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Old 19th August 2012, 10:03 PM   #1
tomchr is offline tomchr  United States
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Default DeathTrap400 : : A Pretty Damn Good 300B Amp

Those of you who have followed my posts for a while know that I've been perfecting my 300B amp for a couple of years now. I think I've finally reached Audio Nirvana.

The main challenge driving a 300B is that the driver will have to swing 160~180 Vpp, cleanly. The ECC99 from JJ fits this bill very nicely. It's a medium tube ( = 22), in current production, and reasonably priced ($15/EA). I've tried several other tubes - 6SN7, 6SL7, d3A, etc. - but they either can't swing 180 Vpp cleanly or can't provide 180 Vpp at all at reasonable bias points. The 6SN7/6J5 comes close, but the ECC99 is much lower THD.

I tried several circuit topologies and ended up settling on using one ECC99 per channel. One half of the ECC99 provides a CCS loaded, LED biased common cathode amplifier stage with a gain of roughly 20 V/V. The other half of the ECC99 is rigged as a cathode follower driving the 300B.
The high gm of the ECC99 (9.5 mA/V) ensures a low output impedance of the cathode follower (Zout = 100 ohm). Hence, the driver's output impedance is dominated by the grid stopper on the 300B.

My first prototype has been playing in my living room for a few months while I've been tweaking it. I'm likin' it. I plan to offer boards for this amp sometime in the fall.

~Tom
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Old 19th August 2012, 10:55 PM   #2
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I assume you call it a death trap because of the exposed circuit. And although I've never owned or used one, have you tried the Russian 6H30 dual triode? It seems like it should also do the job according to what I've read about it.
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Old 20th August 2012, 12:02 AM   #3
Cassiel is offline Cassiel  Libya
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Quote:
I assume you call it a death trap because of the exposed circuit.
And the black and white picture adds that final extra of morbidity. Not long ago I thought about calling one of my amps 'Electro-Cute'. It was cute and dangerous.
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Old 20th August 2012, 12:31 AM   #4
tomchr is offline tomchr  United States
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Quote:
Originally Posted by HollowState View Post
I assume you call it a death trap because of the exposed circuit.
My prototypes are usually built point-to-point on a piece of plywood. Hence, the DeathTrap name... My present amp isn't complete, obviously. It'll need an enclosure and such.

Quote:
Originally Posted by HollowState View Post
And although I've never owned or used one, have you tried the Russian 6H30 dual triode?
I have not. A quick glance at the plate curves reveal that it would be a good candidate as well. The is a bit low (15) so it'd take a preamp with some oomph to drive a 300B into clipping. But it could be interesting to try.

~Tom
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Old 20th August 2012, 12:44 AM   #5
Bigun is offline Bigun  Canada
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You use a CCS load for the common cathode amp stage, why didn't you use one for the common cathode stage instead of R10 too ?

or if you prefer,

Why didn't you use a resistor load for the common cathode amp stage instead of the more complex CCS ?
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Old 20th August 2012, 04:56 AM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Bigun View Post
You use a CCS load for the common cathode amp stage, why didn't you use one for the common cathode stage instead of R10 too ?

or if you prefer,

Why didn't you use a resistor load for the common cathode amp stage instead of the more complex CCS ?
I'm assuming you meant cathode follower.. Not speaking for Tom here, but the load resistor R10 is quite large compared to the internal cathode source impedance, the CCS would provide less benefit here, and if it failed open would take the 300B with it.

There are at least three reasons I can think of to use a CCS as a plate load in a common cathode stage, one is to maximize gain which is needed here, another is for the good linearity it provides, and finally it reduces the required driver supply voltage required rather significantly for comparable performance with a resistive load.. Another obvious advantage of a CCS is the high PSRR it will provide.
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Old 20th August 2012, 08:21 PM   #7
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Originally Posted by tomchr View Post
I've tried several other tubes - 6SN7, 6SL7, d3A, etc. - but they either can't swing 180 Vpp cleanly or can't provide 180 Vpp at all at reasonable bias points.

What's wrong with the D3a? It can swing more than 200Vpp...
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Old 20th August 2012, 08:44 PM   #8
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6N6P for driver?
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Old 21st August 2012, 04:35 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Vincent77 View Post
What's wrong with the D3a? It can swing more than 200Vpp...
I use them to drive GM70 which they do quite well. I have future plans for an IT coupled D3A/300B amp..
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Old 21st August 2012, 09:42 PM   #10
tomchr is offline tomchr  United States
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Quote:
Originally Posted by kevinkr View Post
I'm assuming you meant cathode follower.. Not speaking for Tom here, but the load resistor R10 is quite large compared to the internal cathode source impedance, the CCS would provide less benefit here, and if it failed open would take the 300B with it.

There are at least three reasons I can think of to use a CCS as a plate load in a common cathode stage, one is to maximize gain which is needed here, another is for the good linearity it provides, and finally it reduces the required driver supply voltage required rather significantly for comparable performance with a resistive load.. Another obvious advantage of a CCS is the high PSRR it will provide.
That sums it up quite nicely, actually. On the common cathode stage, I used the CCS load to maximize gain and minimize THD. I tried a CCS on the cathode follower as well but the difference (both in measurements and listening tests) between a CCS and the 33 kOhm resistor was negligible. I figured a resistor beats a CCS on price and complexity any day so I stuck with that.

~Tom
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