fisher 800c phase inverter adjustment 2 - 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 17th May 2007, 11:03 PM   #1
diyAudio Member
 
Join Date: Mar 2007
Thumbs up fisher 800c phase inverter adjustment 2

Is there a way to cheat on the phase inverter adjustment, that is to get it fairly close without all the expensive test equipment. I have an ociliscope and a volt/ohm meter or vtvm.

Also would it be possible to run el 34s in this amp. I realize the bias would have to be adjusted, perhaps a pot for the bias etc. Seems like the pin arangement is the same?


Steve
  Reply With Quote
Old 17th May 2007, 11:52 PM   #2
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
As a first approximation of where to set that pot - use the multimeter.

Some theory.
If you check the schematic you see a 68K in the cathode of the concertina splitter with a 180K going up to B+.
Remember that B+ is an AC (signal) ground so the AC cathode load is 68K in parallel with 180K = 49.35K (You might want to actually measure those resistors and redo the calc).
Now you want the anode load of the concertina (which is 33K + whatever the 25K pot is set at) to equal that same value - so adjust the pot until the combined resistance reads 49.35KOhms.

Once you know where that pot setting is, you can actually screw it a little each way while listening to see if you can detect any difference in sound. My guess is you won't and that setting will be the best.

Cheers
Ian
  Reply With Quote
Old 18th May 2007, 12:27 AM   #3
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
Also would it be possible to run el 34s in this amp.
Not really, for 2 good reasons. EL34 heater current draw is greater than 7591 heater current draw. Do you want to burn the power trafo out? The 2nd reason is drive voltage. The 7591 is as easy to drive as an EL84, voltage wise. Fisher took full advantage of that fact. The 12AX7 voltage gain block into 12AX7 concertina phase splitter is a carry over from EL84 designs. Even if the power trafo could support EL34s, sufficient drive voltage is not available.

Quote:
Is there a way to cheat on the phase inverter adjustment, that is to get it fairly close without all the expensive test equipment. I have an ociliscope and a volt/ohm meter or vtvm.
Unlike some folks, you seem to be willing to make changes and don't lionize Avery Fisher. Remember, 7591s are as easy to drive as "12" W. tubes. If you build an external "12" VDC supply to power the preamp section 'X7 heaters, the 150+ mA./"50" V. negative supply becomes available for other purposes. You then could "import" the differential splitter/driver employed in "El Cheapo". The only instrument needed to adjust that circuit is a multi-meter. BTW, NOS Mullard made 12AT7s are still affordable at about $25 per tube.
__________________
Eli D.
  Reply With Quote
Old 18th May 2007, 12:34 AM   #4
diyAudio Member
 
Join Date: Mar 2007
Cool fisher 800c

Thanks to both of you for the great advice.
  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
fisher 800c intermitant hum slhoward Tubes / Valves 1 28th May 2007 04:16 PM
fisher 800c modifications slhoward Tubes / Valves 1 15th May 2007 11:25 PM
hum in fisher 800c phono section slhoward Tubes / Valves 4 4th March 2007 03:43 PM
Distortion in Fisher 800C Glowin Plates Tubes / Valves 1 2nd November 2005 06:57 PM
Fisher 800C phase inverter adjustment Albert Tubes / Valves 4 18th June 2002 09:42 PM


New To Site? Need Help?

All times are GMT. The time now is 06:10 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