Update :73 Silverface Twin

[Leo_Gnardo]

Volume doesn't kick in until I get past 2.

Blame the volume pot, also Fletcher & Munson. F&M described all the way back in the 20's that human hearing tends to ignore high and low frequencies at low volumes. Hence the "loudness" button on hi fi preamp dashboards, that compensated for this to some degree by boosting hi's and lows.

[Tom Phillips]

My comments:
1) It doesn't mean much to split hairs on the knob numbers when they are set to low values on this amp. (This has been mentioned before in this thread)
2) It's hard to evaluate from the narrative. Would be good if you can actually measure the power or play this amp side-by-side with another Twin Reverb. (And then don't worry if "3" on one is the same as 4.5" on the other)
3) I do agree that a Twin Reverb should sound loud so that's confusing.
Tom

[gtrplayr1976]

My comments:
1) It doesn't mean much to split hairs on the knob numbers when they are set to low values on this amp. (This has been mentioned before in this thread)
2) It's hard to evaluate from the narrative. Would be good if you can actually measure the power or play this amp side-by-side with another Twin Reverb. (And then don't worry if "3" on one is the same as 4.5" on the other)
3) I do agree that a Twin Reverb should sound loud so that's confusing.


Tom

Yes it would be nice to compare it with another side by side. How can you measure the power ? Don't get me wrong. It's loud. I just keep hearing you can't stay in the room with it past 2-3. I didn't hear it before I got it, and didn't get to hear it until I replaced the caps, tubes ,and a burned resistor.

[Tom Phillips]

…How can you measure the power ?...

Basically by connecting a 4Ω resistive dummy load in place of the speaker. Then drive the input with a signal generator while monitoring the voltage across the resistor with a voltmeter and a scope. When the signal just starts to clip then you calculate the power using the formula "Voltage Squared divided by load reistance=Power” That will be the max clean power of the amp can put out. The methodology of doing this will become clear when you get your bench setup as is being discussed in your OScope thread.

…Don't get me wrong. It's loud. I just keep hearing you can't stay in the room with it past 2-3...

There is a huge amount of variation in these amps due to the pots used and even where the knob is attached to the shaft. If your amp needs to be set to 4-5 rather than 2-3 to drive you out of the room then so be it. You can get those variations in a side-by-side amp test too.

[gtrplayr1976]

Basically by connecting a 4Ω resistive dummy load in place of the speaker. Then drive the input with a signal generator while monitoring the voltage across the resistor with a voltmeter and a scope. When the signal just starts to clip then you calculate the power using the formula "Voltage Squared divided by load reistance=Power” That will be the max clean power of the amp can put out. The methodology of doing this will become clear when you get your bench setup as is being discussed in your OScope thread.

There is a huge amount of variation in these amps due to the pots used and even where the knob is attached to the shaft. If your amp needs to be set to 4-5 rather than 2-3 to drive you out of the room then so be it. You can get those variations in a side-by-side amp test too.

Thank you sir. I will understand more as I go ,and this will help a lot.

[gtrplayr1976]

What would make the Vibrato channel sound thin ? I don't have the reverb tank connected ,and the tremolo ,speed,and intensity controls at zero. I've even had all tone controls to 10 ,and backed off the treble (sounds bad like that). The bass control just doesn't seem to change as it should compared to the Normal channel.

[gtrplayr1976]

[QUOTE=Tom Phillips;312010]I think you meant 12AX7s. You can still swap tubes using the X7s from the tremolo and reverb recovery circuits. If the problem is the same with both pre-amps it is less likely to be a first stage preamp tube problem.

I skimmed through the original thread on this amp (It would have been better to add to the existing thread for continuity of the discussion history)

Anyway ... I don't see anywhere that you have taken voltage measurements. Do you have test equipment? If so I'd recommend that you measure the voltages that are listed on the Fender service information. Write down your measurements next to the factory "guidelines" and post them here. I said "guidelines" because you will not get the exact same values. We will be looking for grossly out of whack values and trends.

