filling in the low end, 6bq5 12ax7 circuitry - 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 21st January 2010, 03:06 AM   #1
trigger is offline trigger  United States
diyAudio Member
 
trigger's Avatar
 
Join Date: Dec 2009
Location: michigan
Default filling in the low end, 6bq5 12ax7 circuitry

i salvaged this amp out of an old radio i pulled from the trash, finally got it fired up properly, it sounds amazing now after running an hour, the low end is faint under 80hz.

any thing i can do to improve the low end besides new OT's ?

i dont have a budget to work with, but i do have many vintage radio parts on available.
well all i need do now is get it all pretty with gold terminals and a polished wood case. there is a pre that i also salvaged off the radio i plan to make a small unit that uses the power supply off the power amp. fun stuff! thanks guys for the advices!
Attached Images
File Type: jpg GEDC0192.jpg (515.0 KB, 201 views)
File Type: jpg GEDC3592.jpg (515.1 KB, 186 views)
File Type: jpg ee.jpg (796.6 KB, 181 views)
File Type: jpg bh.jpg (799.9 KB, 164 views)
  Reply With Quote
Old 21st January 2010, 03:09 AM   #2
diyAudio Member
 
kavermei's Avatar
 
Join Date: Jan 2005
Location: Lokeren, Belgium
Send a message via MSN to kavermei
Schematic diagram please!
__________________
Never send a human to do a machine's job. --Agent Smith
  Reply With Quote
Old 21st January 2010, 04:35 AM   #3
diyAudio Member
 
Join Date: Nov 2007
I'd certainly look at replacing the bypass capacitor between the 6BQ5s (black electrolytic). Probably the two other coupling capacitors too.
However, with unknown OTs you are in the lap of the gods as to how far down they will reach.
  Reply With Quote
Old 21st January 2010, 01:04 PM   #4
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Another thing to consider is replacing the PS filter caps. They are AL Electrolytic and have probably lossed considerable capacitance over time. That may effect your low end, and is worth replacing even if it isn't.
  Reply With Quote
Old 21st January 2010, 03:18 PM   #5
diyAudio Member
 
Soonerorlater's Avatar
 
Join Date: Jul 2008
Location: Belfast
Hi Trigger,
Is the third picture a picture of the amp you are taking parts out of?

( It has an mains earth crimp dangling over a valve base)

Best Regards
Bill
  Reply With Quote
Old 21st January 2010, 04:55 PM   #6
diyAudio Member
 
Soonerorlater's Avatar
 
Join Date: Jul 2008
Location: Belfast
Hi trigger,
Rotated the last picture 180 degrees and now I can see your work before and after.

Best Regards
Bill
  Reply With Quote
Old 21st January 2010, 08:20 PM   #7
diyAudio Member
 
Join Date: Nov 2007
Coupling caps should be 0.1uF or greater, 400v or higher rated.
Bypass should be 220uF or greater, 25v or higher rated. Buy a new one for this position.
  Reply With Quote
Old 21st January 2010, 10:40 PM   #8
diyAudio Member
 
Join Date: Sep 2007
I have an integrated UA-1 Heathkit using the same rectifier tube. I installed 120uf 450V caps in the power supply.

For coupling caps I used .22uF K40-Y 1000V Russian PIO. 220uf 50V Nichicon bypass caps. 22uf 100V coupling cap between the 6AN7 (pentode-triode) and the EL84s. I tried doubling the coupling caps to .44uF/ It either didn't make a noticable difference or I didn' leave them in long enough to break in.

I replaced all of the resistors with carbon film except for resistors directly in the audio path, carbon comp. I had a Russian PIO of the correct value for the feed back circuit, so I installed that.

Someone pointed out that replacing the .5watt resistors with 1 watt ones wouldn't cost a lot more, and that they should be quieter.

After rebuilding, the amp sounded AMAZING! Base improves, plus the pace of music improved. The last thing I tried was Belden cable from Parts Express, the RED!!! stuff with just a three conductor plug from Home Depot, another striking improvement!!

I discovered that the black plate square getter white lettering RCA 6AN7 were pretty remarkable. If you find a schematic using a pentode triode combo tube with single EL84 you can take the 12AX7 out and just add a second socket.

I mention that because really good sounding 12AX7s are hard to find, and are a lot more expensive than the RCA 6AN7 or other similar combo 9 pin tubes.

I recently traded some Raytheon black plate two mica NOS 5751 for a pair of Brimar box plate CV4004--finally found a 12AX7 that I really like. Since I like that tube so much, I plan to experiment using a 9 pin pentode to drive it in place of the pentode-triode tubes I like. tubes like D3A pentode are still pretty CHEAP, as are a lot of other very good pentodes.

Enjoy your new iron.

Bob
__________________
Still a novice with electronics.

Last edited by Bob, W7ETA; 21st January 2010 at 10:42 PM.
  Reply With Quote
Old 21st January 2010, 11:38 PM   #9
trigger is offline trigger  United States
diyAudio Member
 
trigger's Avatar
 
Join Date: Dec 2009
Location: michigan
well very sweet this amp has come alive, i replaced the caps that rota and bob mentioned, and thanks guys! nailed it!!!

the amp was missing the low end, i decided to increase the impedance load on the output, to 4 ohm, then 2, and well the problem came to light, a 6bq5 started glowing red, witch showed me the caps were the culprit.
anyways i replaced the two 1mfd (color banded) with two nice fat .39 goodalls, and that black coupling cap with a new one from radioshack, and boom..
the low end is very responsive and deep now, the highs light right up when the volume is at proper max, the amp also showed much more dynamics after the new caps installed.

thank you much!!!

i just need a sweet 12ax7 to rock, i have 3 tungsol 12at7's that were pulled from an organ and a few pair of 12au7's trade anyone?

i will replace the resistors and maybe the large caps on the power side, case the amp in wood and call it a day, half done and sounding sweet...
  Reply With Quote
Old 22nd January 2010, 01:20 AM   #10
diyAudio Member
 
Join Date: Sep 2007
Here you go:
0,22uF +/-10% 1000V PIO Capacitors K40Y-9 NOS Lot of 8 - eBay (item 260529442556 end time Jan-26-10 15:29:47 PST)

Fantastic caps for less than $2 each!
__________________
Still a novice with electronics.
  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
12AX7,6BQ5 PP Amp Search. Troncones Tubes / Valves 13 21st January 2010 02:12 AM
low gain 12ax7 schematic bakpaosiomay Analog Line Level 19 18th August 2009 11:27 AM
low end woofers dalcorn Multi-Way 11 7th April 2006 07:18 AM
high end version of low end phono stage dsavitsk Solid State 20 1st September 2005 11:51 PM
Ideas for 12AX7 / 6BQ5 headphone amp? paulb Headphone Systems 5 18th December 2003 06:44 AM


New To Site? Need Help?

All times are GMT. The time now is 02:52 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