OT transformer secondary issues

[ThisLifeILead]

howdy folks, i got a silly question.
I am restoring an old tube bass amp i found locally on craigslist. Massive hammond iron. I emailed hammond and they indicated the units used in the amp were custom ordered. anywhoo. When trying to determine the windings ratio i found on one of the secondary taps the ratio is way off and its eating up some of the input voltage. (sags to 13.2V from 15.3v the other tap there is no sag in input voltage) one tap gives me a good turn ratio of 18.5:1 (8ohm) the other is giving me 38:1 (which would work for a 2ohm load but its labelled as 16ohms).
Can i just remove the winding that seems to be fried and just use the 8ohm tap or will that eventually lead to more problems down the line?
I just recapped it and i was about to install new tubes after i figured out the winding ratios.

[nickb]

Could you detail exactly how you set up and did these measurements: it's not clear to me what you mean by 'eating up the input voltage' ?

[ThisLifeILead]

using a 12Vac wall wart i injected the voltage through the secondary jacks and measured the voltage across the plate leads. i have to go secondary to primary due to my shitty DMM. I checked the secondary voltage on the wall wart and on the one tap it drops 2vac. ive used this wallwart on dozens of OT's and ive never seen the input voltage sag before. My guess is that secondary winding is shot.
a 38:1 ratio would give you a reflected load of 1441ohms x speaker ohms. (this tap is labelled 16ohms!!)

[J M Fahey]

If "16 ohms" drops more than "8 ohms" then it is not "16 ohms".
Just sayin'
Probably misidentified tap.

or you are loading the secondary (plate side) .
Or vthat amp has protective diodes.

[ThisLifeILead]

If "16 ohms" drops more than "8 ohms" then it is not "16 ohms".
Just sayin'
Probably misidentified tap.

or you are loading the secondary (plate side) .
Or vthat amp has protective diodes.

no diodes. the chassis is silkscreened with impedance.

OT secondary is speaker outs not plate side.

Im just curious if its possible to tape off the suspect OT speaker out and use the winding that tests good?

[nickb]

using a 12Vac wall wart i injected the voltage through the secondary jacks and measured the voltage across the plate leads. i have to go secondary to primary due to my shitty DMM. I checked the secondary voltage on the wall wart and on the one tap it drops 2vac. ive used this wallwart on dozens of OT's and ive never seen the input voltage sag before. My guess is that secondary winding is shot.
a 38:1 ratio would give you a reflected load of 1441ohms x speaker ohms. (this tap is labelled 16ohms!!)

OK. thx.

Just to get a ballpark figure, if I take a 'typical' OT with primary 5K and inductance of 20H then an 8 ohm secondary will have an inductance of 32mH. Ignoring core losses that implies a magnetizing current of about 1A rms. Your 'bad' tap has a about 1/4 the inductance of the first tap so the current will be x4. So, I think you could well be hitting the limits of the power supply's capability.

Not that this helps you much with the impedance labelling anomaly...

What can you tell us about the amp that might help estimate what the primary impedance might have been?

Could the OPT have been replaced / re-wired so that the output Z markings are wrong?

[52 Bill]

When doing your testing, are the windings still connected to the circuit or are they all disconnected?

[ThisLifeILead]

When doing your testing, are the windings still connected to the circuit or are they all disconnected?

everything is connected, but no output tubes. and the two OT secondary taps have their own non shorting jacks. so the only wire really connected is the Primary CT.

[ThisLifeILead]

the OTs were made my hammond on a custom order. ive seen two of these exact bassman style clones locally (including mine that i bought at a salvage yard for 100$). my guess mid to late 70's as they used flea clip board construction. and CDE filter caps. the iron is massive!!! BUT the OT secondary colors are green and yellow. typical 4/8 colors.

[g1]

Try the neon bulb tester on it.
If one tap is bad, then I don't think it will be usable.

