need help trouble shooting the blues junior conversion build

[rsr]

Well, i finished up all the wiring turned it on and....  it works kind of.
It didn't blow up, but there is a significant problem that i would like to get some advice on how to trouble shoot and fix.   

I'm sure i must have either miss-wired something, left something out, or possibly have a bad connection someplace.  As a wise man once said - if it were wired up correctly, it would be working.  That said, i have triple checked things and it appears that all is correct.

I'll go through it again tomorrow with fresh eyes, but in the mean time, was hoping that if i describe the behavior i'm getting, someone with some experience might have a notion of where to focus my attention to locate the problem.  So here goes...

The symptoms are that it sounds like some pretty severe oscillation/static as a note/chord is played and rings.   The initial sound on the attack seems ok, but is a little weak and overly distorted (and not in the good way) but then that oscillation effect kicks in and is very bad as the note dies out, generally getting cut off prematurely.  Also, there is absolutely no clean tone now.  Even with the master high (5) and volume low (2) the sound is full on distortion.  That used to be a nice sparkly clean sound.  In fact, as i've tested it more, even that initial attack is clearly not good either.

When i turn the amp straight off from full power, bypassing the standby,  it immediately surges up to full volume and sounds like it is supposed to, and there appears to be no oscillation effect as played for the 5-10 seconds that it takes to fade away to nothing.  During this brief time, the expected level of hiss is present if not playing, and with master up and volume down, i do appear to be getting the expected clean sound.  The increase in volume when switched off is quite noticeable, which tells me that when in full power mode, everything is not all there. 

Interestingly, when going from full power to standby mode, it fades away over a few seconds, as expected, but does not sound good during that time, as it does when going straight to off.  Then, when subsequently going from standby to off, it again does that thing where it surges up to full volume and sounds good for a few seconds while it fades away again.

The tone controls all appear to be effective, in that when turned each alters about the right frequency band of the noise that comes out.

The FAT switch seems to do it's thing, though it gives a pretty loud pop when switched.  When i leave it on and use the footswitch, it switches silently, so that's good.

The meat control does not appear to have any effect as far as i can tell.

The amp is silent at idle.  By that, I don't mean that it's just quiet.  I mean it is dead silent. No hiss, no nothing.
There is no oscillation or static coming and going of any hiss or other noise.  Just silence until a note is struck. Then there is sound but it sounds pretty crappy.

I was able to adjust the bias to read -3.4v between the top of the large filter cap (B+)  and the brown OT wire on V5 pin 7.  There's quite a wide range in that bias pot so it's pretty touchy. Was easy to get it in to the 3-4v range, very difficult to get from 3 to 3.4 without shooting past and up to 4.

I checked the voltages as follows
In full power mode, i read 350v from the top (positive) leg of the big 47uf filter cap to the chassis.
Turn the amp off, it drops immediately to about 10v.
Turn amp on to standby, it stays at 10v (or whatever it was reading while off)
Turn amp up to full power, it goes back to 350v
Turn amp down to standby. The voltage drops very slowly, eventually reaching that 10v range, but it takes several minutes. I believe this is the normal expected behavior.

Apologies for this being a little long. 
Thanks for reading this far, and i'm really hoping somebody can point me in a useful direction here.

rsr

[rsr]

I have a theory about what might be the issue here, please tell me if this sounds like it would explain the behavior described previously...

The hoffman circuit has V2 jumpered 8-3, 7-2, and 6-1.  In the tube board replacement project, which i did prior, this was described as an optional mod in order to get a little extra push, which i declined to perform as i liked it the way it was.  So i also decided not to jumper these now when doing the full rebuild, same logic.

Thing is, in the original wiring, pins 3, 2, and 1 are all grounded, as i recall.  I just left them open.  Duh.

Having half a tube open seems to me like it might explain why the amp seems to oscillate between being there and not.

Does that compute?   

My thinking is to just ground pins 1,2,3, as it would be a little awkward to get in there and try and jumper those pins at this point.  Plus, like i say, i'm not after that extra push anyway.

