Unfair Bias ?

[Toxophilite]

Hi
I've put up a couple of posts and have found all the input extremely helpful

This question is about a 5F11 vibrolux build I did a month or so ago
I reference this amp in my posting about transformers

As my starting chassis, power transformer didn't have a bias tap I 'stole' the bias from the rectifier plate using a circuit from a Marshall plexi build
here:
http://mhuss.com/Plexi6V6/Plexi6V6c.gif

It was the first thing that came up
Now though I was looking at a Princeton AA964 schematic and it has the same sort of tremelo and steals the bias in a similiar way but with different value components

I'm wondering if the one I used is fine for my amp
The tremelo works great and the amp sounds good, the negative voltage delivered is right (I have a pot on it and can fine tune itit) but the amp did acquire more hum since I added the circuit

Also I took the power from the other plate of the rectifier from the one that normally supplies all the B+ for the amp
Is that a problem noise-wise?

This amp started out as a 5e3 tweed deluxe and in that configuration only had appreciable hum  when it was dimed.

I hope I'm not overposting I just didn't want to jump around from amp to amp and sunbect to subject too much in one post
I apologize because I'm Canadian.....

[HotBluePlates]

... I 'stole' the bias from the rectifier plate using a circuit from a Marshall plexi build ... the amp did acquire more hum since I added the circuit. ...

You did your circuit exactly like Mark Huss'? Complete with a pair of ~50uF caps to filter the bias voltage, with a 15kΩ between them and a bias adjust pot afterwards?

If you didn't filter the bias voltage like Mark, you may be injecting hum directly to your output tube grids. TEST: pull out your phase inverter tube; is the hum still there? A failed bias filter cap causes a hum that doesn't go away when the phase inverter is gone.

If you put the bias filter caps in backwards (- ends to ground, + ends to tube grids), they may explode or they may just fail to filter, in which case you'd get a nice hum (though curiously, the tubes may not redplate).

Since the hum appeared when you added the new bias circuit, the fault probably lies somewhere in the execution of that circuit. No offense, but if a new symptom appears when I make a change, I have to suspect I didn't do my change correctly.

Also I took the power from the other plate of the rectifier from the one that normally supplies all the B+ for the amp
Is that a problem noise-wise?

You mean rectifier "plate" as in red wires from the power transformer secondary? If so, it's not a problem for Marshall or the Fender Princeton, so I imagine it's not a problem for you either.

[Toxophilite]

No offense taken
I make all sorts of mistakes, all the time!

I'm pretty sure I did the circuit just like his with the pair of caps and the 15k resistor (I used a 10 + a 4.7 in series)  ..my guitars are noisy(old gretschs with low output single coils) but there is some inherrent hum in the amp itself and more since this circuit
I think it's time to yank it from it's cab and make sure
The tremelo is working though and nothing has exploded yet and I have used it for a few hours at a go in rehearsal. No 'Magic Smoke' was evident
Also if i'm going to ask questions about it I should have it handy and maybe a picture of the innards

[Toxophilite]

Okay I did the phase inverter test..The hum went away completely
And here are some pictures which may very well expose me to the slings and arrows of design/layout scorn. What the heck. I've played inumerable gigs exposing my innermost thoughts and feelings...this is just another variation.

Another source of hum could possibly be the cramped conditions and proximity of the input tubes and inputs to the power supply.?
I used the layout as is

However as I said before it didn't hum as much as a tweed deluxe and everything is pretty thoroughly star grounded

[Toxophilite]

Hmmm
I'm wondering if this is a difficulty
You can see my B+ is taken from pin 2 of the 5Y3
I've heard this is okay but you might want to pop a 5AR4 in there
however I'm not taking my voltage for my bias from the plate...I've hooked it up to the other filament at pin 8.
I'm going to check that Princetone schematic to see where they steal their bias from..ooops it's from the plate..hmmm NO GOOD!

[Toxophilite]

Hmm
and I had my pot hooked up wrong..however I can't seem to get a decent negative voltage after the diode now..I get a reading in the millivolts and the tremelo is unhappy..bukka bukka bukka bukka
hmm

[stratele52]

Sorry but you poor wiring act like antenna for Hum . You may follow Fender layout .

