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Old 3rd June 2010, 08:38 PM   #21
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Lets see what we can do. Bigun, how many exactly of which tubes do you have? Two or four 6AS7, driven by 6SN7 would make a sweet amp, but you will most likely have to roll a few to get a reasonably matched pair, also, 6SN7 is a great tube, only drawback being somewhat low gain for such an application, I'd get a 6SL7 (or a few, they are a handy tube) to drive the 6AS7's for the higher Voltage swing.

The 6AS7 has a Mu of only two, which makes it somewhat difficult to drive in a tube-stingy SE design, However, you could do a ccs loaded 6SL7 , with an LED in it's cathode, and have your 6AS7 biased about -60~70v (60mA or so through a 1k cathode R) and you should have a decent starting point.
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Old 4th June 2010, 12:37 AM   #22
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Wavebourn View Post
Hi Bigun;
since you are fluent with SS, you may go directly to Gyrator load jumping over chokes.
Yes, I really like this approach and I looked at your particular version a few months back with the idea of using it as a load in a SS amp. Your gyrator is more than a gyrator, very interesting circuit. But my tube journey has to start at the beginning, without any semiconductors (except for the n-type semiconductor coating on the Cathode that is...). No reason why I can't try out a gyrator later.

Quote:
Originally Posted by planet10 View Post
275 x 1.4 = 385 V RAW DC less voltage drop across the rectifier, and the Rs & Ls in the series legs of the PSU filter.
dave
I guess I lose the factor of sqrt(2) in using a choke loaded psu. If I were using caps it would be a different story.


Quote:
Originally Posted by Lingwendil View Post
Lets see what we can do. Bigun, how many exactly of which tubes do you have? Two or four 6AS7, driven by 6SN7 would make a sweet amp, but you will most likely have to roll a few to get a reasonably matched pair, also, 6SN7 is a great tube, only drawback being somewhat low gain for such an application, I'd get a 6SL7 (or a few, they are a handy tube) to drive the 6AS7's for the higher Voltage swing.

The 6AS7 has a Mu of only two, which makes it somewhat difficult to drive in a tube-stingy SE design, However, you could do a ccs loaded 6SL7 , with an LED in it's cathode, and have your 6AS7 biased about -60~70v (60mA or so through a 1k cathode R) and you should have a decent starting point.
I expect to be receiving one 6AS7. That's not to say I can't obtain a second one but how would I use it ? This is really my first project and the power output is not a concern at this point (I want to avoid pushing the OT in order to get the best bandwidth from it). I want to start with SE not PP. But who's to say where this will lead when I come to build another

Now, with a choke load (from the OT) on the output tube I have something that is pretty close to a CCS do I not ? with the added benefit that the choke allows the plate voltage to go above B+ so that my limited rail voltage is less of a problem. Same goes for the input tube which I'm also now going to use a choke load for too.

GAIN
====
I understand that there isn't much gain here, but I'm not sure I need very much gain. With a choke load for the input tube I'm going to get a lot more gain. I'm actually concerned that I will have too much gain and I'm now showing an additional degeneration resistor in case this is necessary to bring the gain back down. It's something I'd like some advice on ?????

The input tube I have been given is apparently a nice one. It's actually a Russian 6H8C of some vintage. It comes in a very old looking unmarked box with a piece of paper inside testifying to the Russian military standards.

No C.T. on the heater secondaries so I guess I need to resistively couple them to gnd to reduce hum ?

I've shown a rail fuse in the schematic - a practice from my SS days, is this recommended for tubes ?
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Last edited by Bigun; 4th June 2010 at 12:39 AM.
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Old 4th June 2010, 08:03 AM   #23
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That would most likely be a very acceptable amplifier as you've drawn it, like before though, you may not have enough gain for full output from most sources, most variations of 6SN7 driven 6AS7 SE that I have seen needed additional gain to reach full output, 6SN7 type tubes simply lack the amount of gain required. If it was me, I'd run a 6SL7 (or equivalent) and then implement a small amount of negative feedback into its cathode to tighten things up and eat some unneeded gain.

