807pp + input phase splitter

[Jaap]

Why does the lower tube not have 100K from grid to ground ?

 

[jane]

Upper EF80 works, and as I can see on the scope, there is a big improvement.. but I can't listen how it sounds because lower EF80 still doesn't split anything !

There is no input signal to the lower tube so there will be no output signal either..

 

[emk2]

Why does the lower tube not have 100K from grid to ground ?

Lower grid is directly connected to ground, why adding a 100K in //

There is no input signal to the lower tube so there will be no output signal either..

It seems that I'm too stupid to understand how a two tubes splitter works, I listened to the left channel like this and the right channel not yet modified (with input transformer). No bass on left channel and strange sound, like out of phase.

 

[Shoog]

Upper EF80 works, and as I can see on the scope, there is a big improvement.. but I can't listen how it sounds because lower EF80 still doesn't split anything ! tube seems to be good, led on, plate voltage ok. But no signal at all, tried to change the probe, same result.

Here is what I did.

You have built a Long Tail Pair phase splitter and then removed the Long Tail by using LED's. The input signal for the bottom pentode should come through the cathode not the grid1. In many LTP's you can simply ground the second Pentode grid (but you lose half the gain). There is no way for the input signal to communicate to the second Lower Pentode. I would advocate using at least a stiff resistor to emulate a long tail (though a CCS is much better). Look at how I achieved a LTP phase splitter with input transformer in my Tabor to understand how you could achieve the same without having to introduce a negative rail or an input capacitor.

Shoog

 

[sajti]

Upper EF80 works, and as I can see on the scope, there is a big improvement.. but I can't listen how it sounds because lower EF80 still doesn't split anything ! tube seems to be good, led on, plate voltage ok. But no signal at all, tried to change the probe, same result.

Here is what I did.

Take away the LEDs from the cathode of the EF80! Connect together the cathodes, and apply resistor to the ground. It will be slightly better.
But the best solution would be ccs in the cathode, like the attached picture.

Sajti

 

[mwiebe]

I’m not sure what’s wrong, the topology works. The LEDs do ground AC which would have an impact on how it sounds, but it should still work.

Have a look at some other similar designs.

JC Morrison’s 6CK4 P-P
http://web.archive.org/web/20101205...fortunecity.com/saints/668/primer/6ck4-pp.gif

Eric Kingsbury’s Music Machine
6V6 Amp Audio Circuit; Parts Expo

Kevin Kennedy’s 6CK4 P-P
http://www.kta-hifi.net/projects/amp_page/6ck4amp/6ck4amp_c.pdf


El Cheapo a slight variation with feedback,
http://www.diyaudio.com/forums/tubes-valves/108399-up-date-el-cheapo-schematic-2.html#post1883565

One version of Gary Pimm’s Tabor,
http://www.pimmlabs.com/web/Tabor3s.gif

Hopefully you will see something that will make a difference.
Matt

 

[Shoog]

Your input transformers make life massively more straightforward.

Shoog

 

[mwiebe]

Shoog,
I used to build with input phase splitting transformers or autoformers, or interstage versions of the same. I kept hearing so many good reports of Kingsbury’s Music Machine that I finally had to try it. So I pulled out the input transformer and now my favorite P-P amp uses that input topology and I couldn’t be happier. But input splitting does work better, vastly better, with very high inductance on the input tube cathodes.
Matt

 

[Shoog]

Shoog,
I used to build with input phase splitting transformers or autoformers, or interstage versions of the same. I kept hearing so many good reports of Kingsbury’s Music Machine that I finally had to try it. So I pulled out the input transformer and now my favorite P-P amp uses that input topology and I couldn’t be happier. But input splitting does work better, vastly better, with very high inductance on the input tube cathodes.
Matt

I always use the Pink Mouse LED referenced CCS circuit for mine now.
The great thing with using CCS to bias up both your input stage and your output stage is that when you get the load resistor values right, everything is always self adjusting regardless of what tubes you plug into it. My last two amps have been DC coupled and universally stable. Its magic of the best kind.

