Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on June 12, 2016, 11:48:03 am
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Jbefumo has build an amp where a paralleled 6SL7 tube feeds an EF86
(http://el34world.com/Forum/index.php?action=dlattach;topic=20353.0;attach=58902;image)
consider that the EF86 is near a 6SJ7 tube
EF86
(http://www.mif.pg.gda.pl/homepages/frank/short/001/e/EF86.gif)
6SJ7
(http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6SJ7.gif)
in the Selmer Zodiac 30 a 12AX7 feeds an EF86 while in the Matcless Clubman a 12AX7 tube feeds a 6SH7 tube
http://el34world.com/charts/Schematics/files/selmer/selmer_zodiac_twin30_.pdf (http://el34world.com/charts/Schematics/files/selmer/selmer_zodiac_twin30_.pdf)
http://el34world.com/charts/Schematics/files/matchless/matchless_clubman.pdf (http://el34world.com/charts/Schematics/files/matchless/matchless_clubman.pdf)
EF86 is near a 6SJ7 but 6SH7 is a bit different tube
6SJ7
(http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6SJ7.gif)
6SH7
(http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6SH7.gif)
the 6SJ7 has a sensibility of 1.65 mA/V and 6SH7 4.9mA/V
So ... what will happen if, in a circuit similar to 5C1, a 6SH7 is used in place of a 6SJ7 tube ?
Will this emulate the use of a tube that precede the 6SJ7 and feed it with a higher signal than the signal that is coming from the guitar ?
Franco
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the 6SJ7 has a sensibility of 1.65 mA/V and 6SH7 4.9mA/V
Not sure on the swapping tubes, my understanding of sensitivity, the SJ needs about 3 times less signal in to get same signal out.
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May be my brain is in confusion, at the moment he say a higher mA/V is relative to a higher sensibility :rolleyes: :rolleyes:
EDIT: OK, now I'm sure higher mA/V = Higher sensibility
this was told by PRR in an old thread
EL84 like half the drive of 6V6; EL84 has nearly twice the sensitivity of most other audio power tubes.
EL84
(http://www.mif.pg.gda.pl/homepages/frank/short/001/e/EL84.gif)
6V6
(http://www.mif.pg.gda.pl/homepages/frank/short/001/6/6V6.gif)
as you can see EL84 is rated near 11mA/V, the 6V6 only 4.1mA/V
Franco
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as you can see EL84 is rated near 11mA/V, the 6V6 only 4.1mA/V
I stand corrected :BangHead:
In my field, we needed to discern signals just above the *grass-noise*, the smallest signal discernible, we called, the receivers' sensitivity, ie, I can *see* 1mV above noise. But I mixed my metaphors, 1mV above noise has NOTHING to do with 11mA/V :think1:
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May be here there is something of interesting to read for you
http://el34world.com/Forum/index.php?topic=16111.msg157242#msg157242 (http://el34world.com/Forum/index.php?topic=16111.msg157242#msg157242)
Franco
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... consider that the EF86 is near a 6SJ7 tube ...
I really recommend Frank's (https://frank.pocnet.net) for tube data sheets.
The EF86 (http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EF86.pdf) and 6SJ7 (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6SJ7.pdf) have similar Gm, but the triode Mu is different; Mug1-g2 of the EF86 is 38, where it is 19 for the 6SJ7.
Why do you care? Because the Mu from grid to screen impacts how the screen controls plate current. As a result, if you wanted to bias the 6SJ7 to the same operating point you have with the EF86 in its circuit, while using the same cathode resistor (and so same plate current/bias voltage), the 6SJ7 screen will need to be at a different voltage.
To give an example of how different the biasing is, look at the "Typical Operation" on the 6SJ7 data sheet linked above, where 250v is on the plate and 100v on the screen. The EF86 data sheet doesn't give the same tabulated condition, but look at the graph on page 6. This graph is for various screen voltages with the plate held at 250v; Look at the Vg2=100v curve... For this condition, the 6SJ7 had a plate current of 3mA with a bias of -3v, but the EF86 with the same bias has a plate current of 0.3mA.