Where would be the best point to ground to?  Does it matter?  The chassis is right there, but that would not be ideal, right?  The closest point i think is the bottom of the left most 1.5K resistor/ 47uf 25v capacitor pair (the top of which is connected to V1 pin 3)  Or, i suppose, any of the lugs that are laced together on that line would work.  It's a straight shot to the bottom of the 1.5K resistor that V2 pin 8 is connected to.  So that's probably where I'll go, unless someone has a different suggestion.

for reference:
http://el34world.com/Hoffman/files/Hoffman_BluesJunior.pdf

Does this sound plausible as the cause of the problem? 

Or should i just leave those V2 pins open and look for the problem elsewhere?

Thanks again

rsr
 

[shooter]

Does this sound plausible as the cause of the problem?

probably not since they should be electrically isolated, sans filaments.  you can just jump grid to ground with gatorclip and see.

I would look more for a 100k where a 10k should be, same for caps.  do all your DC volts match Doug's?

[rsr]

Does this sound plausible as the cause of the problem?

probably not since they should be electrically isolated, sans filaments.  you can just jump grid to ground with gatorclip and see.

I would look more for a 100k where a 10k should be, same for caps.  do all your DC volts match Doug's?

Thanks, i'll do that tonight.   Couple followups please..

I know how to measure resistors with my multimeter, but from what i read, measuring capacitors seems like it can't be done while they are in the circuit.  Is there some method for doing that that i can follow, having just a digital multimeter available?   Before installing these i didn't measure them, but matched up the tiny little writing on each to the part code on the hoffman site and the value listed on the wiring diagram.  So it was a 3-way merge of information and i certainly could have transposed something.  I'll definitely check those again tonight.

I did measure the resistors ahead of time in order to identify which was which but i could have messed up when installing. I did spot check many of them yesterday before i turned it on and all was correct.  Still could have missed one though, so i'll go over them all again tonight as well.

As for measuring the voltages, what's the method there?  One lead clipped to the chassis and the other touching the points to measure?  That's how i measured the B+ lug at 350v.  Is that safe to do at all points shown in read in the schematic then?  And how much variance is OK here?  Like on the schematic it says that it should be 340v at that B+ location, but i get 350v.  Close enough?  My meter may or may not be totally accurate.  Like it still reads 10v after the amp is turned straight off (no standby) and sits for a bit. Should it be 0 at that point?  Maybe a residual of 10 is fine.  Maybe my meter's just off by 10....

Thanks for your help
rsr

[shooter]

measuring capacitors

no measure required, if it says .02uf, and printed on the cap is .2uF........

for now, whatever your meter reads, compare to what it should be and figure a 20% fudge factor is good for now, volts DC

easy way to test your meter, new D cell battery should be close to 1.5vdc, when's the last time your meter battery was changed?

[rsr]

measuring capacitors

no measure required, if it says .02uf, and printed on the cap is .2uF........

for now, whatever your meter reads, compare to what it should be and figure a 20% fudge factor is good for now, volts DC

easy way to test your meter, new D cell battery should be close to 1.5vdc, when's the last time your meter battery was changed?

Thanks for the tips.  Never changed the battery in the meter yet. Possibly it's time. I'll do the battery test and see.

And, the thing about the capacitors is, if it said .022 or .0022 right on there, it'd be fine.  But they don't. They have some barely legible writing on there that includes a code number that appears to match up with a code like ".022uf/630v Xicon-223" as shown on this page http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Capacitors

So that's what i mean about the 3-way merge.  I can easily see having transposed or mixed those up along the way somehow.

[sluckey]

If you look at the bottom of the cap page you just linked you will find an explanation of how to decipher those cap codes.

[shooter]

If you have Excel, or a translator, I have this posted on my bench, might help

[rsr]

If you look at the bottom of the cap page you just linked you will find an explanation of how to decipher those cap codes.

Thanks, i think i had pretty much figured it out, but i'll double check against that tonight.
My real problem was making out the writing on the caps themselves.
Eyes ain't what they used to be you know.
And remembering where i put things.  Wow. Don't even get me started....

