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PT on nuts or washers for heat dissipation ?

I wanted to drop the voltages slightly as well as have a very slow start, this is why I previously added a secont thermistor, for the tubes lifespan.

The appertures looks like that, sorry for the picture, but my amp was plugged so I just lifted it up to take the pic.

1.jpg2.jpg
 
The thermistors don't provide a slow start for the valves - they only suppress the power transformer in-rush, and can reduce the hot turn-on current pulses through the valve rectifier if you turn the mains power off then on quickly. As such, they may allow a lower fuse rating to be used, but it appears that you added the thermistors, so there is no connection with better fusing.

I can't see that the KT88 base is really obstructing air flow through those chassis holes - there is no forced air flow through those holes, only chimney effect air flow.

What other parts are under the chassis that generate heat? Maybe you have degraded the thermal performance of the amp by not appreciating the consequence of adding the thermistors. Perhaps this thread is about trying to fix something that shouldn't need to be fixed in the first place?
 
The thermistors really slows the rise of all components voltages at cold start, both Volts and Current, with all my tubes amps they solw down the heaters to golw, and everything.

With my Yaqin 300B, I don't have a problem with two thermistors, they are in a separate empty compartment under the power transformer, but with this KT88 amp, a second 120 2A ohms thermistor create a hot spot under the choke.

Only the thermistor now, (one on AC input) generate heat under the chassis, in the corner, behind the choke.

Maybe I can put two thermistors in parallel, in series with two other thermistors in parallel.
 
What thermistor part are you using, and what is the AC mains rms current drawn by the amp when operating in idle?
 
NTC 120-D15 (Now MF72 120D15)

I don't have a "Kill A Watt" to measure exactly and I don't know.

Four Thermistors to do the wotk as one, they will each heat less, slightly slower start, and slightly higher vlltage drop.
 
Typically the mains ac current for an NTC should be between about 0.6A (30%) and not too close to 1.8A max rating. A cold 120 ohm in series with mains ac is imho huge, and you originally had two in series. It seems a risky and unsafe action to add parts to the AC mains side of equipment when you don't appear to know whether the part is operating within its ratings or with much understanding of what it is doing apart from lowering the initial heater glow rate.

The NTC will heat up quickly when power is applied, and well before the valves start conducting, and your amp needs to be off for likely at least 2 minutes before turning on again to get the NTC temp back down to near ambient.

NTC's should never be operated in parallel. It seems like you haven't even read through the basic use guides of NTC's.
 
Where did you read that we cannot put thermistors in parallel, ? each will carry half of the current flow.

Even overrated, they are quite hot when in series but not paralleled.

And in series, each will carry the inrush of other.
 
You shoud not put thermistors in parallel to compensate for lhe too low rating of one, but if current doesn't top the rating of one, why not ?
 
mroboto said:
Where did you read that we cannot put thermistors in parallel, ? each will carry half of the current flow.
Click to expand...

mroboto said:
You shoud not put thermistors in parallel to compensate for lhe too low rating of one, but if current doesn't top the rating of one, why not ?
Click to expand...

Because, courtesy of manufacturing variations, one thermistor has a slightly lower resistance and thus passes nearly the entirety of the inrush current. As the current flows through the hotter device (lower resistance path) it becomes even hotter. Remember, an NTC has its resistance drop with temperature, so the hotter device always has lower resistance and thus always carries more current. This means that the other device has been removed from the circuit.

A better way to perform the delay is via an RC delay controlling a MOSFET to ramp up the voltage over time.

The reason for the delay is (a) initially bringing the heaters up to operating temperature, which overloads the filament winding, plus (b) initially charging the filter capacitors on the B+ windings.
 
mroboto said:
The thermistors really slows the rise of all components voltages at cold start, both Volts and Current, with all my tubes amps they solw down the heaters to golw, and everything.
Click to expand...
If you have a SS rectifier or a directly heated tube rectifier, a thermistor will help soften the turn on surge just a tiny bit. But it does not come anywhere near to the slower start which results from using an indirectly heated rectifier tube, ie one that has a cathode.

mroboto said:
With my Yaqin 300B, I don't have a problem with two thermistors, they are in a separate empty compartment under the power transformer, but with this KT88 amp, a second 120 2A ohms thermistor create a hot spot under the choke. . . . Only the thermistor now, (one on AC input) generate heat under the chassis, in the corner, behind the choke.
Click to expand...
If you can post a pic it would be helpful. Perhaps there is a better spot to put it where it won't heat things up as much.
 
maxhifi said:
As was previously mentioned, standing the transformer a bit off the chassis can help with vibration related buzz and hum.
Click to expand...

This ... ^ ^ ^

... and adding another voice to the choir, 128F isn't all that hot. What's lacking here is a clue what you've got for air circulation around the unit? Tube amps especially like to "see the sky" as they say, and placed in an open area. If you can't do that, provide as much air space as you can.
 
sKiZo said:
This ... ^ ^ ^

... and adding another voice to the choir, 128F isn't all that hot. What's lacking here is a clue what you've got for air circulation around the unit? Tube amps especially like to "see the sky" as they say, and placed in an open area. If you can't do that, provide as much air space as you can.
Click to expand...

Not lacking, I replied, he is sitting on a wooden chair, with 1cm rubber feets between the chair and the chassis.

Just , the chassis accumulate heat, not made to run cool from the start.
 
FlaCharlie said:
If you have a SS rectifier or a directly heated tube rectifier, a thermistor will help soften the turn on surge just a tiny bit. But it does not come anywhere near to the slower start which results from using an indirectly heated rectifier tube, ie one that has a cathode.
Click to expand...

Yes, sometimes , I use a 6106 instead of a 5Y3 with my Raphaelite DP84, but I prefer the sound with the RCA 5Y3GT.
 
I added two 1mm M4 teflon washers, and the power transformer is approximately at the same temp as before, 123°F, but the chassis is slightly better vented.

I think I could add a 3rd layer of teflon washers, they are not thick as I believed.
 
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