Inverted Cross Audio Thunderdome Amp WIP

drgonzo1969

drgonzo1969

Well-known member
I came across Inverted Cross Audio in another thread and discovered they had some super reasonably priced PCBs and faceplates for a 90w KT88 amplifier ($43 for both). I have been on the fence about starting an amp build for too long so I decided to grab the PCBs and start the amp building journey. Inverted Cross Audio has links to parts in the BOM but they are based in Spain and shipping would be expensive for all the hardware so I am going to source most of the parts through the usual suspects of Antique Electronic Supply, Tayda, and Mouser. I've found the chassis, most of the resistors and capacitors, but I need help sourcing the output and power transformers. Also, I could use help finding this power switch and impedance selector. PCBs should get here in a week or so. Here is a link to the BOM and the parts I need help sourcing. Thanks in advance for any input and advice.


invertedcrossaudio.com

THUNDERDOME - Inverted Cross Audio

Tour friendly high gain amp with two channels (clean/lead). 90 watts output power provided by 2xKT88 output tubes, 4xECC83 preamp tubes. Flightcase and footswitch included. Controls: clean gain, lead gain, mids, low, high, clean master, lead master, presence. Switches: off/standby/on, channel...
invertedcrossaudio.com invertedcrossaudio.com
www.tubetown.net

Toggle Switch APEM 644 H/2 DP3T - ON-(OFF)-ON

Toggle-Switch, used by Orange-Amps Type: Toggle Switch Version: DP3T - ON-(OFF)-ON Rating: 15A / 250VAC, 10A / 400VAC Contact resistance: 10 mΩ max. at 1A 2VDC Insulation resistance: 1.000 MΩ min. at 500VDC Dielectric strength: ...
www.tubetown.net www.tubetown.net
www.tubetown.net

Rotary Switch 1-Pol / 3-Pos - 4 A @ 250VAC - Impedance / Voltage Selector

High Power Rotary Switch - Can be used as Voltage or Impedance selector 1 pole / 3 position Solder lugs Shaft Diameter 6 mm, D-Shaft Rating: 250 VAC 6(4) A Contact Resistance: 100MOhm Max Dielectric Intensity: 1500VAC / 5S for ...
www.tubetown.net www.tubetown.net
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Your OT transformer needs to go from a high hertz to low hz. You'll need to know the input hz and the desired output hz to be able to decide. Do you have a schematic for this? You could probably call Hammond and tell them what you have and they would know which of their transformers to use.

EDITED:
By Hertz, I really meant Ohms. wth I was thinking I don't know, except I had a juicy In 'n' Out double double and fries I was eating at the time, lmao.

Your circuit is going to be producing an output signal in a specific load, and obviously speakers are 2 ohm to 32 ohm so you're OT has to match that. Power transforming isn't as picky, but each tube uses so much heater in watts and you cannot exceed or the PT will fry. The HT voltage has to be appropriate for the circuit so the plate voltage is correct. Which is why you need a schematic with the voltages and Ohms, or a sales person with knowledge, or even betterer transformer schematics showing all the primary and secondary values.


Pictures being better than words.....
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drgonzo1969 said:
I ordered the Hammond P-T1750U. The build docs are linked on the Inverted Cross Audio Thunderdome page in my first post. 1,700 ohms is what the build docs called for. For the power transformer I ordered P-T290HX. Should get here Saturday.
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I'm remarkably free of the ravages of intelligence today. Allergies. Ugh.

Looks like a fun project, I'm still hacking away at the 5F6-A I started many moons ago. I make my own turret/eyelet boards with G10 board. I was really close to having it done when like a million things around the house seemed to break at the same time. Gaaa.

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Hell yeah! I want to do a turret board amp eventually but both my pedal attempts are still in the troubleshooting box of shame. I really appreciate you guys with more amp experience taking a look and offering advice. This is my first amp build so I could definitely use ALL the advice. I don’t want this amp to end up in the pile of shame 🤣
 
drgonzo1969 said:
Have you built any KT88 amps? Do you have thoughts/opinions on different brands of tubes?
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I built a Marshall Plexi model 1987 (with the year 1967 circuit layout) that I put KT88's in. I had EL34's in it like the original, but then had to change the main resistor and put in a 50k bias pot instead of a 25k to get the wider range I needed to go back and forth between those two tubes. I eventually settled on the KT88's (original factory Winged =C=) because I liked the sweeter sounds I got out of them. The EL34's do break up at lower volume, and that's kinda fun too.

