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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 13, 2017 6:34 pm 
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Hello Kyle
If you are making boards to sell count me in and far as my 5902 I have tube sockets
When get chance I might order the stuff to product my own board but at this time I don't have the time working 5 days a week from 3pm to 11:30
Anyway your boards look great nice work


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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 13, 2017 10:04 pm 
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john8750 wrote:
Guys
How important is the IC gain pot, 20k? I have built several amps using this gain adjustment control. It don't seem to make any difference when I try to calibrate. Just wondering if we could do with out it.
I like it Kyle. Mine will use the 6V6G Coke Bottles. I must have a few sets. Thank You.

John:
We covered this once before on the phone. Perhaps it didn't sink in at that time or you may have forgotten.
The 20k gain pot on the opamp is there to be able to set each op-amp's gain to match the other channel's gain.

The opamp is supposed to have a gain of 100. But as long as both are set identical is the important thing.

The way to do it is to FIRST disconnect the global feedback via .47uf cap temporarily.
Then put in a fixed value of 400hz sine wave (say .1v PP) and adjust the pot such the the output sine wave is at some value approx 100 times greater than the input. (10vPP) .. the exact output value isn't important as long as you feed that exact same value input signal to both channels and then adjust the outputs to the same matching value. So the output can be 9.7vPP or 10vPP or whatever as long as both channels are set to the same value..
Do the same for the other channel.

The idea is to have both channel's gain be set to equal or match each others output for a given fixed input.

If you have no way to measure input or output signal.. just tweak the pots for max resistance. 20k.


After you get them to match... then reconnect the global feed back path.

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Last edited by Pbpix on Feb Mon 13, 2017 11:10 pm, edited 1 time in total.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 13, 2017 10:27 pm 
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dkjones96 wrote:
That feedback pot adjusts the opamp gain from ~83:1 to 100:1. Not a huge change but enough to bring a channel in line if the amp is unbalanced. If all of your tubes are new or equally used I doubt you'd need to change it.

Hi Kyle:
Unless I'm mistaken, I didn't believe that the 20k pot is there to compensate for tube differences.
No. I thought it was just to set the op-amp's gain to match each other.

(Minor tube differences should be ideally compensated for by the auto-self-adjusting bias via the LM317-Constant current source circuit)

As such, the 20k pot would be there only to adjust the opamp's gain separately for each channel.
So I adjust with the tubes out of the signal path by opening the global feedback path. So the global feed back path is temporarily disconnected so as to set each op-275 gain for about 100. (or just match both channels)
The pot will allow the op-amps to be set to equal gain in case the local FB resistors aren't exact or precision values.

So after global FB is disconnected, I feed in a 400hz sine wave at .1vPP and tweak the pot to get approx 10vPP output. But the exact value isn't important since the idea is the have the two channels match. Then reconnect the global FB.

BTW:
With the op-amp rails at +/- 20v DC....
Using a scope on the tube grid, the max PP audio signal you can hope to see on the tube grid (with global FB disconnected) is about 37-38vPP. After that, further input signal increases will result with the PP signal tops starting to flatten.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Tue 14, 2017 12:13 am 
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Just sitting here tonight in "7th heaven" listening to this fantastic 6V6/6aq5 amp.

I have lots of different tubes that I rotate in it for fun.

orig USA 6V6G
Orig USA 6V6GT
orig USA 6AQ5
Orig USA 12V6GT
Orig USA 12AQ5

Russian 6V6GT (6p6s or 6n6c) - excellent ( $8 ea new)
China 6V6GT ($5 ea new)
Chinese "Shuguang 6V6GT" - excellent

Tonight I'm using these really fantastic-sounding Shuguang 6V6GT tubes. ( matched quad $32)

Fantastic... full-bodied 3-dimensionl sound.
Audio heaven.

This amp is just ...pure joy to listen to.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Tue 14, 2017 2:38 am 
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Pbpix wrote:
dkjones96 wrote:
That feedback pot adjusts the opamp gain from ~83:1 to 100:1. Not a huge change but enough to bring a channel in line if the amp is unbalanced. If all of your tubes are new or equally used I doubt you'd need to change it.
Hi Kyle:
Unless I'm mistaken, I didn't believe that the 20k pot is there to compensate for tube differences.
No. I thought it was just to set the op-amp's gain to match each other.

(Minor tube differences should be ideally compensated for by the auto-self-adjusting bias via the LM317-Constant current source circuit)...
I don't know what you mean by that. The output tube CCS simply forces the same idle current through the tubes. It does nothing for differing gain (gm).

I agree with John that the 'gain' pots seem superfluous. While you can 'measure' the effect your ears can't tell a 16% difference, much less .2% (if you used fixed .1% resistors).

Btw, the range of adjustment does suggest it was intended to compensate for the tubes but they're in a feedback loop too, albeit not much, which tends to reduce the overall gain error.


