Ironside build upcoming

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Hi y’all! Just to notify you folks there will be another Ironside build in the upcoming weeks. For those who don’t know: the Ironside is a Tremolo TMB with my personal tweaks.

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While waiting for the parts to arrive I feel the need to give some advice for fresh starting or inexperienced builders.
Building an amp is a lot of complex stuff. Basically you need to follow a plan and a schematic, but it's more so important you understand the plan and the schematic. You should plan the steps of your build in your head or even better: on paper! Let's have a look at my approach.

1. sourcing the parts:

after gathering all information of the parts needed for the build you can order them from the supplier. Don't forget things for you will have to pay extra shipping for the forgotten parts, sometimes those shipping costs will be more than the forgotten part itself. Preferably order some spare components or anticipate on the things you might need to tweak. (i.e. various dropping resistor values, various bias resistor values, various coupling cap values, some extra filter caps in case you mess things up, definitely some extra pf caps, ...)

2. prepping the chassis:

let's assume you buy a predrilled chassis an ready made face plates. First I put in all tube sockets. The chassis is still lightweight and will be easier to handle. Same goes for spacers, solder terminals, pots and jacks, switches, pilot,...
Mount the can capacitor before the transformers because sometimes the transformer is in the way to easily close the cap clamp.
Then I wire the heaters from the 1st tube socket to the last. Don't solder the last socket yet.
Lastly I mount the transformers.

3. the wiring and soldering:

A. let's have a look at the schematic. Start at the AC power inlet. This is an important and often overlooked part of the circuit. It might be confusing, but it's fairly simple. The hot lug (L) from the IEC inlet should be connected to the fuse, the other lug from the fuse is connected to the power on-off switch. The other wire from the on-off switch goes to the 120V or 230V side of the primary winding of the power transformer. The neutral (0V) wire goes directly to the neutral (N) lug of the IEC inlet. The ground lug should be wired to a separate ground terminal, dedicated for this wire only. Do NOT attach this ground lug to a transformer bolt! Leave enough slack on this ground wire in case the IEC inlet would ever come loose from the chassis. The ground wire should be the last wire to break off, after the hot an neutral wires.
When this is connected I wire the Pilot lamp and the heater wires from the last tube socket to the pilot light assembly and from the transformer to this pilot lamp (I always use 6.3VAC pilot lights)
Now you can check if the amp lights up. Watch out! Make sure that no other wires from the transformer are exposed to you or the chassis. Clip the center wire from the heater wiring to the chassis to do this test.

B. completing the power supply... if you have a tube rectifier you should wire the heaters and high voltage secondaries for the tube rectifier. Watch out! There's a wire that comes from the rectifier and goes to the 1st filter cap ( via the standby switch if you use one, which I never do) . On an EZ81 it is pin 3 , so no problem. But on octal rectifiers it is branched off of one of the heater wires, so make sure you have that wire soldered to the tube lug already, along with the 5V heater wiring. Leave this wire long enough to reach the turret board or filter cap.
About the filter cap can... it happens often that you forget a wire and already have soldered the other wires to the cap can. Be aware of that. Most cap cans have 3 lugs (2 hot + 1 ground). Let's call the B+ 1, B+ 2 and ground.
Wires connected to B+1: wire from rectifier, wire to output transformer center tap, 1 lead end of the ground leak resistor (often 220k 2W), wire to the choke OR 1 lead of the 1st dropping resistor
Wires to B+2: other lead of the 1st dropping resistor of choke, wire to the screen grid resistors of the power tubes. You can attach a 3rd wire for the following B+ node (B+3) or you can branch it off the connection to the screen grid resistors.
Ground: other end of the ground leak resistor, wire to the power amp ground lug, center wire of the high voltage secondary winding of the power transformer.
About this ground lug: this should be a dedicated star ground lug for the power amp only. Connections here are: ground from cap can, speaker jack negative wire, wire from the turret board ground buss of the power amp components, heater wiring center wire (although it is sometimes beneficial to connect this wire to the + side of the cathode cap on a cathode biased amp. This elevates the heater voltage and reduces hum sometimes)
Now you should have an operational power supply. You should check the operation by powering the amp on without tubes. Now you can check the HT voltage unloaded. AC and DC. So far so good.

