problem on thorens PR15 (strange filaments circuity) - 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 8th October 2006, 08:14 PM   #1
bembel is offline bembel  Europe
diyAudio Member
 
bembel's Avatar
 
Join Date: Aug 2006
Send a message via Yahoo to bembel
Default problem on thorens PR15 (strange filaments circuity)

Hi all,
today I found a nice wharfedale "rosedale" speaker and decide to recall to service my dusty Thorens PR15.
I notice then (having changed tubes), that I have problem with this strange filament circuit, the symptomes are:
- when music is played softly (~<1W) filaments (of preamp tubes) are way too hot !!! and there is distortion.
- when music is played loud (~>1W) filaments (of preamp tubes) are still hot but distortion is gone.

Here is a schematic (I'm sorry for the bad quality) that I have, but it don't seems to be anymore on internet (if anyone find a link or have it ...)


Any Ideas about the problem ?
Attached Images
File Type: jpg pr15.jpg (82.0 KB, 596 views)
__________________
Plus je pédale moins vite, moins j'avance plus vite !
  Reply With Quote
Old 8th October 2006, 10:49 PM   #2
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
The filaments of the 12AX7's are being used as the source for the bias on the output tubes. I have seen this before (old Fishers I think). They are getting too hot because the output tubes (or something else) is drawing too much current.

I would bet on the coupling capacitors (the .1uF's on the grids of the 6L6's). Check the voltage on either side of the 330K grid resistors. If there is a significant difference the caps are leaking (or your meter has a low input resistance).
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 9th October 2006, 01:03 AM   #3
diyAudio Member
 
HollowState's Avatar
 
Join Date: Aug 2006
Location: Taxland, New Jersey
Yup, what tubelab said. There's one other outside possibility. And that's excessive heater to cathode leakage in the rectifier tube. From the schematic, it looks as if they're running it with the other tubes. They should have used a seperate filament winding for that tube.

Victor
  Reply With Quote
Old 9th October 2006, 01:53 AM   #4
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Victor,
This configuration for the 12AX7 heaters is well within their ratings. I think tubelab has hit the problem on the nose.

-Chris
  Reply With Quote
Old 9th October 2006, 02:30 AM   #5
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Also, one of 6L6 grid resistors may be open.
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers.” -- Plato, 447-367 BCE
  Reply With Quote
Old 9th October 2006, 09:00 AM   #6
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
I've seen something similar in a Bell 6060 amp.
Bell 6060 #1
Bell 6060 #2
  Reply With Quote
Old 9th October 2006, 12:09 PM   #7
bembel is offline bembel  Europe
diyAudio Member
 
bembel's Avatar
 
Join Date: Aug 2006
Send a message via Yahoo to bembel
thx for your replies,
I think Tubelab pointed it too (the usual suspect...),
I had no time to look at it yesterday, but there is a least one error on the schematic: the rectifier is a GZ34.
from the schematic I think bias should be -(12.6+12.6+(39*0.15))
right ? (I am not sure to read correctly the resistor)
__________________
Plus je pédale moins vite, moins j'avance plus vite !
  Reply With Quote
Old 9th October 2006, 01:07 PM   #8
bembel is offline bembel  Europe
diyAudio Member
 
bembel's Avatar
 
Join Date: Aug 2006
Send a message via Yahoo to bembel
Ooops, my memory fools me ... schematic is right rectifier is a EZ81

I just had a look at the beast, and it seems that the series DC filament resistor (that is hard to read (29R ? 39R ?)) is a CTN or CTP !!! Is it original or replacement I don't know

I have to unsolder it and test it (maybe later today, with photos to come)
That could explain some of the facts, But what to do then ? (as I can't read the proper value on the schematic)

Any suggestion ?
__________________
Plus je pédale moins vite, moins j'avance plus vite !
  Reply With Quote
Old 9th October 2006, 03:59 PM   #9
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally posted by DigitalJunkie
I've seen something similar in a Bell 6060 amp.
Bell 6060 #1
Bell 6060 #2
A bulb is a CCS for infrasonic frequencies.

Quote:
Originally posted by bembel
thx for your replies,
I think Tubelab pointed it too (the usual suspect...),
I had no time to look at it yesterday, but there is a least one error on the schematic: the rectifier is a GZ34.
from the schematic I think bias should be -(12.6+12.6+(39*0.15))
right ? (I am not sure to read correctly the resistor)
It should be 22-24V for 6L6 tubes. Don't torture the resistor, if it is bad you'd not see any filaments glowing.
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers.” -- Plato, 447-367 BCE
  Reply With Quote
Old 9th October 2006, 08:40 PM   #10
bembel is offline bembel  Europe
diyAudio Member
 
bembel's Avatar
 
Join Date: Aug 2006
Send a message via Yahoo to bembel
Indeed the resistor is a CTN/CTP (i allways miskate) here is a picture
the filament serie resistor is the closer one with no label.

> Its resistance is 30ohm cold (maybe 29R in the schematic)
> and higher when hot (about 45R with a lighter on for ten secionds)

Also I'm not really sure about current path ? doesthe B+ go back to the - side of HV (so it have to be 150mV !) ?
Attached Images
File Type: jpg resistor.jpg (58.7 KB, 338 views)
__________________
Plus je pédale moins vite, moins j'avance plus vite !
  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
Thorens TD160 MkII armlift problem ddutch Analogue Source 3 29th September 2007 12:44 PM
Thorens TD 150 problem johnsmills Analogue Source 3 11th September 2006 03:04 PM
Help with basic Thorens TD-160 problem Chris Balton Analogue Source 5 5th January 2006 08:08 PM
strange amp problem feelander Car Audio 1 8th June 2005 07:21 PM
Strange little problem (or maybe not so) elizard Chip Amps 8 4th January 2004 06:25 AM


New To Site? Need Help?

All times are GMT. The time now is 12:10 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