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Old 5th February 2013, 02:51 AM   #11
mbeards is offline mbeards  United States
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How about a parallel 6SN7 triode driver for a low impedance source?

Thanks for the great tips.
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Old 5th February 2013, 11:21 PM   #12
tomchr is offline tomchr  United States
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Quote:
Originally Posted by kmaier View Post
my 300B matched pair is WE.
Dang. Nice tubes.

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Originally Posted by kmaier View Post
I've seen some thermal runaway (as you called it)
Yeah, I don't like the term either, but what seems to happen is that the anode gets rather hot (cherry red in some cases). This seems to cause more current to flow. Whether it's because of emission or impact ionization, I don't know. The increased current flow causes the anode to get hotter and the tube races for self-destruction.

So far I've been able to catch runaway tubes before they developed issues, but a warning about this seems in order.

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Originally Posted by kmaier View Post
Driving the 300B... I consider a minimum Pk-to-Pk voltage closer to 250...
Which only increases the driver design challenge....

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I did a low-cost 45/2A3 design a few years back which used a cathode follower driver. Works very well for the 45/2A3, just not for the 300B (again, IMHO).
In my experience, that depends on the cathode follower. I tried a 6SN7 and/or 6J5. Mushy sound. That tube doesn't have a high enough gm to work well as a cathode follower in a 300B driver (IMO). That's one of the reasons I ended up in ECC99 and 6N6P land.

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Originally Posted by mbeards View Post
How about a parallel 6SN7 triode driver for a low impedance source?
I take you mean a standard grounded cathode amplifying stage. The output impedance of that circuit will be fairly high (roughly (rp/2)||Rload) and the circuit cannot source current.

You need a driver with an output impedance of a few hundred ohm or lower. Lower is better. This implies that you're using a tube with a gm over 10k (Rout ~= 1/gm) at the operating point of choice for the cathode follower.

~Tom
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Old 6th February 2013, 01:54 AM   #13
mbeards is offline mbeards  United States
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Quote:
Originally Posted by tomchr View Post

I take you mean a standard grounded cathode amplifying stage. The output impedance of that circuit will be fairly high (roughly (rp/2)||Rload) and the circuit cannot source current.

You need a driver with an output impedance of a few hundred ohm or lower. Lower is better. This implies that you're using a tube with a gm over 10k (Rout ~= 1/gm) at the operating point of choice for the cathode follower.

~Tom

Yes. I was plaining to use an 6J5GT input (essentially 1/2 of a 6SN7) directly coupled to a zener biased 6SN7 with both plates in parallel. This gives an output resistance of around 3.3K.

I am surprised if this output impedance is to high to effectively drive the 300B. Where is all the the current draw coming from to load down the previous stage? The inter-electrode capacitance's of the 300B roll off the output beyond the audible frequencies and the gird leak resistor is somewhere around 300K.

or... am I way off?


Also, dare I say.. why use the 300B at all if it is such a dog of a tube drive?
(yet I want to build one )


Matt
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Old 6th February 2013, 12:45 PM   #14
kmaier is offline kmaier  United States
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Tom,

Good post... re the thermal runaway. I don't know what else to call it either, but it seems appropriate. It could likely be some residual gas in the tube, but the getter flashing is fully intact on my 2A3 tubes.

The 300B bias point I'm using is 455V cathode-to-plate and requires 100 volts of negative grid bias with an idle current ~67ma. At a calculated minimum, 200V Pk-Pk is required for drive. Having decent headroom makes sense, as the driver stage does not reach it's limits when fully driving the output stage. You also have some class A2 capability with a well designed transformer-coupled driver.

EC99 - good tube. I also like the E182CC (7119) For the 45/2A3 amp, I used a 12AT7. First stage is a voltage gain amp direct-coupled to the second stage as a cathode follower. It has good linearity, wide bandwidth and low distortion and can easily swing over 100V Pk-Pk driving the 45 or 2A3. Again, the idea was low-cost and simple.

Mbeards - 6SN7 is a good tube, and many circuits about using one as an input/driver for the 300B. For under 8-watts output, it's fair, but limited. I don't like to parallel tubes, so I've taken a different path. Search the net for "bugle45 amp". There's a PDF article from Gordon Rankin for his Bugle 45 SE amp. Good article and has some good formulae in the sidebar for calculating slew rates and bias currents for driving the grid, bypass values, etc.

Regards, KM
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Old 6th February 2013, 10:07 PM   #15
tomchr is offline tomchr  United States
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Originally Posted by mbeards View Post
Yes. I was plaining to use an 6J5GT input (essentially 1/2 of a 6SN7) directly coupled to a zener biased 6SN7 with both plates in parallel. This gives an output resistance of around 3.3K.
I like the 6J5GT as well. I have a handful of Pinnacle 6J5 tubes.

To avoid blocking distortion, DC coupling to the 300B is needed. This either means that you're using a plate choke on the driver or using a cathode/source follower.

I tried using a d3A in triode with a 70 H plate choke. That works really well, but to minimize the THD of the driver, a variable cathode supply is now needed. Each tube needs it's own supply and the supply voltages are rather different due to tube-to-tube variation. Sounds good, though.

Due to the supply complexities, I opted to use a source follower. Now the challenge becomes finding a good tube for this. One that can handle the signal swing and has high gm. That means d3A, ECC99, 6N6P, and 12BH7A.

