socket to the wiring board and things, but each bit of length acts as an antenna which can lead to pickup of unwanted interference.  
So the meg across the socket is a reference assembly. In the design proces as you extend wires for convenience of layout, you can maintain the same *schematic* as if the resistor were soldered right to the socket. As long as it sounds the same it doesn't have to be true point-to-point to be as good as it gets. But you can add a couple more solder joints in the process and there is room for a couple more errors.  
Now on the 5F1, the first 12AX7 triode does not have exactly what I consider ideal layout, but it is close.  
In order for the #2 jack to attenuate the signal in the conventional way, the 1meg with the short leads is soldered to the jack rather than the socket, and there is an additional 68K between the grid and the 1Meg. Now a 1meg resistor can vary by more than 68K from nominal to begin with if its 10percent tolerance. Plus both 68K's go in parallel when you plug into just jack #1 so that's only an effective 34K the way most people play most guitars. The only real drawback is the few additional solder joints and inches of unshielded wire but that is not normally a problem, this is a well proven amp. If everything is well soldered, this grid sees ground as well as it would having the 1.034megs right there at the socket pin. Provided the jacks are well grounded, the chassis alone may not be good enough and it may help to add a ground wire and perhaps after that isolate the jacks from the chassis occasionally. I think your problem is in the next stage anyway.  
Looking at the second 12AX7 triode on the schematic, there is no resistor from this grid to ground since the 1meg pot is in that position. So the grid sees one meg to ground when the pot is maxed. When volume is less than max, the grid sees less than 1meg between it and ground. Actually the less impedance there is between grid & ground, the more stable the gain stage, but you need nominally 1meg on a grid like this so you don't lose too much signal to ground when you want full volume. So theoretically this pot should work as well or better at any setting compared to a fixed 1meg resistor at the socket terminal, which would leave you stuck at full volume anyway.  
Except one thing. The layout does not guarantee support for the schematic. Looks like you would need factory training, additional assembly notes or bulletins, or just get lucky. Or maybe figure it out using the same logic.  
Looking at the layout of the volume pot, you have the signal coming in from the 0.02 cap to the top terminal of the pot. There is a constant 1megohm between this top terminal and the bottom terminal. The variable wiper terminal goes to the grid. For the grid to see ground, this pot must have the bottom terminal solidly connected to a good clean ground point. On the layout this is not clearly shown. I like to run wire from the bottom pot terminal to a carefully selected low noise ground point. Instead, on the factory units often the terminal was bent to contact the metal pot case and soldered to it. Then when the pot is mounted to the chassis that completes the ground scheme for this stage. If the pot is mounted tightly, using star washers of alloys compatible with the chassis, nut, and pot, and there is an effective chassis ground scheme, you may get performance which is not noticeably any different than if this ground was soldered. And if the performace is good, it may also be as durable as if it was a soldered ground. However you would only know for sure over the long term.  
If this pot terminal has nothing connected to it yet, I would solder a wire from it to the same ground as the 220K from the 6V6. See how that sounds.  

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