| Bas Horneman |
I want to build a preamp with a 300B. Why?
Simple..I want to build a preamp with a DHT tube and don't want to buy another tube for a while. (Cash and too many unused tubes in cupboard already)...at first I was thinking...gonna build a nice choke loaded TJ 205d preamp or 101d...But that's a lot of bucks to fork out...then I thought...but I already have some DHT's "in stock" for years that I have not used (that is where the 300B's come into the picture). Plus I have a Hybrid Gary Pimm CCS built for 60mA (Just need some EL34's..but they are cheap)
I may or may not use a 7n7 SRPP stage or so before...to get some gain first.
Any suggestions...should I parafeed because I have a CCS already?
Manley Labs have done a 300B preamp and so have a company in Singapore (any idea about the topologies of these companies?)
Pete Millet also has a low mu preamp where he uses say a 6AS7 with EL34 current source...I might do something like this as well...substituting the 300B in place of the tubes Pete has suggested. (Anybody built this baby?) http://www.pmillett.com/lowmu_preamp.htm
I have this idea that there are more folks that have built a 300B preamp but are afraid of the Environmental lobby groups...and won't admit to it?
Experiences and suggestions are very welcome. |
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| slowmotion |
Hi Bas, all
I used a 300b linestage for a while,
it sounded very "organic" , dynamic and musical,
but I wouldn't really call it "accurate", whatever that means ;)
I used the 300b chokeloaded, with a cap on the output.
Regulated DC filaments.
cheers ;) |
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| Bas Horneman |
| quote: | | it sounded very "organic" , dynamic and musical, |
Thanks slowmotion!!...I am looking for organic, dynamic and musical... :-)
What value choke did you use? Do you remember what cathode resistor / operating point you used?
Who forced you to say it sounded accurate? GreenPeace?
I also have a DC reg in my cupboard..just waiting to be applied.
Regards,
Bas |
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| slowmotion |
It's been a while,
but if I remember correctly
( fat chance ;) )
probably 300V and 60-80 mA
or something like that.
It's nice to put a bit of current through it.
Regulated DC on the filament with catode return taken from the negative side of the filament only.
Choke was a Tamura output transformer with the secondarys left floating, output cap was a Mundorf I think.
hope this helps
cheers ;) |
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| Bas Horneman |
Thanks Jan!
| quote: | | Regulated DC on the filament with catode return taken from the negative side of the filament only |
Good tip...thanks again. (never used a DHT before ) |
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| planet10 |
... go for it...
look foward to another of your sculptures in wood...
dave |
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| Bas Horneman |
Thanks!
Did you ever see my darling (still not completed)
 |
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| Eli Duttman |
Bas,
Use current regulated DC on the 300Bs' filaments. There are NUMEROUS remarks around about "washed out" sonics when voltage regulated DC is used with DHTs.
FWIW, I would float the filament supply and rig a 4PDT switch to periodically reverse the DC polarity applied to the filaments. The 300B was designed to be heated with AC. The potential gradient associated with DC heating can shorten service life. Flip/flopping the DC polarity about once a month (more often, if you like) will compensate for the problem.
Consider using unbypassed cathode resistors. The 300Bs will be linearized without significantly altering their overall character. |
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| Bas Horneman |
Hi Eli,
Will do...
I've got a voltage reg followed by a current reg...so I might take out the voltage reg part.
The thing I don't really understand about a current reg is this :
With what voltage do you need to supply it? AFAI understand it ...it does not really matter..since the filament will only be supplied with 1,2A? So I can supply 8V or 12V does not matter?
I usually leave out the cathode resistors. But this time round I want to also add a switch so that I can hear myself the advantages/disadvantages.
Thanks and regards,
Bas |
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| SY |
Well, I had suggested a phono preamp using a 6528 some time back, so a 300B linestage is only the second most ridiculous idea I've heard of.