C. connections not going to the turret board: now I start connecting the (shielded) input wire to the tube socket, all wires from the pots to the tube sockets, output transformer wiring to the impedance selector switch and to the anodes of the power tubes, the primary centre wire of the output transformer to the cap can B+1

D. preparing the turret board: install all jumper wires on the turret board. I advise you to do all these wires on the top side. I know it's tidier looking when they're on the under side but it's a PITA when you have to change things after the build is complete. It happens a lot that one or more wires are not going to the correct turret.
Once all wires and the ground buss are connected to the turret board an triple checking you can mount the board in the chassis.

E. jumper wires and pot connections: now I start connecting the wires to the turret board. 1st I do the B+ wire or the HT wires to the rectifier diodes, the screen grid wires and ground connection wires. Then I connect the tube sockets to the turret board. Watch out for wiring errors. This is a very confusing phase.
Then I move on to the connections to the pots. same deal: watch out that the wires go to the correct turrets. I also wire the footswitch jack at this stage.
Be aware of the rules of wiring the tube sockets to the board. All wires should be preferably flat to the chassis. They also should have rather short runs, but leave a little slack for eventual lead dress issues. Keep all wires as separated as possible. Crossings should be close to 90° to avoid cross talk between signal and DC carrying wires.

F. mounting the components: I start by mounting the components of the power amp. Those contain some big parts that are hard to squeeze in between other parts. Follow the schematic and layout closely try to follow the flow of the signal backwards.
Next I start to wire in the components of the preamp stages, starting at the input stage. Follow the flow of the signal on the schematic and learn how this amp is built up. After the preamp components are mounted I wire in the parts of the phase inverter.

G. checking the wiring and start up: after the final wiring I clean up the inside of the amp and my work bench. Now I start checking the build with some hardcopies of the schematic and layout. With a highlighter in my hand I check the entire build, starting from the input. Each time a part of the circuit is checked and verified to be good I mark that with the highlighter on the schematic and layout. I advise you to go over this at least twice, so make sure you have enough copies. Correct faults immediately when spotted.
After all is checked and verified I take detailed pictures of the build. When you look at pictures it's more easy to spot bad solder joints or possible shorts from wire ends you forgot to clip off or other bad things.
When that is all checked I go over the circuit with my DMM to check if there aren't any shorts where they shouldn't be.
Now I start the amp up without tubes. You can see that without tubes all B+ nodes read maximum high voltage, that's normal, unless you have a tube rectifier.. then there's no voltage at the B+ nodes. I use a variac or light bulb current limiter at this stage, so the voltages might be on the lower side. That's normal too.
When all seems fine I put in the rectifier tube. No problems? Ok then the power tubes. No problems? Check the voltages on those tubes. Again, when using a light bulb current limiter or variac the voltages may be lower than spec'd, but in proportion. When the power tubes are fine I put in all preamp tubes. It's preferable to use known good working tubes at this stage to avoid you will start troubleshooting and messing with the circuit while you have a brand new tube that turns out to be bad. Remember the most obvious problems in a tube amp are tube related, so check tubes first, ALWAYS!
Ok, when everything is working without anomalies you can remove the light bulb limiter or variac and power the amp up at the right input voltage. all voltages and write them down on a voltage chart. There are lots of voltage charts in the download area, often with reference spec voltages.
Let the troubleshooting begin!

Oh, and NEVER switch on an amp with power tubes in it without a speaker connected.

[Iain T]

Interesting primer!

As someone who is definitely in the newbie category I've just completed the BoM for my Princeton build. Having a plan from the start is really important.

I have a few things I need to decide on before ordering parts - Modulus here in the UK offer a 'standard' spec wound xformer as well a beefed up spec. Waiting for them to get back of summer holidays to ask a bunch of questions.

Next big decision is: pre-drilled chassis and modify or blank and do from scratch. I'm not using the external filter can cap and am considering giving it a switchable "stokes mod" so it'll have original character vs more clean headroom.

[Bieworm]

Interesting primer!

As someone who is definitely in the newbie category I've just completed the BoM for my Princeton build. Having a plan from the start is really important.

