Winter amp project

[Casey4s]

This is the latest version of the preamp and power amp, and hopefully the last one. I didn't show the 6.3V with the CT on a hot leg as we discussed, it would just make this drawing too busy.

I am going to make a seperate sheet with switch wiring, that 6.3 CT, and some other details that would just clutter the pages up.

I am going to enter the values on my first drawing after I am certian they are correct so I don't have to reference a list.

[Casey4s]

I just added a Post PI Master Volume pot. I forgot to add this to the last revision.

[SomeoneYouKnew]

Hey Casey, your MV will work okay in the cathode bias mode, but will mess with the bias voltage sent to the grids in the fixed bias mode.

It's more direct to replace the 220k resistors with a dual pot. Also, you have misdrawn the connection for the NFB to the transformer secondary.

[Casey4s]

OK, thanks for the tips. I am going to redraw that part of the schematic and make it revision V LOL.

I was looking for a post PI model on the internet someplace, I had seen something like your rendition but I had no luck finding it.

Thanks for saving my ass again.

[Casey4s]

I redrew the offending areas and this is revision V.

[SomeoneYouKnew]

OK, thanks for the tips. I am going to redraw that part of the schematic and make it revision V LOL. I was looking for a post PI model on the internet someplace, I had seen something like your rendition but I had no luck finding it. Thanks for saving my ass again.

I remembered seeing one similar to the way you originally had it, but with additional coupling caps after the wipers of the MV. I thought it to be unnecessarily complicated and came up with the one I posted.

I did a little research after you mentioned seeing something similar on the net. It seems the guys at the amp garage came up with the same idea.

http://ampgarage.com/forum/viewtopic.php?t=10088



The 2.2Meg resistors are for protection. They insure bias will always be applied to the grids of the output tubes in the event of (wiper) contact failure. I think it's a great idea.

[Casey4s]

That looks a lot like what I saw someplace but I seldom go to Amp Garage so it may be the same drawing that showed up on another forum somplace. I added those two resistors to my drawing, I agree it is a good safety measure.

I am going to add a bleeder to the filter circuit too.

[SomeoneYouKnew]

I'd suggest a 330k in series with a 47k for the bleeder. Point (B) on your supply would be as good a place as any. This will take about 5 minutes to discharge the caps to about one volt. You'll have about 53 volts across the 47k during normal operation. You can connect the CT of your heater winding there.

 

You'll have a less than half a watt dissipated in the 330k, but use 1 watt resistors.

[Casey4s]

I added a few things to Rev VI.

Hum Balance pot. (6.3 has no CT)
Pilot light
Bleeder

I also redesigned my SPST and SPDT switches as well as a better looking DPDT for the "Fixed / Cathode" bias switch, and corrected some drawing errors.

[SomeoneYouKnew]

Hum Balance pot. (6.3 has no CT)

The hum balance pot is probably overkill. Even with the wiper grounded, two equal value resistors would work just fine. But leave it in. You'll be able to see something interesting happen.

 

With the wiper grounded, the pot will end up right around the center for minimum hum. But when you connect the wiper to the node between the 330k and the 47k, it won't matter where the pot is set. There is no way you'll get heater emission onto the cathode.

[Casey4s]

The hum balance pot is probably overkill. Even with the wiper grounded, two equal value resistors would work just fine. But leave it in. You'll be able to see something interesting happen. With the wiper grounded, the pot will end up right around the center for minimum hum. But when you connect the wiper to the node between the 330k and the 47k, it won't matter where the pot is set. There is no way you'll get heater emission onto the cathode.

 

 

Then maybe it would be simpler to just add a pair of 100 ohm resistors to ground, or to that resister junction. Since there are so many 9A type tubes I was concerned about heater balance.

[SomeoneYouKnew]

Then maybe it would be simpler to just add a pair of 100 ohm resistors to ground, or to that resister junction. Since there are so many 9A type tubes I was concerned about heater balance.

I would. Resistors are cheap and don't take any panel space. And the idea here is that neither end of the heater ever gets more than half the (heater) supply voltage below ground. If there is a slight mismatch between the two resistors, the difference in hum will be tiny.

