Update :73 Silverface Twin

[gtrplayr1976]

Finally got back on this twin this week. Got everything back to where it was working as good as it was before. The noise I had was indeed a leaking cap. The 250pf. The BIG issue with this is lack of volume. It was like this when I got it the first time ,and replaced the filter caps ,and a burned resistor. I can crank this amp ,and it doesn't seem as loud as my 18 watt ,so I know there is an issue. Maybe speakers ? OT ? I want to find another cab to test the speaker theory.
Any thoughts ,or experience with this in a Twin ?

Thanks folks !

[Leo_Gnardo]

find another cab to test the speaker theory.
Any thoughts ,or experience with this in a Twin ?

Lots of places to lose your signal. Yes, for a quick check run it thru a set of known working good sounding speakers & if that ain't it then look at all the usual suspects & don't forget 1) the right tubes in the right places 1A) a pre tube whose gain isn't what it's supposed to be, 1B) Failed bypass cap in preamp cathode circuit (Twins are getting that old...), and B) a pre tube that's dropped a filament - one of my favorites.

There's lots more. You'll find it!

[gtrplayr1976]

Lots of places to lose your signal. Yes, for a quick check run it thru a set of known working good sounding speakers & if that ain't it then look at all the usual suspects & don't forget 1) the right tubes in the right places 1A) a pre tube whose gain isn't what it's supposed to be, 1B) Failed bypass cap in preamp cathode circuit (Twins are getting that old...), and B) a pre tube that's dropped a filament - one of my favorites.

There's lots more. You'll find it!

Thanks Leo_Gnardo. Everything on the board has been replaced. The tubes have had little use ,but that doesn't mean one isn't bad. I will need to pick up a couple of 12AT7s to fully test the preamp side. Both channels have about the same volume level (low)

[Tom Phillips]

... I will need to pick up a couple of 12AT7s to fully test the preamp side. Both channels have about the same volume level (low)

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

[gtrplayr1976]

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

I actually did add to the existing thread ,but thought it may be easier to start a new one. Shows how much I know. I thought I had posted voltages . I have them on here somewhere I think. They all seemed pretty normal though.

[Tom Phillips]

OK. I found that there was a third thread about this amp at http://music-electronics-forum.com/t32539/
That thread did have posted voltages but they were lost in the server problem of a few months ago.
It would be good if you post a schematic or layout drawing with the voltages filled in. That's much easier to interpret than a narrative or a table of voltages.
What other test equipment do you have. (Scope? Signal generator? Dummy Load?)

[gtrplayr1976]

OK. I found that there was a third thread about this amp at http://music-electronics-forum.com/t32539/
That thread did have posted voltages but they were lost in the server problem of a few months ago.
It would be good if you post a schematic or layout drawing with the voltages filled in. That's much easier to interpret than a narrative or a table of voltages.
What other test equipment do you have. (Scope? Signal generator? Dummy Load?)

All I have right now is a DVM.

[Tom Phillips]

All I have right now is a DVM.

Understood. We'll wait to hear the results of trying different speakers. Also please re-post any voltage readings you can take on the re-built amp.

[gtrplayr1976]

Understood. We'll wait to hear the results of trying different speakers. Also please re-post any voltage readings you can take on the re-built amp.

Will do Tom. I have the preamp voltages for the plates on V1-V4 , and the plate ,and grids for V5 , plate ,grid ,and cathodes for V6. The others on V1-V4 were around 2 volts. All the filaments were good. These are also before I found that I didn't have the Z wire in.
V1 pin 1 plate is low at 186 volts (before I changed the leaky cap), pin 6 is at 246.
V2 pins 1, and 6 are at 235 +/- 1 volt ,
V3 are 446 on both 1 and 6.
V4 are 245/251 ,
V5 1 is 440 ,6 is 388 ,2,and 7 are -37.
V6 1 and 6 are 328/321 , 2and 7 are 111 ,and 3 and 8 are 144.


I'm waiting to get the voltages from the power tubes sent to me. I'm away from home for a couple of weeks.

[gtrplayr1976]

Here are some of the voltage readings added to the schematic. The readings in red are the ones that may be different than what the schematic shows. The readings for the power tubes are all very similar to V7. I don't have some of the readings on the schematic ( I think the ones at the filter caps in the doghouse ) The others are pretty close. Bias ,and B+. I'll be getting my meter in the mail in a couple of days ,so I can take more readings. Thanks all.

[gtrplayr1976]

Ok. Tried a different cabinet. I can't really tell a difference. It seems if the Master is at 10 ,and I use the channel volume to control , it gets pretty loud at 4. Maybe I'm expecting too much ,and it's actually doing what it's suppose to do? I have new voltage readings. Some look a little high ,and some a little low according to the schematic.

[JoeM]

Was this amp modified at some time? Blackfaced? Have you checked the resistor values at the PI?

[gtrplayr1976]

Was this amp modified at some time? Blackfaced? Have you checked the resistor values at the PI?

Was never modified to my knowledge. Nearly all the components have been replaced (resistors ,and caps)
Noticed the Vibrato channel seems thinner that the Normal channel at same volume ,and tone controls the same.

[Leo_Gnardo]

Ok. Tried a different cabinet. I can't really tell a difference. It seems if the Master is at 10 ,and I use the channel volume to control , it gets pretty loud at 4. Maybe I'm expecting too much ,and it's actually doing what it's suppose to do?

That sounds about right. Something else to remember, the 60's-70's Fender amps with mid controls, if you dial all 3 bass mid & treble down to minimum, you get NO signal. Unlike a stock-wired Marshall or Tweed Bassman tone stack which still has a significant amount of mid coming thru. Gotta dial some EQ up to get signal through. That has foxed a lot of folks.

You can get a pretty good clean tone with the MV down to 7 generally, and doing so takes some hiss & hum out of the picture. This line of Fenders never was much good at getting an overdrive grind by dialing down the master and whupping up the pre volume. Although on a couple of occasions I've heard a guitarist get an acceptable tone that way. Definitely not a sizzling overdrive option.

[gtrplayr1976]

That sounds about right. Something else to remember, the 60's-70's Fender amps with mid controls, if you dial all 3 bass mid & treble down to minimum, you get NO signal. Unlike a stock-wired Marshall or Tweed Bassman tone stack which still has a significant amount of mid coming thru. Gotta dial some EQ up to get signal through. That has foxed a lot of folks.

You can get a pretty good clean tone with the MV down to 7 generally, and doing so takes some hiss & hum out of the picture. This line of Fenders never was much good at getting an overdrive grind by dialing down the master and whupping up the pre volume. Although on a couple of occasions I've heard a guitarist get an acceptable tone that way. Definitely not a sizzling overdrive option.

I always knew they didn't have much drive. Just mostly clean ,and loud. Just seems to me it should be louder than it is. The Vibrato channel sounds a little thinner than the Normal also. Volume doesn't kick in until I get past 2.