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Old 27th March 2013, 12:17 PM   #21
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There's nothing jumping out of that OPs schematic that shouts "less bass because of this". It would seem the problem is qualitative, not quantitative.

You can have pretty much flat frequency response, and still have soft, lazy bass with no punch.

First mod I'd do would be to take out the cathode resistor and cap and put in two red LEDs in series. If the stage has more than 10mA current thru it, it's fine just like that - if not, then put a resistor from B+ to the cathode sized so that there's 12 to 15 mA of current thru the LEDs.

Simple thing to do; adds a lot of punch and bass definition.
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Old 27th March 2013, 12:20 PM   #22
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Another great thing to add 'speed' and punch to a gain stage is to change the plate resistor to a MOSFET gyrator.

LED (or fixed bias) and gyrator on top makes for a really really fast and detailed sounding triode stage.
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Old 27th March 2013, 02:14 PM   #23
GoatGuy is offline GoatGuy  United States
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Maybe [DF96] isn't lurking (or talkative) on this but MrCurwen, those two suggestions totally define the triode-as-colorizer camp. They work together to deliver remarkably nonlinear response from the triode, but at least in a "musical" way.

Of course, one can always use a nice high-value inductor as the plate load (which is what, in the end, the gyrator is attempting to model, more cheaply) ... a well regarded if but older fashioned technique for those with money in their pockets to burn.

HOWEVER - your advice regarding "two red LEDs" and "if current too high..." is actually wrong. Current can be tamed by adding more LEDs, period. Unless you're actually trying to achieve a blended cathode response curve ... partially unbypassed resistor for its local negative feedback effect, and partially LED for its 'constant voltage without a capacitor' effect.

Ah, small signal triodes. Wonderful devices, they. But definitely colorful.

The only way I'm aware of that maximizes their linearity is the opposite of your recommendation: unbypassed cathode resistor, constant voltage plate (cascode), two-stage amplification (for at least symmetrically colorizing the signal), possibly tailed with either a MOSFET or 6SN7 cathode follower to drop Z(out) to something useful down-wind.

Ahem. I'm in the other camp. Linear, baby, and accepting a bit, but not enhancing the triode nonlinearity. I much prefer local NFB (i.e. the cathode resistor, unbypassed) to global feedback, and I much prefer working with the symmetric split-phase signal than just single-ended. "Long-tailed pair" is OK too, but doesn't really model the split-phase-amplified notion. Better is to have the first stage contain an interstage transformer as its cascode-side load. That way the signal is exactly split in phase at the outset, for subtle and sweet treatment down the line.

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Old 27th March 2013, 04:59 PM   #24
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Quote:
Originally Posted by GoatGuy View Post
Maybe [DF96] isn't lurking (or talkative) on this but MrCurwen, those two suggestions totally define the triode-as-colorizer camp. They work together to deliver remarkably nonlinear response from the triode, but at least in a "musical" way.
How is that?

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Of course, one can always use a nice high-value inductor as the plate load (which is what, in the end, the gyrator is attempting to model, more cheaply) ... a well regarded if but older fashioned technique for those with money in their pockets to burn.
A well designed MOSFET gyrator can deliver much more consistently higher impedance across the frequency spectrum than most chokes.

A choke has a rising impedance against frequency. It is lowest at the low end of audio - thus distortion is highest in the bass area.

This would be just that qualitative degradation that I'm presuming OP is talking about. The bass is there (if inductance is sufficient) but it is not well defined and detailed, due to distortion.

A well designed MOSFET gyrator doesn't have this problem, and it does in fact deliver more detailed and 'faster' bass than some more conventional approaches.

Quote:
HOWEVER - your advice regarding "two red LEDs" and "if current too high..." is actually wrong. Current can be tamed by adding more LEDs, period. Unless you're actually trying to achieve a blended cathode response curve ... partially unbypassed resistor for its local negative feedback effect, and partially LED for its 'constant voltage without a capacitor' effect.
I don't understand what you're talking about. I simply meant that the LEDs should have at least 12 to 15 mA of current thru them to operate properly, and if the triode doesn't have that, supply it via a seperate resistor from B+.