[sluckey]

I use reading glasses and even sometimes use a big handheld magnifying glass too.

[rsr]

Well, i have done the following:

• tried jumpering V2 pins 1,2,3 to ground.  As predicted, no change
• checked meter - reads 1.5 volts with D battery
• verified all resistor values. thought i had bad one on the input jack, was reading as 0 until i plugged in a cable. then 1M as expected
• verified all capacitors, by code only, not measured
• checked voltages at B+, Z, Y, and X.  All were a little above the listed values, but well within 10%. At X it measured 261v vs 253v
• checked voltages at V4 pin 9 and V5 pin 9.  These were a bit high at about 340v vs 321
• checked voltages at V4 pin 7 and V5 pin 7.  These were also reading high at about 348v vs 338
• checked voltages at V4 pin 3 and V5 pin 3.  Schematic shows 25mv.  Wasn't sure where to measure, but i used the tops of the 1ohm R98 and R99 resistors.  Meter read 0mv.  Is this a problem?

So is there anything to the 0 reading at V4/5 pin3?  Visual inspection of those connections look good, and i have continuity between the tube socket pin and those resistor tops.  Probably i'm just measuring it wrong.

If so, what to do about it? 

If not, any other ideas?

Thanks once more
rsr

[VMS]

You measure that voltage across the 1ohm resistor, you might need to change the scale on your meter to read smaller voltages.

How's your bias voltage, do you get close to -12.3V on the C- point on the schematic?

[rsr]

You measure that voltage across the 1ohm resistor, you might need to change the scale on your meter to read smaller voltages.

How's your bias voltage, do you get close to -12.3V on the C- point on the schematic?

Measured at top and bottom of the 1ohm resistor, getting 0 in both spots.

Measuring the C-, i am getting about -27V

[sluckey]

Adjust the bias pot until you have -12.3vdc at point "C-". Then recheck the voltage on the 1Ω resistors.

[rsr]

Adjust the bias pot until you have -12.3vdc at point "C-". Then recheck the voltage on the 1Ω resistors.

well, well.

Now we are getting somewhere.

Even with the bias pot cracked all the way clockwise, I can only get the voltage down (up?) to -19v
And with that, my prior method of measuring bias (top of B+ to V5 pin7) reads about -.4v instead of the .3.4 i had been shooting for previously.

But, the amp sounds good at that setting.   Don't know about great, i couldn't turn it up and play at this hour, but i can say that the effects i was describing before appear to be resolved.

The voltages at the tops of the 1ohm resistors now read different from eachother, but close - about 3.5 and 4mv. 

Does that tell us anything?   
Is -19 better than -27, but not yet good enough?
Is that a sign of something else being out of whack and i shouldn't use it like this?

This is pretty encouraging though.

Thanks!

[shooter]

- about 3.5 and 4mv. 

25mV IS your target, anything else is just shooting in the dark

[rsr]

- about 3.5 and 4mv. 

25mV IS your target, anything else is just shooting in the dark

Fair enough, but how do i get there?
Is the bias pot not working correctly somehow maybe?  It is turned as far clockwise as it can go at this point. 
Is there too much / too little coming in someplace before that maybe?
I did verify all the resistors.

[shooter]

It is turned as far clockwise as it can go at this point.

pull V4 AND V5.
meter set to DC, connect blk probe to ground, red probe into the tube socket, pin 2
power up, adjust the pot full CCW record value, fully CW, record value.

edit, repeat for other tube

[sluckey]

You don't have a good adjustment range on the bias pot. The fix is easy, but we need to know what range you have right now to advise what to do. So provide the numbers shooter is asking for.

[rsr]

It is turned as far clockwise as it can go at this point.

pull V4 AND V5.
meter set to DC, connect blk probe to ground, red probe into the tube socket, pin 2
power up, adjust the pot full CCW record value, fully CW, record value.

edit, repeat for other tube

will do as soon as i get home tonight.
thank you

[PRR]

>> My real problem was making out the writing on the caps themselves.
>> Eyes ain't what they used to be you know.
>> And remembering where i put things.  Wow. Don't even get me started....
> I use reading glasses and even sometimes use a big handheld magnifying glass too.