I have a few things I need to decide on before ordering parts - Modulus here in the UK offer a 'standard' spec wound xformer as well a beefed up spec. Waiting for them to get back of summer holidays to ask a bunch of questions.

Next big decision is: pre-drilled chassis and modify or blank and do from scratch. I'm not using the external filter can cap and am considering giving it a switchable "stokes mod" so it'll have original character vs more clean headroom.

Why don't you ask John Wilkinson from Primary Windings for transformers. His prices are very competitive and they are UK based.
I have already built a Princeton Reverb with his PWMA01 power transformer. Ok, I had to do a cathode biased version because the HT secondary has no center tap, but it turned out pretty nice! I also went for EL90 power tubes, which are 9 pin equivalents of a 6V6 in the size of an EL84.
The nice features of this transformer are the small and lightweight size + the HT secondary is 0-90-275V. With these taps in a well figured out switching circuit it can give you a switchable B+ (a VVR kinda thing). This gives you the option to switch between 12 - 6 and 1 watt.
Just throwing it out there..

[Iain T]

Why don't you ask John Wilkinson from Primary Windings for transformers. His prices are very competitive and they are UK based.
I have already built a Princeton Reverb with his PWMA01 power transformer. Ok, I had to do a cathode biased version because the HT secondary has no center tap, but it turned out pretty nice! I also went for EL90 power tubes, which are 9 pin equivalents of a 6V6 in the size of an EL84.
The nice features of this transformer are the small and lightweight size + the HT secondary is 0-90-275V. With these taps in a well figured out switching circuit it can give you a switchable B+ (a VVR kinda thing). This gives you the option to switch between 12 - 6 and 1 watt.
Just throwing it out there..

Ooooh, that's awesome stuff, thank you!

I'll start up a thread for this build next week and I'll certainly be quizzing you on some of the above. I should probably build standard before getting too creative though!

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If you’re interested on how to implement the PWMA02 and its B+ switching I will look for my documentation. I will probably have it somewhere

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rotary switching.png

like that:

[Iain T]

Thanks for sharing that - going to take me a minute to work out the switching but that's a great idea.

Is the diagram showing alternate options for the switching between a rotary and 2 switches?

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Thanks for sharing that - going to take me a minute to work out the switching but that's a great idea.

Is the diagram showing alternate options for the switching between a rotary and 2 switches?

The diagram shows 2 options. One is the rotary and the other is the toggle switch. The toggle switch should be a 3 position double pole switch. The rotary is easier to get, but nastier to connect.

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ok here's some progress. I'm in the middle of making the combo cab, and already did the chassis with power supply wired and then some...

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and more

[JMPGuitars]

Everything looks great so far!

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Everything looks great so far!

Thanks. In the meanwhile I wired the tube sockets to the board and soldered most of the components of the tremolo.
I have a question that I wanted to ask for a long time. Maybe the answer will be useful for others? Here goes… on the Tremolo TMB the first preamp stage has a 100k plate resistor with a 220k in series. My monkey brain tells me it’s 320k plate load, which is higher than the usual 100k and 220k in some applications. This pushes the 1st stage into higher gain than the usual preamps. I know the Classic 18W has this config too. Am I correct on this statement? And if so, does it lower the gain when this one has a separate 100k plate resistor? Or is there some other theory at play here?

[JMPGuitars]

It's a different theory. You need to think about how the tremolo works, and why you might want to amplify the tremolo output signal.

[Iain T]

The diagram shows 2 options. One is the rotary and the other is the toggle switch. The toggle switch should be a 3 position double pole switch. The rotary is easier to get, but nastier to connect.

Yay, I understooderated!

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Hooray!!! The amp is finished and works as it should. Now back to the combo cab works…

I was very lucky though. I used a schematic where I drew a full wave bridge rectifier and thus wired one in. But I had a 300-0-300V secondary. I did ground the CT of that winding and also had a ground on the FWB rectifier. When I switched on the amp the transformer started to make strange sounds so I switched it off immediately. After correcting this the amp’s been on for an hour, so fingers crossed but the transformer seems fine.

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Backside

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I lost count of how many of these I have built. But it’s always a pleasure to build this amp, more than any other.

[JMPGuitars]

Great work, as usual.

So, can you teach us about how the signal flows through the tremolo?