Cheers,
Tom[/QUOTE

12AT7s Tom. The phase inverter ,and V3 use AT7s ,but I may not need them in order to check this. I can omit V3 ,and use an AX7 in V6...right ?

[Tom Phillips]

...12AT7s Tom. The phase inverter ,and V3 use AT7s ,but I may not need them in order to check this. I can omit V3 ,and use an AX7 in V6...right ?

I made the X7 comment because you were talking about troubleshooting the preamp. When you say “check this” I assume you mean that you want to determine if a tube problem is causing the vibrato channel to sound weak. I don’t know where you are going with “…I can omit V3, and use an AX7 in V6...right?” but first it’s important to understand what each tube does in the circuit and be able to visualize the signal flow through the circuits.
Starting on the preamp side let’s agree that “V1” is the first small preamp tube and “V10” is the power tube at the other end of the chassis.

The tube functions are:
V1: 12AX7 - Normal Channel preamp
V2: 12AX7 - Vibrato Channel preamp
V3: 12AT7 – Reverb driver
V4: 12AX7 – Reverb recovery and additional vibrato channel gain stage
V5: 12AX7 – Tremolo circuit (no audio signal flows through this tube)
V6: 12AT7 – Phase inverter / Driver

If you pull some tubes then the amp may still work but you just loose the function associated with that tube. A missing tube will also cause the plate voltages on the remaining tubes to rise slightly but this is not really a problem for troubleshooting purposes. You can swap 12AX7s for other 12AX7s and likewise for T7s. You can even swap X7s for T7s & Ts for Xs and the amp will still work with the possible exception of a T7 in the tremolo position.

Now what are you trying to troubleshoot / prove?

It’s a good idea to mark the original positions of all tubes so you can keep track of the changes. This tube swapping is usually done for a quick test when a fresh known good tube is not available. In the shop I would be substituting new tubes for the test.

As to the low power problem the power measurement with a dummy load, signal oscillator and scope that you are working toward will tell what’s really going on. Speaker swap too. Have you verified that your two speakers are wired in phase? Easy to Google that if you need more info. It’s a simple test with a battery.

[gtrplayr1976]

I made the X7 comment because you were talking about troubleshooting the preamp. When you say “check this” I assume you mean that you want to determine if a tube problem is causing the vibrato channel to sound weak. I don’t know where you are going with “…I can omit V3, and use an AX7 in V6...right?” but first it’s important to understand what each tube does in the circuit and be able to visualize the signal flow through the circuits.
Starting on the preamp side let’s agree that “V1” is the first small preamp tube and “V10” is the power tube at the other end of the chassis.

Ok, I think I get it now. V3, V4 ,and V5 only operate the reverb ,and tremolo effects ,so they really wouldn't cause the thin sound,or the low volume issues on the vibrato channel ?

Now what are you trying to troubleshoot / prove?

I am trying to figure why the vibrato channel seems weaker ,and thinner sounding. I may have solved part of this with the bright switch. Seems that the switch may be bad ,and I also had a weak connection.

It’s a good idea to mark the original positions of all tubes so you can keep track of the changes. This tube swapping is usually done for a quick test when a fresh known good tube is not available. In the shop I would be substituting new tubes for the test.

I do have some new x7s ,and swapped them ,but it didn't seem to make a difference.

As to the low power problem the power measurement with a dummy load, signal oscillator and scope that you are working toward will tell what’s really going on. Speaker swap too. Have you verified that your two speakers are wired in phase? Easy to Google that if you need more info. It’s a simple test with a battery.

I have the scope ,and generator. I just need to know where to connect them ,and what the settings should be. I am familiar with the battery test.
I have discovered the the normal channel seems fine. It is the vibrato channel that is the issue.

Thanks Tom !

[Tom Phillips]

Ok, I think I get it now. V3, V4 ,and V5 only operate the reverb ,and tremolo effects ,so they really wouldn't cause the thin sound,or the low volume issues on the vibrato channel ?...

