andyfromdenver

Turns ratio on tube amp pushpull output transformer
« on: March 29, 2017, 12:37:07 PM »
Hi smart people  ;D

For an unknown output transformer (OT), with a primary center tap (CT), I have calculated the turns ratio by applying AC voltage to the two primary ENDS (excluding CT) until measuring 1VAC across the secondary.  Then, I measured the AC voltage from my variac output for a 30:1 ratio.

I know for single ended OTs I simply use this.  I just want to make absolutely sure i'm not missing something with regard to the CT and Push Pull.

I am trying to determine if the "mystery" OT wants an 8ohm load or 4 ohm. (and the schematic/ search engine isn't helping...)

at 30:1 (from primary end to end, no CT) I get

(30x30) x 8 (ohm) = 7.2K reflected primary, this jives very well with 7189 power pentodes (like el84s on steroids, I believe  :o)

(30x30) x 4 (ohm) = 3.6k (of course it does...)

can someone please clarify what the proper speaker load would be for this scenario?  I just haven't applied the formula to PushPull vs Single End.

I'll be reading the RDH4 while my question is posted, in case the info is in there.

Thank you!!!!

Andy


edit:  ok. so...uhm  ::)  ;D

I just measured a transformer I have on hand which is spec'd @ 8k <-> 8ohm. and it is 30:1.

mystery solved. 
« Last Edit: March 30, 2017, 08:51:48 AM by andyfromdenver »
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie


trobbins

You can never really know the primary and secondary impedance specs for an unknown OT.  If you can gather other related details, like what it came out of, manufacturer, relative core size, secondary wire size, ratios of secondary windings if more than 1, and primary inductance, then you may get some more confidence.

You may get a better turns ratio accuracy by applying the ACV to half the primary winding, and measuring the voltage (for turns ratio calculation), from the other half primary winding.  Also best to swap the primary half windings and repeat test, to ensure that the half-primary windings do in fact have same turns.

andyfromdenver

re:
« Reply #2 on: March 29, 2017, 11:06:23 PM »
Thanks so much for taking the time to reply.

The "mystery" OT is in a Leslie 122 Reverb Amp, I was confused because the schematic: http://theatreorgans.com/hammond/faq/schematics.html (click on 122vb scheme) shows a paralleled pair of speakers, which made me think 4ohm.  However, all the pictures I can find of units have a single speaker.  This makes me more hopeful that it will prefer 8ohm.

« Last Edit: April 27, 2017, 06:56:25 PM by andyfromdenver »
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

abbey road d enfer

Thanks so much for taking the time to reply.

The "mystery" OT is in a Leslie 122 Reverb Amp, I was confused because the schematic: http://theatreorgans.com/hammond/faq/schematics.html (click on 122vb scheme) shows a paralleled pair of speakers, which made me think 4ohm.  However, all the pictures I can find of units have a single speaker.  This makes me more hopeful that it will prefer 8ohm.
The Leslie 122 main amp uses 6550's, supposed to see about 4kohm plate-to-plate and the rotary speakers are 16 ohms.
So it's not the main amp, it's the reverb amp, using 7189's, that are supposed to see about 8kohms plate-to-plate. Your xfmr, with 30:1 ratio is adequate for 8 ohms load (2x16 ohms speakers in parallels).
However, loading with 16 ohms (one speaker) is acceptable. Think how many guitar amps have an extension jack that allows connecting a speaker in parallels, or how the AA763 chassis was used in different speaker configurations resulting in quite different nominal impedances.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

