1939 Recording King resistor refurb

[wsscott]

Ok 65ma it is.

[WimWalther]

Sorry, or is it 66.3 ma?

Correct.

If you're dividing Volts by Ohms, the answer will be in Amps. Multiply Amps by 1000 to get mA.

0.0663A x 1000 = 66.3 (mA)

[shooter]

now that we're all happy with 66mA, it's a shared cathode R so each tube is at 33mA, acceptable in my world for 6L6


take the bulb out of circuit and repeat measurement.

[wsscott]

So with the amp plugged directly into the mains, same input signal as before, same cathode resistance of 229 ohms, the cathode voltage is reading 27.4 VDC.  So the bias is 104.8 ma.

[shooter]

or 55ma a tube
what's the data sheet show for PP cathode biased 6L6 specs?

[wsscott]

WimWalther-I checked the resistance between ground and a filament pin, and its reading 279 ohms.  The filament voltage is 6.7 vDC.

[shooter]

ok, i looked it up myself

[wsscott]

Shooter-For two 6L6GC tubes, AB Cathode bias, the specs show 88ma for zero-signal plate current, and 132ms for maximum-signal plate current.

I don't know if that's the info you want.

I also used Rob Robinette's calculator.  For my 6L6GC tubes and using my actual plate to cathode voltage of 379 vDC, his calculator shows 79.2 ma maximum safe dissipation for Cathode bias.  And using my numbers and the cathode voltage drop calculator is shows Plate dissipation per tube of 47.7%.

[shooter]

so now play it like you stole it 

[wsscott]

So Shooter, if there don’t appear to be any shorts and the circuit is ok, then why does the current limiter bulb illuminate and stay illuminated? 

[shooter]

100 mA at the tubes will light the bulb, how much, don't know.

[wsscott]

It seems like the bias voltage is higher than typical, and maybe that's just the way this amp works.

But I checked the DC rectified voltage at the first filter cap coming off the rectifier tube and it is reading 423 VDC.

That test point also has the wire connecting to the input of the Output Transformer.  Is this input voltage higher than normal for an 8 ohm OT?

Since the schematic has no voltages, I have no way of knowing, but you guys might.  If it is high, maybe that's what's affecting the bias, and it should be lowered.

[shooter]

I also used Rob Robinette's calculator.  For my 6L6GC tubes and using my actual plate to cathode voltage of 379 vDC, his calculator shows 79.2 ma maximum safe dissipation for Cathode bias.  And using my numbers and the cathode voltage drop calculator is shows Plate dissipation per tube of 47.7%.

48% plate dissipation is completely fine in a PP self-biased PA


play it for 30 minutes, power down, let cool.  bring it back up measure and re-calculate
if your meter has a Min/Max/Avg feature, clip it across the cathode Resistor, then play for 10 minutes.  use the average value and re-calculate.


to "lower bias"  you can bump up the 229 ohms to 270, 330 ohms.  eventually you'll wind up in a catch-22 loop where B+ goes up as current goes down and the math stays the same.  Then you have to lower B+, re-do the cathode R, repeat, it's fun, trust me 

[sluckey]

if there don’t appear to be any shorts and the circuit is ok, then why does the current limiter bulb illuminate and stay illuminated?

The brightness depends on the wattage of the bulb. Lower wattage bulbs will show some amount of reduced brightness, while a 250W bulb may be so dim you cannot even see it. If your bulb is as bright as it would be if screwed into a household light socket, then you would have a direct short on the primary side of the PT. If the bulb is considerably dimmer, you are probably fine. So, what is the wattage of your bulb and just how bright is it?

BTW, I have a pushbutton switch on my lamp bulb limiter so I can quickly apply full line voltage to the bulb to see just how bright the bulb is. I find this pushbutton to be blindingly useful.   

===========================================
A couple important notes when checking bias...

1. Bias is checked at idle so don't connect any input signal.

2. Amp must be plugged directly into the wall. (But if you have a variac I suggest now is a good time to use it. Plug the amp into the variac and adjust the line voltage to 120VAC.)

