DR One channel -- Strange B+ numbers

[Lectroid]

I just got my newest amp running for the first time, sort of a Hoffman+sluckey's Tweed DR .  This is my first attempt at a fixed bias amp so I wanted to check bias.  I measure the resistance of the OT, (187.6 ohms and 214.9 ohms,) and then measured the B+ by touching the lead to the reservoir cap.  Then the plate voltage at each pin 3 on my 6V6GT's. Here's what I got:

      Res. cap:  335.6V
Outside tube:  337.7V
  Inside tube:  337.1V

I don't understand how those numbers can even exist side-by-side but I've done the measurements ten times (at least) and keep getting answers in that same relationship.

Possibly related:   
1). I've been measuring voltages all day at various times.  This morning, my first measurement was 351V at the reservoir cap, and 349V+ at the plates.  But those voltages have been dropping steadily all day. 

2) I have a 1ohm resistor going to ground from each 6V6 cathode.  The voltage on the outside tube is 5.0mV.  The voltage drop on the inside 6V6 cathode resistor is 10.4mV.

I'll post a schematic and I'll post results of any tests you all can dream up.  I'm stumped.   

[sluckey]

2) I have a 1ohm resistor going to ground from each 6V6 cathode.  The voltage on the outside tube is 5.0mV.  The voltage drop on the inside 6V6 cathode resistor is 10.4mV.

Sounds very cold to me. I would expect 20 to 25mV.

What do you mean by "inside" and "outside" tubes?

[shooter]

^^^


swap inside tube with outside tube n see if the current follows the tube

[Lectroid]

2) I have a 1ohm resistor going to ground from each 6V6 cathode.  The voltage on the outside tube is 5.0mV.  The voltage drop on the inside 6V6 cathode resistor is 10.4mV.

Sounds very cold to me. I would expect 20 to 25mV.

What do you mean by "inside" and "outside" tubes?

Sounds cold to me too.  Reflowed the solder to all connections there but no joy.  I am getting -35V to the grid pins.

Here's another couple of things I forgot to mention.  When I change the bias voltage, the B+ voltage rises and falls slightly along with it.  Also, my current limiter glows dimly steadily now instead of just at turn-on.  100W bulb

Inside/outside:  Uncle Doug's convention for naming power tubes instead of 'right' and 'left.'  The outside tube is whichever is nearest to a side wall of the chassis.

@shooter:  I will try swapping tubes tomorrow and report back.

Forgot the schematic before, here it is. 

Otherwise, amp is working more or less.  A tiny bit of hum, not bad at all.  Very early distortion, and a little ice-picky, but nothing major beyond this B+ nonsense.  Just voicing and normal de-bug.

[sluckey]

Get rid of the lamp limiter, plug the amp straight into the wall, and recheck voltages.

[66Strat]

The .022uF bright cap, if not a typo, is likely the source of the ice pickiness. A much smaller value in the 47pF to 250pF range may better suite your taste.

[PRR]

> Here's what I got:
      Res. cap:  335.6V
Outside tube:  337.7V
  Inside tube:  337.1V

Howcome it is higher (more positive) at the plates than at the B+? We expect a  drop. A rise implies negative current flow. Or the 6V6s are making power out of thin air. OR your three-step process, utility variation, and lamp drop, are making a mockery of your measurements.

The  1 Ohm cathode resistors measures say it is WAY cold. We don't even care if it is 5mA or 10mA; we want 25mA to 40mA. Wake the tubes up.

[pdf64]

I’m thinking either the mains voltage rose a bit between measurements, or probing the anodes is facilitating oscillation.

I suspect that the latter is pretty common.

Whatever, as mentioned, the 1ohm cathode current sensing resistors are providing the necessary info here.

Dunno why folk are so fixated on measuring output anode voltage, when unless the OT is screwed, the anode voltage will be negligibly different to the HT voltage.

[Lectroid]

Get rid of the lamp limiter, plug the amp straight into the wall, and recheck voltages.

@sluckey,
Dang! I cannot believe I forgot about the limiter being a big resistor!  Of course, once I unplugged it, voltages became more reasonable.

How come it is higher (more positive) at the plates than at the B+? We expect a  drop. A rise implies negative current flow. Or the 6V6s are making power out of thin air. OR your three-step process, utility variation, and lamp drop, are making a mockery of your measurements.