So you will not be able to plug a 6SJ7 directly into an EF86 circuit using the same parts values and get similar results. You'll likely need to go through a pentode gain stage design process to select voltages & parts values which cause the 6SJ7 to perform the way the EF86 did in the stage you're replicating. Unfortunately, there are more variables & more work to designing a pentode gain stage than a triode gain stage.
the 6SJ7 has a sensibility of 1.65 mA/V and 6SH7 4.9mA/V
So ... what will happen if, in a circuit similar to 5C1, a 6SH7 is used in place of a 6SJ7 tube ?
Assuming you have accurate Gm figures for the operating point of the tube, then for any pentode gain stage the actual amplification is Gm * RL, where RL is the load resistance (plate load resistor in parallel with any resistance to ground which follows). So the pentode with the higher Gm will exhibit more voltage gain with the same plate load resistor.
However, Gm is not constant; it must be evaluated at the chosen operating point, especially because data sheet Gm figures may be typical of what you'd get in a specialized RF circuit and wildly different than what you could realistically get in a guitar preamp. Gm for any tube usually increases with rising plate current.
If we take the same 250v plate, 100v screen, -3v of bias when evaluating the 6SH7 (http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6SH7.pdf), then the graph on page 2 of that data sheet shows plate current at 0.4mA (rather than the 6SJ7's 3mA), while the upper graph of page 3 shows Gm is only 700 micromhos (0.7mA/V) instead of the 6SJ7's 1.65mA/V.
So a direct-swap of the 6SH7 for the 6SJ7 with no supply voltage or parts changes will result in a stage with less gain, although it is uncertain without a fuller analysis of where plate/screen/bias voltage and plate/screen current will land.
Looks like none of these pentodes are as-similar as they first appeared.
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Many thanks HotBluePlates
My knowledge is so poor and my flight of fancy too ambitious
I always end having reasons to be embarrassed :embarrassed:
too much imagination corroborated by too poor knowledge :think1:
Franco
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I think a 6SH7 can be a fine voltage amplifier. But you will want to adjust the screen and cathode resistors.
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Thanks PRR
I was thinking that the 6SH7 can be used the same way of other tubes (pentodes we see commonly on preamps)
I referred to the Matchless Clubman schematic where Plate Resistor is 220k and Cathode Resistor is 2.2k, values that can be find often when there is a pentode as preamp
Matchless
(http://i.imgur.com/f6bN0Rv.jpg)
Selmer
(http://i.imgur.com/VqMytgU.jpg)
so I assumed (wrongly) that it can be used "exactly" as other pentodes seen in preamps
EDIT: Between the two pentodes arrangement of this schematics there is a big difference, B+
on the Matchless B+ at V1 is 205V and on V2 is 410V
on the Selmer B+ at V1 is 300V and on V2 is 340V
more, V1 has a different arrangement
Franco
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really recommend
+1
That was the 2nd book-mark I saved after finding this place!
thx franco
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Ciao Shooter
The merit is of HotBluePlates, not mine :smiley:
Franco
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not mine
Ya, but you left the breadcrumbs :laugh:
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Many thanks HotBluePlates
My knowledge is so poor and my flight of fancy too ambitious
I always end having reasons to be embarrassed :embarrassed:
too much imagination corroborated by too poor knowledge :think1:
Franco
franco,
Please continue to ask the questions, whether you are going the wrong direction, or not the comments posted from the forum tend to provide learning opportunities.
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... I was thinking that the 6SH7 can be used the same way of other tubes (pentodes we see commonly on preamps)
I referred to the Matchless Clubman schematic where Plate Resistor is 220k and Cathode Resistor is 2.2k, values that can be find often when there is a pentode as preamp ... so I assumed (wrongly) that it can be used "exactly" as other pentodes seen in preamps
EDIT: Between the two pentodes arrangement of this schematics there is a big difference, B+
on the Matchless B+ at V1 is 205V and on V2 is 410V
on the Selmer B+ at V1 is 300V and on V2 is 340V ...
With triodes, it is fairly easy to derive the complete characteristics of the stage with just a knowledge of the resistors used & the supply voltage. Unfortunately, this is not the case with pentodes. I just spent an hour squinting at the 6SH7 data sheet and got nowhere. Without at least a cathode voltage to help derive some other values, there's no way to compare how the two different tube perform, even with the same parts values.
Of course, throwing each on a breadboard would provide the needed (missing) voltages...