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Old 28th February 2011, 08:12 PM   #1
bichela is offline bichela  United States
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Question What audio output xfmr for a 6BM8 pp amp?

I have the schematics for a Realistic SAF-24 amp that I lost some years ago. it shows all the needed info but ONE piece. what is the best audio output transformer for a unit using 6mb8 tubes in a push pull configuration. I want to learn by building this classic stereo amp in stages.

I don't need 16 ohms output. just 8 and 4 ohms. Rated wattage output was 24 watts RMS.

I got the time but funds are a bit tight. the power xformer shows a 230v-0-230v ht with a 76.5 takeoff for the grid bias. If I can find the right 230V ct output can I use a voltage divider to get the 76.5 from that or should I go for broke and find a transformer that is configured exactly to the schematic?
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Old 28th February 2011, 08:53 PM   #2
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You can use a PP transformer for an EL84/6BQ5.

24 watts is optimistic even if it is the sum of both channels. 8-10 watts/channel is what you can expect.

Edcor is the goto for reasonable budget OPTs.

EDCOR Electronics Corporation. XPP10-6-8K

or EDCOR Electronics Corporation. GXPP10-6-8K

would be good for the cheapest. They only have single primaries, 6 ohm is the goto for 4-8 ohms.

Eli will probably have some other suffestions from his research into the El Cheapo project

dave
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Old 28th February 2011, 09:11 PM   #3
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Quote:
Originally Posted by bichela View Post
what is the best audio output transformer for a unit using 6mb8 tubes in a push pull configuration.
Are you sure you don't mean 6BM8 (ECL82)? For that tube something around 8 to 10 thousand ohms plate-to-plate is a good value numbers wise. The Edcors suggested will work well. And for the bias voltage, it's possible to use a series resistor (10 watt wire wound) from one end of the 230V winding to feed a single half wave SS recitfier for the negative voltage you need.
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Old 28th February 2011, 10:02 PM   #4
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Are you sure you don't mean 6BM8 (ECL82)?
I didn't even notice, assumming ECL82.

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Old 28th February 2011, 10:27 PM   #5
bichela is offline bichela  United States
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Thanks for the correction. yes, I meant the 6bm8. Given what I have seen already, I can build one channel at a time. Clearly the PS needs to be built first. (From the schematic it uses a 6ca4 rectifier tube. this is a 9 pin tube, same socket as the 6bm8 tubes. Caps in the PS are rated (as per the orignal schematic) are two 40 MFD and two 25 MFD and one 50 MFD polarized electrolytics. I know the original amp had one of those big can caps. What would be a good choice for modern components that are not too expensive, I made an error at saying the grid take off was 76.5v schematic shows it as 26.5v rectified usng a selenium rectifier. Now, given what little I know of these. that means a voltage drop of .7v. Thus, a modern silicon rectifier means the take off should be reduced to 18.55 volts before the rectfifier. am I right?

I will have to scan the original schematic and put it up. It is well done. now the schematic says that everything is in MFD except items with a star that means mmfd. Is that Microfarads? Remember, this schematic was from the 60s and some things are refered to differently now.
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Old 28th February 2011, 10:28 PM   #6
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If it's SAF 24C then according to this thread it's 6BM8 (ECL82)...........


Realistic SAF 24c - Need some help

Sorry didn't see your post in time
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Old 28th February 2011, 10:40 PM   #7
bichela is offline bichela  United States
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Heres the schematic of my saf-24 amp.

Notice the take near the power lead that ties it to common of the output xfrm and the shield of the power xfrmr. Am I right that this is a BAD idea? Always wondered why I got zapped now and then! Note, put a IEC connector in with a REAL Earth pin! If one REALLY wants to remain two pin. A polarized plug is a GREAT idea!Click the image to open in full size.

Last edited by bichela; 28th February 2011 at 10:48 PM. Reason: Just an observation about the power supply
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Old 1st March 2011, 12:35 AM   #8
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Yes well worth putting in a proper earthed connection.

Mullard prepared at least a nice 10W PP UL stereo amp back in 1962 based on 6BM8/ECL82 which had widespread use. OT was 9k PP with UL tappings at 20%. I'm about to resurrect a stereo amp using that design, but based on a 12AX7 for preamp tube for each channel.

You may want to separate the HT supplies to each channel for better seperation.

Ciao, Tim
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Old 1st March 2011, 12:36 AM   #9
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Holy crowly. what a screwball design. Putting tone controls in the feedback loop from the OPT secondary is a really cheap and dirty way of eliminating a "proper" tone control stage. I guess that's typical Rat-Shack design.

Yes, the secondary must be grounded for the controls to work and also for safety should the OPT primary short to secondary. Rare, but could happen. So grounding here is a good idea. Actually, in this case the four ohm tap is being grouned where usually it's the common (zero) end. And definitely use a three wire hard chassis ground. Without one, that's why you were getting tickled.

If I were making this amplifier, I would add balance adjustments for the bias on the output tubes. A small pot between the two 470K grid resistors where the negative voltage comes in. I'd also add a way of measuring the tube current, like a very small resistor in each 6BM8 cathode to ground.

Oh yea, micromicrofarad is an old term meaning one-thousandth of a microfarad. (nine decimal places) Today it's called picofarad. (one billionth of a farad)
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Old 1st March 2011, 01:08 AM   #10
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The cleverness by which these guys cut costs never ceases to amaze me. Gotta give them credit for thinking outside the box.
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