Clueless Idiots and High Voltage Vacuum Tubes Really Do Mix!

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Medalist Debug – A Test Drive

May 4th, 2011 · 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” Vernazza ( can be heard hammering it on my
YouTube clip here. Sadly, this cuts off early, but the full five minute MP3 is here : JohnnyV puts the medalist through its paces

So $175 with shipping off ebay and $30 or so in parts and I have a pure Class A, SE amp that simply rocks.

I’ll post more on the engineering side soon, but I thought I should make it clear that whether or not I understand it, it sounds fantastic.

Also.. you really should think about lessons from Johnny V if you live in the So Cal OC/SD area.

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Debugging the Medalist – More Weirdness

April 23rd, 2011 · 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, the output tube grid should not be more than 7 volts or so.

The real problem now is that every thing seems to be running to spec (other than that wacky grid problem). Lets break it down.
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Debugging The Medalist – The Output Tranny

April 7th, 2011 · 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 widely, with little by way of information about why or when. We here at Paleoelectronics [snicker] are familiar with that.
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Debugging the Medalist – Mistakes Were Made

April 7th, 2011 · Uncategorized

As I try to figure out what is wrong without blowing more 6BQ5’s to do it, I have to acknowledge that I rushed a couple of things in my excitement. I guess I’ll never really be a radio man.

Whether or not those mistakes caused the failure is not likely to be known. However, those mistakes did cost me in valuable information. If I had gone through the amp a bit more thoroughly, it is possible I would have detected a condition that caused the blowout. It’s very possible that I would have avoided blowing the resistors since I would likely have not run the amp (especially dimed) and it is certain I would have more information now to work with.

What mistakes?

  • I did not measure grid voltages. Especially in the output tube, where the plate and cathode values are fine, the grid input could well have been the problem if indeed the problem was external to the tube.
  • I did not figure out that I knew what the design voltages were until after ( I should have checked the MSM schematic against the one I got. )
  • I did not scope a clean signal through the amp.

These wants shall be rectified.

“Live and don’t learn, I always say!” – Calvin

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Debugging the Medalist – Intelligent Design

April 5th, 2011 · 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 very hard to read. Here is the critical point, that 260V goes through the output transformer and loses a bit before it gets to the plate. Lets presume it drops 10V because it looks like the test reading on the plate is 250V. The 250V source off the 3.3KΩ goes directly to the screen. Those are exactly the GE design center values for this tube. 250/250 and -7.3 grid. These numbers make perfect sense. Most engineers would have used the design center values unless they were forced to change.

Lets see how things looked before the blow out.

Test Point Name Value Units Expected Notes
TP1 PT 2nd 255 VAC 260 Expected is somewhat dubious, very hard to
TP2 PT 2nd 255 VAC 260 read on the schematic
TP3 Rect. Out 284 VDC 260 A bit hot here, probably my AC is hot.
TP4 Heater 6.43 VAC Heater
TP5 POW Screen 262 VDC 250
TP5.5 POW Plate 263.6 VDC Not Given. SB slightly less than TP3
TP6 POW Cathode 7.79 VDC Not readable.

Pretty decent agreement really. I’m running a bit hot, but well within the usual 10% slop that should be tolerated. So I still don’t know what happened, but I am really starting to believe the mechanical failure theory. Parts arrive in a couple of days.. maybe I will get in a power section analysis before then.

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A Tale of Two Schematics

April 4th, 2011 · 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 :


67 (ish) GA-5T Schematic

Here is the one Patrick got :

Skylark GA-5T from MSM

Now… If you’ll recall, I argued that the schematic given to Patrick could not have been correct for a ’67 Skylark, because the MSM was published in ’66.

Well now, that’s just silly isn’t it? Two more pieces to add to the puzzle. One : That whole section of the manual is about Medalist amplifiers so the amp is perhaps ’66 or the manual just included the amps that hadn’t made it to market yet. Two : The schematics are almost identical circuits. I now claim that these are just different versions of the medalist series.