 

If you can make use of of one of your SPDT switches for initial testing, you could switch between having the two 100 ohm resistors being referenced to ground and having them referenced to +50v DC. It should be obvious the difference this makes when you compare the two.

 

The early stages will be the most sensitive to hum from heater emission. The signal there is very small. So any hum that get in there is amplified a ton by the time it reaches the end of the signal chain.

[Casey4s]

I would. Resistors are cheap and don't take any panel space. And the idea here is that neither end of the heater ever gets more than half the (heater) supply voltage below ground. If there is a slight mismatch between the two resistors, the difference in hum will be tiny. If you can make use of of one of your SPDT switches for initial testing, you could switch between having the two 100 ohm resistors being referenced to ground and having them referenced to +50v DC. It should be obvious the difference this makes when you compare the two. The early stages will be the most sensitive to hum from heater emission. The signal there is very small. So any hum that get in there is amplified a ton by the time it reaches the end of the signal chain.

 

 

Sounds like a fun expieriment, I'll do just that. Although I believe you, I like hands on testing of new ideas, that's the only way I learn new things.

[Casey4s]

Here's the power supply with the 100 ohm resistors tied to the two resistors, and a cleaned up preamp drawing. The more I use this CAD software the more creative I am getting. (Check out the new reverb pan)

[Casey4s]

I finally decided on all the componet values and entered them on this page so I don't have to reference to a seperate sheet anymore.

I got the switches in the mail today for the midrange cut selector, and the magic inductor is on the Big Brown truck for tomorrow. As soon as I get my hands on the inductor I can start my layout on paper. I have no idea what that inductor looks like so I can't get started until I see it.

After I get the layout done I can make my shopping list for the rest of the components that I don't already have on hand.

If there are any obvious errors on this final build sheet, just say the word.

[Casey4s]

I finally got the inductor today, it's 1" dia x 1" high on a 1.25" square board. One corner of the board has been cut off as a marker but I am still stumped about which way this thing feeds. I am going to try and contact the Ampeg friend and ask if he knows.

But if anyone here has any thoughts, just post em here.

[SomeoneYouKnew]

the middle two taps will be physically obvious. they both have two wires going to the pins on the board. the ends will only have one.

 

you might be able to use an ohmmeter to determine which end is which, but I suspect the resistance of each section of the inductor is so low that you might not be able to tell the difference.

 

 

on your schematic in the previous post, you have a typo on the coupling cap for V1a's anode. I'm pretty sure you meant .022, not .002

[Casey4s]

the middle two taps will be physically obvious. they both have two wires going to the pins on the board. the ends will only have one. you might be able to use an ohmmeter to determine which end is which, but I suspect the resistance of each section of the inductor is so low that you might not be able to tell the difference. on your schematic in the previous post, you have a typo on the coupling cap for V1a's anode. I'm pretty sure you meant .022, not .002

Yeah I was wondering which was the ground and out leads I am hoping the Ampeg dude can tell me what the cut corner is for. That should be the key.

Good Eye, I did make a goof there and I will revise it to be .022 instead of .0022. It always helps to have another keeneye look things over, thanks.

I tested the switch with a DVM and I got these results which are exactly as you described in your previous post with the drawing.

[Casey4s]

Reported

[Zozobra]

Good to see you got the inductor.
What's with all the spambots in here recently?

[Casey4s]

Good to see you got the inductor. What's with all the spambots in here recently?

 

 

I have no idea about the spam bots, they seem to target this thread for some reason.

[Slunderfungus]

Out of curiosity, whats the point of dual bias?

[Casey4s]

Cathode biased amps and Fixed bias amps have a different "Feel" from one another, just like a SS or Tube rectified amp "Feels" different. This way I can optimaly have 4 different combinations of basic amp systems giving me different statring points with the amp.

[Slunderfungus]

I see how you have that set up in your schematic, but am having a rather difficult time understanding how the switch ing works for the dual bias. It looks like you have two switches that are on/off, where you turn one off then turn the other on, is that right?

[Casey4s]

Yes it's a DPDT toggle controlling the bias feed switching. A DPDT is actually two switches in one. One throw of the switch will perform both functions.