6BM8/ Heathkkit S.88 Has only 3 and 15 ohm O/P..? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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
Reply
 
Thread Tools Search this Thread
Old 9th February 2008, 05:01 PM   #1
diyAudio Member
 
Alastair E's Avatar
 
Join Date: Feb 2004
Location: South Wales
Default 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 P-P 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
  Reply With Quote
Old 9th February 2008, 07:00 PM   #2
diyAudio Member
 
tubewade's Avatar
 
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
  Reply With Quote
Old 9th February 2008, 07:31 PM   #3
Vek
Guest
 
If the OT has two secondaries, wired in parallel for 3 Ohms and wired in series for 15 Ohms, there will be no 3-15-tap.
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
  Reply With Quote
Old 9th February 2008, 07:55 PM   #4
diyAudio Member
 
tubewade's Avatar
 
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
  Reply With Quote
Old 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
  Reply With Quote
Old 11th February 2008, 07:20 PM   #6
diyAudio Member
 
Alastair E's Avatar
 
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' elect-o-lytics 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 5Ka-a, Add summit like a ECC83 to replace the triode sections of the ECL82, along the same lines as the original scheme for phase-split 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
  Reply With Quote
Old 12th February 2008, 12:05 AM   #7
diyAudio Member
 
tubewade's Avatar
 
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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
G2-driven 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?

All times are GMT. The time now is 12:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2