Did your heater wire have the center tap to ground ?

 

[eleventeen]

"I'm wondering if this is a difficulty--You can see my B+ is taken from pin 2 of the 5Y3" <<---shouldn't be a problem. There is definitely a preference for taking your B+ from pin 8 in an indirectly heated cathode rectifier (5AR4/GZ34) and I would make that change if you decide to change rectifier tubes, but it makes zero difference in a 5Y3. It's the correct way for a GZ34 (or a 5V4) but it won't stop the amp from working nor will it harm anything. I would make that change as a matter of good procedure/practice if you are under there with a soldering iron for some other reason. Should you change to a GZ34, your B+ will rise maybe 20-30 volts because there is less drop across a GZ34 or 5V4 vs a 5Y3. In general, you won't notice.

It is correct to take your bias from one or the other plate in the rectifier. Or, it is equally correct to take the bias from either end of the HV transformer winding....you are just using the plate terminal on the rectifier socket as a tie point, the two are (or should be) electrically identical. The bar (cathode) end of the diode must be pointing to that plate. I like to have a resistor of some kind between the plate and the cathode of the diode "just because" (for current limiting)  but assuming you are using 1N4007 (1000v/1A) there is no problem if you do your voltage drop before or after that bias diode...or both. You'll see that "before" resistor in most amp designs. Much more important, that diode's anode has to connect to the MINUS terminal of the bias filter cap (probably through a resistor) and the PLUS of that cap is to be grounded. The diode is, relative to most other applications, "backwards" and the filter cap is backwards. If you are using a 350/450 volt electrolytic to filter the bias (because it was on hand) and it's installed wrong (neg to ground and diode feeding the "+") the 50-60-75 volts you are probably feeding it may well not be enough to blow up the cap, but it won't work right. That could definitely cause your hum problem.

It's also necessary, in 99% of amps, to provide either a real or a synthetic center tap to ground for the heater winding. Not having that will cause serious hum in almost all cases. The two qty 100 ohm resistors tied to ground can be located anywhere, but the most common practice is to locate them near as possible to the power trans. Fender' practice was to mount them on the pilot lamp holder. Close enough.
 
I'm a little confused by your screen resistor wiring on the 6V6 tubes. Maybe I am misinterpreting things. But I *can* see the plate leads for the output tranny, and that shows me where pin 3 is. It looks to me like you have wired the left-6V6 screen resistor to pin 4 (correct) but it sure looks like you have wired it to pin 3 on the right 6V6....but I could be misinterpreting that.

I'm also having a hard time discerning flow on the .1 ufs that feed the 1500 ohm grid resistors for the 6V6 tubes. I see the 220K resistors but I do not see the non-tube side of the 220K's tied together---which would be the bias feed.

Your wiring is rats nesty, but that's how lots of stuff was wired back then.

[sluckey]

...I'm not taking my voltage for my bias from the plate...I've hooked it up to the other filament at pin 8.

...however I can't seem to get a decent negative voltage after the diode now

You cannot take the voltage from pin 8! That's B+.

You must take the voltage for the bias from pin 4 or pin 6, doesn't matter which. Make that change and your negative bias voltage will get happy, ---IF---, the other bias components are wired correctly ---AND--- you didn't destroy the bias caps. The diode should have prevented destruction of the bias caps.

[Toxophilite]

Sigh
As I said when i posted pictures..'And here are some pictures which may very well expose me to the slings and arrows of design/layout scorn.'

Fair enough, I expected it, however I'm using an existing vintage chassis with all the main parts already installed (tube sockets, transformers et)
Perhaps eventually I'll pull everything , build a new chassis, and buy more parts to make it exactly like a fender amp layout, but for now that's not going to happen.. I was having fun with what I had.
For the same reasons I'll probably never build a finger jointed pine box and cover it with laquered tweed..though I think they look very nice. Come to think of it a nice long tweed-covered finger-jointed pine box might be something good to be buried in !


My heater wires don't have a center tap to ground
I've checked a bunch of fender schematics and I can't see the 'synthetic' center tap consisting of 2 100 ohm resisters tied to ground anywhere.
Would you mind explaining what they would attach to (besides ground) I'd appreciate that!

