Danelectro Cadet 123 needs OT

[misterc57]

I just picked up this Dano and opened it up this morning. The previous owner said it had very low output. From what I am seeing at some point the speaker and filter caps were replaced, but I do not see an output transformer. I think the original speaker had the OT attached to it. So whoever replaced the speaker might not have realized they needed to save the OT.

I fear turning this on to hear for myself as I do not want to damage anything. I would imagine the missing OT is the reason there is low output.

I have no experience with missing OTs or how to choose a replacement OT. I may have some sitting around my basement, need to search. What do I need to look for? Winding ratios, resistance, made for a 50C5 output tube, etc?

Schematic attached for Silvertone 1430. The Dano does not have the T1 transformer shown in the schematic. That aside I think everything is identical.

Thank you. MC

[Jazz P Bass]

The output transformer was mounted to the speaker.


FYI: this amp configuration needs an isolation transformer to be operated safely.
The chassis is at mains potential.

[xtian]

Looks like T1’s purpose is to supply 12vAC for the 12AU7’s heaters. So, the amp cannot work without it. I don’t know what’s going on with your amp, but I will say, this amp is deadly and can kill you! If you plug it into your wall voltage with the incorrect orientation of the power plug (50/50 chance...) you’ll have 120vAC on your guitar strings.

EDIT: Jazz beat me to it.

[Jazz P Bass]

Here is a replacement OT: "All American Five" Audio Output Transformer (Item: RDX-300B) - Radio Daze LLC

If you do not have the preamp transformer, then all of the heaters should be in series.


Here is good read on the iso transformer: Isolation Transformer Upgrade for Old Guitar Amps: 11 Steps (with Pictures)

[misterc57]

All the heaters are in series.

Currently my plan is to install a 3 prong cord (to eliminate the shock hazard) and an output transformer.

MC

[misterc57]

In the schematic, what does the 180 ohms on the primary side of the OT represent? Is this a consideration on a replacement?

Thank you

[Jazz P Bass]

Currently my plan is to install a 3 prong cord (to eliminate the shock hazard) MC

That is not going to work.
Line ground & chassis ground are the same point.
You Must install an isolation transformer.
Must.

[misterc57]

That is not going to work.
Line ground & chassis ground are the same point.
You Must install an isolation transformer.
Must.

I see what you mean now. Thank you

[misterc57]

The schematic shows 180 ohms on the primary side of the OT. The replacement AA5 transformer has a spec of 1.6K to 2K ohms. I am trying to understand what this means and if the OT would be suitable.

Mouser has OT with primary impedance ranging from 48 ohms to 10K ohms. I need to learn more about this as I am always over thinking everything.

Thank you

[octal]

The schematic is showing the DC resistance of the winding for test/troubleshooting purposes. For design/replacement output transformer purposes, the only specs which matter are the primary and secondary impedances of the transformer and its power rating.

[Jazz P Bass]

The schematic shows the dc resistance of the transformer primary winding.
Useful in troubleshooting.
Useless in engineering.
It's all about the 'turns ratio' & the reflected load that the speaker puts back onto the output tube.

Without going over the theory (plenty on the web), you want to match the output tube primary load impedance to the secondary load (ie: the speaker)
Attached is the 50C5 datasheet.
The AA5 that I linked to would work very well into a 4 ohm speaker.
(or 8 as it has multi tap output)

50c5.pdf