I have identified only two differences so far. The bias resistor in the MSM is 2.2kΩ and ‘my’ schematic is 4.7kΩ. This results in a slightly higher bias voltage of 1.65V compared to the nominal 1.0V found in earlier amps with the 2.2kΩ. The other difference is actually inside our old friend the 102C84 Sprague network. C2 in the MSM and C6 in mine. In the MSM this is .0047µF and in mine it is .003µF. Very small differences. This is great news.

Why? Because the one in the MSM has Voltages!! Hot diggity, I needed those. Not just voltages but exactly the ones we would expect. They are very hard to read, but they will be invaluable.

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Debugging the Medalist – What Happened?

April 3rd, 2011 · 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 time. I’m not sure, but I might have pulled back the guitar volume and then maxed the amp volume to see how that sounds. Rings a bell, but I’m not sure that is what was happening when it blew. Oh well, let us have look at the damage, starting with the blown tube.

Blow tube

Blown tube with good tube for comparison

As you can see the flashing (chrome looking area on top of the tube) is completely burned off. There are lost of scorch marks on the tube and the metal inside. I was not kidding when I said this thing failed spectacularly. Now the obvious resistor. The big one is R11 and the little one R13.

Blown Resistor

Blown Resistor

So naturally, I went through and re-checked all the resistors. Here is what I got :

Resistor Expected (Ω) Value (Ω) Diff Notes
R11 150.0 275.0 83.33% Visibly Blown
R13 3,300.0 4,150.0 25.76% Way Off
R12 100,000.0 156,800.0 56.80% Way Off
R10 330,000.0 364,800.0 10.55%
R9 100,000.0 105,200.0 5.20%
R4 100,000.0 108,200.0 8.20%
R3 4,700.0 4,960.0 5.53%
R1 47,000.0 44,300.0 -5.74%
R2 47,000.0 44,100.0 -6.17%
R15 68,000.0 73,200.0 7.65%
R16 510,000.0 527,000.0 3.33%
R17 1,500,000.0 1,572,000.0 4.80%
R18 4,700.0 5,030.0 7.02%

Gonna do some shopping as I don’t have those 150Ω 1 Watts handy. Of course I don’t know if I’ve blown any caps as well. If I had a capacitance tester, I could probably test many of them in circuit because pulling the tubes opens a lot of those lines. I did put in the rectifier and and run some quick voltage tests.

Unloaded (no tubes except 6X4 rectifier) :

Test Point Value Units
TP1 270.8 VAC
TP2 270.8 VAC
TP3 375.9 VDC

Again, that’s unloaded (nothing drawing current). Looks like a pretty standard 270 – 0 – 270 transformer so that is fine, but that 375 B+ supply is pretty high. The EL84/6BQ5 is design centered for a plate voltage of 250 and max 300. So I loaded it and watched as the other tubes warmed up. Starts at 375 and then drops to 305 VDC and that is before the filtering stages. It should at least be at or under the max. Generally 10% over max is not considered dangerous.

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Debugging the Medalist – F#%$!!

March 31st, 2011 · 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 is that we didn’t get any voltages from Gibson. That’s not normal as most of the schematics I have seen have a bit of voltage reference.

Blue for a reason!

Ok.. so I took some readings after the caps were in. Sorry to do this as a picture, but I can’t for the life of me get a decent looking table out of wordpress.

Voltage Measurements after recapping

Voltage Measurements after recapping

Basically, everything looks reasonable. Max ratings for this tube are 300V on Plate and Screen with normal Class A operation at 250. The cathode is 7.3 V. No real surprises in the other tubes. So lets see if we have actually cured our little ripple problem. First, please recall that we could not even scope the ripple at the first filter cap before. It was way out of the range of my scope and I didn’t want to blow the damn thing. Here is the new picture at TP3 (right off the rectifier)

10 V AC Ripple after the first filter cap

Oh.. that’s nice. Just like the one we saw when I alligator clipped the cap in place. That’s 10 V (p-p) just like before and it’s at 2 mSec ( ~ 120 hz, double the standard house frequency of 60 hz, just as expected). Ok, great, moving on to TP5 which has the second filter cap (this time a 10μF).