If you are over 20, you are losing near-focus. By 40 you have to admit to yourself; and sometime later, to everybody. (They have the same trend, it's universal.) This is why Reading Glasses have been best-sellers for 700 years.

All your life, closer was better because it was bigger. But as you age, closer is worse because it blurrs. Very frustrating.

You *should* get an eye-check, because there are several other issues. I was bad no matter what lens. Cataracts were messing with everything. Very glad I got them fixed. There's a few other things; the eye-doc/tech will know them all.

What was the question? Oh...

Anyway: simple getting-old poor near-focus is "cured" with a lens. A 1.5 reading power makes 2 feet focus like infinity, which is good for average eyes. 3 makes 13 inches focus like infinity. Somewhere between will focus newspapers and most food directions.

Pill-bottles and modern components ARE printed too darn small. Even with readers. You can stack readers. You look funny but who cares? 1.5 with 3.5 makes 5.0 which is good for 8 inches.

If still too small, magnifier. The numbers on these work different. A "5X" will read about 2 inches away. Hold the magnifier away from your eye and about 2 inches from the object. Adjust object-lens distance (not eye to lens) to focus.

A Real Good magnifier for old folks is the lens off the old SLR camera. Better optics than craft-store magnifiers. Probably sitting idle now. 50mm is 2 inches, good for many small things. Hold the film-side to the object and look in the front side from a distance. If you have a wide-angle, or a small-format enlarger lens, 25mm is 1 inch and about 10X magnification (re: a teenager's view at 10 inches).

[rsr]

>> My real problem was making out the writing on the caps themselves.
>> Eyes ain't what they used to be you know.
>> And remembering where i put things.  Wow. Don't even get me started....
> I use reading glasses and even sometimes use a big handheld magnifying glass too.

If you are over 20, you are losing near-focus. By 40 you have to admit to yourself; and sometime later, to everybody. (They have the same trend, it's universal.) This is why Reading Glasses have been best-sellers for 700 years.

All your life, closer was better because it was bigger. But as you age, closer is worse because it blurrs. Very frustrating.

You *should* get an eye-check, because there are several other issues. I was bad no matter what lens. Cataracts were messing with everything. Very glad I got them fixed. There's a few other things; the eye-doc/tech will know them all.

What was the question? Oh...

Anyway: simple getting-old poor near-focus is "cured" with a lens. A 1.5 reading power makes 2 feet focus like infinity, which is good for average eyes. 3 makes 13 inches focus like infinity. Somewhere between will focus newspapers and most food directions.

Pill-bottles and modern components ARE printed too darn small. Even with readers. You can stack readers. You look funny but who cares? 1.5 with 3.5 makes 5.0 which is good for 8 inches.

If still too small, magnifier. The numbers on these work different. A "5X" will read about 2 inches away. Hold the magnifier away from your eye and about 2 inches from the object. Adjust object-lens distance (not eye to lens) to focus.

A Real Good magnifier for old folks is the lens off the old SLR camera. Better optics than craft-store magnifiers. Probably sitting idle now. 50mm is 2 inches, good for many small things. Hold the film-side to the object and look in the front side from a distance. If you have a wide-angle, or a small-format enlarger lens, 25mm is 1 inch and about 10X magnification (re: a teenager's view at 10 inches).

Ya, i've had 40+ year old eyes for 15 years now, and i know the drill. 
What i would be handy for work like this is one of those lighted magnifying porthole on a stand type of things, ala


And also, a 3rd hand would be super helpful.

[sluckey]

I've had one of those magnifier/lights on my bench for over forty years. It was great when I was younger. I still use it for a light source, but rely more on reading peepers and a handheld magnifier. Plus I wear a LED headband light when working inside a chassis.