[wsscott]

Sluckey-Its a 150 watt incadescent bulb.  Its low level brightness, still an orangish glow.  So certainly not full out.  It looks like this bias is going to be fine and doesn't appear to have any shorts.  So I think I'll give this horse a rest until I can get the replacement pots and resistors installed and see what its like then.

[WimWalther]

A 150W lamp running on 120VAC passes around 1250mA. A typical 6L6GC amp with a tube rectifier and 2-4 12AX7s will idle at under 1A, maybe 800mA.. that's enough for a strong glow, but well below full brightness.

[wsscott]

Can someone tell me what this resistor rating is?  It reads 42K ohms on my meter, but the colors don't look right and I don't see a 42K ohm resistor on the schematic.  There is a 4200 ohm resistor but this is reading 42K.  Thanks.

[WimWalther]

The significant digits appear to be "420", (yellow-red-black) but that's about as far as I get.

It could be "40" (yellow-black) + "00" (red) for a value of 4K. Might also be "42" (yellow-red) + "0" (black) for 420R. Just semi-educated guesses.

Btw, "value" designates resistance, whereas "rating" is power (wattage).

(ETA: Are we sure that's actually a resistor, and not just a really leaky cap?) ;-)

[wsscott]

Thanks for your input.  Yea, I should have chosen my words better.  Looks like Silver is the last band for the "rating".

I've never seen a cap like this.  It doesn't read anything when my meter is set to cap.  So, it's got to be a resistor.  The color bands are what's confusing me.  I don't know if Yellow is just a background color, or it's part of the code.  I've got another one that's similar in size and appearance that has a Green "background" that reads 500K and there's no 500K resistor on the schematic.

And this is an original resistor, so it's been there since 1939.  So the schematic seems to indicate it could be a 20K based on its location in the circuit, but I don't know unless its serving as a resistor for two 20K resistors on the circuit.

[sluckey]

Can someone tell me what this resistor rating is?  It reads 42K ohms on my meter, but the colors don't look right and I don't see a 42K ohm resistor on the schematic.  There is a 4200 ohm resistor but this is reading 42K.  Thanks.

A huge clue would be what it actually connected to in the amp. Then you could look at the schematic and know what it should be.

[wsscott]

I did that, and its connected at one end to the jumped Plates at pins 2 and 5 on the first 6SC7 tube, with the other side going to a terminal type connection on the socket housing (not a lug) of the second 6J7 tube where several wires are mounted.  The only resistor coming off those jumped Plates is a 20K resistor that heads over to the 6SC7. So although I think I know the connection from the schematic, the value shown in the schematic is way different from the actual reading.

But I can't figure out what the value of this old resistor is.  Is it a 20K?  I don't know how to read an old resistor like that. So I don't know if it's a bad 20K resistor, or if a larger resistor was substituted when the amp was built.  There's another one, similar in design but different colors, that reads a little over 500K, but the circuit is showing a 250K.

[shooter]

I did that

and the schematic we're looking at doesn't line up with tubes you have
are you SURE it's not a coupling cap from the plates?  did you find a coupling cap at the same point?

[sluckey]

I did that, and its connected at one end to the jumped Plates at pins 2 and 5 on the first 6SC7 tube

Ain't no 6SC7 tube on the schematic you posted. You really need to confirm which type tubes should be in this amp before proceeding. A 6SC7 and 6N7 are not pin compatible.

I don't know how to read an old resistor like that.

Read this... Your resistor is shown in figure 2. The code is body, end, band. So your resistor is yellow, black, red (or orange). Value would be 4K (if red)or 40K if orange.

[AlNewman]

But I can't figure out what the value of this old resistor is.  Is it a 20K?  I don't know how to read an old resistor like that. So I don't know if it's a bad 20K resistor, or if a larger resistor was substituted when the amp was built.  There's another one, similar in design but different colors, that reads a little over 500K, but the circuit is showing a 250K.

The other resistor is green black and yellow.  If you were to read it as BED,  (Body, End, Dot) it would be a 500k resistor, which is what it reads on your meter.  Which would make this one a 4k resistor, (Yellow, Black, Red).  Are you sure you measured 42k, and not 4200 ohms? 

I'm not saying that's what it is for sure, but it kind of makes sense judging by the other resistor and your measurement.

Edit:  after reading sluckey's response, I suppose it could be orange?

