Help question Building a 102 using a 60 watt crate transformer

[samtech60]

Have a 102 I am building I also have a beefy old Crate Turbo valve 60 power transformer I intend on using its no load unrectified is 398 vac on the secondaries.

I know the Dumble likes to see about 345-355 vac range can I use this without pushing the limits of the values and components in the power supply? Here is a picture of the original Crate schematic. I also have the 60 watt output transformer all made in America back in the 90s . Any input is appreciated Thanks Sam

[martin manning]

Same answer as I gave last time:

You can increase the first dropping resistor after the choke to bring the preamp voltages back in line. A member here built an ODS with 500 V on the power tube plates using a Peavey PT and was very happy with it.

The Peavey schematic confirms 490V loaded. The bias comes from a separate winding, where there is a 25V DC supply taken from a tap. I don't know what that was powering, but it may be adequate for the relay supply. I’m assuming that you are building a 50W (2x 6L6) version of the 102 circuit.

PS posting the same question in multiple areas of the forum is not good form.

[samtech60]

This is not a Peavey it's a Crate and it's about 45 volts higher vac unrectified directly on the secondaries. And this measurement is no load and a typical Dumble 50 watt transformer is between 345 and 355 on the unrectified secondaries no load VAC

[samtech60]

Yes I am new on here I spoke to the Admin and we got that all straightened out my apologies for acclimating myself on here . I am looking for the best solution and sometimes it comes from many minds not just one .Thanks for your time

[samtech60]

This crate also has a nice 60 volt separate bias supply winding

[martin manning]

Yes, sorry the 500V amp I mentioned used a Peavey PT.

Also look into using the buas winding tap for the relays.

[samtech60]

BTW this beautiful transformer also has separate 20vac secondary windings that came stock to power some relays .

It's the perfect transformer in every way I am just trying to drop main B+ abit here is an enclosed 102 power supply section trying to calculate the proper voltage drop and use a heavy enough dropping resistor to compensate for it ?

We have the voltage no load VAC like I said about 395 vac secondaries trying to calculate a formula to drop 50 volts . Could we use a 5watt resistor and proper value adjustment in place of R2 ? What's our mystery value is what we need ? Thanks again

[samtech60]

Here is the Dumble 102 power section do you feel changing Resistor 2.7k after Choke ?

[erwin_ve]

Here is the Dumble 102 power section do you feel changing Resistor 2.7k after Choke ?

Yes that resistor it the place to drop the voltage for the preamps and the PI

Also the screen resistors (470R) need adjustment to make sure your powertubes are not getting fried. 1-1.5k is ok.
I run my HRM Ods with a Peavey PT and OT at 500v on B+. The amp is build in 2009 and still running fine.

[samtech60]

Thank you Erwin appreciate it . Any does any value come to mind maybe 3.7k or 4.7k at 5watts to start?

[martin manning]

BTW this beautiful transformer also has separate 20vac secondary windings that came stock to power some relays and Reverb driver supply .

It's not separate, it's a tap on the separate bias winding. Sounds like it has the current capacity to power the ODS relays. Please post the whole schematic.

It's the perfect transformer in every way I am just trying to drop main B+ ... Could we use a 5watt resistor and proper value adjustment in place of R2 ? What's our mystery value is what we need ?

You want to drop 50V at say 100 mA. That requires 500 Ohms, and 5W power dissipation. With the usual 2x factor the dropping resistor should be a 10W. But I would not go there. Just run the plates at 490V like the Crate amp did, and drop the voltage going into the PI node by increasing the value of the resistor that feeds it.

[erwin_ve]

Thank you Erwin appreciate it . Any does any value come to mind maybe 3.7k or 4.7k at 5watts to start?

Check my build thread, it might be interesting for you regarding voltages etc. I came up with detailed info on the voltages and the guys,back then, were very helpful to get where it needed to be.
https://ampgarage.com/forum/viewtopic.php?f=4&t=10284

[samtech60]

The only thing it shares with the bias is a secondary ground it has more than enough current capacity to drive a few 150ma relays 20volts can be run through a 7812+ volt regulator as it can take up to +35vdc . Basically its still an extra supply winding independent of the bias winding however too much over current could cause problems but , we are not anywhere near exceeding that current draw .

[samtech60]

Thanks Martin and Erwin

[samtech60]

Here's complete schematic