Needing some help understanding a schematic (first amp build)

[Pat_rocks92]

Hello,

I'm building a rob robinette micro bassman 5f6a-M as a first build.

I recieved an Hammond 270 ax power transformer without secondary 6.3v center tap.

As rob asks it i have to make an artificial center tap is my understanding of it correct or not (see attacgements) ?

it is in the middle downside of the schematic :
https://robrobinette.com/images/Guitar/Bassman_Micro/Bassman_Micro_Schematic_small.png

Thank you for you help !

 

[King Fan]

Are you building the cathodyne PI version (the schematic you linked)? Or the LTP? Or even the EF80 version? Still, no matter. Compare the layout to the schematic. I believe for all three Rob shows you how to build the artificial CT on an output tube socket. Here's the LTP version.

Here's the EF80:

Someone smart may tell us if there's much advantage to this socket-mounted approach given the low elevation voltages available here. If not, you could simply ground the 100ohm resistors off the lamp or elsewhere on the heater circuit.

 

[Pat_rocks92]

Are you building the cathodyne PI version (the schematic you linked)? Or the LTP? Or even the EF80 version? Still, no matter. Compare the layout to the schematic. I believe for all three Rob shows you how to build the artificial CT on an output tube socket. Here's the LTP version.

View attachment 980502

Here's the EF80:

View attachment 980503

Someone smart may tell us if there's much advantage to this socket-mounted approach given the low elevation voltages available here. If not, you could simply ground the 100ohm resistors off the lamp or elsewhere on the heater circuit.

Thanks for your help man !

Yes i'm going to build the cathodyne PI version the one i linked.

i just don't get it : isn't the artifical center tap the green yellow wire going there on the cathode v3 resistor ?

https://robrobinette.com/images/Guitar/Bassman_Micro/Bassman_Micro_Eyelet_Layout_With_Rectifier_Tube_small.png

it is written on this wire 6.3v center tap. As i don't have it i must create it ? Or am i confusing things.

what is that wire ? i don't have it on my hammond 270ax. that's the reason if find it weird.

 

[Len058]

The picture has both options. If you have a center tab on the transformer, connect it to the cathode resistor. In this case you do not put the 100ohm resisters on the output tube.
If you don't have a center tab, add the 100ohm resisters to the output tube heater wiring as shown. This is an artificial center tab. If you want to know more about the function of the center tab, artificial or not, google it.

So yes, you have to create it. There are a few options, one is shown in the schematic. They are the two 100ohm resistors on the power tube.

 

[Pat_rocks92]

Thank you ! it is clear now !

I think it might be interesting to learn what each part does in an amp, but i don't have enough time now, which sucks.

another question is how am i supposed to wire the fil 6.3v tube heaters ?

I mean there are 2 green cables with 6.3v.

is it like in the attached file but with cables twisted to avoid hum ?

 

[2L man]

Thank you ! it is clear now !

I think it might be interesting to learn what each part does in an amp, but i don't have enough time now, which sucks.

another question is how am i supposed to wire the fil 6.3v tube heaters ?

I mean there are 2 green cables with 6.3v.

is it like in the attached file but with cables twisted to avoid hum ?

Yes! 12A-series tubes have two triodes and both have own 6,3V filaments which are possible to connect parallel like that tieing sockets 4 and 5 together and using socket 9 to close the circuit.

For 12,6V use they are connect to series feeding between sockets 4 and 5 and not using socket 9.

 

[Pat_rocks92]

Thank you !

As i decided to make a micro build i took a very small chassis and my issue is that i lack place for a switch so i had to take a micro switch 3A 125vac. My last question is will this micro switch be enough for the tone stack bypass (raw switch) ?

The combo cab i made is rather small too.

 

[Len058]

3A 125vac. My last question is will this micro switch be enough for the tone stack bypass (raw switch) ?

The combo cab i made is rather small too.

Yes, it will be enough for the raw switch.

I think it might be interesting to learn what each part does in an amp, but i don't have enough time now, which sucks.