[wsscott]

The tube in the 3rd position is a 6SC7 and the one in the 4th position is a 6SC7MG.

The color I'm pretty sure is orange, so that's 40K, and my meter is reading the resistor, out of circuit, at 42.8k.

As to the other resistor, its colors, based on this chart, says its 500K, and it reads a little over that number.

But there are no such resistors shown on the circuit's schematic that I have and that I have posted to this site.  This amp may have been built on a different schematic design but I don't know where I'd find another schematic.

[sluckey]

I suggest you quit trying to match that schematic to this amp. They clearly don't belong together and will only lead to heartbreak if you continue.

This pic shows the colors better...

[wsscott]

I think you're right about that.

[sluckey]

Thought you may want to see this...

&ab_channel=UncleDoug

[PRR]

The tube in the 3rd position is a 6SC7 and the one in the 4th position is a 6SC7MG.

But are they right??? Lots of amps get stuffed with any old tube. Any idiot can do that. Did that amp ever work for you with this tubeset?

[wsscott]

I’ve watched that video before and during this episode, and actually watched it again today and confirmed that his amp and mine are the same, the tubes are the same as mine, and the questionable resistors are the same as mine. So I’m satisfied that my amp changed from the schematic I got.

This amp worked with these tubes, it just had a lot of noise and scratches and hum.  I think most of that is pot related, which I have replaced.  So will probably light it up tomorrow and see how it sounds.

[sluckey]

Lot's of info on this amp on the net... Everything except a correct schematic. The correct tube line up is 6J7, 6J7, 6SC7, 6SC7, 6L6, 6L6, 5V4.

[AlNewman]

I’ve watched that video before and during this episode, and actually watched it again today and confirmed that his amp and mine are the same, the tubes are the same as mine, and the questionable resistors are the same as mine. So I’m satisfied that my amp changed from the schematic I got.

This amp worked with these tubes, it just had a lot of noise and scratches and hum.  I think most of that is pot related, which I have replaced.  So will probably light it up tomorrow and see how it sounds.

Yeah, light it up, through a limiter first of course.  The main issue, if any, is that there was somebody else in there already.  Did they replace parts from what they saw, or the schematic, (which isn't accurate)?

If it works, fine.  If not, you may have to draw out the circuit you have, and test voltages at every pin to figure out what is actually going on.

[WimWalther]

The color I'm pretty sure is orange, so that's 40K, and my meter is reading the resistor, out of circuit, at 42.8k.

There you have it. The band comes across as very much red on my screen, but orange makes better sense.

[WimWalther]

This pic shows the colors better...

It sure does.. the band I'd identified as red is clearly orange in this pic.

[wsscott]

Got it finished.  I replaced that resistor and 4 or 5 others that were way out of spec, replaced the 3 pots, and the resistors for the input jacks.  Sounds really good and pretty clean.  It can get loud.  No noise until its turned all the way up.  No hum that I can tell.  I didn't install the 100 ohm resistors.

Thanks for everyone's help on this project.  I doubt I'll ever see anything like this one again.  It's been fun and as always a learning experience for me.  So that's good.  Best.

[wsscott]

Before I return it, I noticed this morning that when the amp was turned on and just got warmed up, the rectifier tube "popped" once with a flash, and then settled down and was quiet.  I ran it for a little while, then turned it off for an hour or so, and then turned it on.  This time no pops, flashes, noises, etc.  Sounds fine.

I also tapped the tubes and there seems to be a lot of resonance on the 1st 6SC7 tube, and some on the 2nd 6L6GC tube.  Nothing from the rectifier.

Any thoughts on this?

[sluckey]

Tell your friend that the tubes are old and tired and he should buy a complete spare set.

[wsscott]

That was my thought too.

[wsscott]

I guess "microphonics" is the better word.

[wsscott]

I have a question about the sizing of the Output Transformer in this amp which is running two 6L6GC tubes and is cathode biased.

The OT was installed by someone else to change to a modern design using an 8 ohm speaker.  I can't find any label on the OT.  It has the standard 2 leads for the Primary, and a red center tap lead, and a black and blue lead on the Secondary.

