In Rush current related

[mresistor]

OK guys,   I have a question that is inrush current related  I'm working on 65 style Fender SE Champs & Vibro Champs.  I'm using Classic Tone PTs and I find in these builds I have some resulting audible 120hz hum . So I'd like to add some filtering capacity to the PS. I've got a factory Fender VC Schematic that shows 40uf on the first filter node off the rectifier. But I've read that exceeding the published recommended capacitance of the PS filtering is not good for the life of the tube rectifier. So I'm thinking of using some resistance between the rectifier and the first PS filter cap. I've used a 10 ohm 5 watt sand cast resister on a VC I built ins series with the first node 40uf cap and it seems to work rather well. This seems to quiet the hum a little and also tightens up the low freqs. My question is;  Is this an effective way to minimize some of the increased initial charging current?

[PRR]

> tube rectifier.

Which tube rectifier?

> I've used a 10 ohm

I'm not sure 10 Ohms does anything at all on a Champ.

5V4 data shows like 100 ohms per plate, way up from your 10 Ohms.

> audible 120hz hum

Champish plans should generally have an added R-C filter, unless you are building for mass profit.

[mresistor]

Hi PRR - It's a 5Y3GT which is the stock tube rectifier for this amp. Its got around 394 volts on it.  Attached schematic. I've already built a bunch of these and currently working on two and a Champ version. I'm pretty much sticking to the schemo with some slight variations. But for some reason these last ones have a lot of 120hz hum. I'm trying to eliminate it or reduce it, and increased filtering is effective. But I am worried about using too much capacitance on the first PS filter node, and the harmful effects of charging current on the tube rectifier over the long run. So what I'm asking is, is a series resistance between the rectifier and the first 40uf filter capacitor effective in limiting the initial charging current (when amplifier is turned on) ?
I should note that convention seems to be a 20uf first capacitor. I'm doubling it and also upping the capacitance on the following nodes.
 
 

[eleventeen]

That Bronco schematic shows an EIGHTY volt drop across the OT Primary. 420 > 342.


Is that......real?

[mresistor]

Well , I don't think so, most have 15v or so drop. My copy of this schematic says "blue" and has no voltage listed on the plate of the 6V6. It also says Vibro Champ but I still think it is an AB764 circuit. But  what I'm asking is about limiting charging current because of increased PS filtering.

[mresistor]

Well I guess I could use a capacitor in series with a choke to form a Pi filter, but this is impractical. I'm just adding some series resistance and hope it limits the initial charging current. Since I have found more than one official Fender schematic with 40uf listed as the first filter node, then I think this should be OK to do. Having done it in one amp I can tell it is effective, but I don't know long term effect on the rectifier.

[John]

Courtesy of PRR (I saved this to my Scrapbook folder)


This should help. I've done this on my last couple, and they're very very quiet.

[Willabe]

Champish plans should generally have an added R-C filter, unless you are building for mass profit.

Do like PRR wrote.   

A number of guys here have run into the same problem on a Champ type build and solved it this way.

The deep theory guys have explained it a number of times before.

When the tube sheets say max capacitance for a given rectifier tube it also depends on any/all resistance in the PT's B+ wind, which is not always known. More resistance limits the B+ current inrush.

I don't think it's as much how much charging current a given rect. tube can handle as much as it is the inrush current that at turn on can kill a rect. tube, but I could be wrong.

Adding series resistance, like you brought up will slow down/limit the inrush current, but a pi filter will work better.

Cap/resistor (or a choke)/cap and you've added both resistance and capacitance. 20uf/some R value (40ohms to 100ohms? or a choke)/20uF will be a better filter then 40uf alone. You could go 40uf/R-choke/20uF. Or 40uF/R-choke/40uF. The main thing is enough added resistance between the uF's to slow down the B+ inrush. A choke large enough to handle all the amps B+ current, not just the screens and preamp plates, will slow it down too.

I have 1 of Doug's Fender chokes for a 2x6L6 amp here that measures 105.7 ohms. That size choke will work fine for a single 6V6 Champ type amp for the whole B+ PSU.

If you go with a R instead of a choke then the larger the R value the bigger the wattage rating it will need, maybe 10w to 20w?


                         Brad     

Edit: I was typing when John posted a schemo from PRR, go with his numbers.       
   

         

[mresistor]

Thanks John and Willabe - good information, now to try it out.  ;-) I've got some 8 watt 250 and 300 ohm wire wounds that would easily fit and the JJ cap is 40-20-20-20 so have enough capacitors there.

[Willabe]

I've already built a bunch of these and currently working on two and a Champ version. But for some reason these last ones have a lot of 120hz hum.

What exactly did you change?

Layout? Speaker? Parts brand (caps)? Circuit?


