Grid/screen resistors in Fender Princeton Reverb

[Jim Coash]

Greetings:  I am near the end of my Fender Princeton Reverb project.  I have replaced nearly all of the original resistors and capacitors with new, quality parts.  The filter caps are larger including the one on the bias board.  The rectifier tube socket now houses a pair of IN4007 diodes.  The bias pot modification suggested by Doug is done.  All controls and switches cleaned and lubed.  One final question.  Doug recommends installing Grid/screen resistors in Fender amps that do not have them.  I did replace those when I re-built the Twin Reverb just as suggested but there are no resistors at all on the two 6V6 sockets in the Princeton.  Pin 6 on those tubes has no connection.  Is there a reason to add resistors under those output tubes?  I have my Eico 1078 all ready to go but I am still reluctant to fire it up.  Jim

[sluckey]

Doug recommends installing Grid/screen resistors in Fender amps that do not have them...  ...Is there a reason to add resistors under those output tubes?

Yes. The screen resistors add a bit of over current protection to the screens. The grid (stoppers) may help with parasitic oscillations. I would add them like this...

[Jim Coash]

Thank you.  I have the parts and it will take only a few minutes.  Jim

[Mike_J]

Doug recommends installing Grid/screen resistors in Fender amps that do not have them...  ...Is there a reason to add resistors under those output tubes?

Yes. The screen resistors add a bit of over current protection to the screens. The grid (stoppers) may help with parasitic oscillations. I would add them like this...

That resistor to Pin 5 should be 1.5K not 4.2M if you read it the other way.  Probably obvious to most but maybe not all.  Screen resistor going to Pin 4 would be 470R.  Fender used 1watt carbon comp resistors on the screens but most people today use 5W wirewound or at least something more than one watt and preferably fireproof.  A 1/2 watt works for the 1.5K resistor.  These are the values I used on the AB763 and 5f6a amps and the amps perform well.  What you say about the 1.5K resistor adding stability is dead on.  The 5f6a had motorboating issues before I installed them.  No problems after the resistors were installed on each power tube socket.  Is the 100K resistor from the cathode to ground for measuring bias?

Thanks
Mike

[Jim Coash]

Thanks, again.  I have brown-black-gold 100K ohm 1/2w wire wound precision resistors from each pin 8 to ground, yellow-violet-brown 470 ohm 2w wire wound precision resistors between pins 4 & 6, hung outside the sockets and brown-green-red 1/4w precision wire wound resistors between pins 1 & 5.  I will hang these over the sockets but with some room to breathe.  I will use one of the tubes ground lugs to ground both of the 100k resistors.  BTW, is this a mod I should do for the Sovtek Mig-50 amp?  Jim

[Jim Coash]

Greetings:  I find that both 6V6 pin 8s are grounded directly to the chassis.  Should I remove the ground and install the resistors instead?  Jim

[Ed_Chambley]

Greetings:  I find that both 6V6 pin 8s are grounded directly to the chassis.  Should I remove the ground and install the resistors instead?  Jim

Place the 470ohm how sluckey suggested.  Place a 1ohm resistor from pin 8 to ground replacing the direct ground.  This is how you will measure your bias.

[sluckey]

Is the 100K resistor from the cathode to ground for measuring bias?

The cathode resistor is 1Ω (brown black gold). You're confusing gold for yellow. Look at the 470Ω to get a feel for yellow.

[Jim Coash]

I can do that!  Thanks to you both.  Jim

[Mike_J]

Is the 100K resistor from the cathode to ground for measuring bias?

The cathode resistor is 1Ω (brown black gold). You're confusing gold for yellow. Look at the 470Ω to get a feel for yellow.

I always use 1 ohm bias resistors.  I didn't think the bias resistor would be 100K but wanted to make sure it was clear because it looks a lot like a 100K resistor on the diagram.

Thanks
Mike

[sluckey]

I always use 1 ohm bias resistors.  I didn't think the bias resistor would be 100K but wanted to make sure it was clear because it looks a lot like a 100K resistor on the diagram.

That would be brown - black - yellow. I might call that 'gold' band brown if I didn't have another brown in the pic, but I'd have a hard time calling it yellow.

Different monitors, eyes, etc. Doug writes the value on his resistors specifically to avoid this confusion. Maybe I should do that too. Naw, I like the pretty colors! 

[Mike_J]

I always use 1 ohm bias resistors.  I didn't think the bias resistor would be 100K but wanted to make sure it was clear because it looks a lot like a 100K resistor on the diagram.

That would be brown - black - yellow. I might call that 'gold' band brown if I didn't have another brown in the pic, but I'd have a hard time calling it yellow.

Different monitors, eyes, etc. Doug writes the value on his resistors specifically to avoid this confusion. Maybe I should do that too. Naw, I like the pretty colors! 

