Tweed Princeton 5F2-A build. debug

[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]

5F2-A build

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/s...ead.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/s...ead.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.