OTL designed by Tim Mellow with 4 6C33C? - Page 9 - 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 3rd November 2010, 01:05 AM   #81
djn is offline djn  United States
diyAudio Member
 
djn's Avatar
 
Join Date: Oct 2004
Location: Waterford Michigan
Quote:
Originally Posted by atmasphere View Post
The rectifiers need to be good for several amperes. In addition, the B+ is low as far as tube amps go, so the usual voltage drop that tube rectifiers have is unacceptable. Now if you had enough of them (more of them than power tubes) you might be able to get away with it. That seems like it would be a prodigious project!
OK, so you are saying that tube rectification has a voltage drop that SS does not? So, excuse my ignance, but what is a voltage drop? Thanks for your help.
  Reply With Quote
Old 3rd November 2010, 02:42 AM   #82
diyAudio Member
 
sepolansky's Avatar
 
Join Date: Nov 2007
Location: Maryland
djn,

Simply, when converting AC to pulsating DC there is an internal voltage loss absorbed by the rectifier. In SS, that number is a fraction of a volt, depending on the specific rectifier chosen. In tubes, it can be as little as 11 volts or so, for a damper diode type, or even 30 volts for a more conventional type.
That's part of the problem, but for you, the bigger issue is current. OTL output stages use a lot of current, several amps. Most tube rectifiers can only deliver fractions of an amp. Damper diodes, can deliver up to 2.5 amps or so. Maybe a little more.

Your best bet is a Hexfred type of SS rectifier. Really durable and can deliver about 15 amps of current.

One more word about the current: when charging up a big bank of capacitors, there are big inrush current pulses, until the cap is charged. These pulses can far exceed the rating of the rectifier. I've blown up a few Schottky type SS rectifiers because of the big cap banks I tend towards. The advantage of the Schottkys is that they don't generate RF noise like other SS diodes do, so they are the next best thing to a tube diode from that standpoint. However, they blow up where my Stealth Hexfreds have lived for years.

Stuart
  Reply With Quote
Old 3rd November 2010, 04:41 AM   #83
djn is offline djn  United States
diyAudio Member
 
djn's Avatar
 
Join Date: Oct 2004
Location: Waterford Michigan
Thanks for that Stuart. I think I understand now.....well said.

So will the RF noise show up in the signal path???
  Reply With Quote
Old 3rd November 2010, 09:50 AM   #84
diyAudio Member
 
Join Date: Jan 2002
Location: nowhere
Quote:
Originally Posted by Brit01 View Post
I would have though 200-250mA would have been a better choice for each power tube. If a 6080 is pulling 100mA or so and the bias goes off, then it will shoot up over 200mA or more.

How would a 1.5Amp fuse protect it?

Please correct me if I'm wrong or misunderstood.
1.5amp may be for the 6C33?

6AS7s will put out about 750mA when grid driven a little positive.

I have never bothered with fuses other than mains, and never had problems. I figure I built it, I can fix it, even if it blows up
  Reply With Quote
Old 3rd November 2010, 05:24 PM   #85
djn is offline djn  United States
diyAudio Member
 
djn's Avatar
 
Join Date: Oct 2004
Location: Waterford Michigan
Just checked out the Hexfred diodes....man they sure do look serious.

Q: if a PS needs a certain voltage and amp rating for the diodes, can you go WAY bigger for protection? Or will over sizing them be a problem?

The reason I ask is a friend of mine told me last night that he has a stash of very big ones that I could rob. He didn't know off hand their rating, but was sure it was much higher than what I needed.
  Reply With Quote
Old 3rd November 2010, 05:30 PM   #86
diyAudio Member
 
atmasphere's Avatar
 
Join Date: May 2007
Quote:
Originally Posted by Brit01 View Post
I would have though 200-250mA would have been a better choice for each power tube. If a 6080 is pulling 100mA or so and the bias goes off, then it will shoot up over 200mA or more.

How would a 1.5Amp fuse protect it?

Please correct me if I'm wrong or misunderstood.
You will blow out a 250mA fuse just about right away.