With the two resistors together it is clear one is yellow and the other is gold.  Not so easy on a stand alone basis.  Wouldn't brown - black - gold be 10 ohms?  Seems like black - brown - black - gold would work better.

I think Doug has a good idea writing the values on the resistors. 

I always appreciate your advice it has been invaluable.  Sorry for the nitpicking, just remembering when I was a complete newbie and thought a little clarification would be beneficial.  The quality of your diagram is much better than average.

Thanks
Mike

[sluckey]

With the two resistors together it is clear one is yellow and the other is gold.  Not so easy on a stand alone basis.

Both of those resistors appear twice in the first pic I posted.

Wouldn't brown - black - gold be 10 ohms?  Seems like black - brown - black - gold would work better.

Brown-black-gold is 1Ω.

Black-brown-black-gold is an invalid code.

[jjasilli]

IMHO color coding is a PIA and should be used as a last resort.  Numeric values should always be stated.  Even on old actual resistors the colors may be hard to read given fading, age, tobacco smoke coating, etc:  red/orange? blue/violet? white/grey?


Reality check:  an xxxK Ohm resistor would probably turn off a power tube.  Usual cathode resistor values are usually in the hundreds of Ohms, tops.

[Jim Coash]

I have it done.  Just as in the picture.  The 1 ohm resistors are soldered to the terminal on the tube socket where I removed the ground wire and then soldered to the ground wire which goes directly to the chassis.  I could not have gotten the pool of solder on the chassis hot enough to solder directly to it so I just crimped the end of the resistor and the ground wire and soldered them.  All 6 resistors, 3 on each 6V6 socket are metal film gold banded units.  As recommended, four of them are larger wattage.  I think I am good to go with the Princeton Reverb.  As for the Sovtek Mig-50, any ideas?  Jim

[Willabe]

As for the Sovtek Mig-50, any ideas? 

Why don't you start a new post about what you want to do with the amp?


               Brad     

[Mike_J]

IMHO color coding is a PIA and should be used as a last resort.  Numeric values should always be stated.  Even on old actual resistors the colors may be hard to read given fading, age, tobacco smoke coating, etc:  red/orange? blue/violet? white/grey?


Reality check:  an xxxK Ohm resistor would probably turn off a power tube.  Usual cathode resistor values are usually in the hundreds of Ohms, tops.

I agree with you 100%.  I probably should have come out and said I would use numeric values for the resistors (or a combination of the colors supplemented with numbers).  I knew the cathode resistor should be 1 ohm although I have seen 100 ohm resistors used on the cathodes for bias purposes.  My point was if it is not clear to me then it may not be clear to someone with even less experience than I have.  If a newbie placed 100K resistors from the PT cathodes to ground it may take him forever to figure out why the amp wasn't making any sound.  Unfortunately I did not make my point very well.

Thanks
Mike

[sluckey]

IMHO color coding is a PIA and should be used as a last resort.

I think color coding is still useful in tube amps. And it will be around for quite awhile in wiring standards. Ever wire an RJ45? That would be fun without a color code.

I bet I can accurately sort a shoebox full of Allen Bradley 1/2w CCs faster than you can sort an equal box of those cute Dales with the tiny writing on them. And I wont even have to use reading glasses, magnifying glass, or a headband lamp!   

[Willabe]

My $$'s on Sluckey.   

I do have a good supply (newer) of 1/2w CC's and MF's in labeled parts bin drawers that the color bands very to the point I cant tell what color it is.       The older CC's I've seen in vintage amps the colors where very consistent in color shade and contrast and were easy to read.

And the Dales I have, I agree that they should have made the #'s larger! I need my glasses to see them.     

I think Sluckey's drawings are probably the best I've ever seen (Doug's new 1's are very close.), including all the colors, R's, wire, ect, and I really like the look of the R color bands in them. But because I don't have the color code memorized, I do sometimes wish he had the #'s on them. 

I bet he memorized the R color code when he went to school (maybe even before?) and has used it all his professional life at work, so he knows it like the back of his hand.

It's like a studio player who can read and play what he's reading on the fly. I can read enough to figure out the notes but not in real time on the fly.


                   Brad   

Edit; Added ( ) around newer.   

[sluckey]

I got my first real summertime job when I was 14. Left the farm and headed to town 6 days a week. I was working in the local tv shop. My duties that first summer included toting the tube caddy, loading tvs in the van, rooftop antenna work, lot's of sweeping and dusting! After a few weeks I knew what a resistor and capacitor looked like and fetching parts for the old man was added to my duties. He gave me a pocket calculator to help fetch the right resistor. (See the ebay link at the bottom of the page.)

