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Driver tube suggetions for 300B amp

208.46.240.4
Posted on February 10, 2016 at 09:15:11
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 I'm looking to spice up my now 16 year old 300B SETs, by try out a DHT driver. The current interstage in there is a Hirata Tango NC-20 but I might be willing to try something else here since there are lots of high quality interstages offered these days and I could probably make quite a bit selling them off. May even be able to fund all the upgrades for the amp and cover cost of new IT.

Prefer something that can either be purchased now new (ie 300b, 45) or something where there is plenty of old stocks.

 

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4P1L in filament bias, posted on February 10, 2016 at 13:26:00
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 4P1L is a great tube in triode. Run the filaments in parallel at 2v and use filament bias with a couple of Rod Coleman regs (Google Lyrima). Separate transformers for each tube and preferably a nice Hammond choke input. I use 159ZC. It's worth paying a lot of attention to the filament supply - in filament bias it makes a big difference. No false economies. Look at Ale Moglia's site for details.

Plenty of old stock of 4P1Ls, cheap and rugged as hell. Rp of 2k and mu of 11. What's not to like. Needs to be run at 25-30mA. Ale, myself and several others stopped using 26s in favour of the 4P1L, especially as a driver. It's an extraordinary little tube. To get the best out of it you must use filament bias. Threads on DIY Audio, both as a line stage and as a SET. I use it in the output stage in preference to 300b or 45. Just sounds better to me in filament bias - no cathode bypass caps, just a small resistor. Makes all the difference.

 

RE: 4P1L in filament bias Andy's amp, posted on February 10, 2016 at 14:07:31
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012

Andy's Amp

 

RE: Driver tube suggetions for 300B amp, posted on February 10, 2016 at 14:25:27
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002

How about the PX25
Gain of 9.5
Rp of 1150 (Good with the NC-20)

DanL



 

I'm building something with a 4p1l filament bias driver, posted on February 11, 2016 at 02:22:20
Bas Horneman
Audiophile

Posts: 4084
Joined: March 28, 2001
 but you better have gain somewhere else in the chain. From my first trials...on my test speakers (not efficient) it plays very soft.

 

RE: I'm building something with a 4p1l filament bias driver, posted on February 11, 2016 at 05:24:14
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 What is input sensitivity? I am not using a preamp, so source voltage is 2 vrms max from DAC...

I don't think using an input transformer plus DHT driver tube is a great choice. I'd rather stick with IDHT with no input transformer.

 

RE: 4P1L in filament bias, posted on February 11, 2016 at 05:26:45
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 Thanks Andy. I am not sure about 4p1l I have heard it in filament bias as output and tbh do not like the tone of it with unamplified instruments, in particular instruments with more rich upper harmonics like violin which tend to sound less realistic and more strident.

Big thanks to you and Ale for all your experiments, I have learned a lot

 

RE: Driver tube suggetions for 300B amp, posted on February 11, 2016 at 05:27:43
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 From my searching they are the only manufacturer with current production?

 

RE: Driver tube suggetions for 300B amp, posted on February 11, 2016 at 07:34:21
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 There is a Sophia PX25 but
it does not have "real" PX25 specs.
Higher rp and lower mu.

DanL



 

That circuit is out of date for a few reasons, posted on February 11, 2016 at 10:48:39
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000



I now just use 4P1L into 4P1L. No need at all for the 300b. I have Alpair 10 speakers and a 2v input from a ES9023 DAC. Here's a better and updated schematic. I actually just use the single 4P1L output, though I have another amp with PSE outputs.

In response to "strident violins", Ale and I puzzled for a while over this. There was a hint of sharpness. What I've found is that this was in the filament supply. With a quite large choke input and a good quality EI transformer the sound got smoother. A lot of this is in the filament supply with filament bias. It's a lot more important than many of us thought at first. I'd say you have to hear a choke input supply into Rod's Regs to be able to properly judge filament bias.

 

RE: Driver tube suggetions for 300B amp, posted on February 11, 2016 at 11:48:27
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 How about an EML30A with no NC-20. A pair of MQ Dowdy chokes gapped for 25mA would probably be a fun plate load for the 30A.

 

RE: Driver tube suggetions for 300B amp, posted on February 11, 2016 at 11:54:29
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 Dan, the Full Music, TJ, and Sophia tubes are made in the same factory, just a name game. I know, years ago I imported a bunch, I had the option of putting my own name on them, but thought it BS, so went with the generic factory name (Full Music). I didn't want to further confuse the situation, and wasn't buying into the deceptive marketing scheme. Here in Texas, bidness is a contact sport, in China it's war.

twystd

 

RE: Driver tube suggetions for 300B amp, posted on February 11, 2016 at 13:29:32
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002
 Not from what I got off searching the web.
Maybe from the same factory but not the same tube.
Not like that was never done before.
TJ is the only PX25 with real PX25 specs.
The others have a mu of 6.5 and 1800 rp.

DanL



 

RE: Driver tube suggetions for 300B amp, posted on February 11, 2016 at 14:05:52
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 Have you seen actual curve traces of the TJ? The data they claim is identical to the printed data of the PX25 and the only curves I have ever seen are a direct copy of an original datasheet.

