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Old 10th January 2013, 07:20 PM   #31
GoatGuy is offline GoatGuy  United States
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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).

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Old 10th January 2013, 07:33 PM   #32
GoatGuy is offline GoatGuy  United States
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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.

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Old 10th January 2013, 08:43 PM   #33
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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;

Quote:
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/

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Last edited by Shoog; 10th January 2013 at 08:53 PM.
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Old 10th January 2013, 10:41 PM   #34
6J7 is offline 6J7
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Quote:
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.

Quote:
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.
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Old 10th January 2013, 11:06 PM   #35
jrenkin is offline jrenkin  United States
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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.
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Old 10th January 2013, 11:10 PM   #36
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Quote:
Originally Posted by 6J7 View Post
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
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Old 11th January 2013, 09:00 AM   #37
6J7 is offline 6J7
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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.
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Old 11th January 2013, 02:14 PM   #38
oshifis is offline oshifis  Hungary
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I use 5k:8 OPT with 807 PP and it sounds great.
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Old 12th January 2013, 04:49 AM   #39
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Quote:
Originally Posted by Shoog View Post
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:

Quote:
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).
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Old 12th January 2013, 11:10 AM   #40
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Quote:
Originally Posted by Miles Prower View Post
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
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