Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tectron on October 09, 2011, 06:34:03 pm
-
Hello, this is my first post, this is the best site I have found so far relating to tube amp info, awesome.
I don't have a schematic, but the amp has a 250uf 100v electrolytics in parallel with a 270 ohm 10 watt resistors running from pins 1&8 of el34's to ground, both of which are toast. Am I correct that these have something to do with bias? If so would it be possible to use 220uf caps as replacements, without effecting the overall tone of the amp? 250uf 100v caps are like hens teeth, and I would like to avoid running caps in parallel if possible.
The amp is a Bad Cat "Hot Cat" in case anyone is wondering
Thanks
-
Sure, you can use 220uf 100v (or 100+ v) instead of 250uf/100v. That would be cathode biased.
Having said that, have you resolved the issue that toasted them so it won't happen again?
With respect, Tubenit
-
The toasting (assuming no fault in the amp) is probably due to the fact that they always mounted these caps right next to the cathode resistor. They literally just cook from the heat coming off the resistor.
-
they claim to have 15 in stock.
--DL
http://www.onlinecomponents.com/mallory_tc10251.html?p=11713737 (http://www.onlinecomponents.com/mallory_tc10251.html?p=11713737)
-
From your post I thought you said that both the resistor ad the cap are toast. It's hard to toast a 10W resistor unless your cathode is pulling serious current. How did the EL34 fare? It may be that the resistor just looks toasted beause of reside from the burnt cap, but I would measure its value anyway, and invesigate for other things like burnt tube socket pins, insulation, etc. As for the value of he new cap, 220uF is fine.
-
I am assuming that the cause is what Hotblueplates stated, the resistors were sandwiched right up to the caps, there are obvious burn marks caused by the resistors. The owner also stated it runs so hot that it will burn your hand when you touch the metal switches in the back. There is little/no ventilation built into the cabinet :w2: . I decided to replace the rest of the big electrolytics for good measure. Thanks for that link, I will check them out..
Steve
-
Hey DL. Did you see the price on that cap? :huh:
-
Hey DL. Did you see the price on that cap? :huh:
$40 for a single pot! Worst prices on the Net, IMO. :evil5:
-
Hey DL. Did you see the price on that cap? :huh:
ya, i did... priced like hen dentures... :huh:
-
:huh: Those are salty caps, lol, I opted for some 220 uf 250v caps from Newark $4.50 a piece.
-
From your post I thought you said that both the resistor ad the cap are toast. It's hard to toast a 10W resistor unless your cathode is pulling serious current. How did the EL34 fare? It may be that the resistor just looks toasted beause of reside from the burnt cap, but I would measure its value anyway, and invesigate for other things like burnt tube socket pins, insulation, etc. As for the value of he new cap, 220uF is fine.
The resistors are still functional, they just look toasted. The sockets etc look good, he said the amp probably doesn't have more than 3 hours play time since he bought it in 09 so functionally its not very old.