6BM8 Tube Review

Discussion in 'Audio Hardware' started by allied333, Mar 12, 2018.

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  1. allied333

    allied333 Audiophile Thread Starter

    Location:
    nowhere
    I acquired an Allied 333 receiver. The Allied 333 is the USA import of the Pioneer SX34. The stock receiver is somewhat veiled, but not worse sounding vs a stock Fisher 400. I rebuild the Allied with all new Russian K40Y-9 coupling capacitors including the tone controls. After 50 hours use, the capacitors are for the most part broken in. I also rebuilt the HV PS for less AC ripple. The Allied 333 is a very good sounding receiver after the rebuild.

    I was surprised performing a search for 6BM8 performance from different manufactures on the internet and came up with about nothing. By contrast a 6BQ5 has 100s of reviews. The only reference to the 6BM8 was at google groups (deja.com). A 6BM8 'party' as they call it for a review of 6BM8 tubes. Only issue was the review was limited to 6BM8s manufactured by RT France, Tungsram, Telefunken, Toshiba and Mullard. The result only stated as a brief summary the FR France was 1st place, Telefunken 2nd place and the Tungsram was liked as well. No details was given why this or that tube was better.

    So, based with this dilemma, I purchased the following quad sets of 6BM8-
    Siemens Germany, Mullard England, Matsushita Japan, AWA Thorn Japan, Philips MiniWatt Australia and Amperex Bugle Boy from Holland. All were bought on ebay vs the standard USA tube suppliers. To my amazement no one except Tube Depot and Antique Electric supply has the tubes in stock with more than one of same brand available. This includes large tube suppliers from SND, N9TEW, IERC and vacuumtubes.net. Some stated they had quads last year, but have been sold out as of this year.

    The 6BM8 is not a common tube for audio amplifiers and especially so in the USA. Most receivers and amplifiers were made in Japan & Europe. Some of the 6BM8 gear includes the Sansui 250, Knight KG 250, Amperex model 28, Sargent Raymont SR1040 and a few Pioneers beyond the SX34. I have read about some Audio Note gear using the 6BM8, but believe it is used as a driver tube vs a power tube. A few newer 6BM8 amps have been built in China in the last 15 years.

    The 6BM8 is basically a 12AT7 triode & a 7 watt plate dissipation pentode in one tube. Some have stated there is not a bad sounding 6BM8 and for the most part that is true. I heard a much larger variance in 6BQ5s, but have not heard every 6BM8 manufactured.

    Siemens 6BM8- the Siemens is an excellent tube. The bass, midrange and high frequencies are very detailed. The treble is not shrill, bass not wooly and the midrange is clear & accurate. This is my favorite tube of the lot.

    Matsushita 6BM8- the Matsushita excels with the most musical midrange of all the 6BM8s. Lush and detailed at the same time. It has a refinement that is pleasing. The tubes are not quite as detailed as the Siemens tubes, but very pleasing to the point I give it second place. The treble and bass is very good like the Siemens.

    Amperex Bugle Boy 6BM8- the Amperex is a fine tube and rated very close to the Matsushita in performance. Performance wise it may be a matter of 'taste' in-between the Matsushita and Amperex It is just a little brighter than the Matsushita across the midrange & treble, Bass is taught like the tubes above. At this point I have no problems with any of the tubes above including the Amperex. Any could have a home in my Allied 333 receiver.

    AWA Thorn 6BM8- the AWA Thorn is also a good sounding tube. I would rate the midrange a little 'solid-state' sounding. Although clear sounding the fine details are not quite as present as the tubes listed above. he bass and treble sounds good. Perhaps the entire frequency range is missing some minor details. I think I will pass on the AWA Thorn, but certainly could live with it if the other tubes above were not available.

    Mullard England 6BM8- Usually any Mullard tube is great, but the Mullard 6BM8 are a disappointment. The entire frequency range is veiled & muddy. Although it makes music, it is not pleasing at all. Not recommended and the AWA Thorn way outperforms the Mullard.

    Philips Miniwatt 6BM8- I have not received this brand yet. Likely 2-3 weeks out. But, being made on Mullard tooling in Australia, I suspect it may sound like the Mullard England 6BM8s. However, Matsushita 6BM8s are made on Mullard tooling and sound completely different. If I am impressed I will update this post.
     
    Last edited: Mar 12, 2018
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  2. allied333

    allied333 Audiophile Thread Starter

    Location:
    nowhere
    I want to add Brent Jesse does not have any quads of 6BM8. TubeWorld have a few quads available, but not any Siemens 6BM8.
     
    Last edited: Mar 12, 2018
  3. Rick Bartlett

    Rick Bartlett Forum Resident

    can the 12AT7 swap out the 6BM8?
     
  4. allied333

    allied333 Audiophile Thread Starter

    Location:
    nowhere
    The 12AT7 side of tube is internal to the tube. So, it cannot be swapped out. And, a 12AT7 cannot replace the 6BM8.
     
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  5. SandAndGlass

    SandAndGlass Twilight Forum Resident

    I'm experiencing a similar tube for the first time.

    I recently acquired an Audio Note kit amp (fully assembled). It is EL34 Based and came with some current production Russian made "Mullard" EL34's.

    I did go ahead and order a quad of NOS East German made RFT (Siemans), to replace the power tubes. I should have them in a week or two.

