
Home  Forums  Rules  Articles  The diyAudio Store  Gallery  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
9th February 2008, 05:01 PM  #1 
diyAudio Member
Join Date: Feb 2004
Location: South Wales

6BM8/ Heathkkit S.88 Has only 3 and 15 ohm O/P..?
Had a bit of luck, got hold of a complete Heathkit S.88 amplifier....
It has a PP 6BM8/ECL82 output stage, and the O/P Tx is wired for 15 Ohm or 3 ohm..... There is No 8 ohm tapping.... Any ideas as to how to get the best out of this without changing the O/P Tx....??
__________________
Das Beste Oder Nichts 
9th February 2008, 07:00 PM  #2 
diyAudio Member
Join Date: Jun 2007

Connect your loudspeaker between the 3 and the 15 ohm taps. Would this not be about 9.8 ohms? Should work with an 8 ohm speaker.
Wade 
9th February 2008, 07:31 PM  #3 
Guest

If the OT has two secondaries, wired in parallel for 3 Ohms and wired in series for 15 Ohms, there will be no 315tap.
Then you will have to take the 3 Ohms arrangement. You will loose some power, though ... but in general it would work this way! Greets 
9th February 2008, 07:55 PM  #4 
diyAudio Member
Join Date: Jun 2007

What VEK says is true. But I think the OTs on this particular amplifier have three wires on the secondary side; Black  common, Yellow  3 ohms, and Brown  15 ohms. Try the speaker between the brown and yellow wires.
Wade 
11th February 2008, 02:27 AM  #5 
diyAudio Member
Join Date: Oct 2004
Location: Adelaide South Oz

Alistair and Wade,
You need to remember that with transformers the impedance ratio is the square of the turns ratio. Talking in terms of 4 and 16 Ohms 2 off 4 Ohm Taps in parallel doesn't change the numberr of turns so it still 4 Ohms. 2 off 4 Ohm taps in series doubles the turns so X4 of the impedance that is it becomes a 16 Ohm output. the 3 and 15 Ohms mentioned for your transforemer are nominal impedances. The 3 is probably about 3.5 Ohms. It is extremely likely that the number of turns between the 3 and 15 Ohm taps is the same as between the Common and 3 Ohm tap. That means connecting a load between teh 3 Ohm tap and the 15 Ohm tap will just be another 3 Ohm nominal output. If you wish to connect an 8 Ohm load I'd just hook it to the 3 Ohm output. I had one of these amps  with (I think) the same output trannies. Do yours have ultralinear taps. If so then they are the same as I had. The Raa was about 6K and the Ultralinear Taps were at 20%. I used the trannies in the ECL86 (6GW8) prototype which led directly to the "Baby Huey" design. I ran 8 Ohm speakers of the 3.5 Ohm tap Cheers, Ian 
11th February 2008, 07:20 PM  #6 
diyAudio Member
Join Date: Feb 2004
Location: South Wales

Thanks for all your input guys, I havent picked up the amp yet, but I do know it needs quite some work, Loads of those nasty old 'Plessy' electolytics to go, plus who knows what else...!
Hows this for a mad idea.... Replace the current 6bm8/ECL82 with summit like a EL86, or even a PL508 that 'likes' O/P impedance of 2.5K or 5Kaa, Add summit like a ECC83 to replace the triode sections of the ECL82, along the same lines as the original scheme for phasesplit and gain stage, and run a 8 ohm speaker from the 15 ohm taps.... Just a mad thought...It'll never fly!
__________________
Das Beste Oder Nichts 
12th February 2008, 12:05 AM  #7 
diyAudio Member
Join Date: Jun 2007

Thanks for the input, Gingertube. I did find the error in my numbers but it comes from trying to do the math and calculate square roots in my head. If these are the same transformers you had then I am sure you know more about them than I do. I am not that certain of any of the impedances ratings or turns ratios. What you said makes sense and there may very well be two 3 ohm or 4 ohm windings in series for 12 ohms or 16 ohms. I can't say. One could measure the transformer, if so inclined.
Best 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
6BM8 Equivalent  GlidingDutchman  Tubes / Valves  11  15th April 2009 06:40 PM 
6FD7, 6BM8 or other  mashaffer  Tubes / Valves  1  9th May 2008 02:14 AM 
G2driven 6BM8  wfmali  Tubes / Valves  34  13th February 2008 05:47 PM 
6BM8 Question  Peter Menting  Tubes / Valves  10  18th September 2004 03:08 PM 
New To Site?  Need Help? 