Tuned Circuit? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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 8th August 2013, 11:36 AM   #1
diyAudio Member
 
oldschooltube's Avatar
 
Join Date: May 2011
Location: Quad Cities USA
Default Tuned Circuit?

Greetings all. I have a pair of Altec 1570B amplifiers that I have been slowly rebuilding. I now have them up and screaming and I enjoy them a lot. My question is this: There is a 1000 volt 6uF capacitor in the B+ supply (C8 on the schem) that is a bit leaky. I read on another forum (cant remember where) that this cap value should not be changed because it is a "tuned circuit". Looking at the schematic it looks like a straightforward tube rectified LC supply to me. I know that the cap value for rectifier tubes cannot be too high but I have 2 new 7.5uF caps to replace the 6uF caps. An extra 1.5uF is not enough to harm the tubes but I am puzzled about this tuned circuit bit. Was it just someone blowing smoke or do they know/see something I don't?

Anyway, should I change these caps and bump up the extra 1.5uF?

Attached is a schematic. Fire away, inquiring minds want to know!
Attached Images
File Type: jpg altec-1570b schematic.JPG (80.7 KB, 174 views)
__________________
Marty: "Why don't you just make 10 louder?"
Nigel: "...These go to 11."
  Reply With Quote
Old 8th August 2013, 01:44 PM   #2
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
It's hard to read the writing on the schematic, but it looks like C8 is the electrolytic right after the choke? If so the tolerances on old electrolytics are +/- a bunch so you are probably OK.

On the other hand I have seen HV power supplies used in old Motorola radio transmitters that did use a resonant element. The choke had a cap connected directly across it and there was also a cap from either side to ground. All three caps were in the same can and they were PIO. The schematic warned that the cap and choke were to be replaced as a pair if either one failed.

If it was my choice, I would try the 7.5uF caps.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 8th August 2013, 01:55 PM   #3
diyAudio Member
 
oldschooltube's Avatar
 
Join Date: May 2011
Location: Quad Cities USA
Yes it is the cap right after the choke. I dont see anything "tuned" about this power supply. I will give it a go!

Just like the hungry computer said: "Thanks for your input"!

These old beasts are really great but they sure were ugly! I did a little work on the cosmetics by blacking the entire thing out (except for the big, beautiful tubes of course!) and adding an edge lit plexi front panel I engraved with the Altec logo.

They sure look cool at night!
Attached Images
File Type: jpg Lit Altec.jpg (956.4 KB, 159 views)
__________________
Marty: "Why don't you just make 10 louder?"
Nigel: "...These go to 11."
  Reply With Quote
Old 8th August 2013, 05:08 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You certainly don't want that cap and the choke to form a tuned circuit as that would increase hum. It was probably just the usual ignorant web chatter.
  Reply With Quote
Old 9th August 2013, 06:35 AM   #5
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by oldschooltube View Post
I know that the cap value for rectifier tubes cannot be too high but I have 2 new 7.5uF caps to replace the 6uF caps. An extra 1.5uF is not enough to harm the tubes but I am puzzled about this tuned circuit bit. Was it just someone blowing smoke or do they know/see something I don't?

Anyway, should I change these caps and bump up the extra 1.5uF?
It is an incidental tuned circuit, an L-network to be precise. The only difference between L-network and LPF is the Q-factor. All proper LPFs (Butterworth, Chebychev, Bessel) all have Q < 1.0. That's not an easy thing to accomplish with the usual values of L and C encountered in any ripple filter. (I calculated the values for a Chebychev characteristic and came up with huge values for L and small ones for C -- totally impractical). You need to get that resonant frequency way below the audio band. For my projects, Fo= 4.0Hz, with a Q~= 13. That would make for a nasty filter bounce, but it's highly unlikely the filter would see 4.0Hz. That's 2.3 octaves below the 20Hz bottom of the audio band.

In this case, replacing a 6.0uF filter capacitor with a 7.5uf unit will drive that resonant frequency even lower, give you somewhat better attenuation of the AC ripple frequencies. Besides, electrolytic capacitors aren't that accurate in the first place when it comes to actual capacitance v. nominal capacitance. It won't cause problems.

