Noble Super 990/Magnatone 280A

[davohilts]

I have been working on an old Noble Super 990 that from all appearances is the same as the Magnatone 280a. The amp came with 3 extra 6973 power tubes and when powered up one of the 6973's would redplate. Upon thorough inspection I found that one of the original cap can's 10mfd connections had a 1.2K resistor that had never been soldered to the triangle marked post. Everything inside the amp was unmolested so I figure that the amp came new this way and had not been used much and was probably unreliable in its function.
I was delighted to be able to buy two new 20x10x10 cap cans from Hayseed Hamfest that I could drop in to replace the originals which both tested bad on my ESR meter and DMM.
I also noticed that there were 8 – 100K resistors serving as voltage dividers to the grids 3-6 of the 6973 pairs and also serving the plates and cathodes of V5 (6CG7) and also V6B's plate. These 100K resistors were smaller and rougher looking than any other resistor in the amp and upon testing them they were all way off in value or were infinite when tested. Replacing these resistors solved the power tube redplate issue, although the tube was distroyed and wouldn't pass voltage. The amp is now functioning properly and sounds very good.
The original speakers in this amp are 2-Jensen P12P with a date code of 1958 and there is a crossover network for each output transformer feeding two solid metal basket tweeters.

Now, when I look at the schematic for the Magnatone 280A it shows 6CZ5 or 6973 power tubes. I know that they are similar and by shorting across two pins that they can be interchangeable. Does it stand to reason that since the schematic shows both tube types that the voltages in this amp allows the subsitituion?

I have posted some pictures of the amp on my photobucket located here: Noble Super 990 Photos by davohilts | Photobucket

[EFK]

Yes they are interchangeable but you MUST - and I mean MUST - make sure that the screen voltage for *every single power tube* is fed on pin 1. If the screen voltage for any power tube is taken off pin 8 of another tube, ala many Valco amps, you must run a jumper to pin 1 from another pin 1 or other screen voltage source. 6CZ5 have no pins 1/8 internal connection like 6973 do.

If the screens are all fed via pin 8, you can just tack a piece of jumper in across each socket to pin 1. What you really have to watch out for is the Valco method of using one of the tubes itself (assuming a 6973 w/ internal 1/8 connection was in place) as a "pass through" screen source for subsequent screens downstream.

BTW, I am one of those people who truly believes that they are basically the same tube. I think the confusion has arisen because book values for 6973 are given as max. values, design maximum and 6CZ5 are given as max. values, design center.

[davohilts]

Thanks EFK! I have a bunch of 6CZ5's and I'd love to try a comparison. Next time I'm inside the amp I'll verify the source of the screen voltage for the tubes. There is so much discussion on these tubes and when I saw ON THE SCHEMATIC both tubes listed as equal I thought there must be more to the story.

[Justin Thomas]

Thanks for this thread! I've got a bunch of organ chassis laying around with 6973 tubes. I'll go w. 6CZ5, given the price diff instead of putting in 6V6. Great info here!

Justin

[EFK]

The only caveat that I would insert is that *I have not personally* used 6CZ5 in 6973 designated circuits at voltages higher than approx. 375 (plates). Most of my Valcos, given modern wall VAC, are putting around 340-350 on the plates and maybe 335 or so on the screens. They idle @11-12W dissipation and climb to @ 16-17W when the amp is "dimed." 6CZ5 do just fine in these. Magnatones usually run them a little bit higher (@370-375 on plates) but keep the screens dropped a bit, and as we're discussing here, the 6CZ5 do just fine in those also. There were a few stereo amps here and there that put approx. 420 VDC on the plates but kept the screens (fed from a separate rectifier) down around 330. I've yet to try out 6CZ5 in one of these, although I'd like to try it just to see what would happen. Actually I'd like to try out the new EH 6973 at these higher voltages too, just to see what they can take, b/c I'm not so completely sold on those new ones yet.