Shoog

 

[emk2]

Thanks all for your comments.

I tried again with input transformer but only 4K across secondaries and led's were replaced by unbypassed resistors. Sound has changed, now it is "slow" - not dynamic at all - with boomy bass, more "tubey" but always this unpleasant forward treble. I'll try with bypassed Rk.

 

[GoatGuy]

Sigh... without the bypass capacitor(s) (Rk) on the cathode resistors, you get a lovely amplifier that becomes a cathode-follower ... inverter. It will change the GAIN by quite a bit, typically by a factor of 5 or more.

The LEDs act as "constant voltage sources". LEDs are used 'cuz they're not too far from an optimal bias voltage (2V) (for some tubes, others are quite different!), and also because they are pretty, and light up, and "show" operation, including brightening and dimming as the music program changes. Otherwise, they're not somehow "special" at all. Zeners could serve the same purpose, as could strings of ordinary silicon diodes, in forward conduction, in series to build up the voltage desired.

Resistors in the cathode REQUIRE the Rk in order to maintain mu-dominated gain. Otherwise, the whole front end becomes more linear. (yes, that too!) ... You've effectively linearized the front-end stage.

Also... did you measure plate voltage? I'm betting that there is a significant difference between the tubes. Remember ... in the mu-dominated mode, it really is best to adjust the Rk cathode resistance values so that both tubes are operating near the same plate-voltage point when quiescent (doing nothing).

GoatGuy

 

[GoatGuy]

PS: My "spidey sense" is making little itchy noises - that perhaps your speaker subsystem either isn't all that flat in response, or that things along the audio path are coloring the sound, which you're compensating for at other stages. Like trying to get a bunch of kids to hold a long soapy (wet) water-filled water-balloon. Almost impossible - they're all compensating for each other, and inevitably it bursts. Yet, if they just put it down, with no force whatsoever, it stretches out and assumes a simple, smooth, linear configuration.

I think this with audio as well. The more things added along the path, and the less critical thinking has gone into minimalism, the more likely that all the things contradict and countermand each other. Which... becomes an exercise in futility.

GoatGuy

 

[Shoog]

My guess is your not going to get there without some form of feedback.
My guess is that you are hearing the 3rd to 6th harmonic components brought to the mix by the 807.

Let me quote from someone who attempted to use the 807 in triode mode;

The tubes fell into three categories: "yawn", "very nice", and "great". In the "yawn" category is the 807 tube. I can't recommend this tube in triode mode at all. The RCA manual doesn't recommend it either, and now I know why. It doesn't even work for background listening.

http://www.enjoythemusic.com/magazine/manufacture/1108/

Shoog

 

[emk2]

Also... did you measure plate voltage? I'm betting that there is a significant difference between the tubes. Remember ... in the mu-dominated mode, it really is best to adjust the Rk cathode resistance values so that both tubes are operating near the same plate-voltage point when quiescent (doing nothing).

Difference between the EF80's plate voltage is about 2-3V.

I agree with your next message but there is definitively a problem with this amp, because when I switch on the tripath, all is well.

My guess is your not going to get there without some form of feedback.
My guess is that you are hearing the 3rd to 6th harmonic components brought to the mix by the 807.

Let me quote from someone who attempted to use the 807 in triode mode;

I am open to try other tubes, I have KT66, EL34, 6550,...

OPT's are 4.5K with UL taps.

 

[jrenkin]

I could be off base, but isn't 4.5K load a bit low for any of these tubes, including the 807 in PP? I would think 7-8K is more reasonable and 4.5K would be better for a SE 807.

 

[Shoog]

Difference between the EF80's plate voltage is about 2-3V.

I agree with your next message but there is definitively a problem with this amp, because when I switch on the tripath, all is well.



I am open to try other tubes, I have KT66, EL34, 6550,...

