Tweed Princeton 5F2-A build. debug

[The Captain]

Hi (again)- Im using this diycustomamps.com 5F2-A (with a std 6.3v lamp)http://www.diycustomamps.com/images/...ton_wiring.pdf

I cant understand the termination of the filament centre-tap. It shows going to the + end of the cathode bypass cap/ resistor . Im using a std pilot lamp with a 6.3v centre-tap, and as per other builds have it going to gnd at a PT bolt with the HV centre-tap & 1st filter cap gnd.. is this wrong? I am getting a loud buzz with sound, and trying to debug it.

Also using a SS rectifier as above, and a PT's ~280/ 280v HV secondaries Im getting very high voltages at each filter cap: 425v, 415v and 380v.. is this too high?

Finally Ive removed the neg feedback 22k and connection to the spkr jack.. is this wise, maybe a cause of bad buzz/ loud hum?

Appreciate any help, Captain.

[Chuck H]

Ola El Capitan,

I don't understand what you mean by a 6.3 volt pilot lamp with a center tap. The lamp should only have two wires and they should be connected to either end of the filament winding, not the center tap. This filament winding is hum canceling. That is why the lamp is not grounded and the filament winding center tap is NOT USED OR GROUNDED (that blob on the schematic on the filament winding center tap is tape or shrink tubing NOT solder). Just take the filament winding center tap and cut it clean, shrink tube the end and don't use it. There is a false center tap created by the two resistors connecting either end of the filament winding. That new false center tap is connected to the top of the cathode bypass cap instead of ground. This elevates the filament supply and helps to make it quieter. The rest of the filament supply is wired differently than what you are used to also. Just follow the schematic and it will work fine.

FWIW the noise your hearing is not likely to be the NFB loop. But IMHO that amp may be better without the loop anyhow.

Those voltages are correct for that transformer.

HTH
Chuck

[The Captain]

Thanks Chuck, what I mean by 'my 6.3v centre tap' is what I had thought a std fender principle Ive used on both the 5e3 and BF push-pull Princeton I have you see, and so have followed same procedure with this 5F2-A: ie these builds' PTs have the 2 secondary greens that go to pilot lamps with its green/ yellow centre tap to ground at a PT bolt.

CAN I DOUBLE CHECK THIS IS WRONG for a 5F2-A? & I should as you say cut it/ shrink it away and..

1. INSTEAD use a pair of 100rs from pilot lamp (differing from above layout in that their artificial ct 100r pair DO NOT start from a 6.3V pilot lamp) and..

2. terminate this new 100r pair to the hot + side of the cathode resistor/ bypass cap at end of 6v6 pin 8.

muchos gratias, Captain.

[Chuck H]

"CAN I DOUBLE CHECK THIS IS WRONG for a 5F2-A? & I should as you say cut it/ shrink it away and.."

The schematic you posted does not represent the standard wiring for a 5F2-A. It is an improved circuit. Use it.

"1. INSTEAD use a pair of 100rs from pilot lamp (differing from above layout in that their artificial ct 100r pair DO NOT start from a 6.3V pilot lamp) and.."

Because the pilot lamp will now be wired to the two green leads there is no reason you cannot solder the 100r resistors there as well. Except that the terminals on the pilot lamp will likely be too small, in which case you will be better off installing a small terminal strip or pop three more eyelets into the board for a junction point.

"2. terminate this new 100r pair to the hot + side of the cathode resistor/ bypass cap at end of 6v6 pin 8."

Yes.

So, to sum up:

The yellow filament winding center tap should be isolated and not used at all or connected to anything.

The pilot lamp should be run from the two green filament winding leads and not grounded anywhere.

On each of the two green filament leads connect a 100r. The two unconnected ends of these 100r should be connected together at the top of the 6V6 cathode resistor.

After this, the two green filament leads should go to the tubes as follows:
*NOTE: No tube socket terminals for the filaments of any tube should be grounded for this circuit.

