Parafeeding a Tetrode - 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 25th April 2008, 08:03 AM   #1
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Default Parafeeding a Tetrode

I am working on a little 12V guitar amp with a 12k5 as the output tube. An example of one of these can be seen at http://www.sophtamps.ca/schematics/sopht_12k5-v6.jpg

The problem is that the only output transformer I have around for this cannot have any DC on it. So, I was thinking about adding a plate load resistor and a cap to couple the transformer. The questions is, will this work, and is it a reasonable solution?

On the attached picture, the left side is the way I've seen this tube used, and the right is what I am thinking about doing.
Attached Images
File Type: jpg guitar output.jpg (14.6 KB, 553 views)
  Reply With Quote
Old 25th April 2008, 12:06 PM   #2
oshifis is offline oshifis  Hungary
diyAudio Member
 
Join Date: Mar 2004
Location: Budapest, Hungary
I don't think is would work at such low voltage. You should provide sufficient bias current that mandates a relatively low value of R, but then it shunts the transformer in AC point of view.

I would insert an air gap in the transformer. The lower cutoff frequency will rise then, but guitars don't go down to 20 Hz anyway, are they?
  Reply With Quote
Old 25th April 2008, 08:09 PM   #3
Jeb-D. is offline Jeb-D.  United States
diyAudio Member
 
Jeb-D.'s Avatar
 
Join Date: Nov 2005
Location: SoCal
dsavitsk,
If you want to run parafeed off the 12V you should choke-load the plate. If you do go the resistor route you will have to raise the B+ so that the Plate operates at 12V+, not B+ 12V. Remember a resistive load can't swing above B+ as an inductive load can. Also, it has a much higher DCR than a choke or transformer winding. You will loose much of the available output power too, unless you make B+ large enough to use a plate resistor value that is 10x the value of the transformers primary impedance.

I'd advise against a CCS in case that thought was brewing. The Tetrode acts as a CCS too, so they will fight with each other. You can add a shunt resistor from plate to ground to allow a CCS, but that approch would be janky IMO.

You could hit up edcor for a custom transformer. They say their engineering charge is only $20. And I can't imagine them charging any more than $17 for the transformer itself for that application.
  Reply With Quote
Old 25th April 2008, 09:22 PM   #4
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
I'm not really worried about output power -- I need about 3 to 4mW here. Also, the transformer in question is a $3 1K:8 Radioshack thing that I was hoping to get away with. I did try to add the gap, but they seem to have epoxied the thing shut.

This really is a cheap and goofy project. But, what it sounds like is that while I am going to lose lots of power and while a larger B+ would be ideal, that this will "work" in a sense. That is, this is not an incorrect way to hook up the tube and the transformer, just a fudging of proper operating points. Does that seem right?
  Reply With Quote
Old 26th April 2008, 02:54 AM   #5
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 26th April 2008, 02:31 PM   #6
diyAudio Member
 
Cycline3's Avatar
 
Join Date: Jul 2007
Quote:
Originally posted by tubelab.com
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.
I dunno.. that transformer he's got is small - coin size small. I've seen it. On the other hand, if it dies... it's only a few bucks.

Unless size is a real issue, Parts Express has a $4 transformer that handles 10W and I've built a couple SE guitar amps with 6L6s and it works great for that. Maybe it would work here...

70V Line Transformer for tube output

From the reviews of it:
Ron Banks from LaPorte IN
I keep several of these on hand for old tube radio restoration. The .62 watt tap is approximately 8000 ohms, the 1.25 watt tap is about 4000 ohms and so on. These are well made and have more iron than most of the transformers in the old sets. They don't look out of place on the chassis and allow you to use most any speaker. Using the 1.25 watt as the center tap they work in push pull tube amps as well.
__________________
My site:
http://www.seanrose.com
  Reply With Quote
Old 26th April 2008, 10:05 PM   #7
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Quote:
Originally posted by tubelab.com
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.
I'll give it a try and see what happens. Worst possibility is I am out $3.


Quote:
Originally posted by Cycline3


I dunno.. that transformer he's got is small - coin size small. I've seen it. On the other hand, if it dies... it's only a few bucks.

Unless size is a real issue, Parts Express has a $4 transformer that handles 10W and I've built a couple SE guitar amps with 6L6s and it works great for that. Maybe it would work here...
I've got a couple of 70V line transformers around, so I'll try one of those too. Good suggestion.
  Reply With Quote
Old 29th April 2008, 10:44 PM   #8
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
Quote:
Originally posted by tubelab.com
Before thinking too hard, just hook up the OPT in the normal way. The typical plate current for a 12K5 is 8 mA. Probably not enough to saturate the transformer.
Well, it seems to work fine. Well, even. In fact, it sounds better than the much larger 70V line matchers I have. Who'd a thunk? It is a little *splatty* in the bass, but overall it makes a nice late night guitar practice amp.
  Reply With Quote
Old 29th April 2008, 11:39 PM   #9
Jeb-D. is offline Jeb-D.  United States
diyAudio Member
 
Jeb-D.'s Avatar
 
Join Date: Nov 2005
Location: SoCal
Quote:
Well, it seems to work fine. Well, even.
That's cool. Do you have a driver stage or are you driving the 12k5 straight from the guitar?
  Reply With Quote
Old 30th April 2008, 01:27 AM   #10
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
I used a 12U7. Schematic is basically the same as http://www.thomasdog.com/amps/sopht/ except I used 1M pot as grid leaks between each stage so I had a pre and post control, and I added a brightness switch to switch between a 4.7u and a .01u cathode bypass on the first stage. Also, the wall wart I used is unregulated, so I built a CRCRC filter (1000u, 4.7R, 2200u, 4.7r, 2200u) to get rid of the hum.
  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
TETRODE amplifiers Klimon Tubes / Valves 35 10th January 2011 12:31 PM
807 vs. 1624 tetrode ? coresta Tubes / Valves 33 22nd November 2008 09:27 AM
5763 tetrode on OTL ? coresta Tubes / Valves 1 15th March 2006 12:53 AM
Parafeeding a push pull Gluca Tubes / Valves 3 15th March 2005 12:06 PM
Do I lose power when parafeeding? Bas Horneman Tubes / Valves 2 26th March 2004 02:30 PM


New To Site? Need Help?

All times are GMT. The time now is 06:20 AM.


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