Ab763 simplified design

[Roe]

I'd remove the 470k to ground connected to the channel mixing resistors.

That 470K provides the ground reference for the tube grid. Without it the tube cannot establish bias.

If so, I'd increase the 470k to 1m or more. Or I'd provide a ground reference through the power supply (e.g. a bleeder resistor across a filter cap)

[pdf64]

I'd remove the 470k to ground connected to the channel mixing resistors.

That 470K provides the ground reference for the tube grid. Without it the tube cannot establish bias.

If so, I'd increase the 470k to 1m or more.

Could you explain your thinking

Or I'd provide a ground reference through the power supply (e.g. a bleeder resistor across a filter cap)

I’m confused with what’s meant above. A bleed resistor across a HT decoupling cap doesn’t help a grid that’s floating with respect to its cathode, which is the V4B scenario for the schematic in the OP https://ampgarage.com/forum/download/fi ... &mode=view

[Lynxtrap]

It wasn t my intention, I understand how to adjust the gain now but I prefer to keep the schematic design as genuine as possible an maybe work from there. I did the opposite with my first amp and I had hard time understand what was going on untill I brought everything back to original. I will put it back in its place, the 3.3m resistor I mean.

Besides what Sluckey has pointed out: You would need some kind of master volume after the 3rd stage for the overdrive channel. Otherwise it will be very loud compared to the clean channel.

[Roe]

That 470K provides the ground reference for the tube grid. Without it the tube cannot establish bias.

If so, I'd increase the 470k to 1m or more.

Could you explain your thinking

Or I'd provide a ground reference through the power supply (e.g. a bleeder resistor across a filter cap)

I’m confused with what’s meant above. A bleed resistor across a HT decoupling cap doesn’t help a grid that’s floating with respect to its cathode, which is the V4B scenario for the schematic in the OP https://ampgarage.com/forum/download/fi ... &mode=view

First, I was thinking the 470k provided unnecessary loading that reduces gain somewhat (prefering a higher value to increase gain). Second, I thought that there was already a ground reference here via the anodes of v1b & v2b (at least with a bleed resistor). But this could be wrong.

[Mikante]

It wasn t my intention, I understand how to adjust the gain now but I prefer to keep the schematic design as genuine as possible an maybe work from there. I did the opposite with my first amp and I had hard time understand what was going on untill I brought everything back to original. I will put it back in its place, the 3.3m resistor I mean.

Well it's your project so build it as you like. I was just trying to help fix some fatal flaws with your original schematic and put the two channels in phase so you could jump channels. FYI, that third gain stage (V4) cannot possibly work in your original schematic even if you put the 3.3M back in place.

And you did help but, you drew the new schematic around a mistake, my mistake, so i guess it was not your intention to leave that 3.3m resistor aside.
Anyway, why wouldn t it work? It is the same design but without tremolo and the reverb and for what i am learning here, using the third preamp tube for both channel shouldn t be a problem. I can t share the same knowledge as you do, so all i am saying here is just for talking and learning.
Any further advice will be much appreciated.

[sluckey]

And you did help but, you drew the new schematic around a mistake, my mistake, so i guess it was not your intention to leave that 3.3m resistor aside.
Anyway, why wouldn t it work?

I didn't draw around your mistake. I ***FIXED*** your mistakes. When you ripped the tremolo and reverb circuits you also crippled V4B. It's cathode goes to a point labeled "E" that no longer goes anywhere, so the tube is dead. Also you left the grid of V4B floating with no resistive path to ground. That means that even if you fix the cathode problem you still cannot bias V4B until you fix the floating grid.

Your normal channel is workable, but even if you fix the V4B issues, you still cannot jumper the channels together because they will be out of phase with each other and just cancel each other to the point of sounding very thin.

Anyhow, it seems I just wasted my time trying to help you so I'm just gonna drop out of this thread.

[Lynxtrap]

Anyway, why wouldn t it work? It is the same design but without tremolo and the reverb and for what i am learning here, using the third preamp tube for both channel shouldn t be a problem.

You need to take another look at the 3rd gainstage V4B. On your schematic it has no cathode resistor, no grid leak resistor and no grid resistor. That is why it won't work.

When you put the grid resistor and grid leak in place, you should probably arrange them to form a voltage divider that dumps a significant amount of signal to ground. Otherwise the 3rd gainstage will be hit with way too much gain from the 2nd stage.

