Some of you remember I recently converted an old Motorola radio to be used as a power amp. It has an EZ81 rectifier tube and one 6BM8 per channel. Originally, this yielded roughly 8wpc of output at 8 ohms, but in the process of snipping out the RF circuitry, I somehow am left with only one portion of each tube, and power output is only about 1 and a half wpc at maximum.

Does anyone have a pinout or other info on how I can utilize the pentode portion of these tubes for the full 8wpc of output power? Kind of botched this project up.....

A single 6BM8 pentode side can't really put out 8 watts. It's only got 7 watts dissipation max... and in SE mode, you really can't get much more than half that, as output, before you start overheating the tube.

A pair of them can easily put out 8 watts in push-pull... maybe as much as 15 watts, if you run the tube with high plate voltage and high impedance output transformers...

Regards,
Gordon.

I'm working on a 6BM8 job at present. It's a Japanese made Knight / Allied. The old solder they used is nasty.:thumbsdn:

See what I mean, I'm confused! LOL.

So, the 6BM8 is a dual section tube, with an integrated driver section and a small power amp in the same envelope, right?

Seems like I'm not using both sections...

Do you have a schematic?

http://www.nj7p.org/Tube4.php?tube=6BM8

Thanks a bunch, Jay. Couldn't seem to find a datasheet for some reason...

No schematic on this set unfortunately. But I'm going to continue tinkering and will probably just settle on using this as a headphone amp. Since my cans are 32-ohm (grado sr325i) i'm going to need to work with those small output trannys. Any tips fellers? :scratch2::thmbsp:

See what I mean, I'm confused! LOL.

So, the 6BM8 is a dual section tube, with an integrated driver section and a small power amp in the same envelope, right?

Seems like I'm not using both sections...

I'm just taking this off of the top of my head, so don't take it as gospel!
The 6BM8 is actually two tubes in one envelope. 1/2 of a 12AX7/12AU7, and a 6AQ5 or 6BQ5.

If you are getting sound, you are using the output side of the tube. Depending on its original wiring set-up, the other half would be the pre-driver or AF amp or phase inverter.

The ones I've been picking up lately all seem to be weak on the triode side. It may be very well due to the way some designs were trying to squeeze every last drop out of the tubes. I dunno, I would think that the pentode side would get tired first....:dunno:

I have about 20 6BM8's here that are "used" and the driver portion (triode) on all of them test weaker then the pentode.

Now, I have about 8 NOS sylvanias and the triodes test strongly...

So, I wonder if the match between the triode and pentode is really right.

Who knows....

Given then similarities, as noted, between the triode half of the 6BM8, and the 12AX7/6EU7 family, and the similarities between the pentode half and the 6AQ5/6BQ5 type tubes, you should be able to wire up the amp using something like a Magnavox 8600-type circuit, and have it work fine. Might have to change out the output tube cathode resistor to re-bias it, but that's probably about the extent of the needed changes.

http://www.audiokarma.org/forums/attachment.php?attachmentid=101119&d=1217106020

Regards,
Gordon.

Now that Jay has given you the pin out for the 6BM8 I would start by checking the voltages referenced to ground and see if both the triode and pentode sections are biased at a reasonable level. I would guess that the original is pentode mode with some feedback. If so you may have done something to increase the feedback ratio thus reducing gain.

If you want to do this right I think you need to trace the circuit as is and write it down so you can see where you are. It is time consuming but not all that difficult as such. I would expect a pretty simple circuit in something like that. The triode is probably common cathode with cathode bias and a bypass cap. Probably cap coupled to the pentodes control grid but it could be direct coupled. Easy to check just see if the connection from pin 9 to pin 3 is a wire or a cap. I would expect that the pentode is also cathode biased.

Is the screen (pin 7) connected to the B+ supply (pentode mode), the output transformer (Ultra Linear mode) or the plate (triode mode)?

If there is feedback it would probably be taken from the secondary of the OPT. Are there any connections from the OPT secondary to anything other than the speaker output or ground?

mike

This one used a .047 cap to connect the triode and pentode. This thing sounds mighty good, I never heard a 6BM8 up to snuff before.:music:

This one used a .047 cap to connect the triode and pentode. This thing sounds mighty good, I never heard a 6BM8 up to snuff before.:music:

Design looks similar to the Allied 333 and Pioneer SX-34. Bet it was made by Pioneer...

Design looks similar to the Allied 333 and Pioneer SX-34. Bet it was made by Pioneer...

You had it the first time (almost). It's a Knight Allied 935. Nice little Japanese integrated with tons of leaky caps. It cleaned up nice and has both the steel and wooden case, so I figured it was worth a re-cap. I'm going to re-laminate the wood case in Cherry I think. :scratch2:

This one used a .047 cap to connect the triode and pentode. This thing sounds mighty good, I never heard a 6BM8 up to snuff before.:music:

Pretty amp there. Your "surprise" arrive yet?

You had it the first time (almost). It's a Knight Allied 935....

Actually, the Allied 333 was made by Pioneer; essentially an SX-34...

Sorry for the delay in responding, but I haven't had the time to poke around with this amp.

Let me start by saying thank you to everyone... Excellent information; I'm learning a lot.

Mike, The OPT's secondaries have only the two leads going to the speaker terminals. The screen is connected to the plate via a capacitor.

Being somewhat challenged as a 'start from scratch' designer of tube gear, I think i'm going to try out that Magnavox schematic. Looks like I could make it work. Will report back when I get something done.

(I'd post a pic but I'm not permitted to do so at this time.... when/if the time comes when I can, I'll get some up.)