A safe (?) DC coupled amp - 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 7th July 2004, 10:47 PM   #1
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
Default A safe (?) DC coupled amp

Good evening!

I have a 6B4G SE amp that for various reasons needs a little makeover, so why not have a little fun meanwhile?

Maybe I should begin with telling a bit of its currrent condition:

6B4G SE with Hammond 1627SE OPTs. 5AR4 + CLC PSU, CLC filters for the filaments. Input stage: Choke loaded (Hammond 156C) 6SN7. Thin bass and plenty of hum

My idea is to rebuild it completely, using a new mains transformer that gives +460V after proper filtering. Way too high for a regular 6B4G circuit but ideal for a direct coupled one.

The choice stands between a "Monkey" or "Free Lunch" design that requires expensive plate chokes for the driver or somekind of active load referenced to the cathode of the output tube.

Since the gain was too low with the 6SN7s Im going to replace them with PC86s. I have good experience with mosfets as plate loads in -followers so this might be one way to to.

Have a look and post your opinions!

Cheers
Attached Images
File Type: jpg apa.jpg (18.6 KB, 200 views)
  Reply With Quote
Old 8th July 2004, 11:44 AM   #2
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
Yay for 6B4s! I use them in my SE amp, a totally nice tube. Really need the DC filaments for them though, they just have too much hum otherwise.

I know its heretical, but have you tried driving them with a cathode follower? I have a very simple 6SL7 cathode follower driving mine and just get the most fantastic sound- great bass, very neutral and no hum at all.

Otherwise, have you considered a choke input power supply to drop the B+?
  Reply With Quote
Old 8th July 2004, 01:48 PM   #3
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
Yup, its a great tube!

I used 6SL7s with cathode followers before I changed to 6SN7s.
Sounded good but I know I can get better

The point is that the new power transformer allows DC coupling, which is a good thing. If I would want lower B+ Id use the original (Hammond) transformer.

Ive been thinking a bit about the posted schematic and I feel that a pair of Lundahl plate chokes might be worth the money.
Getting Mosfets involved is a bit heretic...
  Reply With Quote
Old 8th July 2004, 02:40 PM   #4
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
FETs are fine for duties other than voltage amplification. Think heaterless pentode. Frequently, a combination of tubes and FETs does a better job than either device type alone does.

Be pragmatic; it's the music that counts. IF "sand" acting as a "spear carrier" for tubes improves performance, so be it.
__________________
Eli D.
  Reply With Quote
Old 8th July 2004, 07:51 PM   #5
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
Im facing a hard decision here...
Im guite fond of inductive loading but price is definitely an issue.
Two 270H 10mA Lundahl chokes would probably cost me about 160-170 Euros. The mosfets can be bought for like 1/100 of that.

I guess the mosfet circuit would result in a very good frequency response, something that cannot be taken for granted when inductors are involved.

Quote:
IF "sand" acting as a "spear carrier" for tubes improves performance, so be it.
Well said!!
  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
capacitor coupled vs. direct coupled ouput hugobross Solid State 11 21st January 2011 02:56 AM
6sn7 direct coupled and transformer coupled neazoi Tubes / Valves 3 23rd May 2008 04:30 PM
Coupled or de-coupled cabinents troystg Multi-Way 12 20th October 2004 11:34 PM
Hakko 936 - ESD safe vs. non-ESD safe KT Parts 2 20th June 2004 04:58 AM
Can someone explain parafeed & direct coupled & a.c. coupled? mwmitchell Tubes / Valves 2 11th September 2002 02:10 PM


New To Site? Need Help?

All times are GMT. The time now is 11:21 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