Hi at all.
Somebody know if 2a3 could be driver for a 300B ?
I hace 2 x 2a3 and 2 x 300B tubes.
Thanks
Santiago.
Somebody know if 2a3 could be driver for a 300B ?
I hace 2 x 2a3 and 2 x 300B tubes.
Thanks
Santiago.
Several problems:
1. Gain as they are both low gain and you would need a high gain (ie. noisy) preamp to drive it.
2. Each power tube (all 4) would need their own heater supply.
3. Fancy power supplies and big ones for the current needed by 4 power tube.
4 What is wrong with a 5687 or 6N1P as a driver? ( or others? and there are many.)
1. Gain as they are both low gain and you would need a high gain (ie. noisy) preamp to drive it.
2. Each power tube (all 4) would need their own heater supply.
3. Fancy power supplies and big ones for the current needed by 4 power tube.
4 What is wrong with a 5687 or 6N1P as a driver? ( or others? and there are many.)
1. Start with a CD player that has 2.1Vrms out (3V peak), and CDs that play loud (near full scale DAC digital numbers at the crests of the music).
2. Next, use a single triode of a 5687 or a single triode of a 6SN7, with an IXYS current source in the driver plate load, and you will get full output from a 2A3 or 300B amp. You can do stereo with one dual triode input tube as the L and R drivers.
3. Play a CD that has music crests that are 6dB, 10dB, or 12dB down from full scale DAC numbers, and you will not get full output from the 2A3 or 300B.you will need a high output preamp, or 2 stages of 5687 or 6SN7.
One man's Excess gain is another man's way too Low gain.
Just sayin'.
2. Next, use a single triode of a 5687 or a single triode of a 6SN7, with an IXYS current source in the driver plate load, and you will get full output from a 2A3 or 300B amp. You can do stereo with one dual triode input tube as the L and R drivers.
3. Play a CD that has music crests that are 6dB, 10dB, or 12dB down from full scale DAC numbers, and you will not get full output from the 2A3 or 300B.you will need a high output preamp, or 2 stages of 5687 or 6SN7.
One man's Excess gain is another man's way too Low gain.
Just sayin'.
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As I said in PM on FB, the best way to drive 300B by 2A3 is to use in A2 driving through an interstage transformer. Otherwise, forget about 2A3 and use some smaller tubes and A1, as people already suggested.
Obviously, between 2A3 and 300b.
If say we bias 300B at 350V A-K, 80 mA, -70V on G1, we need like 150V minimum P-P swing on it's grid. Let's assume that we have 200V available for A-K of 2A3, so with 1:1 interstage transformer we can merry them perfectly. The transformer must be designed for 50 mA current, 100V on 20 Hz RMS.
But since we use 2A3 that needs like 35V RMS signal, we need one more stage with amplification of 35-40.
Any other thoughts?
If say we bias 300B at 350V A-K, 80 mA, -70V on G1, we need like 150V minimum P-P swing on it's grid. Let's assume that we have 200V available for A-K of 2A3, so with 1:1 interstage transformer we can merry them perfectly. The transformer must be designed for 50 mA current, 100V on 20 Hz RMS.
But since we use 2A3 that needs like 35V RMS signal, we need one more stage with amplification of 35-40.
Any other thoughts?
Attachments
And , would be better a interestage transformer , that increase the signal from the 2a3 ?
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I would not do it.
First, typically 1:1 interstage has wider frequency response than step-up and if bifilar much better than 99% of the output transformers whereas a typical step-up is typically worse than a good output transformer; Second the 2A3 as a driver is linear enough to provide the drive; Third, if you are worried about the gain you can use a medium-to-high gain tube with CCS anode load as input stage. The choice is wide (12AX7, 12AY7, triode-strapped small pentodes etc....). Even in the worst case where you want 100-110 V peak at the 300B grid the input stage will need to provide no more than 30 V peak which is nothing difficult....
If you are looking for a reference (i.e. the best you can get) then you can look at the Tango NC-20, Monolith Magnetics IT-05/25mA if you use the 45 or the IT-03/60mA if you use the 2A3, Hashimoto A-305....
A really good interstage is not cheap.
First, typically 1:1 interstage has wider frequency response than step-up and if bifilar much better than 99% of the output transformers whereas a typical step-up is typically worse than a good output transformer; Second the 2A3 as a driver is linear enough to provide the drive; Third, if you are worried about the gain you can use a medium-to-high gain tube with CCS anode load as input stage. The choice is wide (12AX7, 12AY7, triode-strapped small pentodes etc....). Even in the worst case where you want 100-110 V peak at the 300B grid the input stage will need to provide no more than 30 V peak which is nothing difficult....
If you are looking for a reference (i.e. the best you can get) then you can look at the Tango NC-20, Monolith Magnetics IT-05/25mA if you use the 45 or the IT-03/60mA if you use the 2A3, Hashimoto A-305....
A really good interstage is not cheap.
yes. Actually if you are only looking for 8-9W in class A1 you could use a 71a with 1:1 interstage.
For the 2A3 I would buy the Sowter 9525: Sowter type 9525
This is gapped for 50 mA which is more than enough, has nice 50H inductance which is plenty for the 2A3 driving the 300B and has very good frequency response. In my experience none of the standard Lundahls has got all these desirable features regardless of the cost...
