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Custom 6sn7 preamp runs too hot

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Posted on May 12, 2017 at 10:36:09
Cuernavaca
Audiophile

Posts: 878
Location: NW
Joined: December 23, 2011






A few years back I bought a custom preamp from a midwest builder who seemed to know his stuff. It uses 3 6sn7 tubes and sounds exceptional. I also have a Mapletree preamp to compare it with, and the custom preamp has better/deeper soundstage, more defined bass, etc. But the Mapletree runs cool, the other runs so hot you could fry eggs. I've had to replace the 47uf caps in the power supply, they seem to fail and create hum after a short amount of time. A local engineer rebuilt it 3 years ago and he seems to think the caps were faulty and their failing has nothing to do with the design.

My question is, what is making this run so hot? I may try to install a small fan to help dissipate heat. Tube life hasn't been an issue. Power supply sits on top and doesn't seem to be affected by the heat, it stays just lukewarm.

Any ideas would be appreciated. I can post the schematics if that would help, the original builder has since passed away.

Thomas

"I've never owned a firearm, but I do have an attack parrot!"

 

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Schematic? Shelf?, posted on May 12, 2017 at 10:44:06
Lew
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Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 It would be impossible to know why the circuit gets hot without seeing the schematic, but on the other hand, that cabinet is not ideal for dissipating heat from the tubes, etc. Have you tried sitting it up on the top shelf, with nothing above it to block heat dissipation?

Or if you have no schematic and if you've already ruled out the cabinet as the cause, do you know the plate dissipation of each of those 3 6SN7s? That would certainly give one a clue.

 

RE: Schematic? Shelf?, posted on May 12, 2017 at 10:58:25
Cuernavaca
Audiophile

Posts: 878
Location: NW
Joined: December 23, 2011






I've ruled out the shelf, it runs just as hot in other locations. Will try other tubes, here are schematics.

Thomas
"I've never owned a firearm, but I do have an attack parrot!"

 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 12:10:53
OldNuke
Audiophile

Posts: 141
Location: Silicon Valley
Joined: November 18, 2014
 How tall are the feet on the bottom cover? Doesn't look like there is any room for ventilation/air circulation in your picture.

 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 12:13:35
Tre'
Industry Professional

Posts: 17305
Location: So. Cal.
Joined: February 9, 2002
 There doesn't appear to be any ventilation for that chassis.

When you add up all the heat dissipated by the plate resistors, cathode resistors, drop resistors for the shunt regulation circuits it's a lot of heat.

You might try leaving the bottom plate off and raising the unit up off the shelf with some tallish rubber feet.

BTW That's a very nicely built unit.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

What he said..., posted on May 12, 2017 at 12:44:52
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 Who builds a tube amp with no chassis ventilation? No wonder it gets hot.

 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 12:57:34
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 6SN7 is not a "hot" tube unless it is drawing too much current. Have you personally checked the voltages in your preamp to verify that there is not a problem?

 

Also, wood is a rather good insulator compared to aluminum or steel~nT, posted on May 12, 2017 at 13:32:56
Cleantimestream
Audiophile

Posts: 7551
Location: Kentucky
Joined: June 30, 2005
 ~!
The Mind has No Firewall~ U.S. Army War College.

 

bottom is ventilated, posted on May 12, 2017 at 13:44:10
Cuernavaca
Audiophile

Posts: 878
Location: NW
Joined: December 23, 2011



There are 3/4" tall feet and the bottom plate is well ventilated. Cabinet is not enclosed, there is plenty of air coming in from behind as well. The top aluminum plate where the tube sockets are placed gets very hot. I am going to try a variety of tubes to see if that makes a difference. I had been using some Sylvania 6sn7 WGTA, brown base, chrome domes, these seemed to create more heat than the JAN-CHS Sylvanias.

Thomas
"I've never owned a firearm, but I do have an attack parrot!"

 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 13:46:35
Cuernavaca
Audiophile

Posts: 878
Location: NW
Joined: December 23, 2011
 It was just gone over and voltages are okay. Will work on getting more air ventilation.

Thomas
"I've never owned a firearm, but I do have an attack parrot!"

 

RE: You still might want to post those numbers..., posted on May 12, 2017 at 14:17:22
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 since your piece is custom made. Sometimes even good sounding equipment have flaws that could be passed along. Different eyes can pick up something that was overlooked. I have 3 preamps that use 6sn7 tubes and even though they have bottom plates and no vent holes, none of them run "hot".

 

And that brings up the point..., posted on May 12, 2017 at 14:17:44
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 ...that the 'SN7s probably are running hot, maybe contributing to all this. Look at the anode and cathode resistor values. This is an offshoot of the Blue Velvet preamp. It's a decent topology, but the operating conditions are wrong.

 

RE: bottom is ventilated, posted on May 12, 2017 at 14:19:05
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 It's not the tubes. Heat rises. The chassis wasn't designed for tube gear.



 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 14:31:24
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 "I've had to replace the 47uf caps in the power supply, they seem to fail and create hum after a short amount of time."
This is definitely unusual. I use Solen's all the time in almost everything i build or repair. And i have never had to replace any of them for any problem. So something is out of whack.

 

Shunt zeners, posted on May 12, 2017 at 15:34:37
Salectric
Audiophile

Posts: 1358
Location: East Coast
Joined: February 23, 2003
 The shunt zeners in the power supply are dissipating heat and the heat is trapped inside the chassis. That is adding to the heat issues.

