67 Skylark

175 is a good price and 15 shipping is great. Only red flag is first time seller.

I’m not part of this deal at all (and I don’t need one) but if you do.. you could do worse.

Cap Can Closeup

These are (IIRC) wax paper and foil capacitors and are sealed with wax. It is electrolytic but has a single ground lead. And by the time you read this, it is already dead.

Replacing this (or original discrete capacitors) is the most common fix to these old amps and often is all that is required to get the amp working again.

Mine is 45 years old it it came to me dead (despite the sellers assurances that it plays perfect). No surprise there. I’ve recently had some questions asked about this job and since it is so common, lets cover it in some detail.

First, should you do it? If the amp works fine then it’s already been done or you have magic capacitors. Don’t do anything. If the amp starts off humming (after tube warm up of course) then you probably should do it. If you don’t want to know anything about the guts or don’t have any electronics skills then take it to an amp guy. I can’t recommend any, because I don’t use them, but most towns have someone. Ask at the mom and pop guitar shops, they’ll usually know.

The actual job is unsoldering 4 leads, and then soldering in 3 capacitors, six leads total. The tech will probably want to vacuum/dust in there. He/She should quote you no more than an hour of labor plus parts. The caps are $10 and $11 bucks each for 500V rated Sprague Atoms here and you really don’t need the best in the world. So I don’t know.. maybe a hundred bucks to redo the power caps and they should do some contact cleaning and tightening while they are at it.

However, if you were in the habit of paying professionals to do the job right, you wouldn’t be here. So lets get started.
Safety Preamble – Read me or die.

Buy Caps
I pointed to some replacement caps above and those are great but don’t get fooled by any ‘audiophile’ nonsense here. These are power filters and are not in the audio path. I would love to double blind someone with mediocre filter caps vs. audiophile but anyway, a decent cap should last you at least another 50 years. The general rule of thumb for voltage rating is double the expected voltage. These about 260V tops so 500V is good.

Preparation
You can’t really test the caps in place without de-soldering anyway so you might as well do that. Power down, unplug and bleed every cap you can reach. Lots of people bleed the power caps but leave the death cap energized. That can result in a tasty little shock. Not that I know from personal experience .. lately.

Find Cap Can and Leads
The picture above should be all you need to find the thing.. the blue cans are the replacements I already put in. Here is the power filter section from the MSM.

MSM Power Section GA-5

The cap can has those three capacitors (20/10/10) inside. First, the ground lead is black (you should be able to read it off the cap or from the picture). This is often tied to pin 2 of the 6X4 tube socket. The 6X4 doesn’t use pin 2 so builders would use the extras to avoid having to add another (or longer) tag strip.

De-solder Them
Try to get them out of there without damaging anything else connected.

Solder in the New Ones
You can just note the location of the ground and the other leads and re-solder the new caps in exactly the same spots. This will definitely work and in theory should be no worse than the original performance.

In a perfect world, each sub-circuit (power amp, pre amp, power etc) would have a single ground point (like star grounding) and all of those star ground points would be connected to a single chassis ground (galaxy). The earth ground from the plug would attach to a different point and this ground point can then be ‘lifted’ if you want to add a switch.

Since your amp is certainly not setup this way the best you can do is to ground the new cap with the section it belongs to.

Here is a notional (meaning no real thought) diagram of what grounds would be tied together in star/galaxy mode. If you ground your cap to one of the grounds used with the same color, you should be fine. However, you can’t be sure until you hear it.

Medalist Skylark 5T (2299) with possible star ground scheme

Why ? That’s a good question. I’ll wave my hands at that later.

I Not Dead, I Playing.

First though, lets look at a standard SE output section.

Champ output section

That’s from a late 50′s (5F1) Champ, but they are all the same until the Medalist Skylark shows up. I’ve cleaned it up a little of the wires we don’t care about. So Vs is the highest voltage in the amp. It’s 305 to 315 in most of the old schematics and 375 for the 50′s Gibsons if the schematic is to be believed. Vs flows through the primary of the OT and into the plate of the output tube. Those are DC voltages (in theory) and in a perfect transformer, this would have zero loss and zero effect on the secondary.

