so why a 5751 in the PI not a 12au7 or 12at7?

[nomad100]

so why a 5751 in the PI not a 12au7 or 12at7?

 

[kc2eeb]

Actually, there's no reason you can't use a 12AT7 or 12AU7.
The 12AT7 is what Fender used for years in their amps.
The difference is gain AND current.

 

[Whoopysnorp]

I've been using a 12AT7 in the PI of my RM100 for a little while now. It's a subtle difference (to my ears at least) but it seems to improve the clarity.

 

[BigBrewtus]

I was just reading another thread on this. I believe that mainly the answer is the output of these. 5751s seem to still have very good output but help reduce gain. I use JJ 12DW7 in PI will great results. It is designed to do this very thing. Bit pricey though.

 

[Anomaly]

been looking to get 5751 to tame the gain of my JF Erect, how are the new production (sovteks) compared to the pricey NOS? don't really want to spend much on pre-tubes..

 

[kc2eeb]

BigBrewtus,
A 12DW7 is 1/2 a 12ax7 and 1/2 a 12au7, so 1/2 of the tube has a gain factor of 100 and the other half has a gain factor of 17. So, if you're using a 12DW7 in the PI position, it's unbalanced by a factor of 100 to 17.
How's that sound?

 

[Mattfig]

been looking to get 5751 to tame the gain of my JF Erect, how are the new production (sovteks) compared to the pricey NOS? don't really want to spend much on pre-tubes..

Get something with JAN in it if it's for tone...I know that sounds a bit weird but, in my experience

JAN-Sylvanias are the top
JAN-GEs are great
JAN-Phillips are also great

I would guess though that for simply a PI, a Sovtek or any brand should be fine....

 

[BigBrewtus]

BigBrewtus,
A 12DW7 is 1/2 a 12ax7 and 1/2 a 12au7, so 1/2 of the tube has a gain factor of 100 and the other half has a gain factor of 17. So, if you're using a 12DW7 in the PI position, it's unbalanced by a factor of 100 to 17.
How's that sound?

Great output with clarity added. All modules sound more like themselves. If that makes any since. I use less presence than before and all my mods seem to react to EQ changes better.

 

[nomad100]

Does anyone know the theory behind why it work? Is it less compression?

 

[kc2eeb]

The theory is simple. When you drive a signal into "clipping" the highs
are cut, the wave becomes more square, and it sounds like a fuzz box. A lower gain tube, like a 12AU7 0r 12AT7 will clip less or not at all, so, not compressed.
BigBrewtus-
The PI circuit in a Randall MTS is very similar to a Fender Twin long tail pair. It uses the same 82K and 100K plate resistors. Putting a 12DW7
in the PI position is attenuating one of the wave swings going to the output tubes. It's like switching a humbucker to single coil, (not really, but
the results are similar), thinner sound but more highs.
If you have an oscilloscope, try comparing a 1k sine wave input to the
speaker output with the 12DW7 in place. You will see a very asymetric output waveform.

 

[BigBrewtus]

The theory is simple. When you drive a signal into "clipping" the highs
are cut, the wave becomes more square, and it sounds like a fuzz box. A lower gain tube, like a 12AU7 0r 12AT7 will clip less or not at all, so, not compressed.
BigBrewtus-
The PI circuit in a Randall MTS is very similar to a Fender Twin long tail pair. It uses the same 82K and 100K plate resistors. Putting a 12DW7
in the PI position is attenuating one of the wave swings going to the output tubes. It's like switching a humbucker to single coil, (not really, but
the results are similar), thinner sound but more highs.
If you have an oscilloscope, try comparing a 1k sine wave input to the
speaker output with the 12DW7 in place. You will see a very asymetric output waveform.

Wow. You know your stuff. Dont have a scope. I did notice less over all bass, which is fine because my cabinet produces so much. Does this harm anything?

 

[m0jo]

I believe the difference is that 12AU7's are simply less gain then a 12AX7 bit a 5157 is different in that it puts out less voltage but more current or something?
This isn't helping so far. :lol: But the effect is they both reduce overdrive in that stage but the 5157 should retain the volume I think.

 

[kc2eeb]

Using the 12DW7 shouldn't harm anything and actually is a simple way to
get the effect you need which would otherwise entail a lot of modification to the capacitive/resistance arrangement of the PI to get the same result.
The tube is plug and play.

 

[m0jo]

Using the 12DW7 shouldn't harm anything and actually is a simple way to
get the effect you need which would otherwise entail a lot of modification to the capacitive/resistance arrangement of the PI to get the same result.
The tube is plug and play.

Theorethically it could wear your powertubes unevenly though right?

This is theorethically though, but I'd keep an eye on the bias balance of the tube set

 

[kc2eeb]

You can see the current the output tubes are actually drawing by simply leaving the multimeter connected to the bias check points and playing the amp! It's not completely accurate since it also includes the screen resistor, but for all practical purposes, it's perfectly fine.
With the RM100, if you had 4 meters you could monitor each tube while you're actually playing.
The RM50 or RT2/50 would need a simple "Y" connection for the ground,
and you could monitor each tube.
When you watch that the tubes draw up to 200 ma. depending on the tube,
you can understand why the HV current fuses have the amp rating they have.
Bias is like setting the idle speed on an engine. You only see that when you're standing still.
I don't think I would worry too much about it, though.
Playing a high gain module or using a distortion box starts to approximate a 100% duty cycle which greatly increases the heating of the output transformer windings and speaker voice coils. It starts to act like DC rather than AC. Modern amps like the Randalls are designed to handle that
all day long. So are the speakers.
That's why all the old amps used to blow transformers, speakers, and screen resistors once musicians turned them up. There were no "valleys"
between the "peaks."

 

[withmittens]

I found some old tubes in some old hifi gear left by the side of the road. got myself a tester at a yard sale. I have old telfunken and RCA 12au7a and 12au7 tubes. In P1 I immediately hear less gain, less fizz and less volume. This is allowing me to crank the power section way past what I would have been able to do before. I get great power tube saturation now. huge sound.

another reminder that grind comes from power, no way to fake it with gain. volume rules

 

[nomad100]

Some amps let you mute the signal to one side of the push pull giving a quozi SE power section. Your 12DW7 probably somewhat doing this, and in my opinion i love SE amps.