The grid screen resistors on the 6V6s are hooked up right but I can see how in the photo it doesn't appear that way.
The two 220k resitors are tied together by a black wire
I think perhaps a little lug strip would've been handy in that spot..i might still put one in and tie them all to that in a tidier fashion

The 5Y3 was wired that way (pin 2 = B+)when I got the unit and as at the time I didn't know better I left it that way

And with the Bias I was accidentally taking it from the other filiment and not the Plate..oops

I had a resistor between the diode and the tie point on the 5Y3 and the diode faced the right(or wrong.haha)direction (negative to the tie point) As well the filter caps were positive to ground.
It worked that way..(hooked up to the filiment) and provided the proper negative voltage...go figure
I even did some recording with it and it sounded really nice. ( I don't play loud here though)
I had actually used it in rehearsal as my stereo amp a week or so back and it sounded really nice then at considerably louder volumes..It's a loud amp wth the D130 in it

Humourously when I unhooked it from the filament and switched it to the plate last night...The tremelo stopped working nicely)
So I made a test Princeton AA964 faux bias tap circuit and with a little resister tweaking (22k to ground not 27k) I managed to get a decent -30 ish volts and the tremelo working again
oy yoy yoy!

thanks for all the varied input and I'm really keen to find where I attach the 2 100 ohm resistors to create a synthetic heater center tap!
that seems like a good next idea to try

[sluckey]

Humourously when I unhooked it from the filament and switched it to the plate last night...The tremelo stopped working nicely

You will have to adjust the bias pot for a balance between good bias and good trem. Get the proper negative bias voltage to pin 5 of each output tube (≈-30 to -35 for 6V6 or ≈-40 to -50 for 6L6). YOU MUST CONNECT THE BIAS SUPPLY INPUT TO PIN 4 OR 6 OF THE RECTIFIER TUBE!!! This should be your #1 priority. Then get the tremolo to work.

I'm really keen to find where I attach the 2 100 ohm resistors to create a synthetic heater center tap!
that seems like a good next idea to try

One end of a 100Ω goes to ground the other end connects to one of the 6.3V filament leads. The other resistor connects between the other 6.3V filament lead and ground.

A convenient place to mount the resistors is directly on a power tube socket. I'd use the one without that brown devil. Connect one resistor between pins 2 and 8. Connect the other resistor between pins 7 and 8. Use half watt resistors.

But your #1 priority should be fixing that bias supply!

[eleventeen]

Get those 2 qty 100 ohm resistors in there, bet it cuts your hum by at least 80%. From both sides of the filament winding to ground. Mount them anywhere convenient. Sluckey had the good idea.

[stratele52]

Get those 2 qty 100 ohm resistors in there, bet it cuts your hum by at least 80%. From both sides of the filament winding to ground. Mount them anywhere convenient. Sluckey had the good idea.

   
Re: Unfair Bias ?
« Reply #6 on: January 08, 2014, 05:00:02 am »
Quote  Modify  Remove
Sorry but you poor wiring act like antenna for Hum . You may follow Fender layout .

Did your heater wire have the center tap to ground ?

 

[Toxophilite]

80% hum cut would do it
It's not unbearable hum...it's just mildly annoying and I don't like watching the JBL move in and out with the tremelo on.

I'll get on that and Thanks..I thought about it afterwards and I had a duhhhh moment when I realized,,two resistors , two heaters, they likely run from the heaters to ground, nice to have it confirmed by hose with experience and greater knowledge

I think my other explanation was a little confusing

I did switch the bias steal to the plate of the 5Y3 last night, it was the first thing I did
I then checked the circuit and then spent some time adjusting the pot, multimeter in hand . All I could get was millivolts, albeit negative ones, with the tremelo going bucka bucka bucka really loudly.
 I checked the circuit again, tested the diode etc etc. but no dice

Soo,   as the AA964 princeton bias steal is a much simpler circuit, I quickly rigged one up as a test, unhooked the other circuit, and installed the AA964 circuit and got a decent -35 bolts with a working tremelo (no loud bucka bucka)
I then changed the 27k resistor to ground to a 22k and got -30  volts ( a little above actually) which is closer and the tremelo is working well

I think I will fully install the AA964 circuit as it's a lot simpler and will be less bulky inside.

