Old 0-245V power transformer for octal champ amp

[constantineaudio]

Hi all ,

I found old power transformer from old hi-fi amp , I am planning to build champ/princeton amp by following layout below, I will try to build it with 6SN7 preamp tube instead of 6SJ7 , but problem is there is only 245V - 0V high voltage tab on PT is this suitable to use on that amp usually I using 250-0-250V and this seems very different to me. And preamp stages needs re-adjustment due to the 6SN7’s requirements and which resistors and caps values must be changed ? or do you guys have any proper layout to use with these PT and 6SN7 preamp stage.

 

[NTC]

Regarding an Octal SE amp, the very earliest Princeton use a 6SL7, which is a high gain 6SN7. Look at that schematic. As for the transformer, you need to use a bridge rectifier circuit, so you won't have a tube rectifier.

 

[Lowerleftcoast]

 

[Lowerleftcoast]

I recommend the first filter cap (Reservoir Cap) be at least 50uF.

 

[constantineaudio]

Thank you for all your advices , and I have another question about diode rectifier , should I solder 0V - 245V opposite sides right , I mean one red hv wire is 0V other one is 245V did I understand correct ? And rxcept filter caps what kind of replacements will I need on the preamp tube side for 6SN7 tube.

 

[Lowerleftcoast]

should I solder 0V - 245V opposite sides right , I mean one red hv wire is 0V other one is 245V did I understand correct ?

Yes, just like i have shown it in post #3
Since there is no center tap on your high voltage winding, you will use a Full Wave Bridge Rectifier which is wired as shown in post 3.
Most of the schematics and layouts I have searched for, only show wiring for a Full Wave Rectifier using a center tap from the high voltage winding. Study up on these two types of rectifiers. You will see the differences.

what kind of replacements will I need on the preamp tube side for 6SN7 tube.

First, a 6SN7 does not have the gain most of us would want for this build. (The 6SN7 is a non starter for me.)

If you have decided to swap the 6SJ7 (pentode) to a dual triode, either find a 6SL7 or change to a 9pin 12AX7.

The 6SL7 and 12AX7 are both dual triodes. Besides the different pin outs, they would use the same circuit. The values of the components in the circuit differ depending on which model you are looking at.

Imo. it would be helpful for you to study the similarities and differences in a few different models. These components are relatively inexpensive so you may want to purchase values to be able to try different flavors in this circuit.

Gibson GA-5 with 12AX7:



Fender 5F1:


The version you are working on has a tone circuit inserted where these don't, but simply add the tone circuit like a 5F2A to these.

It may be helpful to also look at the *Mod 102+* layout. It is point to point. It incorporates a different tone stack but you may glean some helpful info in layout positioning for your build. The Mod 102+ is tried and true. Look at the site below:

https://www.modelectronics.com/sites/modelectronics.com/files/media_files/MOD%20102%2B%20Instructions%202021.pdf

Notice it uses a Full Wave Rectifier using a center tap. Yours will use a Bridge Rectifier.
You will see it has all 47uF power filter caps. (Ime, the first cap on this type of circuit does the most for reducing hum. The second two filters can be 8uF to 10uF with no noticeable change in hum.)

 

[jumpbluesdude]

The 50 will tighten up the amp and affect the vintage characteristics of the amp. The bridge rectifier will also tighten the amp up. Your amp won’t get the effects of an under-designed power supply like the tweed amps are known for. If you like it, great, but if you’re missing that vintage feel, you’ll know why. Everything affects tone. EVERYTHING. Layout, lead dress, component choice, wire choice, spacing between components, etc.

 

[Jerry garrcia]

I wouldn’t run it with a 6SN7 as people already told you. Tried an octal champ with that just for fun. No gain and totally lifeless. The gain of 20 mu is really noticeable compared to the 6SL7 with a gain of 70 mu. You might try something like this. An octal variant of a “Vibro-5F2A”. Just remove the tremolo section in the lower left part of the schematic.
Don’t mind the serial connected filter caps in the second filter node. I did just use them since I was trying to get rid of some 275V caps. Since it’s serial connected the voltages doubles and the total capacitance decreases by 50%. So it’s really a 16uF 550V together. The parallel resistors are there for an “equally” effect on the caps.
Since you won’t be using a rectifier Tobe you’ll have an extra tube socket to use. If you widen it to accommodate an octal socket you could use two single triodes like the 6SF5 or a 6SQ7 (ground the two diodes) which both are more or less half a 12ax7.
If you do it that way you’ll end up with a 5F1/5F2A, gain wise, but in octal sockets.
I would really recommend to use a 47-100uF cap for the first filter stage (which has previously been recommended) and adding a NFB switch or make it variable. A MUCH more versatile amp that way. If you consider a switch I’d run with a 3-pole one that also disengage the first bypass cap on V1. Easy to insert and you get three different amps in one.

Here is the schematic @NTC referred to

 

[constantineaudio]

Thanks for all your precious advices, I just finished the amp using with 6SL7GT and played a little while Its very noisy and bassy other hand compare to my other 5F1 champs amps has almost %50 volume, and I ll try another with using 6L6 single ended amp , of course I ll stick to diode bridge with 0-245V but I just take it out from old tube amp 290V-0-290v and 3,15-0-3,15V pt should I go with it or 245V enough for this ?

Another question is does 6L6 and 6SN7 or 6SL7 suitable together ? If so what kind of changes will I need on the layout ? I m sharing the layout picture below.

And I will share the video of 6SL7 champ in a short , It would give an idea to who consider to build this kind of amp and you might hear the noise

 

[radiocaster]

The 245V transformer will put out a lot more than 300V if using a solid-state bridge rectifier.

The 6SN7 usually gives a pretty clean sound, more jazzy than rock. Very nice if that's what you want. It was used in the Gibson BR-9 and also some other amp I can't remember.

 

[constantineaudio]

Noisy but very good amp , This will be my test lab for octal champ layouts. Thank you all.