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2A3 driver

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#81
Another question I have is about the dual triode. Obviously they work, but when electrons flow from the cathode(s). how do they "go" to the correct plate?
Click to expand...

Most twin triodes are 2 separate systems, that share the base, bottle, and heater connections. Notice that each of the plates surrounds the grid and cathode.
 

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#82
Construction

Hi,

I get it, so really the plate in the case shown is really a oval containing sleeve for the cathode, then the grid must also then be a smaller containing sleeve with the cathode being inside of both.
That makes sense.
Thank you sir.

Ron
 
#85
You can test it by running 60mA of DC current through the primary and placing a 150k of resistance in series with the winding. Now run a 1kHz sine wave through both primary winding and 150k resistor, I would think if the primary was 25H then it's reactance at 1kHz would be 157k ohms, the sine wave should drop -3db.
 
#86
Test

Hello Famousmockingbird,

I will get set up to try to do that. My workstation needs a piece of wood on the work surface, it is a glass desk, I have a long power strip along the far edge, my DC power supply, scope, and signal gen are to the left, might need some coaching after a photo post tomorrow. I'll have to post some pictures of the OPTs as I do not have the knowledge to identify the primaries vs the secondaries. Probably running a small current through one side and measuring the other is how to identify, don't want any damage.

I have to say to you ALL and I really mean it, that I am so appreciative of your help, envolvment, coaching, teaching, and patience. As I said before, the world would be a better place with more people like you!!!


Thanks again,

Ron
 
#91
allenrssob said:
Hi,
I heard back about the OPTs, the primary inductance is 25H.
Is there a way to verify this?
Thanks,
Ron
Click to expand...

Is this necessaey ? Seems to me that you have some nice transformers and output tubes and no need to get distracted by them any further. There's lots to do still to build the amp so maybe decide on the two remaining key items 1- the power supply and 2- the input/driver. Then get ordering the remaining bits!

For a traditional design you'll want tube rectifier supply, choke loaded.
The driver stage could be cascaded 6sN7 as suggested or a single tube such as a triode-wired pentode. There are many proven schematics, which are on the web, less risky than reinventing a design.

Some other reading...

Simon's New Adventures In Hi Fi: Design Of A Single Ended 2A3 Valve Amplifier - Part 1

2A3 SET ? TRIODE LAB

I Never Met A 2A3 Amp That I Didn't Like Article By Editor Joe Roberts From Sound Practices Issue 15

https://web.archive.org/web/20111012095255/http://angela.com/angelamodel91amplifier.aspx
 
#92
25H will work good with the 2A3. You don't have to measure them but sometimes it's nice to know how matched they are. The DCR is important to know so you can take voltage drop into consideration when designing the power supply.

I wouldn't advise copying a schematc. A two stage amp will be easy enough to design from scratch with the help from everyone on here. At the end of the day you will hold your head higher knowing you didn't just copy a schematic. Don't worry it will sound lovely.
 
#94
@
http://www.diyaudio.com/forums/tubes-valves/289123-2a3-driver-8.html#post4669270
Yes. but acc. to datas for 6N7 mu=35, Ri=11K per section.
One section is just enough?
with sectopns in parallel the same thing about Cdyn is present, this time for input tube, but double. 🙂
And somehow looking at the datas, this tube is main for +Ug region?
But it could set also in -Ug region, at the first site, it is linear with -2V Ug.
that giving room for 4V p-p input signal... 🙂
 

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#95
allenrssob said:
Hi,
I heard back about the OPTs, the primary inductance is 25H.
Is there a way to verify this?
Thanks,
Ron
Click to expand...

25Hy is the good value for primary inductance with respect to lower end.
Other words You will have more than good bass. And gives You a room for the lower values of Ck in cathode node 2A3 Rk biasing...
I got the -1db@8Hz with bit more than 22Hy with probably 600ohms generator
the 2A3 is as generator something like this value (see Ri from the datas or calculate using graph method from the exact working point from the graph...)
 
#96
Zoran said:
@
http://www.diyaudio.com/forums/tubes-valves/289123-2a3-driver-8.html#post4669270
Yes. but acc. to datas for 6N7 mu=35, Ri=11K per section.
One section is just enough?
with sectopns in parallel the same thing about Cdyn is present, this time for input tube, but double. 🙂
And somehow looking at the datas, this tube is main for +Ug region?
But it could set also in -Ug region, at the first site, it is linear with -2V Ug.
that giving room for 4V p-p input signal... 🙂
Click to expand...


One section isn't enough. Have a look at the curves of each section again at Va=150. Mu/gm.

35/.0015=23,000

The 11k=Ri is for both sections in parallel.
 
#97
Unlike the others, I disagree with the 25Hy value - it does not need to be this high. 10-15Hy is fine enough for single ended 2a3. There are other important factors to consider too.

For example I take the Tango XE-20s which is a very decent OPT but only has primary impedance of something like ~16 Henries on the 3.5k primary windings (into whatever correct load it is reflected into)

The -3dB low frequency cutoff point can be determined by the following formula:

f = Z/(2*Pi*L)

Z is the primary source impedance (reflected impedance in parallel with the source impedance presented by the tube's plate)
L is the primary inductance.

So lets take for example a 2a3 driving a Tango XE-20s on the 3.5K ohm taps. Lets run the 2a3 at 250V A-K biased at -45V on the grid giving an Rp of 800 ohm, just like in the RCA specs sheet.

Z= (3500 *800)/(3500+800) = 651 ohm
L= 16H

f= 651/(2*Pi*16) = ~6.5 Hz

I think you will agree that a 3db low frequency cut-off of around 10Hz or less is good enough.

So what else is important?

Voltage drop is a factor to look at, as well if there is any HF ringing and how well the transients are handled, so you need to consider DCR, leakage inductance and capacitance.

Very good OPT's will exhibit low DCR, low leakage inductance and low capacitance.

Then of course there is power handling and simple mechanical considerations (i.e. does it vibrate). Then you have to consider layout - how to best position it on your chassis. After all this then you have it really sorted.

Ian
 
Last edited:
#99
soulmerchant said:
Unlike the others, I disagree with the 25Hy value - it does not need to be this high. 10-15Hy is fine enough for single ended 2a3. There are other important factors to consider too.

For example I take the Tango XE-20s which is a very decent OPT but only has primary impedance of something like ~16 Henries on the 3.5k primary windings (into whatever correct load it is reflected into)

The -3dB low frequency cutoff point can be determined by the following formula:

f = Z/(2*Pi*L)

Z is the primary source impedance (reflected impedance in parallel with the source impedance presented by the tube's plate)
L is the primary inductance.

So lets take for example a 2a3 driving a Tango XE-20s on the 3.5K ohm taps. Lets run the 2a3 at 250V A-K biased at -45V on the grid giving an Rp of 800 ohm, just like in the RCA specs sheet.

Z= (3500 *800)/(3500+800) = 651 ohm
L= 16H

f= 651/(2*Pi*16) = ~6.5 Hz

I think you will agree that a 3db low frequency cut-off of around 10Hz or less is good enough.

So what else is important?

Voltage drop is a factor to look at, as well if there is any HF ringing and how well the transients are handled, so you need to consider DCR, leakage inductance and capacitance.

Very good OPT's will exhibit low DCR, low leakage inductance and low capacitance.

Then of course there is power handling and simple mechanical considerations (i.e. does it vibrate). Then you have to consider layout - how to best position it on your chassis. After all this then you have it really sorted.

Ian
Click to expand...

Hello
between two OPTs 3K/46H/25W and 3K/29H/10W for application in 2A3 SE How prefer
Τhank you
 
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