New PT , how to implement it?

[uki]

What is the minimal and maximum watts for this resistors ?  I got some that seen to be or 1/2 or 1/4 watts. is that good enough? They seen to be metal film 5% , grey body.

[Ambugaton]

I know you will want 1% tolerance not sure minimum size though

[sluckey]

Those will work. High wattage is not needed. 1% tolerance would be more accurate, especially with your $6 meter. Hoffman sells them for $0.15.

[EKDENTON]

Uki once you install them there is no reason to remove them. You may need them to check your bias  later if you change to a different tube or even a different brand tube.

[uki]

Uki once you install them there is no reason to remove them. You may need them to check your bias  later if you change to a different tube or even a different brand tube.

Got it , thanks Ed !!

I got the resistors in place, and power tubes in place, I tried to read across the resistors with standby off, but I got nothing, does standby need to be on ? What next ?

[sluckey]

Read the link

[uki]

Read the link

Ok I got it ! 

10k bias resistor in place.

B+ is reading 442vdc

Pin3 on all power tubes are reading 433-434vdc

Across the 1ohm resistors I got the following, meter at 200mV dc
V7    6.4
V8    6.3
V9    6.8   (does this difference bother?)
V10  6.3

All tubes were very hot after about 10 minutes.

433 * 6.3 = 2727.9  so that is about 27watts  2.7watts
433 * 6.4 = 2771.2
433 * 6.8 = 2944.4   wow super hot !! ouch on the limit !!! 

also I take notes of the grid stopper resistors resistances, if that matter
V7    1607
V8    1632
V9    1636
V10  1569

What else should I look at ? What next ?

[sluckey]

433 * 6.8 = 2944.4   wow super hot !! ouch on the limit !!!

No. That should be 433 * .0068 = 2.944 watts. That's super cold. You need to increase the value of that 10K.

[uki]

433 * 6.8 = 2944.4   wow super hot !! ouch on the limit !!!

No. That should be 433 * .0068 = 2.944 watts. That's super cold. You need to increase the value of that 10K.

But if that is cold why the tubes are so hot ?

[sluckey]

6L6s get hot just because of the filaments! You don't see any plates glowing red do you?

If I can trust your measurments then your tubes are running super cold. Keep lowering the value of that 10K resistor until you measure 40 to 50mV across each 1Ω resistor.

[uki]

6L6s get hot just because of the filaments! You don't see any plates glowing red do you?

Hotter than before at least that is what I've noticed, is it because the PT have now more mA ? No red spots on the plates, it would be easy to see yes ?

You need to increase the value of that 10K.

Keep lowering the value of that 10K resistor until you measure 40 to 50mV across each 1Ω resistor.

The meter show 6.3mV where did the other 40 something go?

You got me lost now !!!    Up or down ?!

[sluckey]

I was mistaken to say lower. Please forgive me.

The meter show 6.3mV where did the other 40 something go?

When you INCREASE the value of that 10K you will flush it out of hiding.

[uki]

All good Sluckey !! I appreciate very much your help and patience!!!

Been playing for about 20-25 mins watching the power tubes closely, no red spots in the plate.

All tubes are plugged, all hotter than before.
Power tubes are going way hotter than before, due to the new amperage on the PT yes?

The tone is definitely different than the old PT, slightly, bit dry and breaking up sooner than before.
Overall tone is about the same.

How much can the bias resistor go up, what is the safe limit ? 

[PRR]

> That meter only has a 1MΩ input impedance

Or maybe 2MΩ.

Apparently "DT830B" covers several meters from several factories all sold as the same thing. Different colors and even different jack layouts (so not the same molds). A thorough reviewer on Amazon says his is 2Meg while others are 1meg.

[Ambugaton]

When you say "hotter than before" how are you measuring that? The power tubes will be very hot... too hot to touch or hold. This is normal. If biased to about 70% (recommended) or whatever you prefer... and there is no red plating then I wouldn't worry.

When you read 6mV across the 1ohm resistor was the bias pot full CW or CCW?

You should have a decent range from one end to the other with that pot. Maybe 6mV until 30mV+

As you increase the current (measured over the 1ohm resistor) the plate voltage will lower which will change the 70% calculation. So adjust, re-measure, and re calculate as you go.

EDIT: Realize you have just a fixed bias resistor... follow Sluckey's advice and increase it until 40 - 50mV

[2deaf]

Or maybe 2MΩ.

The results posted in reply #13 and reply #25 firmly support 1M.  Not that it matters.

[uki]

When you say "hotter than before" how are you measuring that? The power tubes will be very hot... too hot to touch or hold. This is normal.

