DC heater schematic

[77seriesIII]

Hi all,

I am in the process of building, well ok collecting parts, for the next project. I think I am going the 300B monoblock route that is a derivative of the JE labs as found here: http://homepage.mac.com/parisatnight/ho ... lbum2.html

Now I want to put DC across the heaters and am looking for a schematic. I did find this schematic:

http://www.diyaudio.com/forums/attachme ... 1202158617

Is this overkill, is it a good one? I am guessing that since this schematic takes 6vac and turns it into 6vdc it should work for 5v with the same level of success I know that could push in 6.3 and in playing with resistor values I should be able to get 5vdc.

I guess in short does anyone out there have a good/simple schematic to convert ac to dc or do they know what freakin' terms I should be using to find them on the internet

thanks for the help

erick

[SDS-PAGE]

So, you're building one too, eh? This is what I got so far:
http://www.diytube.com/phpBB2/viewtopic.php?t=2241
got all other parts including some boutique bypass and coupling caps.

It beats me why you'd find it neccessary to add DC across the heaters. You can make your amp run quiet with AC heaters if you add hum balance pots for 300Bs and make sure that your grounding is solid.

SET + KISS (keep it stupid simple) = Yellow_Light_Colorz_PDT_12

[erichayes]

Directly heated power triodes such as the 2A3, 45, 300B, etc., are designed to run with AC on their filaments. While they can be run on DC, tube life can be shortened due to weakening of the filament. As Min says, hum can be designed out of an amp in other ways.

Last year, I restored (actually it was design and build using the existing chassis and speaker) a 1935 guitar amp that used 45s in push pull. I thought I'd screwed something up when I did final testing until I grabbed the (ungrounded) volume control shaft and damn near blew the speaker with hum.

[77seriesIII]

I understand the estoeric differences/debate argument over DC heaters on the older styled triodes but I want to try something because I havent heard the difference between DC and AC. In other words I really havent heard an AC or DC 300B simultaneously on the same AMP build and same speakers and want to see if my 12 years in US Army aviation ears CAN hear the difference. If it doesnt work then I'll switch it to AC and try that. Just trying an experiment and to be honest for about $10-20, for a build that will go big in cost, I can play with DC versus AC I'm willing to try, just to see what happens. I'm not sold on the shorten life expectancy of the tube on AC v DC. AC by its very nature oscillates, DC very little ripple and can be flat maybe both cause stressors but in different ways, Not sure.

SDS-Page...saw your post and am interested in your build? How's it going? Who are you getting your chassis through? You know what I'll ask these questions on your post so as not to have you spread the attention between this thread and your thread. Yellow_Light_Colorz_PDT_16

So has anyone come across a good schematic/recommendations or a method for DC on a 300B?

/e

[SDS-PAGE]

Sure, experimenting is a good portion of DIY tubin'.

SDS-Page...saw your post and am interested in your build? How's it going? Who are you getting your chassis through? You know what I'll ask these questions on your post so as not to have you spread the attention between this thread and your thread. Yellow_Light_Colorz_PDT_16

I am just waiting on the chassis. My friend, for whom I made GSG amp, commissioned me to make a pair of 300Bs monoblocks. He's a professional cabinet maker and has all sorts of hi-tech equipment for woodworking. All cutting is automated. All he has to do it to punch in some numbers into a computer. So not fair! The GSG chassis you see on one of the pictures is his work. You will see that there are little grooves along the edges on top. This is for putting another plate on top to hide all the screws.

The only DC 300B that I came across is the GSG 300B hybrid that I built. You can probably copy the circuit. Just remember that you need to replace 0.75 ohms 3W with 1.5 ohms 3W.




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[77seriesIII]

Ha! I did the same. I have a GSG 300b. I keep bouncing between tube and ss rectifier. I was thinking that as well. If i power the circuit with 5vac it should push through 5vdc, right?

/e

[hembrook]

Directly heated power triodes such as the 2A3, 45, 300B, etc., are designed to run with AC on their filaments. While they can be run on DC, tube life can be shortened due to weakening of the filament. As Min says, hum can be designed out of an amp in other ways.

Hi. Like my homey 77SeriesIII, I have seen this debate on other boards. I have yet to see a single authoritative piece of evidence that shows that filament life of VACUUM TUBES can be shortened by DC. If I am wrong, please provide me with that piece of authoritative information.

There are a lot of wives tales and voodoo out there in Audio, and "DC weakens your filaments" is one of them. Dozens of tube manufacturers. Hundreds of tube manuals over the years. Billions (and billions and billyuns) of tubes made, and not a single source I have found that shows tube life shortened by DC, but there are tons of anecdotes about how people have never had a tube die from a bad filament... AC or DC...

Lets put this one to bed.

Thanks!

[77seriesIII]

I'ld like to give a little Shout Out to my brother from another mother Hembrook...wasssssuppp!

For the rest of it though...is anyone using a DC schematic for heaters that they think is pretty cool and wants to share?

[mesherm]

http://www.tubecad.com/july2000/page10.html

I havent used this yet as my parts for my 2A3 amp haven't arrived yet but I like the concept.

[erichayes]

Hembrook, I thought I'd have time to respond to your throwing of the gauntlet, but, alas, the eve has turned into the dark of night.

I've been involved with "Hi Fi" since 1959 and "Stereo" since 1963.

I'm gonna leave it there for now.

Peace

[EWBrown]

One technique for utilizing DC filaments, that I read about last year, and that was just add a polarity changing (crosswired DPDT) switch or relay so that the DC filament voltage polarity can be occasionally reversed.

Supposedly this can get around the shortened lifetime issue. I haven't tried it, and won't give it any real credibility, but parhaps it works?