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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Tue 14, 2017 3:11 am 
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Flipperhome wrote:
Pbpix wrote:
dkjones96 wrote:
That feedback pot adjusts the opamp gain from ~83:1 to 100:1. Not a huge change but enough to bring a channel in line if the amp is unbalanced. If all of your tubes are new or equally used I doubt you'd need to change it.
Hi Kyle:
Unless I'm mistaken, I didn't believe that the 20k pot is there to compensate for tube differences.
No. I thought it was just to set the op-amp's gain to match each other.

(Minor tube differences should be ideally compensated for by the auto-self-adjusting bias via the LM317-Constant current source circuit)...
I don't know what you mean by that. The output tube CCS simply forces the same idle current through the tubes. It does nothing for differing gain (gm).

I agree with John that the 'gain' pots seem superfluous. While you can 'measure' the effect your ears can't tell a 16% difference, much less .2% (if you used fixed .1% resistors).

Btw, the range of adjustment does suggest it was intended to compensate for the tubes but they're in a feedback loop too, albeit not much, which tends to reduce the overall gain error.

I am not the original designer.
That guy's original design didn't use the CCS bias... He only had normal cathode bias.

I've interpreted the gain pot to be used for tolerance compensation for the op-amp gain matching if one happened to use normal 10-20% resistors. But that's my guess.

The tube's gain may be one part of tube-matching.. yes.
... but additionally if one were to have used either fixed bias or regular cathode bias and tried to have balanced idle current set properly by grid voltage table-values, there is still going to be some wide variation in current, especially with age.
The CCS bias circuit provides an extremely accurate balanced idle current regardless of typical tube variations.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Tue 14, 2017 5:19 am 
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Pbpix wrote:
Flipperhome wrote:
I don't know what you mean by that. The output tube CCS simply forces the same idle current through the tubes. It does nothing for differing gain (gm).

I agree with John that the 'gain' pots seem superfluous. While you can 'measure' the effect your ears can't tell a 16% difference, much less .2% (if you used fixed .1% resistors).

Btw, the range of adjustment does suggest it was intended to compensate for the tubes but they're in a feedback loop too, albeit not much, which tends to reduce the overall gain error.

I am not the original designer.
That guy's original design didn't use the CCS bias... He only had normal cathode bias.

I've interpreted the gain pot to be used for tolerance compensation for the op-amp gain matching if one happened to use normal 10-20% resistors. But that's my guess.

The tube's gain may be one part of tube-matching.. yes.
... but additionally if one were to have used either fixed bias or regular cathode bias and tried to have balanced idle current set properly by grid voltage table-values, there is still going to be some wide variation in current, especially with age.
The CCS bias circuit provides an extremely accurate balanced idle current regardless of typical tube variations.

That's what I said, the CCS forces DC balance but doesn't do a thing for gain, which happens to be the topic under discussion.

Good luck on finding your 'normal' 10-20% resistors. You'll have to pay a premium to get that loose a tolerance.


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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Tue 14, 2017 6:34 pm 
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Hello john
I agree with Peter and yes the gains are go idea
Do as Peter said and both channels will be same level
Anyway Kyle
I would live to get few your boards once you prof extend the design
It would save me buying all material and making my own from scratch


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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Wed 15, 2017 1:50 am 
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Location: Albuquerque, NM
Building your own boards is so much fun but I understand. It has taken me a while to get to the point where I can do my own silkscreens and solder resists. I work ~ 12 hour days when I'm on a trip but our local hosts are off this weekend so as a result so are we. I'll have the board files ready to go this weekend.

The opamps should be spot on with each other as long as the resistors used in the feedback networks match between channels. The pots are only useful for input vs final output adjustment. Even if the tubes have differing gm the global feedback loop should be able to compensate for a lot of that.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Wed 15, 2017 9:06 am 
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Hello Kyle
I don't mind building boards but need to order one of those kits
I used to build boards in the day and really enjoyed it


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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 12:30 am 
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This is the board almost as it is going out. I need to assign numbers to the parts on the schematic and to do that I need a schematic I can edit. For the prototypes, however, this will work. Sending these off to be made this week. Once they come in I will build a 5902, 6AQ5, and 6V6 version of the board to verify they all work.

The tubes stick out of the opposite side of the board as the solid state components. I THINK the holes on each side of the octal sockets are in the right place for your typical chassis mount socket and if utilized with a 1/2" spacer the board can be mounted to the chassis using nothing but the socket screws(saves chassis drilling) otherwise the 5902 and 6AQ5s will need to have 4 holes drilled for mounting(though it occurs to me that I might be able to fit holes for the 7 pin sockets too).

I made sure the opamp was clear on both ends for easier replacement if someone wants to 'roll' some different ones and most of the capacitor areas have multiple holes to allow room for fitting different sizes of capacitors. Added filament balance resistor spots for hum management. All of the preamp signal lines are on the top layer and equidistant(not that it matters at AF) with a mostly uninterrupted ground plane for best shielding.