I mean temperature, measuring just by the touch, not the "hot" term for bias adjust. Ok maybe I was not so sure about temperature before, because the previous PT was overheating due to low specification. The tubes gets indeed too hot to touch or hold.

How much can the bias resistor go up, what is the safe limit ? 18k ?

[sluckey]

Take it up until you are satisfied with the static plate power dissipation. I like 70% of max.

Read the link!

[Ambugaton]

With the bias resistor, as you increase its resistance you are supplying less and less -Vdc to the tube. Less -Vdc = more current. The max safe amount for that resistor is something that gives you the correct bias. Unsafe would mean you have too much current for the 6L6 and it will red plate and/or not last as long as you might want it.

For real though the link that Sluckey posted on page 1 will absolutely explain everything.

[uki]

With the bias resistor, as you increase its resistance you are supplying less and less -Vdc to the tube. Less -Vdc = more current. The max safe amount for that resistor is something that gives you the correct bias. Unsafe would mean you have too much current for the 6L6 and it will red plate and/or not last as long as you might want it.

For real though the link that Sluckey posted on page 1 will absolutely explain everything.

Oh that is good info !!! Thanks !! I got to check out the tube specs also !!

Take it up until you are satisfied with the static plate power dissipation. I like 70% of max.

Read the link!

I been reading sir !! I'am  ,  I'm not sure what is 70% of max , that I get by measuring the 1ohm resistors ?!

[Ambugaton]

If I were you I would either do:

1) Get a trim pot to adjust and find the sweet spot for your liking, measure the resistance over the pot in that position, then hard wire the applicable resistor. (Or keep the pot if you feel you may want to tweak it or change tubes in the future)

2) Use alligator clips to quickly swap resistors until you get the correct readings. This will save you time because going from 6mV to what you need (40 - 50) will change your plate voltage quite a bit. I just biased my amp yesterday after installing a new PT. Original plate voltage was 445ish with bias set at 6mV. I adjusted my trim pot to get 70% dissapation which lowered the plate voltage to around 410. I then had to re-calculate 70% and make another small adjustment.

[sluckey]

I'm not sure what is 70% of max , that I get by measuring the 1ohm resistors ?!

Maximum static plate power dissipation for a 6L6 is 30 watts. 70% of 30 watts is 21 watts.

Now... READ THE LINK! 

[uki]

How many watts is it doing now according to my readings ? 3 watts per tube? That can't be right, is it ?

[Ambugaton]

433 * 6.8 = 2944.4   wow super hot !! ouch on the limit !!!

No. That should be 433 * .0068 = 2.944 watts. That's super cold. You need to increase the value of that 10K.

Sluckey did the math for you above. With your posted readings you are indeed around 3 watts per tube. Like he said... that is suuuper cold and is exactly why the amp sounds very dry and lifeless.

[sluckey]

I GIVE UP

[PRR]

> How many watts is it doing

Idle dissipation has nothing to do with maximum output (especially on a fixed-bias amp).

There may be something you can read.

[uki]

I'm starting to get this !!

I GIVE UP

Sluckey!! https://www.youtube.com/watch?v=B_rL8vLkxjM


Ok this, with 16k resistor the voltage still about the same -49vdc. but the mV on 1ohm resistor doubled 12.3

then I put in a 21k resistor voltage dropped a little more -46.3 and the mV went up good to almost 20, 18.4 in one side of the power tubes and 19.4 in the other side. Then I tried it...


NOW THE AMP SOUNDS GOOD !!!! No break up , good loud clean tone until 9 !!!

 

> How many watts is it doing

Idle dissipation has nothing to do with maximum output (especially on a fixed-bias amp).

There may be something you can read.

This only change the tone ?!

[EKDENTON]

Do you know where to find the maximum plate watt dissipation for whatever tube you are biasing?


From that you can calculate 70% or whatever you want.

[uki]

> How many watts is it doing

Idle dissipation has nothing to do with maximum output (especially on a fixed-bias amp).

There may be something you can read.

That only change the tone ?!

Do you know where to find the maximum plate watt dissipation for whatever tube you are biasing?


From that you can calculate 70% or whatever you want.

I was about to ask that, how to calculate the bias resistor value to get that answer? Is there a formula ?

[EKDENTON]

I don't think their is a formula that will tell you the exact size resistor you need to be at 70% plate watt dissipation. The schematic gives you an approximate starting size.


If you check the tube data sheet you will find the maximum plate watt dissipation.


With the voltage measurement across the resistor you now know your current.