Exactly.  If anyone has a plausible theory about why that happened, I'd love to hear it.  Or maybe I've found a way to violate the Second Law of Thermodynamics and invented perpetual motion.

swap inside tube with outside tube n see if the current follows the tube

Yes, it did.  I tried a variety of used 6V6s I had laying around and many behaved the same way.  Eventually I found one where the cathode voltage discrepancy was only 3mV.  Close enough for testing.

The  1 Ohm cathode resistors measures say it is WAY cold. We don't even care if it is 5mA or 10mA; we want 25mA to 40mA. Wake the tubes up.

Sounds good to me.  Once I had the cathode resistor voltage differences down to 3mV, I turned up the bias pot until the higher one read 35mV and the lower one read around 31mV.  The resistors tested out at 1.1 ohm and 1.2 ohm, (for "1%" tolerance resistors!)  If I"m doing the math right, that 0.1 ohm difference could explain most of the discrepancy between the cathode resistors' voltage drops.

Then I tested the amp.  It is much cleaner at low volumes and from there the master volume could easily take it up to "Way too loud," as my dear wife expresses it. 

Dunno why folk are so fixated on measuring output anode voltage, when unless the OT is screwed, the anode voltage will be negligibly different to the HT voltage.

If you meant me, I was using RobRob's method to get cathode current using the voltage drop across the OT.  I don't have bias probes.  They're On The List.

The .022uF bright cap, if not a typo, is likely the source of the ice pickiness. A much smaller value in the 47pF to 250pF range may better suite your taste.

@66Strat,  Thanks, I'll give it a shot tomorrow.  Voicing the amp and testing out the reverb circuit is next on the list.

Thanks everyone.    I have ordered a matched pair of new 6V6GTs.  I expect they'll bring everything into line.  If not, I'm sure I'll be back with more questions.

Rich

[pdf64]

… If anyone has a plausible theory about why that happened, I'd love to hear it. 

I’m thinking either the mains voltage rose a bit between measurements, or probing the anodes is facilitating oscillation.

I suspect that the latter is pretty common.

Dunno why folk are so fixated on measuring output anode voltage, when unless the OT is screwed, the anode voltage will be negligibly different to the HT voltage.

If you meant me, I was using RobRob's method to get cathode current using the voltage drop across the OT.  I don't have bias probes.  They're On The List.

If I use that method, eg due to fitting 1ohm cathode resistors not being feasible, the probes get securely clipped to the anode and CT terminals.
And to mitigate the risk of oscillation, the signal path at the output valve grids gets muted, perhaps by pulling the valve in the preceding stage.
But you’ve got 1ohm cathode current sensing resistors, so why bother with a bias probe or OT volt drop methods?

[Lectroid]

If I use that method, eg due to fitting 1ohm cathode resistors not being feasible, the probes get securely clipped to the anode and CT terminals.
And to mitigate the risk of oscillation, the signal path at the output valve grids gets muted, perhaps by pulling the valve in the preceding stage.
But you’ve got 1ohm cathode current sensing resistors, so why bother with a bias probe or OT volt drop methods?

Mostly because this amp was my first fixed bias amp and I'm not familiar enough to be comfortable with the 1 ohm resistors method.  I wanted to check bias the old way as a double-check on what I got from the resistors.

Good tips about the clip probes and pulling an upstream tube -  thanks.

Rich

[sluckey]

I turned up the bias pot until the higher one read 35mV and the lower one read around 31mV.

And what are the plate voltages at that bias setting?

[acheld]

The resistors tested out at 1.1 ohm and 1.2 ohm, (for "1%" tolerance resistors!)

The resistors are most likely within spec, but your DVM will have difficulty measuring that low due to its own tolerances, and your test leads have a small resistance as well.

I've seen this with "0.1%" tolerance Vishay-Dale mil-spec resistors.   I trust those resistors more than my test leads.

The good news is that test lead resistance does not change, and becomes negligible at higher resistances.

[PRR]

...The resistors tested out at 1.1 ohm and 1.2 ohm, (for "1%" tolerance resistors!)....

Ordinary ohm-meters can't resolve 1 Ohms well. It is not easy. Imperfect probe contact and meter lead loss. Trust the resistor.

[Lectroid]

I trust those resistors more than my test leads.

Trust the resistor.

@PRR, @acheld,
Will do.  Thanks for the voice of experience, as always.

One other voicing question.  Can I reduce the coupling caps coming off the PI?  Say, from 0.1uf down to 0.022uf or even 0.01uf?  Or is it better to do tone-shaping earlier in the circuit?

@sluckey,
I will post the voltages.

