73 Silver face twin...continuation :)

[gtrplayr1976]

Ok...so I have to turn the volume to at least 3 on master ,and volume to get enough sound to hear. Is this normal for these, or should I be looking for something? I 'm wondering if the speakers are bad, or miswired..or something. I have new tubes, almost all new caps. Checking the speaker jack it shows 3.4 ohms which I would assume is 4.
The amp is really quiet on idle. The only thing besides the volume is when I do play the lower frequency notes tend to have a little buzz/rattle...not a real good bass tone.

Thank you.

[gtrplayr1976]

Does anyone have a parts list for the early 70s Silver face ? I may need a bright switch. When it's switched to bright it makes a really loud hum, and no sound from the guitar...almost like it's grounded out?

[stereomonostereo]

That's not normal. You said it's an early 70's? I think the master volume came out around 1974. What circuit is it?

[gtrplayr1976]

This is a 73. My friend bought it new ,so I know it's correct.

[stereomonostereo]

Are both channels equally quiet? You might wanna check out your phase inverter resistors next.

[gtrplayr1976]

Yeah, they're both about the same as far as volume. My friend tells me it sounds fine. I was just thinking it would be a lot louder. It doesn't seem to get loud until you have both volumes at around 5 .

[Krwkka]

I don't have my '74 Twin tubed right now to compare results with you, but your scenario doesn't sound so weird to me. Turn the Master up to 10 and leave it there. Then put the volume on the channel to 4 or so. It should be quite loud. Crank the channel volume up to 6 and ears might start to melt internally.

Most people leave the master all the way up and we think it sounds best like that.

[gtrplayr1976]

Thanks ...I'll have to check that out when I get at the amp again. I do know it has a bad bright switch. When it's on it has a loud hum...I have a replacement on the way ,and a new cap.

[Krwkka]

It doesn't make sense to me that the bright switch would cause hum. Before you do anything I suggest that you put that master up and try running the amp. I think you'll find that everything is fine as it is.

[gtrplayr1976]

With the amp in normal mode (bright switch off) it's virtually noiseless ,but once you hit the switch..loud hum. I'll take care of that when I get the parts.
With the master at 10 there would be no noticeable boost when you pull the switch out ...correct ?

[Krwkka]

I just hooked up my Twin Reverb and here's what I can tell you.

With the master all the way up there is a noticeable boost in volume of maybe 4 or 5db when you pull the knob. I hear the same boost with the master on 3.

Back to the hum problem ... When I see the word "hum" I think of a deep, low frequency sound. However, when you hit the bright switch on this amp all that happens is that the high frequencies run at full tilt, completely bypassing that channel volume control. The lower you have the channel volume, the more apparent "boost" you will hear in the highs. If you are running the channel volume at 2 and have a poorly shielded guitar or a bad or unshielded patch cable it would certainly make sense that you'd hear a massive increase in "buzz" when you hit that bright switch.

Could this be what's happening?

[gtrplayr1976]

I notice a little boost only if the master is below 10. I couldn't really tell a difference at 10. Why do you think this part could be weak ?

Whatever the noise is when the bright is switched on is bad. It doesn't sound like a normal shield buzz. I'll have to take another listen when I have the opportunity ,so that I can get a better description of the noise, and what type it is. I'm not sure about my friends cable. Sounded fine without the bright switch.

[Krwkka]

Why do you think this part could be weak ?

I have no idea. The boost (as originally designed by Fender) is entirely independent of the master volume. I'm wondering if somebody might have changed the configuration of the boost circuit from its stock form. It's also possible that they changed the function of the bright switch.

Have you compared the wiring to the schematic?

[gtrplayr1976]

As far as I know no one has touched this amp other than me. My friend bought it new. It quit working, and sat for a few years. I told him I would look at it to see if I could get it working. I changed nearly all the caps, added a bias, and new tubes. The bias cap is a different value , but the one that was in it wasn't what was on the layout ,and the one on the schematic was different than that one, so....
Anyway, I will go through the schematic again following the master to see what I can find out.

[Krwkka]

Well good luck with this.

As to the speakers, have you checked to see if they are wired out of phase (this has nothing to do with the hum, of course). That would make for a "thin" sound...