With Ohms law use this formula to find power:  P= current X voltage





So you want to keep adjusting that resistor size trying to get your plate watt dissipation to 70% or under what the data sheet shows for a 6L6 push pull amp.


Hope I got this correct it's been about 8 years since I bought the tube bias checker and it's been that long since I used the 1 ohm resistors. LOL




So earlier you said you measured 6.3mV?
You have 443v for plate voltage?
So 443*.063= 27.9 watts
 I think the max for 6L6 is 30 watts so you should be around 21 if you want 70 %
Someone chime in if this is incorrect, I'm a bit rusty on this.


Oops my mistake it would be 443*.0063=2.79 watts

[uki]

Do you know where to find the maximum plate watt dissipation for whatever tube you are biasing?
From that you can calculate 70% or whatever you want.

I don't know where to find it. Everyone says 70% why exactly have to be that ?

If you check the tube data sheet you will find the maximum plate watt dissipation.

I think this is the maximum negative voltage the tube can take.
Ug1 = -14 V(se)    -17,5 V(pp)

> How many watts is it doing

Idle dissipation has nothing to do with maximum output (especially on a fixed-bias amp).

There may be something you can read.

That only sets up the tone ?!

I don't know what some things in the chart means those letters in the left side, like La, Lg2, N, and some others.

[EKDENTON]

That doesn't give you much information. Try looking for the 6L6 RCA datasheet. The one I see has 23 watts (I think) for max plate watt dissipation for class AB push pull. Your correct though idle and full out will be different that's why you don't want to bias over 70%


I emailed you the tube datasheet

[uki]

Please guys bear with me, time is in short supply here and I go as I can, I did big progress believe me, I'm kinda slow due to the lack of time, got my hands full.

Here:

How about add to the bias circuit a pot to adjust it, would that be a good move ?

...you can tweak the supply to get the mA's that you want at around the midpoint of the pot.

Sure. But first you should get it working right.

I see the need of the value for the bias resistor must be found 1st, then it is possible to tweak the adjustable bias circuit, did I get it right ? The mV should be between 40-50mV is yes?

Refer to Hoffman's schematic. Change R33 from 15K to 10K. Then experiment with the value of R35 (up and down). Can you now get a wider range? Maybe something close to 40 to 60 volts?

Would this work here? Can I follow that schematic?

[EKDENTON]

Uki, something just popped into my mind about  your amp voltage measurements.


You said your measurement across the 1 ohm resistor was 6.3mV? I was thinking about that voltage measurement.


 The filament voltage is 6.3v. Are you sure your 1 ohm resistor is on the correct pin, and also that your taking the voltage reading across that  resistor, and not getting your filament voltage some how?


i guess if the mV measurement is changing as you change your bias resistor, then it was just a coincidence.


 

[uki]

Uki, something just popped into my mind about  your amp voltage measurements.


You said your measurement across the 1 ohm resistor was 6.3mV? I was thinking about that voltage measurement.


 The filament voltage is 6.3v. Are you sure your 1 ohm resistor is on the correct pin, and also that your taking the voltage reading across that  resistor, and not getting your filament voltage some how?


i guess if the mV measurement is changing as you change your bias resistor, then it was just a coincidence.

That 6.3mV was with the 10k resistor, when I put 16k and then 21k the mV went up ! I got new resistors today, with thicker legs, the ones in place are too thin, 1/2 watts and 1%  for more accurate readings !!

[2deaf]

The mV should be between 40-50mV is yes?

Yes.

Would this work here? Can I follow that schematic?

What for?  You are getting close with the one you are using.  Keep going.

[EKDENTON]

Here is a bias calculator. You enter your tube type and the plate to cathode voltage and it calculates approximate.






I think your plate to cathode voltages will change as you change the bias resistor so you will need to check that plate to cathode voltage as you change resistors and recalculate or change the voltage in this bias calculator to get the new mv setting for 70%




https://robrobinette.com/Tube_Bias_Calculator.htm

[uki]

Keep going.

I'am !! as time allow me I keep moving !! In place now a 28k bias resistor, bias voltage at -41,  B+ at 400ish, and the mV in the 1ohms resistor is reading 30mV ish. Got to find some 39k or 42k resistors, I think it will be right in the spot !!!

And this is very interesting the tone improved a lot more , it isn't even "there" yet !! Tone is more round and smooth now!!

About B+ is it the expected behavior to it drop as the bias voltage changes the way it is going ?

[PRR]

> About B+ is it the expected behavior to it drop as the bias voltage changes the way it is going ?

It is cold here tonight. I turn on all my electric heaters. The lights dim. My 125V drops to 115V.