I watched some videos on determining the Impedence Ratio of OT's, and did the following:  After unplugging the amp and removing the Output Tubes, I put a 10 VAC on the black and blue leads, and read 480 Volts AC on the Primary leads.  I determined the Votage Ratio is 48:1.  Then to determine the Impedence Ratio, I squared 48 resulting in an Impendence Ratio of approximately 2300:1.

Ok, if that is correct, and based on the tube chart for the 6L6GC's they have Effective Load Resistance Plate to Plate rating of either 6600, 3800, or 5600 ohms.  I don't know which one to use, or should I be looking at some other spec?

But with this information, can I determine what the ratings on the OT should be when I'm using a 4 ohm or a 8 ohm speaker?

If so, how do I do it?

[sluckey]

But with this information, can I determine what the ratings on the OT should be when I'm using a 4 ohm or a 8 ohm speaker?

If so, how do I do it?

You say the impedance ratio is 2300:1. Same as 9200:4, or 18.4K:8. I would not use that OT. My choice is Hammond 1760J.

[wsscott]

Is that because its too big, or some other reason?

[sluckey]

Is that because its too big, or some other reason?

The impedance ratio is too big. Not optimum for 6L6s.

[wsscott]

Thanks Sluckey for your help on this.

[shooter]

I don't know which one to use, or should I be looking at some other spec?

typically there's 3 columns, the top line is typically plate VDC.
I use as many "close enough" of these values that match what I "expect" for my particular amp to narrow which column I use.

[wsscott]

Shooter-I looked at the spec sheet for the Hammond OT that Sluckey posted, and it says it's 40 watts.

I'm trying to learn more about OT's, and wanted to learn how to calculate the wattage output of an OT.  I'm curious as to what it is for the existing OT that I've got in this amp.  Can you help?

Also, what determines the wattage of a speaker that you want to use with a particular amp?  For example, if you've got a 20 watt amp, what size, ie. watts, speaker should you use.  I've seen speakers, say at Weber, that you can buy in 2 different wattages.  What should one consider in matching a speaker with an amp, or for that matter the amp's OT?

[shooter]

how to calculate the wattage output of an OT.

the simplest method is the data sheet from the OT maker.
an OT is simply a step-down transformer, so the input side will have many a wind of smaller copper, the output has less windings with bigger copper
you can do some crazy math on wire diameter, current ratings of wire diameter over time


so in your case of "unknown OT" in a working amp
work it hard for 15 minutes, get slightly warm, quite warm, hot?
if 1 or 2, work it another 15 minutes, retest hotness.
a proper OT worked hard for an hour will be warm to quite warm but not "to hot to touch"


speakers are one of those "mojo" math things, some guitarists want an under-rated speaker that will distort as part of their "sound"
others want over-rated for fidelity, longevity, etal.
pretty much everyone wants ones that "match" ~~~close to designed wattage of amp, AND have the "sound" they're looking for.
I rolled about 12 speakers through a 2X12 cab til I found 2 that worked well together for "my sound", both were ~~ 40W speakers using a 23W rms amp.  This covers guitar amps, Audiophile speakers and usage is a completely different set of Mojo-math 

[shooter]

here's the data sheet for the PA tubes in my last build
I selected the parameters i would "shoot" for from the data sheet (red box)
build n tweaked amp to both sound good and stay "close" to the datasheet values
notated the datasheet with final measurements


before all that, I knew what PA tubes I wanted by the datasheet "default" values to buy the PT OT.  you'll notice the datasheet "wants" 12k plate-plate.  the one I bought was 10K
part of my "tweaking" was to adjust plate VDC, cathode bias (Vk),  for "biggest" clean signal at speaker

[wsscott]

So am I correct that the Plate Voltage of the OT's will affect the Output Volume of the amp, and the Output Volume is not affected by the Wattage of the OT?  And if that is the case, then the Wattage of the OT could not harm either the Output Tubes or the Speaker?  In essence you want to select a OT that can handle enough Wattage that the Output Tubes produce?

[PRR]

Audio transformers never over-heat. The iron and copper to reproduce bass cleanly is the "power limit". Tastes vary about what is bass and how much distortion to tolerate. But a 100W amp on a 10W transformer may flub-out playing bass runs.