1.5 Amps is used because one of the failure modes of the tube is arcing. You want a fuse that is large enough to allow the amp to clip, but small enough to prevent damage.

BTW, if you get the 6C33s new: they come from the factory untested. You will want to run them for at least 72 hours continuously, **filaments only**, no B+ or bias voltages, to precondition them. This will double their life expectancy, and will substantially reduce the possibility of arcing!
  Reply With Quote
Old 3rd November 2010, 05:48 PM   #87
djn is offline djn  United States
diyAudio Member
 
djn's Avatar
 
Join Date: Oct 2004
Location: Waterford Michigan
BTW, if you get the 6C33s new: they come from the factory untested. You will want to run them for at least 72 hours continuously, **filaments only**, no B+ or bias voltages, to precondition them. This will double their life expectancy, and will substantially reduce the possibility of arcing!

Good to know, thanks.
  Reply With Quote
Old 3rd November 2010, 05:55 PM   #88
Brit01 is offline Brit01  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Quote:
BTW, if you get the 6C33s new:
thxs yes I heard about that. I burnt my 6080's in for a few days before I used B+ on them.

Quote:
1.5 Amps is used
you guys use a fuse on the secondary and primary? Secondary makes sense to me if the bias fails.

But I'm looking into a bias B+ relay protection also.
  Reply With Quote
Old 3rd November 2010, 08:06 PM   #89
diyAudio Member
 
atmasphere's Avatar
 
Join Date: May 2007
Quote:
Originally Posted by Brit01 View Post
you guys use a fuse on the secondary and primary? Secondary makes sense to me if the bias fails.

But I'm looking into a bias B+ relay protection also.
The fuse on the secondary side, if you are using 6C33s, is set up so that there is *one for each tube*.

For the protection relay, a system that detects 0V on the grid with respect to its cathode of either tube is what is needed. I would set it up so that musical transients don't set it off.
  Reply With Quote
Old 3rd November 2010, 09:47 PM   #90
diyAudio Member
 
sepolansky's Avatar
 
Join Date: Nov 2007
Location: Maryland
Quote:
Originally Posted by djn View Post
Just checked out the Hexfred diodes....man they sure do look serious.

Q: if a PS needs a certain voltage and amp rating for the diodes, can you go WAY bigger for protection? Or will over sizing them be a problem?

The reason I ask is a friend of mine told me last night that he has a stash of very big ones that I could rob. He didn't know off hand their rating, but was sure it was much higher than what I needed.
The voltage rating you see on a diode is PIV or Peak Inverse Voltage. Basically, this is the maximum voltage the diode is asked to block in the reversed direction. The actual number is much larger than the DC output of your rectifier, and varies by type of rectifier connection. Using 1000V rectifiers for the 150V supplies will be just fine.

As far as the current goes, as far as I know, the more the merrier. SS devices are nowhere near as forgiving as tubes, as I may have mentioned before. Exceed their ratings and you'll be sniffing up magic smoke. Use the ratings as "never equal or exceed" numbers. If you need a 1.5 amp rectifier, choose at least a 3 amp unit. Remember also that inrush of charging the capacitors will exceed the 1.5 amp steady state rating. I think the Stealth Hexfreds are 1200V 15A or so.

Stuart
  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
OTL design recommendation? (6C33C maybe?) arteitle Tubes / Valves 14 16th January 2012 04:37 PM
6C33C OTL DC schematic needed neazoi Tubes / Valves 29 25th June 2010 05:25 PM
Custom designed transformer for Andrea Ciuffoli 6c33c OTL AMP.... zxcxz Tubes / Valves 2 10th July 2009 05:14 PM
Transformer for 6C33C OTL WALTER BURKHARD Swap Meet 1 29th June 2003 11:06 AM
Availability of 6C33C tubes? I need some for OTL amps tubetvr Tubes / Valves 39 31st January 2003 09:21 PM


New To Site? Need Help?

All times are GMT. The time now is 02:19 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