DRRI bias sweep.

[martin manning]

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

Sure it can. I'm betting the resistor is pretty close to 1R regardless of what the DMM says it is. I have a very good DMM, but I would never assume that I can measure resistances down around 1R accurately with it. Measuring small voltages is a diferent story, and I think you can measure 20mV with reasonably good accuracy.

[Sea Chief]

Possible I have not made my reading clear.

I measured 30mV at pin 8, on both tubes but this has not taken into account the division by 1.6.. which would make it 19mV / 19mA. You mentioned I should aim for 32 with the intention of dividing the result by 1.6 > to equate to 20.

(So Im now thinking should the 30mV fig be instead divided not by 1.6, but 2.6?)

[martin manning]

I do not believe the resistor is anything other than 1R, and you should not be dividing the mV reading at all. That is to say the current is at 30 mA. That is why I suggested measuring the current by using the OT DC resistance. It's not any different from what you are trying to do now

[Sea Chief]

I do not believe the resistor is anything other than 1R, and you should not be dividing the mV reading at all. That is to say the current is at 30 mA. That is why I suggested measuring the current by using the OT DC resistance. It's not any different from what you are trying to do now

Damn I thought I finally had my amp come alive.. now I got to put it back to as it was/ dull as heck. Sigh. Ok I understand why you were advising to divide by 1.6r.. as you thought the resisitor was in actuality a 1.6r.

Bloody well complcates now Im reading the wretched thing as 2.6r. I need to clear my head & start over.

I'll have another read of you OT DC psot, but my head hurt when I read it/ I couldnt understand it. Pls be ready for some Qs on it tho!

[martin manning]

It's pretty simple:
1) Turn power off and switch the standby to play
2) Measure the voltage from the red OT primary lead to ground. This is just to make sure that the power supply caps are drained, so look for a voltage close to zero.
3) Measure the resistance from the red to the brown primary lead and record it as Rbrown.
4) Measure the resistance from the red to the blue primary lead and record it as Rblue.
5) Turn the power on and measure the voltage from the red to the brown primary lead and record that as Vbrown.
6) Measure the voltage from the red to the blue primary lead and record that as Vblue.
7) calculate currents Ibrown = Vbrown/Rbrown and Iblue = Vblue/Rblue.

[Sea Chief]

It's pretty simple:
1) Turn power off and switch the standby to play
2) Measure the voltage from the red OT primary lead to ground. This is just to make sure that the power supply caps are drained, so look for a voltage close to zero.
3) Measure the resistance from the red to the brown primary lead and record it as Rbrown.
4) Measure the resistance from the red to the blue primary lead and record it as Rblue.
5) Turn the power on and measure the voltage from the red to the brown primary lead and record that as Vbrown.
6) Measure the voltage from the red to the blue primary lead and record that as Vblue.
7) calculate currents Ibrown = Vbrown/Rbrown and Iblue = Vblue/Rblue.

I know its simple for you, and most.. but to me its not. I havent done this resisitance across the OT red and brown before, I wouldnt know for a start what to set the DMM Ohm to. Pins 3 of the 6V6's? I dont have a good tether point at the red OT into the board/ its within a series of terminals very close to each other- and I like to croc clip both probes firmly on esp with any HV measuring.

Its back as it was before more or less 21mA measured across the 1r, and its -33V at pin 5 respectively (-37 gets me only 17mA so that would sound even duller than before!). The pro has said he'll do the bias for me sometime & I can see what he does without stress or anything (unless he fries s'thing else.. best not make it fri 4PM then).

Very grateful Martin- Iv'e learnt a fair old bit/ no doubt about that anyway. Cheers Sea Chief.

[martin manning]

No worries, and yes it's best to be safe above all. The blue and brown OT leads will be going to the 6v6 pin 3's, and the resistance from red (the center tap) to either end (blue or brown) should be around 200 ohms.

Seeing 33V at pin 5 at 20 mA current certainly seems reasonable. You could recalculate your dissipation if you measure anode voltage (pin 3). That would be current at pin 8 times anode voltage divided by 12W. It's likely that the anode voltage has moved some. At least you have your bias range working properly.

[Sea Chief]

Hi Martin- if you're out there..

I was with a friend who put his bias meter on (tube adaptor things/ meter etc etc).. & pleased to see the reading matched mine 20/ 22.5, so not such a bad judge this 1r method after all- certainly fine for my needs even if quite a faff to pull amp out etc.