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The only way I'm aware of that maximizes their linearity is the opposite of your recommendation: unbypassed cathode resistor,
Unbypassed cathode resistor does add degeneration thru local feedback, but it also significantly increases plate resistance of the triode. If this isn't countered with a significantly higher plate load also, the result is not satisfactory.

Plate load should be at least 10 times the tubes plate resistance for excellent results.

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constant voltage plate (cascode),
The gyrator plate load offers constant DC plate voltage regardless of tube parameters; thus it's perfect for aging tubes and direct coupled systems.


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Ahem. I'm in the other camp. Linear, baby, and accepting a bit, but not enhancing the triode nonlinearity. I much prefer local NFB (i.e. the cathode resistor, unbypassed) to global feedback
How do LED bias and gyrator plate loads enhance triode nonlinearity?

I don't use any global feedback (or local feedback via cathode degeneration); I prefer using as linear triodes as possible with fixed bias (when SE) and plate gyrators for as horizontal load lines as possible - even in balanced circuits!

Inherently linear tube + horizontal load line = best linearity.
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Old 27th March 2013, 05:22 PM   #25
jgf is offline jgf  United States
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Apparently all this conflicting advice and side-threads have driven the OP away...
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Old 29th March 2013, 09:19 PM   #26
Cassiel is offline Cassiel  Libya
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Quote:
Originally Posted by jgf View Post
Apparently all this conflicting advice and side-threads have driven the OP away...
Sorry, I've been away. My CRC filter is 220uF-2K-180uF. There are more uF's down the line for a total of 680uF.

I have no problem with any poster, on the contrary, maybe I can learn a few things here. I know about power amps and how different topologies and power supplies can affect their bass response, but I'm a complete noob with preamps. 2V of audio signal can't be this troublesome...or so I thought.

Anyway, even though I haven't had the time to try out possible fixes I have spotted a problem with one tube: one of the triode's filaments was underpowered, seriously underpowered. Barely glowing. The triode with problems was facing the wall so I really don't know when the problem started. Turns out my bakelite socket is acting up; my fault really, I have a soft spot for vintage parts and sometimes **** happens. Now I have to replace the sockets and possibly change the schematic. No more news from me in a while. Thanks to all.
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Old 30th March 2013, 09:25 AM   #27
hpeter is offline hpeter  Europe
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changing from 802 to ecc803 increased sensitivity and somehow (?) is not so bass shy anymore /srpp
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Old 30th March 2013, 08:20 PM   #28
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There is a huge difference between using a tube with =100, rp=62K (ECC803) and one with =17, rp=7K (ECC802).

We seem to have many alternatives without explaining any bass-shyness in the original circuit.

Cassiel,

There is no reason to have any bass-shyness in your original circuit; in fact the coupling capacitor could be 0,47F and still go down to 25Hz with a power amplifier input impedance of say >10K. (I do not know of anything that low.) Also not with your power supply values; they could be far lower and still have no influence on low frequencies. (Compare the Williamson power amplifier with 8F power supply caps and still going to 10Hz - and that a power amplifier.)

You have discovered a problem with a tube heater voltage - perhaps an old socket could cause other problems? Otherwise you must look elsewhere, not inside the circuit given by you.

I am still looking for the value of R2, please, although any practical value would not make a difference.
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Old 30th March 2013, 08:36 PM   #29
Loren42 is offline Loren42  United States
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I am still waiting to see quantitative measurements.
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Old 30th March 2013, 09:15 PM   #30
Cassiel is offline Cassiel  Libya
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Quote:
You have discovered a problem with a tube heater voltage
Yeah, one filament gets all the food while the other starves. I wonder what's wrong with this socket, dirt inside the pin I presume. Value of R2 is 130K.

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I am still waiting to see quantitative measurements.
I can understand your distrust, graphs get me dizzy though - you won't see them coming from me. I am merely stating that my preamp's bass is kinda soft in a direct comparison against the Bryston preamp.

What I am going to do is experiment a lot. I have done this with tube output stages and now it's the turn to do it with preamps.
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