Protecting Plitron OPT - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th April 2016, 07:11 PM   #1
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
Default Protecting Plitron OPT

I was reading the thread about using split load op amps to drive a push pull stage. That thread drifted onto the subject of protecting OPTs. which I'm very interested in. Rather than further drive that thread off the original subject, which I'm nonetheless interested in, I'll continue the thread about protecting OPTs here.

I'm in the process of designing an amp that borrows concepts from various places. Unfortunately when you do that sometimes those concepts don't play nice with each other. Tubelab brought up some very good points in that thread. I'm using a Plitron cathode feedback OPT in the design, not the unity gain one, so I can't use cap bypassed resistors to auto bias. If I did it would negate the CFB in the OPT and hold the dynamic as well as static signal in the cathodes at a constant level.

Since the OPT is so expensive I really want to protect them. Tubelab opened my eyes to the fact that if I fuse the cathodes at a common point then when it opens due to overcurrent it will cause a big spark that might destroy the OPT. I'm really concerned about that now. I'm planning on using mosfet source followers to drive the final in AB2 bias. My understanding is that eliminates the use of CCS's in the cathode circuits in the final stage.

My question is this then: what would be the best way to protect the circuit. I generally go for the simplest answer so I'm not looking for something complicated. Just as long as it doesn't affect the sound. Any help is appreciated. Thanks much.
  Reply With Quote
Old 13th April 2016, 07:34 PM   #2
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Wait, are you saying that the CFB winding is one tapped winding and not two separate windings like on the 1070UC?
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 13th April 2016, 07:43 PM   #3
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
http://plitron.com/wp-content/uploads/file/2100CFB.pdf

This one has only 10% signal feedback, not 100% like the unity gain model. There's two other models similar to what I ordered in their specialist line. The reason I went for the 10% feedback one is it doesn't require the driver capability of the unity gain.
  Reply With Quote
Old 13th April 2016, 07:50 PM   #4
diyAudio Member
 
BinaryMike's Avatar
 
Join Date: Apr 2010
Location: Oregon
I suspect that a metal oxide varistor (MOV) is the best answer here.
  Reply With Quote
Old 13th April 2016, 08:13 PM   #5
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by BinaryMike View Post
I suspect that a metal oxide varistor (MOV) is the best answer here.
My only concern would be if it loads the circuit before it reaches its breakdown voltage, especially if it is a non-linear load.

It would be easy enough to put it in there, get the amp working, and then take distortion measurements with it in and out of circuit to see if it adds any ugliness.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 13th April 2016, 08:15 PM   #6
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
I'm guessing here that the best place to position a MOV would be between B+ and ground to protect the OPT. I suppose you would use it in conjunction with a permanent fusing device. Otherwise it might end up in an endless cycle of field collapse and regeneration.

The problem (at least to these eyes) is that if this combination of fuse and transient suppression was a good solution in the final stage of a tube amp then why isn't it a standard practice? I'm sure this problem crops up all the time in non-self biased amps. Does it affect the sound I wondering? This would also apply to gas discharge tubes. I'm really in way over my pay grade here and have no experience whatsoever.
  Reply With Quote
Old 13th April 2016, 08:31 PM   #7
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by exeric View Post
http://plitron.com/wp-content/uploads/file/2100CFB.pdf

This one has only 10% signal feedback, not 100% like the unity gain model. There's two other models similar to what I ordered in their specialist line. The reason I went for the 10% feedback one is it doesn't require the driver capability of the unity gain.
Okay, so you do have separate CFB windings for each tube. My protection circuit is very simple compared to what tubelab was describing for his dream amp.

I have current-sense resistors under each CFB winding, between the winding and ground. I use those for two purposes.

One is a bias servo. I bought the one from Guido Tent and it works great. I had originally designed one, had the cheapest place I could find fab it, only to find that the boards were bad. I then threw money at the problem to get it done.

The second goes to the monitor circuit. I take that signal from the current-sense resistors and also filter it and send it to a comparator that goes high if a reference voltage is crossed. That goes into a microprocessor board that monitors two things, the output of the comparators and the on/off switch. Either of those can cause the amp to shut off. The microprocessor controls power-up/power-down via solid state relays on the transformer primaries.

I recognized that in my design I am putting a lot of trust in the bias servo. That thing has the power to kill my power tubes and output transformers if it wants to. I designed the monitor circuit to put a check on that power.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 13th April 2016, 09:19 PM   #8
diyAudio Member
 
BinaryMike's Avatar
 
Join Date: Apr 2010
Location: Oregon
The concern about distortion increase might be valid. It should be tested, if someone hasn't done this already. Protection against high voltage transients could be achieved with an MOV connected across any winding of a tightly coupled transformer, so it would make sense to use the cathode winding or secondary for this purpose. Loudspeaker loads certainly act to suppress transients and probably save most OPTs from damage in misbehaving hi-fi amps.
  Reply With Quote
Old 13th April 2016, 09:35 PM   #9
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
I would be concerned about capacitance of MOV devices. Especially if it is varying with V. Presumably the datasheet would have this info. I've got some MOVs here somewhere. I'll check one on the C meter.

I just checked a 175L10 (probably 175V rating) and its got 836 pf on the meter. That would spoil most OT HF responses.

Idea: Use a HV Mosfet across the B+ to plate winding on each side of the OT with a series HV diode (Source at one end, then to Drain and diode at the other). Put voltage divider resistors across the Drain to Source, with the mid-point going to the gate. Scale the divider so it turns on the Mosfet at some given HV across the Mosfet. You could put them in both polarities on each side of the OT. The HV diodes prevent them from conducting when reverse polarity. One could get more accurate firing V using a trigger diode, like used for SCR firing.

Last edited by smoking-amp; 13th April 2016 at 09:55 PM.
  Reply With Quote
Old 13th April 2016, 10:34 PM   #10
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
Quote:
Originally Posted by smoking-amp View Post
I would be concerned about capacitance of MOV devices. Especially if it is varying with V. Presumably the datasheet would have this info. I've got some MOVs here somewhere. I'll check one on the C meter.

I just checked a 175L10 (probably 175V rating) and its got 836 pf on the meter. That would spoil most OT HF responses.
Unfortunately the hard drive died on my computer that had the LT Spice model of my circuit. Luckily I've got a printed copy of it. If I still had a working LT Spice model I'd just plug in capacitor values one at a time across different windings of the OPT and see what it simulates as an output. Now that's out the window. Of course, that doesn't model varying individual capacitances but one might get a pretty good idea by just plugging in a range of different cap values at each individual position. To bad I can't do it now.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Protecting Wire spatten Tubes / Valves 7 22nd February 2014 04:25 AM
Protecting Ribbons exurbia Planars & Exotics 1 14th April 2010 04:24 PM
Plitron LOW Power OPT... chrismercurio Tubes / Valves 2 22nd July 2009 03:16 PM
Protecting a microcontroller rtarbell Parts 10 11th January 2008 02:23 AM
why is my amp protecting? mbugua Class D 2 19th January 2007 01:58 PM


New To Site? Need Help?

All times are GMT. The time now is 12:49 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki