Original Circuit topology, advice? Power supply and biasing question...

[Steelwitch]

Here is a basic topology for an amp i'm working on. You will see that a lot of the values are left blank, that's intentional! The end result for this amp will hopefully be something that sounds unique, with a very simple point-to-point construction and pretty streamlined circuit. I am really trying to construct a stiff power supply with only minimal if no sag, with a lot of touch sensitivity, almost in line with some dumble elements. I am very intentionally wanting to build a medium gain amp. I wanted to see if anyone here has any suggestions for the power supply. The transformer I have is 680vac with CT @ 120mA, a 50v bias supply which I will use for the fixed bias (I need to see how this can potentially be beneficial, it's there so i would love to use it, but if it sounds better cathode biased I just wont use it). 6.3vac heater supply as well.

I will probably filter all of the stages separately. Does anyone have suggestions of how I should bias the preamp and LTP? What would be a good starting point for an amp that favors overdrive coming from the LTP and Output tubes? I think that's the voicing I'm going for. Hopefully resulting in something that has a gradual range of clean to overdrive, if that makes sense. Or if anyone can think of any amps that might sound similar, please feel free to link a schematic. I'm using a two phase rectifier-- I've seen some with 2 diodes and some with 3, is this to avoid voltage sag?

also please excuse some connections in the output section that are technically incorrect, the program I was using didn't have the tube symbol i needed!




Opry Audio Minnow.pdf

[Chuck H]

That's actually A LOT of inquiry! Some of the answers (from myself and others) may be from a subjective perspective.

I am really trying to construct a stiff power supply with only minimal if no sag,..

With a diode rectifier you aren't limited on how many uF's you use for filtering. So maybe go with 100uf, then a 100R 15W resistor, and then another 100uf (this arrangement is often called a pi filter because it looks like the symbol for pi when you draw it on a schematic). Use this for the main, first filter.

Then comes the screen node. For a "stiff" power supply a choke would typically be used. This would add another transformer to the chassis and some expense to the project. So you could do that or just go with a resistor to feed the screen node. The resistor will be less stiff, but IME not saggy in any extreme way that you might find objectionable. Either way I'd just use 22uf for this node. YMMV and the choice is yours.

Then just go with 22uf for the following nodes. Start with something like 5k resistors in between PI and preamp nodes just because it's a typical value. You can always adjust for specific voltages later if needed. I think that should be stiff without being stringently so.

...a 50v bias supply which I will use for the fixed bias

I actually doubt that will be enough bias voltage for your plate voltage if you use 6V6 or 6L6 power tubes. You'll be ok if you use EL34's but good ones are harder to come by than good 6l6 types. I don't know what tubes you have in mind.

I will probably filter all of the stages separately.

Totally unnecessary to filter both preamp stages separately. You should only need four filter nodes. Plates, screens, PI, preamp tube.

Does anyone have suggestions of how I should bias the preamp and LTP?

Well, since you mentioned Dumble, why not start there for your cathode resistor and bypass cap values.?. Just look up the schematic on line. The topology is similar enough to yours that "where to use what" should be obvious.

I'm using a two phase rectifier-- I've seen some with 2 diodes and some with 3, is this to avoid voltage sag?

This isn't a sag issue and the number of diodes here won't have any affect on that. The number of diodes in series from either leg of the rectifier is for additional voltage handling. You should be fine with two 1n4007 types in series from each end of the winding.

[Steelwitch]

That's actually A LOT of inquiry! Some of the answers (from myself and others) may be from a subjective perspective.



With a diode rectifier you aren't limited on how many uF's you use for filtering. So maybe go with 100uf, then a 100R 15W resistor, and then another 100uf (this arrangement is often called a pi filter because it looks like the symbol for pi when you draw it on a schematic). Use this for the main, first filter.

Then comes the screen node. For a "stiff" power supply a choke would typically be used. This would add another transformer to the chassis and some expense to the project. So you could do that or just go with a resistor to feed the screen node. The resistor will be less stiff, but IME not saggy in any extreme way that you might find objectionable. Either way I'd just use 22uf for this node. YMMV and the choice is yours.

Then just go with 22uf for the following nodes. Start with something like 5k resistors in between PI and preamp nodes just because it's a typical value. You can always adjust for specific voltages later if needed. I think that should be stiff without being stringently so.



I actually doubt that will be enough bias voltage for your plate voltage if you use 6V6 or 6L6 power tubes. You'll be ok if you use EL34's but good ones are harder to come by than good 6l6 types. I don't know what tubes you have in mind.



Totally unnecessary to filter both preamp stages separately. You should only need four filter nodes. Plates, screens, PI, preamp tube.



Well, since you mentioned Dumble, why not start there for your cathode resistor and bypass cap values.?. Just look up the schematic on line. The topology is similar enough to yours that "where to use what" should be obvious.



This isn't a sag issue and the number of diodes here won't have any affect on that. The number of diodes in series from either leg of the rectifier is for additional voltage handling. You should be fine with two 1n4007 types in series from each end of the winding.

I was planning on using 6V6’s. Is there any way for me to increase the bias supply? I’m surprised because this transformer is spec’d for a tweed deluxe should I consider tapping the B+ for a bias supply instead? Or a combination of both?

For the rectifier, I will use only 2 diodes per side. Did you mean a 5K 5w voltage drop resistor? Not 15w.

Also could you clarify about the preamp power supply? I would have to filter them separately unless they were cascaded right? As in biased with a single cathode resistor

[Dave H]

I was planning on using 6V6’s. Is there any way for me to increase the bias supply? I’m surprised because this transformer is spec’d for a tweed deluxe should I consider tapping the B+ for a bias supply instead? Or a combination of both?

