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General Discussions => The Lab => Topic started by: ed rees on September 12, 2010, 12:53:30 AM

Title: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: ed rees on September 12, 2010, 12:53:30 AM
Hi,
Trying to figure out the 'Neon bulb balancing trick' used on EMI Altecs. You guys have pulled through so far for me, hopefully this wont be very hard to find an answer to.  ;)

The UA175 and EMI/Altec both had plate balance on the 6BC8, but the EMI used a neon bulb to create a ticking that was canceled out when perfectly balanced.

Here is a more through explanation by someone on GS:

"As i understand from "Recording The Beatles", the Balance was a way to synchronize the two amplifiers in the "push/pull" design of the RS124. it was done with a neon Bulb that was wired to the circuit, when the technician pushed the momentary switch (the one you called balance button) the bulb would create a repetitive low frequency "ticking" sound. the sound was sent through both amplifiers. since one was in reverse phase the two amplifiers would null if they were perfectly balanced.
but if you could hear the " tickling" noise you knew the amplifiers were not properly balance. in that case you could adjust the amplifiers balance with the screw hole in the upper left corner above the balance. "

Anyone have an electrical explanation, or know any similar applications I can look at to see exactly how this was implemented?

Thanks!
Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: rotheu on September 12, 2010, 01:39:59 AM
Hi,
Trying to figure out the 'Neon bulb balancing trick' used on EMI Altecs. You guys have pulled through so far for me, hopefully this wont be very hard to find an answer to.  ;)

The UA175 and EMI/Altec both had plate balance on the 6BC8, but the EMI used a neon bulb to create a ticking that was canceled out when perfectly balanced.

Here is a more through explanation by someone on GS:

"As i understand from "Recording The Beatles", the Balance was a way to synchronize the two amplifiers in the "push/pull" design of the RS124. it was done with a neon Bulb that was wired to the circuit, when the technician pushed the momentary switch (the one you called balance button) the bulb would create a repetitive low frequency "ticking" sound. the sound was sent through both amplifiers. since one was in reverse phase the two amplifiers would null if they were perfectly balanced.
but if you could hear the " tickling" noise you knew the amplifiers were not properly balance. in that case you could adjust the amplifiers balance with the screw hole in the upper left corner above the balance. "

Anyone have an electrical explanation, or know any similar applications I can look at to see exactly how this was implemented?

Thanks!


Look in the Lab for "Another Poor Man's Fairchild 660/670", on the page 12 there is a power supply and balance circuit schematic. This balance circuit is the one from EMI RS-124. Ignore the DPDT switch and double output, that's for two channels. Just use SPST switch.
Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: ed rees on September 12, 2010, 09:01:13 AM
Beautiful, and thank you! Looks just as simple as I expected.  

Your PM670 looks GREAT btw.
Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: gary o on September 15, 2010, 06:55:42 AM
Hi Ed, I built a RCA BA6A a while back it has a built in circuit that basically puts 6.3 VAC thru the input and out to the meter for you to adjust with the balance pots......I had to change my version a little so that i hear the hum and can adjust by ear.....its works great....so is similar to RS124.

I believe the BA6A schematic is in the metas if you want a look

heres a relaxation osc prob similar to wot they used on RS124 either ways its only used to balance circuit has nothin to do with sound...

http://www.cliftonlaboratories.com/relaxation_oscillator.htm

Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: eskimo on September 15, 2010, 12:35:22 PM
Hi Ed, I built a RCA BA6A a while back it has a built in circuit that basically puts 6.3 VAC thru the input and out to the meter for you to adjust with the balance pots......I had to change my version a little so that i hear the hum and can adjust by ear.....its works great....so is similar to RS124.

More specifics on that? Schem? Sounds interesting.
Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: gary o on September 15, 2010, 08:20:04 PM
http://www.sowter.co.uk/schematics/rca_ba6a.jpg

this schematics has some wrong resistor values

http://www.groupdiy.com/index.php?topic=4570

Theres more info if you search for BA6A I think some 1 has made one recently too it was thanks to CJs work and my money that Sowter make the transformer
Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: eskimo on September 15, 2010, 08:56:18 PM
Oh, I meant your balance hum introducer. :)
Title: Re: 6bc8 plate balance in UA175 and Altec/RS124?
Post by: gary o on October 08, 2010, 09:55:23 PM
The hum inducer from the BA6A ?..........if so its in the schematic.. basically 6.3 VAC....