DRRI bias sweep.

[martin manning]

Unless stated otherwise, voltages are always with respect to zero.

Agreed. Some reference point in the circuit is chosen and assigned the numerical value of zero volts.

You can increase away from zero in two directions.

It is true that the potential difference can be made larger going either way, but increasing the numerical value of the voltage and moving away from zero is only possible if you are at or above zero.

More is increase. So how can "more negative" be a decrease?

The potential difference is larger, but the voltage with respect to the reference is lower.

[Sea Chief]

g1 martin said in the last but one sentence 'less negative, or closer to zero', which couldn't be clearer.

Martin I understand I need more current flowing at pin 8. But 2 Q's as Im slightly confused again.

1) In order to get more current flowing you say I need to increase the bias voltage on pin 5: I'm under the impression that to get this, its a result of doing something else, namely tweaking R59. Is this correct, or am I somehow increacing the bias V at pin 5 primarily.. and as a result of this (whatever I do to pin 5), the current at pin 8 increaces?

2) is the byproduct of tweaking the resistor at R59 primarily to increace in current available for pin 8? or is it primarily to increace th bias V at pin 5?

3) I was under the impression (its stated so before, I think in Leo's posts) that if you increace bias current, a byproduct is -a decrease- in bias V. IE he mentioned my bias V was hovvering around -30V and a better figure was -35V, and a worse figure ('dont go below -30V' was the advice) was -25V, which an increase in bias V as you are saying (which is what you say Im to expect if I tweak R59 to get more current at pin 8).

Can anyone just pick up on these 1) 2) 3) for me. Id appreciate that: the reason is there is a query over the state of my 6V6's after the red-plate affair, and 'dont go below -30V bias V' as was the advice earlier on, is exactly what Martin is saying will happen in #40 (unless Im mistaken).

[martin manning]

g1 martin said in the last but one sentence 'less negative, or closer to zero', which couldn't be clearer.

Martin I understand I need more current flowing at pin 8. But 2 Q's as Im slightly confused again.

1) In order to get more current flowing you say I need to increase the bias voltage on pin 5: I'm under the impression that to get this, its a result of doing something else, namely tweaking R59. Is this correct, or am I somehow increacing the bias V at pin 5 primarily.. and as a result of this (whatever I do to pin 5), the current at pin 8 increaces?

Correct. Given that a voltage is applied at the anode (pin 3), and at the screen (pin 4), the voltage on the control grid (pin 5) is like the handle on a water valve. With pin 5 at -50 volts, the electronic valve is shut off, with essentially no current flowing. With pin 5 at zero volts, the valve is full-on, or wide-open (there are some subtitles around full-on, but keep it simple for now).

2) is the byproduct of tweaking the resistor at R59 primarily to increace in current available for pin 8? or is it primarily to increace th bias V at pin 5?

Primarily for increasing the voltage at pin 5, which causes more current to flow at pin 8, as described in the answer to question 1, above.

3) I was under the impression (its stated so before, I think in Leo's posts) that if you increace bias current, a byproduct is -a decrease- in bias V. IE he mentioned my bias V was hovvering around -30V and a better figure was -35V, and a worse figure ('dont go below -30V' was the advice) was -25V, which an increase in bias V as you are saying (which is what you say Im to expect if I tweak R59 to get more current at pin 8).

As I say in the answer to question 1, -50V on pin 5 is full-off, and 0V on pin 5 is full on. For a new, nominal 6V6 with 400V on the anode and screen, I would expect 20 mA to be flowing through the valve when there is about -36V on pin 5. An old or damaged valve might require a less negative voltage to hit 20 mA, say -30V. If it's in really bad shape, -25V. "Don't go below 30V" was meaning to say that if the voltage required on pin 5 is less negative than -30V (-25V say), then there is reason to suspect that the valves have been damaged.

[Sea Chief]

Hi Martin, Ok thanks for that. So Im primarily increacing the V at pin 5 (and a byproduct is an increase also in the current).. but Im not reading the result of this procedure at pin5, but only at the end-result-point @ pin 8..?

Ive just been emailed by the pro & yes both 6v6 tubes were tested after the brief red-plate affair, and both checked out fine. So I hope not to get a reading of less negative pin 5 of -25V for eg. 1st I need to check my DMM tho..

I was also told [by the pro] that you cant really rely on my DMM to accurately read 1r, in my case as I read before (had noted) as 1.6r.

i just tested it again and they both read 2.6r. So I thought, I therefore need to re-reference my DMM to see what it "adds" to a reading of zero (my fab brain in action you see). All my 10r set of resisitors read 12.6r on the nose, for eg.

What am I concluding from this- my heads a mess now! 1.6, 2.6 as the figure to divide the current reading by-?

[martin manning]

So Im primarily increacing the V at pin 5 (and a byproduct is an increase also in the current).. but Im not reading the result of this procedure at pin5, but only at the end-result-point @ pin 8..?

Correct. The cathode (pin 8) current is what you are targeting. The exact voltage required on pin 5 is not important.

Ive just been emailed by the pro & yes both 6v6 tubes were tested after the brief red-plate affair, and both checked out fine. So I hope not to get a reading of less negative pin 5 of -25V for eg. 1st I need to check my DMM tho..

Ok, good.

I was also told [by the pro] that you cant really rely on my DMM to accurately read 1r, in my case as I read before (had noted) as 1.6r.
i just tested it again and they both read 2.6r. So I thought, I therefore need to re-reference my DMM to see what it "adds" to a reading of zero (my fab brain in action you see). All my 10r set of resisitors read 12.6r on the nose, for eg.

I said the same thing above. The potential for error is way too big, even if you are compensating for it. If the resistors you have are marked 1R then I would assume that they are 1R. If you are not sure, you could use 10R resistors. In that case 200 mV measured on the cathode would indicate 20 mA flowing through the resistor. Ohm's law again.

[Sea Chief]

Hi Martin. Ok Im getting somewhere..

Redid a 10K onto R59 22k. Measuring across now i set pot innitially midway, and gently up'd and voila I can reach 32mV and left it at that (albeit 90% on the pot rotation- I guess this is immaterial).

Would I now measure the -V figure at pin 5, just to see what's going on there? or is this oin 5 reading "whatever it is".. "the main thing is the pin 8 -32mV reading".

[Sea Chief]

Still confused.

If I measure my 1r as 2.6r.. what am I dividing the 32 reading by.. 1.6 or 2.6?

[martin manning]

Ok, good, that's progress! Yes, measure the voltage on pin 5's to see where you are (just for academic interest).

I'm still not convinced that your resistors are 1.6R though. If you are game to try, there is another way to measure the current, and that is by measuring the DC resistance of the OT primary, red to blue and red to brown with the amp off (leave the standby on, and measure voltage at the red lead to make sure the power supply caps are drained), and then using those resistances to calculate the current by measuring the voltage drops across the same leads when powered up. That is a bit more risky than measuring at the cathode, but you can measure from each lead to ground and then calculate the voltage difference. Divide that by voltage by the resistance (it'll be a bit different on each side), and you have the current.

[Sea Chief]

And Im only reading -27.8v on pin 5.

(Not that Im following him & yourself at same time/ Im following your advice, but I was told by the pro to "if DMM cant be relied on, then just set pin 5 V to -37v".

How would someone go about doing/ changing this figure? ).

Im still not sure if this -V @ pin5 is the priority and the pin 8 mA is a product-of.. or the pin 8 mA is the priority and the pin 5 -V is the product-of.

[martin manning]

And Im only reading -27.8v on pin 5.

(Not that Im following him & yourself at same time/ Im following your advice, but I was told by the pro to "if DMM cant be relied on, then just set pin 5 V to -37v".

How would someone go about doing/ changing this figure? ).

Im still not sure if this -V @ pin5 is the priority and the pin 8 mA is a product-of.. or the pin 8 mA is the priority and the pin 5 -V is the product-of.

Your tech guy is saying ignore the cathode current at pin 8 and just set the voltage on pin 5 with the trim pot. If I'm not mistaken, -37V is what appears on the Fender schematic. If your tubes are on-spec then that should work fine. Further, if your tubes are on-spec, then the fact that you have -28V on pin 5 now suggests that your 1.6R might not be accurate. I would get a second measurement of the cathode current using the OT primary DC resistance as I described.

