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/princeton/tweed_princeton_wiring.pdf

I cant understand the termination of the filament centre-tap. It shows going to the + end of the cathode bypass cap/ resistor :confused:. 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. ChuckI 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!).

http://i102.photobucket.com/albums/m106/TheCaptain_02/Picture390.jpg

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.

[Chuck H]

"no change to hum when just preamp tube pulled out"

Since the transformer orientation seems OK (You've done it before, I've seen it before...no problems reported) I'm suspecting the filament supply or filter grounding scheme. How are your power filters grounded???

Chuck

[The Captain]

Hi there Chuck,

I have two paralled 16ufs (lhs on board), the left one grounded at the bttm rhs PT bolt.. then another 16uf grounded independently to the same PT bolt.. and a 3rd 16uf (preamp iirc, rhs filter cap).

The only difference on schemos is my 1st paralled '32uf' pair (tied - ends obviously) not having the PT centretap as a gnd; instead I have this ct independently to the same PT bolt: so 3 whatnots only gnd at this PT bolt.

Thanks Captain

[Bruce / Mission Amps]

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

I'm 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.Steve M. is a friend of mine and many years ago, as a novice builder, he use to come over to my shop all the time for a few parts and some pointers.
I think you could email him too and ask what kind of hum noise he has with this project... I wouldn't be surprised to find out I've seen it or maybe even worked on it at some time!

[dai h.]

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



dunno if this is what you followed, but from looking at the "5F2-A Tweed Princeton Wiring" pdf file, the grounding scheme doesn't look the most ideal. I would for example, move the tone cap and vol. pot ground to the right to the input jack (super low level current returns, so group together). Also, if that isn't an isolated jack that wouldn't seem good. Also the preamp filter and power tube cathode R grounds, and the spk. out doesn't say if you are to use an isolated jack or not.

[The Captain]

I had similar reservations in regard to the power tube gnd being linked to the rhs preamp/ pot stuff (although Steve does mention his amps like this to be dead quiet). It is though the basic general layout I used, butt Ive separated the preamp gnd stuff/ pots/ jacks/ preamp filter gnd (all to a pot buss bar)-- from the power filter cap gnds/ HV ct/ filament ct (to their own PT chassis bolt). This method Ive used for 2 builds and dead quiet they are too. This one's still humming like mad.

[dai h.]

is there a picture of the chassis that can be looked at to see how things are hooked up?

[The Captain]

Yes indeed.. 1st/ 2nd filter caps paralled (blue atom and next one) & next filter cap & HV ct are together to own PT bolt. All other gnds go to the pot bussbar extending/ linking the input jacks gnd lugs. The filament ct (100ohm pair) goes to the 25uf cap/ 470r (via pin 8/ pin 1 of a 6v6 unused pins). The 2 metal spkr jacks' grounded where they are (tried them to the '3' at the PT bolt without any change in hum).

http://i102.photobucket.com/albums/m106/TheCaptain_02/Picture418.jpg

[TD_Madden]

might be a dumb suggestion, but would it help to remove that black finish at points where connections are tied to ground...or am I just not seeing that?

[MWJB]

TD's right any ground points should have the paint scraped away so there is good contact with the chassis material.

Are you using both ungrounded terminals on the master pot? You should just be using the middle terminal. I'm not keen on the way the wire from the master goes over the heaters on the way to 6V6 pin 5, in fact, I'd consider mounting the master on the underside/back of the chassis and running the coupling cap from V1, pin 6 horizontally to shorten the path.

Not crazy about the routing of that red hi voltage wire feeding the preamp plates, it would be nice if this was under the board.

[The Captain]

might be a dumb suggestion, but would it help to remove that black finish at points where connections are tied to ground...or am I just not seeing that?

Perfectly sensible Q.. the first thing I did with the blank chassis is test across it for continuity- none, so I knew I was in for lots of scraping- which Ive done at each jack, pot, grounding bolt, switches, etc.. literally hours of work so at each now there's an excellent connection to the chassis, and all components very tightly secured on.

[The Captain]

Hi Mark, I know wiring it not optimal here and there, but I have the exact same awful hum with or without MV pot wired in (same as Steve's site, and as I wired in one to great effect in a Vjr 5w; here tho its not sounding great- but this is not part of my concern at the mo), and routing other wiring exactly as you've suggested for trials- exactly the same hum, not even slightly different.

Appreciate the thoughts of course, Capt.

