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A 6SN7 direct coupled preamp & regulated supply for everyone's perusal.

Hi all. I wanted some opinions and vetting of a preamp & supply. Here ya go:
 

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ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
Suggestions for the preamp:

I'd connect a resistor of, say, 1Meg between the slider of the volume control and ground, to prevent unpleasant noises in case the track has imperfections that could cause momentary open circuit.

With the resistor values and B+ voltage shown on your schematic, the plate current in each triode will be only about 2.5mA, which is rather low. As you say, the plate voltage of the first triode will be 75v, which I also think is too low.

If you can increase B+ to 300v, using the same resistor values you will have a plate current of 5mA in each triode and a plate voltage on the first triode of 150v. This is a better operating point and both triodes will be dissipating only 750mW, which is well within their ratings.

Of course, there are all sorts of clever things you could do with a 6SN7 to achieve similar ends, including SRPP and mu-follower. My suggestions are just aimed at improving the results with your own perfectly good topology.

Suggestions for the PSU:

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA. This give 350v with a cap inout filter but will only let you draw 30mA (you can only use 60% of the transformer's AC current rating with a cap input filter). That's not going to be enough. I would get a 250v tranny with 100mA rating.

You will need two OD3 VR tubes in series in each regulator. Driving the VR tubes from a constant current source, as you have in your schematic, is a nice refinement but probably not really necessary unless your line voltage is highly variable.

I can't comment much on the CCS, since I don't know the device you're using, except to say that you need to set it to deliver 20mA for each channel. That allows 10mA for the preamp and another 10mA for the VR tubes in each channel. 5mA is the absolute minimum current for an OD3 to work so, to allow for low line voltage, you need to feed it more than that under 'average' conditions. It is happy with up to 40mA, so 10mA is a reasonable figure. That's why you need a bigger tranny.
 

hotbottle

Member
2008-03-16 6:31 am
If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA. This give 350v with a cap inout filter but will only let you draw 30mA (you can only use 60% of the transformer's AC current rating with a cap input filter). That's not going to be enough. I would get a 250v tranny with 100mA rating.

I was just interested in this calculation. Is it just a rule of thumb? and is it dependent on the type of rectifier? Also, what causes the limitation, inrush current of the cap?
 
ray_moth said:

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA.

Hello Ray:

I have to disagree with you on this one. The transformer is rated 40VA, and comes with a 6.3V 2A aux winding. That leaves 27.4VA left for the secondary, which at 250V is 110mA. It looks like its rated for a full bridge operation.

Also, he actually doesn't have a cap input filter, it's more of a hybrid, riding the fence between choke and cap input. His transformer current is about as nice as you can get without the harsh turn-off of the choke input supply. So the transformer looks pretty perfect to me.

Of course, if he pursues a 300V supply instead, it would be useless, but looks good for 150V. PSUD confirms, at least.

Cheers.
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
Sorry, zigzag, I misread the schematic! I now see that the cap is, indeed, 0.33uF, not 33uF. In that case he has a choke-input filter, to all intents and puposes, and he must be using the full winding already. Becasue it is a choke-input filter, he can probably get 225vDC @50mA.

A pair of OC3 VR tubes in series would give 210v B+, allowing 15v to be dropped across the CCS and providing for a somewhat better op point for the 6SN7s than a 150v B+.
 
ray_moth said:
Sorry, zigzag, I misread the schematic! I now see that the cap is, indeed, 0.33uF, not 33uF. In that case he has a choke-input filter, to all intents and puposes, and he must be using the full winding already. Becasue it is a choke-input filter, he can probably get 225vDC @50mA.

A pair of OC3 VR tubes in series would give 210v B+, allowing 15v to be dropped across the CCS and providing for a somewhat better op point for the 6SN7s than a 150v B+.


I don't understand what you are saying Ray. The transformer is 125 - 0 - 125 @ 100mA not 50 mA. The transformer does not have two 50mA windings on the secondary. It has one center tapped 100mA winding. Am I missing something?
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
What it is, G, is that the 100mA rating assumes that you willuse the tranny as it was designed to be used, with the center tap as negative and a diode on each end of the winding to give the positive. This will give 125v @ 100mA.

