Hi!
Are you familiar with this project?
What can you say about it?
https://audioxpress.com/article/the-2a3-tube-amplifier-a-tribute-to-simplicity-and-warm-sound
Are you familiar with this project?
What can you say about it?
https://audioxpress.com/article/the-2a3-tube-amplifier-a-tribute-to-simplicity-and-warm-sound
Loftin-White yes.
12AX7 as driver (even paralleled) NO. IMHO.
12AX7 as driver (even paralleled) NO. IMHO.
GZ34 is limited to 60uF input capacitance. The 100uF (C4) cap is too large and shoul be decreased. Typical value is 47uF.
That's the original Shishido design with minor tweaks to resistors mainly due to 30V lower supply voltage. The effective anode voltage of the 2A3 is only 220V and anode load 3.3K instead of 2.5K. Although the Sovtek is not really like the original 2A3, I am not sure it can do 3.5W. The 12AX7 works in about the same conditions as the original.
With 450-460V supply voltage the resistors can be chosen to make a good quality current production 12AX7 work at 1 mA per side and get 2mA in total with 100K common anode resistor at about 150V plate voltage. A red led diode could be used in place of the RC for about 1.6V cathode bias. The 2A3 cathode bypass could be made with DC Link film caps.
With 1 mA I guess it will barely make 20KHz at -3 dB. With 2 mA will be more like -1 dB down which is acceptable. The paralleled 12AX7 operative conditions could be also adjusted, mainly for anode current, between 1 and 2 mA for minimum THD.
The available anode voltage for the Sovtek 2A3 would be about 250V. AC heating will result in significant IMD.
With 450-460V supply voltage the resistors can be chosen to make a good quality current production 12AX7 work at 1 mA per side and get 2mA in total with 100K common anode resistor at about 150V plate voltage. A red led diode could be used in place of the RC for about 1.6V cathode bias. The 2A3 cathode bypass could be made with DC Link film caps.
With 1 mA I guess it will barely make 20KHz at -3 dB. With 2 mA will be more like -1 dB down which is acceptable. The paralleled 12AX7 operative conditions could be also adjusted, mainly for anode current, between 1 and 2 mA for minimum THD.
The available anode voltage for the Sovtek 2A3 would be about 250V. AC heating will result in significant IMD.
To add to the previous comments.
The cathode resistors dissipate about 12W each, They may be rated 20W, but I learned that that is only when mounted to a proper heatsink. The way they are mounted now makes them very hot, with the two decoupling caps in their middle.... Still not ideal, but would be better to have mounted the resistors onto the chassis, so it acts as heatsink and better transfer of heat to the outer world.
The cathode resistors dissipate about 12W each, They may be rated 20W, but I learned that that is only when mounted to a proper heatsink. The way they are mounted now makes them very hot, with the two decoupling caps in their middle.... Still not ideal, but would be better to have mounted the resistors onto the chassis, so it acts as heatsink and better transfer of heat to the outer world.
Well said Erik. It's actually more than 12W because the cathode resistor gets the 60 mA from the 2A3 plus 4-5mA from the resistive network that sets the 12AX7 anode voltage and grid-to-cathode bias for the 2A3.
I would use 2 or more resistors in parallel of suitable value and rating to distribute the power. I do not like attaching stuff to the chassis.
I would use 2 or more resistors in parallel of suitable value and rating to distribute the power. I do not like attaching stuff to the chassis.
With Sovtek 2A3 SE and a Gomes circuit, 12AX7, I got the following power (3% THD)
Vdc 260 volt, Ia = 55mA
Fixed bias
static
16 ohm: 4,76 w
8 ohm: 5,27 w
5,3 ohm: 3,35 w
4 ohm: 1,94 w
I use a single secondary set at 6 ohm
Vdc 260 volt, Ia = 55mA
Fixed bias
static
16 ohm: 4,76 w
8 ohm: 5,27 w
5,3 ohm: 3,35 w
4 ohm: 1,94 w
I use a single secondary set at 6 ohm
Best use CCCS load or even nicer a gyrator on Triode wired C3g. You can use E280F, D3a, E810F, etc. too.
All will be much better than the wimpy 12ax7 (even when paralleled as in this schematic).
Also, consider a power supply with a decent sized choke input. Power supply is very important in single ended amplifiers.
All will be much better than the wimpy 12ax7 (even when paralleled as in this schematic).
Also, consider a power supply with a decent sized choke input. Power supply is very important in single ended amplifiers.
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Indirectly heated drivers are a no-no for me. The 2a3 deserves a DHT driver stage to make it sing. Choice of e.g. 2P29L, 4P1L, 26, 112A, 10Y all in filament bias. 2P29L the easiest to build with Rod Coleman filament regs. You will need a 1:4 step-up transformer on the input and I use a Hammond studio series 1140-LN-C. Very happy with this - my own amp has a 10Y input stage.
Choice of 2a3 tubes? I'd like some feedback. I have EH which are clearly better than my pair of Sovtek, more midrange and treble detail.
