6BM8 SE STEREO AMP

[TIMBO]

Hi guys, Looking for things to do... well not really.. kinda
Digging into my pile of junk and found this on the shelf



So, first off, health check....
PT, good, approx. 300v and heaters 6.7v
OT's, BAD
They appear to have a UL tap and are open between the UL tap and the plate....
Found a couple of replacements.
What to build...
Google Found me this.
https://el34world.com/Forum/index.php?topic=25086.msg276357#msg276357
Thanks TOXO, I borrowed some ideas.

So now the questions.
The replacement OT's are similar size to those mounted on the chassis.
One has marked on the side 5000/15ohm and the other by calculation is approx. 7000/15ohm.
As per datasheets, 6BM8 like this range, so...
I have quite a lot of 8" speakers of 4,8 and 16ohm.
For purely shits and giggles I want to load each OT with three speakers each OR possibly 4 and 2 depending on how loud each channels are.
Loading each channels with three speakers (for the moment) I can get approx. 5.5k per OT with different ohm combo's.
Any problem loading that many speakers per OT and primary impedance.
Thanks to TOXO adding a reverb in this manor is new to me and not much info on how well it works.
Local builder did it and again on way of knowing if it worked.

I can only imagine an 8 or 10ohm input tank would be used

[shooter]

an 8 ohm is "the load" regardless of speaker count.

and ya, that's a wonky reverb 1st quick looks like it's part of the NFB?

[kagliostro]

Ciao TIMBO

I don't see how the reverb channel can work

Where is the signal input on the reverb channel ?


To me, if you want someting coming out of the reverb channel you must connect the input of the reverb tank to the other OT

Franco

[TIMBO]

Hey Franco, How are you...
You are right connected to wrong output.
Schematic fixed.

[kagliostro]

Ciao TIMBO

I'm a bit stressed but at home we are in health, what about you and family ?

I was thinking to the Gibson GA-1RT-1

https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga1rt.pdf




Franco

[TIMBO]

Yeah, We are all good.
I looked at that and wasn't sure how it worked.

[kagliostro]

Yeah, We are all good.

 

--

I looked at that and wasn't sure how it worked.

If you mean you give a look to the Gibson schematic .... the red and black input of the tank are the speaker connections

---

On the attached .pdf file you can find info about power distribution using different speakers connected together, unfortunately text is italian, not english


Franco

[PRR]

> They appear to have a UL tap

No. That is a small (~~5%) hum-buck tap, to save a penny on the filtering of the low-level stages.

Leave that area strictly stock, or just ignore the extra lead.

[TIMBO]

Thanks PRR, The OT has Red-B+ Yellow-screen and Blue-plate
There is NO continuity between red and blue, but there is between red and yellow.
Appling 9v to the red and blue I don't get a voltage reading, no DC resistance either.
I think there DEAD.

[kagliostro]

I cooked one of those transformers (it was new old stock) trying to measure it

I give 10 V AC to the "choke" + "principal winding" without any precaution about current and ...... 

the choke winding burnt

Franco

[Pietro]

Gibson...or offcoarse a univox 45B.

[shooter]

NO continuity between red and blue

opens are better than shorts 
before the scraper's get it look close for a broke whisker, might have to cut the wrap a small bit, if it's not in the 1st inch of the tab, it's worth 3.7cents a pound