My goal is to get as clean of a sound as possible. If you're more into "sound shaping" than I am, then the 6SN7 or 6J5 are good candidates. It's also possible that they'd work well in a configuration where the 6J5 provides the gain up front and a high gm tube is used as a cathode follower. I know they work well with a source follower. BTDT.

The 300B is drawing a little bit of grid current as Vgk approaches 0 V. If your driver can't deliver this current - or it results in a signal dependent voltage sag across the output impedance of the driver - high THD will follow. With a 6J5 grounded cathode driver, you'll probably get 5-ish W at 3 % THD. I seem to recall measuring 0.6 % THD @ 1 W in such an amp. With a driver with lower output impedance, you'll be able to get more power out. That's how I'm getting 10~11 W @ 3 % THD and 0.1~0.2 % THD @ 1 W.

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Also, dare I say.. why use the 300B at all if it is such a dog of a tube drive?
(yet I want to build one )
Yes. Some of us are masochists...
Some of us get into 300B designs because of the sound quality of such designs. We later realize that it's a royal pain to drive that tube. Once I came to that realization and wrote down the requirements for the driver circuit, the circuit topology became rather obvious and only a small handful of tubes fell out as possible candidates.

I actually rather enjoyed the challenge and design process. But then I'm used to the multivariable optimization problems bound by the laws of physics from my day job.

I suspect some like the 300B amps because with a non-ideal driver, you can get a sweet tone from it. That's the "sound shaping" aspect. Each to their own I guess.

~Tom
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Old 6th February 2013, 11:44 PM   #16
mbeards is offline mbeards  United States
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I actually rather enjoyed the challenge and design process.

~Tom
Couldn't agree more.

Thanks for all the help. If you couldn't tell, this is my first attempt with a 300B circuit. I usually also stick to mid power P-P amps. For now I will devise a few preamp designs, get parts and try them all.

Matt
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Old 7th February 2013, 03:43 AM   #17
PGDO is offline PGDO  Netherlands
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Originally Posted by tomchr View Post

I seem to recall measuring 0.6 % THD @ 1 W in such an amp. With a driver with lower output impedance, you'll be able to get more power out. That's how I'm getting 10~11 W @ 3 % THD and 0.1~0.2 % THD @ 1 W.

~Tom
Hi Tom,

The 0.6% you mention is that at 1kHz?

I am busy with my first built, a Herb Reichert "Flesh and Blood" derived 300b so 6SN7 GTB cascade. The power supply is of my own design, a low DCR design with four 5V3 tubes and a CLCLC with 150mH and 2.6H chokes optimized for low impedance and fast impulse recovery.

It is currently on auto bias but I will change that to fixed bias once I have the EML 300b-XLS (500VDC B+).

What do you think of the distortion plot taken at 1 W and 84mA plate current? Is it normal that the second harmonic is curved like it is?

Regards,
Peter
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Old 7th February 2013, 07:31 AM   #18
tomchr is offline tomchr  United States
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The 0.6% you mention is that at 1kHz?
Sorry for not being specific. Yes. All distortion numbers are THD+N at 1 kHz measured using an HP 8903A.

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What do you think of the distortion plot taken at 1 W and 84mA plate current? Is it normal that the second harmonic is curved like it is?
I haven't looked at the individual harmonics in detail in my amp. But from your plot, the distortion is clearly dominated by 2nd and 3rd harmonics. That's the case in my amp as well. I would expect the THD to go up at lower frequencies as the OPT gets closer to saturation and any magnetic nonlinearities would tend to show here. I seem to recall my design with 6J5 having higher THD at higher frequencies as well, but I'd have to check my notes to make sure. Bottom line: Your 2nd and 3rd harmonics look normal to me.

In my current design, the THD is highest at low frequency, flat (at 0.15~0.2 %) from 200-ish Hz through 10-ish kHz. then it drops slightly at higher frequencies. The bandwidth of the amp is 32 kHz.

In your plot, the fourth and certainly fifth harmonics seem to be below the noise floor of your equipment. I'm guessing, but that's what the graph looks like.

~Tom
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Old 7th February 2013, 09:15 AM   #19
PGDO is offline PGDO  Netherlands
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Bottom line: Your 2nd and 3rd harmonics look normal to me.
Thanks for confirming that I was a bit worried about the high frequency distortion level.


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In your plot, the fourth and certainly fifth harmonics seem to be below the noise floor of your equipment. I'm guessing, but that's what the graph looks like.
Yes, my 192kHz soundcard broke down so measurements were done with a 96kHz soundcard sampling rate. I just bought a second hand EMU 1616 as replacement but I am currently sailing in the Indian Ocean. I will be back home in three weeks time then I can do some better measurements and install the 300b-XLS and connect the fixed bias.

I have a Tentlabs bias module but did all my simulations with autobias not realizing that the Tentlabs is of course a fixed bias. Stupid mistake, now I have a anode voltage of 500VDC in fixed bias, to much for a standard 300B. The EML 300B-XLS should work well on that voltage.

The low 2nd harmonic at the mid freq. band could be the result of cancellation, the 2nd stage 6SN7 actually has quite high 2nd harmonics distortion.

I can start a new thread if anybody is interested, as said the circuit is reasonable standard except for the low DCR power supply.

Peter
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