Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on January 21, 2013, 03:38:23 am
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Hi guys, Sorry to have to go over this again,but it seems that there is mixed results with using a PPIMV with a Fixed Bias.
I have a standard PPIMV circuit that you would use with a cathode bias amp and a switch that i have used to flick between fixed and cathode bias.(without the PPIMV) I don't believe these circuits are a new thing.
BUT there does NOT seem to be anything concrete when it comes to using them together.
I have the circuit in the build below and it is displaying some odd things (but maybe they are not odd :dontknow: )
In cathode mode my readings are good as per the weber bias calculator and as calculated approx. 32w
In fixed mode things get a little crazy
In the amp in standby (in fixed mode) i can get a great range on the bias pot -15v to -56v and checked that on pin5 of power tubes and set at -46v that all is good
And as per the schem the switch changes the grounding of the cathode(in fixed mode )to the cap/resistor (in cathode mode) this works well
With the PPIMV set on "0" the bias reading on pin5 reads -46v and with ppimv at "10" the reading changes to -36v
I kinda understand why this is happening, in a normal settup(no ppimv) the bias is connected to the 220k gird leak resistors to deliver the -voltage the the grids
Adding the ppimv as i have it connected i can see why the bias voltage drops when you turn up the ppimv
The bias voltage is correct when the pot is at "0" because this is the easiest path to the wiper in the pot but when the pot is turned to "10" the easiest path is still through the pot but with the added resistance of the pot , reducing the voltage.
I took a reading across the 1ohm resistor at both "0" and "10" on the ppimv and they were the same .028v, i thought this odd as i would have thought it would change when you change the setting on the ppimv
Please correct me if i am wrong, because some have used this circuit and have said the it works well
Is there a way to tell if it is working cause the only thing that changes is the -voltage at pin5 :think1: :dontknow:
So either i am doing something wrong or the PPIMV DOES NOT work with a fixed bias with this circuit
Bruce Collins says that his circuit works but there seems that some dought that it works either :sad2:
So what do you think boys.
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Ciao TIMBO
Today for me is not the best day as to say someting, I have a fever
I give a quick look to the schematic you posted
it seems to me you are missing a pair of capacitors, a PPIMV for cathode biased amps is very similar to those for fixed bias, but you must prevent DC to reach the potentiometers
See schematics
https://taweber.powweb.com/store/6d30_schem.jpg (https://taweber.powweb.com/store/6d30_schem.jpg)
K
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I think you have an unusual circumstance.
I have a standard PPIMV circuit that you would use with a cathode bias amp and a switch that i have used to flick between fixed and cathode bias.(without the PPIMV) I don't believe these circuits are a new thing.
There's nothing wrong with your schematic and now I believe there is nothing wrong with your actual circuit either (except 'maybe' the bias power supply). That PPIMV works very well with fixed bias. That PPIMV will not change the bias on pin 5.
I took a reading across the 1ohm resistor at both "0" and "10" on the ppimv and they were the same .028v, i thought this odd as i would have thought it would change when you change the setting on the ppimv
This is a BIGGGGG clue. The negative voltage on the grids is NOT changing when you crank the MV.
But, when you connect your meter to pin 5, the bias is changing! How can that be? It be because your meter is now providing a resistance from pin 5 to ground. This resistance forms a voltage divider with the MV pot. Current now flows thru the pot (and your meter) and this current flow will cause a voltage drop across the MV pot, which makes the voltage at pin 5 (and your meter) change.
A high impedance (10Meg or so) DMM should not have that much effect on the bias circuit. You should be able to crank the MV end to end without seeing a change. Unless... The meter is not a high impedance input, or,,, the bias supply impedance is really, really high.
Try another DMM. If bias at pin 5 does not change, then your other meter is low impedance or defective. If the bias at pin 5 STILL changes, then you have an error in your wiring (I don't really suspect this) or your whacky bias supply is very high impedance and needs to be redesigned. If that's the case, then disconnect the AC input for your bias supply from that voltage quadrupler, and get a more conventional AC source.
I would like for you to check one thing though, before you make any changes. It will require 2 meters. Connect a second meter across your 1Ω cathode resistor. The voltage should be about 28mV and cranking the PPIMV pot should not change the 28mV reading. Now connect your original meter to pin 5 to measure the bias voltage. If cranking on the MV pot changes the bias voltage, I bet the 28mV across the 1Ω resistor changes too. Does it?
At this point you may also want to just ignore the fact that the PPIMV changes the bias voltage on pin 5 when you have that meter connected to pin 5. Just measure the bias at point "#" but adjust the bias voltage while monitoring the voltage across your 1Ω resistors.
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Thanks Sluckey, Again your knolledge has been spot on :worthy1: and i am left :BangHead: to even think that my MM was telling me lies and left me thinking that something was a miss. So a new MM and fresh batterys has got me a better result.
There is still a difference between the bias pot (-45.7v) and pin5 (-44.7v) regardless, the voltage across the 1ohm resistor remains constant when PPIMV is turned.
So i guess (i do alot of that) the question is, which setting do i believe and how do calculate the power output.Thanks :icon_biggrin: