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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 1:00 am 
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Pbpix wrote:
Flipperhome wrote:
Here's the PS for the amp section, including OPA604s, that I proposed. The rest of the solid state stuff, MP3 booster, RIAA phono preamp, and tone controls, would then have it's own power supply.

Attachment:
Doubler - heaters - opamp N-77U.jpg


Transformer current loading (752 mA) is based on bias being 25 mA per tube. The N-77U could handle the 'normal' load of 37 mA per tube at nominal line voltage but at +6% it's current rating would be exceeded. Note, the N-77U rating is at 25C. I don't know what derating should be applied for other temps.

The zeners might be able to take a no load condition (dissipation is 1 Watt under no load) but I don't recommend it, at least not for long.

Note, on some previous schematics D6 is backwards. It's correct in this one but I didn't go back and edit the previous ones.

So Flip:
What will the ripple level be on the B+?
I'm using much larger filter caps (820uf) and I get only 20mv PP

Just trying to follow along with the same calcs:
On the 24v supply : How are you calculating the current for the dropper cap?
Do you calculate for the 32ma max for the full cycle?

In the spread sheet I used ... it indicates that 2.7uf will deliver 40v @ 120ma with 2v dropped by the 22ohm surge resistor.
So... I'm not sure with (+/- ) type supplies how to properly use that spread sheet accordingly.

I use this spread sheet:
http://www.vintage-radio.com/download/v ... ations.zip

I entered:
120v Mains
60 Hz
40v Heater volts
0.12A Heater current
3v surge volts = 25 ohms

So with that input it indicates 2.7uf is the dropper value.

So as I calculated it:
If the cap does deliver 40v and you desire 24v ... at a max of say 34ma ... then the 470k resistor drops 15v @ 34ma .... right? So if the chips require 28ma the Zener will take 6ma. Right?

I just don't get why it indicates that 2.7uf will limit the current to 120ma ? Seems too high... no?
B+ ripple is estimated to be around 23mV.

You can't use that spreadsheet to calculate (two) half wave filter(s). It's for a resistive load.

For the +-24 V supply, the 32 mA max is over line voltage variation, leaving a mA flowing through the zener (on the low end). Current through the 470 Ohm resistor is 40 mA at 120VAC and, of course, there's two of them (+- 24 V) for 80 mA total. Voltage on each filter (+-) is around 42 V (at 120VAC). Transformer loading is around 110 mA (at 120VAC).


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 3:45 am 
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Pbpix wrote:
Flipperhome wrote:
john8750 wrote:
Thanks Flip. I will build this version. Could you please calculate what the 24volt loads, in resistance, be for testing.
Guys, the level drops if I use the tone board. Hooked straight in,I have a higher level. Must be why Peter can nuke the walls with his 6V6, and I cant. Would another booster work?
The load is easy. It's just 24 V divided by 32 mA (if you're loading it to the max). That's 750 Ohm (on each leg). 1k ohm would probably be representative of 'typical'.

If you're still using the original circuit for the 6V6 it's the low input impedance that's killing your tone control. That won't happen with the new circuit design.

I'm using the new design... and I can't remember whether or not I had the same wall shaking, output power before ... but it certainly does the job now.
So... maybe Flip is right and that 5uf NP input cap made a difference?
It's not the cap, per see, but the fantastically low input impedance.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:31 am 
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I need to order a few parts to build the new power supply. Will start striping the chassis of the old circuits and PS.
So the total load on the new PT will be about 700ma or so. Not much room left.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:37 am 
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So why the N77-u @ $32
instead of the FD8-120 @ only $20

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:41 am 
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BTW:
If you're thinking of building a separate supply for all the other chips.. and you have to buy transformers or little supplies anyway... why not just run the filaments on a small 25 or 12 or 6v transformer then the Main transformer is free to power up all op-amp chips everywhere you want... no?

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Last edited by Pbpix on Jan Fri 12, 2018 5:43 am, edited 1 time in total.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:41 am 
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Pbpix wrote:
So why the N77-u @ $32
instead of the FD8-120 @ only $20
Because the FD8-120 is open terminal.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:43 am 
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Pbpix wrote:
BTW:
If you run the filaments on a small 25 or 12 or 6v transformer then the Main transformer is free to power up all op-amp chips everywhere you want... no?
Of course, if he has the room and it would cost. It would take a 2 amp 6.3 VAC transformer.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:47 am 
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Flipperhome wrote:
Pbpix wrote:
So why the N77-u @ $32
instead of the FD8-120 @ only $20
Because the FD8-120 is open terminal.

But whats the difference? Where will it be mounted? The wires are still sticking out of the N77-u as an open frame ugly thing... and the FD8 looks more finished-looking could have the wires soldered on and heat shrink over the pins.
So... seems the Fd8 looks nicer...
How about mounting under the chassis or cut a hole and change the mount.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:50 am 
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Flipperhome wrote:
Pbpix wrote:
BTW:
If you run the filaments on a small 25 or 12 or 6v transformer then the Main transformer is free to power up all op-amp chips everywhere you want... no?
Of course, if he has the room and it would cost. It would take a 2 amp 6.3 VAC transformer.

or a 1/2 amp 25v

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:54 am 
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Pbpix wrote:
Flipperhome wrote:
Pbpix wrote:
So why the N77-u @ $32
instead of the FD8-120 @ only $20
Because the FD8-120 is open terminal.

But whats the difference? Where will it be mounted? The wires are still sticking out of the N77-u as an open frame ugly thing... and the FD8 looks more finished-looking could have the wires soldered on and heat shrink over the pins.
So... seems the Fd8 looks nicer...
How about mounting under the chassis or cut a hole and change the mount.
The 'difference' is the same 'difference' why he didn't want to use the 130 Watt, 27 buck, VPS230-570 that also solves 'all the problems'.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 5:56 am 
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https://www.jameco.com/z/RS-25-12-MEAN- ... hfEALw_wcB

http://www.nextwarehouse.com/item/?1738998_g10e

https://www.alliedelec.com/triad-magnet ... VpEALw_wcB

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 7:00 am 
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I already have the N77U. And would like to use what I have. I also have an 85watt iso transformer. Might be overloaded though. I will try to mount the N77U terminals down. Cut a square hole in chassis.
And have the small 12volt components for the rest.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 7:39 am 
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No, I have the FD8-120, not the N77U. So mounting will be a job.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 8:00 am 
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john8750 wrote:
No, I have the FD8-120, not the N77U. So mounting will be a job.

Doesn't that metal mounting fame come off?
If so ... perhaps there are holes in the lamination corners for mounting screws to a chassis face down.
But it's not big... can't you mount it below the chassis on a side wall or upside down hanging from the deck?

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Fri 12, 2018 6:17 pm 
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Pbpix wrote:
john8750 wrote:
No, I have the FD8-120, not the N77U. So mounting will be a job.

Doesn't that metal mounting fame come off?
If so ... perhaps there are holes in the lamination corners for mounting screws to a chassis face down.
But it's not big... can't you mount it below the chassis on a side wall or upside down hanging from the deck?



That's a thought. Let me check that out.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Sat 13, 2018 12:48 am 
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Here's a little bit of bench measurement feedback. FWIW

Here's the actual voltage and current measurements live from my 6V6amp's dual supply for the OP25s.

So John it's nice to see and understand how the voltage is developed and how current is divided between the two op275 chips as a load, ... and how the excess current is going to the Zener.

Now:
Flip you indicated that the spreadsheet I use (to calculate the correct value dropper cap), is not meant for a dual +/- supply ... but only for a fixed resistive load.
Are you sure about that? If so why?

Doesn't the whole entire +/- supply and its related loads represent a single combined load to the dropper cap? It sure seems that it does

Because when I use it to compare these results for a dropper cap value it's almost right on.

In the spread sheet I enter:
120v mains
60Hz
37v heater voltage
40ma heater current
4v surge limit

The result sows a .98uf capacitor and a 100 ohm surge resistor


Attachment:
op275 dual supply data.jpg
op275 dual supply data.jpg [ 121 KiB | Viewed 251 times ]

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Sat 13, 2018 9:58 pm 
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Man you guys. That simple little dropper circuit is quite complicated. I thought you would just start with a dummy load, swap caps till you got what is needed.

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Sat 13, 2018 10:35 pm 
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john8750 wrote:
Man you guys. That simple little dropper circuit is quite complicated. I thought you would just start with a dummy load, swap caps till you got what is needed.

For "dropper caps" calculation : ...On a simple single AC resistive load like just tube filaments ... you could do that.

But a dropper cap with a regulated DC supply with a Zener is more to think about because it takes a little more thought because now you add the design consideration for a Zener regulator after the dropper cap.

(Heres a video that taught me how to design a Zener regulator supply)
https://www.youtube.com/watch?v=6xGCOrPBL4s

And for a dual supply you just do it twice.

But I also use a free Excel spread sheet that already has all the "Dropper-Cap" calculation formulas built in it to calculate the proper cap depending on the load voltage and current needed. So that gets me close for the 1st pass.... then I trim on the bench for the real world load.

Bench testing it always provides the accuracy and confidence that my final component choices will work.

This page will explain three ways to drop voltage for filaments.
http://www.vintage-radio.com/repair-res ... calcs.html

1. Resistive
2. Diode
3. Capacitive dropper

Read it and get some idea about it all.
But scroll to the bottom 1/3rd of the page about dropper cap calcs.
Involved formulas... but then he gives you a FREE link to the Excel spread sheet that does the math for you.
Try reading it John... you'll appreciate it.
http://www.vintage-radio.com/repair-res ... calcs.html

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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Sun 14, 2018 12:27 am 
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Pbpix wrote:
...Now:
Flip you indicated that the spreadsheet I use (to calculate the correct value dropper cap), is not meant for a dual +/- supply ... but only for a fixed resistive load.
Are you sure about that? If so why?

Doesn't the whole entire +/- supply and its related loads represent a single combined load to the dropper cap? It sure seems that it does

Because when I use it to compare these results for a dropper cap value it's almost right on.

In the spread sheet I enter:
120v mains
60Hz
37v heater voltage
40ma heater current
4v surge limit

The result sows a .98uf capacitor and a 100 ohm surge resistor...
You didn't do the calculation correctly and that it's "almost right on" is a serendipitous confluence of those errors, and the fact it's already a working design. For example, how do you know it'll regulate? There's nothing to take into account ripple. E.g. put a 1 uF cap for C2/3 and it falls apart but your calculations are perfectly happy with that.

But you've also overestimated the load. I don't know where the 40 mA comes from but, in any case (I'll use your numbers), that's not right and has the effect of overestimating the load by twice. I.e. Two 20 mA (max) supplies at 18.5 V is .0.37 W x 2 = 0.74 W, not your 37 V x 40 mA = 1.48 W.


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 Post subject: Re: 6V6 amp project- with a twist
PostPosted: Jan Sun 14, 2018 1:50 am 
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Not enough room to mount the power transformer under the chassis. The power supply components are already there.
Attachment:
20180113_162134.jpg
20180113_162134.jpg [ 113.87 KiB | Viewed 222 times ]

Plenty of room on top. Will need to make a cover over the terminals. Maybe a wood box over the transformer. It could match the chassis. Vent holes in the floor and in rear.
Attachment:
20180113_162305.jpg
20180113_162305.jpg [ 107.89 KiB | Viewed 222 times ]

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