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Clueless Idiots and High Voltage Vacuum Tubes Really Do Mix!

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Entries Tagged as 'Medalist Debug'

Replacing the Cap Can

February 20th, 2012 · 8 Comments · Amp Repair, Medalist Debug

The first things to go generally on these old amps is the power filter capacitors and these are often all rolled into one single ‘Cap Can’. Here’s mine in place in a GA-5 Medalist. These are (IIRC) wax paper and foil capacitors and are sealed with wax. It is electrolytic but has a single ground […]

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Medalist Debug – A Test Drive

May 4th, 2011 · No Comments · Amp Repair, Medalist Debug, sounds

So, I’m still working on posts for the calculations and all that and I’m still uncomfortable with the running voltages but it seems to be doing what it was designed to do, so hell, let’s hear it! As luck would have it, I got a genuine guitar legend to try the thing out! Johnny “V” […]

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Debugging the Medalist – More Weirdness

April 23rd, 2011 · No Comments · Amp Repair, calculations, Medalist Debug

So I replaced the bad resistors and took so measurements. Basically, everything looks perfectly reasonable, and it sounds great at low volumes, but I have one major problem. Anywhere past 3 or 4 on the volume knob slams the output tube with up to 30 V (rms) on the grid! For reasons I may show, […]

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Debugging The Medalist – The Output Tranny

April 7th, 2011 · 1 Comment · Amp Repair, calculations, Medalist Debug

Well, the parts have arrived, but first, I want to understand that output section a bit better so let us examine the OT. The always helpful Patrick at Mercury Magnetics suggested that the drop in replacement they make is the GA5-0 which has an 8kΩ primary to 8Ω secondary. He also noted that they varied […]

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Debugging the Medalist – Intelligent Design

April 5th, 2011 · No Comments · Amp Repair, calculations, Medalist Debug

As we now have some reference voltages to use (see previous post) we can take a lot of the guesswork out of our debugging. So here is the power section from the Master Service Manual. Those are design values. The test readings are in the top corner of that page (319 in the MSM) and […]

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A Tale of Two Schematics

April 4th, 2011 · No Comments · Amp Repair, calculations, Medalist Debug, Schematics

In the course of working on two different 5T ‘Medalist’ Skylarks, we have come across two different schematics. One of them came from blogfriend Patrick and is found in the 1966 publication of the Gibson Master Service Manual. The other was attached to the amp I bought. Here is the one I got : Here […]

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Debugging the Medalist – What Happened?

April 3rd, 2011 · 1 Comment · Amp Repair, calculations, Medalist Debug

I don’t really know what happened. My suspicion is that the tube blew, but the other possibility is that the cathode resistor blew, causing the tube to blow. Again it could very well be the 44 year old original tube and I might well be the first guy to run it for a long long […]

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Debugging the Medalist – F#%$!!

March 31st, 2011 · No Comments · Amp Repair, Medalist Debug

Ok. When last we met (do I say that every time?), we had decided that the power caps were bad. At least C4A and since those crap cans go bad all the time, I decided to go for them all. First lets have a go at the schematic with test points marked in. One thing […]

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Debugging the Medalist – Power Supply Filtering

March 28th, 2011 · 2 Comments · Amp Repair, Medalist Debug

As you might recall from two posts ago (“Scoping the Power Supply”) we were unable even to scope the power supply ripple at TP3 and saw a hideous deformed thing at TP5. The lack of filtering certainly indicates that C4A (the 20μF from the cap can) was blown. Since that big funky cap can itself […]

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Sidebar : Rectification and Filtering

March 28th, 2011 · No Comments · Amp Repair, calculations, Medalist Debug

Here is a nice little explanation of rectification and smoothing. Here is one that focuses on smoothing. This and most amps have full wave rectifiers. Our 6X4 tube is a twin diode and the PT is has a center tap on the secondary. Fun fact : The first vacuum tube was a diode. Triode is […]

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