I haven't heard Monolith OPTS but hear good things about them. I use NP Acoustic amorphous core 3.5K from Vietnam. Very detailed - they bring the magic out, which you will certainly get with a 10Y input stage.
Choice of 2a3 tubes? I'd like some feedback. I have EH which are clearly better than my pair of Sovtek, more midrange and treble detail.
I haven't heard Monolith OPTS but hear good things about them. I use NP Acoustic amorphous core 3.5K from Vietnam. Very detailed - they bring the magic out, which you will certainly get with a 10Y input stage.
Indirectly heated triodes work absolutely great in a right driver circuit.
It's more important not to pair the 2A3 amps with cheap & crappy low-resolution bookshelf speakers.
It's more important not to pair the 2A3 amps with cheap & crappy low-resolution bookshelf speakers.
<< Indirectly heated triodes work absolutely great in a right driver circuit.>>
I've tried out endless varieties of indirectly heated drivers, the best of which were probably esoteric 6J5 variants like CV1135. Went through the usual 9 pin varieties the best of which were probably E80CC, E180CC and 6N1P '60s versions.
Have you actually tried some DHTs as drivers?
I've tried out endless varieties of indirectly heated drivers, the best of which were probably esoteric 6J5 variants like CV1135. Went through the usual 9 pin varieties the best of which were probably E80CC, E180CC and 6N1P '60s versions.
Have you actually tried some DHTs as drivers?
Loftin-White and other DC-coupled schemes sound great, but suffer from bias drift unless there is a DC servo. They require frequent bias adjustment. Transformer coupled sounds as good (with quality interstate transformer) but is worry-free.
Should be checked by the parameters?
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For input/driver for 2A3 choose the tube with amplification factor according to the bias of the 2A3.
And wanted input sensitivity gor the max power.
Say You want 4V p-p signal at the input, and for the example if the bias is say -45V
that is 90V / 4V = 22.5 times for the gain of input stage.
Because in the circuit amplification of the tube stage always be smaller than the amplification factor of the same tube, mju has to be slight higher... So choose the tube mju = 25 to 30.
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Next internal resistance of the tube Ri as the driver. should be matched to drive dynamic capacitances of the 2A3. That should be calculated.
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Next insure that linear region of operation is in the higher value of -Ug than the 1/2 of max input signal. For these 4V p-p that will be more than -2V og Ug say -3V or -4V. This could be tricky for the LW SE, because it limiting the anode voltage in the first stage.
The higher anode voltage in the first stage will resulting the higher voltage supply of the output stage...
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12AX7 ECC83 has mju of 100, in the circuit it will be as 65-70 times of gain and small values for biasing the input. Internal resistance is huge, and that is the worst case for the any input/driver tube.
At proper design, the trioded C3g (mainly if it's active loaded) beefy driver for even larger tubes than 2A3.
It also needs a higher B+. with the bias scheme for the driver tube, the 2A3 roughly has 200V across it (410-209). No wonder he referred to it as having a "Warm Sound".
That is for sure better solution.
BUT mju of C3g in triode mode is 40 X. That implicates for case of max driving signal foe 2A3 of 90V p-p input, that the input sensitivity will be about 3V p-p or 1V RMS. With in circuit cca 30X of gain... So with these 1V rms 2A3 will reach the full power.
That is very low sensitivity baring in mind that almost every source has about 2V RMS or even more at the output... 🙁
And when we add preamplifier optionaly the situation getting worse.
So that is the only tech issue.
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C3g has from the datas low internal resistance of 2.3Kohm, so the output resistance will be lower than this value
(cca paralleld with R LOAD of 39K it will be cca 2.17Kohm)
That is very good.
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the good settings for C3g in triode mode are: -2V=Ug, 3.5mA=Ia, 150V=Ua.
Rload=36K-39K cca. (from TFK datasheet PDF).
BUT Vbattery is higher about 300V. Note that the tube is already working with say mid or lower Va of 150V.
For additional lowering the Vb, tube has to be set at the lower Va, which is not good - tube is the voltage device.
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BUT maybe is the solution choke load? L has to be minimum (should be calculated) 150Hy for these 2.3K of Ri.
Rdc of the L is way smaller than Rload so voltage drop across L will be minimal. say the Rdc of L is 350ohms voltage drop with 3.5mA will be only 1.3V cca
BUT resistor for lowering the voltage from 2A3 power supply has to be super large in W because that drop will be huge... 🙁
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C3g is high transconductance tube with open anode. That is giving specific kind of sound. Some may like it some are not...
For instance 2A3 is totly opposite in these parameters and construction - anode is one closed box. 🙂
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For 2,12V rms input, 6V p-p
we just need 15 times amplification to 90V p-p
...
In that case power amp will be compatibile for preamp in the system?
we just need 15 times amplification to 90V p-p
...
In that case power amp will be compatibile for preamp in the system?
Right.
1V RMS up to A1 region of 2A3.
In my practice this driver it's perfect for amplifier, has enough headroom if power tube can working even in A2.
C3g as VAS stage capable even 220Vpp (below 2V RMS input) with enough low distortion, so using with larger power tubes (for example 211) is possible.
Driving 2A3 is "easier job".
sample:
