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Old 26th August 2003, 11:51 PM   #11
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Quote:
Originally posted by Saurav
So what would it take to convert my driver stage from 6SL7 SRPP to 6SN7 SRPP? And is there any reason why the 6SN7 would not be a good choice for SRPP? Would you recommend a 6SN7 SRPP driver for a 2A3?
You've misunderstood. I wouldn't use an SRPP at all.

I played with the simple totem pole arrangement that the JE uses in TubeCAD with 6SN7's and settled on Ia = 5.5mA, Ra and Rk(bypassed) = 750R which gave an output swing with a couple of dB headroom.

Quote:
* Heater current 600mA vs. 300mA - I think I'm good there, my PT is rated for a few amps on its 6.3V tap
Yes, you'd need to ensure the power tx could do it.

Quote:
* Plate current 9mA vs. 2.3mA - again, I think my PT can handle this. I'll have to adjust the dropper resistor in the PS.
As I mentioned above, I chose 5.5mA as 9mA didn't give enough output swing to drive the 2A3 and have some headroom left. Vg = - 4.2V

FTR, the JE 6SL7 stage is running about 0.73mA, not 2.3, which to my mind is waaaaay too gutless. It's not even enough to get decent performance out of the 6SL7, let alone drive the 2A3 grid as well.

1.6V bias/2200 ohms = 0.73mA

Quote:
* Plate voltage is 250V for both, and my SRPP operates at around 300V (so 150 drop per tube). I'd have to check if this is a poor operating point for a 6SN7 though.
My sims are for a B+ = 300V. Change the dropper to a 2k 2W, assuming a shared PS between channels.

Quote:
So... other than changing PS resistors to get 300V at the SRPP B+ with 9mA flowing through it, it doesn't look like much else will need to change. Or am I completely missing something here.

Is this something you would recommend?
I bet it would sound a whole lot better than the 6SL7. If you have the tubes, or can borrow some, it's only an hour's work to change the cct to suit. You must not that the gain with the 6SN7 SRPP is much lower at 14, vs 48 for the 6SL7 version.

A final note.
I tried a 5965 in the sim and it looks like it might be even better in the same SRPP as the 6SN7. Gain is 38 and there's a good 6dB of headroom. (6SL7 in brackets)
B+ = 300V
Ia = 8mA (0.73mA)
Vg = - 2.65V (-1.6V)
Ra = Rk (bypassed) = 750 (2200)
Vout = +/- 101V (73V)
Zout = 1.25k (7.1k)
The gain difference is 2dB, but the 5965 can drive the 2A3 with ease and the input bias to the 5965 is enough so that it will only marginally clip on peaks from a CDP whereas the 6SL7 will clip easily.
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Old 27th August 2003, 12:10 AM   #12
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Thanks, this is all very interesting.

First, this sim tool you talk of - is this something that can be downloaded from the TubeCAD website? I'll go look for it there if it is.

Quote:
I played with the simple totem pole arrangement that the JE uses in TubeCAD with 6SN7's and settled on Ia = 5.5mA, Ra and Rk(bypassed) = 750R which gave an output swing with a couple of dB headroom.
I'll look for that. I'm not too sure what a totem pole circuit is, but I can look that up as well. And I'll hold off on some of my questions until I can see the circuit that you're talking about.

Quote:
FTR, the JE 6SL7 stage is running about 0.73mA, not 2.3,

...

1.6V bias/2200 ohms = 0.73mA
Understood. I was just looking at max ratings to make sure I wouldn't need to start from scratch with a new PS transformer.

My sims are for a B+ = 300V. Change the dropper to a 2k 2W, assuming a shared PS between channels.

Quote:
You must note that the gain with the 6SN7 SRPP is much lower at 14, vs 48 for the 6SL7 version.
That would be a problem, because I have a passive linestage which I'd prefer not to wire for voltage gain (it's built with autoformers). I've been considering raising the output voltages of my sources to see if that'll make things louder for when I want those volume levels. And maybe that's not where the problem is, maybe the signal level/gain structure is fine and I just need more sensitive speakers for the output power I have.

