No bias pot, and amp runs hot

[Gearjoneser]

A couple years ago, Bad Cat Cougar amps were discontinued and sold really cheap on Musicians Friend.

I really love the Cougar 50 I have, as a hotrodded EL-34 flamethrower, but always thought it didn't have enough midgain range. It's at full gain at 10:00.

Even though it was made in China, it was designed by James Brown who did the 5150, and it's well made. However, it has no bias pot. I just checked the bias with a meter, and the EL-34's are running at 46.3mA....it should be around 36mA.
No wonder it sounds too pushed and the back panel heats up too much.

I remember Trevorus returning his because of this.

Nobody services these amps anymore, but I'm thinking of Emailing James Brown and seeing what he has to say. The other option is paying a local tech to install a bias pot, or jumpering the bias resistors with more resistance.....or figuring out how to do it myself.

I know Jeremy and a couple others here bought Bad Cat Cougars.
The amps sound great, but I'll bet they'll sound perfect once we get upgraded tube sets perfectly biased. Seriously, who designs a tube amp without a bias pot or self biasing circuit???

 

[allbutromeo]

Re: No bias pot, and amp runs hot

Seriously, who designs a tube amp without a bias pot or self biasing circuit???

Every Mesa

 

[Darg1911]

Re: No bias pot, and amp runs hot

I don't know that amp and this may be a dumb question, but have you checked online to see if there are any mods posted for a bias pot mod? I have a PV Valve King 112. The first version. Also no bias pot but it generally ran the 6L6's cold. I installed a pair of test ports on the back of the chassis and a bias pot out of the way, underneath. I have a little bit of experience with mods and builds, but if I recall, it was a pretty easy mod. I believe that I ended up cutting a trace (or two) to do it, but it was worth it to be able to bias the tubes. The mod for the VK was posted on the net. I meant to also install a plate voltage test point but never got around to it. I upgraded the speaker which was a big improvement, but still rarely play the thing. If I take enough interest in it again, I'll probably install that test point.

At any rate, it may be worth searching to see if there is a mod posted on the net, if you are handy enough to do it yourself. Or having a tech do it if you like the amp.

 

[crusty philtrum]

Re: No bias pot, and amp runs hot

Seriously, who designs a tube amp without a bias pot or self biasing circuit???

Mesa, as mentioned, and also a lot of Fenders ... many had/ have bias balance pots, bias Voltage was/ is adjusted by changing a resistor (same with Mesas). No doubt others, it wasn't uncommon in the bad old days. Many or most single-ended amps probably still run cathode bias, usually adjusted by changing the cathode resistor.

 

[Kamanda~SD]

Re: No bias pot, and amp runs hot

The 5150 is fixed bias too, so not surprising here.

I would say go and do a bias mod to it, any competent tech should be able to do this for you.

 

[jeremy]

Re: No bias pot, and amp runs hot

i wonder if mine is biased colder than yours since my back panel doesnt get all that hot... ill check this weekend when i get a chance.

 

[JeffB]

Re: No bias pot, and amp runs hot

almost as poorly biased as my dsl. but at least Marshall has the trim pots.

makes no sense for a modern amp to be this way. unless like MESA you want to have everyone buy your tubes too.

 

[Rand-O-Monium]

Re: No bias pot, and amp runs hot

If you really like the amp, I'd think it'd be worth the mod...

 

[DankStar]

Re: No bias pot, and amp runs hot

Does that mean they'll burn out faster if run as is?

I bet you could figure out how to bias with a resistor - you're halfway there & know your stuff

 

[Kamanda~SD]

Re: No bias pot, and amp runs hot

^^ Generally yes, the hotter the bias the less the life

 

[Demanic]

Re: No bias pot, and amp runs hot

Isn't it possible to put a trim pot in parallel with the original resistor?

Sent from my MotoE2(4G-LTE) using Tapatalk

 

[tone4days]

Re: No bias pot, and amp runs hot

what about tweaking the resistance at the gain pot to move the 10:00 position up to something like 3:00 or 4:00?

 

[Gearjoneser]

Re: No bias pot, and amp runs hot

what about tweaking the resistance at the gain pot to move the 10:00 position up to something like 3:00 or 4:00?

That's why I'd like to ask the designer of the amp a couple questions, because that's also another separate issue.

The Cougar 50 is a well thought out high gain amp that has a searing lead channel and a clean channel similar to a US Bad Cat. The only 3 issues I have are the lack of a bias pot, gain knob that's at full tilt too early, and transformers that are OK, but could be better....like a Mercury Magnetics upgrade.

Think of the Cougar as a budget built Bogner, Friedman, Voodoo, or Splawn. Definitely worth the money, despite a few minor issues.