I say, build it, then tweak as needed. That's most of the fun in this hobby!
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Old 4th June 2010, 03:47 PM   #24
Bigun is offline Bigun  Canada
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Let's say I don't add any degeneration (by removing R3) in the recent schematic I've posted - how much gain will the 6H8C give me when choke loaded ?
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Old 4th June 2010, 04:06 PM   #25
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It is better to connect 5V filament to cathode (just one side directly) of the rectifier.

Fuses are used in high power amps. However, it won't harm when you solder and tweak your amp.
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Old 4th June 2010, 04:39 PM   #26
Bigun is offline Bigun  Canada
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Thanks Lingwendil, I'll keep on trucking then !


Quote:
Originally Posted by Bigun View Post
Let's say I don't add any degeneration (by removing R3) in the recent schematic I've posted - how much gain will the 6H8C give me when choke loaded ?
Let me ask if my own understanding is correct here.

Gain of input stage is maximum with a CCS and no unbypassed cathode resistor. The gain is effectively equal to mu. With a choke load I am pretty much in CCS territory which means I can ignore the plate resistance ?

So my maximum gain is equal to mu, or around 21dB. I think this is enough gain for my application with the source I will be using (YBA CD player and Magnum Dynalab analogue FM).

Does this sound reasonable ?


Wavebourn - I believe you are saying I should not be using any resistors on the secondary for the rectifier heater, there is not a hum concern there and internally the rectifier has the cathode tied to the heater at one end anyhow.


p.s. my 6H8C is from 1957
p.p.s I have been given a nice bag of capacitors, Sprague Atom 20uF each 500V rated
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Last edited by Bigun; 4th June 2010 at 04:47 PM.
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Old 4th June 2010, 04:46 PM   #27
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Quote:
Originally Posted by Lingwendil View Post
and then implement a small amount of negative feedback into its cathode to tighten things up and eat some unneeded gain.
Small amount?

Small amount slightly reducing 2'nd order distortions turns them into grass of high order ones. Feedback is like a gun: if you took it use it, or don't take at all instead of using "small amount".
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Old 4th June 2010, 05:51 PM   #28
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My five cents: Build this amp with the parts you have now. You'll love it;


The 6AS7 gets a lot of unfair discredit. It is a very linear tube, but it does require a very large drive signal.
Another thing is with the low rp and usually relatively high currents the power supply will normally have high ripple that shows at the output.
Both the above can be easily fixed.
True, the two triode sections are poorly matched, but I've never had a big problem with that.
I've used these tubes for years, and they never fail.
The tube makes musical amps and has great bass in all amps I've used it with.
Here's a fft of my SE amp with this tube, and ok, the second is high, but way below audible, and everyone who's heard it loves the sound. It uses zero NFB.

I'll admit it, the 2A3 is better, but for the price the 6AS7 is great.
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Old 4th June 2010, 05:57 PM   #29
Cassiel is offline Cassiel  Libya
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Quote:
Originally Posted by Wavebourn View Post
Small amount?

Small amount slightly reducing 2'nd order distortions turns them into grass of high order ones. Feedback is like a gun: if you took it use it, or don't take at all instead of using "small amount".
Yeah, that amp needs at least 20dB of feedback.

The more I see the less I like, 6H8C and 6as7... you won't feel like making another tube amp after hearing this one.

Seriously, you need more gain. Change the driver tube.
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Old 4th June 2010, 06:02 PM   #30
Cassiel is offline Cassiel  Libya
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Quote:
Originally Posted by SemperFi View Post
My five cents: Build this amp with the parts you have now. You'll love it;


The 6AS7 gets a lot of unfair discredit. It is a very linear tube, but it does require a very large drive signal.
Another thing is with the low rp and usually relatively high currents the power supply will normally have high ripple that shows at the output.
Both the above can be easily fixed.
True, the two triode sections are poorly matched, but I've never had a big problem with that.
I've used these tubes for years, and they never fail.
The tube makes musical amps and has great bass in all amps I've used it with.
Here's a fft of my SE amp with this tube, and ok, the second is high, but way below audible, and everyone who's heard it loves the sound. It uses zero NFB.

I'll admit it, the 2A3 is better, but for the price the 6AS7 is great.
What output transformer are you using?
I remember that bass was good but the rest wasn't.
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