5F4 Head Build Questions

[Rocketrob]

I'm building a 5F4 cuircut in a head with stand up transformers and choke. I have not yet built an amp with fixed adjustable bias and a PI filter choke, so a few questions need to be answered please.

The grounding scheme: I'm planning to use three grounding points; One for the AC line in ground (near the point where the cable enters the chassis), one for power supply section grounds, including the PT center taps, and first filter cap (on one of the PT mounting bolts), the third ground point will be for the pre amp section, including v1 and v2 cathodes, input jacks, and volume and tone pots ( near the input jacks).

This is what I'm not sure about:
Where to ground the two filter caps after the choke?
Where to ground the Presence pot?
Where to ground the output jacks?
Where to ground pin 8 on the output tubes (6L6GC or 5881)?

I know most builders have their own favorite way of doing these grounds, so your ideas are much appreciated. RD

[djembutz]

You just answered your question. If you visit ceriatone.com website, there's huge and proven ground scheme. Pick what you needed.
About your question :
Where to ground the two filter caps after the choke? -> Same as PT and OT CT grounded
Where to ground the Presence pot? Same as PT and OT CT grounded
Where to ground the output jacks? Same as PT and OT CT grounded
Where to ground pin 8 on the output tubes (6L6GC or 5881) -> I put mine right on 6L6 socket screw with washer

I guest you just build and reporting here in forum if you had a trouble.
Many members here will happy to help..
Cheers.. and happy building

[loudthud]

First cap after the choke and pin 8 of the 6L6s goes to the center tap ground. Presence control, output jacks and filter cap for the PI go to the preamp ground.

[Rocketrob]

Thanks for the replies.

I have a copy of the ceriatone layout, and weber's and fender's. Only a few small differences between them.

Loudthud: your suggestion to ground the presence pot and the output jacks(OT CT) to the pre amp ground surprised me. could you elaborate?

Thanks, RD

[loudthud]

RD, I'm not a member of the "CT to transformer bolt" cult. Since it's rare to break people from the grasp of this cult, and you seem to be a member, I have tried to work within the limitations imposed by this evil practice and make suggestions about your purposed build that will result in success. But first let me say there are many ways to connect grounds. Some are better than others, and many of them are good enough. I haven't tried them all. It would be easier to connect a ground buss wire to the input jack and route it behind the pots picking up grounds throughout the preamp as you go and terminate the wire at the Presence control. But whether you do that or try to make a star at the input jacks is up to you.

It all starts with the HV center tap wire. The current pulses in that wire are what makes the buzzy 120Hz hum. But it doesn't stop when you connect that wire to the chassis. Those current pulses must find their way to the first filter cap, through that cap and back to the transformer via the rectifier(s). So the wire that goes from the chassis to the first filter cap has those current pulses in it and, although tiny, a small voltage drop of a couple of millivolts of buzzzzzzzzzzzz appears between the chassis and the ground side of the first filter cap. If your input jacks ground to the chassis but you input tube's cathode resistor goes to the buzzy ground, you've got a buzz in series with the guitar signal. To eliminate the buzz, the ground for the first tube's cathode resistor must go to the input jack so that there is no little buzzy voltage between the tube and the guitar. This same idea is used until the signal gets to the output of the phase inverter.

Note that so far only ground has been mentioned. If any of the filter caps for the preamp connect to ground at the buzzy ground, that buzz will get onto the B+. Filter caps are pretty good at conducting the 120Hz. So the preamp filter caps need to go to the quiet preamp ground that comes from the input jack.

In the ideal situation, the feedback around the power amp comes back to the phase inverter where it is compared to the incoming signal. Grounding the speaker jacks there means you are not injecting any power supply buzz into the feedback loop. The output transformer isolates the power supply buzz from the secondary winding. (Note: ground the secondary at the output jack, then run a wire to ground.) And, it is a good practice to transition from the quiet preamp ground to the noisey power amp ground where the signal is differential (with plus and minus phases) such as the inputs to the power tubes. The noise between grounds is rejected as a common mode signal. Also, the filter caps should be connected so that the one for the phase inverter is on the quiet ground and the one for the screen grids is on the noisey power amp ground. The resistor that separates these two filter caps plays an important role in keeping buzz off of the quiet ground and from contaminating B+ for the preamp.

Grounding of the output tubes is not overly critical. Naturally you don't want to connect them to the quiet ground because it may cause a strange oscillation. But as far a hum goes, it's hard to make a mistake because the push pull nature rejects ground and B+ noise pretty well. In a fixed bias amp you can connect the ground for the bias supply to the chassis or the output tube cathodes it won't make a huge difference.