Doc Bottlehead uses unfiltered DC filament voltage in the "Paramount" 300B / 2A3 (convertible) amps, the AC filament voltage is rectified and then fed through a choke, but there is no capacitor "smoothing". Supposedly this gets around the "pure" DC voltage and filament lifetime issues.

just my dos pesos' worth...

/ed B in NH

[hembrook]

The simple fact is that I have been looking at hifi for a while, not as long as you, but a while, and I have yet to find a single document from anyone that says DC is bad for your heaters in vacuum tubes. Scuttlebutt, sure. No authoritative document.

Given the massive investment over many decades of tube manufacture, if there was validity to the idea, someone would have published something. I have nothing.

What do you have which would "clarify" (if I may use so diplomatic a word) your view that there is a harmful effect of DC heaters on the life of vacuum tube filaments?

Thanks,

[erichayes]

Hi Hembrook,

No offense taken at your post--a legitimate point-- and none was intended with my rejoinder.

I preface my admittedly anecdotal "proof" with a couple of statements that most of the folks on this forum who've been around for a while have figured out.

First, I have low regard for the wine-and-cheese crowd who've invaded the Hi Fi world in the last couple of decades. Unidirectional speaker cables and gold plated contacts don't even leave home plate, let alone get to first base with me. I build, along with Bud Wyatt of (among others) Sheffield Labs infamy, power amplifiers for recording studios, so snake oil doesn't impress me.

Second, this "experience" occurred in 1960 or '61, when nobody but the military gave a damn.

My electronics mentor was a UCB college buddy of my folks in the early '40s. He wound up in the Army signal corps during the war (WWII) and later worked for the Navy at Hunter's Point in San Francisco as an electronics maintenance specialist.

One weekend, when he came to visit, he saw that I'd inherited a Heathkit variable power supply with a bad PT. The pass tubes were 1619s, which were was a directly heated 2.5 volt version of a 6L6 used prolifically in signal corps equipment. This prompted a monologue on the failure rate of the 1619 and his part in the analysis of same.

Baldy (his surname was Balderson) told me that there was an inexplicably high failure rate of 1619s in mobile equipment, as opposed to stationary gear. His job was to find out why.

He (and his coworkers) found that, almost without exception, the failures were caused by the negative-connected end of the filament breaking. Their conclusion was that, due to the relatively unregulated voltages available to mobile equipment, and the fact that the tube filament was designed to operate on AC, the filament suffered from a byproduct of the Edison Effect.

Being the Baby Boomer version of Soren, I asked him if this was why directly heated tubes intended to be operated only on DC had polarity stated on the basing diagrams. He said "probably", but there were other factors to be taken into consideration, such as which leg the suppressor grid was connected to in pentodes, or where the diode plate(s) was located in a multifunction tube. I guess my question intrigued him, because he told me on a later visit that he'd talked with an RCA engineer and was told that they tapered the filaments on those tubes, with the "fat" end of the filament being connected to the negative pin.

I don't read other tube fora anymore; I got tired and disgusted with the "facts" that are propagated, spewed and flatulated on them. If someone will back up the phenomenon here, or on another forum, I, too, would appreciate some corroboration. All I have is a story that's almost fifty years old, told by someone who didn't really give a rat's ass about tube life. My guess as to why it wasn't publicized is that the JAN tube manufacturers were (duh) in the business of making tubes, the military wanted same--regardless of failure rate--and juicy contracts would have been jeopardized if someone had blown the whistle.

Sound familiar?

My point is, if you're going to invest a considerable amount of money in a pair of 2A3s or 300Bs, think about tube longevity vs hum problems that can be addressed by means other than running the filaments on DC.

[EWBrown]

There was a posting quite a while ago (probably on Bottlehead, as it is about the only tube-related website other than this one and Magnequest, that I frequent) about how DC filament voltage on 2A3s and 300Bs and other DHTs could possibly lead ti uneven "stripping" of the emissive material on the filament and that the polarity was the culprit.

That is where someone came up with occasionally reversing the DC voltage or using unfiltered DC. (as used on the Bottlehead Paramount monoblocks.

Another approach (which could be easily implemented into the GSG and other SET amps with DC filaments) is to wire one tube (assuming 2A3 or 300B in this case) with pin 1 positive and 4 negative, and wire the other tube with pin 1 negative and 4 positive, and then simply swap the two tubes on occasions. Apply to pins 2 and 7 for teh 6B4Gs and 6A5Gs.

I have one of the same "1619" Heapkit regulated PSUs, I got it cheap at a recent swapmeet, along with the manual. It works perfectly, and appears to have had very little use. Apparently Heath, in their early days, used a lot of WWII era surplus parts and tubes in their kits, and this one reflects that very well... Ditto for a cap checker, which uses WWII vintage tubes, including using a 1626 as a rectifier :o Yellow_Light_Colorz_PDT_09

1619s should still be relatively inexpensive, if they are still available.


/ed B in NH

[hembrook]

Eric:

I appreciate the level of detail in your answer. In the end I can see why you feel the way that you do and see no reason for you to change your way.

I hope that you won't take offense at me saying that this is still sort of "Uncle Freddish" (if that is a word, if not, I just maybe coined it) for an experience that you clearly lived thru and learned from, but was not really scientific. It is still good stuff, however, and interesting to know.

I agree with your comments on the "wine and cheese" types I think there is a lot of voodoo. In addition to making amps for fun, I used to (still do, rarely) make guitars for a living. If you really want to talk about voodoo, man there is the place for it. Those folks are full of it! (voodoo that is) Every misconception, uncle fred, or "well, that's the way they designed it" fills me with laughs.

So, I won't dissuade your thinking, but I still think I need more to get into the belief that DC will kill your heaters.

Thanks for the opportunity to share.

Robert