Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dunner84 on October 29, 2020, 06:20:13 pm
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Hello all,
I just finished a spare parts only 1978 JMP 2204 build, and I'm having an issue with PT heat.
The build went smoothly, and the only hiccups were some minor adjustments in the bias circuit to get things centered nicely.
The amp sounds amazing, and is dead quiet, but I noticed after 2 hours of playing the PT was scorching hot. I have a very high heat tolerance in my hands, and it was cooking.
The PT is a weber W025130, and is rated for 150mA on the HT winding, and 5 amps on the heater winding. 2 el34, and 3 12ax7 should he around 4 amps.
I let the amp cool down, turned it on again with no guitar plugged in, and after 2 hours it was cooking hot again which makes me thing the heater tap is being over taxed.
I have two small 6.3v transformers, one rated for 1A, and one for 1.5A
I am thinking about adding one to run the 12ax7s, and take some load off the PT.
Question 1 - can I get away with the 1A transformer? 3 12ax7 is Around .9A draw.
Question 2 - where should I place it.
I can squeeze it beside the PT
Or I can set it beside the OT
The benefit to placing it beside the OT are aesthetics, and the heater winding will line right up with the first 12ax7. A big con is having the primary winding cross my power tube sockets.
Which transformer, and which placement option should I go with?
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I think that PT is very capable of supplying the filaments. But 150mA on the HT seems a bit light for a 2204.
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That was my initial thought too, but that was the original Marshall spec, so I thought I'd go for it. I have more robust transformers on hand that would maybe be better, but I went with this one for a few reasons:
1 - the voltages, and current ratings were in line with the original schematic I was using.
2 - The secondary winding has two taps, and I was going to put them on a switch to try high plate voltages vs low plate voltages, as the internet seems divided on whats best for this circuit, but I couldn't source a switch with the proper voltage rating.... :(
3 - it lined up almost perfectly with the hole that was cut in this chassis.. (laziness)
I have no problems changing it if the HT current spec is at fault, but if it's heater current, I would like to use one of these filament transformers.
I tried ruling that out by seeing how hot the PT got by just letting it idle, but maybe that's not correct?
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Some amps just get hot. May not be a problem at all.
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How about a fan? Fairly cheap possible solution?
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Ok I'm blabbering!
The magnet wire used to wind the transformer is what will limit or determine the max operating temperature. So if it is wound with 150 class it would be limited to 150 degrees Celsius if it is 200 class it is limited to 200 Celsius ect....
That's mighty warm! But efficiency is going to be affected. Also the more air around the core will affect cooling.
The reason many transformers die is because the insulation on the wire breaks down from vibration and or heat. Then shorts out.
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Thanks for all the feedback.
I tried clipping in the dedicated filament transformer on the 3 preamp tubes, and let the amp idle for a couple of hours. It still got pretty hot, but not as much as before. With the dedicated transformer clipped in, the Heater voltages were 6.6V on both transformers. I put things back to normal, and with the PT connected to all 5 tubes, the heater voltage was 6.5V.. No crazy loading or anything weird there. I think the transformer must just get hot by design. It is still worrisome, so I am going to install one of those PC fans in the cabinet to encourage good airflow.
I am going to be playing for about 4 hours tonight, so it should be a good test. I will have the fan running, and I will take some IR temp readings. I am curious to know how hot it really is because it was burning hot to touch.