Say I have a pack horse. I put on the saddle, OK. Then I put a 100W amp in one side and a drum-kit in the other side. The horse sags.

All power sources sag when loaded hard.

You seem to be working-up from 6mV in 1 Ohm (6mA) to now 30mA. Or since you should be measuring one of four similar tubes, 24mA to 120mA. That's 5 times more current than you had before. The B+ voltage will sag some. When played HARD, a 6G8 may pull over 400mA. But as long as the sag now is not huge, it will be fine. (We hope.)

[Ambugaton]

When you say the tone is "right there" it is very subjective.  What are you after?

[uki]

Many Thanks PRR !!
 

When you say the tone is "right there" it is very subjective.  What are you after?

I was talking about the mV something between 40-50mV , sorry I was unclear.

Well I found a 38k resistor, the results are:  bias voltage -36 , B+ 400 ish(405) , mV -46.6   

But the tubes are super hot now    even the chassis where the tubes are is getting very hot, and I've noticed as the values been moving up the amp gets a little noisy, some small hum, due to the current increasing maybe?

I did like a lot the result when the 28k bias resistor was in place.

Played for a little over 5 mins with this setting was late and didn't had the chance to really feel the tone, but still it is way too hot. On the good side the PT barely warm up !!

Seen like the range is  for the bias resistor is 10k to 39k, Now with all those numbers, what to do next ?! 

[EKDENTON]

Put the 28k back in (or even better a 5k with a 25k pot in series)


Adjust it until you get around 28-39 mV.


Then do the math or use the calculator I emailed to see what your plates are dissipating.


Then adjust the pot until you like the way it sounds but if you don't want to have to replace the tubes often
You should try to stay under 70%


The cooler they are biased the longer they should last but also if you bias too cold the amp does not sound like we want a guitar amp to sound.

[2deaf]

Are the bias voltages posted in reply #76 and #90 taken at the 56K resistor?

[uki]

Are the bias voltages posted in reply #77 and #91 taken at the 56K resistor?

The one in the junction(orange dot) with cap 8µF, 56k resistor and the other one that I've been swapping, initially 10k, then 16k, 21k, 28k, 38k.

Got a pic.

[2deaf]

That's what I figured.  Attached are two adjustable bias ideas now that I am reasonably certain that I know what you mean.

[uki]

What is the difference between the two ? Are those similar to the previous examples you posted earlier in the thread, more ripple less ripple ?

[2deaf]

The one with the 25K pot. does have less ripple, but that was not the main criteria for the designs.  I designed them using mostly resistors that you have on hand.  10K is a really common pot. for bias supplies, so I designed a circuit with that and your resistors.  If you happen to have a 25K pot., I included a drawing for that using your 21K resistor.

I like the numbers you posted with the bias voltage at -36 and you like the sound at -41V.  Both of the variable bias supplies should easily have both values within their ranges.

[uki]

No 25k pots around here, usually 1, 2, 5, 10, 20, 50, 100 and some bigger ones that is usually what the stores have. I do also recycle old electronic equipment like tube screen TVs, or anything that is obsolete but have usable parts, and I use the part to make pedals !!

I got 3 trimmers, 10k, 50k and 100k, the small ones, and some 10k pots too, I can do the 10k one , I have all resistors and the pot !

[uki]

Been reading the amp voltages while playing on it,

I put back the 21k resistor, readings are:

across 1ohm resistor, peak is about 70mV , it did happen while playing really hard a 6 strings chord, that was about two times in about 5 minutes tops, average mV was around 40-50mV,  the amp chassis goes really hot at the power tubes area, and after an hour playing the PT also get hot.

idle plate voltage 413 , and when hitting hard a 6 strings chord the voltage dropped to about 390.

So that is 27 WATTS

390 X .070mA

That seems like possibly that if those measurements are correct the tubes should be redplating at over 23 watts.

Seems like their is too much current swing from just sitting idle to when you play on it pretty hard. 

Maybe post again that your bias setting is  below the 70% plate dissipation but when you push the amp hard with your guitar it seems to be going well over 25 watts dissipation.

Is that the expected behavior ?

[EKDENTON]

Did you say the idle voltage measurement across the 1 ohm was around 30mV?


I think at idle you are around 14 or a bit more watts. Thats around 30%



It seems like a pretty big swing from 30mV to 70mV, almost 140% more,  To go from 30% to over maximum for the tube. But if it is only momentary...??


To me it looks like if you tried to bias it at idle around 70% you will be way way over while playing.


Maybe someone more knowledgeable on this bias circuit and current swing will chime in?  If it is okay momentarily to go beyond what the tube is designed for, but on average is below the max for the tube?