• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

My 6SL7, 2A3 SE design

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#63
Joel said:
James, I'm having trouble finding reference to slew rate other than being device dependant. In other words, active devices (opamps, etc) with a quoted V/mS. I've never seen that value given in data sheets for any tubes. Am I looking in the wrong place?
I would assume that the device's slew rate would be the starting point for any further calculations.😕
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Slew rate specification of a single component (for example Valve), does not have any meaning. It is the way that driving power is supplied to this component that produces slew rate limiting. We can only be warned from the driving requirements of a particular tube that it may show slew rate problems if not driven properly. On the other side, operational amplifiers are not single components, as they incorporate many amplification stages, one driving the other. On some of them there is provision to connect external components to compensate for the Miller capacitance of the Vc stage

Regards
George
 
#65
Thanks George. The question at hand though seems to be why my numbers are half those that James got (and presumably Brett since he agreed wholeheartedly).

They calculated a distortion point of 10kHz with a .9mA driver current.
I get a 20kHz point with only .8mA available. And that was a worst-case-scenario of an input capacitance of 80pF.
Can you shed any light on that?

the 71A has Cgk of 3.7pF, Cgp of 7.4pF, mu of 3... 80V swing on the grid.
 
#66
Re: Toroids and E/S screens

EC8010 said:
I don't see a fire risk with copper tape from a hobby shop. After all, you weren't intending to let the transformer get significantly hot were you?
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No, but accidents do happen. Additionally, I'm uncertain about the VA rating for the core. There were no specifications printed on it and I couldn't track down anything using the part number.

It seems like it should handle the 12.5VA I need. It's a bit over 60mm in diameter and 25mm tall with a 35mm central hole. Weighs 300 grams. This is the same diameter and weight of some 15VA trans I found dimensions for on line. It's about 15% shorter though.

A problem I do foresee is the winding of tape becoming a shorted turn. You might want to use kitchen foil to cover one side of the toroid and the circumference, then run a strip of insulating tape all the way round the circumference of the toroid, then use foil to cover the other side. Solder that works on aluminium can be readily bought, but it contaminates the tip of your iron, so keep that tip for aluminium jobs.
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I have considered the shorted turn scenario. I was intending to leave a small gap in the shielding. I like your idea better though. Another idea might be to use the copper foil adhesive tape with an insulating tape completely covering the surface of the foil as it is wrapped on to the core. Earth only one end.
 
#67
SPECCING BRANDS.

Hi,

(Anyone who specs brands for the components on a schematic ain' quite right in the brain IMVHO......)
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Another of those pulled out of context things.:cannotbe:

What the author means is that he specced components for a given result.
You deviate from it and the circuit becomes yours,not his.

Cheers,😉
 
#69
the design is fine

Well, it turns out that a 6SL7 is more than enough to drive the input capacitance of a 2A3. Actually, only 1.38mA is required to get up to 20kHz, and with 90V pk-pk input swing.
:nod:
See the thread "Slew Rate Limiting..." if you want to see how I got the final number.

Cheers

Joel
 
#71
CONFUCIUS SAID...

Hi,

What happend to all you "high current" guys?
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Why do I get the feeling that most of the members that commented against the 6SL7 have missed out on the fact (at least I think it is) that the two sections of that tube are in //?

In which case I'm pretty sure it will drive a single 2A3 adequately...deep sigh.:nod:

Cheers,😉
 
#73
hmm think im gettin the jist of it. i do somewhat understand the advantages for high current for a low mu output tube, but not for driver(refer to prior posts). also, a big misunderstanding was that I DID LEAVE OUT BYPASS CAPS, i did my gain and distortion figuring as if they were in place. as for not having them and that lowering distortion, im going to order the caps and just see which setup sounds right to me. thank you for the help.
 
#74
NOW HE'S TELLING US...

Hi,

I DID LEAVE OUT BYPASS CAPS, i did my gain and distortion figuring as if they were in place. as for not having them and that lowering distortion, im going to order the caps and just see which setup sounds right to me. thank you for the help.
Click to expand...

While you're at it also try the following configuration:

Bypass the cathode of the 2A3 and not the cathode of the 6SL7.

In theory it should give you less gain from the input stage and a better coupling from the output tube to the OPT.

Cheers,😉
 
#75
mig-ru said:
ok this is the final version of my 6SL7, 2A3/6A3/6B4G Single Ended design. feel free to critique. also was wondering if this will drive 91dB speakers to sufficient volume. and seeing as to what happened to my last post, if we could stick to the matter at hand... thx.
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I know it has been a long time since this post but how did the amp turn-out? I am currently looking for a 6SL7 driver stage for my KT120s I thought I may use yours.
 
#76
#78
I looked at the 6SL7 for my cellini amp hoping to improve the voltage swing into the output triode. I only want a simple two stage amp with single triode driving a single output triode so SRPP was out of the question.

Every way I looked at the 6SL7 it just didn't work, the plate resistance is just too high at 44k. With a choke load I'd have needed 810H to get good bass response. With a resistive load I'd need at least 100k and then I have so much voltage drop across this resistor that my plate voltage drops or I need a large B+

I'm a newbie with these things so I may have missed something obvious, but to me the 6SL7 makes a poor choice for a two stage amp.
 
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#80
I too want to only have a two stage amp with only three tubes. I am currently using a 6N1P to drive my KT120 SE UL power amp. The 6N1P does an excellent job but I have built this power amp with remote driver stages all running from the one PS. So I can inter-change driver stages.

It is my intention to also make a 12AX7 driver stage.

Vacuum Tube (Valve) Audio Projects - Be Bamp with KT120 Tung-Sol tubes - DIY Audio Projects Photo Gallery

Here is the 6N1P remote driver stage:

Vacuum Tube (Valve) Audio Projects - Remote driver stage for Be Bamp - rear view - DIY Audio Projects Photo Gallery
 
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