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Clueless Idiots and High Voltage Vacuum Tubes Really Do Mix!

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Entries Tagged as 'Amp Repair'

I Not Dead Still

October 7th, 2012 · No Comments · Amp Repair, Debug

A rusty pass through a Crestline debug leaves us with more questions than we started with.

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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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Sidebar : Graphing Resistances

May 22nd, 2011 · No Comments · Amp Repair, calculations, Datasheets, Schematics

I want to talk about the power section of my Medalist in detail (and some other things), but before I do that I’m going to give myself a refresher course on tube data sheets and graphing resistances. Recall, if you will, that V = I * R.  An equation in three variables.  If any one […]

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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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