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Single Ended 2A3/6A3/6B4G DC coupled & Transformer choices
Single Ended 2A3/6A3/6B4G DC coupled & Transformer choices
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Old 25th February 2018, 07:57 AM   #21
soulmerchant is offline soulmerchant  Switzerland
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Quote:
Originally Posted by RajkoM View Post
Hi soulmerchant,

It is hard to beat the German post small-signal tube as an input/driver (C3g, C3m, D3a .... etc).

What is the input voltage in your DC C3g-2A3 amp for maximum output power?
About 1Vrms, or so ??
Maybe C3m would be more applicable, imho.

For me, in amplifier good input/driver stage is more important than Oriented HiB, Amorphous or M3 grain oriented core in OPT.
I use pro audio input levels, so input for maximum output power is about 1.22v rms. But to be honest, even standard consumer input works well on my setup. I'm using 2-way speakers (external crossover) with 94dB sensitivity.

C3m is very nice if you can manage the 20v heater requirement.
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Old 25th February 2018, 08:54 AM   #22
soulmerchant is offline soulmerchant  Switzerland
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Originally Posted by Perfectusaudio View Post
Very clever but what about the D3A
Any comments on that tube
D3a is getting too expensive imho, but that is probably due to people using it for phono or maybe headphone amp duty.


Here is the struggle you will get into with direct coupling most of these little triode strapped pentodes:

You get the idea that direct coupling will save all your 'troubles'. No more 'boutique expensive audiophool coupling caps'. No more heavy expensive interstage transformers that (due to your inadequate calculations) roll off your frequencies. But hey.. its just not that simple.

For D3a, take a HARD look at the specs sheet. Really hard... Those german engineers created very detailed specs sheets for these impressive little valves...

You see that your triode strapped D3a has nice low internal resistance when triode strapped, but be careful! That's biased for 24mA and 160V on the plate! For direct coupling, 160V is too high.... so you will want a lower plate voltage... and don't forget, you must still bias it to get decent headroom.

At first you imagine that a plate voltage of 130V will be OK. So you look at the triode curves and you really don't want to bias into the knees of the curves, right? You think, hmm.... maybe -1.25V bias will be ok. Yes, If you are willing to accept an input coupling capacitor, then you can do a nice some grid bias (and solve a LOT of headache at this point) but if you are some kind of audio direct-coupling purist and don't like that idea so are mostly stuck with a cathode resistor, etc.

Now look at the D3a specs sheet for these values and you will see you need 17mA through your CCS. You look at your internal resistance for 17mA and you see that its about 2.5Kohm. Sounds OK so far?

However when you set it up on your breadboard.. and discover... hmmmm sound is NOT so High-End. Kind of 'Tubey' sound. You realize that bias at -1.25V really isn't enough. Its just too little headroom even for consumer audio transient responses. You can see this on your scope too.

So then you take your D3a and want to bias at -1.5V you will suddenly need to push the current way down....to say 8mA. The specs sheets don't lie and the original manufacturer measurements were very good. The internal resistance at 8mA is about 3.8Kohm and on a steep incline if your current for any reason would drops lower. You discover that bias at -1.5V still didn't provide enough headroom even for consumer audio... The sound is just not there... kinda bla....with such low current on this valve either... ugh..

At this stage you might also think.. ok then maybe I can accept a higher plate voltage, but then your direct coupling requires bigger cathode resistor on your 2a3 or 300b, etc. Maybe the input coupling cap is worth testing out (*hint*hint*).

Just my feedback.

Ian

Last edited by soulmerchant; 25th February 2018 at 09:00 AM. Reason: typo
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Old 25th February 2018, 09:02 AM   #23
soulmerchant is offline soulmerchant  Switzerland
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Quote:
Originally Posted by RajkoM View Post

For me, in amplifier good input/driver stage is more important than Oriented HiB, Amorphous or M3 grain oriented core in OPT.
Absolutely agree.
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Old 25th February 2018, 10:19 AM   #24
RajkoM is offline RajkoM  Bosnia and Herzegovina
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Thanks for info.
20V heating for C3m is not problem. 2x6,3Vac windings in series then voltage doubler.

I think C3m is "born" for DC 2A3 amp.
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Old 25th February 2018, 11:13 AM   #25
soulmerchant is offline soulmerchant  Switzerland
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Quote:
Originally Posted by RajkoM View Post
Thanks for info.
20V heating for C3m is not problem. 2x6,3Vac windings in series then voltage doubler.

I think C3m is "born" for DC 2A3 amp.
You didn't find the huge plate resistance to be detrimental? In triode the gain is simply too low, but I hear that triode with an IT is a nice combination.
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Old 25th February 2018, 11:37 AM   #26
RajkoM is offline RajkoM  Bosnia and Herzegovina
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No. C3m plate resistance will be about the same as for C3g (about 2.5-2.6 kΩ in triode mode). See triode curves for both tubes.

No. With CCS (or gyrator in your case) in C3m anode, and with about 1.7Vrms at the input, you get full power from DC 2A3 amplifier.

To drive 2A3 (about 85Vpp) each tube with μ ~~ 20 is quite sufficient.

Last edited by RajkoM; 25th February 2018 at 11:48 AM.
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Old 25th February 2018, 12:01 PM   #27
RajkoM is offline RajkoM  Bosnia and Herzegovina
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One finished very very good sounding DC 2A3 amp from my friend.
Some values in PSU have been changed to obtain correct B+ of 465V.
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File Type: gif EL84-DC-2A3.gif (26.9 KB, 307 views)
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Old 25th February 2018, 12:03 PM   #28
Hanze Khronye is offline Hanze Khronye  Australia
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Quote:
Originally Posted by soulmerchant View Post
D3a is getting too expensive imho, but that is probably due to people using it for phono or maybe headphone amp duty.<snip>

For one with D3a on the bench at the moment it does come down to bandwidth when triode strapped. I run the tube at a couple hundred mV of bias, fixed 10mA Ip, and drive signal is ~150 mV p-p. CCS load.

With mu 75, even 40 ohms Rk adds >3k to the 2k Rp for 5k3 (you are suggesting even higher?), and it does roll-off around 15-18kHz on the scope. Reduce Rk to 10R and increase Ip to 15mA?


Hanze.

Last edited by Hanze Khronye; 25th February 2018 at 12:11 PM.
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Old 25th February 2018, 12:18 PM   #29
soulmerchant is offline soulmerchant  Switzerland
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I did try c3m in triode but for me the gain was too low for the wife's little silver disk player. Sound was indeed very good though.

If you ever try c3g then you won't be sorry. Even beat up old c3g's can still be good.

Ian
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Old 25th February 2018, 12:28 PM   #30
RajkoM is offline RajkoM  Bosnia and Herzegovina
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I have C3g in my 300B amplifier
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