[pdf64]

I turned up the bias pot until the higher one read 35mV and the lower one read around 31mV.

And what are the plate voltages at that bias setting?

Or the HT voltage, ie at the OT CT or reservoir cap.

Regarding trusting the 1ohm resistor values, the only time to be suspicious of their accuracy is after a valve has shorted.

[Lectroid]

I turned up the bias pot until the higher one read 35mV and the lower one read around 31mV.

And what are the plate voltages at that bias setting?

Plate voltages were 389.1V and 386.8V.  Using my measured values and trusting the resistors, I believe that means:

       389.1V * 36mA  = 13.6W
       386.8V * 32mA  = 12.4W

RobRob puts that at 97% of max dissipation so I dialed the bias back down to 28mA/26mA.  This gives me:

       389.1V * 28mA  = 11.0W
       386.8V * 26mA  =   9.7W

RobRob says this is ca. 78% of max.  Have I got that correct? 

At this level the amp is still quite loud and pretty clear if still too crisp and jangly, and it starts to break up pretty quickly with the volume knob at around 2.5.  And now there's a random oscillation I have to track down but not much hum at all.

[sluckey]

Plate voltage should have increased when you dialed down on the bias, but you show the same numbers. Did you recheck the plate voltage after adjusting the bias? You should always do that.

[pdf64]

So what's the HT voltage?
I suggest to avoid probing output valve anodes, as doing so can facilitate oscillation, eg see an obviously extreme example https://www.tdpri.com/threads/blues-junior-extremely-high-voltage-problem-1300-vdc-on-el84-plate.1107031/

14W is the design max rating for 6V6. That means it's the worst case limit condition, not the normal condition. In this context, the limit condition would have the amp operating with the mains voltage at its upper limit and the load impedance at its lower limit
If designing for the normal condition, design centre ratings are the appropriate choice, ie 12W for 6V6.
https://www.one-electron.com/Archives/RCA/RCA-AppNotes/RCA%201958%20AN-174%20Design-Maximum%20System%20for%20Rating%20Electron%20Tubes.pdf

Dunno why Rob has 14W listed for 6V6GT, as all the GT info I've come was published prior to the design max system being introduced, and hence the 12W design centre rating is noted?

[sluckey]

In 1959 the RCA RC-19 tube manual lists the 6V6GT as 12W max plate dissipation. One year later RC-20 lists the 6V6GT as 14W max plate dissipation. Has remained at 14W ever since. I choose to use 14W. Looks like Rob does too.

[pdf64]

In 1959 the RCA RC-19 tube manual lists the 6V6GT as 12W max plate dissipation. One year later RC-20 lists the 6V6GT as 14W max plate dissipation. Has remained at 14W ever since. I choose to use 14W. Looks like Rob does too.

Ah, right, RCA AN-174 which introduced the design max system is dated August 1958, I guess RC-19 was drafted prior to that.

Using design max ratings is obviously fine, very clever engineers devised it.
However, I don't get the impression such ratings are used in the way that those engineers intended, ie as you describe, most folk seem to think the valve types just got 15-20% more beefy somehow and make no distinction between design centre and design max ratings.
I guess that's fine, valves are tough, and if their working conditions are such that operational life is curtailed a tad, fortunately they're easily replaced 

[Lectroid]

@pdf,
HT voltage = ca. 400V after 10 minutes of playing.
A:  400V
B:  390V
C:  372V
D:  345V

Plate voltage should have increased when you dialed down on the bias, but you show the same numbers. Did you recheck the plate voltage after adjusting the bias? You should always do that.

Thanks.  That was a question I had from earlier; I had noticed the effect but wondered if it was normal or unusual. Why do we re-check it?

In 1959 the RCA RC-19 tube manual lists the 6V6GT ...

I truly enjoyed the discussion of Design Center standards but still have some questions outstanding.  Could someone please answer some of them?  Thanks.

RobRob says this is ca. 78% of max.  Have I got that correct? 

2. Trying to tame the jangly vibe. Can I reduce the coupling caps coming off the PI?  Say, from 0.1uf down to 0.022uf or even 0.01uf?  Or is it better to do tone-shaping earlier in the circuit?

3. And is 78% just Too Damn High?  Is 70% a good maximum to shoot for?

4. If one tube measures 10W and the other output tube 8W, does that mean the amp is an 18 Watt amp?  Is that how it's determined, just adding the two tubes' dissipations together?

