• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

High Grid-Cathode Voltage in LTP

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fleian1

Member
Joined 2010
#1
Alright folks,

Maybe got a weird one for you today...

I'm working with a typical guitar amp, LTP phase inverter.

Setup:

V_B+: 380V
V_plate: 270V
R_plates: 100k
R_cathode: 680
R_tail: 43k (I've tried 24k to 43k with the same effect)
R_grids: 1Meg
C_input_couple: 22n

When I meter the grid to cathode voltages, I get -1.2V. When I meter the grid voltage (grid to gnd), I get about 55.5V. When I meter the cathode voltage (cathode to gnd) I get about 85V !!!

I can't for the life of me figure out what's going on! I measure the voltage -> where it should be. I measure the differences to ground and BAM, weirdness.

I've swapped tubes and input caps so far. The voltages on the "other side" of the caps is 0V as it should be.

Any ideas? Is this magic, or have I had my head in the chassis too long and am having a huge ol' brain fart?

Thanks in advance!
 
#3
@rayma

Will do!

For what it's worth, it's not the whole schematic...just the part that should matter/have an effect
 

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#4
The high value of R9 elevates all the tube electrode voltages re ground upward by (2 x Iplate x R9).
The Vgrid to Vcathode voltages will remain normal, though. Remember all voltages are relative.

Not sure why the grid to grid voltage measurement (should be zero) is erroneous.
Did you measure directly at the tubes' grid pins?

At any rate, just measure the Vgk for each tube separately, and then subtract them (they're about equal).
 
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#5
Your meter loads the circuit by an amount that's significant at high impedance points (grids) and less significant at lower impedance points (cathodes). You can confirm this by connecting another voltmeter from anode to ground, and watching it change when you measure grid voltage to ground.


All good fortune,
Chris
 
#6
@ rayma

Yeah, I know that the voltages should be "high" (40V-90V, or thereabouts, dependent upon tube of course) for this topology--but yeah--I was thrown off by the disparity between the direct g-k measurement and the indirect k-to-gnd minus g-to-gnd method

I will definitely measure at the pins, and report back.

Thanks!
 
#8
@Chris Hornbeck

Thanks for your response!

So assuming I haven't discovered some hole in the universe or something, I am thinking what you are, where the coupling/loading introduced by the meter is "fixing" the problem somehow.

I will certainly try what you suggested, and report. Additionally, I think I also want to alligator clip a 10Meg or so resistor between the grid and cathode, and meter the grid-gnd and cathode-gnd. Given the ~85V cathode voltage, I'm expecting that the grid voltage will rise to about 85 - (2mA * 680) = 83.6V
 
#9
If you have two identical voltmeters, use one to measure the resistance of the other. I find that most cheap meters have an input resistance of about 1 megohm.

Now think about what will happen to your circuit when you stick a 1 megohm resistor from either grid to ground. It will make a parallel circuit with the 330K and 24K resistors which will lower the voltage on the grid.

Your circuit has about 85 volts on the cathode, and 83.5 volts on the grid until you stick that extra resistor (your meter) from grid to ground.

Back when these circuits were originally developed we used a VTVM (vacuum tube volt meter) that had an 11 meg ohm input resistance. Even still they would mess up the readings in a grid leak biased stage like those found in a FC1 Champ.
 
#10
All,

Just for some additional information:


  1. This came up due to a problem that I'm trying to remedy. Namely, with the gain pots earlier in the circuit (not shown), when I adjust the master pot--the 100k pot {U2}--I get an increasing amount of 60Hz hum until almost exactly halfway (if not exactly), and then the hum drops completely. The pot has been replaced--same deal. The series 1Meg resistor {R8} has been swapped for both a 1.5Meg and 680Meg resistor (for testing)...same deal: exactly half-way through it's rotation.
  2. The 22n coupling caps {C2, C7} have been replaced.
  3. The 100n coupling caps {C1, C9} have been replaced. They also were originally 22n...problem persists
  4. The schematic shows the grid leak resistors {R5, R12} as 330k. These have been swapped with 1Meg...problem persists
  5. The schematic shows the tail resistor {R9} as a 24k. This has been tried as well as a 43k resistor...problem persists.
  6. All pots shown and mentioned are log pots (A-type)
  7. NOTEWORTHY: If I swap the grid bias resistor {R10} with a 1k, the master-volume-halfway-hum-thing goes away. I have not metered this circuit version yet, though.
  8. NOTEWORTHY: If I drop the gain pots down from max to about 75% (not shown), The hum consistently increases in volume through the whole master pot rotation. I have not metered this either.
 
#11
@Tubelab_com

Thank you for the reply. This makes sense to me.

I have two meters. One (relatively) cheaper boy, and a fluke (115?). For what it's worth, the fluke datasheet, which I used for the measurements, lists the Volt DC input impedance as >10M and <100pf.
 
#15
fleian1 said:
Alright folks,

Maybe got a weird one for you today...

I'm working with a typical guitar amp, LTP phase inverter.

Setup:

V_B+: 380V
V_plate: 270V
R_plates: 100k
R_cathode: 680
R_tail: 43k (I've tried 24k to 43k with the same effect)
R_grids: 1Meg
C_input_couple: 22n

When I meter the grid to cathode voltages, I get -1.2V. When I meter the grid voltage (grid to gnd), I get about 55.5V. When I meter the cathode voltage (cathode to gnd) I get about 85V !!!

I can't for the life of me figure out what's going on! I measure the voltage -> where it should be. I measure the differences to ground and BAM, weirdness.

I've swapped tubes and input caps so far. The voltages on the "other side" of the caps is 0V as it should be.

Any ideas? Is this magic, or have I had my head in the chassis too long and am having a huge ol' brain fart?

Thanks in advance!
Click to expand...

I had a similar problem in a Sony broadcast camera back in the '80s. It was only a 200V supply but the 11
Meg loading of the Fluke 8060 made the measurements invalid. The 'cure' was to get the 40kV probe which
changes the loading to 1.5G ohm which was enough to get valid readings. As you've found, MOST things are
fine with an 11 meg load but not ALL. You would have a similar issue with a scope probe.

G²
 
#16
Sounds like you have a ground loop problem, changing components randomly won't help, lifting the odd ground connection and moving it with a jump lead might. I empathise, chasing hum is a pain in the maximus glutonus.

The hum level is changing due to an impedance change when you turn the volume knob.

Andy.
 
#18
@Diabolical Artificer

I'm pretty sure that's what's going on. And yeah, I've been trying to find a ground problem for about three weeks. As for the component replacement: this amp was built with the intention of having an amp to mess around with (I'd rather "play" with my own amp with my schematic than some $3000 one I bought new/used). As such, the part replacement WAS part of troubleshooting, but also part of the tweaking process.


AND


@Kay Pirinha

This is definitely right, given the back-and-forth I've had here regarding the meter-induced effects.

Thanks all!
 
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