[Mikante]

Anyway, why wouldn t it work? It is the same design but without tremolo and the reverb and for what i am learning here, using the third preamp tube for both channel shouldn t be a problem.

You need to take another look at the 3rd gainstage V4B. On your schematic it has no cathode resistor, no grid leak resistor and no grid resistor. That is why it won't work.

When you put the grid resistor and grid leak in place, you should probably arrange them to form a voltage divider that dumps a significant amount of signal to ground. Otherwise the 3rd gainstage will be hit with way too much gain from the 2nd stage.

Hello, thank you.
I actually didn t notice it, i took the original schematic and erased both tremolo and reverb and doing so i left the cap and cathode resistor out by mistake, the v4 is the reverb recovery tube and it has those two components but they just show up in the reverb circuit draw. It was just an oversight from me.
My intention is to only leave the cathode cap in both first stages, for the second stage there will only be a 1.5k resistor, like an early 5f1 design.
Then, it is not mandatory for me to have a third gain stage, so i can get rid of the v4 and just go straight into the Ltp.
I m interested in the deluxe reverb design mostly for its early power tube distortion.

[Lynxtrap]

Hello, thank you.
I actually didn t notice it, i took the original schematic and erased both tremolo and reverb and doing so i left the cap and cathode resistor out by mistake, the v4 is the reverb recovery tube and it has those two components but they just show up in the reverb circuit draw. It was just an oversight from me.
My intention is to only leave the cathode cap in both first stages, for the second stage there will only be a 1.5k resistor, like an early 5f1 design.
Then, it is not mandatory for me to have a third gain stage, so i can get rid of the v4 and just go straight into the Ltp.
I m interested in the deluxe reverb design mostly for its early power tube distortion.

OK.
There is actually a way to have one more gainstage in one channel and still have the channels play "in phase". It can be done by connecting one channel to the "lower" half of the phase inverter, like this:
https://www.thetubestore.com/lib/thetub ... ematic.pdf

But you can NOT have negative feedback connected to the phase inverter if you do that.

[Mikante]

Thank you, i see, the feedback is pretty important to me.
the bassman a165 is pretty close to what i want.

[Mikante]

Here the new schematic.
as suggested, i will have two 5m trim pots in place of the channel mixing resistors, fine tune and then use the proper resistor.
This should make testing a lot easier and give me the the possibility to adjust gain for each channel.
I removed the second cathode cap for the normal channel.
I d like to use a 3 way switch for the feedback, any idea on different values?

F090792A-FAED-4FE1-98B0-320D9E3D28FC.jpeg

[Lynxtrap]

To vary the NFB, you should be looking at the 820R resistor (where it says "Feedback" on the schematic). IMO, a potentiometer in series with the 820R resistor is more flexible than a switch with set values.

[Mikante]

I have revised the schematic.
For the NFB, would you go for 25k or 50k?
I m confused about the 470k resistor down the v4 grid, providing grounding has been mentioned.
Also, that “C” on the v6b, what does it means?

[Lynxtrap]

I have revised the schematic.
For the NFB, would you go for 25k or 50k?
I m confused about the 470k resistor down the v4 grid, providing grounding has been mentioned.
Also, that “C” on the v6b, what does it means?

If you don't need to be able to cut off the NFB completely, 25k should be enough to give you a usable range. Try it out to see what you like. If you don't need more than 10k, use a 10k pot to get usable sounds from the whole sweep of the pot.

The 470k resistor is the grid leak for V4 and part of a voltage divider for the Normal channel. I would move it to the actual grid so that it comes after the grid stoppers (trimpots on the schematic) to make it act as a divider on both channels. As it is now it doesn't act as such on the Vibrato channel, while the grid stoppers (trimpots) for both channels are part of the voltage divider for the Normal channel, with the grid stopper for the Vibrato channel being in series with the grid leak as the "ground leg" of the divider.

For instance, look at the AB165 Bassman to see how the channels connect to the shared 3rd gain stage (ignoring the fact that the coupling cap in that one is common to both channels and placed between the stoppers and the leak).
https://www.thetubestore.com/lib/thetub ... ematic.pdf

Look up "voltage divider" on the net if you're not sure how it works.

The C is what powers your PI. The junction of the 82k/100k plate resistors for the PI (V6A and V6B) is connected to node C on the B+ line.
B to the screens, C to the PI, D to the preamp.

[Mikante]

Thank you very much for these infos.
New schematic.

1A4E3115-71AF-427F-8549-12DB832BCCED.jpeg