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Cathode resistors on M60 output tubes

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Posted on August 25, 2009 at 20:45:36
TerryC
Audiophile

Posts: 29
Joined: November 1, 2004
 Just to move the feather with a very faint breath of life, I'll ask Tyutyu who said

Quote --even if you did put the 5ohm on the cathiodes,i like it better with out them,that me,you can pull the 6AS7s so hard thay all most POP --end quote

what is it about the sound that you like without the cathode resistors?

I built the M60 kits without resistors on the output tubes, then put in resistors on all the plates as part of the initial mods. I liked the sound earlier because it was really lively. I couldn't tell whether it was the resistors or one of the other mods which smoothed out the sound.

Ralph said that putting resistors on the cathodes of the output tubes helps tube life and makes smoother detailed sound. I've hesitated all this time to do the change because I wasn't sure I wanted smoother sound. (Plus, it's a lot of work.) But perhaps it's better sound than resistors on the plates. Are there other opinions about this? Thanks.

RE: Cathode resistors on M60 output tubes, posted on August 27, 2009 at 00:35:51
TerryC
Audiophile

Posts: 29
Joined: November 1, 2004
 Thank you for the replies and for the link to Rushton's post.

Actually I have intended to install the cathode resistors, because I had confidence in Ralph's recommendations, but I was waiting until I could install teflon coupling capacitors, too, which I haven't been able to afford yet. The capacitors aren't difficult so I'll do them later.

The change in sound that I remember was like the difference between ordinary CD and 20-bit remastered CD. The first CD of Don Ellis' 'Electric Bath', for example, has an energy that goes with the performance; the 20-bit re-issue is smoother but takes away some of the liveliness. On those recordings (from Columbia), I prefer the original CDs. I read in Audio magazine that the changed sound of 20-bit wasn't so much due to the technology as to the re-mastering. This has nothing to do with the M60 amps, but sort of describes the difference in sound that came with the early modifications of the amps. Part of the problem was the line stage I was using. It went through tubes pretty fast. Things would sound great for a while then end up sounding only ordinary, until it was re-tubed.

Despite Tyutyu's comments I am looking forward to the improvements.

plate vs cathode, posted on August 26, 2009 at 14:03:07
Ralph
Manufacturer

Posts: 1157
Location: Minnesota
Joined: April 24, 2002
Contributor
  Since: April 1, 2002
 I got talked into using plate resistors by a former associate. The claim was that it would make the amp more stable. We never had stability problems, FWIW.

The problem is that the plate circuit is not a good place to make input to any tube. The idea is that we want the tubes to operate more harmoniously together, IOW to reduce 'current hogging' in a way that happens in transistor amps. All transistor amps have 'emitter resistors' not 'collector resistors' and for good reason.

Putting the 1 ohm resistors in the cathode was an instant improvement. We did not loose any liveliness, but THD did drop.

Now the same associate had told me that he had tested a variety of resistors, and that 1 ohm was ideal for the Russian 6AS7s. I found out later that that simply was not true- its my assumption to this day that he made it up, because had he actually tested them, he would have found that 1 ohm was no-where near optimized!

So we started putting in 5 ohm 5 watt units instead of 1 ohm units (going much higher than that gets into other issues). Instantaneously we got better results. Here's a link to Rushton's post on this forum that includes this change.

The output impedance of the amplifier, with the resistors installed, actually goes down slightly as the tubes are working together. It is a bit of work, but it does pay off, both with better sound and better tube reliability.

RE: Cathode resistors on M60 output tubes, posted on August 26, 2009 at 08:24:37
Lew
Audiophile

Posts: 2686
Location: Bethesda, Maryland
Joined: December 11, 2000
 Thanks for the interesting post. I am not sure what you are saying, however. Do you like the sound better with resistors on the plates vs with no resistors? You seem to be saying that the sound was more "lively" without the resistors and is "smoother" with them. Which do you prefer? As you may know, placing the resistors on the plates is not analogous to placing the (5-ohm) resistors on the cathodes. The two options have entirely different effects on output impedance. My own reservations about the cathode resistors has/had to do with their effect on slightly increasing the output impedance, but I have become convinced that the effect is trivial, if the other benefits are great. That said, I have yet to install cathode resistors on my MA240s. Since the output tubes on my amps are 6C33Cs, which have a lower Rp than do 6AS7s, the value of the cathode resistor recommended by Ralph is commensurately lower, 1.5 to 3 ohms. I bought the resistors but have not installed them.

RE: Cathode resistors on M60 output tubes, posted on August 28, 2009 at 05:12:57
brownw10
 Interesting to note that in a data sheet on the 6c3cc reckons there should be a cathode resistor if paralelling valves up - and the max current derated slightly.

Also says that the vlaves don't have to be specially selected and can share the same bias on the grids . .although on the monster I'm getting the bits together for, it's going to be done seperately.

WALLACE.

RE: Cathode resistors on M60 output tubes, posted on September 2, 2009 at 08:23:26
Lew
Audiophile

Posts: 2686
Location: Bethesda, Maryland
Joined: December 11, 2000
 Yes, the recommended value for Rk to promote current sharing when two or more tubes are used is probably higher than that recommended by Ralph, but the value was probably chosen without regard for the trade-off in the effect of the resistor to increase output impedance, which is important in an OTL output stage but not so important in the usual use of the 6C33C in a voltage regulator. I struggled with these concepts myself. In the OTL, you want to use as low a value for Rk as you can get away with and still have some effect.

RE: Cathode resistors on M60 output tubes, posted on September 3, 2009 at 11:16:38
brownw10
 And also somewhere convenient to measure the current through the valve !

I'll post a pic sometime of the mock-up of what's being knocked up - the only problem is it's HUGE. I'm thinking of calling it 'Colossus' after the WW2 code breaking machine trhat had about 2000 valves in it.. .

WALLACE

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