Thanks again to everyone!     I don't mean to sound pushy or demanding, just trying to back-fill my huge gaps of ignorance as much as I can while I've got a few wizards on the line.

[sluckey]

Why do we re-check it?

Because it changes! Adjusting the bias pot changes the current flowing through the tube which changes the voltage drop across the tube.

Can I reduce the coupling caps coming off the PI?

Yes. Many people use .047µF caps in Fender amps to reduce the bass boom. You still hear the bass notes but they sound sharper, more focused.

And is 78% just Too Damn High?  Is 70% a good maximum to shoot for?

70% is the most commonly accepted number. 60% is a good number too. Try both settings and let your ears decide. If you don't hear a difference then go with the lower number and extend tube life.

If one tube measures 10W and the other output tube 8W, does that mean the amp is an 18 Watt amp?

No. Those are simply the dissipation numbers for each tube. This is actually wasted power. The output power rating for an amp is measured by injecting a test signal (usually sine wave) and measuring the max undistorted AC RMS voltage across a dummy load connected to the speaker jack. For example 16V_rms across an 8Ω load would be 16x16/8 = 32W_rms

[pdf64]

…70% is the most commonly accepted number. 60% is a good number too…

For a given valve type, from what I can discern, 70% of its design centre rating equates to 60% of its design max rating.
So from that, for 6V6 in fixed bias AB, it’s probably best not to idle hotter than 8.5W.

[PRR]

... Using design max ratings is obviously fine, very clever engineers devised it. However, I don't get the impression such ratings are used in the way that those engineers intended, ....

Center has tons of margin, for people who mass-produce stuff without measuring.

MAX is for people who measure (or apply a ton of tests and thinking before production).

People here debating 37mA and 365V can probably run right up to the MAX number, 14W. Remembering it is on THEM to allow for wall-voltage, 113V at home and 127V at the Hot-Cha Club. (More likely the other way, but better check... and I have worked Lodi NJ's industrial zone night/weekend when all the factories were down and it did seem awful high from light loading.)

[pdf64]

... Using design max ratings is obviously fine, very clever engineers devised it. However, I don't get the impression such ratings are used in the way that those engineers intended, ....

Center has tons of margin, for people who mass-produce stuff without measuring.

Dunno about tons of margin? As I recall, design centre ratings tend to be about 15% lower than design max.
That’s typically only sufficient to cover the likely increase in dissipation caused by the mains voltage rising from its nominal to its upper limit.

MAX is for people who measure (or apply a ton of tests and thinking before production).

People here debating 37mA and 365V can probably run right up to the MAX number, 14W. Remembering it is on THEM to allow for wall-voltage, 113V at home and 127V at the Hot-Cha Club…

Indeed, it’s just that the typical diy builder doesn’t seem to realise that the design max system puts that responsibility on them, or if they do, how they fulfil that responsibility.
There can be a ton of measuring, but still only fuzzy understanding.
Hence the common forum query along the lines of ‘my anode voltage was 400V yesterday but today it’s 420V, what gives?’

[shooter]

the typical diy builder doesn’t seem to realize

The 1st PP repair that came my way was a pricey "collectors" amp.
once it was working and idle bias set cold, I hooked up a couple meters, plugged in the guitar and started hammerin strings.
I was amazed at the huge jumps in current, kept tweakin n hammering til I was satisfied the amp "stayed withing the margins"


If the guitarist wanted more, i'd ALWAYS let them know, "any damage will be on you" 

[Lectroid]

Since this is my first fixed bias amp, it's been a steep learning curve.  Thank you all for answering my dumb questions even as we veered off-thread.  And for demystifying the Max/Design Center values into real-world parameters.  Every thread I've started here has taught me a lot but this one has been a real kick-start.

Yesterday I spent swapping out coupling caps and just listening.  I changed a few values and the amp sound is much tighter and well-rounded.  It's also much louder than I was expecting.  I put in a Master Volume but can barely get it past 3.  The Volume knob gets into overdrive around 5.  I don't remember Deluxe Reverbs being this loud. But with my hearing and memory loss, who knows?   

@sluckey,
Thank you for explaining how to calculate an amp's power output.  That will be useful.

Why do we re-check it?  Because it changes! ... changes the voltage drop across the tube.

And because of that voltage drop change, we have to recalculate the dissipation every time we change the bias setting, right?  Or am I missing something?

Everyone, thanks again!

[acheld]

The Volume knob gets into overdrive around 5.  I don't remember Deluxe Reverbs being this loud.

That's about right. 