I can also use the pot I installed for the Mike Wuss circuit as it's about 25K I believe, perfect for fine tuning the bias voltage
Thanks again
I'll do that stuff and repot back

[HotBluePlates]

Good job on fixing the bias supply.

Try this approach instead of willy-nilly looking for hum cures:
You pulled the phase inverter tube, and the hum stopped. Now pull the next tube in line closer to the input. See if hum stops or continues.

What you are doing here is breaking the circuit into sections. When  you pull the phase inverter, the signal connection from it to the output tube grids is broken, and only the output tubes (and any circuit directly connected to them) is attached to the speaker. When the hum stopped, you know the hum must be at or before the phase inverter tube.

By successively pulling tubes, you can hear exactly how much hum each stage contributes (or doesn't) to the problem. Once you've got it localized to a given tube, you can look at the circuit, components, connections, grounds, etc for that tube (and whatever is connected to it).

[Toxophilite]

A sound and sensible suggestion! (excuse the illiteration)
Easy too
I'll try that as well
THanks

[Toxophilite]

Can I get away 90 ohms to ground on each heater?
I can only find one 100 ohm resistor..(How I don't know..they come in packs of 10!)
I have a couple of fat ones on my other build chassis..not sure if I want to yank them yet

I do have some 180 ohm resisitors that I can put in paralell to get 90 ohms

Would that suffice or is the 10 ohm difference too crucial

i haven't put anything in yet


As well I tried pulling tubes to isolate sections

I got the most hum reduction from pulling the first preamp tube (It was hum but higher in tone)
With just the phase inverter in there was still a very low frequency hum but very quiet
THis is with the amp turned up all the way

[HotBluePlates]

Can I get away 90 ohms to ground on each heater?

90Ω is fine.

If you notice the 12AX7 Hum Balance control from your other thread... well, it's a 100Ω pot from one heater wire to the other. With the pot wiper in the center of its travel, that's the same as a 50Ω resistor from each side of the heater to ground.

By the way, your amp doesn't already have a center-tap for that heater winding, does it? If it does, the 100Ω resistors won't be doing anything (no harm in installing them though).

[Toxophilite]

I only have one brown wire leading from the power transformer to the star ground point

[Toxophilite]

So I fininshed installing the AA964 version of the bias steal
I had a little difficulty till I put a bigger filter cap on it (47 uf 160 vdc) With that and the 27k resistor going to ground it produces a steady -35 volts which gives me a nice deep clean sounding tremelo.

Without a guitar plugged in the amp isn't very noisy untill you hit around 9 on the volume control
My guitars are both 50 years old this year and have low output single coil pickups. One's position has a lot to do with noise!
i tried the resistors going to ground and it seemed to produce no result in reducing hum
However it's only noisy with a guitar plugged in and then only at the highest volumes
my guitar is noisy through my 1980 super (last of the silverfaces)at higher volumes too..add in a space echo and who can tell anymore!

I also might mess around with the 1 meg volume pot as it seems to get most of it's volume between 1 and 3 on the pot and not much after..Like it's a linear pot. I'm pretty sure it isn't and I might swap it out.
As well I pulled and cleaned up a lug strip (name?) to use between the Phase inverter and the power tubes to make that a little less messy eventually.

[sluckey]

Just curious... Why did you name this thread "Unfair Bias ?"

[Toxophilite]

Well
With no depth great depth of thought actually
I thought 'bias' and immdediately I thought 'unfair bias'
I was questioning the more elaborate bias setup I had in my amp
So it seemed kind of appropriate and silly
Admittedly I have a strong appreciation for stupid humour!

I was quite happy and thankful for the information I got, Even if I didn't implement it all, I certainly learned a fair deal and advanced my limited understanding somewhat.

I was wondering, as you seem to be very knowledgable if you could give me a pointer in my other thread, my current build regarding placement of preamp/ vibrato circuit etc.
Also I was curious as to your AB763 lite ( Ithink that's the name. a fellow refernces it but somehow I couldn't view the particular schematic posted

I'm going to go post my question now