The Sophia Electric curves are the only actual traced curves I have seen and I have a hunch that they might be representative of all the PX25's.

This hunch also holds true for the WE 101 and 205 replicas. Never do you see actual traced curves, all that can be found is a cut and paste from the original data sheet which doesn't convince me of actual measured behavior.

dave

 

You got it right Dave. (nt), posted on February 11, 2016 at 15:59:04
twystd
Audiophile

Posts: 2723
Location: Austin,Texas
Joined: December 9, 2000
 .

 

Adding what I remeber from back in the day..., posted on February 11, 2016 at 22:06:26
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 The 4V filament TJ PX25 was closer to the traditional PX25A or some such. The 5V filament TJ PX25 was a little different and its curves/ ideal operating points did not resemble any particular tube. Still, it was used in the DIY HiFi LadyDay amps with only minor mods (cathode R change to lower plate current) and 5K load. IIRC, apparently it sounded similar to the TJ 300B run at cool operating points and 5K load.

All this was going on about 10 years ago on the DIY HiFi forum, back when Brian Cherry owned it and Thorsten was in the mix. Could probably be interweb searched.

Edit, this form 2006:

"I have just seen a reply by Brian Cherry claiming that the 4V TJ PX25 does meet the PX25 specs... I've seen the measurements on a 5V one and it quite a way off mu=9, Gm=2.5 instead of 7.5mA/V..."

Cheers,
91

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

The alternative is a 4P1L/high gain output stage, posted on February 12, 2016 at 00:06:09
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 The problem with a 300b is that it's a low gain tube. Hence people resort to using IDHT drivers, so you lose the all-DHT sound. Personally I'm not willing to use IDHTs - for me they subtly veil the sound and lack that DHT see-through transparency.

So my solution would be to use an output tube with at least a gain of 8 or 10. With a 4P1L driver this gives you a 2-stage amp that works with speakers of moderate sensitivity. Not necessarily horns - full-range units like Alpairs or Fostex or Lowther work okay.

Output stage doesn't have to be 4P1L - could be another higher gain tube like PX25, 813, 211, 801.

 

HY40, posted on February 12, 2016 at 08:08:15
PakProtector
Audiophile

Posts: 12365
Joined: May 14, 2002
 or HY51A. mu is 20, and their gm is respectable for TT cathode power tubes. Of course it is a bit ironic to consider the '51A as it has more plate dissipation than the tube it is driving...LOL
cheers,
Douglas
( and no, you can't have any ov mine )


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Adding what I remeber from back in the day..., posted on February 12, 2016 at 10:53:26
dave slagle
Manufacturer

Posts: 5430
Location: NYC
Joined: April 27, 2001
 The basic pattern of behavior that I have seen from the chinese tubes is that the bias point and filament voltage determines the tube type. If it has a 2.5V filament and biases up at 60ma with 250V P-K and -45V bias it is a 2A3 and the transconductance never comes into the picture.

The sophia electric site has a number of actual traced curves and here is how they compare to the "original"

The 45




A real 45 biases up at 250V P-K -50V on the grid and runs 34ma of current with a mu of 3.5 and an rP of 1750 which translates to a transconductance of 2000.

The above tube nails the operating point but has a higher Mu 4 and a lower Rp 1260 which equates to a higher transconductance of 3190

Now for the mesh 300B.





Again it biases up really close to where a 300B should but at that point a "real" 300B should have an Rp of 750 ohms a mu of 3.9 which translates to a transconductance of 5300u and this tube has the correct Rp but the mu and Gm are higher.

Both of these cases are within the realm of sample to sample tube variation so things aren't that bad (yet)

Now lets look at the PX25 that got this all started.





Again the bias point is almost nuts on however instead of a mu of 9.5 and an rp of 1256 ((gm of 7500) we get a mu of 6.9 and an Rp of 1880 for a Gm of 3670 which isn't close.

Now lets look at the 50.





Yup the bias points are spot on but instead of a mu of 3.8 and a Rp of 1800 ohms (Gm of 2100) we get a Mu of 5.11 and an Rp of 740 ohms for a Gm of 6880.

To me it sure looks like the 50 and the 300B are the same tube with a different filament voltage and the PX25 actually looks more like a 50.

I do want to give credit to Richard at Sophia for actually publishing this info and while it could be that just his versions of the tubes behave this way, it would take compelling info from someone who does not make or sell the tubes to make me believe it.

dave

 

RE: The alternative is a 4P1L/high gain output stage, posted on February 12, 2016 at 14:23:06
Al Noakes
 Why not try a 4P1L as pentode driving 300B ?

Al

 

RE: Adding what I remeber from back in the day..., posted on February 12, 2016 at 17:39:26
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Well spotted Dave - thanks for posting.

As a tube manufacturer it would be tempting to rationalise product lines, tooling etc. to minimise costs - maybe that is what is going on. Problem is, although the tubes will "work", they won't really be what the amp was designed for. Seems a "profits first" business model.