One green lead goes to pin 2 of the 6V6 socket and pins 4 & 5 of the 12AX7 socket.

The other green lead runs to pin 7 of the 6V6 socket and then to pin 9 of the 12AX7 socket.

The two green filament winding leads should be twisted together as they run from from point to point and between tube sockets.

There will be no more "so if I???" or "then should I???". This is not the filament wiring your familiar with. The one you know cannot be hybridized in any way with this new circuit. Wire your filament supply just like I have said. Use the schematic and simply wire the pilot lamp to the two green filament leads.

HTH

Chuck

[Enzo]

In common Fender amps, the two 100 ohm resistors take the place of a center tap. SInce he already does have a center tap, why would he prefer to use a pair of 100 ohm resistors to the cathode instead of just using the center tap they replace?

Or do I misunderstand. Do you actually have a center tap wire from the transformer - green with a yellow stripe? Or are you mis-using the term center tap to refer to the two resistors?

The circuit as draw is a good one, as Chuck says. If there IS a green/yellow center tap wire, I see no reason to add the 100 ohm resistors, I'd just connect the center tap to the cathode.

But DO NOT connect resistors to a chassis bolt AND wire these things to the cathode.

I believe the yellow blob to represent unused 5v wires, not that it matters much.

[Bruce / Mission Amps]

I agree with others here.
There is no reason to use the 100 ohm resistors as drawn if you have a center tapped 6.3v winding.
So, Enzo's comment about connecting the CT G/Y wire right to the cathode of the power tubes (and not bother using the 100 ohm resistors) is right.
However, I usually install a 1/2 watt, 47 ohm to 100 ohm resistor in the 6.3v center tap lead if grounded.
Actually I connect that to the 6V6 cathode biasing resistor too.
In the case of the 6.3v CT wire being grounded, the 1/2w resistor offers some kind of fusing action if the B+ rail shorts from lugs 3 to 2 on the power tube socket.

One more thing, (this might not apply to you)....
if any of you are using a tube rectifier and have a 5v winding with a center tap, you can use that 5V center tap wire to feed the B+ to the amp's B+ rail and not use the lug on the rectifier tube socket.
It looks neat and always creates questions! ha ha...

[Chuck H]

Enzo, the only reason I suggested the false ctr tap is because it does provide some "fuse" protection in the event of a short. I have never seen a case where it would have actually saved an amp, but it still made sense to me. I think Bruce's idea with the 47r between the actual ctr tap and ground is a good one. I just never thought to do it that way before. Good call on the yellow wires being the 5V winding. Because of the discussion at hand I figured those were meant to represent the ctr tap. You have a knack for seeing past whats said and figuring out whats actually going on. Your ability to do that helped me a great deal with a power supply issue once.

Chuck

[The Captain]

Enzo, the only reason I suggested the false ctr tap is because it does provide some "fuse" protection in the event of a short. I have never seen a case where it would have actually saved an amp, but it still made sense to me. I think Bruce's idea with the 47r between the actual ctr tap and ground is a good one. I just never thought to do it that way before. Good call on the yellow wires being the 5V winding. Because of the discussion at hand I figured those were meant to represent the ctr tap. You have a knack for seeing past whats said and figuring out whats actually going on. Your ability to do that helped me a great deal with a power supply issue once.

Chuck

Chuck your help, and others above too as per usual second to none. I understand things clearer now and have the grounds all hopefully good now (pot wire for some, HV ct and others on a bolt- and the grn/ yellow in Q to the cathode r/ 470w.

So its up and running- great! (sounds very sweet too, biggish hum but hey I can live with it). My innitial Q was raised as all I got was a loud buzz/ brrr from spkr: turns out the numbers on the 6v6 valve socket (1, 2...8) were all exactly one wrong! ie made and labelled as 1 where 2 should be etc etc took me 6 hours of head-sctaching and swearing to find it (a proper 'eureka' moment!!). Anyone heard of such daftness?? the socket, not neccessarily me I refer too.

Terrific help folks- Captain..