You might think about incorporating that into this preamp. If you're going to make your electricity provider rich, may as well go all the way. |
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| Bas Horneman |
| quote: | | phono preamp using a 6528 |
Suddenly a 300B preamplifier looks like a perfectly normal thing to do.:D |
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| janneman |
| quote: | Originally posted by Bas Horneman
[snip]The thing I don't really understand about a current reg is this :
With what voltage do you need to supply it? AFAI understand it ...it does not really matter..since the filament will only be supplied with 1,2A? So I can supply 8V or 12V does not matter?
[snip]Bas |
Hi Bas,
I don't know your particular current reg, but there will be a minimum voltage it needs to function - depending on implementation. And then there's the voltage across the load (heater). So, if the nominal heater voltage is 5V, and the current reg needs 6V to function, you need to supply it with at the minimum 11V. Obviously you should stay above that, but anything higher will lead to more dissipation in the current reg, that's just V x I, so you'd want to minimize that. (The V being the voltage between the two terminals of the current reg of course).
Jan Didden |
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| Bas Horneman |
| quote: | | Obviously you should stay above that, but anything higher will lead to more dissipation in the current reg |
Thanks Jan,
That makes it crystal clear.. Thanks!
By the way it is a simple current reg on Doede's boards.
http://www.dddac.de/at05.htm |
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| Alkis |
Hi Bas,
Same thought came to me several months ago for a 300B line pre in order to drive my 845 monoblocks.The idea was to built a whole DHT system,from the beginning all way to the end.I wonder,what would be the sound of a projeckt like this after building preamplifiers with ECL-86,5687 and currently 6BX7.
300B is one of the most linear tubes has been ever made,it's a DHT,and as they say, the sound or the colouration of a stage is infected by the earlier stage.
An interesting approach is here,
http://www10.big.or.jp/~dh/work/93072.html ,which I thought to modify.
Anyway,I'm looking forward to seen the outcome of your search |
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| Bas Horneman |
| quote: | | Anyway,I'm looking forward to seen the outcome of your search |
Thanks Alkis,
I will report my findings here.
Thanks for the link as well...wow..preamp with a 211!
Regards,
Bas |
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| 845 |
| This is my 300B preamps (with STU-7003 Tamura 5k/150Ohm Line output transformers). The driver is C3m. |
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| Bas Horneman |
I knew there would be diy'er out there who had built a 300B preamp! :)
So you have opted for a gain stage before the 300B.. have you tried other topologies?:)
What do you think of your 300B preamp? |
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| 845 |
In my case the best result i had with C3m tubes (I used transformer input 845 power amps).I use local feedback in gane stage.
This is best preamp for my Power amps.
Iam very sorry for my Inglish:rolleyes: |
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| Bas Horneman |
| quote: | | In my case the best result i had with C3m tubes (I used transformer input 845 power amps).I use local feedback in gane stage. |
Nothing wrong with your english. ...I understand what you are saying perfectly:D |
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| jeapel |
hi
On my side 6as7 in my preamp for a very very
good sound equivalent to a 300b and low zout
give a plus.
www.loginnovation.com/jeapel
if you don t like cathode follower a 6as7 + ccs
on plate and maybe 200 or 250v supply
would be a good solution
bye :-) |
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| SY |
| quote: | Originally posted by 845
In my case the best result i had with C3m tubes (I used transformer input 845 power amps).I use local feedback in gane stage.
This is best preamp for my Power amps.
Iam very sorry for my Inglish:rolleyes: |
Any chance that you could post a schematic?
Re: English, Ani lo m'daber Ivrit. |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Eli Duttman
The 300B was designed to be heated with AC. |
Well, early WE Datasheets specifies the 300B for DC heaters explicitly so, I think that view is highly debatable.
Sayonara |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Bas Horneman
I want to build a preamp with a 300B. Why?
