AA864 power issue troubleshoot

[ETMusic777]

I have not had a chance to crack open the amp but I will this weekend. I turned on my AA864 Bassman tonight, it was connected to a proper load (Fryette PS100 Power Station attenuator) and the pilot light briefly flickered on, and then went out and there was no sound output. All tubes, including the 6L6s are heating up, and the amp is drawing current so there is power there. The amp is stock except for a replacement Output Transformer, which a prior owner put on there. The power tube sockets have also been changed and its been recapped. The OT is a 2ohm ClassicTone which is actually meant for a tweed Bassman. However I've owned this for 6 months and have played it, and its been great. Before I get in there this weekend, I just wanted to see if anyone had any thoughts on where I should start with the diagnosis? The 2A fuse is fine, and since its drawing power is not a factor.

Im not amp tech but know my way around a schematic, soldering gun and multimeter. Attached are photos of the internals and externals which I took several months ago when I got the amp, they are not current. Power Transformer and Choke are original. When I can get in there, I am going to rewire the 3 prong cord so that it bypasses the aux output totally. I was planning on replacing the OT at some point down the road and will do it now, if needed.

Thanks in advance, amp Gurus!

 

[Jon Snell]

The pilot lamp is connected directly across the heater supplu.
How can you have no lamp but the heaters glow ... unless the bulb has stopped working for some reason.
Probably a faulty lamp and it coincided with an anode load resistor going open circuit or a bad connection on a valve socket.
Sorry but a proper load is a loud speaker!
A resistive load is just what is says on the tin, somewhere to drain the power and turn it into heat.

 

[ETMusic777]

Yes, I saw the schematic and was wondering the same questions. I will check every resistor in that section this weekend. So a Fryette PS 1000 attenuator is not a proper load? First time I have heard of one possibly causing damage. It was set to the 2/4 ohm input setting.

"bad connection on a valve socket." This may be it, Im in there poking around and some connections seem loose.

 

[peteb]

I just wanted to see if anyone had any thoughts on where I should start with the diagnosis?

a voltage chart is a common first step.

start with the power tubes.

write down the DC voltage at pins 3,4,5 and 8.

write down the AC voltage at pins 2 and 7.

I also suggest studying the pin out diagram of the 6L6 so that you are familiar with the function of each pin in the 6L6.

share your results.

 

[2L man]

I don't have any amp which I don't hear a pop from speaker when I touch power tube anode using MM probe. If not problem is power tube(s) and onwards. In used amps loudspeaker plug circuit is high on propable fault when filaments glow, fuses are OK and if other power tubes do not help. Multimeter should show very close to 0 ohms on speaker cable hot to cold but there cable and OT secondary resistance can make 1 or 2 ohms.

 

[Jon Snell]

Yes, I saw the schematic and was wondering the same questions. I will check every resistor in that section this weekend. So a Fryette PS 1000 attenuator is not a proper load? First time I have heard of one possibly causing damage. It was set to the 2/4 ohm input setting.

"bad connection on a valve socket." This may be it, Im in there poking around and some connections seem loose.

I was not infering it would cause damage, just that it is not an inductive load, that's all.
There is a difference when measuring the performance of an amplifier with an output transformer as the load varies over frequency. A resistive load is a constant load that does not vary.

 

[tubedude]

Check if you are getting any HV. That inductor looks like it's had a life of high heat. It might be open.

 

[ETMusic777]

Thanks for everyone's reply. Some good news: I swapped out the bulb for the bulb from my 67 Bassman, and it works and now I have sound. The bulb was burned out and for whatever reason there was no sound and the heaters were not getting full heat with the burned out bulb. With the replacement bulb the tubes are getting heated and there is now sound output. I am going to order some replacement bulbs.

So it sounded good but there was a noise, that sounded like "wind rustling over the prairie" I swapped out the tubes and that did not solve the problems. I then worked the pots a bit, and finally switched the "Deep" switch on and the noise disappeared! It comes back when I switch the switch on and off. I Deoxited it earlier and maybe now there is some electrical gremlin in the switch? Weird. Anyway I am going to get in there tomorrow and measure the voltages and resistances while watching football.

EDIT: I checked all resistors today and everything checks out fine within spec. I played the amp for an hour and its noise free and sounds amazing. I guess a burned out bulb can affect the 6.3v heaters to the point of no output. I found some old threads in the forum here where others had similar experiences. I will keep a small supply of bulbs on hand.