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1/2 12BH7??



 
 
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  #1  
Old January 18th 16, 06:52 PM posted to rec.audio.tubes
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Default 1/2 12BH7??

Hello!
Which tube is equivalent to 1/2 12BH7?

Thanks!

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  #2  
Old January 19th 16, 12:14 AM posted to rec.audio.tubes
Big Bad Bob
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Default 1/2 12BH7??

On 01/18/16 10:52, so wittily quipped:
> Hello!
> Which tube is equivalent to 1/2 12BH7?
>
> Thanks!
>


that's an interesting question. I looked at specs for 12BH7 and though
it looks a LOT like a 12AU7 or something similar, the max current and
voltage ratings are pretty high for it being a triode. It's suppposed
to be a deflection amplifier for a TV so apparently it's oriented
towards pulsed operation.

typically to get a nice sawtooth deflection wave, you'd discharge an
inductor by essentially pulsing it, then allow it to charge at a
somewhat constant current (so that voltage across it has a nice flat slope).

so a tube like that would be inclined to have a relatively constant
current with a widely varying plate voltage at some linear point.

normally the vertical output could be a relatively low power, whereas
the horizontal output needed more guts because it was also used for the
flyback high voltage anode on the TV tube.

not sure if the twin triodes are intended to be paralleled or not, but
they could be.


not sure what would be half of one of those. what are you planning on
using it for?

NOTE: 6AH4GT and 6S4 are listed in the GE tube book as a vertical
deflection amplifier triodes - maybe those?


  #3  
Old January 19th 16, 04:14 PM posted to rec.audio.tubes
Peter Wieck
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Default 1/2 12BH7??

On Monday, January 18, 2016 at 1:52:28 PM UTC-5, wrote:
> Hello!
> Which tube is equivalent to 1/2 12BH7?
>
> Thanks!


The standard equivalent in use today is the 12AU7, with perhaps a little bit less gain on the latter.

There are those who find that they are indistinguishable as drivers in low-power amps, favoring the 12BH7 over the 12AU in higher-power amps.

Peter Wieck
Melrose Park, PA
  #4  
Old January 20th 16, 06:51 AM posted to rec.audio.tubes
Patrick Turner
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Default 1/2 12BH7??

On Tuesday, 19 January 2016 05:52:28 UTC+11, wrote:
> Hello!
> Which tube is equivalent to 1/2 12BH7?
>
> Thanks!


Quite a few comments on 12BH7.
Its about equal to having two 12AU7 in parallel.
The Ra for 1/2 12BH7 is half the value for 1/2 12AU7, but is nearly equal, so BH7 has higher Gm.

This means that for a given B+, RLa and Ea and Ia, BH7 can swing a wider voltage before Ea swing is limited by Ra curve for Eg1 = 0V. The lower Ra means F response extends to higher F. Because it swings wide, its an ideal driver for output stages needing high Ea swing, such as in McIntosh amps.
I've used BH7 often for rebuilds when I found I had them around - its a very good driver, IMHO.

The only equal tube I know is 6BL7, but that's an octal tube, but then a quite fabulous twin triode with fabulous reputation same as 6SN7, or big plate 6CG7 which are 9 pinnie versions of 6SN7s.

When I first built a decent preamp in 1994, I tried SRPP and I could not tell any difference in sound between 12BH7, 12AU7 or 12AT7 in a line stage amp; you can build the SRPP to suit all 3 without changing Rk. IN SRPP, there is no RLa, the tubes are in series with no anode resistor. But signal is less than 1Vrms, so no matter what you use, it works, and you only have to worry about gain.

Patrick Turner

  #5  
Old January 20th 16, 08:55 AM posted to rec.audio.tubes
Big Bad Bob
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Default 1/2 12BH7??

On 01/19/16 22:51, Patrick Turner so wittily quipped:
> This means that for a given B+, RLa and Ea and Ia, BH7 can swing a wider voltage before Ea swing is limited by Ra curve for Eg1 = 0V. The lower Ra means F response extends to higher F. Because it swings wide, its an ideal driver for output stages needing high Ea swing, such as in McIntosh amps.


that's an interesting description of possible usage in audio amps. I
was looking more at its intended function, but that being said,
deflection amplifiers would need to have large signal linearity or you
get a distorted picture.

Being designed for (in essence) a current source for horizontal or
vertical deflection oscillator [or apparently a vertical deflection
output stage?] it would have to use a larger linear region to make the
sweep flat. That being said, "wider voltage swing" makes sense.

