a directly heated monkey? - 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 27th April 2006, 03:20 AM   #1
Rocky is offline Rocky  Norway
diyAudio Member
 
Rocky's Avatar
 
Join Date: Apr 2004
Location: Kongsberg/Oslo
Default a directly heated monkey?

As I finally have got online with my DHT amp, I am thinking about ways to rewire it for further experiments.. I'm not going to rewire it right away, but when I'm ready I'd like to have a plan. Using parts I have in the amp plus two caps and few resistors, I can get online with the schematic shown below.. It will require quite a bit of tweaking to get the bias points right, but I really like the idea of getting rid of the coupling cap, and I should also get some free filtering of the driver B+. Any words of advice? I realize the output tube will fry if the driver doesn't conduct as it looses the bias I'd like to try it anywayz..

Any recommendations on the cathode bypass cap for the 71A? I'm thinking a Black Gate VK would do well..

(all heaters will still be DC, batteries)

EDIT: the "free lunch" topology by Jeremy Epstein would perhaps be just as good or even better. It is basically same as the scheme above but drops voltage over a resistor in series with the plate choke instead of the resistive divider in the cathode circuit..
Attached Images
File Type: gif signal.gif (5.8 KB, 1090 views)
  Reply With Quote
Old 27th April 2006, 03:38 AM   #2
Rocky is offline Rocky  Norway
diyAudio Member
 
Rocky's Avatar
 
Join Date: Apr 2004
Location: Kongsberg/Oslo
The free lunch variant would look like this..
Attached Images
File Type: gif signal.gif (5.6 KB, 949 views)
  Reply With Quote
Old 27th April 2006, 08:20 AM   #3
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
I´ve built a "Free Lunch" with 6B4G, E81L, LL1664 and LL1621 (IT wired as plate choke).
490V B+ rectified by two 6AU4GTA.
Attached Images
File Type: jpg ampkompl.jpg (38.1 KB, 825 views)
  Reply With Quote
Old 27th April 2006, 10:13 AM   #4
Rocky is offline Rocky  Norway
diyAudio Member
 
Rocky's Avatar
 
Join Date: Apr 2004
Location: Kongsberg/Oslo
Nice work Fuling. I'm curious about how you like it, compared to more conventional SE topologies (with coupling caps)? is the direct coupling a very apparent sonical advantage in your opinion?
  Reply With Quote
Old 27th April 2006, 10:22 AM   #5
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
This is the best amp I´ve ever built, no question.
Direct coupling to a choke loaded driver stage is definitely the way to go!
This is my second "Free lunch", the first one used much cheaper iron and IDHT´s and even that one sounded great.
Search also for DRD, Direct Reactance Drive, that´s the same topology.
  Reply With Quote
Old 27th April 2006, 02:13 PM   #6
Rocky is offline Rocky  Norway
diyAudio Member
 
Rocky's Avatar
 
Join Date: Apr 2004
Location: Kongsberg/Oslo
Most useful information, thanks alot. Your search tip turned up alot of information that previously has slipped me by, I have alot more background info to work on now. This looks more and more like a really interresting topology, I'm pretty sure this is the trick for my next mod.. I need a higher voltage power supply though.. Tell me, do you use ultrapath cap or conventional cathode bypass? I did read an article (linked below) about very poor immunity against B+ ripple in "conventional ultrapath".. not sure wich route to take. I'm not really worried about hum though, The 71A output tube is low mu, and is followed by a 17.67:1 step down in the OPT.. That should significantly step down any potential hum too.. What do you think about the matter?

"Ultrapath Line Stage"
http://www.geocities.com/dmitrynizh/Ultrapath.htm
  Reply With Quote
Old 27th April 2006, 04:32 PM   #7
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
I use conventional cathode bypassing, mostly because there´s no room for such big oil caps in my chassis.
  Reply With Quote
Old 28th April 2006, 01:34 PM   #8
Rocky is offline Rocky  Norway
diyAudio Member
 
Rocky's Avatar
 
Join Date: Apr 2004
Location: Kongsberg/Oslo
Tell me about it. I fell asleep yesterday staring at the amp in the audio rack puzzled by how I was supposed to fit those oil caps. No room on top, no space inside.. Pretty easy mathematics I think I'll go for the cathode bypass cap too, for the same reason as you. I'll leave the ultrapath for a later tweak, when the amp is ready to move into a new box. What bypass caps are you using? Tried Black Gate VK?
  Reply With Quote
Old 28th April 2006, 11:06 PM   #9
Rocky is offline Rocky  Norway
diyAudio Member
 
Rocky's Avatar
 
Join Date: Apr 2004
Location: Kongsberg/Oslo
I put down some thoughts on paper here, feel free to comment them. I went a little bananas saving on the PSU chokes, they have very high DCR but are in fact rated for ample power as I only draw 20mA (that is, unless I upgrade to another power tube). PSU capacitors will be cheap Solen PP or oil bargains on eBay, I was thinking I'd make AE wind the power trannies as I couldn't find any 350-0-350 xformer that also had a 4V winding. There is a hammond with a 5V winding though, I suppose it wouldn't hurt sound quality reducing it to 4V using drop resistors one in each end? Filaments are battery powered, all resistors aren't calculated yet but they are pieces of cake I just didn't bother. The supply itself is modelled in Duncan's excellent PSU designer. I put a 5mA dropper resistor in there to discharge capacitors as a cheap health insurance..

Any thoughts on the scheme?
Attached Images
File Type: gif compl_amp.gif (7.4 KB, 683 views)
  Reply With Quote
Old 29th April 2006, 12:29 AM   #10
diyAudio Member
 
Join Date: Feb 2002
Rocky - looks kosher, I think you can't go wrong with any of those sketches. You're basically going through what I went through as I developed the Free Lunch originally - starting with a basic amp and then making some bigger changes bit by bit. Should provide a lot of fun!

-j
  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
A directly heated triode or Pentode SE amp on a Budget avisa Tubes / Valves 24 7th March 2009 11:16 PM
Directly heated cathodes, but why? Hesky Tubes / Valves 12 8th June 2004 12:18 AM


New To Site? Need Help?

All times are GMT. The time now is 09:49 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