Yes me too. I will have just the 2A3's. Having the VR would also look great! I will think about in september.
Ciao
Ciao
Konnichiwa,
Yes, driving the 102 from a low impedance source is a good idea, though among the various TVC's readily available it is more compatible with a wide range of source impedances than most.
Still, using an SE 45, 2A3, or 300B to drive a TX-102 in "WE parallel Feed" will give a defined and low source impedance (< 1K with 2A3 & 300B). There will be plenty of overload margin in the active stage and a reasonable one in the TX-102, as long as sources have normal full scale levels (meaning CDP with 2V RMS @ 0dbfs and LP Setups with 400mV @ 5cm/S plus/minus 6db).
The circuit could be rather simple and effective indeed.
Sayonara
analog_sa said:
Yes, driving the 102 from a low impedance source is a good idea, though among the various TVC's readily available it is more compatible with a wide range of source impedances than most.
Still, using an SE 45, 2A3, or 300B to drive a TX-102 in "WE parallel Feed" will give a defined and low source impedance (< 1K with 2A3 & 300B). There will be plenty of overload margin in the active stage and a reasonable one in the TX-102, as long as sources have normal full scale levels (meaning CDP with 2V RMS @ 0dbfs and LP Setups with 400mV @ 5cm/S plus/minus 6db).
The circuit could be rather simple and effective indeed.
Sayonara
Konnichiwa,
Yes, however with certain limits:
1) Maximum Primary Level +20dbu (around 8V RMS)
2) Low source impedance (otherwise save yourself the trouble and use the source directly).
On a totally crazy note, a 6AS7 SRPP will have very low output impedance and a gain of 1...approaching 2 (depending upon bypassing the lower Vlaves cathode resistor or not), or equally, a 6AS7 can be resistor or CCS Loaded to drive a TX-102.
How does that sound for a linestage? Crazy and heretical enough?
Sayonara
Gluca said:
Yes, however with certain limits:
1) Maximum Primary Level +20dbu (around 8V RMS)
2) Low source impedance (otherwise save yourself the trouble and use the source directly).
On a totally crazy note, a 6AS7 SRPP will have very low output impedance and a gain of 1...approaching 2 (depending upon bypassing the lower Vlaves cathode resistor or not), or equally, a 6AS7 can be resistor or CCS Loaded to drive a TX-102.
How does that sound for a linestage? Crazy and heretical enough?
Sayonara
Heretical indeed! And it will be crazy as I plan to use 2A3's (PP maybe).
BAS we can do that, can't we?
Gianluca
BAS we can do that, can't we?
Gianluca
... I am still laughing ... but I just started collecting all the parts.
Suddenly a doubt passed by my mind.
Suppose we bias the 300b at, say, 45 V-250V letting something like 50mA pass through the cathode. One would use a 900R bias resistor Rk or half that value for a Push Pull with common cathode resistor. The 300B would present roughly 700R as internal resistance Ra.
But 😕 the output resitance seen at output transformer would be
Rout = Ra + (1+u) Rk
where u is the amplification factor. Well that's what M Jones is writing ... I found something similar, in a different argument, on Radiotron but I didnt worked the formula out as I am too lazy.
Given these facts, the tranny would see 5k as SE and half that value in the PP.
Am I wrong or what?
And more. Suppose we go for the PP, as I am actually, is it safe to have just 1 parafeed capacitor connected between the two windings of the primary of S&B102TX or, as I believe, we'd better to place 2 capacitors at the plates of the tubes?
Gianluca
Suddenly a doubt passed by my mind.
Suppose we bias the 300b at, say, 45 V-250V letting something like 50mA pass through the cathode. One would use a 900R bias resistor Rk or half that value for a Push Pull with common cathode resistor. The 300B would present roughly 700R as internal resistance Ra.
But 😕 the output resitance seen at output transformer would be
Rout = Ra + (1+u) Rk
where u is the amplification factor. Well that's what M Jones is writing ... I found something similar, in a different argument, on Radiotron but I didnt worked the formula out as I am too lazy.
Given these facts, the tranny would see 5k as SE and half that value in the PP.
Am I wrong or what?