I wouldn't be surprised to see these in old B&W sets from the 1950's.


  #6  
Old January 23rd 16, 11:23 PM posted to rec.audio.tubes
Patrick Turner
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Default 1/2 12BH7??

On Tuesday, 19 January 2016 05:52:28 UTC+11, wrote:
> Hello!
> Which tube is equivalent to 1/2 12BH7?
>

Mr Wieck typed....

"The standard equivalent in use today is the 12AU7, with perhaps a little bit less gain on the latter.
There are those who find that they are indistinguishable as drivers in low-power amps, favoring the 12BH7 over the 12AU in higher-power amps."

Just who says the 12AU7 "the standard equivalent" to 12BH7? The 12BH7 has a much lower Ra and higher Gm, and although both BH7 and AU7 are able to be used in the same audio circuit along with 6CG7, the BH7 has a significantly higher Ea swing because the Ra curve for EG1=0 is a more vertical line than in the AU7 or CG7.
I built and sold countless new and re-engineered amps to my customers, and some used BH7 for driver tubes, especially where drive voltage to output grids exceeded 50Vrms max. McIntosh used 12BH7 to drive 6550 output tubes where drive voltage could exceed 150Vrms, because the 6550 was set up with its anode and cathode Vac being from 1/2 of the two OPT primary windings for anode and cathode, ie, where Ea = +450V, Va-k in each 6550 could be 400Vpk, ie 280Vrms, so Va = Vk = 140Vrms, with opposite phases.
If the 6550 gain = 10, then Vg-k = 28Vrms, so Vg-0V = 28+140 = 168Vrms!!
To achieve this easily, the dc carrying anode RL of BH7 in MC60 = 12k and fed from a same phase point on OPT with 140Vrms, ie, the BH7 anode was bootstrapped to make its anode load an effectively a much higher number of ohms, about 100k, while keeping Ea of BH7 about +380V. Its a case of mild positive FB, which tends to boost THD, but because the BH7 has low Ra, the PFB effect is minimized because the Ra of BH7 and Rla 12k form a resistance divider reducing PFB by -12dB approx.
But no matter how how high the value of BH7 RLa, the triode still has to swing the rather high anode swing, and my load line analysis reveals BH7 is better than AU7 at this job. I figured BH7 could swing just over 230Vrms max, and AU7 over 205Vrms. McIntosh would have sussed out all this long ago. But now most ppl will miss making distinctions between tube types because they have forgotten how to do load line analysis and they refuse to ask WHY very much and they make a statement based on superficial assessment of tube properties.

I have not included the consequences of McIntosh's use of a cathode follower after BH7 to directly drive output tube grids, but they do have CF and with bootstrapped cathode RL, so that output tubes can be driven a bit AB2, so squeeze the very most anyone can from a 6550, while keeping the total anode load ohms for BH7 high as humanly possible, and affordable, without using a separate choke feed to BH7 anodes. The CF has high gain but follower connection reduces this to just under 1.0, so its gain effects on BH7 gain may be neglected here in this context.

The AU7 used to replace BH7 in McIntosh MC60 will mean gain of driver stage is reduced slightly, because Gain A = x RL / ( RL + Ra ) and the higher Ra for AU7 always means gain will be lower. If the Va = 168Vrms max, and BH7 gain = 16, then its grid drive = 10.5Vrms, and AU7 might need 12Vrms, but all this is while the OPT voltages have not sagged much with load; when they do sag, the AU7 gets less bootstrapping so its gain falls more than BH7,and correctional effects of GNFB is reduced. Its just another reason why McIntosh favoured BH7.


So just how you use 12BH7 or 12AU7 should be carefully considered if you want the best possible performance, ie, wide Va swing, low THD, and reliability, and low Ra effect on bandwidth and FB.

But where the BH7 or AU7 are used in circuits for low Va swings including preamps, their function is very similar, and both are similar to 6CG7 / 6SN7 / 6BL7 and maybe a few ECC euro twin triodes. All these medium triodes produce hi-fi audio amplification so easily......

Patrick Turner.

  #7  
Old January 25th 16, 08:08 AM posted to rec.audio.tubes
Big Bad Bob
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Posts: 366
Default 1/2 12BH7??

