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driving a sb220
- Thread starter Number15
- Start date
It Could Happen... No, REALLY!!!
You know, Mr.X is someone I've known for ~40 years.
Mr.X was hanging around with his friend Bill Eitel (the Ei in Eimac) one day and heard Bill say, "Ah, those 3-500Zs are capable of at least a good 20% over their rating..." - They were talking about Plate dissipation.
OK, so, given the 3-500Z is rated at 400 mil, a pair of them at 800 mil, plus about 25% (remember Bill said "At least..." so let's push them to +25%, why not, we're CBers right?) - or ONE FULL AMP, and at +25% of their 4KV rated Plate voltage, or 4500VOLTS, - the input from a choked 4500V supply, driven hard enough, could reach 4500watts input, plus the 300w drive as we are talking about a GG (Grounded Grid) amp, so ~4800w total.
At ~62% typical GG efficiency, that's right about 2976w output.
The problem is the tubes would melt due to excessive plate dissipation plus inadequate cooling. The 220 was designed to dissipate right about 1300w output, PEP, SSB service, @ ~62% efficiency, or about 400w AM.
NOW, you COULD get ~3Kw out but you would have to mod it 'a little'.
First., since you no longer have need for the power supply as you have chosen to build a choked 4500V supply (and went ahead and moved the filament transformer outboard also) now you have a big hole where the transformer, filter / doubler caps and Diode stack were located.
- This is a good thing because you'll also have to remove the tuning caps as they would arc out with more than ~1800w pep, so now you will have to add nice large vaccuum caps where all those holes are.
So far so good? ... but wait, you can no longer use the 120w handling tuned input circuit so now you will have to tear that out and pick up an outboard 1kw PEP/ 300w AM antenna tuner for your input in order to handle the 300w or so AM peaks you're now driving the amp with.
oops, I forgot, more power means a new hand-wrapped larger guage set of chokes, so pull the Plate and filament chokes, wrap & install a pair of new, heavier-duty chokes.
Oh, wait! Those crappy little porcelain sockets won't handle the new hotter tube pin temperature so you'll have to remove those also and add a set of alloy air sockets & chimneys you'll need to keep the tubes from total melt down at that 'max-redline power level, and since the original fan for your SB-220 is a joke in comparison to the outboard blower you'll need to force air into those sockets for adequate cooling, you can remove that too!
OK, so, yes it could happen that a pair of 3-500Z triodes could output ~3kw in an SB-220 CABINET, but that's all that would be left of an SB-220, THE CABINET!!!
One more thing, how long would your engine last if you constantly drove it at red-line?
How long do you expect a pair of $300 tubes to last running them at full-bore-PLUS?
...and I haven't even touched on the parasitic nightmare you'ld be looking at with that level of power from a set of 3-500Zs on '10'
meters...
Good luck!
You know, Mr.X is someone I've known for ~40 years.
Mr.X was hanging around with his friend Bill Eitel (the Ei in Eimac) one day and heard Bill say, "Ah, those 3-500Zs are capable of at least a good 20% over their rating..." - They were talking about Plate dissipation.
OK, so, given the 3-500Z is rated at 400 mil, a pair of them at 800 mil, plus about 25% (remember Bill said "At least..." so let's push them to +25%, why not, we're CBers right?) - or ONE FULL AMP, and at +25% of their 4KV rated Plate voltage, or 4500VOLTS, - the input from a choked 4500V supply, driven hard enough, could reach 4500watts input, plus the 300w drive as we are talking about a GG (Grounded Grid) amp, so ~4800w total.
At ~62% typical GG efficiency, that's right about 2976w output.
The problem is the tubes would melt due to excessive plate dissipation plus inadequate cooling. The 220 was designed to dissipate right about 1300w output, PEP, SSB service, @ ~62% efficiency, or about 400w AM.
NOW, you COULD get ~3Kw out but you would have to mod it 'a little'.
First., since you no longer have need for the power supply as you have chosen to build a choked 4500V supply (and went ahead and moved the filament transformer outboard also) now you have a big hole where the transformer, filter / doubler caps and Diode stack were located.