[Sea Chief]

Your tech guy is saying ignore the cathode current at pin 8 and just set the voltage on pin 5 with the trim pot. If I'm not mistaken, -37V is what appears on the Fender schematic. If your tubes are on-spec then that should work fine. Further, if your tubes are on-spec, then the fact that you have -28V on pin 5 now suggests that your 1.6R might not be accurate. I would get a second measurement of the cathode current using the OT primary DC resistance as I described.

Ok understood- but thats going to take me a whole afternoon figuring out how to do it. When it comes to standby switches my heart sinks.. its a nightmare trying to establish if ON means 'engaged/ flipped up/ amp plays' or standby ON means it is in 'standby mode/ not on yet/ amp doesnt play'. Both positions can be said to be ON.

Anyway progress at the figure now adjusted to 30mA (altho whatever that actually is in mA: I dont know whether I'm dividing the measured 1r resistor as I see it as 2.6r............ by 2.6? or by 1.6? thats a heck of a difference between 11mA and 19mA you see).. as it sounds a heck of alot better. The amp now sounds actually alive, for the 1st time in 6 months or however long since I built it. Not a dead cardboard box sound (sounded nothing more interesting than a s/h £20 ss amp before). Good progress at last! The attack on the strings has appeared, the bass notes arent farting out and woolly mess (tho I struggle with the bass on every amp I have being OTT, tried 2x10 to try and tame it but still sounds voiced like a cello, which I detest with a vengence. Even got a tele for an SG and still the bass totally dominates the sound: not what a gtr should be voiced as IMO).

Ok I must try and find out what the bias actaully is, and why the figure of -28V. I might be some time, or I might have to take back to the pro guy (why he measured 19mA at the time with it as it was.. I cannot begin to undersatnd) to get a proper bias reading if I cant do it.

Thanks SC

[martin manning]

I was going to add a comment about the standby. I meant put in "play" mode. And yes you can see that you really need to have a close-tolerance resistor for current sensing. The OT primary DC resistance will be on the order of 100 ohms, so the potential for error using your meter to measure it is much smaller.

[Sea Chief]

Hi Martin,

alas I just dont understand the other OT way you outlined. Im just going to ask Q after Q about it before I can even start, and drive you mad. You've been great so far & Im most grateful: so I think its best I take to a pro for a proper bias reading.

As I said Im reading the resisitor not as 1.6r. Im reading it as 2.6r. It is a 1r resisitor though. So the DMM adds 1.6r onto any reading (eg a known set of many same, new on a bullet-belt, 10r each measures 12.6 Ohms).

Can you just tell me before we sign off, if I do have a 1r resisitor lets just say it actually is... and I measure it as 2.6r with my crappy DMM.... what am I dividing the mV pin 8 reading by: 1.6 or 2.6?

Thanks, SC

[martin manning]

As I said Im reading the resisitor not as 1.6r. Im reading it as 2.6r. It is a 1r resisitor though. So the DMM adds 1.6r onto any reading (eg a known set of many same, new on a bullet-belt, 10r each measures 12.6 Ohms).

Can you just tell me before we sign off, if I do have a 1r resisitor lets just say it actually is... and I measure it as 2.6r with my crappy DMM.... what am I dividing the mV pin 8 reading by: 1.6 or 2.6?

You would divide the voltage reading on pin 8 by the resistor's value to get the current: I = V/R. If the resistors are marked 1R, then I'd assume that they are 1R, and you don't have to do any division since anything divided by 1 is unchanged. Measured mV is equal to the current in mA.

I'm thinking you are biased a bit too hot right now, around 100% of maximum dissipation.

Dial it back to 22 mV on pin 8 and see what you get for voltage on pin 5.

[Sea Chief]

Hi Martin. I understand you'd simply read the same mV number off the DMM and convert 1: 1 for the value in mA -IF- the DMM read the 1r as 1r. Lets assume the 1r is actually a 1r. But my DMM doesnt read it accurately:

But its reading A) as I previously thought 1.6r, and B) now it seems (maybe a new crappy DMM since then/ prob the disparity) all of 2.6r.. then Im stumped as to what Im dividing the mV reading by.

It can't be 1:1 with the DMM not registering the 1r as 1r, can it?