DR's are loud, especially at home with she who must be obeyed present.

[sluckey]

And because of that voltage drop change, we have to recalculate the dissipation every time we change the bias setting, right?

That's right.

[Lectroid]

@sluckey,
Thanks.  Over and out.

DR's are loud, especially at home with she who must be obeyed present.

 

[thetragichero]

22w is pretty loud regardless, but especially through modern high efficiency speakers

[shooter]

^^ yep!
build a SE KT88, 2X12 cab, 24W rms at speaker.  had about 5 guitarist test drive evaluate, they wouldn't believe the math, I tried, gave up agreed it was a 40W and I wanted CASH 

[pdf64]

… I put in a Master Volume but can barely get it past 3.  …

More details, eg what type? Type 3 perform poorly at low settings, may leave a lot to be desired, especially so on 12AT7 LTPs.

[Lectroid]

@pdf,
It's 100K log taper.  I don't know about Type 3 (or any other type).  I've been thinking about replacing it with a linear taper to get more low-end control.  Would that work?  Yes, it's driving a 12AT7 LTP.s

[sluckey]

I put in a Master Volume but can barely get it past 3.

I've been thinking about replacing it with a linear taper to get more low-end control.  Would that work?

That will make it worse.

[pdf64]

@pdf,
It's 100K log taper.  I don't know about Type 3 (or any other type).  I've been thinking about replacing it with a linear taper to get more low-end control.  Would that work?  Yes, it's driving a 12AT7 LTP.s

Are you sure it’s a log (audio) taper?
Your description is that of a linear taper volume control.

It seems you may have a type 4 master vol, ie before the phase splitter.
For the nomenclature and details, see p31 onwards of the Trainwreck Pages https://robrobinette.com/The_Trainwreck_Pages.htm

[sluckey]

The schematic is in reply #3.

[pdf64]

Oops, thanks 
Yes, it’s a type 4, Fender style, as per their 70s models.

[Lectroid]

I got the TW pages and thanks for the reference, lots of good stuff.

I tried to copy my master volume from sluckey's Tweed DR project, sitting between the preamp third stage and the PI.  But mine doesn't look much like his example of Type 4 although it does look like the "cbs_45w_MstrVol" over at Schematic Heaven.  Should I rip mine out and use the Trainwreck type 4?

I've been thinking about replacing it with a linear taper to get more low-end control.  Would that work?

That will make it worse.

Would a 250K be a better choice?

[pdf64]

I got the TW pages and thanks for the reference, lots of good stuff.

I tried to copy my master volume from sluckey's Tweed DR project, sitting between the preamp third stage and the PI.  But mine doesn't look much like his example of Type 4 …

What difference do you perceive? As I see it, yours looks the same as http://sluckeyamps.com/tdr/tdr.pdf

With your master volume set at 50% rotation (eg 5.5 with a typical Fender 1-10 knob), what resistances does it measure from the hot track end to the wiper, and wiper to cold track end?

[nandrewjackson]

What "low end control" are you looking for?


If it's lacking low end, I'd blame the 0.001uf that is between the master volume and phase inverter.

[sluckey]

What "low end control" are you looking for?


If it's lacking low end, I'd blame the 0.001uf that is between the master volume and phase inverter.

Not talking about low frequency. He's saying the amp is too loud by the time the MV is rotated to 3 on the dial.

[Lectroid]

What difference do you perceive? As I see it, yours looks the same as http://sluckeyamps.com/tdr/tdr.pdf

With your master volume set at 50% rotation (eg 5.5 with a typical Fender 1-10 knob), what resistances does it measure from the hot track end to the wiper, and wiper to cold track end?

@pdf64,

Sorry, I think my post was confusing, let me put it this way.  I'm pretty sure I built sluckey's MV control correctly as sluckey drew it.  What I meant to say is that sluckey's version from his TDR doesn't look (to me) like the Trainwreck's Type 4 diagram. Trainwreck's method looks interesting though.

Resistance at 50% rotation is 15-20K.  I don't get to 50K until ca. 75% rotation.

Also, yesterday I raised the P/S "D" node dropping resistor from 4.7K to 18K.  This was to drop the voltage to the preamp stages 1 and 2, to get their voltages down to something near Fender's 180V-170V, and also to maybe throttle back the amp a little.  With the 4.7K resistor in that spot I was getting 225V, with the 18K I'm reading 190V.  Close enough to Fender.  I don't think it affected what we're talking about.  The behavior of my MV control did not change perceptibly after the resistor swap.