Harmony H303A

[tubelab.com]

I looked in the RCA RC-30 tube manual, and my guess was right the 6CU5 is the same as a 50C5. The RC-30 is available at Antique Electronics Supply.

I would not think of running any tube at the extremes that I ran some of those 6AQ5's, but they weren't much good for anything else. As with any tube, pushing them harder than spec shortens their life. In this case the power was dropping off and the distortion was rising after a short time, maybe 1/2 hour. The extreme operating temperature causes out gassing from everything in the tube destroying the vacuum. A tube operating like this will usually suffer a catastrophic failure long before melting. The fact that I was able to melt two of them (both GE 6005's) says that they were well made tubes with good vacuum and clean materials.

If you decide to push any tubes, examine them carefully in a dark room. Check the plates for red spots on all sides, and examine the screen grid wires. Glowing grid wires will kill a tube quickly, often with bad results. Even if things are not glowing, the lifetime of the tubes may be shortened. It is also a good idea to put a fuse in the cathode connection to deal with catastrophic tube failures. Don't push tubes too hard in an amp that you have to rely on. Flaming amplifiers may get a lot of applause at a gig, but will generally ruin your day.

I have been running a pair of Shuguang 6L6GC's just into the pale red glow zone on and off for over a year. They are just beginning to show the effects of this abuse. This is an unusual case especially since these are $5 tubes.

That being said, I read somewhere that amps sound better on lower plate voltages and higher currents, instead of upping the plate voltage.. Am I missing something here?

I am of the other opinion. I like higher voltages and lower currents, myself. It is a matter of personal taste. If you are using the Hammond 125 series transformers, you have the option of intentionally mis matching the speaker impedance to gain sound flexibility. WARNING, operating a tube at high plate supply voltage, and a high load impedance, can result in a tube or socket arc. This can blow up a lot of things! Don't try a 16 ohm load on the 4 ohm tap near the max voltage spec for any tube.

[ThSpeakerDude88]

The reason I chose the 125DSE was flexibility, and if I ever scrapped the beautiful amp I built .. ( unlikley!) I'd be able to use it in something else. The schematic it comes with lists the differeent primary to secondary ratios, but it only shows them with one common lead on the secondary, the black one. I wonder, is the resistance different between each different lead, say putting a speaker between the 8 ohm tap and the 32 ohm tap to get a different primary impeadance? Or are they all just multiples of 4/8/16?

Anyway.. I have JJs in my amp so their not valuable NOS or anything I'd be afraid the blow up. I have thought about fusing the cathode and that seems like a really good idea if I wanted to run the tube hot. I'm not sure what the JJ's can handle , their russian made arent they? I've heard that the russian made ones don't do well when you push them to their limits. As far as pushing them, I dont mean really hard, just at their maximum limits and maybe a tad more plate dissipation if it can. I did notice that in the tubes there is a very faint blue glow coming from inside the plate, but it is not very noticable unless you are really looking hard. Does screen dissipation increase as plate dissipation does, or not since it is seperatly controlled? IE, if I raise the plate voltage and lower the cathode resistor, and raise the grid resistor to compensate won't that protect the grid?

[kevinkr]

JJ tubes are made in the Slovak Republic. (Formerly part of Czechoslvakia) Not Russian at all.

6P14P are Russian made variants of the 6BQ5 and purportedly are quite rugged. (Similar to the 7189) I'll let you know when I get mine and plug them into my notoriously hot running Realistic Stereodyne 40 which is a 6BQ5 killer.