[ThisLifeILead]

Try the neon bulb tester on it.
If one tap is bad, then I don't think it will be usable.

im going to plug the wallwart into my bulb current limiter and test it. i got a feeling the 16ohm tap is friend. prob got a 4ohm speaker plugged into it and fried. It worked for alittle while before blowing the fuse. it hummed so thats why i recapped it but it was friggin loud!
Its just weird to see the standard yellow/green (8/4) taps but labelled 16/8. i mean it could have been a mislabel but i just dont think so. the wiring is very neat and laid out well BUT everything has its own chassis ground. almost seems like someone with limited knowledge built it. they grounded both ends of the shielded wire. but yet they used sealed ohmite pots and high end russian tone caps.

[52 Bill]

No negative feedback?

[pdf64]

im going to plug the wallwart into my bulb current limiter and test it. i got a feeling the 16ohm tap is friend. prob got a 4ohm speaker plugged into it and fried. It worked for alittle while before blowing the fuse. it hummed so thats why i recapped it but it was friggin loud!
Its just weird to see the standard yellow/green (8/4) taps but labelled 16/8. i mean it could have been a mislabel but i just dont think so. the wiring is very neat and laid out well BUT everything has its own chassis ground. almost seems like someone with limited knowledge built it. they grounded both ends of the shielded wire. but yet they used sealed ohmite pots and high end russian tone caps.

See http://www.hammondmfg.com/pdf/EDB1760L.pdf and http://www.hammondmfg.com/pdf/EDB1750M.pdf
White on the 1760L is 16 ohms, whereas it's 2 ohms on the 1750M, 8 ohms is green on one, yellow on the other.
The apparent mis-wiring may have been a mix up between Hammond and the manufacturer.
So the colour is no big deal, just work with what you've got according to the ratios measured.

Regarding the ratio measurement, how do they work out if you feed the 12Vac into the primary?

[Enzo]

When he mentioned the neon bulb test, he was NOT referring to the light bulb limiter, he was talking about a ringing test. Go to RG's Geofex site and find the transformer tester. Very simple.

I doubt your 16 ohm winding is fried. All the windings are linked, so if one winding or winding segment is shunted or otherwise screwed up, it will affect ALL the windings. Transformers are like gears, if you jam one gear, all the gears stop.

Fahey's earlier point was you could be loading the primary side. You are driving the secondary with your test voltage, but the primary is still linked to the secondary, and things like NFB etc can affect it as well as stuff connected to the primary directly.

My own opinion, for what it is worth, is that your one mystery winding checked out as if it were 2 ohms, and also loads your test supply, yet it is labelled 16 ohms. I'd wager on that 16 ohms being incorrect, or the wrong wire is connected to that jack. Or someone changed wires to the jacks for their purpose, and THEY knew which jack was which even if it no longer matched the printed legend. If it were indeed the 2 ohm tap, then it matches your readings, and also explains it loading down the test supply.

In any case, you could send a test signal - something steady - through the amp, at a low level so it doesn;t threaten the OT. Measure output voltage between the common and each tap. The voltage will increase as impedance steps up. You can order the taps by voltage, and easily see which tap is which. or apply a voltage to the 16 tap and common, and use the OT like an autotransformer, measuring voltage from common to the other taps, which will again be in order of impedance voltage wise.

[ThisLifeILead]

I removed the NFB and retested. same results. I think you guys are right about the mislabelling. the "8ohm" tap reflected into an 8 ohm load results in an impedance of 2800. 4ohms would be 1500. but the "16ohm" tap if it was actually 2ohms would reflect back 2800 ohms. is it possible it was wired with 2 and 8 ohm taps?
I installed and biased up new JJ 6l6 tubes and took it for a rip! SUPER LOUD! but the tone controls are wonky and a quick glance indicates it was all miswired. looks like i will need to go back through the preamp. an impedance switch is in order as there is only 1 jack per secondary tap. need two to get down to 2ohms.

If i load the primary with a 12v wallwart i get similar results but not as accurate, as my DMM has no AC resolution. only 1 decimal place.