• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Masco C-10 Mic/Phono Amp

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Jump to Latest
C

CraigNS

Member
Joined 2012
Hi,

I have an old Masco C-10 Amp I was trying to get going. I've tried reforming the caps and I've replaced the burnt tubes (specifically the twin diode rectifier tube). I was getting a good output but the transformer was heating up. So I then replaced the multi-cap can with some individual capacitors. Still heating up. I then drew out the schematic and noticed the 20k ceramic resistor going to ground. I had a look and it's on a two terminal block bolted to the chasis and the terminal has been busted off. It is now held in place with a wire to the adjacent terminal. It looks like to me this shouldn't be going to ground. Maybe someone with more knowlege than I can have a look at this circuit and tell me what they think.

Thanks,
Craig
 

Attachments

  • Masco C-10.jpg
    Masco C-10.jpg
    156.4 KB · Views: 75
jazbo8

jazbo8

Ex-Moderator
Joined 2011
CraigNS said:
Hi,

I have an old Masco C-10 Amp I was trying to get going. I've tried reforming the caps and I've replaced the burnt tubes (specifically the twin diode rectifier tube). I was getting a good output but the transformer was heating up. So I then replaced the multi-cap can with some individual capacitors. Still heating up. I then drew out the schematic and noticed the 20k ceramic resistor going to ground. I had a look and it's on a two terminal block bolted to the chasis and the terminal has been busted off. It is now held in place with a wire to the adjacent terminal. It looks like to me this shouldn't be going to ground. Maybe someone with more knowlege than I can have a look at this circuit and tell me what they think.

Thanks,
Craig
Click to expand...

The 20K as shown would pull a lot of current that's why the transformer is heating up, a more reasonable schematic is shown below, perhaps the 20K is sub'd for the choke?

An externally hosted image should be here but it was not working when we last tested it.


Jaz
 
Last edited:
C

CraigNS

Member
Joined 2012
jazbo8 said:
The 20K as shown would pull a lot of current that's why the transformer is heating up, a more reasonable schematic is shown below, perhaps the 20K is sub'd for the choke?

An externally hosted image should be here but it was not working when we last tested it.


Jaz
Click to expand...

Right now, the center tap from the B+ transformer is going to chasis ground. Also it's using a multi-cap can, so no way to grab the negative of the 10uf/20uf cap. Do you think the center tap should be going to this resistor?
 
J

jjman

Member
Joined 2009
The 20k is a bleeder for safety, assuming the scheme in post#1 is accurate. It drains the dc after turning off. You can use a larger value if you are concerned it's pulling too much current. I'm actually using a 1meg on an amp but that's overly "slow."
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Top Bottom