                   Brad   

[Willabe]

Doug's Fender type 2x6L6 choke is listed at 90mA, 4H.

A standard Champ type build is ~40mA to 50mA for the 6V6 plate current, add a few mA's for the 2x12AX7 plates and there's still plenty current left over from the choke.

If you have enough heater current available to use a 6L6 or EL34 it's still enough because using the same B+ PSU and OT will not really give anymore output power. 


                    Brad        

[mresistor]

They are minor changes - ones you know about like we have to use 50KRA pot for intensity as the 25 isn't available, upping one of the .01uf caps to .02 to slow down the trem. Increase value of fb resistor. 6V6 grid stopper and screen resistor. Stuff like that. I also tried switching in 15K in parallel with the 18k on the bass pot for a more tweed like sound, but that didn't do much and I like the regular sound better. Problem with adding a choke is where to mount it on a Fender Champ chassis, I'd rather avoid that.

[Willabe]

Just looking for why your other builds were quieter?

Yeah, Champ chassis are on the small side. 

You can go with an R instead of a choke, it'll work fine, just an option.

Some like a choke better, sound/feel wise, some like the R better.      I think to get the ~same B+ filtering with a choke or R in the middle of a pi filter the R's resistance will be larger then the chokes series resistance?

But it might not matter that much with a single ended class A amp because the amp is already drawing a larger idle current then a PP class AB amp? So extra sag from added resistance in the PSU will not be as noticeable as in a PP amp.       


                              Brad          

[HotBluePlates]

That Bronco schematic shows an EIGHTY volt drop across the OT Primary. 420 > 342.

Is that......real?

Probably not. Look at the screen node... says 410v at the filter cap, but 340v at the screen. Across no resistor...

[mresistor]

Just looking for why your other builds were quieter?

That's what I'd like to know. The ones I built before didn't have any 120hz hum. Same house,  same electric service. One thing different is the iron. These last three all have Classic Tone. So that has me wondering about them. None of my original Fender Champs/Vibro Champs hum like this either. I had one Deluxe Reverb build that had a tiny bit of audible 120hz hum but the other push/pull amps I've built are quiet.

I implemented the Pi filter and the amp is really quiet now. Works great. 6V6 plate volts are now at 359v and seems more in tune with the blackface AA764 circuit which is really want I'm looking for. The amp sounds nice when playing through it with a lot of the Champ tone.

[Willabe]

One thing different is the iron. These last three all have Classic Tone. So that has me wondering about them. None of my original Fender Champs/Vibro Champs hum like this either.

What exactly did you change? Layout? Speaker? Parts brand (caps)? Circuit?

  Well I would say that's a change.

I was trying to ask what did you change on the 3 new builds that have noise from the builds before that are quite? Not any changes from the Fender schemo, like adding a cap to slow down the trem speed or adding grid stoppers and screen grid R's. If anything those stops should make the amp quieter.

For some reason(s) your earlier builds of the same amp are quite, so something in the new builds must have changed.

Even (what you might think are) little things can be very important, like; 

Are the PT/OT cores aligned differently on the Classic Tone iron then the other PT/OT's on the quite builds?

Did you mount them differently? Do they have steel end bells? Did you go from an 8" speaker to a 12" speaker. (Cheap little 8" speakers won't/can't reproduce the low 120hz so it's not as noticed.)

Did you change any wires in you layout lead dress?


                                  Brad      

[HotBluePlates]

What Willabe is really getting at is several folks on this forum built Champ-style amps, but realized that even ~4w of power through big 12-inch speakers (or a couple of them) makes a sufficiently-big racket.

Then they noticed hum, even though the amp was perfectly-assembled according to the original schematic. We eventually figured out the bigger speakers had deeper low-end response, and they were hearing hum for the first time that's present in all Champs, but which a puny 8" speaker and tiny cabinet naturally attenuate.

The answer has almost universally been to add a stage of filtering before the filter cap where the output tube plate & OT take power (in other words, the PRR sketch that John posted).

You'll generally get much better filtering of hum by adding a C & R as in the sketch, rather than making the first filter cap several times bigger. This is because a series resistor and the following cap form a divider which cuts a.c. ripple (hum) while leaving d.c. mostly-unchanged. A choke does better than a resistor at preserving d.c. voltage, but space, weight and cost may argue against using the choke.

[Willabe]

What Willabe is really getting at is

Yeah, what he said!

I couldn't have said it better myself and I didn't!