 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 16:53:52
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 The power supply voltages do not make sense - total supply is 34mA, of which 23mA goes to each line amp, and the cathode followers draw 16mA - a total of 62mA.

The really big issue is the first Zeners, which are supplied through the 470//680 ohm resistors. With only 20v drop, they are extremely sensitive to the power line voltage.

 

RE: Custom 6sn7 preamp runs too hot, posted on May 12, 2017 at 17:42:10
Johno
Audiophile

Posts: 388
Joined: June 9, 2002
 Pretty amp I love wooden chassis.

Looking at schematic the valves appear to be dissipating around 4.5W per channel, 9W collectively. A 6SN7GTB has a absolute maximum combined plate dissipation of 7.5W so you are well within those ratings collectively across 3 x 6SN7. This assumes the valves are operating as per schematic this can be confirmed with a DVM.

But as Tre & TK suggested the combined heat generated from plate, Kathode & power supply dropping resistors and with no ventilation holes on top plate, heat ramps up quickly.

I would try running the amp with the bottom on & also removed & elevated for comparison. Measure the top & internal temperature with a IR laser gun. This will tell what the inside chassis temp is running at. I suspect those PS caps are failing due to excessive internal temperature (they are located close to top plate)also note that aluminum top looks to be no more than 1-2mm thick, this heats up quickly. Remember there are 2 ways heat can be removed air convection (fan) or radiation (reflection)

A small fan run above the amp top will certainly help.

Johno

 

RE: Schematic? Shelf?, posted on May 13, 2017 at 01:11:52
awesterner
Audiophile

Posts: 23
Location: Seattle
Joined: May 7, 2017
 The design IMO sux. The anode resistor value should be 47K. The voltage is WAY too low. 6SN7 sounds better with a hefty B+, like at LEAST 350 VDC. I know there is a strong belief by many that a shunt regulator is the sheet, but, it wastes energy, creates a lot of heat needlessly, and doesn't sound as good as a pass transistor regulator. The Cathode resistor value might be correct for the rather low B+, but, if B+ were where it belongs the cathode resistors would need to revised.

My experiences with those 100 and 150 volt Zeners was a high failure rate, and I suspect that due to inductive kickback. When they fail, they short, and the output voltage will go low AND there will be a lot of heat generated as a result of increased current through those power resistors. . Consider replacing them with a string of lower value zeners and use very high voltage capacitors to absorb the inductive kickback when the power is shut off. The very rapid collapse of the magnetic field causes a very high voltage spike that kills solid state devices. MY solution was to use voltage regulator TUBES instead. They looking F-ing kewl, and they don't get taken out by inductive kickback.

 

RE: Schematic? Shelf?, posted on May 13, 2017 at 01:36:08
awesterner
Audiophile

Posts: 23
Location: Seattle
Joined: May 7, 2017
 ALSO, since you are using SS rectifier, you can go much higher than 47 UF for the PI filter. I'm betting one of the zeners is blown, The power supply should be well over 300 volts if the zener leg was absent.

 

holes in top plate, posted on May 13, 2017 at 05:23:50
RayP
Audiophile

Posts: 727
Location: Maryland
Joined: June 30, 2005
 Since heat rises it would help to have some way for some of the heat to escape through the top plate.

It looks like there are two holes towards the sides that have a screw or bolt that are not really doing anything. As an experiment you could try to remove them to see if it helps.

It's a lovely looking piece of equipment but I would be tempted to drill some holes in inconspicuous places.

ray

 

RE: What he said..., posted on May 13, 2017 at 05:43:59
deafbykhorns
Audiophile

Posts: 1067
Location: Florida
Joined: October 17, 2003
 "Who builds a tube amp with no chassis ventilation? No wonder it gets hot."
One of those uneducated voters?

 

Thanks everybody for the help., posted on May 13, 2017 at 09:04:31
Cuernavaca
Audiophile

Posts: 878
Location: NW
Joined: December 23, 2011
 I will give that a try, plus I am going to install a small 40mm fan with a variable speed. It's been on all week during the day despite the heat and sounds great, zero hum, deep soundstage.

As far as changing the design, i.e. components, I will leave that to someone more qualified in the future.

Thanks for all the comments and help.

Thomas
"I've never owned a firearm, but I do have an attack parrot!"

 

Pull it out from enclosed area, posted on May 13, 2017 at 09:17:06
Charlie8521
Audiophile

Posts: 914
Location: South East Michigan
Joined: October 2, 2004
 Your pic seems to show it in an semi-enclosed area. Might not hurt to put the preamp where the heat from it can freely dissipate. All of my tube gear is in areas where the heat can freely dissipate. Just a thought.

 

RE: Thanks everybody for the help., posted on May 14, 2017 at 18:35:34
GEO
Audiophile

Posts: 4749
Joined: April 7, 2000
Contributor
  Since: September 9, 2000
 If you have failed components, you should have it fixed.

 

Preamp revisited, posted on August 2, 2017 at 11:40:40
Cuernavaca
Audiophile

Posts: 878
Location: NW
Joined: December 23, 2011






So after another failure due to heat(burnt resistors), I took it back to the engineer for a power supply rebuild. Redesigned power supply in a separate box with 5AR4 tube rectification. Preamp box uses 0A2 and 0B2 tube voltage regulators. Runs cool an so far so good.

Thomas

"I've never owned a firearm, but I do have an attack parrot!"

 

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