#183 B+....

[mrdarwin]

Ok.
What about the value of the resistor to pull down the negative bias voltage?
How can I calculate this value?

[martin manning]

Probably trial and error. I suggest a 6k8 in front of the diode and a 15k from the bias pot to ground. That should result in a negative voltage range from about -30 to -60VDC with a 98VAC input.

Edit: Revised resistor to ground targeting -50VDC at mid range

[mrdarwin]

I found this PT:
http://www.tubeampdoctor.com/en/shop_Ma ... an_100_111

Same specifications than the MOJO, available in UE and less expensive...

On the 183schem, the MOJO is an option for PT. But 6A for the heaters... Is it enough?

[martin manning]

But 6A for the heaters... Is it enough?

Hard to say without knowing anything about who is making transformers for TAD. You could ask them. I see that TAD also has a 100W Marshall PT in lay-down mount. That should fit in your chassis a very little bit of rework.

[Synchu]

I am using http://www.inmadout.com/ in Italy for sourcing transformers. Best pricing I found, quality stuff and they do custom work.
US transformers are very expensive by the time they get to Europe. Edcors are coming at 3x their US price and its not because of the resellers - shipping (unless they load a ship container), customs, taxes add up quite quickly.
Hammonds are a bit better, I guess they are more popular and are sourced in larger quantities, but inmadout work great for me at least.
Niki

[jestaudio]

I am using http://www.inmadout.com/ in Italy for sourcing transformers. Best pricing I found, quality stuff and they do custom work.
US transformers are very expensive by the time they get to Europe. Edcors are coming at 3x their US price and its not because of the resellers - shipping (unless they load a ship container), customs, taxes add up quite quickly.
Hammonds are a bit better, I guess they are more popular and are sourced in larger quantities, but inmadout work great for me at least.
Niki

+1 on the In mad out, bloody good kit and very easy to deal with the company

[martin manning]

I don't know... In mad out has a 100W Twin PT, but on the spec sheet it has either non standard mounting hole centers, and/or there is some error since the c-c dimensions of the mounting holes are the same as the outside dimensions of the lam's. In the "tools" section they have copied a page showing rectifier circuits from Hammond's catalog, with Hammond name and contact info deleted.

[norburybrook]

InMadOut custom made me a 100w OT transformer for my BLuesmaster.


When I powered up there was so little noise I thought it was broken....truly outstanding transformers.




Marcus

[JazzGuitarGimp]

InMadOut custom made me a 100w OT transformer for my BLuesmaster.


When I powered up there was so little noise I thought it was broken....truly outstanding transformers.




Marcus

Or perhaps an outstanding build?

[mrdarwin]

Mmmm...
I will try with a Hammond 100w plexi PT and mod PSU and bias section.
I will tell you! 6A for heaters looks a little low for 4 EL34+3 ECC83.

[martin manning]

Can you get a Hammond 100W Plexi/JCM800 in a lay-down mount? I see only the upright 290HX, which I think would work if you drill new mounting holes, but check carefully for possible interference. You would want to orient the laminations as shown, so they are at 90 degrees to the OT laminations. Maybe the leads could be passed through the Fender cut-out?

The circuit mods should be easy if you are using an eyelet board for the bias/rectifier as shown in the 183 layout; you already have four diodes for the FWB. Don't forget you will not be grounding the center tap. Instead, add a lead to the rectifier board for the main ground.

[mrdarwin]

Thanks Martin for the drawing!
My OT is on the other side of the chassis, no problem for interference...

[martin manning]

Sure; it's interesting to explore this alternative. You can check for hum at the speaker output before you wire it all in by mounting the PT and powering it up with just the primary connected. The hole pattern is square, so you can rotate the PT 90 degrees if necessary.

[norburybrook]

InMadOut custom made me a 100w OT transformer for my BLuesmaster.


When I powered up there was so little noise I thought it was broken....truly outstanding transformers.




Marcus

Or perhaps an outstanding build?

thank you Lou, but in my case my builds are 'functional' as I'm still very much a novice

Marcus

[strat100]

Just got an email from Inmadout stating they will custom make the tatwin100 with Primary, 115V-230V-240V@50Hz
Secondary, 340V-60V-0-340V@400mA
Secondary, 3,15V-0-3,15V@7,5A

for a cost of 94.50 euro with shipping to uk for 18 euro so a total of 112.50 euro,seems like a good deal to me so i am going to jump on it.