Attachment:
Mono Board Layout RSS.gif
Mono Board Layout RSS.gif [ 66.19 KiB | Viewed 2866 times ]


There are things I would like to change in this schematic but I will do that as a part of my own thing later and start a thread on that.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 12:44 am 
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dkjones96 wrote:
This is the board almost as it is going out. I need to assign numbers to the parts on the schematic and to do that I need a schematic I can edit.

Looks good.
What do you mean "need a schematic I can edit" ?

I'd be glad to buy some or several or a bunch...Kyle as soon as you're ready.

How much effort would it take to make a PC board for the power supply as I have mine.. transformerless.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 1:01 am 
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I don't have a printer here and this schematic is one thing I forgot to print before coming to Brazil and so as a result I don't have a schematic to write on. I'll end up redoing the schematic in a cad program I just haven't gotten to it yet.

A power supply board should be pretty easy. If I have spare time tomorrow(I likely will) I can layout a supply board.

I'm only ordering 3 at the moment because I want to make sure everything is lined up and works. I will start a classified thread when the time comes to order them so I know how many I'll need. I'll be lucky to have a solder mask on these first ones!

EDIT: Honestly, it only costs a dollar more to get 10pcs. I'll go over these with a fine toothed comb before having them made and I can have 2 extra pairs for others to build too.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 1:51 am 
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dkjones96 wrote:
I don't have a printer here and this schematic is one thing I forgot to print before coming to Brazil and so as a result I don't have a schematic to write on. I'll end up redoing the schematic in a cad program I just haven't gotten to it yet.

A power supply board should be pretty easy. If I have spare time tomorrow(I likely will) I can layout a supply board.

I'm only ordering 3 at the moment because I want to make sure everything is lined up and works. I will start a classified thread when the time comes to order them so I know how many I'll need. I'll be lucky to have a solder mask on these first ones!

EDIT: Honestly, it only costs a dollar more to get 10pcs. I'll go over these with a fine toothed comb before having them made and I can have 2 extra pairs for others to build too.

Wow! Thanks Kyle... you are a champ!
Very encouraging..

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 3:25 am 
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Pbpix wrote:
dkjones96 wrote:
I don't have a printer here and this schematic is one thing I forgot to print before coming to Brazil and so as a result I don't have a schematic to write on. I'll end up redoing the schematic in a cad program I just haven't gotten to it yet.

A power supply board should be pretty easy. If I have spare time tomorrow(I likely will) I can layout a supply board.

I'm only ordering 3 at the moment because I want to make sure everything is lined up and works. I will start a classified thread when the time comes to order them so I know how many I'll need. I'll be lucky to have a solder mask on these first ones!

EDIT: Honestly, it only costs a dollar more to get 10pcs. I'll go over these with a fine toothed comb before having them made and I can have 2 extra pairs for others to build too.

Wow! Thanks Kyle... you are a champ!
Very encouraging..



Thanks from me also Kyle. A big thanks. I will need to add a tone amp to mine. I like body in my music.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 6:32 am 
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Hello Kyle
Yes count me in I would buy a few boards
Anyway keep up the wonderful work


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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 7:59 pm 
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Peter, do you have a link or datasheet for the 5w and 20w resistors used in the power section? Need some specs for lead spacing.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 11:32 pm 
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dkjones96 wrote:
Peter, do you have a link or datasheet for the 5w and 20w resistors used in the power section? Need some specs for lead spacing.

I totally OVER rated that 20watt resistor for super extra cool operation.
The reality is that it drops 22 volts DC.
So 22v / 220 ohms = .1A

0.1A x 22v = 2.2 watts

2.2 watts tripled for safety = 6.6 watts.

Closest would be 10 watts. And that would still be VERY VERY safe and cool.
These are ceramic body. Should be mounted a bit above the board surface for good cooling.
aprox. 2" long
7/16 x 7/16 square
Here's some on Ebay with dimensions:
http://www.ebay.com/itm/10PCS-220-ohm-2 ... xy9X5TaQNO

here's the 10uf 180vAC filament dropping capacitor that I use...with dimensions :
http://www.ebay.com/itm/AC-180V-10uF-5- ... Swys5WVnnJ

BTW:
There are other 10uf film caps rated 150vAC such as these and others I'm sure elsewhere.
http://www.ebay.com/itm/5pcs-150-V-AC-1 ... SwVL1WBkGE

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Last edited by Pbpix on Feb Mon 20, 2017 11:49 pm, edited 1 time in total.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 11:46 pm 
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Very nice, thanks. I need to try and stay awake until midnight local time if I am to readjust properly to flying back tomorrow so this is on the list of things to keep me awake.

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 Post subject: Re: 6V6 / 6AQ5 PP Stereo Amp
PostPosted: Feb Mon 20, 2017 11:51 pm 
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dkjones96 wrote:
Very nice, thanks. I need to try and stay awake until midnight local time if I am to readjust properly to flying back tomorrow so this is on the list of things to keep me awake.

thank you... I just added another type of 10uf film cap ... but I like the one I'm now using. They simply have to be rated for AC voltage approx 150v or more.

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