Grounding for the center tap for the heater supply can be a little tricky. Sometimes you connect it to the quiet ground and it makes it noisey. Sometimes you connect it to the noisey ground and it's fine. I suggest a separate chassis connection but be prepaired to experiment. Hum from the heater supply is almost always a 60Hz sinewave, not a 120Hz buzz.

Star grounding works in theory because those buzzy currents from the HV center tap only go to the first filter cap and don't contaminate the other grounds. A stack of spade lugs on a transformer bolt is far from ideal. The order in which you stack the spade lugs will make a big difference. It's a better idea to know where the ground currents need to go and prevent them from talking to one another by keeping their paths separate.

[txstrat]

loudthud
Wow, this is quite a great description. I think I've learned a lot.
You don't have sketches or a layout to illustrate your suggested ground points, do you?

[Rocketrob]

Wow, thank you for the in depth answer. I'm trying to digest the ideas and information that you presented. I'll post a drawing later to see if I understand what you are saying. RD

[loudthud]

Here's a modernized schematic with grounding details. The resistor values in the bias supply will probably need adjustment to get the pot in range depending on the exact transformer used. There are people who don't like PI B+ filtering so I drew the output transformer connection as an option. I changed a couple of other things that shouldn't impact the tone significantly.

[Rocketrob]

Thanks Loudthud!

Here is a sketch I made before you posted your schem. I think i'm on the right track here. Sorry for the bad drawing. Also on my chassis the vol, tone and presence pots and the input and output jacks are not isolated from the chassis. Will this create more round loops?

One more question. The PT i'm using is a Hammond 276X (320-0-320 150ma, 5VCT-3A, 6.3VCT-5A) there is no bias tap, so I'll have to take it off pin 4 or 6 on the rectifier. I'm using a 10K-L bias pot. What values should the resistors be in the bias circuit? One in series with the diode and one out of the bias pot to ground. RD

[loudthud]

5V CT does not go to ground! That will short out B+. The 5V CT is ment to be used with 5U4 and 5Y3 rectifiers to pick off B+ in a balanced fashon because those tubes do not have a cathode. The GZ34 has a cathode connected to pin 8 so B+ should come from that pin when a GZ34 is used. Most people just connect B+ to pin 8 and leave the 5V CT un-connected. Put some heat shrink tubing over the end, coil up the wire and tie-wrap it to the yellow twisted pair close to the transformer.

[Rocketrob]

Doh! I should have remembered that from the last build. Thanks for the heads up.

[cminor9]

One more question. The PT i'm using is a Hammond 276X (320-0-320 150ma, 5VCT-3A, 6.3VCT-5A) there is no bias tap, so I'll have to take it off pin 4 or 6 on the rectifier. I'm using a 10K-L bias pot. What values should the resistors be in the bias circuit? One in series with the diode and one out of the bias pot to ground. RD

I'd start with about 100k on the resistor in series with the diode, and 20k for the one in parallel with the capacitor to ground. I did the same thing for a 6G3 build, and needed roughly the same voltage that the 5F4 calls for. I got right in that ballpark with this configuration. Use that as a starting point and work from there. If you need lot more voltage, then decrease the resistor in series with the diode. If you need a little more, increase the size of the resistor in parallel with the cap to ground.

Hope that helps.

[loudthud]

Thanks Loudthud!

Also on my chassis the vol, tone and presence pots and the input and output jacks are not isolated from the chassis. Will this create more round loops?

One more question. The PT i'm using is a Hammond 276X (320-0-320 150ma, 5VCT-3A, 6.3VCT-5A) there is no bias tap, so I'll have to take it off pin 4 or 6 on the rectifier. I'm using a 10K-L bias pot. What values should the resistors be in the bias circuit? One in series with the diode and one out of the bias pot to ground. RD

Pots should have their cases grounded just for shieldiing reasons. You can probably get away with grounded speaker jacks.

The problem that I see in the bias supply is related to the pot value. Normally I like to use at least a 20K. To get very much range on the pot, you have to burn a bunch in the series resistor (6800 on my schematic). Start with 100K 2W flame proof if there is no tap and it must be connected to the HV winding. There will be a resistor in series with the pot to limit it's range, 56K in my schematic. You may have to experiment with the value of that resistor to properly bias the amp. One possible solution used a lot by Kevin O'Connor is to use a resistor and rectifier from each side of the HV winding.

[Rocketrob]

Thanks for keeping my head above water as I move along with this build. The bias supply seems clearer to me know.

I've made some progress over the weekend; PT, OT, choke, tube sockets, pots, output jacks, power socket all mounted. OT wired up, PT wired up, heater supply too. The board is not quite done yet. RD