Using the full winding with a bridge rectifier, as in your schematic, will give twice the voltage but only half the current, so you get 250v @ 50mA.
 
sorenj07 said:
The transformers' current ratings are for capacitor input. Choke input quite possibly would free up more current.

http://hammondmfg.com/pdf/5c007.pdf


Thanks for the link but I don't think more current needs to be freed up. The transformer is rated at 40VA. The filament winding accounts for 6.3 x 2 amps = 12.6VA, the HT winding accounts for 250 x .1 amps = 25VA. 12.6 + 25 = 37.5VA. If one half of the HT winding is rated for 100mA then the other half of the winding is also.
 
G said:
Hi all. I wanted some opinions and vetting of a preamp & supply. Here ya go:

Loadline

Not real good; not too bad. It's not my preference, and you can do considerably better in the THD dept. No idea how this will sound. If most of the estimated THD is h2, you may prefer that type of sonic colouration. If there's a considerable h3 or higher order harominc profile, then may be not so much.

If you really need all that gain, there's a better way to get it, and with considerably more linearity. Bootstrap Mod. The additional coupling capacitor can be a 0.47uF unit, and make it a good one, like an AuriCap.

This gives virtually CCS loading on the input stage, and a very shallow loadline that'll perform much better in the THD dept. The DC operation does not change. It gives a lower Zo, and doesn't cost as much in Vpp as does either an SRPP or a u-stage.

Definitely something to consider.
 
bequerel said:
Cool project G!
If sticking to Hammond iron for this project. Will the standad Hammond chokes like eg. the Hammond 158M ([email protected]) work in both choke and cap input mode, or will choke input dictate another type of choke?

Hi Bequerel. Thank you for the compliment but others will disagree with you. The choke in the supply is a Hammond 193B. I had a LOT of help with the supply calculations from Zigzagflux and I'm pretty sure that you will have to change either C1 or C2 if you change the choke to a 10H/100mA choke. Hopefully he will chime in and tell you what you need to change. I'm not the best one to direct questions to when it comes to power supply calculations, or any questions other than the most basic, when it comes to tube electronics. I am little more than a rank amatuer.
 
Re: Re: A 6SN7 direct coupled preamp & regulated supply for everyone's perusal.

Miles Prower said:
If you really need all that gain, there's a better way to get it, and with considerably more linearity. Bootstrap Mod. The additional coupling capacitor can be a 0.47uF unit, and make it a good one, like an AuriCap.

This gives virtually CCS loading on the input stage, and a very shallow loadline that'll perform much better in the THD dept. The DC operation does not change. It gives a lower Zo, and doesn't cost as much in Vpp as does either an SRPP or a u-stage.

Bootstrapping is a neat trick, but the output impedance of the CF will rise a bit due to positive feedback. The positive feedback depends on the plate resistor (15k) and rp of the tube in the input stage. Since the cathode resistor (800R) not is decoupled the apparent rp will be quite high which will increase the positive feedback and Zout.

Jan E Veiset
 
Re: Re: A 6SN7 direct coupled preamp & regulated supply for everyone's perusal.

Miles Prower said:


Loadline

Not real good; not too bad. It's not my preference, and you can do considerably better in the THD dept. No idea how this will sound. If most of the estimated THD is h2, you may prefer that type of sonic colouration. If there's a considerable h3 or higher order harominc profile, then may be not so much.

If you really need all that gain, there's a better way to get it, and with considerably more linearity. Bootstrap Mod. The additional coupling capacitor can be a 0.47uF unit, and make it a good one, like an AuriCap.

This gives virtually CCS loading on the input stage, and a very shallow loadline that'll perform much better in the THD dept. The DC operation does not change. It gives a lower Zo, and doesn't cost as much in Vpp as does either an SRPP or a u-stage.

Definitely something to consider.