So the DC voltage runs through the transformer to the plate. We are now past my understanding of the transformers action in one regard. Transformers with Vs through them are called energized and I’m not sure why this is done. With no signal, that is DC and there should be no energy in the magnet or coil. But amp guys always talk about an ‘energized’ transformer. I’ll dig into the RDH4 soon and maybe get an answer.

I would have thought that it would be better to capacitively couple the transformer, which I would have thought would result in lower requirements for the transformer.. but ok. What do I know?

As I said, in theory, this is a DC voltage but in fact it is only after the first filter capacitor. You might recall that there is significant ripple on the DC from the first filter (see here where I got 10 V pp on a 260VDC line). I wonder if that alone isn’t enough to create output hum.

So anyway.. signal comes in, gets amplified and shows up on the plate of the output tube. This varying current energizes the tranny which transfers energy to the speaker. The screen voltage is set to a value slightly lower than the plate for maximum output. And you must always have resistor (1k will do) on that screen.

However, the tube/tranny area is fertile ground for amp tweaking, primarily by lowering the screen voltage. There are a few ways to do this but I’ve bored you enough for now.

That was easy! Ok .. moving on.. Wait.. What’s that you say? You want to know why? Really?

Well, alright.. if you are that bored.

As we saw before there is little variability out there for the output tube in these amps. 6V6 and 6BQ5 are very different tubes but they are rated roughly the same. I dug through a lot of low power tubes looking for, a) low power and b) availability and proven use. Didn’t really find much. 6G6 and stuff like that looked promising enough, but I had an epiphany that led me to the 6AQ5: I have a box, with the right sized Iron for 6AQ5s, with a socket in place, and all the room I need.

I’ll talk about the tube at length later, but since I am converting an amp that had two of them in P-P, I know I have the heater current, source voltages and OT ability to run one and then some. When I realized that this made some sense, I grabbed the 6AQ5c – EL90 Datasheet to see what kind of power the thing puts out. I knew the amp ran 8-10 watts in P-P so I figured the tube would be a little louder than I was hoping for. In fact it’s maximum signal output power is 4.5 watts @250V and 2 Watts @180V. The 6v6 is almost exactly the same tube but it can take a higher voltage on the plate (315V) and push 5.5 watts. Both Fender and Gibson SE amps of the 50′s pushed near 315V on a 6V6.

So, I would have liked to have used a tube with less output (so I can drive it hard at low volumes) but this will do nicely as everything is already set up for me.

The Bedroom Blaster.

My goal here is that creamy SE distortion at low bedroom volumes. I want low watt output, preferably even lower than the 5 Watt Skylarks and Champs. But maybe.. in the back of my mind.. I might want to boost that a bit sometimes. I also want a lot of flexibility in the amp topology. Basically, anything that can be switched, will.

Here is what most small SE amps look like :

Basic SE Topology

Input to preamp to power amp to speaker. Just about as basic as it gets. Very little changes between the various SE amps (at least the ones that I fool with).

Because I want to drive it hard, and I know a bit about SE amp topologies, I know that I want to add a second preamp stage to my amp. A single 12AX7 with its two triodes is easily capable of blasting past the limits of a 6V6 output tube, so we know we will have to find a way to contain two full stages.

Now if I want to double the output, I could, in theory have a second output tube in parallel and maintain Class A1 amplification. This is not the same as Push-Pull, which most two tube amps do, and is Class AB1 (usually). I think the GA-6 ‘Gibsonette’ used parallel output tubes in Class A SE config.

Here is a slightly expanded block diagram :

Songbird Block Diagram

No decisions yet about that second output tube, but I have an extra tube socket..

So now it’s time to design my dream SE bedroom amp. I’ve mentioned decisions and design considerations before, and following the songbird category will let you slog through all that if you like.