This was back in 2012, and I used real Mustard caps like the originals. The .022's are *extremely* rare, but I have around 600 of them (for PCB board use) that I got from a dealer in Greece when the economy hit the skids back in the early 00's. Dude wanted to put food on the table, and I was more than happy to oblige!!!

On a dare, I used Plexi glass to make the circuit board!!! Note you can see the wires running under the board, lol. Pain in the butt as you have to use heat sinks to keep the turrets from melting the board. The actual melting point is higher than what I run my soldering gun at, but the BENDING point is well below. Doh.

Old tubes definitely have a magic that newer tubes lack. Tesla, =C=, RCA, Mullard, Valvo, Sylvania are all great older brands. I have Tesla EL34's in my home built DR504 and it's insanely delicious. I don't use kits to make amps, preferring to make layouts in Visio so I can print out the board only to use as a template to drill the holes for the turrets. That's actually my favorite part :)

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This is the Bassman 5F6-A board. Oh, and yes that's weed above the board, lol.
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My layout (this is a hot link to postimg where you can download a VERY high resolution image if you want by clicking on the image)
 

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That's a lot of eye candy! Killer Buds!!!! Sexy plexiglass plexi! You should probably name that amp Lexi.... Lexi the sexy plexiglass plexi. I will have one each plexi and buds please and thank you hahahaha
 
drgonzo1969 said:
That's a lot of eye candy! Killer Buds!!!! Sexy plexiglass plexi! You should probably name that amp Lexi.... Lexi the sexy plexiglass plexi. I will have one each plexi and buds please and thank you hahahaha
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That was the 2023 crop and I ended up naming the strain 23 Pineapple Road.

It has a very pineapple smell and I like the name 23 pineapple road. Huge thick rock hard buds!!!!
 
Alright, I have most of the parts on order from Antique Electronic Supply except for a few odds and ends. I am having trouble finding a few parts still. I need six 220uf 450v electrolytic capacitors. I found some on Mouser but I will have to really double check the spacing and size. They are expensive and I don't want to get stuck with MORE big ass capacitors I will never use hahaha. Also this DP3T toggle switch is eluding me still. Links below are for the parts listed in build doc.
www.tubetown.net

Elektrolyt 220 µF @ 450V

Type: Electrolytic can capacitor Capacitance: 220 µF Voltage Rating: 450 V Tolerance: - 20 % / +20 % Operating Temperature Range: - 40°C / + 105 °C Dimensions: Ø 30 mm x 35 mm Termination Style: radial, Snap-In
www.tubetown.net www.tubetown.net
www.tubetown.net

Toggle Switch APEM 644 H/2 DP3T - ON-(OFF)-ON

Toggle-Switch, used by Orange-Amps
www.tubetown.net www.tubetown.net
 
drgonzo1969 said:
No luck at Mojotone. I think I found some Vishay electrolytics from mouser that will fit. Just gotta find that damn switch. https://www.tubetown.net/ttstore/en/toggle-switch-apem-644-h-2-dp3t-on-off-on.html
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I believe the problem is in their notation. DP3T appears to be identical to DPDT ON/OFF/ON

This switch is higher rated voltage/amps and is dual polarity on/off/on
 
 
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According to the datasheet, the Apem 644H/2 is actually ON-ON-ON, not ON-OFF-ON as listed on the Tube-Town store. Best check the schematic to determine what is needed here. The actual ON-OFF-ON part is Apem 648H/2. I found both of these on Mouser but they're super expensive. Mouser part numbers 642-648H2 (ON-OFF-ON) and 642-644H2 (ON-ON-ON).
And here's a Mouser search for 220uF/450V electrolytics that should fit, sorted by lowest price: https://www.mouser.com/c/passive-co...-snap-in|~Voltage Rating DC|~Diameter|~Length
 
Didn’t startup introduce first caps to 1.5-2X B+ at first cycles before tubes start to conduct? Is 450VDC rating enough for this build? What’s B+ on schematic?
 
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