Noob with tubes, how hot should the EL84 be in an EVJ?

[Southpaw Tele]

I bought the combo (ver. 3) a couple weeks ago (I think?) and I had the thing on for an hour and a 1/2 at church last Sunday. I felt the front panel of the amp and it was relatively hot to the touch. Now I know there's going to be some heat there, but is there anyway to check if it's too hot?

[vjf1968]

Well, if it sounds fine than I wouldn't worry about it tube amps tend to get a little hot. You can check the tubes when it is on and see if they are glowing orange and not red. If it really bothers you, you can get a small fan and blow air into the amp as you play or take it to tech and have them look at it.

[GUITARmole]

Tube amps tend to get a LOT hot!

[Billm]

An EL84 will burn your fingers --blisters -- easily. It can make the whole amp hot. A single-ended amp like the Valve Junior runs the tube hotter than one with push-pull output.

The 12AX7 is just warm by comparison.

[muchxs]

Just take a peek in there and make sure the plates aren't glowing cherry red. Stock VJs run too much voltage and incorrect bias. Some EL84s redplate in conservative applications. Hint: I don't use EI EL84s for anything.

[marshman]

And there's not great space for circulation of fresh air in those little amps--just that little hole in the back...allows heat to build up in there.

[JCSouthpawtele]

Originally Posted by muchxs

Just take a peek in there and make sure the plates aren't glowing cherry red. Stock VJs run too much voltage and incorrect bias. Some EL84s redplate in conservative applications. Hint: I don't use EI EL84s for anything.

I don't believe there is a bias on a single tube. its fixed..you only need bias for balancing two tubes to the same voltage. In a dual tube design one tube gets the first half on the wave (up)the second gets the(down) to complete the cycle.. single ended is just that a single wave through one tube..hence a fixed bias via the design.. a pro amp guy might want to jump in to confirm this.
I have a blackheart Little Giant 5 head and its basically the same thing. If you think its time to replace the tube just by a matched set of EL84's and keep the other one as a spare.

[Southpaw Tele]

Thanks for all the replies gang. It's glowing orange when on for awhile, so I may just need to get used to the heat. As far as cooling, I wonder if anyone has added a cooling fan to the interior? I'll check out sewatt.com later and see. Also, I was under the impression that the EVJ is self-biasing...how does that affect temps inside? Thanks for your patience!

[muchxs]

Originally Posted by JCSouthpawtele

I don't believe there is a bias on a single tube. its fixed..you only need bias for balancing two tubes to the same voltage. In a dual tube design one tube gets the first half on the wave (up)the second gets the(down) to complete the cycle.. single ended is just that a single wave through one tube..hence a fixed bias via the design.. a pro amp guy might want to jump in to confirm this.

It's a free country and you can believe what you like. You're right, your Blackheart uses an output stage similar to the VJ right down to the 220 ohm cathode resistor. That's your bias resistor. If your supply voltage is too high or the value of the resistor is incorrect your EL84 will exceed its dissipation capabilities (too much heat) and the plate (the large structure visible inside the tube) will glow cherry red. I could provide you with a list of component values that would run the EL84 so hot it would melt its glass shell. I won't, though.

Bias in a P-P (plate to plate or push-pull, the classic Williamson circuit) doesn't balance unmatched tubes. Doesn't, won't, can't. It can if you go to a split bias scheme and provide an individual bias network or supply for each PA tube but that's atypical. Most P-P audio amps built prior to the late '50s use cathode bias with a shared cathode network, the cathodes are actually connected and share the network. Same with P-P fixed bias, the cathodes share a common ground with a negative bias voltage applied to the grids.

Originally Posted by JCSouthpawtele

I have a blackheart Little Giant 5 head and its basically the same thing. If you think its time to replace the tube just by a matched set of EL84's and keep the other one as a spare.

A Version 1 VJ will cook them relatively quickly.

[JCSouthpawtele]

Originally Posted by muchxs

It's a free country and you can believe what you like. You're right, your Blackheart uses an output stage similar to the VJ right down to the 220 ohm cathode resistor. That's your bias resistor. If your supply voltage is too high or the value of the resistor is incorrect your EL84 will exceed its dissipation capabilities (too much heat) and the plate (the large structure visible inside the tube) will glow cherry red. I could provide you with a list of component values that would run the EL84 so hot it would melt its glass shell. I won't, though.

Bias in a P-P (plate to plate or push-pull, the classic Williamson circuit) doesn't balance unmatched tubes. Doesn't, won't, can't. It can if you go to a split bias scheme and provide an individual bias network or supply for each PA tube but that's atypical. Most P-P audio amps built prior to the late '50s use cathode bias with a shared cathode network, the cathodes are actually connected and share the network. Same with P-P fixed bias, the cathodes share a common ground with a negative bias voltage applied to the grids.



A Version 1 VJ will cook them relatively quickly.

I keep lurking in the Shock Bros. section trying to learn. I'm thinking about doing a 18 watt Marshall type from Weber. I'll keep learning....

[Southpaw Tele]

Well, I'll hold off and play with it for awhile and see how long the EL84 lasts in there. I'm sure it's no biggie. I'm not quite ready (safety wise) to go digging in there to install a fan.

[Southpaw Tele]

ok, I ran the little guy for 1.5 hours at practice last night. I put my fingers on the front panel and it was hot enough that I didn't want to leave my digits on there for more than a couple seconds.

[f13ndus]

I don't know how you feel about customizing the amp cabinet, but if you really don't like the heat, you might want to think about putting a vent in the top of the amp. Because the chassis is mounted "Fender style" on the top of the cabinet, with the tubes underneath, all the heat coming off the tubes (and from the rest of the circuit, transformers, etc.) rises into the chassis and has nowhere to go. A vent in the top of the amp would allow that heat to escape, lowering your running temperatures, with the added benefit of longer life for the amp (heat over time is bad for the longevity of most electronics.)

Or course, if you often have a frosty beverage on top of your amp - it's a terrible idea to have an opening for liquids to pour directly onto your circuit board! But then again, it's not a good idea to have a frosty beverage on top of your amp, for the same reason - liquids and electricity do not mix well.

[Southpaw Tele]

Plus a mojito or beer on top of my amp in church might not go over so well.

I'd love to do a transplant into another, larger cab (with a 12" speaker). Until then, I may add a computer cooling fan (a 240v running on 120 is supposed to be relatively quiet, I think).

Thanks for the vent idea; I'm going to look into that, as well.

[JohnnyCrash]

Tubes get hot. Preamp tubes gte warm. Unless they're redplating or starting fires, its normal and comes with the territory.

Suckers can welt the old fingers if you try pulling them before they cool down.

Originally Posted by JCSouthpawtele

I keep lurking in the Shock Bros. section trying to learn. I'm thinking about doing a 18 watt Marshall type from Weber. I'll keep learning....

You will not regret it. Fantastic rock amps. They're also cathode biased. Learn safety, then don't worry about having to understand everything in circuits just yet - just build them and things will accumulate in your brain eventually :)