Quote:
I tried a 5965 in the sim and it looks like it might be even better in the same SRPP as the 6SN7. Gain is 38
Now this looks like a much better solution for me. And I know what an SRPP circuit looks like

Quote:
Ra = Rk (bypassed) = 750 (2200)
Leaving Rk unbypassed would reduce the gain, right. I took the bypass caps off in my current amp, and I think it sounded cleaner that way, so I left them off. I could put them back on easily, of course.

Thanks for this post. This is a change I could make very easily (looks like all I need to do is change the values of 6 resistors - 2 PS droppers and Ra and Rk), which makes it a very attractive experiment.

Edit: Ooh, the 5965 is a 9-pin tube. Darn it! Is there any way around this other than re-wiring new sockets?
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Old 27th August 2003, 12:43 AM   #13
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Default STOP PRESS...

Hi,

Don't use a 5965 in SRPP here.
It just doesn't sound very well that way, rather veiled IME.

Quote:
That would be a problem, because I have a passive linestage which I'd prefer not to wire for voltage gain
I don't think it would be a problem; you'd just have a less sensitive amp so you'll be turning up the volctl a few more notches.

Cheers,
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Old 27th August 2003, 12:57 AM   #14
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Quote:
I don't think it would be a problem; you'd just have a less sensitive amp so you'll be turning up the volctl a few more notches.
There aren't that many notches left, that's the problem

If not a 5965, how about Brett's original suggestion of a 6SN7?
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Old 27th August 2003, 01:07 AM   #15
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Hi,

Quote:
If not a 5965, how about Brett's original suggestion of a 6SN7?
The 6SN7 is what I'd have used in the first place.... used as a -follower to optimise gain.

Cheers,
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Old 27th August 2003, 01:39 AM   #16
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Quote:
Originally posted by Saurav
First, this sim tool you talk of - is this something that can be downloaded from the TubeCAD website? I'll go look for it there if it is.
TubeCAD and SE Amp CAD are $US39 ea and can be purchased from AES. Well worth the money if you play with tube designs.

Quote:
I'll look for that. I'm not too sure what a totem pole circuit is, but I can look that up as well.
A totem pole is the variation of SRPP's that the JE Labs uses.

Quote:
That would be a problem, because I have a passive linestage which I'd prefer not to wire for voltage gain (it's built with autoformers). I've been considering raising the output voltages of my sources to see if that'll make things louder for when I want those volume levels. And maybe that's not where the problem is, maybe the signal level/gain structure is fine and I just need more sensitive speakers for the output power I have.
I use S&B's so I know what you mean. I design all my amps to match the system gain structure I have. If you have a signal generator, you can test and measure the levels to see how close you are to clipping at any given signal level from your source. If you don't have a sig gen, then there are a number of freeware signal generator programs, and burn some test tones onto a CD.

My guess is you'll need about 2V peak to drive your amps satisfactorily, as a minimum. With that in mind,an ECC99 with a Pimm BBMCCS (easy and cheap to build) as anode load with Va = 150V, Ia = 22mA would be much, much, much, much better than the 6SL7 SRPP.

Quote:
Leaving Rk unbypassed would reduce the gain, right. I took the bypass caps off in my current amp, and I think it sounded cleaner that way, so I left them off. I could put them back on easily, of course.
Gains, bypassed/unbypassed
6SL7 SRPP 48 / 23.5
6SN7 SRPP 14 / 9
5965 SRPP 38 / 21.3

ECC99 with CCS load - 22 (my choice)
5965 with CCS load - 47 if you need the gain.

Quote:
Edit: Ooh, the 5965 is a 9-pin tube. Darn it! Is there any way around this other than re-wiring new sockets?
Not really. Throw a couple of 9 pin sockets onto some blocks of wood, sit them on top of the chassis and tack solder some wires in to connect it up temporarily. If you like the change make it permanent.
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Old 27th August 2003, 01:40 AM   #17
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Default Re: STOP PRESS...

