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What audio output xfmr for a 6BM8 pp amp?

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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?
 
frugal-phile™
Joined 2001
Paid Member
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
 
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.
 
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.
 
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!
saf24amp.jpg
 
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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
 
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)
 
Some of this stuff I haven't messed with since high school. Heres one thing that puzzes me. (remember, I graduated in 1987) a .001 MFD is a a a 10mmfd cap right. I mean lets see if my brain can wrap around these decimal points.
a MFD is .0001 right? and a mmfd is .0001MFD. so a .001MFD would be 100mmfd right?

I am not trying to be dumb (despite being a whitehead) but I want to get it right

If I remembered, the amp that I had, had 1/2 watt resistors throughout. but what wvdc is minimum for the b+ supplies? I guestimate that 400 wvdc would be fine.

Lastly, where can I find these components new, at one source, that isn't going to charge me an arm and leg? I rather do one stop shopping and Ebay isn't going to help. Maybe the original designers were cutting corners but $13 for a large axle lead 20uf filter cap on Ebay is NUTS! I am building this to prove I can do it. I am not building it by taking a loan from my bank!

Does anyone know how many of each part I am going to need? I got overwhelmed counting the resistors and capactiors of this schematic.
 
Antique Radio Supply does have a power transformer from hammond that has voltages very close to the original. I didn't see a grid take off on the item but then I was looking at the catalog pages very quickly. at $40 or so it wasn't that bad of a price either!

Some of these transformers have some really SHOCKING! voltages and prices to match! But then if you need a specialized marshall or fender transformer... you pay for it. ;)

Okay, heres what I can summarize what I am going to need.
(it is vague as I haven't counted up all the components yet)
7 9 pin tube sockets.
1 1A silicon power rectifier for the grid bias
1 transformer with 240-0-240 output and/or a 6.3 filament transformer
4 6bm8 tubes
2 12ax7 tubes for the riaa preamp
1 rectifier tube. (number slipped my mind. its on the schematic though)
several 40MFD and 24MFD electrolytic caps

Given my economic status.. it will take me a few years to get this project built and powered up.
 
Hi Bichela,

Just to recap, is it fair to say that all you have so far is a schematic of an amp that some here may feel is mediocre. If this is the case then you are as close to starting from scratch as you near as damn can be. Not trying to put you off here just pointing out that this may be a great opportunity through the help of this forum to get a lot more quality for your buck.

I personally recommend Gingertubes "Baby Huey" but there are many more well respected designs from members of this forum.

Once you've decided on a schematic the cheapest way to go IMHO is to look out for a cheap donor amp that has the valve compliment that you want.

The amp could be built first. RIAA preamp and tone controls could be added later.

Another route is to buy a kit.

The K502 would be a cheap option. It use 11BM8 valves which are 11v heater version of the 6BM8. George Fathauer's design is simple but IMHO sounds a lot better than it should and is easily modded.

Alternatively you could source all the parts yourself and build a 6BM8 version if you really want to use this valve. I have a copy of the schematic.


Really trying to encourage you here..........:D

Brgds Bill

Baby Huey Thread www.diyaudio.com/forums/tubes-valves/72536-el84-amp-baby-huey.html

K502 kit here http://tubesandmore.com/ page 79 of catalogue
 
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frugal-phile™
Joined 2001
Paid Member
I would be willing to go with a less complex layout if I can find one.

Attached are 2 schema pulled from my collection that show the 2 topologies these tubes are usually used in. I'm partial to the LTP phase splitter. A CCS or a big B- makes the input trafo moot.

dave
 

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Attached are 2 schema pulled from my collection that show the 2 topologies these tubes are usually used in. I'm partial to the LTP phase splitter. A CCS or a big B- makes the input trafo moot.

dave


Cool! It looks less intimidating. I did also find a simple amp using a 2a3 tube that is a SE amp. However... being this is using the tubes I want to use. and I can usually get them inexpensively. I can build it in stages.

The SAF-24 isn't a bad amp. I had mine for YEARS until the electrolytic cap in the PS started showing its age. Being that I didn't know of a source for a new cap. It got mothballed. Unfortunate for me, my landlord's building went into foreclosure and I was forced out, by the bank. There was no room in my travel trailer that I moved to for all my stuff. and the amp ended up in the trash when the bank cleaned out the place for resale.

I may combine some of my SAF-24 schematic with what you sent me. Especially the Power Supply section as I prefer a rectifier tube section. The SAF-24 RIAA preamp using the two 12ax7 tubes had a great sound and had lots of output from my R25XT moving magnet cartridge. Bass? A bit low but that could have been due to old caps. or lousy speakers. I believe the caps were the main issue.
 
For converting capacitance values, something like this chart is useful.

micro= millionth (10^-6)
nano= 10^-9 (remember in the UK and US 'billion' is different....)
pico=10^-12
1000000 uF= 1F
1000nF=1 uF
mmF=pF

A common source of confusion (as posts attest....)
That chart was AWSOME! and it definitely is the Rosetta stone that I was looking for. (Talking about the link you posted.) I was right that a .001MFD cap is a 1000mmfd or 1000pf cap ;) Time to heat those pokers and melt some Rosin!
 
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