Simple..I want to build a preamp with a DHT tube and don't want to buy another tube for a while. (Cash and too many unused tubes in cupboard already)...at first I was thinking...gonna build a nice choke loaded TJ 205d preamp or 101d...But that's a lot of bucks to fork out...then I thought...but I already have some DHT's "in stock" for years that I have not used (that is where the 300B's come into the picture). |
Well, given that many valves exist that fit on a U4 Socket and many of these have a mu between 3 (45 which would give a gain of around 9db) and 8 (10 and derivates and 205 which would give a gain of around 18db) and most of these Valves have lowish anode impedances allowing them to drive a line output directly, why not build the thing universal.
You do not need to run the 300B at 80mA and 450V for example, a much more relaxed 180V anode voltage will work for any number of DHT Valves, including some mostly (and unjustly) ignored.
You can then add options for 20/30/40mA Anode current via CCS. I would use fixed bias, which then in combination with a suitable CCS can be used to set the Anode voltage.
For the heaters, simply (again) make them adjustable and include in the case a suitable panel meter switchable to measure Left Anode Voltage, Right Anode Voltage, Left Heater Voltage, Right Heater Voltage.
The result a simple, common cathode, line stage amplification stage. I would probably place the attenuator after the Linestage, with a 10Y or the like, you want a 10K load, all others can see lower loads with no problems, so a 10K attenuator as load after the Valve would be my choice.
Then all you need is a stock of all sorts of DHT's. With heaters up to 7.5V/1.5A you have the following linestage Valves (list may not be complete though) as options:
From the Cunningham/RCA derived series
10/10Y/VT25
26
45
71A
2A3
801A/VT62
From WE:
300B
101D
205D
The actual active circuit could look like this:
100nF PTFE input coupling Cap and 250k gridleak resistor to fixed bias with wide adjustment range.
Valve with CCS anode load.
2.2uF Output Coupling Cap (Mundorf or Audyn KP-SN would be my "starter" choice) folloowed by 10K Attenuator.
Done.
Sayonara |
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| planet10 |
| quote: | Originally posted by Bas Horneman
Did you ever see my darling (still not completed) |
There, another Basian glowing sculpture...
dave |
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| Bas Horneman |
Not at all a bad idea!...
Now if someone can point me to nice looking voltage meters (retro looking). I'll get going.
And maybe a good clean fixed bias (alterable) supply. (One side note...what about the voices that say cathode bias sounds better?)
:D |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Bas Horneman
Not at all a bad idea!... |
I've been thinking and tinking with these ideas for a while. If you use a stepdown output transformer most of the usual suspect DHT's have no gain, forcing you to add a driver stage (like Manley does). If you end up with a parallel feed output transformer you might as well do without.
| quote: | Originally posted by Bas Horneman
Now if someone can point me to nice looking voltage meters (retro looking). I'll get going. |
Sorry, no luck there from me, but I'd use a modern digital LCD Meter behind a nice round metal ring and switch offable backlight....
| quote: | Originally posted by Bas Horneman
And maybe a good clean fixed bias (alterable) supply. |
Easy, rectifiy the same voltage as for the HT, add some capacitors in series to the rectifiers to drop the voltage and the use a multiple RC cell filter chain onto 10K multiturn Trmimmers (you get nice panel mount hardware for them). If you use 4 - 6 RC Cells (even just 1K/22u) you knock any noise flat as a pancake.
| quote: | Originally posted by Bas Horneman
(One side note...what about the voices that say cathode bias sounds better?) |
This is a by far too simplified argument. If well implemented both have their own sets of advantages and much will depend upon what you want. For a linestage I have tried both several times (before giving up and going passive) and I found that fixed bias is reliably MUCH better, even when comparing to BG cathode bypass cap's and all that.
Sayonara |
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| Bas Horneman |
| quote: | | Easy, rectifiy the same voltage as for the HT |
Don't I need a seperate transformer so I can make a negative bias? (Putting up the positive rail of the second supply on the earth of the first supply ) so I end up with a negative supply? |
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| analog_sa |
| quote: | | Don't I need a seperate transformer so I can make a negative bias? |
Not really. Just use a separate diode connected with the appropriate polarity. |
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| Bas Horneman |
| quote: | | Not really. Just use a separate diode connected with the appropriate polarity. |
Ah yes. Tis al coming back to me now...(Boy do I feel like Mike Schutte now...(actually am starting to look like him as well)) |
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| Miniwatt |
It must get awfully hot in there, 845, extra airco??