[rsr]

It is turned as far clockwise as it can go at this point.

pull V4 AND V5.
meter set to DC, connect blk probe to ground, red probe into the tube socket, pin 2
power up, adjust the pot full CCW record value, fully CW, record value.

edit, repeat for other tube

Here we go:
V4 CCW (min): -27.47V
V4  CW (max): -19.46V
V5 CCW (min): -27.45V
V5  CW (max): -19.42V

I assume the fix is going to be to add/change a resistor someplace, but if the problem is just that the pot is bad,
i do have the trimpot that i used with the billm mod available, if that would work better.
It did seem to be much more precise than this pot.  It is described as a "50K 25 turn" trim pot, so i guess it is equivalent to the 50kl pot that is there now, right? (and what does the l in kl stand for, btw?)

Anyway, thanks once more

[shooter]

So I would verify the bias circuit is as it should be, use all the optics you want.

I can't get all the smart guys words outta my head; "If you built it like Doug did, it would be working now"

that said, I believe the snippet below should work. I broke the connection from the "top" of the pot and the diode/ cap junction, then added a 10K resistor where the "wire" was
Re-do the measurements CW & CCW, AND DON"T put in the 84's till someone weighs in.

[davidwpack]

L=linear taper

[shooter]

I case you're wondering, the jist here is to set the bias at a happy place then VERIFY by monitoring the 25mV.
your plate Voltage DC * .025A = watts your tubes are sweating.  so if you're 300ish*.025=7.5W, happy place being ~ 60 -70% of whatever the tubes spec is for max plate

[sluckey]

Shooter's suggestion should work. I would just change R62 from 22K down to 10K. The idea is to be able to adjust the pot to get about -12v at point "C-". If you rather use that 50K 25 turn pot, do so. I probably would if it can be mounted so it can easily be adjusted. Hoffman's pot is plenty good too.

[rsr]

Shooter's suggestion should work. I would just change R62 from 22K down to 10K. The idea is to be able to adjust the pot to get about -12v at point "C-". If you rather use that 50K 25 turn pot, do so. I probably would if it can be mounted so it can easily be adjusted. Hoffman's pot is plenty good too.

Oh ya, that would have been cleaner for sure, but i just finished shooter's suggestion before i read this.
But, with shooter's 10K resistor in place, and with the trim pot at full clockwise, i get down to -15V when measuring C-
and 19mv at the 1ohm resistory.

Also, interestingly, the input voltage at B+ is now at 338V. Recall that it was 350V before.

When checking the bias with my old method (B+ and V5 pin 7)  i am now getting -2V drop.  Not quite the 3.4 i'm looking for, but in the neighborhood.

And most importantly, the amp sounds good.  How good?  Still haven't had a chance to really play it, but pretty darn good from a very quick run through that i just did.

So my question now is - quit and claim victory or keep adjusting to try and get to that -12.3 at C-?

I'm guessing that as it is, the tubes are running a bit hotter than they should, so maybe it's worth continuing?
And, this is with the bias pot fully max'd, so i can't lower it any further.

So do you think it is close enough to let it be at this point?  Most importantly, would it sound better, or be quieter (it is actually pretty quiet as it is right not) at 12.3 than 15?

If i were to try to get further, my guess would be to remove shooter's 10K  and replace the 22K at R61 with something a bit lower than 10K sluckey suggested?  Am i in the ballpark there?

Thanks again everybody.
rsr

[VMS]

Those 1ohm resistors are there so you can bias your amp more easily, there is part of that here:


http://el34world.com/charts/fenderservice5.htm

Also, interestingly, the input voltage at B+ is now at 338V. Recall that it was 350V before.

More current is flowing now so more voltage is dropping on the resistance of the OT winding, U=RI


19mV across 1 ohm resistor is 19mA, P=UI so 19mA times plate voltage tells you the dissipation of the tube and i think it's still little on the low side. I would still adjust the bias circuit.

[sluckey]

Leave shooter's 10K resistor alone and also change the 22K to 10K. That should do it.

When checking the bias with my old method (B+ and V5 pin 7)  i am now getting -2V drop.  Not quite the 3.4 i'm looking for, but in the neighborhood.

I suggest you quit doing this check and only use the 1Ω cathode resistors to check the bias.