0.3 Volts ripple at TP5

Had to dial it way back to find this ripple. 0.2 V per division makes that about .3 V ripple. On a 262V line, that’s essentially nothing, unless of course you can hear it. Recall that this was large and weird looking previously. Not sure exactly why this looks more curvy than the sawtooth above, but I think that’s the right curve, I saw it on the internets.

Finally, on to TP9 (I skipped 7 because it should be exactly the same as 9). TP9 is the most sensitive part of the amp, the first preamp stage getting tiny signals from the guitar. Any interference here will be amplified a hundredfold.

TP9 AC Ripple 0.01 or less

So.. that’s 0.01 or less ripple on a 173 V DC rail. I think we can say we successfully cleaned up the power supply, so we button her up and see if we can make some music.

And we can! Sweet sweet music. Much louder than I expected. Very clean, remarkably clear note definition. Bass is good, and the amp is much brighter than the Crestline and the current GA20RVT. The tremolo is outstanding! Couldn’t dime it at 12:30 pm, so I’m not sure, but I don’t think this thing breaks up very early.

Well.. let me amend that statement.. it broke up all right.. after about 10 minutes of noodling the power tube failed spectacularly. Arcing and flashing, horrible noises..

F#%$!! It could well be the original tube, which would make it 44 years old, oh well, back to the bench.

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Debugging the Medalist – Power Supply Filtering

March 28th, 2011 · 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 wasn’t deformed or leaking and since other parts weren’t exploding it seemed reasonable to assume that the cap had blown open.

I figured a good test that I could do without breaking out the iron would be to clip lead a 20μF into the circuit right there. I’m sure there are good reasons not to do this. However, I don’t know them so it must be safe! Clip it in, on goes the amp and scope (on TP3) and voila! Look what we get.

TP3 with 20mF clipped in

Lookit that beauty! Now that is a rectified signal! I wonder if that is how signal generators used to make a sawtooth.. I couldn’t even measure ripple here before and now I’m seeing a very nicely tamed signal. That’s 10 volts peak to peak, well under 5% ripple for the 256 VDC we got here last time we measured. That’s got win written all over it. It doesn’t curve on the up swing like the simulations in the previous post but that is a flaw in those sketches. The cap charges essentially linearly (for that short a time span) and that is what we see here.

Now, this isn’t terribly surprising and I even fingered the culprit when I opened the case for the first time. Those funky caps just go. I’m glad I kept the old 6X4 tube, maybe it was fine after all. The other sections of the can might be good but it’s only a matter of time and that thing is 44 years old. Luckily, I keep a supply of 500V 20μF and 10μF caps around for just such an occasion. Maybe tomorrow night.

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Sidebar : Rectification and Filtering

March 28th, 2011 · 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 just adding one more ‘ode (as in anode and cathode).

Just for fun, lets have a look at a rectified signal and it’s smoothed version (click to go big):

A full wave rectified signal and it's smoothed output

So the dashed line is our AC sine wave that has come out of the rectifier. In our case, it is 256 V (peak) (@120Hz as expected) and you can think about it as the original sine wave from our 120V 60Hz wall socket stepped up in voltage and folded on itself so that the negative peaks become positive (read about full wave rectification or bridges to understand how that works). We expected something like that red line, and what we got was ugly.

That dashed line would be a total disaster in our amps because the amplified sound signal is carried on the same lines as the DC voltage used to power the tubes. We need that smoothed, and the capacitor does that for us, in fact, there are three of them (all in that suspect cap can) that progressively smooth the signal. Lets check the schematic (as always, click it for full size) :

The B+ Rail and it's filtering caps

Next up.. back to the scope.

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