ANOTHER circuit question

[mresistor]

Hi guys. Am I going to have any trouble with hooking the feedback wire going to the first stage of the 6SL7 directly to the 16 ohm tap of this Baldwin case amp. I'm only planning to use the 8 ohm output. The connector the OT goes to is gone as I'm converting this amp to a guitar amp.

[sluckey]

No problem. Just don't forget the 3.9K resistor connected to the 16Ω tap.

[mresistor]

Is there any reason any of you can see that would blow a 5U4 rectifier when turning the amp on to test voltages on a lamp limiter with only the rectifier installed?  

I guess its possible that the old 5U4 that was in the organ fried the last time I turned on the organ. Don't know how or why but that old 5U4 is bad now. I only have two vintage 5U4s on hand and I don't want to ruin one testing this amp.

 

[mresistor]

Ah well - I couldn't wait - tried a known good 5U4 and have about 475vdc on the output. Next going to try it out connected to a load and with the 6L6G's in it.

[HotBluePlates]

I need someone like PRR to check me on this...

This Baldwin case amp, there's an actual preamp "down below" the section you posted, right?

The feedback loop runs from speaker to input cathode; the 3.9k, 390Ω sets the gain at 10 inside the loop. Guessing 25w output, that's 20v across a 16Ω load (or 20v at your 16Ω tap). 20v/10 = 2v input needed for full output.

If you plugged into just that 1st half of the 6SL7, you will only get clean sounds out and no where near full power.

[DummyLoad]

lose the NFB loop and overall gain of the paraphase as is should be around 18-20. won't get too gritty, but maybe loud and clean depending on the pickups and pick force. add another stage... 6J5/6SC7/12AX7/12AY7 etc.. better still graft a pentode (6SJ7/EF86/6AU6) to the front end.

joo don't need no stinking NFB! ;) 

--DL

[Tyrannocaster]

I agree - why would you want the NFB loop for guitar? Personally, I think these old organ amps (and hifi amps, too) sound a lot better without NFB but I also like the sound of cathode biased output tubes. Not everybody does, I have to keep reminding myself.

[tubeswell]

I vote for keeping the loop - you can easily disconnect it if you so decide. What's a few cents worth of resistors

[mresistor]

HPB - the preamp in the organ was up on the top of the console right behind the keyboards. It used a 6SN7 and a 5879 in the preamp. It is gone dismantled. Instead, I have a 6SL7 octal preamp built inside the case amp now, with a Fender tone stack. In my "what to build with this" thread there is a pic of the amp as it is now. I don't think the 16 ohm output was used for anything in the organ as the speaker was tied to the 8 ohm wire output. I am going to take pics of the inside and post in that thread as well. I will be trying it with no feedback soon.

[HotBluePlates]

HPB - the preamp in the organ was up on the top of the console right behind the keyboards. It used a 6SN7 and a 5879 in the preamp. It is gone dismantled.

I meant in terms of the schematic fragment posted. But it is good to know there is a preamp that's no longer available.

You have a paraphase inverter, output tubes and power supply. You'll need to graft a preamp of some kind to this output section.

You can slap whatever preamp you like ahead of the 50k pot feeding the 1st 6SL7 section. You'll probably want to swap that 50k for a 500k-1M pot to avoid loading down your preamp. Design or select your preamp to supply 2v or more to the output section.

I don't think the 16 ohm output was used for anything in the organ as the speaker was tied to the 8 ohm wire output.

It's used as the source for the NFB loop.

If you fire up a power transformer, the filament winding has voltage present even if you only use the high voltage winding. In the same way, all taps on the OT have a signal voltage present in proportion to their intended load impedance, even though only one is attached to a speaker.

The 16Ω tap is being used to derive the source voltage for the NFB loop, regardless of which tap you're using.

[mresistor]

HPB - I meant, 16 ohm tap not used for anything ELSE. Sorry.

See my other thread, I have a simple fender tone stack working in the amp already.

:-)

[HotBluePlates]

HPB - I meant, 16 ohm tap not used for anything ELSE. Sorry.

See my other thread, I have a simple fender tone stack working in the amp already.

:-)

Sorry. I hadn't read the other thread until now. Nice aluminum bracket for mounting the preamp!

PPR correct me if I am wrong,

You could always add a presence pot to the NFB loop. 

Yes, it could be done. The effectiveness of the added presence pot is directly proportional to how much gain the feedback loop throws away. If the loop is modded to the pot it doesn't provide much feedback, the presence will also seem like it barely boosts treble (if at all).

The value of the presence pot would be dictated by the shunt feedback resistor (390Ω in this case). Seems like a 5k pot will work nicely.