Quote:
On the other hand I have seen HV power supplies used in old Motorola radio transmitters that did use a resonant element. The choke had a cap connected directly across it and there was also a cap from either side to ground. All three caps were in the same can and they were PIO. The schematic warned that the cap and choke were to be replaced as a pair if either one failed.
There was once a vogue for doing that: making the inductor into a parallel resonant trap. However, that's a classic SLAGIATT: sure, it will attenuate the hell out of the fundamental of the AC ripple, but ripple has a large harmonic content, and those harmonic frequencies are going to see a much lower impedance from the parallel tuned circuit than would be the case for an LC LPF. It will let more of those ripple harmonics through, and you don't want those polluting the DC rail.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 9th August 2013, 10:21 AM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by Miles Prower
For my projects, Fo= 4.0Hz, with a Q~= 13. That would make for a nasty filter bounce, but it's highly unlikely the filter would see 4.0Hz. That's 2.3 octaves below the 20Hz bottom of the audio band.
Subsonics could come from a turntable, or from mains voltage variations. Fortunately, the load (i.e. the amplifier circuit) adds significant resistive damping so the actual Q is likely to be much lower than your calculation (based on choke resistance?). Still needs thinking about, though, but often ignored in PSU design - especially by the type of designer who think that if something is good then more of it must be better?
  Reply With Quote
Old 9th August 2013, 07:07 PM   #7
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by DF96 View Post
Subsonics could come from a turntable, or from mains voltage variations. Fortunately, the load (i.e. the amplifier circuit) adds significant resistive damping so the actual Q is likely to be much lower than your calculation (based on choke resistance?). Still needs thinking about, though, but often ignored in PSU design - especially by the type of designer who think that if something is good then more of it must be better?
Since I don't have a turntable, it's not a prob. If I did have one, I'm sure the pre (either self designed or other) would include a rumble filter to keep that garbage out of the amp. Here are the results of the actual filter values I used for an LC ripple filter:

L= 7.0H
C= 220uF
R= 2K8 (350Vdc rail; Il= 125mA)

For a second order LCR LPF:

A(w)= [(1 - LCw^2)^2 + (wL/R)^2]^-0.5

Xfer equation for series L at the input, with parallel load R and C at the output.

That peak occurs at 4.0Hz, with the -3.0db frequency at 6.3Hz. That resonance peak is well away from the bottom of the audio band to not be a prob.

A proper LPF with Q < 1.0 would need L ~= 100H and C= ~14uF. A 100H inductor that could handle 125mA would be very large and very heavy. Big capacitors are both easier and cheaper than big inductors.
Attached Images
File Type: gif RippleFltr.gif (103.8 KB, 50 views)
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 9th August 2013, 07:43 PM   #8
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
OK. In case of difficulty a CR 'snubber' could be added in parallel with the cap to further damp the subsonic resonance.
  Reply With Quote
Old 9th August 2013, 08:10 PM   #9
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by DF96 View Post
OK. In case of difficulty a CR 'snubber' could be added in parallel with the cap to further damp the subsonic resonance.
That won't work, since these snubbers dampen out high frequencies, not low ones. At DC, the snubber simply disappears, and becomes increasingly visible as frequencies increase, and Xc decreases.

The only way to dampen out that low frequency resonance is to decrease the load resistance, and that just makes for worse ripple, and wastes a whole bunch of power. Your only options are RC ripple filtration (half as effective, and compromises voltage regulation -- useful where you have relatively constant load currents, and excessive voltage capability) or using big reservoir capacitors (not possible with vacuum diodes, due to limited Isurge capability, and inadvisable when using vintage PTXs since these weren't designed with huge Isurge in mind). You could also try active decoupling if it's gonna be a problem.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com

Last edited by Miles Prower; 9th August 2013 at 08:13 PM.
  Reply With Quote
Old 9th August 2013, 09:05 PM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A snubber could work at 4Hz. It will have less effect at higher and lower frequencies, but there its effect is not needed anyway.

For example, put 100uF in series with 400R then place in parallel with the 220uF cap. At 4Hz this looks like 50uF in parallel with 800R (if my arithmetic is right). The 50uF slightly drops the resonance frequency, but not by much. The 800R damps the resonance as it appears in parallel with your 2k8 load.
  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
Fonken-tuned floorstander type Planet 10 hifi 4 9th December 2011 07:47 PM
Could it be tuned as claimed? (27Hz) michaelpage26 Multi-Way 6 17th June 2008 08:45 AM
PPI tuned off! skooter Car Audio 4 24th November 2007 03:08 PM
PR tuned to different freq mashaffer Multi-Way 4 3rd August 2007 01:10 AM


New To Site? Need Help?

All times are GMT. The time now is 03:03 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2