And more. Suppose we go for the PP, as I am actually, is it safe to have just 1 parafeed capacitor connected between the two windings of the primary of S&B102TX or, as I believe, we'd better to place 2 capacitors at the plates of the tubes?
Gianluca
Konnichiwa,
You are right under the following conditions:
1) Cathode R unbypassed and both cathode circuits are seperate in case of PP.
2) Transformer for output is not returned to the cathodes but to ground (SE Parallel feed or series feed) or connected between the two anodes without reference to the cathodes (PP)
I think you got the official answer from S&B on that score. The 102 is not designed to withstand high levels of DC between primary & secondary. It may do fine (given the insulation rating of modern magnet wire) but you are on your own.
Now, what I PERSONALLY would do is likely to use a 6AS7 SRPP (Or 2A3 SRPP?), which should give a drive impedance to the 102 of a few 100 Ohm worst case.
Or use standard SE and use WE parallel feed and if you must even resistive anode load (I suspect a hybrid CCS with a IXYS CCS & an EL34 per channel would be better.
Sayonara
Gluca said:
You are right under the following conditions:
1) Cathode R unbypassed and both cathode circuits are seperate in case of PP.
2) Transformer for output is not returned to the cathodes but to ground (SE Parallel feed or series feed) or connected between the two anodes without reference to the cathodes (PP)
Gluca said:
I think you got the official answer from S&B on that score. The 102 is not designed to withstand high levels of DC between primary & secondary. It may do fine (given the insulation rating of modern magnet wire) but you are on your own.
Now, what I PERSONALLY would do is likely to use a 6AS7 SRPP (Or 2A3 SRPP?), which should give a drive impedance to the 102 of a few 100 Ohm worst case.
Or use standard SE and use WE parallel feed and if you must even resistive anode load (I suspect a hybrid CCS with a IXYS CCS & an EL34 per channel would be better.
Sayonara
Oh thanks, your replies are always most valuable.
What I am currently thinking to do is to use the 102TX at the input ... but it will be fed, well in the next future as that project is really far away to be completed, directly by a DAC: PCM1794 from BB. I have a bunch of these chips. Probably mono-operated.
Hooked up this way the 102TX would see the I/V resistor as source, I guess it will be a few 100s hom, and the grid leak resistors will load the secondary.
Its going to be a weird preamp!! Lets say that its a preamp for digital sources or a crazy DAC with an unusual (and expensive) tube output stage.
Do you see anything odd/wrong??
I would stay with the Parafeed PP (choke loaded) at the output as I like this simple topology. CCS and SRPP are too complex to me now, but yes! I hope to give them a try as I get more experienced.
And ... I will go for a common unbypassed cathode resistor ... I am listening my amps built around that topology and I find them quite nice sounding.
During the next days I will ask Bas to provide me with a pair of these irons from S&B.
I am still considering what chokes I should pick up... I noticed Lundahl has got in its catalog anode chokes with two windings: you can cancel DC current out when operated in PP (not considering the imbalances between tubes). It seems to be quite a good idea...
A 2:1 step down tranny at the output would complete the preamp lowering the output resitance.
Ciao
Gianluca
What I am currently thinking to do is to use the 102TX at the input ... but it will be fed, well in the next future as that project is really far away to be completed, directly by a DAC: PCM1794 from BB. I have a bunch of these chips. Probably mono-operated.
Hooked up this way the 102TX would see the I/V resistor as source, I guess it will be a few 100s hom, and the grid leak resistors will load the secondary.
Its going to be a weird preamp!! Lets say that its a preamp for digital sources or a crazy DAC with an unusual (and expensive) tube output stage.
Do you see anything odd/wrong??
I would stay with the Parafeed PP (choke loaded) at the output as I like this simple topology. CCS and SRPP are too complex to me now, but yes! I hope to give them a try as I get more experienced.
And ... I will go for a common unbypassed cathode resistor ... I am listening my amps built around that topology and I find them quite nice sounding.
During the next days I will ask Bas to provide me with a pair of these irons from S&B.
I am still considering what chokes I should pick up... I noticed Lundahl has got in its catalog anode chokes with two windings: you can cancel DC current out when operated in PP (not considering the imbalances between tubes). It seems to be quite a good idea...
A 2:1 step down tranny at the output would complete the preamp lowering the output resitance.
Ciao
Gianluca
Konnichiwa,
I'm not sure how much voltage the PCM1794 can tolerate on the output. You might want more stepup than the 102 can deliver.
How about a stepup transformer upfront, then 2A3 Push-Pull with a common cathode resistor and driven from a NEGATIVE supply, so you don't have much voltage across the TX102 on the output.
A 2A3 PP Stage will have around 1K5 to 2K output imedance.
Sayonara
Gluca said:
I'm not sure how much voltage the PCM1794 can tolerate on the output. You might want more stepup than the 102 can deliver.
How about a stepup transformer upfront, then 2A3 Push-Pull with a common cathode resistor and driven from a NEGATIVE supply, so you don't have much voltage across the TX102 on the output.
A 2A3 PP Stage will have around 1K5 to 2K output imedance.
Sayonara
That was the original idea and my favorite topology ... I changed my mind as 1.5k-2k seems to be quite an high source for the TX102. Hooked up this way, the load on TX102 will be the input impedance of the amps, say > 20k.
We can lower the impedance of the PP using fixed bias!?! But this would require a more complex circuit ...
If you think it would be happy with that impedances I'll go this way.
PCM1794 can give 8mA and when mono operated you get 16mA: with a I/V resistor and a 1:2 step-up, or even a 1:1, you can go easily to 1-2Vrms.
Grazie. Ciao.
Gianluca
We can lower the impedance of the PP using fixed bias!?! But this would require a more complex circuit ...
If you think it would be happy with that impedances I'll go this way.
PCM1794 can give 8mA and when mono operated you get 16mA: with a I/V resistor and a 1:2 step-up, or even a 1:1, you can go easily to 1-2Vrms.
Grazie. Ciao.
Gianluca
Konnichiwa,
My Phonostage has an output impedance that high and I use it to drive the 102 in +6db stepup mode, so not too bad.
No, what you get is basically both anodes in series.
But this would require a more complex circuit ...
But how much voltage can it tolerate? Usually the swing needs to be kept quite low on current output DAC's....
Sayonara
Gluca said:
My Phonostage has an output impedance that high and I use it to drive the 102 in +6db stepup mode, so not too bad.
Gluca said:
No, what you get is basically both anodes in series.
But this would require a more complex circuit ...
Gluca said:
But how much voltage can it tolerate? Usually the swing needs to be kept quite low on current output DAC's....
Sayonara
OK then. I'll go for it.
-input step-up tran (or if you like output from the DAC)
-PP of 300B's or 2A3 supplied via a negative rail (I believe I will stay with low plate voltage/bias)
-common unbypassed cathode resitor
-choke on top of the tubes (anything > 50H would be probably an overkill)
-TX102 with a cap in the middle
-XLR out
Max signal feeding the TX to be < 6Vrms to be on the safe side.
Thanks again.
Ciao
Gianluca
-input step-up tran (or if you like output from the DAC)
-PP of 300B's or 2A3 supplied via a negative rail (I believe I will stay with low plate voltage/bias)
-common unbypassed cathode resitor
-choke on top of the tubes (anything > 50H would be probably an overkill)
-TX102 with a cap in the middle
-XLR out
Max signal feeding the TX to be < 6Vrms to be on the safe side.
Thanks again.
Ciao
Gianluca
Konnichiwa,
Transformer is needed for insulation, make sure it is rated for 300V insulation. With the DAC, I would recommend for a balanced output to keep the overall swing to no more 2V P-P, which with 8mA P-P current suggests 250R I/V.
With a 2A3 or 300B having a gain of around 4 and our desire to have 6V RMS at digital full scale (or 17V P-P) we need 4.25V P-P on the grids and thus a 1:2.125 stepup.
This would make a nice "direct digital input" preamp I think.
Sayonara
Gluca said:
Transformer is needed for insulation, make sure it is rated for 300V insulation. With the DAC, I would recommend for a balanced output to keep the overall swing to no more 2V P-P, which with 8mA P-P current suggests 250R I/V.
With a 2A3 or 300B having a gain of around 4 and our desire to have 6V RMS at digital full scale (or 17V P-P) we need 4.25V P-P on the grids and thus a 1:2.125 stepup.
This would make a nice "direct digital input" preamp I think.
Sayonara
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