I would advise to take your time to understand what you're doing. I may sound like a Boomer but the project you started isn't a jigsaw puzzle. A lot can go wrong and if you don't understand the basics you can be very disappointed in the end. You might not make it to the end if you make certain mistakes.

Go watch some youtube video's about amp building and repairs. Not knowing about the filament wiring or the signal strength on the tone stack, doesn't sound good.
Take your time. As someone with adhd I learned the hard way.

 

[Pat_rocks92]

Yes, it will be enough for the raw switch.



I would advise to take your time to understand what you're doing. I may sound like a Boomer but the project you started isn't a jigsaw puzzle. A lot can go wrong and if you don't understand the basics you can be very disappointed in the end. You might not make it to the end if you make certain mistakes.

Go watch some youtube video's about amp building and repairs. Not knowing about the filament wiring or the signal strength on the tone stack, doesn't sound good.
Take your time. As someone with adhd I learned the hard way.

I'll try to take your advice i know that guitar amps are lethal and that mistakes can be costly on components too (specially on transformers).

i might wire everything and then take it to a guitar amp tech to test it.

 

[Len058]

Staying alive is very important but it's quite easy to drain filter caps, if you know where they are. But building an amp is more than prevent elektric shocks or setting fire to a transformer.

When I look at your pictures I see the raw switch, right next to the pilot light. That's where the filament wiring runs. It has AC, so it will probably introduce hum in your signal.
The second thing I see is the fuse holder is at one end and the AC socket is on the other end. So you'll have to run long ac wire from the socket to the fuse. This will probably cause hum. Having the fuse close to I presume V1 will make it worse.

You see why you need to learn before you build?

 

[Len058]

Did you read this?

"My recommended power transformers are the Hammond 270AX for the US and Hammond 370AX (available here) for international builders (has 100, 110, 120, 200, 220 and 240 volt primaries, see this for wiring instructions). They have 240-0-240V high voltage secondary output and 6.3V heater output. If you use solid state rectification you may need to add a 2 watt voltage dropping resistor before the first filter cap to bring the voltage down to around 315V DC at the first filter cap."

I believe you're at 230V in Switzerland, you've got the 270ax PT?

 

[Pat_rocks92]

Did you read this?

"My recommended power transformers are the Hammond 270AX for the US and Hammond 370AX (available here) for international builders (has 100, 110, 120, 200, 220 and 240 volt primaries, see this for wiring instructions). They have 240-0-240V high voltage secondary output and 6.3V heater output. If you use solid state rectification you may need to add a 2 watt voltage dropping resistor before the first filter cap to bring the voltage down to around 315V DC at the first filter cap."

I believe you're at 230V in Switzerland, you've got the 270ax PT?

Yes i already know it but i decided to use a transformer to downgrade 240v to 120v. I Have a Bray 4550 and i believe it sounds different when it is switched to 240v i wanted to switch to 120 to test it.

thank you for your help.

As i will mostly build small amps built by americans on forums i will go with their specs. I prefer having to use a device to downgrade voltage than having to change transformers as it can affect tone.

if the build with the Micro bassman goes well i will try to build a 1w plexi/trainwreck amp :

TMC1 – Chasing Tone

chasingtone.com

that should be my next build : i already have the tranformers

it really sounds like a plexi on steroids :

 

[Len058]

Ok, good luck and have fun!

 

[martinlb]

Yes i already know it but i decided to use a transformer to downgrade 240v to 120v. I Have a Bray 4550 and i believe it sounds different when it is switched to 240v i wanted to switch to 120 to test it.

thank you for your help.

As i will mostly build small amps built by americans on forums i will go with their specs. I prefer having to use a device to downgrade voltage than having to change transformers as it can affect tone.

if the build with the Micro bassman goes well i will try to build a 1w plexi/trainwreck amp :

TMC1 – Chasing Tone

chasingtone.com

that should be my next build : i already have the tranformers

it really sounds like a plexi on steroids :

Sounds great!