Update :73 Silverface Twin

[g1]

Many people bypass tone stacks to let more signal through. The downside being you lose the ability to shape the tone.
So it is up to you to decide which is more important, "fuller" sound versus adjustable tone.
You haven't mentioned how the tone controls are working (or not working) in Ch.1.

[gtrplayr1976]

Many people bypass tone stacks to let more signal through. The downside being you lose the ability to shape the tone.
So it is up to you to decide which is more important, "fuller" sound versus adjustable tone.
You haven't mentioned how the tone controls are working (or not working) in Ch.1.

I'll have to check that when I get off work this evening. The Vibrato channel has all the controls wired ,but it's so thin sounding compared to the Normal channel. You can see in this image how the controls are wired. The schematic shows the wipers of the bass ,and middle controls not connected.

[g1]

I'll have to check that when I get off work this evening. The Vibrato channel has all the controls wired ,but it's so thin sounding compared to the Normal channel. You can see in this image how the controls are wired. The schematic shows the wipers of the bass ,and middle controls not connected.

They are wired correctly (see layout drawing). The schematic symbol shows that they are wired as rheostats rather than potentiometers. For rheostats, one outside leg is either left unused or shorted to the wiper with a jumper.
So it sounds like it's not so much that ch.1 is fuller, but ch.2 is quite thin? This sounds more like the problem is in ch.2 than ch.1.

[gtrplayr1976]

They are wired correctly (see layout drawing). The schematic symbol shows that they are wired as rheostats rather than potentiometers. For rheostats, one outside leg is either left unused or shorted to the wiper with a jumper.
So it sounds like it's not so much that ch.1 is fuller, but ch.2 is quite thin? This sounds more like the problem is in ch.2 than ch.1.

I thought it was a problem with channel 2 also. I need to get channel 1 straight ,and then compare the two.

[gtrplayr1976]

The easiest solution is to use power resistor mounting brackets as shown on the bottom of the page at OHMITE Power Resistors . I have seen through rods used too but they were kept centered with large shoulder washers.The body of the resistor is insulated from the solder tab.

So I ordered 4 of these from Mouser about $16 with shipping. They sent me 400. LOL

[gtrplayr1976]

The easiest solution is to use power resistor mounting brackets as shown on the bottom of the page at OHMITE Power Resistors . I have seen through rods used too but they were kept centered with large shoulder washers.The body of the resistor is insulated from the solder tab.

Tom, I got everything up ,and running. I have the signal at the input, the scope ,and meter at the load. I powered up the Twin,and got the following. Looking at the photos the volume started showing at 2 continued to 3 ,4 ,then 6 ,8 ,10. The Master ,and other controls at 10 (except Reverb,and Tremolo at 0 ). I'm not seeing what I expected. The wave form is distorted after 3 ,and doesn't change much after 4 except the time ? seems to get a little closer together. It was like this on both channels. The voltage only went to 2.4 Vac ,and the wave didn't change whether I had the scope set at AC ,or DC , but there was no reading on the meter at DC. I blew the main fuse on the second run. Now I fear I had something connected wrong ,but I don't know what. This amp has run off ,and on for the past 3 years without blowing a fuse although I don't think it ever sounded like it should.

[g1]

Any chance you have your speaker cable reversed? I don't see any marking to show you which wire is hot and which is ground, you should mark them to make things easier for you. If your scope ground ends up connected to the tip of the speaker connector, you could be shorting the output to ground.

[gtrplayr1976]

Any chance you have your speaker cable reversed? I don't see any marking to show you which wire is hot and which is ground, you should mark them to make things easier for you. If your scope ground ends up connected to the tip of the speaker connector, you could be shorting the output to ground.

Never thought of that. I figured since the resistor wasn't positive/negative it wouldn't make a difference,but I guess it would to the scope. It only blew the fuse the second time I powered it up ,and after I was bringing the volume up ,and down .

[52 Bill]

The photos show something that I didn't know or didn't remember, this is a turret rebuild amp, so everything is suspect. Did you do the original board replacement?

There have been so many test equipment tutorials here I forgot what the real problem was.

[gtrplayr1976]

The photos show something that I didn't know or didn't remember, this is a turret rebuild amp, so everything is suspect. Did you do the original board replacement?

There have been so many test equipment tutorials here I forgot what the real problem was.

What I was trying to do here Bill was check the wattage output. I had thought this amp wasn't putting out near 100 watts. Yes sir I replaced the board, but it's very similar in layout to the original. Another issue I was hearing was the Vibrato channel sounded a lot thinner compared to the Normal. I discovered I left the .047 cap out of the circuit on the Normal channel ,so that could be the reason for that.

[Tom Phillips]

…I have the signal at the input, the scope ,and meter at the load. I powered up the Twin,and got the following. Looking at the photos the volume started showing at 2 continued to 3 ,4 ,then 6 ,8 ,10. .

What is your signal level at the input?

…The wave form is distorted after 3 ,and doesn't change much after 4... .

That could be normal depending upon the level of your input signal.

…voltage only went to 2.4 Vac ...

Any chance you have your speaker cable reversed? I don't see any marking to show you which wire is hot and which is ground, you should mark them to make things easier for you. If your scope ground ends up connected to the tip of the speaker connector, you could be shorting the output to ground.

I think g-one’s idea is good. The negative lead of most scopes is connected to chassis ground. Therefore, if the scope's negative is connected to the speaker hot lead then the amp's output is shorted. Since the short isn’t perfect you will still see some voltage. I wouldn’t expect it to reach 2.4V but who knows. Check the leads as g-one said before you do more testing.

…the wave didn't change whether I had the scope set at AC ,or DC , but there was no reading on the meter at DC….

That’s all normal. There should be no DC voltage on the output.

[gtrplayr1976]

What is your signal level at the input?

It should be 1K and 100mv (maybe 200) I'll check again. I could actually hear the tone . How could that happen without a speaker ?

I think g-one’s idea is good. The negative lead of most scopes is connected to chassis ground. Therefore, if the scope's negative is connected to the speaker hot lead then the amp's output is shorted. Since the short isn’t perfect you will still see some voltage. I wouldn’t expect it to reach 2.4V but who knows. Check the leads as g-one said before you do more testing.

So I connect the positive lead of the scope ,and the DVM to the positive lead from the speaker (Resistor load) ,and the negative lead of both to chassis ground ?

Thanks gentlemen !

[Jazz P Bass]

Keep the input signal at 100Mv's.
200 is too much.

The sound you heard was the output transformer 'singing'.

[gtrplayr1976]

Keep the input signal at 100Mv's.
200 is too much.

The sound you heard was the output transformer 'singing'.

I'll check that signal again. I thought I had it set at 100 ,but I may have changed it when I was experimenting.

[Tom Phillips]

...So I connect the positive lead of the scope ,and the DVM to the positive lead from the speaker (Resistor load) ,and the negative lead of both to chassis ground ? ...

Yes. However the DVM will not cause a problem even if the leads are reversed because the inputs are floating. The hookup of the DVM is important when you are measuring DC voltage and you want the polarity sign on the display to be correct. Also when you are testing diodes.

If you can hear the OT singing that's a pretty good indication that the amp is developing significant power. It's apparently not getting through to your test load resistor.

Always good to check everything.
Measure the resistance of your test load with it disconnected from the amp and verify it is correct.
Verify that you are connecting to the main speaker jack, not the external speaker jack.
Verify that the amp is still loud when you play into the real speaker.

Tom