[shooter]

by my math, .019A*338vdc = 6.4W which is ~6.4W / 14W (max plate) *100 = 46% which is cold biased

with 25mV you should be in the 60-70% sweet spot for bias, you can button it up, play til the neighbors hate you, or book you

[rsr]

by my math, .019A*338vdc = 6.4W which is ~6.4W / 14W (max plate) *100 = 46% which is cold biased

with 25mV you should be in the 60-70% sweet spot for bias, you can button it up, play til the neighbors hate you, or book you

Ok, so after adding that 10K resistor i am at
-15V when measuring C-  and 19mv at the 1ohm resistor.

To get all the way to -12.3V and 25mv respectively, what do i need to do?
Change that 10K to something larger, then back off the bias pot maybe?
A 22K resistor there perhaps?

Thanks

[shooter]

did you do what Sluckey said in post #30?

[rsr]

did you do what Sluckey said in post #30?

Leave shooter's 10K resistor alone and also change the 22K to 10K. That should do it.

actually, i misread his suggestion as meaning i should change that 22K to 10K instead of your adding your 10K.
But in fact, i think he was actually saying to change that 22K to 10K in addition to adding your 10K.

So no, at this point i have only added the 10K you suggested, and that is what got me closer to the target values, but not quite there.

So is the recommendation to now also do the 22/10 change to get the rest of the way there then?

[PRR]

> Here we go:
> V4 CCW (min): -27.47V
> V4  CW (max): -19.46V

This is not right; not what I see on Doug's plan and probably not on Fender's. I do not see the actual bias divider values YOU used; is there another rock to look under or am I missing it?

Assuming 20V AC does give ~28V DC, OK. Doug's values of 50K pot and 22K end resistor should trim 27V to 9V. Your 19V limit suggests (still ass-uming 50K pot) a 122K end-resistor.

Either the 50K is not 50K, or the 22K is not 22K. Bright light, magnifier, less-obscurely-marked parts, or ohm-meter should lead you to a "Who put THIS here?" moment.

[rsr]

> Here we go:
> V4 CCW (min): -27.47V
> V4  CW (max): -19.46V

This is not right; not what I see on Doug's plan and probably not on Fender's. I do not see the actual bias divider values YOU used; is there another rock to look under or am I missing it?

Assuming 20V AC does give ~28V DC, OK. Doug's values of 50K pot and 22K end resistor should trim 27V to 9V. Your 19V limit suggests (still ass-uming 50K pot) a 122K end-resistor.

Either the 50K is not 50K, or the 22K is not 22K. Bright light, magnifier, less-obscurely-marked parts, or ohm-meter should lead you to a "Who put THIS here?" moment.

I agree that something is not right.  I have just re-re-re-checked all the resistors, against what is shown in the PDF wiring diagram (which, unfortunately imo, does not note which is R1, R2 etc....) using my multi-meter, and they are all in the correct spots. I then re-re-checked again the 22K resistor that is at the bottom of the bias pot, and yep, it is very stubbornly insisting that it is in fact a 22K resistor.

(I will say that there are a few that seem to be a bit high.  The 1 ohm resistors nearest the bias pot both read more like 1.3  to 1.5, and the 220K pair near the C- connection read about 227.   The rest are at or very very near what is shown)

I'm not sure how to test the bias pot itself, but with the amp off and the multi-meter connected from the C- point to either of the two other legs, it measures 0 - 10K as i turn the pot.  I'm not sure if that makes it a 10K pot (which would make it wrong) or this is representing a 20% range of a 50K pot (which would be correct).

[sluckey]

Put a 50K pot in it, remove the resistor that shooter had you install, and be sure there is a 22K resistor attached to the pot! IOW, wire it IAW Hoffman's documents.

[rsr]

Put a 50K pot in it, remove the resistor that shooter had you install, and be sure there is a 22K resistor attached to the pot! IOW, wire it IAW Hoffman's documents.

