SE 7868 Guitar Amp - some issues... - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

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 19th April 2009, 07:08 PM   #1
diyAudio Member
 
Stormrider's Avatar
 
Join Date: Apr 2006
Location: Connecticut
Blog Entries: 2
Send a message via AIM to Stormrider Send a message via MSN to Stormrider
Default SE 7868 Guitar Amp - some issues...

I've put together an SE guitar amp from some junk parts i had around and, for the most part, it seems to work pretty good. The amp sounds great except for this distortion I'm getting. Instead of going from clean to a slight overdrive, it goes from clean straight into this nasty, buzzy, raw heavy distortion... It just sounds very bad and unnatural... I'm hoping you guys can point out something I'm missing here.

I'm running it at a 230Vdc B+, very low power, with a 6.6K primary load. The OT is a 35W unit intended for P-P operation... It's my understanding that P-P output transformers can work for SE if they are over sized enough. The primary CT is left unconnected.

Here is the schematic:
http://i120.photobucket.com/albums/o.../7868seamp.jpg

Since the drawing I've changed the 1K screen resistor to a 3.2K. It's just a Fender 5E1 preamp with a 7868 output stage.

Voltages:
7868:
plate = 225Vdc
cathode = ~7.4V
screen = 217Vdc
heaters = 6.4Vac
__________________
AJ
  Reply With Quote
Old 19th April 2009, 08:53 PM   #2
PRNDL is offline PRNDL  United States
diyAudio Member
 
Join Date: Oct 2006
In my experience, the 7868 is very sensitive to bias.
I set the bias initially to what was shown on other schematics, but it had a strange distortion similar to what you describe. I fine tuned it by ear while playing guitar, and once set correctly it sounds great.

I built something similar to an Ampeg Portaflex with PP 7868's, which I call The Tank.
http://www.naturdoctor.com/Chapters/Amps/JazzTank.html
  Reply With Quote
Old 28th April 2009, 07:33 PM   #3
Zibi is offline Zibi  Poland
diyAudio Member
 
Join Date: Jan 2006
Location: Bialystok
Use Ra=3k output transformer with air gap. Change R9 value to increase quiescent current.
http://www.tubezone.net/pdf/7868.pdf
Click the image to open in full size.
  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
Using the 7591 or 7868 andyjevans Tubes / Valves 11 21st June 2010 04:53 AM
7868 amp sounds good but low volume PRNDL Tubes / Valves 9 9th December 2007 08:53 PM
7868 tubes - replace with 6L6's? PRNDL Tubes / Valves 6 29th November 2007 07:11 PM
7868 substitutes AmpKiller66 Tubes / Valves 7 21st November 2007 03:19 PM
7868 tubes in Bogen CHB100 thedoctor Tubes / Valves 6 22nd July 2005 05:36 PM


New To Site? Need Help?

All times are GMT. The time now is 03:57 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