Ameritron 811 Woes

[thedirtmover]

Would anyone have any ideas on what may have blown? I had a tube short out and it popped one of the fuses in the back. I replaced all 3 tubes with new ones and now I have no plate current and the tubes will not light. Of course I replaced the fuse. I would like to try to fix it myself if it is an easy fix, if I am unable, who would you recomend and can you set me up with them?

All posts appreciated......Thanks

 

[nomadradio]

Hi Dirtmover,
The schemo I have of the 3-tube AL-811 shows two fuses. Was only one of them blown? The whole amplifier runs from one large transformer, so I'd be looking at the line-voltage circuits for this problem.

It's been too long since I last looked inside one of those, but there's an interlock switch on the diagram. Usually placed so that the top cover holds it "on". I'd check that for surge damage, and the main power switch, too. Sometimes a switch will be damaged before a fuse can blow. For that matter, if you found a fuse larger than stock in this amplifier, this gets more likely.

73

 

[thedirtmover]

Thanks for the response Nomad. It indeed has 2 fuses. It indeed only blew 1 fuse. As I recall however, I think I am responsible for its demise. There are 2 large sockets and 2 small sockets on the tube. I think I put 1 tube in backwards which caused the amps demise. I will check the interlock switch and the power switch as those should be pretty easy. The fuses were stock and I have had the amp for about a year and a half. The amp came from a hamm estate and it appeared that he took very good care of his equipment. I have had it on a shelf ever since it took the big plunge. I would like to fix it and use it as a backup to my SB220 as I have 3 nice 572B tubes here for it.

Also wondering if anyone has instructions on upgrading one of these to a 4 tuber like the 811H?

 

[Captain Kilowatt]

I believe that the 811 amp has the same power supply components as the 811H.You could probably simply load the amp a bit heavier using the 572B's in it.Normally 572B's will not give you any more power output when simply used as an 811A replacement but when the power supply will handle a heavier load you can run a higher plate current with them and thus gain more power output.How much more is the question but I would suspect 33% more which would still be within spec for the components.572B's like a higher plate voltage to really start talking however,something around 2200-2400 volts.Do NOT change the taps on the power transformer to gain a bit more HV because it will also raise the filament voltage.Not a good thing.

 

[thedirtmover]

Upon further testing, the inline voltage switch is good as well as the power switch.

 

[paws264]

......As I recall however, I think I am responsible for its demise. There are 2 large sockets and 2 small sockets on the tube. I think I put 1 tube in backwards which caused the amps demise.............

Placing the 811A in "backwards" (180 degrees out, both big pins jammed into the small holes) should not have been the reason for this amp to have problems because of the internal construction of the tube.

The E.I.A. Vacuum-Tube Base list this tube as a 3G. The 3G base has the filament voltage on pins 1 and 4 (the large pins), the control grid on pin 3 and no connection on pin number 2.

If the tubes was put in backwards, pins 2 and 3 would be at filament voltage levels but because there is no internal connection between the pins nothing would happen; pin 1 would be at ground potential and pin 4 would not have any connection to it as it is jammed into socket 2.

Even inserting the tubes 1/4 turn off would not hurt the tube.

Well, you say that you want to fix it yourself; first thing to do is make sure that you have the tubes inserted properly.

I DO NOT recommend that non-qualified folks poke around in stuff that they don't have any business in..........having said that:

Check the filament voltage at the bottom of the tubes on pins 1 and 4, you should expect to see 6.3 volts AC. While looking at the bottom of the amp, make sure that pin 3 is grounded. If there is voltage there, the tubes should light up; no voltage, check wiring to transformer.

Meter #1 should show high voltage (B+). There are 5 diodes D-19 thru D-24 in the center tap of the filament transformer that sets you operating bias for the tubes (these appear to be 1N5408), check them to make sure that they are not shorted or open.

The roadmap for this amplifier can be found at http://www.ameritron.com/man/pdf/AL-811.pdf

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