He had a 5E3 (amonst many other tweeds he's built), and we tried his RCA NOS 6v6 in my amp, in case my EH 6v6's were the cause of my flat sound (no, they sounded the same to my ears).. but saw his 5E3 bias with these tubes in he set for 38/40mA. I understand the 5E3 is a fixed bias, but considering his and mine are both deluxes albeit mine 22w his 15w, why the huge discrepency in bias figures? you mentioned not to set mine up at 30mA or the tubes might not last too long.. so how does this other deluxe get away at 38/40mA? (and a std figure for a 5E3 so I read).

Puzzling.

[martin manning]

Good to hear that you got some confirmation of your measurements. Re the time and trouble to remove the chassis, on DIY and some production amps it's common to see bias test jacks and access to the bias trimmer on the outside of the chassis, which makes checking and setting bias a breeze.

I'll try to make a long story short here. Check out a 5E3 schematic for the features described below.

Your Deluxe Reverb has a fixed bias output stage. It gets that name since a constant (fixed) negative DC voltage is applied at the power valve grids to bias them. In your case this fixed voltage is adjustable by means of the bias trimmer, but once it is dialed in it remains fixed.

The 5E3 has a cathode biased output stage, meaning that the bias voltage is generated by placing a resistor (250 ohms) between the cathodes and ground. The current flowing through the tubes (they share the 250R) brings the voltage at the cathodes up to 20 volts or so, the same way the current flowing through your 1R resistors caused the voltage at the cathodes to rise to around 20 mV.

The 5E3's grids are grounded (at 0V), so the bias voltage (Vg - Vk) is about -20V. The grids are not directly grounded, though, since the signal voltage would be grounded out and eliminated. Instead, they are referenced to ground through 220k resistors, a separate one for each grid. At idle and non-overdrive playing conditions there is almost no current flowing through the 220k's, so the DC voltage at the grid remains at zero volts.

So how do cathode biased amps get away with that hot bias? The cathode resistor has a large capacitor (25uF in the case of the 5E3) connected in parallel with it so that when the AC signal voltage is applied, the cathode voltage (and therefore the bias) does not swing up and down with each peak and valley of the signal (the cathode capacitor is allowing AC to "bypass" the cathode resistor). Without the bypass capacitor, the current swing at the output transformer would be dramatically reduced, as would the power output. What does happen, though, is that when a signal is applied the bypass capacitor charges up and the average cathode voltage rises. This effectively lowers the bias current and allows the stage to run cooler than in the fixed bias case. Because of this, cathode biased amps can be idled at around 100% of maximum anode dissipation instead of the 60-70% seen in fixed bias amps, and the bias shift is largely responsible for the difference in sound quality.

[Sea Chief]

Bloody hell- knew I shouldnt have asked. And thats the short version already?

[martin manning]

Bloody hell- knew I shouldnt have asked. And thats the short version already?

Well, the really short version would be: Your Deluxe Reverb is fixed bias, and so the output valves are held at a consistent operating voltage and bias point when played. Cathode biased amps like the 5E3 naturally migrate to a cooler bias condition when played, so they can start out hotter.

[pdf64]

...Cathode biased amps like the 5E3 naturally migrate to a cooler bias condition when played, so they can start out hotter.

I'm not sure that's always true for class AB cathode bias, although for the 5E3 it may be?
The plate and especially screen grid HT supplies of the 5E3 are so saggy that it' difficult to be sure / demonstrate it either way.

[martin manning]

Hi Pete, remember the sims I put together a few years ago showing transient behaviors for fixed vs. cathode bias? Those were done with fixed-voltage supplies, and the shift for cathode bias was still clearly evident. In fixed bias the operating point held steady.

[Sea Chief]

Well, the really short version would be: Your Deluxe Reverb is fixed bias, and so the output valves are held at a consistent operating voltage and bias point when played. Cathode biased amps like the 5E3 naturally migrate to a cooler bias condition when played, so they can start out hotter.

I was mildly kidding, your short (long) version is somewhat understandable but I'll be going over it over xmas to absorb more/ most helpful. I got ot wrong first off then by saying 'I know the 5E3 is fixed bias..'/ sort of get the difference.

Class A, A/B ive never understood a word of mind you!

[pdf64]

Hey Martin, that rings a bell now that you mention it. If you happen across the info again anytime, I'd be grateful for a reminder.