Clueless Idiots and High Voltage Vacuum Tubes Really Do Mix!

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Still not dead!

June 12th, 2015 · Uncategorized

In fact.. I’m not even mostly dead. I have a couple of amps to show you and something fun coming from a interesting place. I never did pimp the record I was on, or show of the pics of me and famous people.

Not going to rush, but warming up to this again finally.

See you soon.

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I Not Dead Still

October 7th, 2012 · Amp Repair, Debug

So again.. not dead. Opened an amp for the first time in a long time today. Crestline 5T. This is the one I’ve been threatening to mod heavily. I decided today to just get her checked out for the fun of it. When last I saw her, she had burned a fuse holder (don’t ask me how, I guess a short). So I put the new one in and had to go digging around to find the tubes for her.
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A good looking amp on Ebay

March 13th, 2012 · Uncategorized

I’ve asked the guy to post pics of the inside, but as is this a good looking 67 (Medalist Style) Skylark. No Trem, so it should have a little more punch.

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.

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Replacing the Cap Can

February 20th, 2012 · 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.

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.

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I Not Dead!

February 20th, 2012 · Uncategorized

Ok, sure it’s been a long time and yeah, I still need to fix something in the RDF links but I am here and will post some more stuff shortly. Of course the primary reason for building amps is to play and that’s what I’ve been doing. Before too long I will go into some detail on my newest guitar, The Loar 309 and my plans for other guitars. Additionally, I might even get some new sound snippets up before too long.

I Not Dead, I Playing.

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Songbird : Choosing The Output Tranny

June 5th, 2011 · calculations, Design, Songbird

Actually, it is already chosen. It is the existing 8k:8 center tapped primary that is already in the Crestline amp. However, I am excited by the choice, and at the risk of sounding like a dangerous lunatic, I’ll explain why.

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

Champ 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.

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SongBird : Output Tube(s)

June 3rd, 2011 · calculations, Design, Songbird


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.
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SongBird : Block Diagram

May 30th, 2011 · Design, Songbird

Getting started designing an Amp means making quite a few taste decisions up front. The only thing you really know about the amp is there is going to be a guitar plugged into it. The next few moves are up to you before you can start with circuit design.

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

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).

Amp Preamp
Champ 5C1 6SJ7 6V6 Pentode Preamp
Champ 5E1 12AX7 6V6
Champ 5F1 12AX7 6V6
Gibson Skylark (50’s) 12AX7 6V6
Gibson Skylark (Medalist) 12AX7 6BQ5

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

Songbird Block Diagram

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

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SongBird : Time To Get Busy

May 29th, 2011 · Design, Songbird

So that Medalist is working ok. Sound is a bit thin for lead work but that’s mostly because it is still too loud for me to crank it at full. Beautiful sounding amp if you pickups can put out some sound or if you boost leading in to the amp.

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.
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Sidebar : Graphing Resistances

May 22nd, 2011 · 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 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

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

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

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.

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