Yes. V3: 12AT7 - Drives the reverb pan and would not cause your symptom.

Expanding on V4: 12AX7 – It is used for reverb recovery and an additional vibrato channel gain stage. Look at the schematic you attached to post #10. The first triode section of V4, V4a, amplifies the small signal from the output of the reverb tank. That signal is summed with the output of the Vibrato channel and then passes through an additional gain stage that uses the second half of V4, V4b. Therefore, all the signal from the vibrato channel passes through V4b. V4b and the associated passive components could cause your problem. One way to test if that is the case would be to inject a signal into the vibrato channel and then trace the amplitude changes as it passes through the circuits. That’s what you are going to learn to do with your new test equipment. Have you started collecting YouTube tutorials about using your equipment?

I am trying to figure why the vibrato channel seems weaker ,and thinner sounding. I may have solved part of this with the bright switch. Seems that the switch may be bad ,and I also had a weak connection. ...

Although the Vibrato channel signal does pass through V4b, that is not the only place there could be a problem. Read on for a more likely cause.

The light dependent resistor (LDR) in the tremolo light bug can fail such that the dark resistance is too low. That loads down the Vibrato channel signal. An easy test for this is to unsolder the wire connected to the middle terminal of the tremolo intensity pot. Try that and tell us if the Vibrato channel comes back to full life. For further information, while you have the wire disconnected, with the amp off, measure the resistance from that wire to chassis ground and report the value. Cover the light bug to keep the light out when you make the measurement.

[gtrplayr1976]

Expanding on V4: 12AX7 – It is used for reverb recovery and an additional vibrato channel gain stage. Look at the schematic you attached to post #10. The first triode section of V4, V4a, amplifies the small signal from the output of the reverb tank. That signal is summed with the output of the Vibrato channel and then passes through an additional gain stage that uses the second half of V4, V4b. Therefore, all the signal from the vibrato channel passes through V4b. V4b and the associated passive components could cause your problem. One way to test if that is the case would be to inject a signal into the vibrato channel and then trace the amplitude changes as it passes through the circuits. That’s what you are going to learn to do with your new test equipment. Have you started collecting YouTube tutorials about using your equipment?

I do have a signal generator (which I am unsure how to set) ,and a scope. I have been watching some videos on scopes ,but I don't really see anything that helps me with the signal generator. It's a Heathkit IG 72.

The light dependent resistor (LDR) in the tremolo light bug can fail such that the dark resistance is too low. That loads down the Vibrato channel signal. An easy test for this is to unsolder the wire connected to the middle terminal of the tremolo intensity pot. Try that and tell us if the Vibrato channel comes back to full life. For further information, while you have the wire disconnected, with the amp off, measure the resistance from that wire to chassis ground and report the value. Cover the light bug to keep the light out when you make the measurement.

I will give this a try when I get home this evening.

[Jazz P Bass]

Why are we 'watching some videos on scopes'?
Hook it up to something.
The scope is simply a visual volt meter.
Red to test point, Black to ground.
One knob will adjust the height & another will adjust the width.

[gtrplayr1976]

Why are we 'watching some videos on scopes'?
Hook it up to something.
The scope is simply a visual volt meter.
Red to test point, Black to ground.
One knob will adjust the height & another will adjust the width.

I had used one years ago Jazz ,but I had a guide to go by. Told me where to hook it , what the settings were , and what to look for. I'm sure I can hook it to something. I'm just not sure what I would be looking for ,but it would be a learning experience.

[Jazz P Bass]

In audio, I generally use a scope to look at a Vac signal.
Hook up the two leads to a speaker on any amp & play with the scope knobs.
It's not like you are going to hurt it.
I like the way an actual guitar signal looks.
As opposed to a pure syne wave.

[Tom Phillips]

You can also just connect your scope to the output of your signal generator and experiment with the settings on each to learn what the various controls do.