Interesting.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Driver tube suggetions for 300B amp, posted on February 13, 2016 at 06:52:22
hifipaul
Audiophile

Posts: 735
Location: NY
Joined: December 22, 2008
I've been using a 6AH4. The circuit goes; #76-6AH4-xfmr-300b. It is easy to implement and sounds good. If I were to experiment and change out the 6AH4 I'd probably try a #46.

 

and excluding ole TT triodes..., posted on February 13, 2016 at 09:58:14
PakProtector
Audiophile

Posts: 12365
Joined: May 14, 2002
 The type EL84 makes one heck of a signal tube...:)
cheers,
Douglas


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Driver tube suggetions for 300B amp, posted on February 14, 2016 at 00:04:52
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 Wouldn't a 20a or 20b be more suitable as a driver tube?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Driver tube suggetions for 300B amp, posted on February 14, 2016 at 08:11:42
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
Joined: October 4, 2006
 45s, 46s and 2A3s are my choices. 2.5 V filament supplies can run any and you can decide your sonic flavors. There is magic when using an output tube to drive output tubes...

ST versus GLobe ? You choose...How about the 10 family ? Too high an impedance for your IT ? If you can find them, try that very forgiving original RCA 801 with the carbon plate and porcelain base ! 45s are without a doubt THE choice for your application; especially the original Globes. Many folks covet the newer "mesh plate" variety; but they are as or more costly than new testing 245 or 345 GL balloons ! The magical naturalness of the original GL types are peerless. Think of all the folks that love their ST 45s. I cannot even imagine their joy after hearing some balloon 45s.

For some adventures with IDHT, may I recommend the 6F6 or 6V6 as a triode driver ? How about the "almost DHT" 6A5 ? Enjoy the journey !

 

excluding ole True Triodes..., posted on February 14, 2016 at 08:16:35
Interstage Tranny
Audiophile

Posts: 3063
Location: Eastern
Joined: October 4, 2006
 ...I agree...EL 84s sound great and sing well in triode (or even SE Pentode output with a bit of feedback). I also like the 6F6 and 6V6 as triode connected drivers. The 6A5 might take the cake with the OP though...

 

3A5, posted on February 14, 2016 at 08:26:35
RayP
Audiophile

Posts: 727
Location: Maryland
Joined: June 30, 2005
 You might find this thread about using a 3A5 interesting, particularly post #8.

ray

 

RE: 3A5, posted on February 14, 2016 at 09:39:04
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 This looks very interesting, thank you for sharing. 3A5 being true triode should mean it should not have the odd order dominance (due to local feedback when wiring pentodes as triode??)

 

RE: Driver tube suggetions for 300B amp, posted on February 14, 2016 at 11:20:01
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 Kinda depends on whether you want to go with 2 or 3 stages. The mu of ~30 or so on the 30A would make it a nice candidate for a 2 stage amp. For a 3 stage amp, the 20A/20B would do well in the second stage.

 

I've used 3a5 - pretty good though not "great", posted on February 14, 2016 at 12:59:22
andy evans
Audiophile

Posts: 4382
Joined: October 20, 2000
 I've used 3a5 - I found it pretty good though not one of the "great" DHTs. I'd probably prefer it to a 6SN7 for instance. Mu of around 15 which is handy. Not specially microphonic. Handy little tube. Can also be used in phono sections - there's an ingenious phono stage with it.

 

RE: I've used 3a5 - pretty good though not "great", posted on February 14, 2016 at 17:08:06
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 Had 6sn7 as the driver when I first got the amp. Completely underwhelming sound.

 

RE: Driver tube suggetions for 300B amp, posted on February 14, 2016 at 17:09:35
FlyCast
Audiophile

Posts: 65
Location: New Haven
Joined: September 5, 2015
 Those EML designed tubes look like the perfect drivers, but I dislike using tubes that are offered from only a single manufacturer in case they go under. Not a big deal with ECC99 I currently have in there since they are cheap and stocked up on a bunch. Stocking up on expensive EMLs isn't an option.

 

RE: Driver tube suggetions for 300B amp, posted on February 14, 2016 at 17:23:45
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 Yeah, no arguments there!

 

For Thoriated Tungsten..., posted on February 15, 2016 at 03:33:12
PakProtector
Audiophile

Posts: 12365
Joined: May 14, 2002
 there is also the HY75...there is also a DH pentode of 12BY7 proportions that should have mu around 20 and significant gm to drop its plate Z into the 'very useful' category.

Pete Millet put up some fine measurements of some smaller pentodes wrt output voltage at a distortion limit, and also connected as triodes...very nice stuff.
cheers,
Douglas


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

What plate voltage? What current?, posted on February 16, 2016 at 12:42:48
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 Any tube can sound mediocre under certain conditions imposed upon it by the circuit.

 

Good idea..nt, posted on February 17, 2016 at 03:50:51
Bas Horneman
Audiophile

Posts: 4084
Joined: March 28, 2001
 .

 

RE: Driver tube suggetions for 300B amp, posted on February 24, 2016 at 23:21:33
shane.
Audiophile

Posts: 394
Location: S.A.
Joined: March 27, 2004
 I think you have about the highest quality interstage transformer available.

My vote is for CX-371 or UX-171

Shane

 

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