Low Volume 5150

[Valvehead]

early model 5150 head..very low volume..sounds like about 5 watts output...not 120w. if i run it fx loop send into a different power amp , i have full volume. I found a screen resistor open on a power tube socket ---replaced it .

Swapped all tubes , cleaned sockets , cleaned fx loop jacks , all socket voltages / bias are there , poked and prodded , moved wires ...

heres 1 thing - the standby LED does not light up . Ive checked R 70 / 72 / 73 . I see that Q6 is right in the path of that Led. I do have 8vdc at 1 leg of the standby Led.

i remember reading that there might be a bad "muting" ? transistor that might cause this ? If the standby led is bad , can it kill signal ??

so for transistors on the pcb, we have Q6 / Q7 /Q3 and CR23

I know Ive read about this volume problem before , somewhere...... here is the scem and layout

[The Dude]

What is the voltage across the LED- not referenced to ground? Check the LED with your DVM. Do you have screen voltage at the output tube sockets?

[Valvehead]

8.5 vdc

or -8.5 vdc reversing leads

yes all voltages are at sockets

[The Dude]

If you have 8.5V across an LED and it's the correct polarity, the LED is open.

[Valvehead]

If you have 8.5V across an LED and it's the correct polarity, the LED is open.

ok--looking at the scem--could that cause my volume issue?

[Jazz P Bass]

Check the Vac at the output tube grids (100Mv/ 1K input, all controls maxed)
A 50 watt amp should be seeing about 50Vac at the grids. (double that for a 100W)

If you have that, then the OT is suspect.

[The Dude]

ok--looking at the scem--could that cause my volume issue?

Nope, not if you have screen voltage. When screen voltage comes up (standby flipped), it turns on Q6 giving the LED a ground path. IMO, your volume issue is unrelated,....... again assuming you have screen voltage and you said you did.

Edit: You could also have cracked/cold solder on the LED, which is why I asked to test the LED with a DVM.

[Valvehead]

Check the Vac at the output tube grids (100Mv/ 1K input, all controls maxed)
A 50 watt amp should be seeing about 50Vac at the grids. (double that for a 100W)

If you have that, then the OT is suspect.

so volt meter from pin4 to pin 5 (6l6 ) ac ?

[Helmholtz]

A 50 watt amp should be seeing about 50Vac at the grids. (double that for a 100W)

The power tubes' grid signal voltage for full/max output is solely determined by the bias voltage (Vbias) - independent of number of tubes or rated output power.
E.g., with a bias of 40V, required grid signal voltage is Vbias x 0.707 = 28Vrms or 80Vpp. No difference between 50W or 100W versions as long as bias is the same.

[Valvehead]

power tubes-
plates are 476vdc
pin 4 476 vdc
pin 5 -54 vdc

from pin 4 to 5 i have 0 vac


both channels have low volume and all controls function normal

[DFuller]

Oh, the infamous 5150 volume drop. Check the ribbon cable attaching the power tube board to the main board.

[Valvehead]

Oh, the infamous 5150 volume drop. Check the ribbon cable attaching the power tube board to the main board.

Checked , cable is good , checked continuity

[Valvehead]

pulled the main pcb--looks great--all joints ok , pulled the riveted in power tube pcb---all ok there .

pulled all ribbon cables -- they look good and test good

tested the OT primary , i have 28 / 60 / 32 ohms , and no shorts to the secondary

I have bias and all normal voltages at all tube sockets

speaker jacks are good , as is the imp. switch ........cleaned all jacks , pots

could the OT be on its death bed ?? I just cant find anything wrong

[DFuller]

EDIT: Actually, a thought. These amps have Flyback prevention diodes. I wonder if they've gone conductive?

[Enzo]

Easy enough to check. But usually if those fail, it blows fuses.


So did you ever remove Q7? That is my main go-to for this symptom.