Mullard 5-10 input stage needed?

[Schmitz77]

Schmitz,
I've seen, and heard, an amp that started with an LTP splitter then had a gain/driver stage for the power tubes. A bit different I grant you, but it sounded pretty darn good.

Yes, there are all exotic kinds of circuits possible for amplifiers.
I have seen a schemo where the output transformer of a power amp sits right at the cathode of the output tube. Does that make sense at all or is something that is achievable? Maybe not, but for practice, it seems to work either.

 

[Francois G]

In the Mullard 5-10 topology if I do not need the gain of the first stage can I apply the input signal directly to the longtail splitter? I am thinking 12AT7 splitter with CCS.

If you drop the first gain stage, and you certainly could, you no longer have the Mullard 5-10 topology, but something else. I don’t think you mind something else

There are many examples of amplifiers designs like you have in mind. Search the Tube Forum for “Baby Huey”, “The Musical Machine”, and “El Cheapo”, which actually uses a 12at7 as LTP with CCS. All of these designs are reported to sound very good. You can find the basic “El Cheapo” schematic below, originally intended for 12AQ5 outputs, but this design had been used successfully with a variety of output tubes, including EL84.

https://www.diyaudio.com/forums/att...eapo-builders-thread-elcheapo-23jun06-map-gif

 

[jderimig]

Nobody removes the input gain stage in front of a phase splitter in a tube amp.
It may be changed to different gain, but never removed.
If you have problems with too high input voltage, reduce it like everyone else does it.

Never read of impedance matching in a tube amp and under what circumstances each stage in an amp functions best? Read some of the basic tube educational literature, its free available on the net.
Have you've ever seen an amp that starts in topology with a phase splitter stage?

No, its not. What defines the output stage of your source. The type of tube you use or the type of circuit it uses? I would say your using a cathode follower stage at the output.
Does this match quite good with a phase splitter input?
You have to think in block schematics and to study what each block has input and output impedances, loadlines and headroom for signal amplifying. Then you will reach the goal.

So the "El Cheapo" looks to be exactly the input stage I was discussing in the open. So is this a bad design because its missing an input stage? I am tempted to build this but

with different output tubes.

 

[dgta]

El Cheapo is a good design. So are many others that consist of just one LTP and one PP output stage. There is no such thing as "missing the input stage". A good design can have any number of stages.
Caution: If you don't know what you're doing when it comes to design, build the exact design that was published. Don't modify it in any way without getting knowledgeable advice.

 

[jderimig]

Caution: If you don't know what you're doing when it comes to design, build the exact design that was published. Don't modify it in any way without getting knowledgeable advice.

That is always great advice!



Look forward to my new upcoming thread "El Cheapo with 1625 output tubes".

 

[Schmitz77]

So the "El Cheapo" looks to be exactly the input stage I was discussing in the open. So is this a bad design because its missing an input stage? I am tempted to build this but

with different output tubes.

You also may ask: Is the Ford Model T (Tin Lizzy) a bad design, because its only two door, little space for luggage, bad driving comfort, no air suspension and yes, its able to drive on a road and bring people from A to B.


My answer: Yes, to me its a bad, El Cheapo design and it doesn't look like a whole, complete amp because the input stage is missing.
I would never build such an amp, because there are many other options to match levels to any amplifier. But I do think, you don't know any of those many options to match any amplifier to any signal level so you have to go such strange ways.

 

[jderimig]

But I do think, you don't know any of those many options to match any amplifier to any signal level so you have to go such strange ways.

]


No I don't have to in such strange ways. Just trying to learn. And I have learned some in this thread.

 

[dgta]

..."El Cheapo with 1625 output tubes".

Tell us why you think that's a good idea.

 

[jderimig]

Because I have a whole bunch of them.

 

[tikiroo]

The baby huey / el cheapo / musical machine topology only works well with a high sensitivity tube like the el84, preferably triode or ultralinear connected as there is not enough gain for much global negative feedback. 1625 / 807 type tubes are not the best choice for this.

 

[duncan2]

807,s were used during WW2 in communications receivers like the R1155 by RAF Lancasters to provide VHF transmitter communication between bomber crew.



Also used by radio "Hams " , not much gain as Tikiroo says --best with a ceramic base .

 

[jderimig]

The baby huey / el cheapo / musical machine topology only works well with a high sensitivity tube like the el84, preferably triode or ultralinear connected as there is not enough gain for much global negative feedback. 1625 / 807 type tubes are not the best choice for this.

I am finishing up an RH807se project now. Well, I have a bunch of EL84's and EL34's too.

 

[dgta]

The baby huey / el cheapo / musical machine topology only works well with a high sensitivity tube like the el84, preferably triode or ultralinear connected as there is not enough gain for much global negative feedback. 1625 / 807 type tubes are not the best choice for this.

Really? Did you look up the El Cheapo schematic and notice it's using 12AQ5 and not EL84? Do you know how the gm of 12AQ5 compares to that of the 1625? How about mu in triode?

 

[tikiroo]

That advice was from Eli, the designer

 

[dgta]

That advice was from Eli, the designer

Can you show exactly where he said that?

 

[dgta]

That advice was from Eli, the designer

I'll help you out. Is this the advice you saw?

The 12AQ5 was chosen for its low cost. Remember, this is a tightwad's project. ANY member of the 6V6 clan will work using unchanged signal path parts values.

LOL. I don't see anything about the gm/mu of an EL84 being required. Do you?

El Cheapo, builders thread

 

[jderimig]

From a quick glance at the datasheets the 6V6 gm ~4000, the 807 is ~6000. If these number are correct does this mean the 807 has higher sensitivity than a 6V6? Or do I have this backwards?

 

[Schmitz77]

Really? Did you look up the El Cheapo schematic and notice it's using 12AQ5 and not EL84? Do you know how the gm of 12AQ5 compares to that of the 1625? How about mu in triode?

Of course it do, because the voltage gain stage has been eliminated and the phase splitter has gain approx. one.
It makes sense to build a complete amp and not a part of it.

 

[tikiroo]

No, it has been repeated by him many times when people raise the idea of different output tubes. It's not sensitivity per se, but the voltage gain required to drive to full output. The 12AT7 LTP has a gain of about 18. With a 2Vrms input signal (Eli's design paramater as that is what a CD player outputs), maximum driver swing is 51V peak with no gNFB, so this represents the maximum bias voltage you can have to drive to full output with no gNFB. With typical 12 dB gNFB, 2V rms will result in a peak drive voltage of 12.7V. A 12AQ5 triode connected and an el84 both happen to bias up near there. I've done it with a triode connected 7951 biased at 17V with reduced gNFB.

A triode connected 1625/807 will likely run at ~25V bias, so only 6dB gNFB will be available to maintain the 2Vrms sensitivity. Or you could increase the drive voltage. Of course this might be acceptable but it's moving away from the design parameters that Eli was using.

 

[jderimig]

A triode connected 1625/807 will likely run at ~25V bias, so only 6dB gNFB will be available to maintain the 2Vrms sensitivity. Or you could increase the drive voltage. Of course this might be acceptable but it's moving away from the design parameters that Eli was using.

What if the 1625 is not triode connected but instead running the screen just under 300v like the RH807 topology?