Alamo Futura

[stereomonostereo]

Hey guys,

Been working on a 1960's Alamo Futura that's had some butchering done to it. Came in with no vibrato or reverb, and not sounding too healthy. Long story short, I replaced the incorrect output transformer, and sorted out the vibrato circuit to get that working again. I've traced the reverb problem to a bad transistor in the return feed. It's a tube amp, but the transistor is used in the reverb amplification circuit. Anyhow, the markings on the transistor read, 'T1417 980421'. I can't seem to find a replacement, or a suitable cross reference. Any help would be greatly appreciated. Thanks!

[52 Bill]

Anyhow, the markings on the transistor read, 'T1417 980421'. I can't seem to find a replacement, or a suitable cross reference. Any help would be greatly appreciated. Thanks!

You need to answer a few questions to figure out what to replace it with. What type of device is it? Silicon, germanium, bipolar, fet, etc.

Polarity? NPN, PNP

Voltage rating?

I know that schematics are rare for these amps, but you could draw out the reverb return circuit and guesstimate from there what the circuit needs to work. If you share the schematic and a photo or two, we can all add our two cents worth.

[stereomonostereo]

It's silicon, and I 'believe' it's NPN.

My voltages are:
C: 1.8v
B: 11.8v
E: 1.2v

That's all I've got. I can't find a data sheet anywhere. I'll try to draw up the circuit when I get a chance. Thanks for the reply

[gbono]

VCC=?
What does the package too like? metal can, a "milk dud" with 3 leads, etc

[stereomonostereo]

Typical silicon, I believe. It's near the center of the photo:

[Enzo]

Looks like silicon, and if your voltages are positive, then NPN. You won't find a data sheet for that house number.

Here is a different Alamo tube amp with transistor reverb, is it close enough?

http://www.webphix.com/schematic%20h...o_jet_2564.pdf

Find power supply, but with 12v on the collector, I imagine a 25, 35, 50v part would work. Most any audio type transistor.

Turn the amp on and the reverb up. Touch the base lead of the return transistor you suspect. NO hum out the speaker? If you hear it, the part is working. Monitor the collector with a scope or signal tracer and see if the signal is there when you touch the base.

[stereomonostereo]

So you're saying I could sub in most any silicon NPN amplifier transistor, provided it's in the realm of 25-50v?

[Enzo]

Pretty much. I have a large selection here, so I would probably grab a low noise type, but even common 2N3904 ought to work. If your old ones were 2Nxxxx, then they should drop in. If your old ones were BC series or 2SC series, the legs are in different order.


However, I'd do enough tests to KNOW it was the transistor itself.

[gbono]

Have you checked the reverb tank?

[stereomonostereo]

I went with the 2N222 and was getting a signal, but it was much weaker than it should have been. Hunted around a bit more and ended up using a 2N5461. It brought it right back to life. Now, the 2N5461 is listed as a P-Chan, which I believe is different from a NPN? (Still relatively new to transistors)

[gbono]

2N5461 = P channel JFET (junction field effect transistor) very similar to a tube in terms of how you bias it. The pinout is source, drain and gate - from left to right "flat" side of package facing you (leads down - i.e. top view). Very different than a 2N2222 bipolar transistor.

[stereomonostereo]

Thanks a ton guys. Still learning everyday!