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PINOUT FOR 6BM8

11/11/1999 8:31 PM
Trace	PINOUT FOR 6BM8 Hey gang; I'm looking for a pinout for a 6BM8. I can't seem to find it in my RCA books. I greatly appreciate it!! Thanks in advance Trace

11/11/1999 8:46 PM
Carl B.	Trace, I got that from Svet's site (svetlana.com) and also Duncan's tube engined. Tried those? Duncan's site has this reference to about 10 pages or so of philips data on that tube. Very hip. - Carl B.

11/11/1999 9:00 PM
Carlo	6BM8 HIGH-MU TRIODE -- POWER PENTODE 1 control grid (triode) 2 cathode or suppressor grid (pentode) 3 control grid (pentode) 4 heater 5 heater 6 plate (pentode) 7 screen grid (pentode) 8 cathode (triode) 9 plate (triode)

11/11/1999 11:57 PM
Trace	Thanks Carlo you rule!! Carl: I did try th Duncan site under data sheets but I didn't see anything listed. Then again I didn't search through the entire site. I thought it was odd that the RCA reveiving tube manual didn't list it. Reid: thanks for the link!!! I'm checking it out now For what it's worth I just got in an old Univox 1/12 combo that has 2=6BM8's, 1=6X4, and one 12AX7. It has volume , tone and tremelo along with three input jacks. The fellow wants it "hopped up a bit" for studio recording. It has a Jensen speaker C12S in it. :^) Trace

11/11/1999 11:57 PM

Trace

Thanks Carlo you rule!!

Carl: I did try th Duncan site under data sheets but I didn't see anything listed. Then again I didn't search through the entire site. I thought it was odd that the RCA reveiving tube manual didn't list it.

Reid: thanks for the link!!! I'm checking it out now

For what it's worth I just got in an old Univox 1/12 combo that has 2=6BM8's, 1=6X4, and one 12AX7. It has volume , tone and tremelo along with three input jacks. The fellow wants it "hopped up a bit" for studio recording. It has a Jensen speaker C12S in it.

:^)
Trace

11/12/1999 3:57 PM
Doc	Trace: I have one of these amps, exactly what you describe. I have the original paperwork with it, including the schematic. This amp needs ALL those gray (Elna?) coupling caps replaced with your choice of other film types. I tried to slow down the tremelo with extra phase shift capacitance, which kept it from taking off, but didn't slow it down as far as i'd like it. (Kind of just left it there for the time being.) I did do some minor mods on the input jack network, and maybe cathode resistors (can't recall clearly). The amp sounds great with a little stompbox delay up front. Mine was in a hardly used at all condition. Even the factory tag was hanging from the handle. The amp puts out a good 10 watts. I think it was a good choice on part of Univox to build it using a 12" speaker. It doesn't take much, just changing a few small parts, to turn this into a usable guitar amp. It lacks reverb, and to my ears doesn't sound very good until you add some delay or reverb to the guitar signal. A boss RV3, or even an older RV2 does the job, as will any AD or DD pedal. It really does make a huge difference in the overall sound of this amp. I guess you could disable the tremelo, and use that 12AX7 for another gain stage. But the triode in the 6BM8/ECL82 is high gain (u=70), and the power pentode doesn't require much grid signal voltage (16v) to max it out. Results obtained by adding another gain stage might be disappointing. Just changing a few resistor values and replacing bad caps will do it. You're welcome to a copy of the schematic. I'll have to send it by snail mail, though. You can get Svetlana's 6BM8 data sheet, and some representative tech articles using the 6BM8, at their website. (I've contemplated making a SE headphone amp with these tubes.) Everything is nicely done in Acrobat. Some basic data, but no curves, can be found in RCA tube manuals (ex:RC-23, p.183). Required load resistance for a typical push-pull circuit can be found in GE essential characteristics book. Doc

11/12/1999 3:57 PM

Doc

Trace:

I have one of these amps, exactly what you describe. I have the original paperwork with it, including the schematic.

This amp needs ALL those gray (Elna?) coupling caps replaced with your choice of other film types. I tried to slow down the tremelo with extra phase shift capacitance, which kept it from taking off, but didn't slow it down as far as i'd like it. (Kind of just left it there for the time being.)

I did do some minor mods on the input jack network, and maybe cathode resistors (can't recall clearly). The amp sounds great with a little stompbox delay up front. Mine was in a hardly used at all condition. Even the factory tag was hanging from the handle. The amp puts out a good 10 watts. I think it was a good choice on part of Univox to build it using a 12" speaker. It doesn't take much, just changing a few small parts, to turn this into a usable guitar amp. It lacks reverb, and to my ears doesn't sound very good until you add some delay or reverb to the guitar signal. A boss RV3, or even an older RV2 does the job, as will any AD or DD pedal. It really does make a huge difference in the overall sound of this amp. I guess you could disable the tremelo, and use that 12AX7 for another gain stage. But the triode in the 6BM8/ECL82 is high gain (u=70), and the power pentode doesn't require much grid signal voltage (16v) to max it out. Results obtained by adding another gain stage might be disappointing. Just changing a few resistor values and replacing bad caps will do it.

You're welcome to a copy of the schematic. I'll have to send it by snail mail, though.

You can get Svetlana's 6BM8 data sheet, and some representative tech articles using the 6BM8, at their website. (I've contemplated making a SE headphone amp with these tubes.) Everything is nicely done in Acrobat. Some basic data, but no curves, can be found in RCA tube manuals (ex:RC-23, p.183). Required load resistance for a typical push-pull circuit can be found in GE essential characteristics book.

Doc

11/12/1999 10:02 PM

Trace

quote:
"I have one of these amps, exactly what you describe. I have the original paperwork with it, including the schematic."

Now what are the odds on that! (ha, ha) We both have one. :-)

quote:
"This amp needs ALL those gray (Elna?) coupling caps replaced with your choice of other film types. I tried to slow down the tremelo with extra phase shift capacitance, which kept it from taking off, but didn't slow it down as far as i'd like it. (Kind of just left it there for the time being.)"

I did replace all those caps. They are also oil filled, well the caps in this amp are anyway.

quote:
"I did do some minor mods on the input jack network, and maybe cathode resistors (can't recall clearly)."

This amp has both cathodes connected to a 3.3K / 10uF cap. I seperated these and it made the amp sound better right away. The touch-response was much better.

quote:
"I guess you could disable the tremelo, and use that 12AX7 for another gain stage. But the triode in the 6BM8/ECL82 is high gain (u=70), and the power pentode doesn't require much grid signal voltage (16v) to max it out. Results obtained by adding another gain stage might be disappointing."

I did disable the trem and utilized the other side of the 12AX7. The tone is amazing but as you said it doesn't take much to over drive those power tubes. It's got the electric-frying bacon sound right now! (ha, ha) I obviously have to tame that down.

quote:
"Just changing a few resistor values and replacing bad caps will do it."

I also added Bass, Treble, and Mids. Since there isn't a lot of gain stages this make sthe EQ pretty flexible.

quote:
"You're welcome to a copy of the schematic. I'll have to send it by snail mail, though."

That would be great Doc! Do you have my address? If not I can email it to you.

quote:
"You can get Svetlana's 6BM8 data sheet, and some representative tech articles using the 6BM8, at their website. (I've contemplated making a SE headphone amp with these tubes.) Everything is nicely done in Acrobat. Some basic data, but no curves, can be found in RCA tube manuals (ex:RC-23, p.183). Required load resistance for a typical push-pull circuit can be found in GE essential characteristics book."

Thanks so much for the info Doc! I'll try to grab the info from Svetlana and GE. If there's anything you need please feel free to hollar!!

Trace

11/16/1999 2:07 PM
Doc	Trace: I found the schematic. My amp is a model U-45B. This amp puts out about 7w clean and 10w dirty. It may be slightly different than yours, because ther's no shared cathode resistors. The initial preamp stage (1/2- 12AX7) is set up in grid leak bias. The second gain stage is a 6BM8 triode with its own 3.3k cathode resistor, and a 10uf cap in parallel. That 3.3k may be the typical recommended value for this tube because the other 6BM8 triode is set up as a split load inverter using 100k loads, but there is a stage bias cathode resistor of 3.3k above the lower 100k. (Grid leak 470k goes to the junction.) The stock input jack network is highly attenuative, which is necessary so as to not swamp the sensitive grid leak preamp. It has 3 inputs with shorting jacks and 47k resistors. I think what I did was rewire similar to a typical fender stage, with 1.5k cathode bias and 1meg grid return, and reduced input resistor attenuation (maybe 47k/1meg, can't recall). It looks like you could rework the preamp easily into a typical BF/SF type w/ tone controls by employing the other half-12AX7 from the tremelo. To cut cascade gain, I would start with deleting the cathode bypass cap on the next stage 6BM8 triode, maybe apply local plate to grid feedback or try a tapped load resistor into the PI stage. I don't have your mailing address, so please let me know. Doc

11/16/1999 2:07 PM

Doc

Trace:

I found the schematic. My amp is a model U-45B. This amp puts out about 7w clean and 10w dirty. It may be slightly different than yours, because ther's no shared cathode resistors. The initial preamp stage (1/2- 12AX7) is set up in grid leak bias. The second gain stage is a 6BM8 triode with its own 3.3k cathode resistor, and a 10uf cap in parallel. That 3.3k may be the typical recommended value for this tube because the other 6BM8 triode is set up as a split load inverter using 100k loads, but there is a stage bias cathode resistor of 3.3k above the lower 100k. (Grid leak 470k goes to the junction.)

The stock input jack network is highly attenuative, which is necessary so as to not swamp the sensitive grid leak preamp. It has 3 inputs with shorting jacks and 47k resistors. I think what I did was rewire similar to a typical fender stage, with 1.5k cathode bias and 1meg grid return, and reduced input resistor attenuation (maybe 47k/1meg, can't recall). It looks like you could rework the preamp easily into a typical BF/SF type w/ tone controls by employing the other half-12AX7 from the tremelo. To cut cascade gain, I would start with deleting the cathode bypass cap on the next stage 6BM8 triode, maybe apply local plate to grid feedback or try a tapped load resistor into the PI stage.

I don't have your mailing address, so please let me know.

Doc

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