    But, in the meantime, I am listening to the new manufacture EL34's and I have to say, that I am quite impressed with their sound. The midrange is awesome, the highs are clear and detailed, without being harsh and they have more bass energy than in my similar powerful Prima Luna Prologue Five, which uses KT88's.

    But, on to the two signal tubes. It seems that they are ECF80's, which I had not previously been familiar with. Upon some checking, I found out this.

    ECF80 = 6BL8 = 6C16 = CV5215 = 6Ф1П

    After some reading, it would appear that the internal construction of the ECF80 / 6BL8 is similar to your 6BM8.

    I found out that the ECF80 are some sort of odd triode/pentode combination, just like your 6BM8's.

    [​IMG]

    What the actual differences between the 6BL8's and the 6BM8's are, I have no idea.

    Not wanting to be caught short, if I lost a signal tube, I ordered (and received today) a pair of GE 6BL8 / ECF80's. They are NOS tubes and ran $8/ea.

    The original tubes aren't clearly labeled as to the manufacturer but they do say Made in Japan. We're guessing that they should be a pair of quality tubes.

    As I am very fond of the amp, I would say both the small signal tubes and the large power tubes are doing what they were designed to do, as far as performance goes.

    I noticed on eBay, the Telefunken NOS tubes appear to be the most rare and highly sought after at over $100/pr.

    I do intend to do some additional research and order some additional signal tubes for this amp.
     
  6. allied333

    allied333 Audiophile Thread Starter

    Location:
    nowhere
    Best EL34 in a Marantz model 8 was Mullard.
     
  7. allied333

    allied333 Audiophile Thread Starter

    Location:
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    6BL8 is a 12AU7 triode and a small 1.7 watt plate dissipation pentode plate. That is a receiving type, not a power tube. I think I seen one in a Fisher 600 receiver.
     
  8. allied333

    allied333 Audiophile Thread Starter

    Location:
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    I never did follow up on the Philips 6BM8. It sounds identical to the Mullard. These tubes came from Australia.
     
    Rick Bartlett likes this.
  9. Mike Stamper

    Mike Stamper New Member

    Location:
    Windsor, CT
    6BM8s are also used by the Lafayette KT-615 (mono) and KT-630 (stereo) . I just finished restoring the KT-615 I picked-up on ebay (for $9; best bargain ever!). The KT-630 was my very first hi-fi kit, which I built as a teenager in 1965. If anyone out there ever has to service a KT-615 or KT-630, the circuit is practically identical to an Allied KG-250, schematics for which are available on line for free. The 6BM8 is a clever idea for a power tube. I'm surprised there aren't more 1960s amps using it.
     
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  10. allied333

    allied333 Audiophile Thread Starter

    Location:
    nowhere
    Update. The Svetlana 6BM8 is now in second place and the Matsushita 6BM8 are in third place.
     
  11. Rick Bartlett

    Rick Bartlett Forum Resident

    Ahhh! Philips 6BM8, those are a little closer to home. I got a few of those!
    Getting harder to find though.
     
  12. BrentB

    BrentB Urban Angler

    Location:
    Midwestern US
    I have a stash of 6BM8 tubes stored away for my Rheem preamp. I have used the Mullards for several years and have not really done any rolling. Based on your assessment of the Mullards I may have to now! Funny that the 6BM8 is like a 12AT7 as the pre used 2 of those as well. Or actually GE 6201 are plugged in there now. Also a 6CA7 rectifier. Can't remember what make though.
     
  13. billyclaire

    billyclaire Member

    Location:
    Marlborough MA USA
    I just was handed a 1965Univox U-45B guitar amp. It has a pair of 6BM8's as the output tubes. I suspect they need to be replaced because the amp overall doesn't sound very good. I don't think my Orange tube tester has a setting for these, but I'm really eager to test the original Mullard 12AX7 that is in it. It also has a 6X4 rectifier tube. I was looking for replacement 6BM8's but trying to figure out according to your results what would be the best version for a guitar amp. Probably the more mid-rangeyones...

    [​IMG]
     
  14. KT88

    KT88 Senior Member

    Hi Billy,
    Long time no hear from you, as an old Orange Forum member. The Electro-Harmonix tubes are good current production units. You should replace all of the 10uf and 3uf caps in there (the blue ones). Then maybe the grey ones as well. I'd test after replacing the blue electrolytics first. Some of those old PIO caps can go bad, and with the Japanese variety, that seemed to be a common issue.
    -Bill
     
  15. CRITTYONE

    CRITTYONE Member

    Location:
    NYC
    I got the Matsushita based on your advice. How would you describe the Svetlana?
     
  16. allied333

    allied333 Audiophile Thread Starter

    Location:
    nowhere
    Svetlana a little more clear sounding. The Siemens at times are found in RCA box label and state made in Germany. Some are Telefunken though. You have to look at pictures of Siemens. The Siemens is most clear sounding. The Matsushita are good sounding though.
    My tests were using very expensive speakers. and a highly modified amplifier with Mullard 12AX7 long plate tubes that have no comparison to other 12AX7s. Average sounding speakers, amp and average 12AX7 tubes may hear no differences in the three tubes mentioned above.
     
    Last edited: Aug 2, 2020
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