High V1 Cathode Voltages*

[plexi50]

2204 & AA864 Combo board build

What in the name of Batman and Robin could make my V1A & B cathode voltages sky high on the 2204 channel?

V1A is 4.95VDC and V1B is 5.24 VDC / This is the 2204 preamp tube. The AA864 Bassman channel is deep and wonderful with no issues

The 2204 channel will squeel to haids with just a small amount of volume or gain being used

I have never seen a cathode voltage this high before in any preamp section

I have swapped differrent tubes out for V1 and the voltages do change but are still outa sight

[Fresh_Start]

OK - assuming a 1.5K cathode resistor, current would have to be 3.3ma per triode (5 VDC).  Are the plate voltages normal?  Unless plate voltage is really low, you're way over rated dissipation for a 12AX7.  Check plate (and cathode) resistor values.  Power rail voltage at the last supply node?

Any chance there's a short at the tube socket between plate & cathode or heater & cathode?  Tube socket wired wrong?

Sorry... this isn't anything other than the obvious.

Chip

[eleventeen]

Fresh has it right. The tube is drawing waaaaay too much current. However, my suspicions would launch into either a leaky coupling cap before the tube; or a bad/incorrect resistor or set of resistors setting up the bias for the tube.

[tubesornothing]

Post the pin voltages - all will be revealed.

[plexi50]

My B+ with a SS bridge is 617 VDC on the EL34 plates. This wont last long. Like 30 minutes? INSANE!
With a 5U4GB the B+ is 585+-

I put a 22K 3 watt through the screens to the 82K-100k coupling and got the B+ down enough to get the cathode voltage to go down as well from 4.95VDC pin3 and 5.24 VDC pin8

Here are my V1A&B voltages below: I know a full amp voltage chart is needed to accuratley get to the root of the problem. I will write them all down tommorrow /  I like that all will be revealed stuff!

 V1:
1-219 VDC
2-0
3-1.907 VDC
4-6.38 VAC
5-6.38 VAC
6-241 VDC
7-0
8-2.267 VDC

[Fresh_Start]

Now you're pulling 1/2 watt through Section 2 (pins 1-2-3), assuming an 820 cathode resistor.  Section 2 is pulling .20 watts, assuming a 2.7K cathode resistor.  However, you must have been pulling a whole lot more current/power through those components before you dropped your B+.  If you're using 1/2 watt plate & cathode resistors, I'd check each one's value - especially if carbon comp.  Knowing the voltage drop across the plate resistors will help.

Chip

[plexi50]

B+ VDC to 100K plates of V1 is 284VDC. Voltage drop on pin #1 is 214VDC / pin #2 is 235VDC

Cathode pin 3 is 2.7K / Cathode pin 8 is 4.7K (more gain)

Cat and plate resistors are dead on there correct values / Metal film

[Fresh_Start]

B+ VDC to 100K plates of V1 is 284VDC. Voltage drop on pin #1 is 214VDC / pin #2 is 235VDC

Cathode pin 3 is 2.7K / Cathode pin 8 is 4.7K (more gain)

Cat and plate resistors are dead on there correct values / Metal film

OK.  A 4.7K cathode resistor produces LESS gain, not more.  It also raises the DC voltage on the tube's cathode, all other things being equal. 

I don't understand where the "voltage drop" numbers come from if B+ is 284.  In biasing a tube, we are concerned with the voltage differential between the control grid and the cathode.  All current passes through the plate resistor and the cathode resistor, so you can measure plate current through either (a) voltage drop across plate resistor, or (b) voltage drop across cathode resistor. Ohm's law says that V / R = I (current), where V is the voltage drop across R.

You might want to review Merlin's Common Cathode, Triode Gain Stage essay.  It's pretty techinical (at least for me), but the load line plots show clearly why a cathode voltage of 4+ VDC for a 12AX7 wouldn't sound very musical under normal guitar amp conditions.

Jack Darr's book might also be helpful for basic triode theory.  Ditto with The Ultimate Tone Vol. 1 by Kevin O'Connor.  You can download Darr's book for free - should be a link in the References sticky.  KOC's books are NOT free but worth it IMHO.  Oh, and Merlin has a book too in addition to the Valve Wizard website.

HTH

Chip

[plexi50]

Well i wound up replacing the 2.7K with a 1K and the 4.7K for a 1.5K

The Cat voltages now are 1.000 & 1.350 VDC. The OT wires were reversed as well making for that loud sonic pos feedback boom

Installed a pilot light and a few black plugs on the front panel  / Both channels sound killer and is looking about complete

Now to change the name on the panel / FRANKENSTEIN II