Negative Feedback

[WA5VGO]

I am well aware of the benefits of using negative feedback in modulators utilizing tetrodes. It certainly reduces distortion and smoothes out the frequency response.

What I would like to know is, are there any benefits to utilizing it in a speech amplifier/modulator the uses all triodes?

Darrell

[N2DTS]

Yes.
It does all the same things it does for tetrodes.
Lowers distortion, improves response.

I think they use it a lot around the driver tubes that draw grid current to lower the distortion and plate resistance?

Brett

[W2XR]

The other positive attribute of using triodes vs. tetrodes in the audio path, particularly in the high level stages (i.e. audio driver tubes and modulator tubes), is that significantly less negative feedback (NFB) is required to maintain acceptable linearity and low distortion, consistent with maintaining a low driver stage source impedance. High levels of NFB can create their own set of problems, such as instability due to phase shift, transient intermodulation distortion, etc.

For example, the Gates 1000 watt BC-1T AM BC rig used a a quad of 807s driving a pair of 833A modulator tubes. I believe the amount of NFB between the 833A plates and the (2) push-pull 807 voltage amplifiers was on the order of 23 dB. The earlier generation Gates BC-1F 1000 watt AM BC rig, used all triodes in the audio path: (2) triode-connected 6J7s, (2) 6J5s, (2) 845s, and a pair of 833As. Only 6 dB of NFB were used between the modulator tubes and the first voltage amplifier stage in that rig. This is due to the fact that triodes are inherently more linear amplifying devices compared to tetrodes or pentodes, even when tetrodes or pentodes are triode-connected. The big advantage that tetrodes and pentodes offer (compared to triodes) is higher voltage or power gain, with (generally) an attendent reduction in the number of stages required for a given amount of gain, but the trade-off is reduced linearity, which is usually corrected by the judicious application of NFB. No free lunch here.

Whether the audio driver tubes are triodes, tetrodes, or pentodes, NFB between the modulator and driver stages also improves the regulation of the driver stage when the driver is driving class B modulator tubes. This is essential for reducing the distortion that can be generated in the driver stage as the class B modulator grids are driven positive.

I use 6 dB of NFB in my rig between the 833A plates and the 6J7 first voltage amplifier stage, and the improvement is very noticeable. My driver/modulator circuit is all triodes.

So to answer your question, yes, NFB does provide the benefits of reduced distortion with wider and flatter frequency response when utilized with triodes.

Hope this helps!

73,

Bruce

[k4kyv]

Most of my audio stuff uses triodes throughout.  I try to avoid negative feedback, using it only as a last resort, preferring to keep things as simple as possible.

I believe that negative feedback is more important with pentodes and tetrodes, than with triodes.

[W2XR]

Most of my audio stuff uses triodes throughout.  I try to avoid negative feedback, using it only as a last resort, preferring to keep things as simple as possible.

I believe that negative feedback is more important with pentodes and tetrodes, than with triodes.

Don,

NFB should never be used as a band-aid for a circuit that is not properly designed or executed. In a circuit that is inherently quite linear, there are benefits frequently to be derived when the NFB is applied, and applied carefully. Too much of anything is not a good thing.

73,

Bruce

[WD8BIL]

The most common NFB I get is that I'm ugly!

[KB2WIG]

 " The most common NFB I get is that I'm ugly! '

What  kind of  NFB do the impolite people give you?.... ..

klc

[WD8BIL]

Here's some NFB for yu Kevin.

I saw your need for a Dow Key. I have one in the parts bin. After that question it's just liable ta stay there!!!

[WD5JKO]

Darrell;

   Take a peek at Patrick Turner's three stage ALL triode power amplifier. The output tubes are a pair of 845's operating in parallel, class A mode. He keeps the distortion down in the first stage by use of a constant current anode supply, and then stages 2 and 3 each have similar distortion (2H) such that they largely cancel out. Then Patrick uses only 8 db of global NFB to achieve some pretty respectable specifications.

http://www.turneraudio.com.au/monobloc845se55.html

Patrick uses the EL-84/6BQ5 extensively in triode connection with good results. The constant current anode supplies apparently really make a difference. Sometimes our low level stages have 1-2% THD, so the power amp that follows usually just adds to that number. We often forget about the low level stages when designing for low distortion.

Jim
WD5JKO

[KB2WIG]

 " I saw your need for a Dow Key. I have one in the parts bin. After that question it's just liable ta stay there!!! "


Foiled again! ! !

klc