5E3 Deluxe Build Running VERY Hot

[JTM]

Hi Everybody,

Was hoping somebody could offer some help with my tweed 5E3 build.

Bought a Mojo kit several years back and found that when finished, it sounded ok, but ran really hot.

When I say hot, I mean that after running idle for 15 minutes with no input signal, the top panel gets so hot that you can only touch it for 3-4 seconds (particularly near the power switch and pilot light). After a lot of research experimentation, I could not solve the problem and shelved the amp.

Fast forward a year, decided to gut the amp and retrofit with Mercury PT, OT, choke and a new turret board (used the same components otherwise). The results were the same. The amp ran incredibly hot, even at idle. The amp was begrudgingly shelved again.

So here I am again, after several other successful builds…. trying to make her right.

As I monitor the progression of the heat across the chassis, it emanates from the bottom of the chassis near the 5Y3 ad 6V6’s (as it logically would), creeps up toward the PT (assuming it’s acting like a heat sink) and then to the top panel. This all makes sense…. what doesn’t make sense is the intensity of the heat. The bottom panel can barely be touched; the PT and top panel can only be touched for a few seconds. (This behavior was the same with both the stock and Mercury transformers)


PT: Mercury FTDP-MM, 335V-0-335V (I selected the lowest B+ they offered in effort to minimize heat)

OT: FTDO59-M w/dual primaries for 6V6 and 6L6 (wired to a switch in my amp). Currently only running 6V6's.

Choke: PCF22707 Hammond? (Note after I installed the choke, I removed the 5K dropping resistor between the first and second filter caps which bumped the screens approx. 10V above the plates. I added the 470ohm screen resistors with no change, so I then added an additional 5W 1K resistor between the screens and second filter/choke. This solved the problem.)


Tubes: Sovtek 5Y3GT, JAN Philips black plate 6V6’s, EH 12AX7, and a Sylvania 12AY7 in the first position.

6V6 Bias current: 39mA per each tube

Voltages:

V5 (5Y3) – Pins 4 and 6 @ 318V, Pins 2 and 8 @ 361V

V3 and V4 (6V6) – Plates 358V, Screens 359V, Cathode, 21V

V2 (12AX7) – Pin1 @ 173V, Pin2 @ 211V

V1 (12AY7) - Pin1 @128V, Pin2 @130V

All other component values and circuits are true to the original 5E3 circuit.

The next obvious step would be to add a fan, but I’m struggling with idea due to the fact that the amp really shouldn’t be running this hot to begin with… plus my obsessive nature is making me feel like I NEED to identify the cause ☺

Any thoughts? Bad Mains filter cap?

Thanks again guys!

[stokes]

You are biased very hot.At 337 plate volts (358 minus the 21 cathode volts)you want about 25ma's for the 6v6's to be biased at about 70%.The 5E3 is usually biased a bit hotter than 70%,but you have a "clone" not a real Fender.Where are your screen volts now?I would like to see a bit lower than the plate volts.

[JTM]

Thanks for the reply Stokes!

I've considered that I could bump up the cathode resistor value, but didn't think that made sense given that 270 Ohms seems to be the standard cathode value used for plate voltages below 380V. Plus I thought the 5E3 was supposed to be biased hot.

Well maybe I'll give it a shot after work tomorrow anyway and see what happens.

Screens are at 359V. 1V higher than the plates.

Thanks again, really appreciate it.

[txstrat]

A cathode biased amp usually develops more heat than a fixed bias IMHE. If you play the amp instead leaving it at idle it might not get as hot.
270 ohms is a good value (instead of the 250 ohms the genuine Fenders ran). Not too hot not too cold with your given voltages.
I built a cathode biased amp with similar voltages and it gets REALLY hot when left at idle. My advantage is a stand by switch I could throw if I don't play.
What I've learned from Bruce (Mission Amps) is, that there's a sort of a too big hype about screen voltages.
When the screen voltage is close to the plates it's fine. Not THAT important to keep it UNDER the plates.
Just my 0.02$

[stokes]

Yes the 5E3's are biased pretty hot,but you are biased at close to 100% at idle,thats hot even for a 5E3.Are your plates glowing?Does the amp sound okay?You say it is too hot to touch,this amp is known to put out some heat,but if the amount of heat makes you worry,just raise that cathode resistor.I've used 300 ohms here with good results.The screen thing is Bruces feeling,and I aint saying he's wrong,but there are many,Fender included who think otherwise.