The only downside of the Sowter is that they do not recommend more than 300V DC between primary and secondary. The Monolith Magnetics for example can take up to 375V without problems but are also more expensive.
That depends on you design choice. If you think you are going for class A2 I would not buy it. If you only want up to 9W, possibly 10W in class A1 then you can use self bias for the 300B and there is no problem as you can run the 2A3 somewhere between 200V and 250V plate voltage without problems.
Hi Wavebourn,
Nice to read your posts. Just last week, I breadboarded a 2a3 driving 35TG (in zero bias) through ≈ 3.5KΩ 1:1 interstage.
It generally sounds quite nice but just as you say, it needs an input tube to get enough gain.
In this case, the 35TG has high rp (just under 10KΩ) and high mu (39) for a dht so am thinking about Schade's IT/RC network. That would increase the gain needed for the input so I'm guessing at gain of roughly 50.
Also thinking about trying cathode drive of the 35TG but first have to make sure the secondary of the little interstage will take the current.
I'm in now way an expert on this, but on first sight i don't see how a low gain triode can be used as driver. But if you want an all DHT amp, i've seen a pair of 26's drive a 300B in a custom design, and that was one of the better DHT amps i've ever heared. Don't ask me how it worked, i just listened to it for a few hours, but those monoblocks had a pair of 26's and a 300B as final stage and a 5U4G rectifier tube. The transformers were Tango's and the speakers that i heared with it were Tannoy Arden's.
No offense but this is exactly what I would never do!
The 26 is a low gain tube as well. If mu is less than 10 it is a low gain tube. Tubes with mu roughly in the 10-30 range are considered medium gain and above 30 high gain. So you need 3 stages anyway and it is not as capable as a 45 or 2A3 as a 300B driver.
The 26 is also very microphonic and needs some selection and lots of attentions regarding vibrations isolation. If you take care of this then it might be a good driver for a 45 or a 2A3 in class A1.
Last but not least, because it has considerably higher plate resistance transformer coupling is more demanding in terms of primary inductance.
The 26 is a low gain tube as well. If mu is less than 10 it is a low gain tube. Tubes with mu roughly in the 10-30 range are considered medium gain and above 30 high gain. So you need 3 stages anyway and it is not as capable as a 45 or 2A3 as a 300B driver.
The 26 is also very microphonic and needs some selection and lots of attentions regarding vibrations isolation. If you take care of this then it might be a good driver for a 45 or a 2A3 in class A1.
Last but not least, because it has considerably higher plate resistance transformer coupling is more demanding in terms of primary inductance.
For 2A3 driving 300B I would use a gyrator loaded triode strapped d3a or 6J52P tube, Gyrator-loaded. Don't be afraid of gyrators, people on the forum sell kits. In case of excess gain I would add some feedback, to make the gain stable, less dependent on particular tubes that you use.
Your SVCS is perfect for this. Makes it very easy to set bias on the following tube in a directly coupled circuit.
Im looking in to something similar to do with my parts bin.
I did build a 102d input, 45/2a3 interstage driving an 845 on low voltage (550v, -70v). I used a Hammond 126b. Both driver and power valves on AC, 3mv hum (by fluke, 3 null pots per side took a long time to get there). Complete overkill i know and no better than simpler 300b SE i've heard. But its diy and fun, I'd still be tinkering with it if the 102d werent bashed by the kids and the price to replace isnt worth it.
Anycase, I'm considering using the cheap IT for a 45/2a3 driving a 300b. I have an Aikido 9pin pre and with a 12ax7 at the input has a gain of 50, 12at7=30, 12au7=10. The circuit is dead quiet and should make just a 2a3>IT>300b power amp driveable. I'm also considering trying Broskie's noise cancellation techniques with output valves. Still need to look deeper in to this.
I did build a 102d input, 45/2a3 interstage driving an 845 on low voltage (550v, -70v). I used a Hammond 126b. Both driver and power valves on AC, 3mv hum (by fluke, 3 null pots per side took a long time to get there). Complete overkill i know and no better than simpler 300b SE i've heard. But its diy and fun, I'd still be tinkering with it if the 102d werent bashed by the kids and the price to replace isnt worth it.
Anycase, I'm considering using the cheap IT for a 45/2a3 driving a 300b. I have an Aikido 9pin pre and with a 12ax7 at the input has a gain of 50, 12at7=30, 12au7=10. The circuit is dead quiet and should make just a 2a3>IT>300b power amp driveable. I'm also considering trying Broskie's noise cancellation techniques with output valves. Still need to look deeper in to this.
I have similar ideas, a 10K:10K should work with the 2A3 (my case: 10Y). I bought this one for the purpose: Amorf C core 10 K: 10 K single ended evenwichtige converter transformator frequentierespons: 20 HZ 26 KHZ 2DB-in Transformatoren van Woninginrichting op Aliexpress.com | Alibaba Groep
It will handle the about 45 mA with ease.
And you can also have a transformer input that gives at least +6 dB (600:2K4/10K).
my SE 300B driven by Hammond 126B 30mA by 6C4C (=2A3) running close to 33mA its OK, perfectly linear , grid choke and 6C45E input . do not buy china IT, do not need 300USD IT.