A 50V winding will be enough for the bias supply for 6V6.
V1B grid doesn't have a DC path to ground. C5 could be replaced by a short.

[eschertron]

V1B grid doesn't have a DC path to ground. C5 could be replaced by a short.

^this. As drawn, V1b won't bias properly. C5 is unnecessary. Maybe you were thinking about a grid stop there instead?

[Chuck H]

I was planning on using 6V6’s. Is there any way for me to increase the bias supply?

On second consideration, Dave is correct. You'll be fine with a 50V supply for 6V6's.

I’m surprised because this transformer is spec’d for a tweed deluxe

I dare say it's not. More likely a Deluxe Reverb. The tweed Deluxe would have a lower B+ and doesn't need a bias supply because it's cathode biased. Your amp will probably have a B+ around 430Vp or more.

Did you mean a 5K 5w voltage drop resistor? Not 15w.

I actually didn't stipulate wattage ratings for the screen, PI or preamp node dropping resistors. The 15W resistor in question would be the 100R used in the pi filter for your first filter node. If you choose to use a resistor in the B+ rail for your screen supply use something like a 1k 5W. Then to the PI would be 5k 3W, then to the preamp would be 5k 3W.

Also could you clarify about the preamp power supply? I would have to filter them separately unless they were cascaded right? As in biased with a single cathode resistor

Your preamp stages are cascaded. Sharing a cathode circuit would actually be a bad idea for most cascade gain stages. You can feed both plate resistors in the preamp from the same node without any problems at all.

[Chuck H]

And here is an alteration of your above schematic with the observations made by Dave and eschertron noted. Along with a couple more of my own. Alterations are circled or noted in red.

The 1M resistor at the input needs to go to 0V. as it was shown it was paralleled by a short and going to nothing.

C5 isn't needed and in fact would foul the bias of the second gain stage.

The volume control looked to be wired as a variable resistor. It's better to use it as a voltage divider because loading the tone stack with a variable resistor would foul it's operation at low volume settings.

The NFB circuit at the tail of your PI is usually put on a variable resistor as a "presence" control because having it hardwired like do is usually too bright. YMMV

I'm showing the rectifier as two diodes per winding end just because we discussed it above. Truthfully two or three doesn't really make any difference to the sound.

I moved your fuse to the switched side of the AC into the PT primary. The fuse and the switch should be on the hot AC lead (black) and not neutral (white). This is recognized as the right, and safest way to do it. Ignore that a million amps have been done as you originally drew it up and that some still do. Safety first.

[Dave H]

The Treble control should also be a voltage divider with its wiper connected to the vol pot CW terminal.

[Chuck H]

The Treble control should also be a voltage divider with its wiper connected to the vol pot CW terminal.

OOoo... Missed that one I'll alter the alteration and reload it.

On that note... I also notice that the tone stack has no mid elevation resistor. This would be like having a mid control all the way down all the time. (I'll make a note on the modified schem).

Between the stiff power supply, the presence on 10 circuit thing and mid on zero circuit thing, along with the typically bassy BF type topology, I would expect this design to sound very mid scooped and maybe a little raspy and boomy. Unique indeed!

[eschertron]

The entire tone stack circuit should be modeled on (copied from?) a Fender stack, at least initially, so that the components have a value and the pots are all connected properly. Note the "bass" pot still has it's wiper floating. There are many variations on the Fender tone stack, use whatever you want. Duncan's TSC comes in handy for this.

Note that specifying the correct source and load impedance is important for any model to represent accurately. Your preamp topology puts the tone stack into the "Fender" design family, impedance-wise.

[Steelwitch]

This all makes a lot of sense! Thanks for your help!

As for the NFB, I am not totally100% that it will even be used, but something I will experiment with. I would love to have only a volume, treble and bass control and have a fixed NFB which I may dial in with a pot and then install a fixed resistor where it sounds most natural, if I use it at all.

Thanks for the corrections on the tonestack! I am ultimately looking to have the treble knob as sort of a tone control, and then have the bass control quite a shallow boost/cut from 0 - 500Hz if that makes sense. So in that sense it would give you some control over the mids, as sort of a way to cut some muddy bass frequencies when overdriven and can easily sit in the right place when mixed with a band. I would love some TB tonestack suggestions!

[Steelwitch]

The entire tone stack circuit should be modeled on (copied from?) a Fender stack, at least initially, so that the components have a value and the pots are all connected properly. Note the "bass" pot still has it's wiper floating. There are many variations on the Fender tone stack, use whatever you want. Duncan's TSC comes in handy for this.

I love the TSC just something I missed when drawing up the schematic!

[Steelwitch]

Here is a revised schematic with a lot of it filled in, at least theoretically this should work and sound decent. I would love to have a good place to tweak from. Do I have other options in terms of power supply? Let's say I was not overly concerned with stiffness-- could I drop voltage without a 15w resistor in place of the choke? More like a tweed style power supply? I'm shooting to feed the PI 350v and have 100v on the tail. Would this configuration work?

[Chuck H]

Another good CAD program from Duncan:

http://www.duncanamps.com/psud2/

It takes some tech to use because you need to be aware of things like the PT winding ohms, different configuration options and what they mean and what current each node will draw, etc. It's a learning tool as well as a design tool. Not hard to get all the pieces together and you'll grow as a designer by learning it. I've said before that an amplifier is little more than a modulated power supply. In that light it's strange that the preamps and power amps get all the attention and the power supplies get treated like peripheral circuitry.

[Dave H]

I notice the output stage feedback is now connected to V2A grid. Is that correct?