andyfromdenver

re:
« Reply #4 on: April 27, 2017, 06:55:07 PM »
The Leslie 122 main amp uses 6550's, supposed to see about 4kohm plate-to-plate and the rotary speakers are 16 ohms.
So it's not the main amp, it's the reverb amp, using 7189's, that are supposed to see about 8kohms plate-to-plate. Your xfmr, with 30:1 ratio is adequate for 8 ohms load (2x16 ohms speakers in parallels).
However, loading with 16 ohms (one speaker) is acceptable. Think how many guitar amps have an extension jack that allows connecting a speaker in parallels, or how the AA763 chassis was used in different speaker configurations resulting in quite different nominal impedances.
Thank you for the reply!
Yeah, I thought I was going to do a restore/recap, but I got roped into a lil' conversion. In my defense vs. infuriating the Leslie crowd, he only had this chassis from an ebay deal, and a broken 7189 (Russian 6P14Ps to the rescue).
Also, it's a radio-hit band friend, so it was an easy "yes" job   ;D (i'm a sucker for high profile clients!)

The chassis is teeny, and since you saw the schematic you can see how it doesn't translate willingly into a complete tube amp. There was no room to add a proper tube pre without risking a lot of $ and a possibly unsatisfying noise floor d/t chassis constraints.

I recapped, rotated the OT to a low hum position (on test), added pwr toggle, fuse, indicator, checked/optimized bias,  and modified the input to the tube stages.
I told him he can use any transistor pedal/ pre and jack-in to his new/nice power amp.

We hooked it to his single 12" leslie cab; sounded swirly and good :-)

Here's my conversion scheme (please compare to the afformentioned in link).
If you are bored and have any suggestions, I'd be a willing listener :-)

Take care!
Andy

p.s. I removed the "please delete/ignore" from the main title, but i guess forgot from the subsequent posts, oops.
« Last Edit: April 27, 2017, 08:51:51 PM by andyfromdenver »
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie

abbey road d enfer

Re: re:
« Reply #5 on: April 27, 2017, 09:27:48 PM »
The chassis is teeny, and since you saw the schematic you can see how it doesn't translate willingly into a complete tube amp. There was no room to add a proper tube pre without risking a lot of $ and a possibly unsatisfying noise floor d/t chassis constraints.

I recapped, rotated the OT to a low hum position (on test), added pwr toggle, fuse, indicator, checked/optimized bias,  and modified the input to the tube stages.
I told him he can use any transistor pedal/ pre and jack-in to his new/nice power amp.
Well, one possibility would be reconfiguring the SS recovery amp for guitar input...

Quote
We hooked it to his single 12" leslie cab; sounded swirly and good :-)
No doubt it would sound good on a guitar cabinet too; I like the hybrid fixed/cathode bias.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

andyfromdenver

Re: re: New
« Reply #6 on: April 28, 2017, 11:20:34 AM »
Well, one possibility would be reconfiguring the SS recovery amp for guitar input...
 No doubt it would sound good on a guitar cabinet too; I like the hybrid fixed/cathode bias.
I did consider implementing a simple transistor or two stage, but it seemed like a lot of effort (time: because it is not my forte)/ expense, when he could use pedals.  Plus, I had a little thought of adding a little "side car" fab'd 12ax7 stage, and was set on that, when I realized it could quickly balloon out of my estimate for work since I had to do so much already in the power section plus buying a matched pair of output tubes.

There is a separate pcb for the tube stages and the transistor stages.  I carefully removed the transistor pcb and gave it to him for safe keeping, maybe down the road I can rework it for him and jack it back in.

He was happy enough to have the direct coupled triode to PI, and liked that it was quiet  :o ha!
We had a talk about "you know how when you use a boost pedal you usually never turn it up past noon cause it's too much volume...well... now you can" and I showed him what it sounded like to drive the output to break up with two clean boosts.

It is an interesting circuit with the dual bias and the voltage regulator tube.
I learned from reading that there is an intentionally undersized resistor in series after the regulator to act as a fuse.  That's cool and old school.  I made sure to keep the same wattage and replaced it because it was looking a little brittle.

Here's a pic of the lil guy  ;D
« Last Edit: April 28, 2017, 11:28:59 AM by andyfromdenver »
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie