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Hammond el84 tubes

T

tube-a-lou

Addicted Member
Hi all,
I have a matching quad of EL84 hammond tubes made in Germany.
I did try them out in my Scott LK48B and does sound wonderful but I'm
told they won't hold up. The voltage is 417 volts across the four tubes
and bias is -20volts. Would these old tubes hold up or should I get a
set of EL84M's or other tubes.
 
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Unless you use the higher rated tube like the 7189 or EL84M, you run the risk of red plating the tubes.
 
Keep an eye on them, and they should be ok. They are running up at the limit of what they will do but as long as they maintain bias they should do it for a few years I would think...
 
Considering official EL84 ratings are for 300v, I'd be pretty leery of running them north of 400v if I wanted them to survive. This is 7189 territory. EL84M is the modern sub.
 
tube-a-lou said:
Hi all,
I have a matching quad of EL84 hammond tubes made in Germany.
I did try them out in my Scott LK48B and does sound wonderful but I'm
told they won't hold up. The voltage is 417 volts across the four tubes
and bias is -20volts. Would these old tubes hold up or should I get a
set of EL84M's or other tubes.
Click to expand...
gadget73 said:
Considering official EL84 ratings are for 300v, I'd be pretty leery of running them north of 400v if I wanted them to survive. This is 7189 territory. EL84M is the modern sub.
Click to expand...
Yes, the 7189 data sheet says it can be used with up to 400v on the plate while the 6BQ5 is rated at 300v but I believe that the more important spec is plate dissipation, which is determined by the combination of voltage and current, rather than voltage alone or current alone.

If you look at the data sheets you'll notice that the maximum plate dissipation for the 7189 and the 6BQ5 is the same - 12 watts. So if you run the 7189 at higher voltage you need to run lower current in order to stay under 12w. The screen (G2) dissipation rating is also the same.

For what it's worth, I recall seeing claims by the former owner of Triode Electronics that there was actually no difference between the 7189 and 6BQ5. What construction differences would account for a higher plate voltage rating yet not allow a higher screen voltage rating or a higher plate or screen dissipation rating?

I couldn't find a schematic that shows voltages. How hard, in terms of dissipation, does the Scott run the tubes?

Nothing wrong with EL84Ms. Just get a set from someone who actually matches them for transconductance and current, such as Jim McShane, rather than whatever passes for "matched" on eBay or elsewhere.
 
Sovtek EL 84m are top of the heap at the moment. You need to measure the current to keep them in the safe zone. If your looking for the best, these are it....
 
Your Hammond tubes will work but they won't last as long as if they were running with rated voltage. I would definitely not risk a quad of good old tubes. In the 60's, I would have said, "no problems" because tubes were plentiful and affordable.
 
They really weren't more affordable. Adjusting for inflation, tubes in the 60s were more expensive than they are now. Not grossly so, but they weren't exactly peanuts either.
 
Good info here. My Fisher X-100 has lettering on the chassis that indicates either 7189 or 6BQ5 can be used, which I guess "works" but isn't ideal. Later chassis then only say 7189, so bad things must have happened in that transition period!
 
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the voltage a tube "sees" is always measured from the cathode. 380v at the plate and 40v at the cathode is effectively 340v. Screen is effectively at 280v, which allows for higher plate voltage.

you can extrapolate the current in the output stage with this one too. Schematic says 160 ma off the rectifier, which is the load of all tubes. The 320v source uses a 2.2k resistor to drop 70v, which means 32ma is used by the screens and the front end tubes. That gives us 128ma of current used by the plates of the 4 output tubes. Divided by 4 is 32ma per tube. 32ma * 340v is 10.9 watts plate dissipation, below the 12 watts allowed in the datasheet.

Going a little further, the 260v source uses a 3.3k resistor to drop 60v, so 18ma is used by all of the small signal tubes. That means 14ma is used by the screens, or 4.7ma per tube. At 280v thats 1.3 watts of screen dissipation, also below the 2w datasheet limits.

so there is more to it than just voltages. the Fisher looks a little rough at first blush but realistically it wouldn't be that awful on tubes.
 
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604man said:
I rebuilt a Scott integrated amp that was designed for 7189A tubes but he had 6BQ5's. still works fine after all these years.
Click to expand...

Not all vintage 6BQ5 are alike. The 1950-60s GE are about as tough as they come. More like a 7189A, with a 6BQ5 label. :) Hammond amps I've worked on, are well-made. However IIRC, they are hard on EL84/6BQ5. I like 6V6GT organ amps over the EL84. Think that the 6V6GT is a tougher tube.
 
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