Hi Miles. Thank you very much for posting a reply. Here is the story about this circuit. I don't think I really need all that gain but maybe you can tell me after I tell you where I am coming from. I have been planning to build a matching (chassis wise) preamp and amplifier for a while. I actually decided on the amp first. It is a 12B4A gain stage and a 12B4A output stage. The input stage will be choke loaded and so will have a gain of 5 or so and the output stage uses a output transformer with a 40H 5K load and I am assuming that it will also have a gain of 4 or 5. I could be wrong about the gain of the output stage, or the input stage for that matter, but that is the amp that I am going to use this preamp with. I do like a "warm sounding" preamp. I have been using a "Foreplay" clone for the last two years. I decided though that I wanted to use a better tube for a future preamp and so I "designed" the circuit that I have posted using a 6SN7. I kept the supply voltage low so that I could lift the heaters to 80 or 85 volts above ground, using a single filament winding, and still be within the 6SN7s heater positive to cathode rating of 100v. I would like to claim that I designed the power supply but that was all Zigzagflux. I merely decided that I wanted a regulated supply for this preamp. So, you see, I do need some gain but I doubt I will ever utilize all of the gain this circuit has to offer. I do, however, need the low zout that his type of circuit offers. The load that I will be driving with this preamp might be as low as 9K so I need a output Z lower than 1K. That is the long and short of it. I could just use a 50K resistor from the input of the preamp to the pot and cut the input voltage in half to lower the output of the preamp although I am not sure how that will impact the sound. Thoughts?
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
If the winding has the ampacity to carry 100mA from either end of the winding to the center tap then it should have the ampacity to carry 100mA through the whole winding shouldn't it?

I apologize, G! I misinterpreted the transformer rating from your schematic because I thought the rating was expressed in AC terms, not in terms of its ability to supply DC. I looked up the Hammond website and I now see that The Hammond spec for the 369AX says: "Secondary: High voltage designed for full wave C.T. use. See chart below (High voltage current is D.C. using C.T. Full Wave rectifier - capacitor input)."

So, Hammmond gives an AC rating for the voltage but a DC rating for the current! :xeye: I guess this must mean that the 369AX gives 125 x 1.414 volts = 176v @ 100mA DC using a C-input filter. This is approximately equivalent to an AC rating of 125-0-125v @ 170mA RMS. (With C-input, you only get ~60% of the rated AC Current).

Using the full winding with a bridge rectifier, you get twice the AC voltage and half the current, which is 250v @ 85mA RMS. The choke-input filter in your schematic should be able to give a DC voltage = 0.9 x the RMS voltage @ approximatley the same current, i.e. 225v @ 85mA before regulation.

The reason you can only use half the rated current over the full secondary winding, BTW, is because with full wave CT rectification each half of the secondary works 50% of the time but with a bridge rectifier the whole winding works 100% of the time.
 
ray_moth said:


I apologize, G! I misinterpreted the transformer rating from your schematic because I thought the rating was expressed in AC terms, not in terms of its ability to supply DC. I looked up the Hammond website and I now see that The Hammond spec for the 369AX says: "Secondary: High voltage designed for full wave C.T. use. See chart below (High voltage current is D.C. using C.T. Full Wave rectifier - capacitor input)."

So, Hammmond gives an AC rating for the voltage but a DC rating for the current! :xeye: I guess this must mean that the 369AX gives 125 x 1.414 volts = 176v @ 100mA DC using a C-input filter. This is approximately equivalent to an AC rating of 125-0-125v @ 170mA RMS. (With C-input, you only get ~60% of the rated AC Current).

Using the full winding with a bridge rectifier, you get twice the AC voltage and half the current, which is 250v @ 85mA RMS. The choke-input filter in your schematic should be able to give a DC voltage = 0.9 x the RMS voltage @ approximatley the same current, i.e. 225v @ 85mA before regulation.

The reason you can only use half the rated current over the full secondary winding, BTW, is because with full wave CT rectification each half of the secondary works 50% of the time but with a bridge rectifier the whole winding works 100% of the time.

No need for a apology Ray. I, in my ignorance, did not fully understand what the specs were telling me. I still have a lot to learn. I know just enough to be dangerous.:) Thank you for the explaination and I'm glad that I can use the transformer if I do decide to go ahead with this design. That remains to be seen though.