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AL-1500 2x mod
- Thread starter Dr_DX
- Start date
I did one many years ago. The right way to do it is to double everything from the plate transformer to the tubes, including the blower. Once you do that, get ready for the tank circuit to fail if you try and get twice the power from it. If you're not trying to get twice the power out, just leave it alone. You' ll only complicate your ability to replace matched tubes in the future. Want more durability, just upgrade the weak blower.
It's a great amplifier as it comes from the factory other that cutting that Green wire for 10/12 meter operation.I would not change anything because if it ain't broke why mess with it?
SIX-SHOOTER
SIX-SHOOTER
I have read a fair amount about the amp online but haven't seen anyone mentioning the blower as "weak". I have seen where they complained about the noise but haven't seen it as a "weak" point. I have seen a lot complaints about the meter lights.
Did you have a personal bad experience with a blower on one?
For most hams that use ssb the blower is fine. For high duty cycle modes and high power the blower is barely adequate. It's rated at 26.5 cfm
https://www.mouser.com/ProductDetail/ebm-papst/G2S085-AA19-15?qs=DB9cKZ5ZWfU1xjbiKNbhww==
Below are the specs from the 8877 data sheet. If you're going to run legal limit (1500 watts pep) that blower is usually fine. If you're trying to make a 3kw+ amp you need more cooling, more plate iron and the tank circuit may not hold up.
The 3cx1200 in the al1200 has a less efficient cooler and it's more of a problem with that tube on AM. The one thing CB amp builders usually do right is going overkill with the blower.
The thing to remember about the airflow spec for a full 1500 Watts is the back pressure. Point forty-one inches of back pressure requires one of two things, or both. A high speed blower wheel, or one that's enormous. The high speed will be noisy. No way around that. And fitting a really large one will be awkward at best.
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PA stages I'm familiar with in FM broadcast, use that tube with a blower about 3 times the size or more on a single tube. That is overkill for ICS duty cycle but the tube is expensive enough to warrant a minimum, of the maximum required CFM according to the data sheet. That is twice the size of the stock blower.
Sadly, when a ceramic tube of this size or more is being properly cooled, you do not want to be in the same room as the noisy blower. Large filters on the air intake and outside exhaust do help. People don't complain that the blower is inadequate but they do complain when the tube fails and sometimes that happens due to the seal, overheating. Just running 25% hotter, over time will reduce tube life.
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All,
Thanks everyone for your sage advice! I spent $80 in an effort to increase the durability with a drop in replacement blower that has 55 CFM output and 35 CFM @ .4 In/H. Seemed like $$ well spent. Now I just need to figure out how to quiet it down.
https://www.ebay.com/itm/Ebmpapst-T...G2E085-AA05-21-115V-60Hz-28W-FAN/291700634931
Thanks everyone for your sage advice! I spent $80 in an effort to increase the durability with a drop in replacement blower that has 55 CFM output and 35 CFM @ .4 In/H. Seemed like $$ well spent. Now I just need to figure out how to quiet it down.
https://www.ebay.com/itm/Ebmpapst-T...G2E085-AA05-21-115V-60Hz-28W-FAN/291700634931
Sounds like a shit-ton of work and expense to gain an extra half an S-unit at best.
You guys have convinced me to not do the 2x mod. I am just going to beef up a few things and call it a day.
Some will disagree, but I upgraded the filter caps and put the plate transformer on the 220 tap. I would at least test the caps and bleeders if the amp is older. You can also replace the rectifier diodes and ditch the ceramic bypass caps. Modern diodes off the same tape don't need them.
With 240v mains, going to the 220 tap will put around 4k on the plate. The amp will have a little more gain and run lower grid current. Depending on the tube you may need a couple more diodes in series with the bias to lower the quiescent current. 6a10s work fine.
With 240v mains, going to the 220 tap will put around 4k on the plate. The amp will have a little more gain and run lower grid current. Depending on the tube you may need a couple more diodes in series with the bias to lower the quiescent current. 6a10s work fine.
That amp uses a common plate/filament transformer. Boosting the plate voltage also boosts the filament voltage. Not a good thing pretty much all the time unless you want shortened tube life. Using simple ratios that would boost the filament voltage to 5.5 volts.YMMV but I wouldn't do it unless you reduce the filament voltage.
It doesn't. I know this because I replaced the filament transformer in my al1200 when I put the 8877 in it. The al1500, al1200 and al82 use the same plate transformer.
Ooops you are correct. My bad. I opened up the wrong .pdf file and was looking at the wrong amp.
Years ago I threatened to award a "most dollars spent per S-unit" for folks chasing 'just a little' more wattmeter reading.
Doubling up a pair of amplifiers that size should get you in the running for a championship score.
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Doubling up a pair of amplifiers that size should get you in the running for a championship score.
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