[dai h.]

the workmanship looks pretty good but the grounding looks kind of discombobulated. I think it should be redone. The power tube cathode R/bypass ground looks wrong, and there isn't enough detail to see where everything is going. I would resort and move the grounds off the chassis except at the input.

[The Captain]

Thanks, but I stated exactly where the grounds are before the pic, there aren't so many of them & with the pic it must be clear..

They go either to one PT bolt, or the pot bussbar, bar the filament ct going to bypass cap/470r as per the layout.

Cheers Capt

[dai h.]

well the details matter like where the B+ center tap is, and where from the ground line you are setting down to the chassis (i.e. how you connecting to the rest of the ground line). Maybe that portion is from Randall Aiken's article where he uses two points down on the chassis and the way he does it has a logic behind it. If that logic is not understood, then you could be putting a noisy part down through the chassis. I see bits and pieces of things but not one unified logical system. For example, couple of the grounds after the rectifier are to the bolt but the power tube cathode/bypass are towards the preamp (high current plus also a big signal through the bypass cap) which looks like it could be problematic. It really helps to see clearly where are things are connected now to be able to suggest a change.

[Bruce / Mission Amps]

Thanks, but I stated exactly where the grounds are before the pic, there aren't so many of them & with the pic it must be clear..

They go either to one PT bolt, or the pot bussbar, bar the filament ct going to bypass cap/470r as per the layout.

Cheers CaptThe only thing that should be grounded to a chassis bolt or PT bolt is the power cord's green earth ground wire.
Take the red/yellow high voltage secondary center tap wire off that chassis bolt and move it to the negative lead of the first main B+ filter cap.... same thing with the power tube's cathode resistor and it's bypass cap... and it wouldn't hurt to tie the grounded lead of the OT to the same place as the power tube cathode resistor is grounded.

And... describe how all that unnecessary shielded cable is grounded.

These amps are really simple and it is very unusual to have bad hum problems.
So after those mods, you still have a hum issue, it has to be lead dress or something else unrelated to grounds... well, unless you really don't have proper grounds due to that coating on the chassis.

[The Captain]

Appreciate your thoughts Dai.h. Unfortunately I can't think of a clearer way to describe my groundings other than my brief descriptions with a photo- alas as best a pic as my camera can take. Just to reitherrate in case Ive missed summat..

I have 2 grounding points, a PT bolt (lets say "A"), and a busswire across pots ("B").

To "A": The PTs secondary High Voltage centre tap/ the 1st & 2nd filter caps (paralled; blue Atom & next cap)/ & lastly the next filter cap (3rd from left).

To "B": The 4th cap which is the preamp filter cap/ the 25uf & 470r bypass cap & resistor/ the next resistor along/ preamp 25uf & 1k5 bypass cap & resistor (last on rhs of board)/ then the pots obviously/ lastly the busswire snakes through the input jacks ground lugs.

Finally, the filament artificial centre tap, two 100ohm resistors, connect to the + end of the cathode 25uf/ 470r bypass cap & resistor (centre on board) VIA pin 1 and pin 8 of the 6v6 (for wiring convenience).

Apologies if this wasn't clear before, thanks for any more ideas Capt.

[MWJB]

This grounding scheme looks fine, I'd be surprised if it was the source of your hum.

[dai h.]

The way that Bruce described is exactly the way I'd do it for those points. Connect to chassis at the input end plus a redundant point very near to the input jack with a nylock nut just in case it comes loose. In order to do a good scheme you need to have a basic idea of what is happening in the ground lines, sort of like this:

http://www.flickr.com/photos/7878340@N05/1467701518/

this is not instructions for that amp but just to show how mentally you need to sort out what is happening in order to decide what to do with the grounds (it's an exercise I did for myself--sort about the low res.--there is a 5Meg limit and I haven't learned to use a drawing program yet). If someone didn't know, I could see them easily doing things wrong even though it might not seem like it.

[WholeToneMusic]

I am building a 5F2-A circuit also. The plate voltage is stable for a minute and then the amp makes an audible click after a minute or so and the plate voltage starts drifting and is un-readable. While playing the amp it sounds fine played clean, however when it is cranked up and played hard the distortion sounds weak, I checked all of the connections to make sure nothing was loose. Any recommendations?

[dai h.]

I would check again for iffy connections since it kind of sounds like the problem has something to do with that. Bad joint, or an unsoldered joint. Maybe goes open as you crank it up and demand more from the power supply and current draw goes up, then something goes open and then you get the weird voltage readings (zero?).