[JTM]

Firstly, I really appreciate you guys being so responsive.

txstrat - Agreed that "Class A" amps will run hotter, but this is a bit excessive. I've got a good amount of experience with AC15's, AC30's, and various Matchless models...and I've not found any of them to emanate this kind of heat. I've read the same thing about screen voltages...but when you get opposing views from two respected sources, it's hard to know who is right :-)

stokes - No redplating. Sounds ok...but not great to my ears. I've read about the 5E3 giving off some heat, but this seems to go beyond the anecdotal mentions of high temps that I've seen.

I'll try raising the cathode resistor and will see what happens. May just have to suck it up and install a fan as well. Was really hoping to identify the issue.

Will keep you posted and thanks again.

[Bruce / Mission Amps]

Try using a NOS rectifier tube if you are using a MOJO or MM PT.

A real 5E3, and good clone, will probably have it's screen voltage at least 15v to as much as 20v less the plate voltage.
Yours is too high.
Put your 5k resistor back in where it belongs.
Use the choke if you want after the first filter cap but use the choke in series with the resistor too.
The power tubes need to be idling around 10-11 watts each... they get hot and so does the chassis... DO NOT compare this to your AC amps.
Those tubes in the VOX amps have much better air circulating around them

[JTM]

Another big thanks, Bruce.

I'll try to get my hands on an NOS 5Y3 for testing.

Will yank the choke and reinstall the 5K dropping resistor and post my results.

Thanks for your help!

[JTM]

Ok...disconnected the choke, reinstalled the 5K dropping resistor, and left the 470 screen grid resistors in case I wanted to switch to 6L6's. (Reminder; I have a switchable primary on my OT for both tube types).

Also installed a 470 Ohm cathode resistor (don't have anything lower for now... other than 270 Ohm).

Ended up with 376V on the plates, 351V on the grids (assuming the screen grid resistors dropped an extra few volts) and 25.6V at the cathode. Using a simple Ohms law calculation, the bias should then be approx. 27mA per tube.

Seems like this is where the bias should roughly be. But I don't understand why such a high cathode resistor value is needed to get it in this range. Maybe a severely mismatched tube?

Anyway, the amp still eventually got really hot, but took a lot longer to do so. Maybe 45 min, instead of 15. What's really bothering me is how hot the PT gets.

Hoping to borrow a bias probe to check the bias current on each individual tube tomorrow. Also hoping to get my hands on an NOS 5Y3.

Will again post my findings. Thanks guys....

[MWJB]

"Seems like this is where the bias should roughly be. But I don't understand why such a high cathode resistor value is needed to get it in this range. Maybe a severely mismatched tube?" It's not the "value needed", neither is it where your "bias should be".

27mA per tube is too cold for most folks. Go back to 250ohms or 300ohms. Mid to late 30's is typical.

What is "hot" for you to touch is not necessarily "hot" as far as the PT and tubes are concerned. Cathode biased tweed Fenders do get hot. Do not fit 6L6 tubes unless your power transformer was selected to specifically allow their use.

[JTM]

Thanks MWJB.

Interesting. Seems like there's some discord about the "correct" bias range for the 5E3. Or maybe it's a matter of the traditional range (30mA and up) vs. the "electronically correct" rating (20mA and up)?

The Weber bias calculator seems to lean towards the latter....

I'd honestly rather the amp sound better than sacrificing tone for tube longevity and reduced heat.

Anybody have a healthy 5E3 or accurate clone they can offer a bias current rating from? Bruce? :-)

Sorry for all the questions. You guys are a great crew.

