Magnatone Troubadour 113 Output Transformer Repair/Replacement

[FPALB]

A 113 came in with a damaged output transformer (see pictures) primaries measured (with a Fluke DMM) 400k on one side and 500ohms on the other - both to C.T.. Once I dissembled the OT, cleaned up the burns, and found the lead now both sides measure around 500ohms to CT. To my knowledge a push-pull 6v6 transformer should have around an 8k to C.T. primary side - my question: this 500ohm to C.T. im measuring from the 113 isn't normal is it? Want to find if this is repairable or if I should replace it with a clone or with a hammond 1750E ( link also below), which is a 15W 8500 C.T. Princeton output transformer and what I feel like is a bit more standard for this type of amp.

The clone is pricey from Mercury Magnetics, but im unsure if the hammond would work at all!

Only schematic I could find is for the 213 troubadour which has a stage that includes the Magnatone varistors. - the 113 does not have this stage.

https://www.hammfg.com/part/1750E

THanks!

 

[fazer]

Your measurements are dc resistance you quote. 8k is the primary impedance. It’s turns ratio derived from knowing impedance (not DC resistance) of the secondary. Search testing an output transformer on internet. Plenty of articles.

 

[Rusan]

On that type of transformer, you'll see anywhere from about 300 to 500 ohms from the CT to the ends, but they'll never share two precisely identical ohms readings because of the way the primary is wound. As fazer said, what you want is the reflected secondary-to-primary impedance, not the DC resistance. Here's a good, well-written resource explaining it.

Personally, I'm not a fan of Mercury OPTs and would only use one here if no other option existed. If there's not a Hammond cross-reference and the repair time isn't critical, I'd call Heyboer with the physical dimensions and desired leadout colors, and have them build one. CJ is the resident expert on guitar amp iron; maybe he'll chime in here with a better solution.

 

[FPALB]

Thanks for letting me know about measuring and calculating correctly.

Sent in 5V on the primary got .160V on the secondary
Turns Ratio: 5/.160=31.25
Impedance ratio = Turns ratio squared = 961
Load impedance x transformer impedance = 8 x 961 = 7688
and there is our 8k.

thanks folks.

In this case it seems like when I cleaned up the transformer's burns the measurement also cleaned up - would I be safe to re install and try it out again? Ive already added a new lead, dressed the outside and re coated in polyurethane.

The hammond 1750E is 8500 ohm impedance is that close enough?

 

[Rusan]

In this case it seems like when I cleaned up the transformer's burns the measurement also cleaned up - would I be safe to re install and try it out again? Ive already added a new lead, dressed the outside and re coated in polyurethane.

Don't forget to check that the end-to-end primary DCR is indeed exactly the sum of the two CT-to-end readings, that no shorts to the secondary exist, and that there's an ohm or two DCR across the secondary. Since everything worked out okay with the winding ratio test, I'm sure all is good. Still, even if all looks good when checked with a DMM, there might be a significant danger of arcing when presented with the full B+ voltage, especially now that some of the primary winding's insulation has been overheated and compromised.

The hammond 1750E is 8500 ohm impedance is that close enough?

I'd say it's probably plenty close enough. The only hesitation I'd have is that unless the Hammond's footprint makes it a drop-in replacement, I'd go another route. I personally refuse to drill new holes in cool old vintage gear, unless there's just absolutely no other way at all. But, that's me; I'm psychotically OC about that stuff...

Although it won't really affect anything here, in my way of thinking there's something else worth mentioning. Strive to leave no stone unturned when replacing a component with one that's not an exact equivalent electrically, because sometimes it does matter. Because this amp was built back when the North American AC line standard was 110 volts, the B+ is already theoretically about 9% higher than the OEM intended when operated on today's 120 VAC. The Hammond 1750E's primary DCR is much lower than the stock OPT's, which means less Ohm's Law voltage drop across it, and a yet even higher voltage at the 6V6's plates. The cathode bias here will negate most of it, but there'll still be a subtle shift in the operating point. It may affect the tone, though the new OPT almost surely will.

Were it a grid-biased amp, it would at the least necessitate rebiasing, plus the fact that the actual plate voltage is the plate to 0V ground voltage, not the plate-to-cathode voltage as it is with cathode bias. With today's tube production, that can become an area of concern on amps that already run at super high voltages anyway, like silverface Fenders with ultralinear output stages, and many vintage Ampegs.

Good luck!

 

[CJ]

You might have a problem with the amplifier, used your repaired transformer to find out. Plug it in and see if one of the power tubes is running hot is red playing. If power tubes lose bias or run with positive grid voltage from a leaky coupling CSP then on side o the push pull circuit will go into thermal runaway and cook one side of the output transformer. But since you now have equal resistance from leg to leg I am thinking that it might have been an arc over on the lead splice cardboard. If that us the case, you might he good to go. Make sure you get all the carbon out if there because it will conduct like a carbon resistor. I would replace the lead plate, un-staple or take the tape tape the leads and place a new piece of cardboard under there. Then tape it up and hit it with the varnish.

 

[FPALB]

Replaced couplers because they were discs and slightly microphonic on one side. No red plating at the moment cleaned as much of the carbon out as I could.

Im now getting what sounds like some sort of feedback when I have the input stage pre amp tube in the circuit. When I remove it the sound goes away. Volume controls on both channels are also now interacting?? Seems very unrelated to the original issue, but im having trouble tracking it down.

As stated in the intro ,the schematic doesn't completely match - the big difference im seeing is that there is only preamp tube stages for instrument in, microphone in, and the oscillator circuit tremolo circuit which outputted to the cathode of the A side of the pre amp tube that is shared with the phase inverter. Im redrawing it by hand now to try and get some clarity.

 

[FPALB]

Here is the preamp section and a sound bite of what im hearing. This is with just the mic/inst input tube in phase inverter tube. I am running a 1k signal into the instrument in. Same issue when I plug in fresh 12ax7

There also a significant amount of 60hz im hearing, but im not sure if that was always an issue or not - main lines look well filtered with about 3v of triangle ac ripple on power tube B+

This is my first time working with elevated heaters, so I could be missing something. The heater center tap is attached to the cathode of the power tubes - ~6V, but its more of a jumpy saw wave

 

[FPALB]

Ah - chopsticking around is showing show big changes in noise when I move wires. I will swap to shielded and report back.

 

[CJ]

microphonic tube?, chopstick the tubes, wires from OPT swapped?
grounds? tighten pots,

 

[FPALB]

microphonic tube?, chopstick the tubes, wires from OPT swapped?
grounds? tighten pots,

The heater on Pin 9 of V1 was causing all sorts of havoc on the input stages. I cut the heater wire and moved it far away as it was sorta in the thick of the wire spaghetti and that helped at lower volumes, but not when the input was cranked. I attached another drawing of the input stage - the amp has a capacitor in parallel with the cathode resistor of each input preamp stage. Im assuming these are voicing filters for the different channels? Once I removed the capacitor on each cathode the heater noise died completely. Is this because the capacitor and the resistor are creating an RF loop due to the component layout? Attached a picture of how the capacitor is piggy backing the resistor.