Standing in front of my rack with two 211 monoblocks I get very hot knees...:hot: |
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| Gluca |
I am in! I think I will start with a passive preamp based on the S&B 102TX an later on I can add an active stage based on a parafeed push-pull of 2A3.
What do you think of the schematics I think to follow??!! I still need to work on it but your impressions would be very welcome.
Gianluca |
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| Bas Horneman |
| The first thing that comes to mind is : "Mama Mia!" :D |
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| analog_sa |
| quote: | | What do you think of the schematics I think to follow??!! |
Like a Raven, only better :) But why the SS PS? Btw, the 'ultrapath' cap in Raven changes the sound for the better IME.
I wonder if the TX102 won't be happier driven by a capable driver rather than being left to the mercy of whatever source is plugged in. Of course this means further complications and there is nothing minimalist about the approach. |
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| Gluca |
Mama mia ;) ;)
SS PS ... well just to stay simple and to give the hexfreds a try but yes changing the PS would be straightforward. I noticed I missed a cap in the RC!!
Source ... I just bought on e-bay a Tascam CD-R for 250eur ... happily waiting for it. I will had a source selector anyway and probably a +6db selector for the TX102
Like a Raven ... yes indeed. I like Push-Pull
Caps ... I am not a part picker but I fear I will need to test different caps
???DC (current) heater???
I am not going to series connect the heaters: I think the best thing to do is to have a separate supply (probably just the last current regulator) feeding each of the valve so each channel should have a bridge + CLC + V regulator + 2 parallel current regulator
Anything wrong???
gluca |
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| Bas Horneman |
Hi Gianluca,
I saw that preamp when I was looking for an example of the 300B preamps on the net..I'm just not too keen on the gain stage of the preamp...with the 6sl7 and all...
Nice preamp though huh:)
Regards,
Bas |
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| Gluca |
Yes me too. I will have just the 2A3's. Having the VR would also look great! I will think about in september.
Ciao |
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| Bas Horneman |
| Yes..the VR glow is very nice...someone over at the bottlehead forum called it Purple Haze!....:smash: |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by analog_sa
I wonder if the TX102 won't be happier driven by a capable driver rather than being left to the mercy of whatever source is plugged in. |
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 |
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| Gluca |
Do you mean, to make it simple, to use the TX102 as an output (parafeed) tranny?
Gluca |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Gluca
Do you mean, to make it simple, to use the TX102 as an output (parafeed) tranny? |
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 |
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| Gluca |
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 |
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| Bas Horneman |
| quote: | | BAS we can do that, can't we? |
Hell yes..we can do anything we want to! ;) |
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| Bas Horneman |
| I'd rather go SE though since I want to use my 300b's |
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| Gluca |
... 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 :confused: 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 |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Gluca
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 :confused: 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?
|
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)
| quote: | Originally posted by Gluca
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? |
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 |
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| Gluca |
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 |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Gluca
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. |
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 |
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| Gluca |
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 |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Gluca
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. |
My Phonostage has an output impedance that high and I use it to drive the 102 in +6db stepup mode, so not too bad.
| quote: | Originally posted by Gluca
We can lower the impedance of the PP using fixed bias!?! |
No, what you get is basically both anodes in series.
But this would require a more complex circuit ...
| quote: | Originally posted by Gluca
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. |
But how much voltage can it tolerate? Usually the swing needs to be kept quite low on current output DAC's....
Sayonara |
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| Gluca |
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 |
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| Kuei Yang Wang |
Konnichiwa,
| quote: | Originally posted by Gluca
-input step-up tran (or if you like output from the DAC)
|
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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| Gluca |
Yes, I am procuring a 1:2 step up at the input.
Ciao
Gianluca |
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