On 01/23/16 15:23, Patrick Turner so wittily quipped:
> On Tuesday, 19 January 2016 05:52:28 UTC+11, wrote:
>> Hello!
>> Which tube is equivalent to 1/2 12BH7?
>>

> But no matter how how high the value of BH7 RLa, the triode still has to swing the rather high anode swing, and my load line analysis reveals BH7 is better than AU7 at this job.


probably because it was designed specifically FOR that (vertical
amplifier/oscillator in a TV, requires large linear region for wide
plate voltage swing)

makes sense to me. I'll consider this tube as a driver for beam power
tubes in the future.

  #8  
Old January 28th 16, 03:52 AM posted to rec.audio.tubes
Patrick Turner
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Posts: 3,964
Default 1/2 12BH7??

Big Bad Bob
25 Jan


probably because it was designed specifically FOR that (vertical
amplifier/oscillator in a TV, requires large linear region for wide
plate voltage swing)

makes sense to me. I'll consider this tube as a driver for beam power
tubes in the future.

Its good, even if you can't make any sense of this now.
McIntosh reckoned they were just right in their amps.

Tubes were rarely ever limited to ONE purpose the designers may have had in mind.

And I still make gear just to fill in time, no particular purpose at all, and last month I got an RF gene going real well, with AM and FM, and I used tubes which seem quite out of place, such as 6CW5/EL86 for a triode concertina phase splitter & buffer for RF up to 2MHz. Worked better than 12AU7 and other tubes I tried. Sometimes, if you think outside the square, you look back at the square, and you see its a circle.......

Patrick Turner.
  #9  
Old January 28th 16, 02:36 PM posted to rec.audio.tubes
Peter Wieck
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Posts: 2,418
Default 1/2 12BH7??

On Saturday, January 23, 2016 at 6:23:51 PM UTC-5, Patrick Turner wrote:
> On Tuesday, 19 January 2016 05:52:28 UTC+11, wrote:
> > Hello!
> > Which tube is equivalent to 1/2 12BH7?
> >

> Mr Wieck typed....
>
> "The standard equivalent in use today is the 12AU7, with perhaps a little bit less gain on the latter.
> There are those who find that they are indistinguishable as drivers in low-power amps, favoring the 12BH7 over the 12AU in higher-power amps."
>
> Just who says the 12AU7 "the standard equivalent" to 12BH7? The 12BH7 has a much lower Ra and higher Gm, and although both BH7 and AU7 are able to be used in the same audio circuit along with 6CG7, the BH7 has a significantly higher Ea swing because the Ra curve for EG1=0 is a more vertical line than in the AU7 or CG7.


Snippage:

> Patrick Turner.


Two people on our little block are instrument amp types.

Explains much.

Peter Wieck
Melrose Park, PA

  #10  
Old February 2nd 16, 02:48 AM posted to rec.audio.tubes
Big Bad Bob
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Posts: 366
Default 1/2 12BH7??

On 01/28/16 06:36, Peter Wieck so wittily quipped:
> On Saturday, January 23, 2016 at 6:23:51 PM UTC-5, Patrick Turner wrote:
>> On Tuesday, 19 January 2016 05:52:28 UTC+11, wrote:
>>> Hello!
>>> Which tube is equivalent to 1/2 12BH7?
>>>

>> Mr Wieck typed....
>>
>> "The standard equivalent in use today is the 12AU7, with perhaps a little bit less gain on the latter.
>> There are those who find that they are indistinguishable as drivers in low-power amps, favoring the 12BH7 over the 12AU in higher-power amps."
>>
>> Just who says the 12AU7 "the standard equivalent" to 12BH7? The 12BH7 has a much lower Ra and higher Gm, and although both BH7 and AU7 are able to be used in the same audio circuit along with 6CG7, the BH7 has a significantly higher Ea swing because the Ra curve for EG1=0 is a more vertical line than in the AU7 or CG7.

>
> Snippage:
>
>> Patrick Turner.

>
> Two people on our little block are instrument amp types.
>
> Explains much.


probably - it's something I'd want to do [again].

looking for output transformers, ran across THIS one.

https://www.amplifiedparts.com/products/P-T125A

it's a 3W somewhat-universal output transformer [push pull] with a
complicated secondary winding that lets you configure the primary Z,
apparently.

My guess is that it would make an interesting TRIODE OUTPUT STAGE using
the 12BH7 and this transformer.

bass response isn't ideal, perhaps, but it might work well as a small
P.A. or "distortion" guitar amp where you want to create a really nice
overdrive sound, but at a relatively low output power...



 




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