5F1 Pre-Build Questions - Grounding layout?

[Wittgenstein]

I currently have two builds going, this and a 5E3. I was wondering if I should ground my Champ in a similar manor to the way that I will for my 5E3. I know there is no hoffman style board I can copy, so I'm working on drawing up my own.

In the mean time I drew up my proposed grounding scheme using Ceritone's layout (similar to Fenders layout) and then editing in paint. Is there a better way to do this, any opinions or ideas before I start this build?

[MWJB]

Use the farthest right input jack ground tag as your preamp ground point. Use switchcraft metal jack sockets that ground directly to the chassis. Run each preamp ground (12AX7 cathodes, preamp filter cap) wires directly to this point, separate wire for each (cap & resistor at 12AX7 pin 3 can share a wire).

Ground the volume pot LH tab to the pot body.

You should be OK grounding the 470ohm cathode resistor & 12AX7 pin 8 cathode resistor to the preamp ground, don't ground the 12AX7 cathode to the power amp ground. If you want to ground the 6V6 cathode to the power amp ground, separate the 1.5K & 470ohm ground connection.

[Wittgenstein]

Use the farthest right input jack ground tag as your preamp ground point. Use switchcraft metal jack sockets that ground directly to the chassis. Run each preamp ground (12AX7 cathodes, preamp filter cap) wires directly to this point, separate wire for each (cap & resistor at 12AX7 pin 3 can share a wire).
Ground the volume pot LH tab to the pot body.
You should be OK grounding the 470ohm cathode resistor & 12AX7 pin 8 cathode resistor to the preamp ground, don't ground the 12AX7 cathode to the power amp ground. If you want to ground the 6V6 cathode to the power amp ground, separate the 1.5K & 470ohm ground connection.

Updated layout... is this what you mean. Wouldn't I want to run the 1.5k & 470Ohm cathode resistor from the 6v6 back to the main power star ground, like I have it? That way only the pre-amp section is going back to the input ground. I did seperate the 1.5K resitor running from the cathode (pin 8) of the 12axy and gave it it's own turret to run a sparate wire back to the second star ground. I plan on using insulated jacks for everything except the output jack, so that is why it all runs to a star pint on the far right of the chassis. Is there anything I'm missing?

[MWJB]

Yes, that looks better. I'm not sure why you want to use insulated jacks. Vol pot ground can be soldered to the body of the pot, remove any varnish from the pot body before soldering. You could, if you wanted run a buss wire from the pot back to the input jack grounds (you mentioned Hoffman earlier, this is a technique he uses).

[Wittgenstein]

Thanks MWJB and Tubeswell for the suggestions. I finished wiring it up the other night, but it is giving me some troubles. I fired it up for the first time and there is sound but it is not very loud. With the volume and guitar all the way up, it sounds like it is half volume. I did notice that when I turned the volume from 0-3 it sounds scratchy, but is fine after that. The big problem however, is my heater voltages. I checked and double checked and I'm getting a reading of 60 to 72 Volts AC of my heaters. I do not know how this is even possible, and I quickly shut the amp off to prevent damage. I can add some pictures of the build later tonight, but does anyone have any ideas what could be causing this. I checked all pin connections to make sure a wire is not touching somewhere and it all looks good. I tested my Fluke DMM at the wall and read 122VAC from the socket, so I know that is working.

I have a hammond 290ax power transformer with 2 yellow wires (5V rectifer heaters). 2 red wires (325V) secondaries with a center tapped red/yellow wire, 2 black primary wires (120V), and the 2 green (6.3V) heater wires. I know that I should add two 100ohm resistors to the 6V6 to create an artifical center tap, but that should only reduce hum. I do not see how that could affect the voltages that much.

My B+ voltages look to be pretty reasonable considering the 122V coming from the wall. At the first 16uf filter cap the B+ measured 373VDC. At the second and third filter caps (8uf) as well as the two 100K anode resistors of the 12ax7, measure 343 volts DC.

[MWJB]

What do you get from pin 2 to pin 7 of the 6V6? If you have wired the green heater wires as a twisted pair, but have not got a centre tap, nor a pair of 100ohm resistors, then you have no ground reference, the difference between pins 2 & 7 might be ~6.3VAC, but that 6.3VAC could be floating above ground?

Sure you have the right value resistor between the 2x 8uf caps? It should be 22K (twenty two thousand ohms, red/red/or), even if your 12AX7 is only drawing 1mA per triode that equates to a 44v drop between the 2x 8ufs.

[Wittgenstein]

What do you get from pin 2 to pin 7 of the 6V6? If you have wired the green heater wires as a twisted pair, but have not got a centre tap, nor a pair of 100ohm resistors, then you have no ground reference, the difference between pins 2 & 7 might be ~6.3VAC, but that 6.3VAC could be floating above ground?
Sure you have the right value resistor between the 2x 8uf caps? It should be 22K (twenty two thousand ohms, red/red/or), even if your 12AX7 is only drawing 1mA per triode that equates to a 44v drop between the 2x 8ufs.

