Check my transformer math please

[Toxophilite]

I'm asking someone to confirm this for me because it's different from the schematic specs.
On the princeton deluxe reverb I built I used a Bogen VP20 amplifier chassis and transformers as a starting point.
In the schematic (and in the instructions) it shows two taps for the OT, one for 8ohms for a single speaker and one for 2.5 ohms for a three 8 ohm speaker setup.

Measuring the turns ratio on the OT (variac at 10VAC on the secondaries)
I get 30-1 on the yellow tap (300vac)
and 22.7 - 1 on the green tap (227 vac)
It seems like the yellow tap would be the one to use with 6V6s (or EL84s as the original had) with a 8 ohm speaker
While with 6V6s or El84s the green tap would be more suited for a 16 ohm speaker (or pair of 8s in series what have you)

Is this math right?

I'm just wiring up an external speaker jack and I want to confirm this.


Thanks!

[PRR]

Your measurements don't align with Bogen's notes: how did they get 2.5 Ohms?

We expect to see a ratio nearer 56:1.

Putting 10VAC on a speaker winding is pretty high level. Try 10V on the *primary* and measure all three possible pairs of secondary connections. The secondaries will be a few tenths of a Volt. A modern DMM will read this fine if the frequency is below 400Hz.

[Toxophilite]

Yes that's why I was looking for confirmation as it doesn't match up with the schematic, unless the OT is a replacement. It was fastened to the chassis with sheet metal screws rather than bolts but that could've been original.


I would use 1 VAC but on my variac it's easier to get an accurate 10VAC
I wonder if both taps were meant to be used simultaneously to achieve  a 3 speaker set up?

[sluckey]

I would use 1 VAC but on my variac it's easier to get an accurate 10VAC

TIP... Connect a filament transformer to the variac when you need  precision low voltage from a variac. Adjust the variac for the precision low voltage on the secondary of the transformer. Any stepdown transformer will do, even a doorbell transformer.

[Toxophilite]

Tried S.Luckey's tip (thanks!) regarding a stepdown transformer after the variac and put 1 VAC on the secondaries and got similar results of 22 VAC (22-1) for the green tap and 29 VAC (29-1) for the yellow tap.


I'm thinking this is either a replacement OT or Bogen changed the design for whatever reason (parts availability, rethink whatever) sometime after this manual was issued.

This OT has another wire (orange) on the secondary side which was hooked up somewhere (stupidly I forgot to label it before detaching) but it is also not shown on the schematic at all.

[sluckey]

This OT has another wire (orange) on the secondary side which was hooked up somewhere (stupidly I forgot to label it before detaching) but it is also not shown on the schematic at all.

Connect one ohm meter probe to the orange wire then see if it has continuity to any other wires. Also check for continuity to the metal parts of the OT. Just may get a clue about that wire.

[Toxophilite]

The orange wire has continuity to all the secondary wires (both taps and ground) and of course chassis ground.

[Toxophilite]

OKay mystery likely solved..duh! Ran 50 VAC into the primaries and got .93VAC....53-1 from the orange wire...to make sure tried it on the yellow wire and got 1.65 VAC (30-1) as expected


Kind of embarrassing but  something must've happened during my initial test that skewed my thinking about this


It seems the orange wire is the 2.5 ohm tap
Just tested it through the speaker and bingo.
So the non-listed tap on the schematic would seem to be the 16 ohm tap
What an interesting selection of taps, now I just need a three position switch. I could run both my mini leslies!

I am currently trying to debug my smart car injector rail..I only wish it were this straight forward....