120 Ohm Resistors.

[halfbackstrat]

Hi guys, I'm planning a 124 build (and have been for faaaar too long haha)

I'm comparing the 124 Skyliner Schematic with the layout diagram available on this site, to really get a feel for how the amp is laid out.

So far I've been through the entire preamp section and am now doing the power supply section, Still got the Power Amp section to go.

On the Power on LED circuit there are two 120 Ohm 2W resistors and one is marked as going to Ground 2 and V4 Pin 8, however on the diagram that I can't find those two resistors, and where i think they should be one leads to V7 Pin 7, and the other one leads to V6 Pin 2.

Is anyone able to offer any insight?

Thanks for taking the time to read this.

Matt

[martin manning]

Those two resistors make an artificial center tap for the filament circuit. All pin 7's on the power tubes should be connected to one side of the filament winding, and all pin 2's should be connected to the other side. The 120R's can then be placed one from any pin 2 to ground, and the other from any pin 7 to ground. Pin 8 is a cathode, and so is the same as a ground, give or take a 1R current sensing resistor if there is one installed between it and ground.

[halfbackstrat]

Thanks for the reply Martin,

This is my first amp build so I'm probably going to sound like an absolute amateur! Apologies in advance.

Just to clarify is the filament circuit the same as the heater circuit?

I see the 120R's now! They're on V4 2 and V4 7 going to grounding point 2.

This makes much more sense. Thank you very much.


Another query I have. In the preamp there is a resistor between the Normal input and pin 7 on V1. In the circuit diagram this is valued at 33k and on the layout it's 22k. I'm guessing either value will work fine but i wondered what the reason/cause of this is. I.E tone preference, component availability (although I think 22k and 33k are both readily available).

Matt

[martin manning]

Yes, Filament = heater. The FET input goes through that series resistor to the grid of V1a too. Its purpose is to keep very high frequencies (RF noise) out of the input by working with the input capacitance at the grid to form a low-pass filter. That capacitance is about 100pF for a typical 12AX7 circuit, so 33k will have a -3dB corner at 48kHz, and for a 22k it would be 72kHz. No audible difference, in theory.

This is a complex amp for a first attempt unless you have other electronic expeience. Sounds like you are taking a methodical approach and trying to understand the whys and wherefores rather than just "painting-by-numbers, which is a good thing.

[amplifiednation]

First build, eh? You are jumping off the deep end! Make sure to read up on some lead dress practice and tube amp theory. There is a LOT going on in these amps.

I was told before I built mine that it takes about 12 of these builds before you truly understand what's going on!!!

Also be prepared to do some troubleshooting. I would be very careful with component soldering to the boards, and follow the grounding schemes perfectly. If you do the research and have the time I bet you'll be surprised with what a great amp you will have.

Good luck and let us know what questions you have

[halfbackstrat]

I see, that makes much more sense. Thank you. I'll check the pictures online and see what the original has, that's probably a good thing to base it on.

Firstly, thank you very much for the concern, I really appreciate that.

I'm a product of the Scottish Education system, and am now a Chemistry student at Uni. I've studied quite a bit of Physics in the past few years and have a good understanding of electrical circuits.

My Dad worked in TV repair for 32 years and now, has moved to a different job in electronics, so I've picked up quite a bit over the years (However I don't want to rely on him).

I have built a guitar pedal (TS-808 clone with a few mods) and I've modified my current Amp (Fender Blues Jr).

What kind of experience did you guys have before you started your first ODS clone (if this is something you have done).

I've been playing with the idea of using 6V6's instead of 6L6's, so that the amp can run at 50W or 25W rather than 100W / 50W. What modifications would need to be made to do this?

Thanks very much,
Matt

[telentubes]

"What kind of experience did you guys have before you started your first ODS clone (if this is something you have done)".

I started building amps 2 years ago and built 10 amps before trying a 3rd gen. 50 Watt ODS. Even with those other projects under my belt, it was a handful. There's a LOT going on.
But I took my time and studied each section as I went along, and of course got coached by the generous and knowledgeable brain-trust at Amp Garage. I took about 6 weeks to build the amp, which fired right up. Last night was its first bar gig, and through a 2X12 cab, sounded friggin' awesome. I have no background in electronics, so a lot of the concepts were/are hard for me to wrap my mind around, but I love detailed work, and I'm persistent, so it can be done. But as I said before, there's a lot going on in those amps and I would recommend taking it slowly if it's a first build. Best of luck. The tone is worth it.

[ToneMerc]

I've been playing with the idea of using 6V6's instead of 6L6's, so that the amp can run at 50W or 25W rather than 100W / 50W. What modifications would need to be made to do this?

Thanks very much,
Matt

I've had good success with 4-6V6's using 50W marshall iron and PT's with slightly lower B+ (405-425V) voltages. This PT should put you around 425V. You could drop the volatges a bit by using a 25W 125ohm power resistor on the center tap.

http://www.mojotone.com/amp-parts/amp-t ... xport-Taps

Mike

[halfbackstrat]

Thanks for the shared experience. My biggest worry is missing something really obvious and ruining it on start up.
10 Amps is a lot! Maybe I'm biting off more than i can chew. What sort of amps did you build? Were they from kits?

Tone Merc, thanks for the info, I might look into this.

I've finished off noting down every part in the build this afternoon. And I'm now transferring it into a spreadsheet to start sourcing parts. I have a question in switches.

I thought that the Rock & Jazz switch was DPDT so as you could set it in the middle and activate the PAB. However in the diagram it's switch schematic is exactly the same as the Mid Boost switch, however I thought this was a DPST switch so that the Boost was only On or Off.
Any thoughts?

Thanks

[rogb]

Going from pedals to a 4 tube ODS build is a tough learning curve as a kit build, where the boards are made up and all parts are there. To source, fabricate and populate your own boards and chassis is not impossible but given the complexity of the build (relays, lead dress etc etc) you may have some problems troubleshooting.
You can get great boards and chassis from guys on here, though.

I built an ODS kit as my second build, but I had spent plenty of time modding and tweaking my 1st 5w Ampmaker, which gave me the confidence.

If you can source and build it, you will be immensely proud, no doubt

[amplifiednation]

I did four amp builds before my ODS, including two Express builds. I read 4-5 books before starting my first amp, and had built some pedals, Leslie motor switches, and a ton of guitar mods before that! I never I could have learned so quickly without the resources here at TAG.

There were more than a few times I got a bit overwhelmed with the Dumble build, even sourcing parts is a weeks work. It took me a few months to get all my parts and then 5 weeks to build the amp. I worked hard at it. Mine fired right up and sounds awesome!!

It sounds like you have a good background and some resources available though. Just know its a big project!! I can't wait to do it again!!! I'm already building boards...

[ToneMerc]

I thought that the Rock & Jazz switch was DPDT so as you could set it in the middle and activate the PAB. However in the diagram it's switch schematic is exactly the same as the Mid Boost switch, however I thought this was a DPST switch so that the Boost was only On or Off.
Any thoughts?

Thanks

I prefer to use an on/on dpdt switch like #183 keeping the PAB functionally on the rear mini switch. If you want the #124 scheme, use a on-off-on DPDT.

TM

[halfbackstrat]

RogB: When i say source everything i meant components, Chassis and Boards would definitely be a purchase. I have my eye on the C&E Chassis, the only problem is that i'm in the UK. I'll look at getting some made this side of the pond before importing from the States/Canada.

Also in reference to Lead Dress, this is something I haven't hugely aware of before, I'm reading up on it atm, and the principle seems to be it's all interference with wires. I think my approach to this will be to identify and categorize every wire in terms of if it carries a high load, or it is sensitive to having a signal inducted into it. Then make sure they are separated from each other in the build.

Thanks for all the feedback guys! This is really great stuff. I might go and order a book on tube amps from amazon. Which books would you guys recommend?

[alvarezh]

Hi, here are very good ones:

http://www.lulu.com/spotlight/valve_wizard

All the best!

[overtone]

...I have my eye on the C&E Chassis, the only problem is that i'm in the UK. I'll look at getting some made this side of the pond before importing from the States/Canada...

there is a chassis in Germany, I have not tried it and it is not drilled. The company is incredibly quick and has a load of other parts that will make your sourcing from the UK side of the pond easier. I have no affiliation with the company. Apart from RS Components I rely on them really a lot because the prices are Ok and the delivery is so quick:
http://www.tube-town.net/ttstore/produc ... umble.html