2 8ohm 10 watt resistors

[Analogluvr]

Has anyone replaced the two 8 ohm 10 W resistors with higher end boutique versions? If so what were the results? If I were to do this would it be better to go with one 5 ohm 20 W or stick with the current values in parallel?  If I go with the single is that the right calculation?
Basically I have replaced the coupling capacitors and gone with a 6A3 aftermarket tube.  I have also gone with a nice Teflon tube socket offset from the top plate with a rubber grommet. What would be the next best thing to improve in your opinion?

 

[Paul Birkeland]

There's a 10,000uF capacitor that bypasses that paralleled pair of resistors.

At 4Hz, half the signal current will go through that capacitor, and half through the resistors.
At 8Hz, two thirds of the signal current will go through that capacitor, and one third through the resistors.
At 16Hz, 80% of the signal current will go through that capacitor, and 20% through the resistors.
At 32 Hz, 88% of the signal current will go through that capacitor, and 12% though the resistors.

When one talks about high quality power resistors, the differences tend to be in how inductive the resistors are.  A resistor that's 8 Ohms but with an inductive wirewound construction might measure something over 8 Ohms at very high frequencies.  The ones provided with the kit exhibit very low inductance, and even if there is some inductance at high frequencies, that 10,000uF capacitor is going to look like a dead short in comparison to that pair of resistors, so I would really focus on that capacitor above all else (which we also did in the kit, the cap provided in that position is quite nice).

 

[Analogluvr]

Thanks for the explanation. Do you know what the inductance is of those resistors you have chosen? Would a wirewound be better? The Mundorf 20 W supreme resistors guarantee they're under 75 pf  of  inductance. However they only list an 8.2 R value. The duelands  list and 8 R value in their 10 W but do not indicate the inductance.

 

[Doc B]

under 75 pf  of  inductance

Does not compute. pF is capacitance.

 

[Analogluvr]

Sorry you're right it was 75 pH. And it is not exactly a P it looks like a combination between p and a u.  So I assume pico Henry's?

 

[Paul Joppa]

The stock resistors are wirewound.

Those concrete-coffin power resistors are usually wirewound for low resistance values, and metal oxide for larger values. The switchover is usually in the range 50-1000 ohms, if I recall correctly - it's usually specified in the data sheet, so you can probably check.

 

[Paul Joppa]

Sorry you're right it was 75 pH. And it is not exactly a P it looks like a combination between p and a u.  So I assume pico Henry's?

Probably the Greek letter mu (μ), which would make it microhenries.

 

[Paul Birkeland]

Thanks for the explanation. Do you know what the inductance is of those resistors you have chosen?

I tried to measure a 4 Ohm version of the same resistor here, and it wouldn't register on the one tool I have to measure such things (which will go to 0.030mH, or 30uH).

In any event, the purpose of my last post was to suggest that the inductance of a resistor with a big capacitor sitting across it wasn't particularly relevant (for audio frequencies at least).

-PB

 

[Analogluvr]

So if I was going to replace that with 20W resistor it should be a 4ohm? Or  am I missing something and is that not a good idea?

 

[Paul Birkeland]

So if I was going to replace that with 20W resistor it should be a 4ohm? Or  am I missing something and is that not a good idea?

A single 4 Ohm resistor should be 18+ watts as a replacement.  I would not suggest using any of the aluminum housed resistors, or the TO-220 packaged "20 watt" resistors for this application; both will overheat.

My comments were not to say that this isn't a good idea, but rather that the 10,000uF capacitor that sits across these resistors is what you're hearing when you're listening to any sound above about 10Hz, so that is where I would (and have for my BeePre) spent my money. 

 

[Chris65]

........so that is where I would (and have for my BeePre) spend my money.

On? if I may ask.. ;D

 

[Paul Birkeland]

On? if I may ask.. ;D

The only caps that I could find that I thought might be better than what we provide stock were the Nichicon KA series caps, but that's debatable.

 

[Chris65]

Thanks.

 

[Analogluvr]

The only caps that I could find that I thought might be better than what we provide stock were the Nichicon KA series caps, but that's debatable.

Did you look at stupidly priced boutique caps as well?  Thanks so much for the help, by the way, the beepre is fantastic and I've not heard anything that can top it imho.

 

[johnsonad]

What are you trying to get by upgrading these resistors?  If it's better sound, then you should consider the amount of return on the item and with these resistors, you probably aren't going to do much other than potentially decrease the reliability of the preamp.  I thought the same but it ended up a waste of time.  In my build, I tried 30 watt thick film Caddock resistors on heatsinks.  It worked but it didn't change the sound or increase my level of enjoyment of the Preamp.  I also tried adding chokes in the PS, this too was a waste of time.  The KA capacitor that PB recommended is what I used and it's best capacitor out there for this position but I couldn't say it made much of a difference either.  Both of these have low level returns.

If you goal is to change the sound of the preamp and not increase reliability or experiment with parts swaps then there are two things to try with large returns on time and effort.  They are swapping output tubes and the output capacitor.  If you read back through my thread, I used clip leads and rolled quite a few different caps until we found one that sounded great in the system. Later as the system changed, we rolled in more caps.  It's an easy swap and makes a large change in the character of the sound.  Second is the output tube.  Again in that thread I rolled 4 or five different 300B's, 6b4g's, 6a3's and such and settled on a tube that sounded great in my system at the time.  These give an instant change to the character of the sound and though expensive, give a high rate of return. 

 

[Doc B]

If you have not installed the BeeQuiet, I would put that at the top of the list. And I would spend time maximizing isolation from mechanical vibration with lossy mass on the signal tube envelopes, lossy mass on the chassis, and squishy feet under the base.

 

[Analogluvr]

I have done all that. I am running tongsul 6A3 and have the attenuator upgrade. I tried Mundorf coupling caps and now have clarity ones in.  I am waiting for a 6B4G adaptor and then I can try some of those tubes that I have laying around.  Do you find those to be less microphonic than the 6A3?

 

[johnsonad]

The Russian 6C4C's were the least microphonic that I found.  Using Dan's method of Herbies Tender Feet tall, double weight rings and dynamat extreme under the chassis  This this didn't due too much) was about as quite as I got it.  Those Russian tubes beat anything I tried for quiet tubes.  Look for a PM from me.