Ahhh... the 290ax power transformer just has a high votage center tap (red/yellow wire), but not a center tap for the heaters and like I said, I have yet to add a artifial center tap. But what you are saying is that it has no referance back to ground, and therefore I cannot get an accurate reading?

I am currently working, but I will make sure to measure the voltage from pin 2 to pin 7 on the 6V6 when I get home and post it here. I will also double check the resitor values on between the filter caps, but I am almost possitive (99.9999%) they are correct.

From what I have read, and please correct me if I am wrong, the best way to add the artifical center tap is right at the 6V6 pins like in the Ceritone layout (see attachment).

[MWJB]

No, it's not the "best" way, it might be adequate..it might not. All Fender champs were built with a sigle wire daisy chain, the 2x100ohm resistor method is usually OK, but try hooking them to the cathode too, whatever works, well "works". If I was dc biasing my 6.3VAC I'd supply it from a voltage divider with 35-40vdc (your cathode won't have anything like that) from the B+ supply...I have built SE amps like this and there is no audible clue that they are turned on at idle, vol full up.

[loudthud]

From what I have read, and please correct me if I am wrong, the best way to add the artifical center tap is right at the 6V6 pins like in the Ceritone layout (see attachment).

You are correct.

The 60 to 70 VAC you were measuring is due to capacitive coupling within the transformer. The centertap connection will eliminate it.

[Wittgenstein]

You are correct.
The 60 to 70 VAC you were measuring is due to capacitive coupling within the transformer. The centertap connection will eliminate it.

Could this also be causing the amp to be so quiet as well? When I turrn the volume all the way up it's about half as loud as what I would expect if not less.

The fender 5f1 layout shows a voltage drop from 340V at the first filter cap, to 295V at the second, and 250V down the line, so I was not too worried about my drop from 373 to 343 volts, but I do wonder why the third filter cap down is so high. I will make sure to measure again once I have instaled the artifical center tap. Am I right in thinking it is best to measure all voltages with the volume at 0?

[MWJB]

Measure the plate voltage at pins 1 & 6 of the 12AX7, also measure the cathode voltages at pins 3 & 8. Yes measure dc voltages at idle, no signal, you don't have to have the volume at "0" but this reduces alarming pops when you probe the preamp plates (do this at the junction of the 100K plate resistors & their 0.022 uf coupling caps, it's easier than trying to probe the tube pin itself).

[Wittgenstein]

Installed the 100ohm resistors (artificial filament center tap) to the 6V6 today, and it seems to have reference to ground now. But, there is still not enough output. The amp makes almost no sound until it is turned up past 8 and then it is very quite. It also emits a high pitch constant noise when at full volume, which I assume is oscillation?

What do you get from pin 2 to pin 7 of the 6V6?

Pin 2 to 7 = 6.6VAC

Measure the plate voltage at pins 1 & 6 of the 12AX7, also measure the cathode voltages at pins 3 & 8.

Pin 1 = 194VDC
Pin 3 = 1.59VDC
Pin 6 = 102.4VDC
Pin 8 = -.125vdc ---- does not seem right. I tested the wire running under the board that joins where the 22K and 1,500 resistors to pin 8, and it shows continuity.


Also looking at the fender layout, it shows pin 1 and 6 both being 150V. I know that my amp is running hot, but I would still expect both to be the same 190V or so. Any ideas why the one side of the triode is pulling more volts then the other side?



6V6GT
Pin 8 = 22V

B+ goes from 5Y3 to 16uf cap at 420V, then drops to 364V where it meets the 8uf and 22K resistor, and finally drops to 298.7V for the last 8uf and 100ohm grid connection.

[Wittgenstein]

I figured it out! I'm not sure how I missed it, but I had switched the wires from pin 7 and pin 8 on the 12ax7. After checking for contiunity once again, I noticed my mistake. After swapping those wires out, I plugged it back in for a test. Now the guitar was louder even with the volume low, but there was a very loud high pitched whine. Turning the volume pot changed the sound but it was just as loud when the volume was at 0 as at 11. So I figured I would try swappig the secondaries from the output transformer, and viola..... it works!!!!! With no guitar plugged in, the amp is silent.... no hum.

The one negative however, is that when I turn the volume all the way up, I can hear a high pitched tone, much like a computer monitor that is going bad. Any ideas what might be causing this? I know that I still have some fine tuning to do, but I am just happy to have it playing.

[Wittgenstein]

Anyone?

[loudthud]

Microphonic feedback is an acoustic feedback from the speaker to the preamp tube or other sensitive component. Try the amp with the speaker on the other side of the room ot confirm. Turn the volume up to where the feedback just starts and give the preamp tube a little flick with you finger. Do you hear a pinging sound at the same frequency? The only cure is to get a different preamp tube. Another possibility is that there is a microphonic resistor or capacitor. Poke around with a chop stick to look for a component that seems to be sensitive.

There is a low probability that the feedback is electronic either from lead dress of a bad ground. Try moving wires around with your chop stick to see if movement of any are sensiitive.