5F1 Champ ---- some problems

[calis]

Hi guys.

A couple weeks ago, I finished building a 5F1 clone (my first build). I have noticed a few problems since then. I should mention, this is not a combo; I built the 5F1 circuitry into a head that I am presently running into a 212 cabinet. I realize it's atypical, but it actually sounds really good through 12s and I think I'll keep it this way. Anyway. Here are my problems...

(1) A low hum.
This hum occurs even when nothing is plugged into the amp and the volume is turned all the way down. At first, I wondered if it was the transformers or rectifier or something like that, but I am positive that it is coming out of the speakers instead. It is not the same kind of hum that you get with single coil pickups. It is a much lower/bassier type of hum. It does not increase in volume or intensity as the volume is turned up on the amp. The general noise level of the amp is very low aside from this one humming. And the humming is not very loud, but it is noticeable and I have not heard it in any other amp that I have ever played through. Any thoughts?

(2) Loss of connection with the higher gain input.
Suppose I plug my guitar into the higher gain input. Then, I put my hand on the left side of the cable that is plugged into the amp input and put some gentle rightward pressure on the cable, the amp will buzz suddenly and lose connection with the guitar until I release pressure. The lower gain input does not do this. I assume this is the sign of either bad wiring or bad components or a bad cable. I checked my wiring and it looks good. I tried the cable in a different amp on several different inputs and could not replicate the problem. So, I wonder if the jack was bad from the start. Any thoughts?

Thanks a lot guys. As usual, I really appreciate all of your help and advice.

Justin

[tubeswell]

My guesses would be:

1) normal champ type hum in the power supply. You can increase the filtering to try to reduce the ripple, either by:
a) upping the reservoir to 40uF (if you haven't done so already), and/or

b) replacing the first (10k) supply resistor with a small (say 40mA) choke (if you didn't build it with one already) - in which case you might want to add a 470R to 1k5 3W screen grid resistor if the screen gets more than about 3-4V above the plate voltage (This will also give the amp more 'bite') and/or

c) making another RC filter stage before the OT supply node (5E7 style), with either with a (slightly) larger (say 50mA min) choke or a resistor with a 10uF to 20uF reservoir filter cap - then the choke - then (say) 50-60uF at the OT/2nd filter cap node

d) the other thing is it could be your heater AC coupling causing hum. Check that your 6.3V supply is away from signal wires and where possible only crosses them at right angles. How have you wired the heaters? If you wire them as per the Fender layout diagram (i.e.; with one side of the 6.3V winding to ground and the one wire on the other side going to the filament pin 2 of the output tube and pins 4 & 5 of the pre-amp tube, and with pin 7 of the output tube and pin 9 of the preamp tube going to ground, then you are likely to get more hum, than if you have wired the heaters as a twisted pair with one side of the winding going to pins 2 and 4/5 and the other side going to pins 7 and 9 respectively, with a heater winding centre tap to ground (or with the 100R 'dummy grounds' from each side of the winding to ground)

e) grounding which could be improved upon

2) Dodgy socket wiring - go back and fix it.

[Chuck H]

I have modded Champs with a humbucking filament arrangement. Some call it a twisted pair, but twisting the wires is not the main difference. As tubeswell described, one side of the 6.3v wind goes to pins 2 of the 6v6 and pins 4&5 of the 12ax7, the other side of the wind goes to pin 7 of the 6v6 and pin 9 of the 12ax7. Obviously if you have one side of the 6.3v winding grounded now you will have to unground it to do this. Double check the pilot lamp wiring to be sure one side of the 6.3v winding is not grounded there. Now you must use the 6.3v winding center tap. If you don't have a center tap you can make a false center tap by soldering a 100 ohm resistor to each end of the 6.3v winding and then solder the free ends of those resistors together. Where the resistors are soldered together will act as a center tap. But instead of grounding this center tap you should solder it to the top (ungrounded end) of the 6v6 cathode resistor. This elevates the winding from ground and provides quieter operation. This has worked very well for me.

But you may also want to look into how you have the different amplifier circuits grounded. I'd think grounding would be easy in an amp like the 5f1 but strangely it's the little single ended amps that seem to get fouled up on the grounding scheme by builders.

Replace the bad jack.

Chuck

[Big Daddy]

A little secret.....

B+ to the choke, use a fender deluxe type. Then the plate of your power tube, a 1K resistor, screen then a 22K resistor for the preamp. You can go as high as a 27K or as low as a 10K.

In a push pull amp the plate goes at the same node as the choke. This setup works better for an SE amp.

The 1K resistor can be changed to anything up to 10K even and you will notice an effect people swear you cannot get in an SE amp, SAG. I also use a 470-1K screen resistor directly to the tube socket and at least a 1.5K grid stopper.

Run your heaters like the later model fender with twisted pairs and 2 100 ohm resistors to ground either at the green AC ground wire at the transformer or to lift it to a higher level at the actual cathode of the power tube before the resistor and cap get grounded.

I use 4 20mfd caps for my power supply. You could try different values but I would not go over 30 for a 5Y3 and 47 for a 5AR4/GZ34. Everything you do to your B+ will effect the sound/tone of a champ drastically. Also you can mess with the preamp plate resistors a bit too for more gain as well as the cathode resistors.

Use chop sticks with the amp on and volume all the way up and move things around a bit to find any bad solder joints or improper wiring as in lead dress.