a directly heated monkey?

Rocky · 27th April 2006, 03:20 AM

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..

Rocky · 27th April 2006, 03:38 AM

The free lunch variant would look like this..

Fuling · 27th April 2006, 08:20 AM

I´ve built a "Free Lunch" with 6B4G, E81L, LL1664 and LL1621 (IT wired as plate choke).
490V B+ rectified by two 6AU4GTA.

Rocky · 27th April 2006, 10:13 AM

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?

Fuling · 27th April 2006, 10:22 AM

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.

Rocky · 27th April 2006, 02:13 PM

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

Fuling · 27th April 2006, 04:32 PM

I use conventional cathode bypassing, mostly because there´s no room for such big oil caps in my chassis.

Rocky · 28th April 2006, 01:34 PM

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?

Rocky · 28th April 2006, 11:06 PM

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?

J Epstein · 29th April 2006, 12:29 AM

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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

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?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

27th April 2006, 10:13 AM	#4
Rocky diyAudio Member 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?

27th April 2006, 10:22 AM	#5
Fuling 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.

27th April 2006, 02:13 PM	#6
Rocky diyAudio Member 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

27th April 2006, 04:32 PM	#7
Fuling 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.

28th April 2006, 01:34 PM	#8
Rocky diyAudio Member 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?

29th April 2006, 12:29 AM	#10
J Epstein 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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)