       
                    Brad     

[mresistor]

I'm sure I used Heyboers in the previous builds and the orientation was the same. I have always built them in stock Champ chassis, from Mojotone or real Fenders. My lead dress has been the same for all. Nothing really fancy but not ugly either.  I am running a 12" speaker; specifically a 12" Alnico P-232 Magnavox in a special 12" Champ cab. And it sounds glorious. ;-) However, the previous champs I built were head cabs and I ran those through my 2x12 cabinet and they were quiet. So this really has had me shaking my head. I have a closet full of C's and VC's and none of them are as noisy with the 120hz hum as these. I don't have the 3rd one built yet, and I plan on including the Pi filter from the get go on that one. I find that the 8watt wire wounds are not even getting hot and that's a good sign.
So, in the back of my mind I am asking the question, is there something about this ClassT iron that is different? Or did these three come from a batch that had problems? I really don't want to think it is the iron, but everything else seems to be the same. Its a ClassT set for Vibro Champ/Champ and I would think those guys know what they're doing and know how they are going to be oriented on a Champ/VC chassis. I haven't actually talked to them about this.
Don't know that I will.

[mresistor]

I should add this one is an original 1968 Bronco chassis. The one in the 12" cabinet is a chassis constructed in Thailand and I made it serial number 10  . LOL  It is almost identical to original 1964 Fender chassis. The one I am now working on is a late 70's Champ chassis that someone gutted and had chromed.
I am going to hot rod it a little and build a head cab for it. I'm going to call it "Chimp" and I am hoping to get a sticker for the front made to say that.

[Willabe]

I am running a 12" speaker; specifically a 12" Alnico P-232 Magnavox in a special 12" Champ cab. And it sounds glorious. ;-) However, the previous champs I built were head cabs and I ran those through my 2x12 cabinet and they were quiet.

Have you tried running the new (noisy) Champs through the 2x12" cab? And the quite Champs through the 1x12"?

I'd try doing both.


                                      Brad   

[mresistor]

It's a little late for that  as they are now quiet.  No worries, thank you for the assistance. Btw, the cloth covered wire I used in the amp in the pics above is from 1957. I like to recycle...       

Champish plans should generally have an added R-C filter, unless you are building for mass profit.

That was very good info PR and I read the same at Valve Wizards site.   Thank you sir. 

[Willabe]

It's a little late for that  as they are now quiet. 

All 3 of them? And it still doesn't answer why the older builds are quite without the added C/R.

So, in the back of my mind I am asking the question, is there something about this ClassT iron that is different?

Seems like there were 2 major changes, 1; PT/OT, 2; speakers.

What about trying 1 of the quite Champs through the new speaker and cab? See if it now is noisy, if so it's the speaker and cab. If not it might be the PT/OT.   

OR If you temporarily disconnect the added C/R stage you added and swap speaker cabs you might find something out?

I'd do both to see if I could find out why the others are quite without the added C/R.


                    Brad   

[mresistor]

Brad - For now, I got a "68 Fender Champ out and plugged it into the 2x12. It's turned on now, there is nothing else but this puter going in this room. And the amp is dead quiet. I will check a few others for ya. But, I'm saying .... these stock 45-55 year old amps are very very quiet. Not a peep. I could hear a cricket f**t.    Gonna call it a day.   

[HotBluePlates]

What's the switch on the volume pot?

[mresistor]

It is to connect/disconnect the negative feedback.

[Willabe]

Their your amps and you can call it a day, that's fine.   

But you said this;

I am running a 12" speaker; specifically a 12" Alnico P-232 Magnavox in a special 12" Champ cab. And it sounds glorious. ;-) However, the previous champs I built were head cabs and I ran those through my 2x12 cabinet and they were quiet. So this really has had me shaking my head.

And this;

So, in the back of my mind I am asking the question, is there something about this ClassT iron that is different?

And;

Just looking for why your other builds were quieter?

That's what I'd like to know. The ones I built before didn't have any 120hz hum.

So I was trying to suggest a test to get at if it's the speaker(s) or maybe the new PT/OT in the new builds that's causing noise.

Now you post this;

For now, I got a "68 Fender Champ out and plugged it into the 2x12.

And the amp is dead quiet. But, I'm saying .... these stock 45-55 year old amps are very very quiet. Not a peep.

But that's the 2x12" cab that you used on the quite new builds not the 1x12" cab you have with the noisy new builds. And it's Fenders PT/OT.

Not the same test's at all.

I will check a few others for ya.

Nope, not for me, you asked the questions.



                                 Brad     

[mresistor]

Thanks for your help. Its not speakers, further testing is pointless. I've already tested every which way with speakers. No need to do it again.  Appreciate the observations of my written words, your summary is excellent. The help and confirmation I came here for has been received and put to use and the problem solved. I have no more 120hz hum, and I appreciate that. Now it's time to put this to rest. Thank you again .

[Willabe]

Glad you got it worked out.



           Brad