[Paul P]

Bruce, you wrote a while back :

The only thing that should be grounded to a chassis bolt or PT bolt is the power cord's green earth ground wire.
Take the red/yellow high voltage secondary center tap wire off that chassis bolt and move it to the negative lead of the first main B+ filter cap.... same thing with the power tube's cathode resistor and it's bypass cap... and it wouldn't hurt to tie the grounded lead of the OT to the same place as the power tube cathode resistor is grounded.

If I read you correctly, that the circuit grounds are completely independent
from the chassis + green earth wire, then I've been thinking the same thing but
this is not standard practice, is it. Doesn't this approach require shielding
of all jacks from the chassis ? (or use shielded twisted-pair cables) The
result is that the entire amp past the power transformer primary is not
referenced to earth ground. I see this as a feature safety-wise but
what happens to interference/noise entering the system from the guitar
end of things ?

I started a thread on this subject a few weeks ago :

http://music-electronics-forum.com/showthread.php?t=4973

and the consensus seemed to be that the circuit grounds and the earth
ground should be connected together and that the cable shields should be
connected to the chassis.

Paul P

[Mark Hammer]

Re: 22k negative feedback resistor.

I've done the experiment of lifting that component on mine (original, not clone) and as you'd expect, it has the net effect of "unsmoothing" the tone.

There is value in tinkering with the amount of negative feedback, rather than simply having it as a yes/no option. Seems to me that, given the wide range of possible values between 22k and open circuit, aiming for something in between stock and no negative feedback would be best achieved with something like a 6PST rotary switch and selected resistor values, rather than a pot. Some of those positions could be used for "global" negative feedback, and others used for tone-shaped negative feedback. For instance, a 22k in series with a paralleled 100k and selected cap would provide normal negative feedback for the top end that can pass through the cap unabated, but less negative feedback for that frequency content forced to go through the 100k resistor. In essence, you can play with the negative feedback as if it were a sort of tone control.

[dai h.]

Bruce, you wrote a while back :



If I read you correctly, that the circuit grounds are completely independent
from the chassis + green earth wire, then I've been thinking the same thing but
this is not standard practice, is it. Doesn't this approach require shielding
of all jacks from the chassis ? (or use shielded twisted-pair cables) The
result is that the entire amp past the power transformer primary is not
referenced to earth ground. I see this as a feature safety-wise but
what happens to interference/noise entering the system from the guitar
end of things ?

I started a thread on this subject a few weeks ago :

http://music-electronics-forum.com/showthread.php?t=4973

and the consensus seemed to be that the circuit grounds and the earth
ground should be connected together and that the cable shields should be
connected to the chassis.

Paul P

I think you misunderstood. The circuit ground should be connected to chassis prob'ly at the preamp end. The reason for not putting that PART (i.e. that doesn't mean not putting the cct.grnd down on the chassis at all) is to keep the noisiest portion of the ground (pulsing AC ripple--100 120Hz--still there before it has gone through some of the RC or choke C filtering stages) off the chassis (so it doesn't go through the chassis) and keep that noisy loop closed (wired so it least effects the ground line and thus the other returns esp. the sensitive ones detrimentally and physically so the physical loop area is small). You seem to have a strong interest in this so you might want to check out this article and the links listed:

http://ampgarage.com/forum/viewtopic.php?t=220

in addition to the links listed, the Rane site has some good stuff as well as ax84 (try searching for Randall Aiken and Paul Ruby posts--they are not the only people with good info of course, but these should lead to some good discussions re: grounding). Also epanorama IIRC.

[WholeToneMusic]

The Voltage reading on a Digital meter set on auto bounces around but does not stay at one voltage. I will go through and check all the connections again, check the continuity of the sockets to make sure none of them are shorting out, check all the grounds to make sure they are zeroing out, check for cold solder joints etc.
Also when I was checking the bias and waiting for it to stabalize, it never did, Last I remember it was around 40 mv, which seems very high for this circuit.

[dai h.]

any chance the meter probes are dirty? Sometimes my clip things get oxidized and don't read well and I have to clean them using a drop of Deoxit (which as I understand is naphtha and some other stuff). Or have you tried setting on a specific range instead of the autoranging setting?

[WholeToneMusic]

I will clean the probes just in case. I may re-solder pin 3 on the 6V6, it may have gotten contaminated. It seems like there is a coating of some sort on the solder.


Brian

[MWJB]

40mA is not a problem for this amp.