Resistor power rating

[catalin gramada]

Hello. Would be a 5w cement wirewound resistor suitable to be used as plate load which dissipate continuous 2w at iddle, please ? Thanks.

[mhuss]

Sure, if it's a class A amp. Just make sure it has plenty or air (below as well as around).

[catalin gramada]

Thanks. My concern regards to not smoke too much. Usualy use to dissipate no more than 25%. Have no experience how hot get if use it at half...

[catalin gramada]

I know the power rating is dependent by ambient temperature. Just wonder if a cement one can perform at half of nominal max power into a warm environment as a tube amp chassis is?

[mhuss]

I find half rated power is usually a safe upper limit. Why not try it (breadboard if needed) and check the temp?

[SoulFetish]

I know the power rating is dependent by ambient temperature. Just wonder if a cement one can perform at half of nominal max power into a warm environment as a tube amp chassis is?

Manufacturers usually (almost always) include some sort of graphical info on temperature derating in the datasheet

[catalin gramada]

Manufacturers usually (almost always) include some sort of graphical info on temperature derating in the datasheet

Yes, cement wirewound have kind of large ecart of what is called power derating. Seems it can be used at nominal power till 70C and slide off till max acceptable 150C / 300F ambient when still are able to dissipate a half. That meant into a 300F environment a 5w cement wirewound still be used if not dissipate more than 2.5- 3W. That is at least what I understood from the graphs...

[catalin gramada]

This is my layout. Those 33k/5w ceramic wirewound resistors hoovering one inch below the driver socket. I intend to dissipate ca. 1.6W on each. Do You think holes around socket will be necessary for proper ventilation, please?

[SoulFetish]

This is my layout. Those 33k/5w ceramic wirewound resistors hoovering half of inch below the driver socket. I intend to dissipate ca. 1.6W on each. Do You think holes around socket will be necessary for proper ventilation, please?

[ATTACH=CONFIG]58670[/ATTACH]

Necessary? Probably not.
If it were me, and I were concerned with the proximity of the resistor to the leads around the socket, I might increase the distance between them another 1/4-1/2” and wire those onto the top of the turrets, and the caps around the sides.
But I can get insufferable about that shit.
Ive seen way worse.
Even your layout, as is, I see the worst case scenario is the Lead insulation dries up and gets brittle earlier than the rest of it. You’re not running those at the rails, and it’s not like they’re butting up again an electrolytic cap or anything. Probably good to go either wa.

[SoulFetish]

Oh yeah, are those 100k resistors on the filter caps KOA Speer 5W Metal Oxide’s?

[catalin gramada]

Oh yeah, are those 100k resistors on the filter caps KOA Speer 5W Metal Oxide’s?

Thanks. I was thinking aso. The problem are short leads of those resistors. I preferred to do a strong loop around terminals instead to gain a litle bit length to space more. And Yes balance resistors are Speer MO. Found 100k well balance 2x470 uF - 2x470 uF at 700V around. Each parallel section get exactly 350V. The driver plates will be supplied from the horizontal capacitors on the top left side. I will put some pics I will do some progress.

[SoulFetish]

Nice work on the lead dress by the way. Looks good

[catalin gramada]

Thanks.Modified inverter/driver stage on test. Original was an whimpy 12ax7 ltpi with no capability to proper drive 4xkt88. It was replaced with 12at7 dc coupled cathodyne followed by a hefty 12bh7 driver. It is a test in progress. Measure good , sounds good. Still thinking what to do with 6 (six) identical 12ax7 tubes in preamp stages which are mixed together? Think I.ll put each chanell onto chanell selector and modify different each chanell for different voicing...

[catalin gramada]

The circuit

[catalin gramada]

Get 650-625 V at iddle (mains voltage depends). I don't have a proper load to do a full throttle test for the moment, but I suspect the voltage will slide more than 580-560V when pushed hard. The power stage is 4xKT88 into 2300k UL OT. 470 ohm was original fitted into screen grids.Looks pretty safe for me but for sake of safety wanna ask You: Should I have any concern, please?