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Help hooking up replacment transformer

F

fmjunkie

Member
Hey gang. I usually stick to working on turntables, but I got a great deal on a set of Arte Forma 300B monoblocks. One has a bad power transformer, but I decided to take these on as a project and try to learn something so have a newbie question.

The original power transformer has secondaries for two 5V heaters: 0-5 and 0-5. That transformer was made by MCT and there are no off the shelf products that match. Edcor has a transformer with almost the exact same specs except it has heater secondaries that are center-tapped: 2.5-0-2.5 and 2.5-0-2.5. I'm under the assumption that I connect the 2.5 secondaries to my heaters and ground the center tap to get the 5V I need (without blowing up anything). Is that correct? Thanks and any assistance is welcome.
 
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fmjunkie said:
Hey gang. I usually stick to working on turntables, but I got a great deal on a set of Arte Forma 300B monoblocks. One has a bad power transformer, but I decided to take these on as a project and try to learn something so have a newbie question.

The original power transformer has secondaries for two 5V heaters: 0-5 and 0-5. That transformer was made by MCT and there are no off the shelf products that match. Edcor has a transformer with almost the exact same specs except it has heater secondaries that are center-tapped: 2.5-0-2.5 and 2.5-0-2.5. I'm under the assumption that I connect the 2.5 secondaries to my heaters and ground the center tap to get the 5V I need (without blowing up anything). Is that correct? Thanks and any assistance is welcome.
Click to expand...

It should work fine, but don't ground the center taps unless the design calls for the 300B filaments to be grounded. They may need to float, but I'm not familiar with the design.
 
fmjunkie said:
Edcor has a transformer with almost the exact same specs except it has heater secondaries that are center-tapped: 2.5-0-2.5 and 2.5-0-2.5. I'm under the assumption that I connect the 2.5 secondaries to my heaters and ground the center tap to get the 5V I need (without blowing up anything). Is that correct?
Click to expand...
This ^^^^. If the stock PT secondaries are not center tapped you should just connect the two ends and leave the center tap disconnected.
 
Guessing this has a balance pot across the 300b filaments? You basically have to use either a pot or a center tap but never both.
 
Gadget73, you right. It does have a hum pot. One of the monoblocks worked, but had a bit of hum. I already replaced both transformers, and all seems good but still have that bit of hum on both monoblocks. Any suggestions for cutting down on that?
 
They also sound a little "thin". There is not adjustment for bias, but I'll double check it to see if the tubes are running cold.
 
hum could be a lot of things, maybe power supply without enough filtering, an ailing cap, a ground in the wrong spot, wire routing, etc. Knowing if its 60hz or 120hz would help to narrow that down some.
 
Definitely a 60Hz hum. Not bad. But there. The hum pot reduces it, but it is still fairly audible. Doesn't change with the volume. Haven't had a chance to check the bias with holiday company and whatnot, but there has to be some room for improvement.
 
The hum could just be the nature of the circuit. There's a lot of room in the chassis and I was thinking about installing a DC filament supply like the Tentlabs 2.5A. Is there a good way to trace the source of the hum before I start throwing more money into this project?
 
Volume independant hum is usually power supply capacitors. It doesn't matter if an amp is tube or solid state. It is not supposed to hum. All of my tube amps are so quiet that you can only hear a little bit of hiss with your ear right up to the speaker.
 
300B uses a 5 volt heater

Does the hum balance adjustment do anything for you? If thats not set right it will hum. Since thats AC powered right off the 60hz transformer output, the hum will be 60hz. Power supply will be 120hz if its a full wave rectifier setup.
 
gadget73 said:
300B uses a 5 volt heater

Does the hum balance adjustment do anything for you? If thats not set right it will hum. Since thats AC powered right off the 60hz transformer output, the hum will be 60hz. Power supply will be 120hz if its a full wave rectifier setup.
Click to expand...

The 300B filaments in these amps are fed by AC directly from the transformer outputs. Hum balance adjustment lowers the hum, but it's still audible--too audible for my liking.
 
does it stop if you pull the tube immediately before the 300B ? Not sure what it is, googling that amp brings up several very different looking amplifiers. Either way if you pull whatever is right upstream of the 300B, if the hum remains we know its something in or connected to the output stage, if it shuts up the problem is before that point.
 
Short the input RCA between the center pin and the outside sleeve. See if the hum goes away. If no then work the filament hum pot back and forth a few times with the amp off then recenter it before power up. Check power supply caps on the HV supply. Is then hum level constant or does it go up with the gain control?
 
fmjunkie said:
The 300B filaments in these amps are fed by AC directly from the transformer outputs. Hum balance adjustment lowers the hum, but it's still audible--too audible for my liking.
Click to expand...

Just to confirm--this is a parallel SE 300B amp? And there is a hum pot for each 300B? Can you take a picture of the underside so we can see the 300B filament and cathode bias arrangement?
 
If these have 5VAC on the 300B filaments, you can only null it so much. DC is the only way to reduce the hum all the way.
 
Thanks for the tips. I don't know enough about tube amps to know if this is a parallel 300B or something else. There is a 6V6GT just before the 300B, pulling that does not eliminate the hum. I have not shorted the input and worked backwards yet...Tried posting some photos but they aren't going through. Will try to figure it out and repost.
 
parallel 300B meaning 2 per channel. That could be push-pull or single ended parallel but some pics of the guts would answer that. If one per channel, its just straight up single ended.


drag and drop the pics into the reply window. If you have several, drop them all, then click the "all thumbnail" option
 
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