So you're saying it's not really a 50K pot after all then, is that right?
Cuz it says on my invoice that that is what i bought and that's sure what i thought i was installing.
Just want to be sure. 
It's only $1.50 but it sure did cost me a lot of time and worry.
And wasted alot of you guy's time as well, which i'm very sorry about.
I might try and salvage the 50K pot from the billm circuit.  The legs aren't aligned right so it might not work out, but i  might give it a try.

thanks again, as always

[rsr]

Well, i took a closer look at the side of the bias pot and guess what, it says right on there - 10KL 650 (something i can't make out)

So i guess that would have been the easy way to verify that

Now my question is different, how do you do an RMA / return with the hoffman parts store?  It's clear that i was shipped the wrong thing, and i don't care about the $1.50, but it is a principle situation....

[rsr]

Ha.
So i was able to salvage the billm bias pot after all, and it had just enough legs left to fit into the hoffman bias pot spot.

Removed the shooter 10K and was able to dial in exactly -12.3V at C-,  although with that it reads 30mv at the 1ohm resistor.  It also reads as a 3.0V drop from C+ to V5pin7 (i believe it is supposed to be 3.4ish).

But, most importantly, it now sounds certifiably awesome, although i still haven't been able to play at a decent volume yet.   The tone is sweet, the meat pot seems to provide a nice range of colors, tone controls do what they're supposed to, fat switch works, and it really is suprisingly quiet when not playing, though that could change when i am able to get the MV up past about 3.

All in all though, very happy with the result right now.

Am curious as to opinions on whether it is more important to hit exactly the -12.3V  or the 25mv, or my 3.4V drop target - or should i try and split the difference?

Would also like to once again give my sincere thanks to everyone that offered comments and questions here.
I'm actually kind of glad there was some issue that had to be worked through.  I learned so much more by going through this than if it had just worked straight off.  Not that i know what i'm doing now, or can understand half of what you guys say here, but the half i was able to understand got me to a good place with this project.

Thanks again.

rsr

[VMS]

Am curious as to opinions on whether it is more important to hit exactly the -12.3V  or the 25mv, or my 3.4V drop target - or should i try and split the difference?

Usually fixed-bias amps are biased to 70% of maximum dissipation of power tube. EL84 is 12W tube so 12*0,7=8,4W. You adjust the negative bias voltage so that when you multiply the mV voltage across the 1 ohm resistor with the plate voltage of that tube your answer is that 8,4W


I'm not completely sure how screen current affects that math so there might be little adjustment there. I'm sure other guys will chime in with more accurate answer.

[shooter]

or the 25mv

READ, post #15

[sluckey]

Now my question is different, how do you do an RMA / return with the hoffman parts store?  It's clear that i was shipped the wrong thing, and i don't care about the $1.50, but it is a principle situation....

This is a private matter. Only one person has the correct answer.

[rsr]

Now my question is different, how do you do an RMA / return with the hoffman parts store?  It's clear that i was shipped the wrong thing, and i don't care about the $1.50, but it is a principle situation....

This is a private matter. Only one person has the correct answer.

I've talked to doug about it separately.
Thanks.

[PRR]

> i took a closer look at the side of the bias pot and guess what, it says right on there - 10KL

As you say: "it sure did cost me a lot of time and worry. And wasted alot of you guy's time".

I trust the invoice 99% and the part-mark 99.9%, but mistakes happen. Look! Measure!

> The 1 ohm resistors nearest the bias pot both read more like 1.3  to 1.5

What does the meter say when you press its tips together? Most bench meters have a part-Ohm of internal resistance, which is in addition to the thing being measured. The tips have a variable amount of oxide and that adds some tents unless pressed very hard. (For more fun, a few digi-meters round-down any low reading to mask this error.) Standard bench meters will tell 10r from 1r, but will not tell 1r from 1.1r or 0.9r with any confidence. For "checking" a low-Ohm part, look at the part, then try the meter with the part and with direct contact. If it says 1.0 on the body, and 1.3-1.5 on the meter, which reads 0.2-0.6 on "zero", then I'd ass-ume the part IS 1.0 Ohms for our purposes.

For more precision you use a "4 point measurement". This is not a standard meter and I have never seen one in the flesh.