problem on thorens PR15 (strange filaments circuity)

[bembel]

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 ?

[tubelab.com]

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).

[HollowState]

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

[anatech]

Hi Victor,
This configuration for the 12AX7 heaters is well within their ratings. I think tubelab has hit the problem on the nose.

-Chris

[Wavebourn]

Also, one of 6L6 grid resistors may be open.

[DigitalJunkie]

I've seen something similar in a Bell 6060 amp.
Bell 6060 #1
Bell 6060 #2

[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)

[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 ?

[Wavebourn]

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.

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.

[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 !) ?