Maco 300 tubes

[Cable Guy]

Hi, I'm looking at the above amp and the final tubes are not 8950s, instead 26lx6 tubes. Now, the 8950 is a 13 volt tube, and the 26lx6 states, well, 26 volts, with less current draw. The transformer is supposed to put out 12v for the heaters, I haven't checked yet. I'm studying tube amps right now, so let's keep this scholastic.

Would the 26 volt tubes run at a lower voltage, assuming they can be supplied with more current? Is it likely someone has added another transformer or modified existing with a voltage doubler of sorts?

Thanks for the look.

 

[BJ radionut]

Is it likely someone has added another transformer or modified existing with a voltage doubler of sorts?

Yep...good logic...without modified info, hard to say how the amp circuitry has been change. Maybe post Top and Bottom interior chassis ...Hi-res pics?

https://cbtricks.org/Amp/maco/maco_300b/graphics/maco_300b_redraw_sch.pdf

Maco 300

I have a Maco 300 which takes seven 8950 tubes is there another tube that can replace the 8950. They are hard to find and expensive. Thanks

www.worldwidedx.com

 

[Cable Guy]

It appears the 26v tubes are used for drivers too. According to measurements, all sockets provide 26v AC to the heaters.
The low volt transformer has both 13v secondaries tied in series to produce 26 volts.

One of the drivers are shorted and must be replaced. I have 2 6LF6 tubes, 6 volt. Check my logic on this, as it is mostly comprised of pouring through threads here. If I rewired the drivers heaters in series, couldn't I grab 12 volts from one of the secondaries and use the 6 volt tubes?

Would the 6LF6 drivers work pushing 5 26LX6's? The capacitances are very close. I will put up some pics later, I'm mainly getting educated before making any moves. It works, after replacing the hv diode rectifier strings, but looks like someone used it to anchor a barge in the Mississippi river gumbo lol.

 

[Cable Guy]

Maybe post Top and Bottom interior chassis ...Hi-res pics?

I snapped some when I first opened it. Here they are:

 

[nomadradio]

The amplifier was built with only one power transformer. Looks to me as if a second filament transformer has been added, to put in series with the 13 Volts coming from the large one.

That is one rusty specimen on the inside. Reminds me of a 1950s ham transmitter in the Hamvention flea market. Sign said "Retired from service anchoring my boat".

73

 

[Cable Guy]

Okay, I will check and see if the transformers are daisy-chained. If the 26v is coming just from the added trans, I could also take my needed driver voltage from the original 12v winding, assuming it is still viable. Would keep the different voltages more isolated I guess.

 

[Cable Guy]

I rewired the drivers to the 12v tap and in series. She didn't blow up, and amp is working. Thanks for lending me your ear.

 

[nomadradio]

Rock on! Gotta love it when a plan comes together.

Congratulations for sticking with it.

73

 

[Cable Guy]

Yeah, ain't it great? Aside from the possibility of death from electric shock, these sweep tube amps aren't that scary, once demystified.

The hv transformer only had 3 wires on the secondary, 2 on primary, so im not sure its the factory installed xfmr as there was no low volt secondary windings. The smaller xfmr was an add on as well, as you said. I think someone overhauled this amp, retrofitting as they went.

73's

 

[Shadetree Mechanic]

Yeah, ain't it great? Aside from the possibility of death from electric shock, these sweep tube amps aren't that scary, once demystified.

The hv transformer only had 3 wires on the secondary, 2 on primary, so im not sure its the factory installed xfmr as there was no low volt secondary windings. The smaller xfmr was an add on as well, as you said. I think someone overhauled this amp, retrofitting as they went.

73's

Glad you got it going. I know the first time I saw the inside of a tube amp, I was surprised at how few parts were inside.

To the guys here sharing their knowledge freely, MANY THANKS from all the people trying to fix their stuff!