Got an NOS 5Y3 and a bias probe for taking individual bias readings on each tube. Will let you know what I find...

[trem]

Ok...disconnected the choke, reinstalled the 5K dropping resistor, and left the 470 screen grid resistors in case I wanted to switch to 6L6's. (Reminder; I have a switchable primary on my OT for both tube types).

Also installed a 470 Ohm cathode resistor (don't have anything lower for now... other than 270 Ohm).

Ended up with 376V on the plates, 351V on the grids (assuming the screen grid resistors dropped an extra few volts) and 25.6V at the cathode. Using a simple Ohms law calculation, the bias should then be approx. 27mA per tube.

Seems like this is where the bias should roughly be. But I don't understand why such a high cathode resistor value is needed to get it in this range. Maybe a severely mismatched tube?

Anyway, the amp still eventually got really hot, but took a lot longer to do so. Maybe 45 min, instead of 15. What's really bothering me is how hot the PT gets.

Hoping to borrow a bias probe to check the bias current on each individual tube tomorrow. Also hoping to get my hands on an NOS 5Y3.

Will again post my findings. Thanks guys....

I frequently see 480 ohm cat resistors in a 5E3. I would not get too hung up on what percent the amp idles at.....58%....70% whatever. The more important question is how does your amp sound and play with the bigger value resistor?
Good Luck

[JTM]

Hi trem.....point taken. It's easy to get lost in the details.

Ok, reinstalled the 270 Ohm cathode resistor and installed an NOS 5Y3.

This gave me 330V on the plates, 18V at the cathode, and 32mA bias current on each tube (perfectly matched as it turns out).

This puts me at a 10 Watt dissipation, exactly where Bruce suggested. The amps still runs damn hot, but takes a lot longer to reach that point. I think I'm just going to accept that's how it is and install a fan.

Something I want to point out. The Sovtek 5Y3 put out 20V more than the NOS 5Y3. Seems that this was a big part of my problem. Kinda scary.

At this point the only thing that seems "off" is the screen voltage, which is at 300V. Guess I'll remove the 470 Ohm screen resistors and see what happens.

Thanks again for everyone's help!

[Bruce / Mission Amps]

Hi trem.....point taken. It's easy to get lost in the details.

Ok, reinstalled the 270 Ohm cathode resistor and installed an NOS 5Y3.

This gave me 330V on the plates, 18V at the cathode, and 32mA bias current on each tube (perfectly matched as it turns out).

This puts me at a 10 Watt dissipation, exactly where Bruce suggested. The amps still runs damn hot, but takes a lot longer to reach that point. I think I'm just going to accept that's how it is and install a fan.

Something I want to point out. The Sovtek 5Y3 put out 20V more than the NOS 5Y3. Seems that this was a big part of my problem. Kinda scary.

At this point the only thing that seems "off" is the screen voltage, which is at 300V. Guess I'll remove the 470 Ohm screen resistors and see what happens.

Thanks again for everyone's help!

Get yourself a 20 watt light bulb and stick it in the chassis.... come back an hour later and the chassis will be hot too. The tweed amp chassis' get very hot.
But too hot is really hot.. like 60 degrees centigrade and that would be pretty damn hot.
I think all my trannys are spec'd to 85 degrees centigrade at full power output and I suspect yours are too.

I think you are just about where you need to be now.
Personally, I don't like the 470 ohm resistors on each socket in these amps.
With the screen voltage being 15 to 20v below the lowish plate voltage, it is unlikely to be an issue anyhow so I'd yank them off and leave them off.
Plus, there is a very subtle tone change with the screen resistors installed in conjunction with the standard 4k7-5k1 screen node dropping resistors on these amps.
Although hard to define, ... I think the largest value I've used one the sockets where I could not hear or feel anything different was about 100 ohms.
Regardless ... don't sweat the heat. Hot in these amps is where they live.

[JTM]

Thanks, Bruce.

I'm pretty happy with where I'm at.

Gonna stop being such an ninny about the heat and just accept it.

Thanks again to everybody!