[Chuck H]

Thats great that it's "working" now. Ha ha. The hum surprises me though. I've wired up a few Champs and VibroChamps with this filament circuit and they are very quiet. Could be a grounding issue.

As for the tube socket...I've seen that before. It's actually numbered correctly, but if you don't start counting at "1" (so you can see where the marker for number "1" is) it actually looks like the numerals are assigned to the following pin...Lame.

If the amp hums with nothing plugged into it:

Make sure that nothing is grounded twice. Each thing that is grounded should have only one path to an acatual ground. Daisy chaining grounds is bad. On a small amp like that one you should use a "star ground" technique. That would mean that everything is grounded to the same point (very near the chassis ground) and each ground uses it's own individual ground lead. You may need to isolate the input and output jacks from the chassis also. But my guess is that the hum is a ground loop or something is grounded in the wrong place.

If the amp doesn't hum with nothing plugged in:

Your guitar is noisy.

HTH

Chuck

[The Captain]

Chuck, re big hum.. my grounding at the mo is this (basic idea from a super-quiet BF Princeton push-pull scratch build recently made): the 1st filter cap & the HV ct to one PT bolt, and a pot wire across back of pots where iirc everything else goes (bar the 100ohm pair from pilot lamp obviously now). Maybe I should separate the jacks and ground the preamp bypass cap (pin 3) to the jack shield to make a third reference point? it does show these together separately on layouts I must say. I also have a hi/ low jack pair to V1 pin 2 as per other fender layouts, but this extra input couldn't be hum inducing could it?

thanks, Captain.

[Chuck H]

"but this extra input couldn't be hum inducing could it?"

Yes, it could. But only if you don't have it on a ground fault jack.

Your grounding schem sounds OK. On a small amp like that one, as long as you have the power supply and OT ctr. grounded closer to the actual chassis ground than the preamp is, and each grounded thing has only one path to ground, you are fine.

I've never had the hum issue with chassis mounted jacks. But it does happen. The chassis mount can present a second path to ground. Sometimes isolating all the jacks from the chassis can help with hum.

How are your power and output transformers oriented with respect to each other??

Chuck

[Bruce / Mission Amps]

Your grounding schem sounds OK. On a small amp like that one, as long as you have the power supply and OT ctr. grounded closer to the actual chassis ground than the preamp is, and each grounded thing has only one path to ground, you are fine. Chuck

I think you meant the PT high voltage secondary center tap didn't you?

[The Captain]

Sometimes isolating all the jacks from the chassis can help with hum.

How are your power and output transformers oriented with respect to each other??

Chuck

I'll try isolating each jach then- they were to the back of pot wire, now separated from this but still together (along with the preamp bypass cap gnd in each case): this change didnt help the hum though..

PT & OT orientation (where I was hoping the hum wasn't due to): PT on chassis 'top' far lhs sidewards, and OT's middle 4.5" away from the PT's edge again on 'top'. The 6v6 sits on 'top' also between the two Ts twds the back (but closeby- the chassis 14" across); the 12ax7 ~3" from the OT far rhs twds the back; the 6v6 then is closeish to the side of the T's (a hammond 125ESE and as big as the damn PT!).



Thanks folks, captain.

[Chuck H]

Bruce, yes I meant PT ctr. Thanks. Captain seems to have enough on the ball that I wouldn't have sent him on a snipe hunt with that one. But it's good to be accurate...Too many beer.

Captain, I have never oriented my trannies like that before. I might suspect the OTs orientation with the 12AX7. But I was really interested in how the laminates were oriented from tranny to tranny. Try isolating the jacks I guess. But after that, I'm outta chops without more info.

Chuck

[The Captain]

Chuck the chassis design is based very similarly to the last (yellow one) example from this very good site- http://www.diycustomamps.com/princeton.htm

Im fairly sure, albeit an educated guess, that even if I shifted A from B on top of the chassis the hum would still persist.. oh and no change to hum when just preamp tube pulled out too.

Thanks for thoughts so far, Capt.