• 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.

Need help to restore Sansui 500A

I am restoring a Sansui 500A amp sections.
Use a cd player to play a 1Khz sinewave.
Measure waveform from output transformer with 8ohm load, distortion found.

Installed 2 6aq8, measure waveform at C707, C716, waveform looks good.

Installed 2 6bl8, measure waveform at C804, C805, C809, C810, the waveform looks different between R&L channels. The voltage I got is quite some different from reference voltage from the schematic.

Voltage after the rectifier measure good according to schematic.
If I want to replace C804, C805, C809 and C810, what type of capacitor to use?

Sansui 500A Service Manual
 

LinuksGuru

Member
2008-08-16 4:57 pm
Hi, fastvideo,

Although I don't have Sansui 500A, but rather Sansui AU-70 (waiting for restoration), and AU-111 (to be completely rebuilt), advices you are asking for are common for vintage equipment restorers.

I would advise you to learn tube schematic a little bit to avoid common problems and mistakes.

I have comprehensive walk-through in my blog, for Pioneer SM-83 and Fisher 400. Sansui 500A is very close to these two.

Pioneer SM-83 Vacuum Tube Amplifier Restoration | MacGuru HQ
Fisher 400 Vacuum Tube/Valve Tuner/Amplifier Restoration | MacGuru HQ
 
Hi, fastvideo,

Although I don't have Sansui 500A, but rather Sansui AU-70 (waiting for restoration), and AU-111 (to be completely rebuilt), advices you are asking for are common for vintage equipment restorers.

I would advise you to learn tube schematic a little bit to avoid common problems and mistakes.

I have comprehensive walk-through in my blog, for Pioneer SM-83 and Fisher 400. Sansui 500A is very close to these two.

Pioneer SM-83 Vacuum Tube Amplifier Restoration | MacGuru HQ
Fisher 400 Vacuum Tube/Valve Tuner/Amplifier Restoration | MacGuru HQ

Thank you, it is very useful.
On the Pioneer Amp, you replaced 7189A with 6P14P-EV.
I just got 4 6P14P-EV, but it did not work with my unit.
Is this pin to pin replacement tube?
 
OK, then you need to rewire the O/P tube sockets to match the 7189 data sheet (nothing attaches to pins 1/6/8) and reduce the "idle" current slightly by increasing the bias voltage (C1 - C4) to -19 V.
Thank you.
There is nothing attach to pin 1/6/8 from the output socket, put the 6P14P-EV in only very weak and distorted output.
I measured the original 7189A tube pin 1,2 short together, pin 6,9 short together. But 6P14P-EV pin 1,2 open, pin 6,9 open.
I rewired the tube socket with pin 1,2 together and pin 6,9 together, install all 4 6P14P-EV, I have good sound output, (with C1-C4 bias voltage at -17) one of output becomes red hot on each channel.
I am stuck from here now.
 
Thank you.
There is nothing attach to pin 1/6/8 from the output socket, put the 6P14P-EV in only very weak and distorted output.
I measured the original 7189A tube pin 1,2 short together, pin 6,9 short together. But 6P14P-EV pin 1,2 open, pin 6,9 open.
I rewired the tube socket with pin 1,2 together and pin 6,9 together, install all 4 6P14P-EV, I have good sound output, (with C1-C4 bias voltage at -17) one of output becomes red hot on each channel.
I am stuck from here now.

See post #12 - you need to increase the negative bias to -19V. Also have you replaced all coupling caps to the output tubes as suggested?
 
I am just starting my 500A rebuild. I may just change it to use el84 instead of 7189A. Cost of 7189A is a little steep to justify the benefits to me. Let's keep each other informed on the build progress and leads/deals on replacement parts.


Use the Russian 6P14P-EV which as Eli states is a 7189 replacement, they are rugged and will live in applications where most modern 6BQ5 may not.
They're not very expensive at all, and sound quite good. (I use them in one of my amps which was melting JJ 6BQ5 on a regular basis.)

Note that you may need to jumper some pins as noted in post # 14. You may also need to adjust the bias voltage.
 
I rewired each output tube socket with pin 1,2 together and pin 6,9 together, install all 4 6P14P-EV, I have good sound output, (with C1-C4 bias voltage at -19) one of output still becomes red hot on each channel.
The tube that connected to C804 and the other that connected to C809.

Have you replaced those caps? Measure the grid bias voltage with a good dvm directly at the grid of each output tube and compare the good and the bad. Try swapping the tubes and see if it stays with the socket or the tube.

It could be that you need even more negative bias or you need to slightly reduce the screen voltage. (A few % will usually do it.)
 
EL84

You can't replace the 7189A with EL84's, they maybe look a same, they have a same pinout, but they would only function for a short time in this amplifier.
Check the the tube data, for max plate voltage , max plate dissipation and the rest.

I am just starting my 500A rebuild. I may just change it to use el84 instead of 7189A. Cost of 7189A is a little steep to justify the benefits to me. Let's keep each other informed on the build progress and leads/deals on replacement parts.