I’ve changed my mind on a few things and come up with some ideas. I’ll sketch here and some of these will be explored at length later.

• Low Power – This is a bedroom rocker
• Not going to build it as the amp computer, but will provide lots of switches and options. Too many, almost surely but that will be ok.
• I’m going to convert my Crestline Skylark, rather that tweaking a 50′s blonde Skylark. Couple of reasons here :
  1. I am adding a second gain stage, so I need the space.
  2. I have one here
  3. The transformers are perfect for what I want to do
  4. I want the extra space to work with, the chassis is almost double in size.
  5. I want the 10″ speaker (minimum)
• Second gain stage using a remote cutoff Pentode
• Pentode, Triode and lots of configurations for the output tube mode

The big thing here is the realization that the Crestline Skylark sitting next to me right now has half of the things I want in my new SE amp. Bigger speaker than the 8″ in the blonde, Plenty of space and lots of good iron. I was looking at damn near 200 bucks to just put iron into a box. For reasons I’ll make clear later, I have everything I need in front of me.

Recall, if you will, that V = I * R.  An equation in three variables.  If any one of them is held fixed, then you have a simple linear equation.  For example if we held R constant at say 4600 Ω, then we would have V = 4600 * I.    For giggles, I will choose to make I a function of V,  so I(V) = V / 4600. I choose a value for V and the equation tells me I. Lets look at a picture. Here we see resistances graphed for 500, 1000 and 5000Ω. Just like any other graph, you pick a value on the x axis, go straight up for to which ever resistance line you like, and then look across to read off the current through that resistance for the given voltage.

Simple graph of Various Resistances

That’s all well and good. The key point is that resistance is the slope of the line. The only thing is, in general, we graph voltage drops across resistors.

Circuit that needs graphing

In the example above, the bottom part of the circuit is likely connected to the plate of a tube or something and we frequently need to plot how the current changes the voltage across the resistor (thus telling us the actual plate voltage).

In order to graph a linear equation, we need two points. Let us calculate two simple points using v = i*R. We know R. We pick i = 0. No current flowing. With no current flowing, there is no drop in voltage so the voltage at the bottom of our little circuit is the same as that as the supply (200V). We can graph V=200, i=0 and that is one point. For the second point, we choose V=0, which means that all of the voltage has dropped through the resistor. i = V/R = 200/100k = 2mA. We can now draw a line between those two points and read any values we need from there.

Graphing Loads

Notice I also converted to milliamps.   By now it is surely obvious why I did all this, I want to start reading data sheets, at night.. to my kids.

67Skylark

As luck would have it, I got a genuine guitar legend to try the thing out! Johnny “V” Vernazza (http://johnnyv.net) 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.

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

Here is a look the amp in block form. I’ve left out the trem because it doesn’t (or shouldn’t) have much to do with this.

GA-5T Medalist Block Diagram

Input

Input is from the guitar pickups. Generally these range between 100mV and 1 V rms (1 volt would be powered active pickups). I’ve been using 200mV sine waves @ 1Khz ( I use Signal Suite for this but there are dozens of ways to do it.)

PRE1

PRE1 takes the input and boosts it to raise the voltage. This is the first half of V1 (called V1B on the schematic). The 12AX7 is capable of nearly 100 times gain per triode. Here, I put in 200 mV and I get out 7.24 V. That’s a voltage gain of 36. If I put 1 V on the grid then I can expect to get 36 Volts out.

Tone

Tone is the tone stack, this is a passive tone stack, what it really does is just roll off treble at successively lower frequencies as you turn the dial. It can only take signal away. We put 7.24 V into the tone stack and coming out we have 2.759 V.

PRE2

PRE2 re-boosts the signal from the tone stack. Input is 2.759 V and output depends on the volume control attached the grid (this is V1A). I’ll probably have to chart it out later but with the volume at 2, I get 7.19 V out of PRE2. That is damn near all I should be getting with the volume up to 11. Essentially, this appears to be where the problem lies, but I will stop here and in the next post look it detail at the power section to see why this is a problem.