Quote:
Originally posted by fdegrove
Hi,

Don't use a 5965 in SRPP here.
It just doesn't sound very well that way, rather veiled IME.
There is no way it could be worse than the 6SL7 SRPP
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Old 27th August 2003, 02:32 AM   #18
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Pursuing the subject of a 6SN7 mu follower for a second, for no other reason than the fact that I won't have to put in a noval socket into an octal punch hole...

I found a few 6SN7 mu follower schematics, (some for line stages). Do you guys have any specific recommendations for operating points / R/C values? I think one of my criteria would be to keep the B+ around 300 or less, so I don't have to completely re-do my PS. Here are some schematics:


This
is as a 2A3 driver, with a way different operating point from Brett's, and the author ended up preferring a single 6SN7.

This (2nd from the bottom of the page) is for a linestage, but it has 300V/7.5mA, which seems closer to Brett's numbers. So I guess I could take this one and re-calculate the R values for 5.5mA. Or just keep the same circuit design and just change the resistors to 750 ohms so I'll get 5.5mA in my SRPP circuit. That sounds like the least amount of work It'll also be lower gain. I'll be building a DAC next, and I think I should build an analog stage that'll give me 3-4V output or something. And then I'll need to build another phono stage. Oh well...

OK, now for the other ideas. I thought my PS trafo was 200mA, but it turns out it's 250 (I remember now, the 250mA one was cheaper), so adding 2x22mA to the 2x2A3 won't cause problems like I thought it might. I looked at Grry Pimm's CCS, and that doesn't look too hard to build.

Choices choices...

Quote:
If you like the change make it permanent.
That's the part that I'm not too confident about. My amps look bad enough as they are, and noval sockets are smaller than octal sockets, right? Though the chances of me ever giving/selling this to anyone else are pretty slim, so...

Thanks,
Saurav
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Old 27th August 2003, 03:49 AM   #19
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Quote:
Originally posted by Saurav
Pursuing the subject of a 6SN7 mu follower for a second, for no other reason than the fact that I won't have to put in a noval socket into an octal punch hole...

I found a few 6SN7 mu follower schematics, (some for line stages). Do you guys have any specific recommendations for operating points / R/C values? I think one of my criteria would be to keep the B+ around 300 or less, so I don't have to completely re-do my PS. Here are some schematics:

This (2nd from the bottom of the page) is for a linestage, but it has 300V/7.5mA, which seems closer to Brett's numbers.
Try that one as is, and if it were I, I'd up the current further (maybe 10mA). And you won't even need to change the socket, so it should be an easy experiment.
But I still think the ECC99 grounded cathode will be better.

Gary's circuits are easy to build, and if you feel lazy his PCBs are cheap.

Quote:
My amps look bad enough as they are, and noval sockets are smaller than octal sockets, right? Though the chances of me ever giving/selling this to anyone else are pretty slim, so...
If you're not going to sell it (and you never even get a fraction of your investment back) who cares? Real DIY amps are built on bits of scrap plywood.
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Old 27th August 2003, 04:58 AM   #20
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Quote:
But I still think the ECC99 grounded cathode will be better.
Well, gotta leave something for future fiddling, right?

Quote:
Gary's circuits are easy to build, and if you feel lazy his PCBs are cheap.
That's good to know that he sells PCBs. I saw the diagram on the website and wondered if he did.

Quote:
If you're not going to sell it (and you never even get a fraction of your investment back)
I was wondering about that (though that's a topic for a whole different thread). So far I've sold "name brand DIY" a couple of times - a Bottlehead Foreplay preamp, and ASL Wave amps that I added Auricaps and FREDs to. In both cases, there was some name-brand recognition in the non-DIY community for what I did, and in both cases I made my parts cost back. If I can do that, I'll be satisfied, because I'm not looking to make a living off of this, so the time I spent on it isn't important.

So I was just wondering if I could consistently make my parts cost back. As long as I stick with popular designs (and the JE Labs ones certainly seem to be popular), I guess the chances are higher. And then I'd have to try to sell it by saying "I built the JE Labs version and then improved on it" or something, and hope for the best.

Just thinking out loud. I don't think I'll be able to afford having multiple amps just sitting around. And at the same time, I doubt this will be the last amp I'll ever build.
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