At the end of the summer he asked me if I liked working there. "YES SIR!" He said good, I can use you on the weekends during the school year if your folks are OK with it. "OH HELL YEAH!" I thought, but tried to be cool. Hey, I would have done it for free!

So he handed me a brand new pocket protector with the name of his tv shop on the front. A brand new Xcelite 1/8" pocket screwdriver and 1/4" nut driver were in the pouch. He said just one more thing... Give me that silly pocket calculator. You'll never need that again. And then he spoke the perfect words for a 14 year old boy to hear to learn that resistor color code...

"Bad boys rape our young girls, but Violet gives willingly!"

So, now I've been working with resistor color codes for over 50 years. I never gave another thought about that pocket calculator except when I'd see a new guy pull one out and scratch his head. Then I'd say, "Wanna hear something neat?"

Here's the exact calculator I had that first summer...

http://www.ebay.com/itm/VTG-1951-IRC-Resist-O-Guide-ohms-resistance-resistor-gauge-color-code-reader-/321631206505?pt=LH_DefaultDomain_0&hash=item4ae2b6bc69

[Willabe]

Good story. I knew you worked at a TV repair shop when you were young, so I wondered if you learned the color code back then before school. Now we know.     

I still use my Rat Shack color code finder with the little 3 wheels, it works.


                  Brad   

[jjasilli]

I too sort them by color.  But I don't use them by color. 


If a cad program auto inserts resistors into a schematic or layout with color markings, that's a plus.  But I would always show the numeric value.

[Ed_Chambley]

IMHO color coding is a PIA and should be used as a last resort.

I think color coding is still useful in tube amps. And it will be around for quite awhile in wiring standards. Ever wire an RJ45? That would be fun without a color code.

I bet I can accurately sort a shoebox full of Allen Bradley 1/2w CCs faster than you can sort an equal box of those cute Dales with the tiny writing on them. And I wont even have to use reading glasses, magnifying glass, or a headband lamp!   

What makes Dale resistors cute?  The tiny little numbers.
Really, what difference does it make?  Sometimes the colors do shift a little, sometimes confusing red and orange on an old one.  New ones no problem.  After a while all I had to do is look at them and the numbers pop in your head.  I can sort A/B CC fast as well, but some of those tiny 1/8 watt and 1/4 watt Film I  have to magnify.

I too started working in a TV shop that fixed radio and amps.  Difference being the old man I worked for fixed one things then had 4 or 5 beers.  Fix another thing and repeat the process.  By the afternoon he was running people off and watching porno he found stuck in old VCR's.

Ahh yea, good times.

How about each one having a tag stuck to it about the size of a postage stamp with 15 point type on it and color codes.  No glasses required.

[Willabe]

Sometimes the colors do shift a little, sometimes confusing red and orange on an old one.  New ones no problem. 

Not true, at least with the R's I bought.

I do have a good supply (newer) of 1/2w CC's and MF's in labeled parts bin drawers that the color bands very to the point I cant tell what color it is.       

How about each one having a tag stuck to it about the size of a postage stamp with 15 point type on it and color codes.  No glasses required.

     



                  Brad   

[eleventeen]

Tell you what, the color bands on the new blue-bodied resistors I find very hard to discern, but my eyes are quite a bit older than they were back in Allen-Bradley 1/2 watt days. I often have trouble deciphering the colors even on the 2 & 3 watt ones. The blue background, I think, does not help, but the colors are kind of washed out in many cases.


I never used to do this, but I now measure resistors before soldering them into something. Maybe its because my eyes are so crappy now, but on the other hand, the jar full of older A-B resistors I pulled out of a CONN organ, like Sluckey said, I can identify those instantly. The colors are just so vibrant and distinct. 

[Jim Coash]

I also have more and more trouble deciphering the codes on all parts.  Despite looking for charts that offer help I often find the information confusing and illegible at least to my old eyes.  I use my glasses and a magnifier light.  In the past I rarely checked the value of new parts before I installed them but reading about your experiences has made me increasingly wary.  My practice has been to simply remove and replace nearly everything on the turret board of my amp projects with identical values.  Naturally, I always buy the very best high tolerance parts and I do occasionally take the advice found on sites like this as far as going with recommended improvements and upgrades.  So far, I have succeeded in making each project come out according to my plan.  My "improved" amps are quieter, cleaner, more powerful and more dynamic than they were before I started.  I am sure they are "better" than new by my standards which are heavily influenced by my years of audio/video sales and service.  Many find my efforts counter to theirs.  When I want more "distortion" or "effects" I add them with outboard devices.  I have added a NFL selector switch to amps in place of the ground switch and I like the way that works.  I am aware that some offer criticism to my penchant for using E/V speakers in every case but I love the "improvements" at least from my perspective.