Need advice on SCA 35 cathode resistor

[K-MAN]

Havent been on here for awhile but hope someone can help with my question. After using my Dynaco SCA35 for about half a year with no problems I did a bit of checks on it to see how it was standing up. I just noticed though looking under the filter caps that the cathode resistor for the 6BQ5's is 150ohm 5W and not the 95ohm that the schematics call for. I don't know if the previous owner did this on purpose or if it was a mistake since I have no idea who put it together. So it's been working fine this way but should I try switching it back to the factory spec? How much does the value of this resistor effect the performance of the output tubes? Here is a picture I took of the problem, it's the wirewound on the right. Thanks for any info you guys have.

[mesherm]

What voltage across the 150 ohm resistor do you measure?
That should tell you what the current draw is.

[erichayes]

The net effect of having that resistor in there is an amp that puts out around 11 to 12 watts per channel at clipping, a slightly higher harmonic distortion component (probably not discernable) at normal listening levels, and output tubes that will last a lot longer than they would with the 95Ω resistor.

[K-MAN]

I'm not so much concerned about tube life as getting the best sound with minimal distortion. I'm still new at this so I find the concept of bias and all that a bit confusing. So will lowering the value of the cathode resistor improve the sound and increase power but will wear out the tube faster? What types of mods can you do to the bias of these amps to get better sound quality? My dream amp is a class A as I hear it has the warmest sound even though it has little power compared to the other classes of amps. I dont suppose Dynaco's can be made into class A amps? sorry if these are dumb questions but I'm still a dummy at this Yellow_Light_Colorz_PDT_09

[Shannon Parks]

I'm not so much concerned about tube life as getting the best sound with minimal distortion.

Change it to a 100 ohm 5W or 10W then. I would. :) Life is too short and the tube supply still plentiful.

[Ty_Bower]

Check your line voltage before you swap out the 150 ohm part. My AC runs high, often in the 124~126 VAC range. I've decided a lot of vintage tube gear must have been designed to work properly with a 115 VAC supply, because the B+ on my stock tube amps always seems to be too high. The output tubes draw too much current, and they're dissipating too much power on the plates. I often get a faint red glow, indicating the tubes are overstressed.

I find that a variac will always solve the problem. I've sometimes found that increasing the resistance at the cathode helps tame things a bit, too.

[K-MAN]

I checked the outlet voltage here and it sits around 120-121v. I noticed that the different tube manufactures make the 6BQ5's with slightly different specs so some run hotter than others. I'm using JJ/Tesla's so would they tend to run hotter or cooler than the stock tubes?

[K-MAN]

I've been looking now at ways my amp could be tweaked while I have it open and the possibilities are a bit overwhelming Yellow_Light_Colorz_PDT_23
One thing that seems appealing to me is the negative feedback and lowering it as I understand less feedback usually gives more of that warm rich tube sound that people love even if it does have more distortion in some areas. How do I go about changing the amount of NF in my amp? I've tried searching the forum but cant really find a simple answer. Thanks again.

[Shannon Parks]

I'm using JJ/Tesla's so would they tend to run hotter or cooler than the stock tubes?

I believe JJs do run hotter than many (including NOS), so that is a consideration. Go ahead and take a bias voltage measurement and let's see where they are currently.

[mesherm]

If you want to lower the NF, a simple way to do it would be to disconnect the feedback wires from the left and right 16 ohm speaker taps and connect them to the 8 ohm taps. That will save wear and tear on the feedback resistor solder pads and traces should you want to return it to normal.
If you want a little different sound you could try running the 6BQ5s in triode mode which will sacrifice some power.

[EWBrown]

The 150 ohm resistor would be equivalent to having 600 ohms at each cathode (presuming that the tubes are well-matched). The tubes like to see around 13VDC cathode above ground (can be between 12 and 14VDC) so a 95 ohm resistor would be pretty close to 35 mA per tube.

With 150 ohms, the total current will be around 86 mA, so each tube gets 21.5 mA, which is fairly low, and running the tubes closer to class B than AB1. So power will be less, and crossover distortion can be higher.

Easy fix: leave the 150 ohms resistor in place and connect a 270 to 300 ohm 3 to 5 watt resistor across the 150. 300 ohms will lower things to 100 ohms, 270 will get you closer to 95. I found that most JJs are happiest with 400 ohms per tube, so the 300 ohms in parallel with the existing 150 will give you 100 ohms, which should be spot-on.

Somewhat more complex fix: If you have the time and inclination, a good fix is to eliminate that 95 ohms resistor, and use an individual 390-400 ohm, 2W resistor at each tube's cathode (pin 3) , and bypass each with a 220-470 uF, 25V electrolytic, and bring all of the negative leads together at the can cap's negative tab (single point grounding). The 150 ohm resistor can be left in place, as there is nothing connected to its positive end after this modification. With JJs, use 400 ohms, with EIs and the Russian 6P14Ps, 390 ohms is better.

HTH

/ed B in NH

[K-MAN]

Thanks for all the great replies. Mesherm I like your tip about lowering the NF and it's simple so I'm gonna give that a try and see how it goes. Thanks again.

[Shannon Parks]

Mesherm I like your tip about lowering the NF and it's simple so I'm gonna give that a try and see how it goes.

It is indeed an excellent idea.

[dhuebert]

I like Eds' suggestion about individual cathode resistors. I have a receiver that has 6BM8 outputs with their cathodes all tied to the same resistor. This was a cost cutting measure and not a good idea IMHO. If you want a simple effective solution, look at the way Shannon biases his 6BQ5s on the Diytube ST-35. This gives you individual control, the schematic is here and you can keep things balanced as the tubes age.

Don

[K-MAN]

Ok I admit my ignorance here, but how can I measure how much current the cathodes are drawing? Someone said to measure the voltage across the resistor but could you tell me how step by step. I'm a bit weary just because of the high voltages with it running and I'm not sure what to do to keep a load on the outputs so the amp doesnt fry itself during testing.