Help me understand plate voltages

[Garydean]

Lafayette Bass Head late 60's

In this schematic, plate voltages are listed. When I take plate voltage readings from the pins. Should the readings I get be within 10% of what is listed in the schematic? Where should I look if the plate voltage measurements are much higher then what is listed on the schematic?

specifically I'm referring to the 12ax7 and the two 12au7's

thank you,

Gary

[Old Tele man]

...yes, but remember that was with 115VAC line input, if your wall power is 125VAC, your readings will be proportionately higher by the PT-secondary voltage-stepup ratio!

[Garydean]

very true...I did actually dial up a line voltage of only 115VAC. I have to go back and measure again, but if I remember correctly, it was over 225 on pin 1 of the 12ax7.

thanks again.

[52 Bill]

Are the measured B+ voltages the same as shown on the schematic? Are the plate resistors within tolerances?

The first place I'd look, would be the cathode circuits. If the tube isn't drawing enough current through the plate resistors there will not be a large voltage drop and you'll see high plate voltages.

[Garydean]

Thanks for writing. I'm going to check and write down all my readings tonight. If things look funny, I'll lift a few resistors and go from there. Thanks again.

Gary

[Garydean]

Sure enough the plate voltages on V2 are high, 260. I lifted the cathode resistor off V2 and it measured 3.3K yet the code is 1.5K. According to the schematic it should be 15K. I checked the RCA manual under resistance coupled amplifiers and 1.5K makes more sense. Resistors tend to drift lower yet this 1.5K has doubled. Would this be a reason why the plate voltages are high?

I also noticed the plate voltages on V3 are less than half of what is called for in the schemo.

B1, B2 and B3 voltages are right on the money.

thank you again.

[drewl]

No, resistors "drift" higher, that is over time the resistance increases due to heat and stress.

[Mars Amp Repair]

What is actually wrong with the operation of the amp? No sound? low volume? I never saw that mentioned anywhere....glen

[Garydean]

Thanks for your reply. Actually, the amp came in with no tubes and a few visually troubled caps. I put in a 3 prong, replaced the caps and a few resistors. I fired it up to check the voltages, no signal and no load. I've yet to test drive it. I'm hung up on the high voltages at this point. Are you thinking perhaps the voltages are fine?

Gary

[jcollins]

Voltages with tubes actually in the sockets?

[Garydean]

yes

[Garydean]

I fired up the amp yesterday and it does sound good, however, the output plates are showing a bit of orange. Nothing that I've done has changed the plate voltage of the ax7.

V1 Resistance
cathode 1 to ground 3.4k
cathode 2 to ground 3.6k

V2 Resistance
cathode to ground 13.6k

V3 Resistance
cathode to ground 468 ohms

Plate voltages.
V1 40V & 42.3V
V2 258V & 256V
V3 105V & 108V

Cathode voltages
V1, V2, V3
1.88, 18.5, 3.5

I adjusted the bias pot to give me -32 and the above voltages dropped about 2%.

I do have 158V on the other side of the 220K plate resistor on V1.

I'm reading 275V on the other side of the plate resistors of V2.

I subbed in 3 different 12ax7's and no change.

I lifted the 5K 2watt resistor and it measured 5,050 ohms. Volts going into that resistor is 327 and coming out is 277.

Obviously the 277V is a bit high, it should be 260V.

I replaced both screen grid resistors for the 6L6's. Both measured 110K. I replaced both of them with a 100K resistor. Should I have gone for 82K resistors instead?

thanks!!!