Princeton Reverb Clone Preamp Bias

[plexi50]

I built a princeton reverb clone using all carbon film resistors. I think it sounded great. That was until i decided to swap all the cathode carbon film resistors out for carbon comp resistors. Now i have been a believer in using CC resistors and have had good success in the past using them. But this time the amp lost it's sweet tone and note response sounding dull and lifeless. Even the breakup was crappy. I switched back to all carbon film and the amp is wonderful again. I need a confirmation on a theory i have using CC resistors:

I checked all of the preamp tubes bias voltages and noticed that they all varied a little more than normal.
Like 1.200 volts film vs 2.220 CC. Same 1.5k cathode resistor value. After i removed the CC resistors i clipped my DVM to each one and put a lighter flame under them to see what would happen after they got hot. Immediatley all CC resistors dropped in value. I thought they would go up in value. Anywayz heres my question:

Does a CC resistor have an influence over the tone when used in the cathode position? I would say no because that resistor is only setting the bias of the preamp tube

But i would also say yes because of the instability of CC resistors when hot tend to drift. How much drift? Cathode r's value not measurable under working load but the tone proved to suck bad
Could it be that i just got a bunch of bad resistors?

[spacelabstudio]

That is indeed a very big difference on the bias in the preamp.  That will have a big effect on gain and tone.  You might need to some of your own QC and sorting if you really want to use CC resistors.

RG Keen has a very good article about CC resistors in guitar amps:

http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

The short version is you might get some extra even harmonics on a load resistor.  Most other locations you're mostly just adding noise and variation. 

[plexi50]

Thanks spacelabstudio. Good reading. I quess i just got some odd acting CC resistors.
I havent really run into this exact problem before. The CC resistors were 1.5k dead on when i put them in and 2 days later after i removed them were reading 1.650+ -.
Thats why i check them before installing them. They must have been acting up under working load and floating all over

[Ed_Chambley]

I built a 1164 Princeton and used all 1 watt dale resistors, metal film.  I love it so much I have changed many of my other amps.  They are dead quiet.  I have even changed my marshall builds to them.  The only amp I have left with CC is one of my 5f4's which is so punchy and clean I did not want to change anything.  I have a Brown Princeton I changed as well and like it much better.  YMMV.

[HotBluePlates]

... That was until i decided to swap all the cathode carbon film resistors out for carbon comp resistors. ...

I checked all of the preamp tubes bias voltages and noticed that they all varied a little more than normal.
Like 1.200 volts film vs 2.220 CC. Same 1.5k cathode resistor value. ...

The resistance of the two sets of resistors had to be different, because the voltage measurements were different. That, or your wall voltage fluctuated significantly. You're talking and almost 100% increase in voltage...

The tonal difference you heard was likely due to difference in the resulting bias of the tubes in question, not magic in carbon film vs carbon comp.

That said, like Ed, I used Dale metal films in my last amp build. I did it because metal film has the lowest noise of all resistors except wirewound. It's difficult to get wirewound resistors in high values like those used in plate loads. The amp sounds the same as it otherwise would with carbon comp, except less noise and the resistor values will likely stay more stable with heat and time.

[plexi50]

So even though the CC resistor values were the same,the voltage,reaction and distortion were different and floaty?  Thats a new word.

Im just glad it is sounding great again. Had me worried and pissed for a day. The chassis is mounted in a yamaha 1x12 cab. Looks to be the same cab as a Mesa Mark cab. I was concerned about using a 12" speaker because i didnt want to loose the sweet compressed tone of a 10" speaker. Still have to redo the faceplate but thats for another month.
 

[HotBluePlates]

So even though the CC resistor values were the same,the voltage,reaction and distortion were different and floaty? 

They couldn't have been the same. Or, they didn't stay the same value.

If the voltage across the different kinds of resistors was different, then due to ohm's law either the resistance must have changed, the current must have changed or both. If the tube/plate resistors weren't changed, then the only thing left is the cathode resistor.

However, if you swapped even 1 preamp tube that got measured during this (or pulled any tubes out of their sockets), then all bets are off. Tubes can vary enough to cause the kinds of changed readings you saw.

Like 1.200 volts film vs 2.220 CC.

Not to belabor a point, but... If you have -35v bias on your output tubes, nearly double that to -65v. How does your amp sound now?

It would take a lot to convince me that any tonal change was due to the resistor type and not the changed bias. I've used tweak audio parts before and found the claimed sonic differences to be almost pure hype.

[plexi50]

Never pulled or swapped the preamp tubes around. The CC resistors read 1.5k on my fluke meter. Thats why i was saying i thought they were floaty or something. Not stable under working conditions. And a day later after i removed all the 1.5K CC cathode resistors i measured them again and they read from 1.600 - 1.650K. I thought that was odd. As soon as i put the carbon film back in the bias on all preamp tubes were back to the cathode voltages as shown on the princeton reverb schematic

All i can come up with is like you are suggesting that the resistance of the resistors or the current had changed
The bias of the power tubes was steady at -34.4 and never changed. I discovered this by checking the cathode voltages with carbon film and then CC resistors after i had installed them. The tone was horrible and i knew there was something that had to be going on with the CC i had just put in

I havent ever seen 1.5k change its value to 1.600 = 1.650K after just 20 minutes at most of playing an amplifier

[HotBluePlates]

That's only 10% of the marked value, but it is troubling that it would change that much, that fast.