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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Sep Thu 24, 2020 2:31 am 
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Joined: Sep Sun 06, 2020 11:02 pm
Posts: 16
Thats cool! The front end looks pretty similar- will try that instead of hacking on the device. Copy on the bluetooth wallwart noise, the one I have there is suspiciously small... :)


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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Oct Wed 07, 2020 2:37 am 
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Joined: Sep Sun 06, 2020 11:02 pm
Posts: 16
Work interrupted progress on the unit, but I did get all the caps done. But I've some time to spend on it so now testing parts for the documented resistances- the only bad wirewound resistor is the Bleeder #1- looks like someone else had been fixing on it a bit, but it looks overheated and the card is damaged with old tape on it. The other wirewounds are OK, though I did have to re-squeeze the rivets as has been pointed out. I'd like to replace with modern resistors- is the dissipation of Bleeder #1 known?


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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Oct Mon 12, 2020 4:41 am 
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Joined: Sep Sun 06, 2020 11:02 pm
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Made up a replacement for Bleeder #1 and checked all the inductors; coils OK, 2nd AF xformer OK, but primary and one half of the secondary of 1st AF xformer are open. I have the transformer can off, will melt out the transformers tomorrow. If the 1st AF xformer can't be trivially repaired I'll replace it with a modern unit.

The parts sheets suggest AF #1 is a step-up on the order of 1:7 at least from a DC resistance standpoint, is there an argument against a 1:1 or 1:2 replacement?


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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Oct Thu 29, 2020 3:18 am 
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Joined: Sep Sun 06, 2020 11:02 pm
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So I've been moving through the set; I got tired of the wirewound and ceramic tube resistors- some of them had drifted quite a ways, so I replaced them all with modern resistors of double the wattage. Caps replaced, the 1st and 2nd AF coupling transformers ended up bad; #1 had an open winding and #2 started unwrapping as I melted out the tar. So I replaced both with a Hammond P-T1750AB and a P-T156 interstage both from Antique Electronic Supply- they look like pretty good matches. I put a 10watt non-inductive 1.1k resistor in to replace the field coil.

I powered up gingerly via dim-bulb and variac, all the filaments good, rectifier tube powered up and started generating the supplies, even the front panel lamp started lighting up. I progressed to 80% line voltage and the plates are coming up in reasonable proportion, not running away.

So far so good :) Will bring it fully up tomorrow, start tracing signals through and checking the new transformers.


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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Oct Sat 31, 2020 11:15 pm 
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Joined: Sep Sun 06, 2020 11:02 pm
Posts: 16
Got the unit working with the circuit mod shown here, playing line level audio very well. There is still some tuning to do, the combination of input level and the 5k plate resistor can put the 1st stage into clipping. Reducing the 5k reduces the 2R gain, my interpretation is 2R's plate resistance thru the plate resistor forms a voltage divider, their proportion affecting the voltage swing on 1A's gate. (looking at the schematic again, the B+ I'm using for the 5k is after the 2500 ohm choke- on the rail supplying 10e/9/1h/5- I am not using the speaker coupling xformer centertap supply).

The volume control is excellent, though at 0 there is still a trace of audio coming thru, we'll see how that plays out as I get the tuning worked out. The input coupling transformer is a Triad TY-142P, though a 1:1 version would be fine I think- 10k to 10k would be plenty.

I've switched between my beater bench speaker to a nice old Stromberg/Carlson unit my dad built when he was in college- its a sensitive speaker and good match for this amp.

From watching the B+ on powerup the 55C could definitely use a soft-start, the plate voltage surges to 300v before the tubes get conducting but it settles down very nicely.

Have to say working on tube systems is pretty fun... maybe I'll have find another one :)


Attachment:
atk-55c-mod.jpg
atk-55c-mod.jpg [ 145.49 KiB | Viewed 123 times ]


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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Nov Thu 05, 2020 3:56 am 
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Joined: Sep Sun 06, 2020 11:02 pm
Posts: 16
Just a followup for posterity. I paralleled the 24's to improve the 1st stage gain, I have the set in the condition shown below- switches in the "aux input" mode. I hooked up a sine wave generator to the line level input, then peaked the input level and volume. With the output stage working into a load resistor I obtained 5.5watts peak and about 29ma of plate current on each of the 45's.

I experimented with paralleling the 27's and adjusting resistor #13 (1st AF filter resistor) to get more signal thru the transformer into the grids- no luck, I'm guessing coupling into the power amp is limited by transformer reactance. Decreasing grid bias into the 45's didn't help either.

I have some 1:1 input transformers coming so will re-evaluate the input stage- right now its a 10k:2k. The 10k plate resistor on the paralleled 24's is approximately the happy place, no distortion at max volume and good apparently loudness into high efficiency speakers.


Attachment:
atk-55c-mod.jpg
atk-55c-mod.jpg [ 176.38 KiB | Viewed 83 times ]


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 Post subject: Re: Atwater Kent 55C - the usual found-in-attic
PostPosted: Nov Tue 10, 2020 3:28 am 
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Joined: Sep Sun 06, 2020 11:02 pm
Posts: 16
I've continued messing around with the unit. I changed from paralleled 24's to cascading, as below;

Attachment:
atk-55c-mod-rev2.jpg
atk-55c-mod-rev2.jpg [ 229.05 KiB | Viewed 40 times ]


(input coupling xformer in the schematic is a drawing artifact- not present in the circuit)

Volume control in this configuration is substantially better- peak power is about 5.5 watts (measured over a load resistor at 1kc, input level and volume peaked) but the cascaded gain helps to bring up the quiet stuff. Decreasing the 2nd 24's plate resistor reduces total gain to avoid overdriving the 27. The coupling caps could as easily be .01uf, though given the fairly bright spectrum this amp exhibits anything to strengthen the lower frequencies is helpful. The quality condenser rolls off a lot of treble, definitely helpful for the cabinet speaker I selected and for the sensitive Stromberg/Carlson monitors. The unit does an ok job with regular 8ohm speakers but clearly doesn't have a lot of drive.


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