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Oscillation in tube amps
Oscillation in tube amps
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Old 18th March 2018, 11:32 AM   #151
macbobny is offline macbobny  United States
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Originally Posted by macbobny View Post
In the end, how did you resolve this? The photo in your original post is EXACTLY what my amp looks like.
In my case I found that there was a parallel RC network forming part of the feedback loop and reducing capacitance there killed the HF oscillation. In doing so I lost some high end distortion performance. I didnít spend too much time on it after that but there may be a middle ground where I could have both.
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Old 19th March 2018, 04:01 PM   #152
DF96 is offline DF96  England
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Quote:
Originally Posted by DualTriode
The Miller Theorem tells us that we can add external capacitance and or resistance and reduce the bandwidth. That is what we are talking about we speak grid stopper.

Unfortunately when we add grid stopper resistance we are also adding hiss kind of noise.
The grid stopper is not necessarily stopping oscillation by limiting bandwidth by working with the Miller capacitance; it may also act directly to damp a resonance in the grid circuit. A typical grid stopper will not add much thermal noise.
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Old 27th March 2018, 03:21 PM   #153
MagicBus is offline MagicBus  Greece
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I read a lot about grid stopper resistors but little about anode stoppers. In a resent build I fixed an oscillation with a resistor between the power tube anode and the OPT. An ordinary cement type wirewound worked best so my conclusion is that here inductance is needed. I've seen others to use the resistor only at one tube of the push pull pair so I did the same. Is that the right thing to do? Also I would like to ask what should be more appropriate to use: higher resistanse with lower inductance or the opposite? Or perhaps something else? I have at hand these inductors SMSC-6R0M-01 Fastron | Mouser Greece Should they work better than the resistor?
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Old 27th March 2018, 04:58 PM   #154
DF96 is offline DF96  England
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Anode stoppers are usually inductors, damped by a parallel resistance.
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Old 27th March 2018, 05:11 PM   #155
MagicBus is offline MagicBus  Greece
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So, for audio, what range of inductance and resistance should we look at?
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Old 27th March 2018, 05:17 PM   #156
jhstewart9 is offline jhstewart9  Canada
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Quote:
Originally Posted by DF96 View Post
Anode stoppers are usually inductors, damped by a parallel resistance.
And broadly resonant at the frequency of interest.
Attached Files
File Type: pdf Ohmite Suppressors.pdf (385.6 KB, 30 views)
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Old 27th March 2018, 05:31 PM   #157
MagicBus is offline MagicBus  Greece
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OK... As much inductance and resistance to get resonance at the frequency found with an oscilloscope. For the moment the cement will stay... Thanks!
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Old 27th March 2018, 06:27 PM   #158
jhstewart9 is offline jhstewart9  Canada
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Originally Posted by multi View Post
Has anyone built an amp using uhf mega steep tubes or just using theory?? Can you use the normal ways to control oscillation?
EFP60, not a power tube but developed for TV IF has a very large gain/bandwidth product. So it is able to provide a very low drive point impedance (16K) in this cct to the following grid. And still provide sufficient gain. In an RC coupled cct this results in lower distortion.

Used lots of them in a research project when they were available but never in an audio amp. Another possibility is the 7788. Was used in the HP 1402A vertical amp plug-in for the HP140 Series scopes.

Neither one a project for the inexperienced. Some may have seen this on another thread.
Attached Files
File Type: pdf EFP60 Philips.pdf (127.6 KB, 17 views)
File Type: pdf 7788 E810F Philips.pdf (338.3 KB, 21 views)
File Type: pdf EFP60 Driver Stage Schematic.pdf (437.8 KB, 29 views)
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Old 27th March 2018, 09:36 PM   #159
multi is offline multi  Australia
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THANKS
jhstuart very interesting have hundreds or 7788
phil
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Old 25th April 2018, 04:06 PM   #160
DualTriode is offline DualTriode  United States
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Quote:
Originally Posted by multi View Post
THANKS
jhstuart very interesting have hundreds or 7788
phil
FYI found on web
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File Type: jpg E810F.jpg (67.9 KB, 84 views)
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