OPT's are 4.5K with UL taps.

I use the 807 in pentode mode with plate to plate feedback and it works out fine - a lovely sounding amp.
You have a number of different issues here;
- the comparison with the tripath is not relevant because the tripath has a damping factor probably in the 10 to 100's where as your amp probably in the 0.1 or less range. This means it cannot control speaker resonances or impedence excursions due to crossovers, the right amount of feedback will solve this
- your input transformer has been ringing which has been part of your harsh sound issue, getting the loading right will solve this.
-your other harshness issue is from high order harmonic distortion in the 807, since you overdamped your input transformer you are now hearing this residual. Again the right amount of feedback will solve this.

You cannot make the sort of amp you are attempting without using feedback or very low impedence triodes such as the 6080 or a 300B. You are going to have to compromise somewhere along the line. The easiest compromise is to use plate to plate feedback as global feedback will generally sound worse and will be difficult to inject into a fully differential amp with both inputs of the input LTP driven.

i have built four similar amps at this stage and this is an approach which works well when you get the details right.

Shoog

 

[emk2]

Waiting for 150pF styroflex to try to adjust the behaviour of the input transformer. OK we listen to music and not squares or triangles but what I can see on the scope is horrible with the trans.. from 4K to 20K across secondaries.

When I unsuccessfully tried phase splitting without trans, signal was clean at the upper EF80.

Tried plate to plate local feedback with 300K resistors, no improvement at all, probably because of the input trans problem. I'll test it again with differents values..

I don't know if 4.5K is well suited for 807 PP.

 

[lcsaszar]

I use 5k:8 OPT with 807 PP and it sounds great.

 

[Miles Prower]

My guess is your not going to get there without some form of feedback.
My guess is that you are hearing the 3rd to 6th harmonic components brought to the mix by the 807.

Let me quote from someone who attempted to use the 807 in triode mode;



Comparing Eight Tubes For Low Power Amplifiers Article Submitted By Mike Zivkovic Of Teresonic

Shoog

I question his judgement here:

In the "yawn" category is the 807 tube. I can't recommend this tube in triode mode at all...

In the "very nice" category are all the 6V6 style beam power tubes. In fact, with the upgraded Beast power supply it was hard to find a distinct difference between the 6AQ5, 6V6, and 6L6. This is not surprising since they all have very similar construction. The 6AQ5 is a lower power 6V6 in a 7 pin package used originally in car radios, and the 6L6 is an upgraded 6V6 with higher voltage and higher current...

He claims the 6L6 is "very nice", and yet the 807 is "yawn"?! They're the same tube! The 807 is a 6L6 with the glass envelop and then-standard five pin base adapted for RF use. Otherwise, internally the same (as is the 6BG6 -- 807 with octal base).

The 6AQ5 is not a "lower power 6V6", the specs are identical. The 6L6 is not an "upgraded" 6V6, since the 6L6 came first. He either can't read a spec sheet, or was too lazy to do so. Strongly suspect the "differences" exist in his imagination.

Not that I particularly care, as I wouldn't use any of the VTs he mentions as pseudotriodes anyway (except 6AQ5s as series pass devices for screen voltage regulators).

 

[Shoog]

I question his judgement here:



He claims the 6L6 is "very nice", and yet the 807 is "yawn"?! They're the same tube! The 807 is a 6L6 with the glass envelop and then-standard five pin base adapted for RF use. Otherwise, internally the same (as is the 6BG6 -- 807 with octal base).

The 6AQ5 is not a "lower power 6V6", the specs are identical. The 6L6 is not an "upgraded" 6V6, since the 6L6 came first. He either can't read a spec sheet, or was too lazy to do so. Strongly suspect the "differences" exist in his imagination.

Not that I particularly care, as I wouldn't use any of the VTs he mentions as pseudotriodes anyway (except 6AQ5s as series pass devices for screen voltage regulators).

I agree - the 807 should only be run as a Pentode in this arrangement.

Shoog