- This is a good thing because you'll also have to remove the tuning caps as they would arc out with more than ~1800w pep, so now you will have to add nice large vaccuum caps where all those holes are.
So far so good? ... but wait, you can no longer use the 120w handling tuned input circuit so now you will have to tear that out and pick up an outboard 1kw PEP/ 300w AM antenna tuner for your input in order to handle the 300w or so AM peaks you're now driving the amp with.
oops, I forgot, more power means a new hand-wrapped larger guage set of chokes, so pull the Plate and filament chokes, wrap & install a pair of new, heavier-duty chokes.
Oh, wait! Those crappy little porcelain sockets won't handle the new hotter tube pin temperature so you'll have to remove those also and add a set of alloy air sockets & chimneys you'll need to keep the tubes from total melt down at that 'max-redline power level, and since the original fan for your SB-220 is a joke in comparison to the outboard blower you'll need to force air into those sockets for adequate cooling, you can remove that too!
OK, so, yes it could happen that a pair of 3-500Z triodes could output ~3kw in an SB-220 CABINET, but that's all that would be left of an SB-220, THE CABINET!!!
One more thing, how long would your engine last if you constantly drove it at red-line?
How long do you expect a pair of $300 tubes to last running them at full-bore-PLUS?
...and I haven't even touched on the parasitic nightmare you'ld be looking at with that level of power from a set of 3-500Zs on '10'
meters...Good luck!
Re: It Could Happen... No, REALLY!!!
Couldn't agree more on that either!!
OK, so, yes it could happen that a pair of 3-500Z triodes could output ~3kw in an SB-220 CABINET, but that's all that would be left of an SB-220, THE CABINET!!!
...and I haven't even touched on the parasitic nightmare you'ld be looking at with that level of power from a set of 3-500Zs on '10'
Good luck!
Couldn't agree more on that either!!
Toll_Free
Well-Known Member
W5LZ said:
1. Dahl is outta bidneth for helping the amateur community, so no more drop in or external Dahl's. Better get some piggies.
2. Your right, you have to remove the plate tune cap, double the spacing (halving the max capacitance), making the amp basically a 10 and 15 meter amp. Only way you'll get it to withstand the 4500 volts you need on the anodes to see that 3 grand.
3. Better redo the tuned input. Heath doesn't even have a Q of 1, I believe. Eimac states 2 or 3 for glass tubes (usually) and 5 for ceramics. Just redoing the tuned input on a SB220 with a Q of 3 and new caps (those old caps get lossy after 30 years) will net you a couple hundred watts PEP increase.
4. The stock xformer is a .6kva. That's right, it's good for 600 watts average draw. Three times that is 1800 watts, which is just about what the SB220 runs INPUT.
5. The SB220 in today's world has too much voltage on both the fils and the internal xformers. Pole voltages and wall voltages have both climbed since it was designed.
This is a basic rundown. Bias section must be modified for as close to Class C (yes, these are tubes, and we run them actually class C, not B as grounded base xisters are) as you can stand. Then, you can hit 3 grand with an SB220. It takes an HONEST 2 days of work (14 hours), and someone who isn't an idiot. And a bit of money. The transformer alone is a couple hundred bucks, and my labor isn't cheap.
Stock trim, the SB220 is a 1200 watt PEP amplifier, when driven correctly, on 11 meters. The input is run out of tune, the relay in it sucks and it just isn't much better. Tuned for 11 meters, they will run at close to 1600 pep out. There just isn't any more transformer than that in them. And 1600 is pushing it.... 1600 out is > 2400 in... REALLY taxing that 600 watt transformer.
--Toll_Free
Toll_Free
Well-Known Member
High Miler said:
You have to remember, they aren't using these amps on 10 meters, where the input and output networks are tuned.
200 watts PEP in on 11 meters will produce about the same grid current as 130 on 10. Still hammering the SB220 transformer, but the tubes can take that just fine.
--Toll_Free
