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Old 12th October 2011, 12:07 PM   #1121
tubekit is offline tubekit  United States
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Default The wire, down to, and wiring . . .

Hi tubelab! Thanks for the "secret sauce" recipes! I hope you're mending well.



@Printer2:
Quote:
". . . want it clean I have to keep the input volume down to 1 or 2 ans still have the gain I want when turned up to 10."
I'm not quite sure if I know just exactly what you mean by this (which is why I did not reply earlier) but it is either the same problem I have now decided to address, or the opposite of it, I can't be sure.

I still WANT the gain I HAVE when turned up to 10.

I had set things up so much to favor a great deal of distortion at high gain settings, that I couldn't get clean except at very low gain pot settings. This really limited the volume of clean coming out when the master volume was all the way up.

So I went back into the Radio Engineers Handbook and reviewed the material on pentode amps, coming across a graph showing a series of screen voltage curves superimposed on a graph with axes showing plate current and grid 1 voltage. Ah yes, approaching it that way! Choose the plate current and bias voltage FIRST, and then set the plate current using the screen voltage. Use a pot on the screen during development to find the resistance for the screen resistor that gives the right screen voltage! What a technique!

I needed a stronger clean signal, so I upped the plate load 150% and calculated the new plate current level that put one minus 62% of the B+ on the plate with that load. Then I chose my own grid bias to give the input to the tube more headroom. The reasoning was twofold: (a) I needed more clean gain (bigger RL) and (b) I needed clean to hang around longer as I turned the gain pot up, i.e., more headroom going into the stage before clipping, i.e., a bigger bias voltage. With the new values chosen I began to play with it, and tweak until I wasn't risking melting the screen grid (the 62% of Ebb across RL recommended book value for max gain wasn't going to work because it required exceeding the screen's rating. I ended up nearer to 50% of Ebb across the load at the sweet spot) and, voila!

It gives me clean for at least about 30% of the gain pot.

But now I've gotta get my upper end distortion back. Ah, no problem. I'll use a little DPDT switch and flip between clean at 30% gain, and fuzz grunge meltdown at 100% gain, flipping the bias resistor on one pole of the switch, and the screen resistor on the other to keep the plate current constant!

Or something.

Quote:
"And I thought I almost had this wrapped up."
I hear you! Now I've gotta find a way to keep from over-saturating the reverb's input transducer when these hotter signals come out the other side of the master volume.
Cause that is one ugly sound!

Let's see . . . can I fit another resistor under the hood here?
How about over there?
Hmmm, then again, how's it gonna look on the BOM?
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Old 12th October 2011, 07:44 PM   #1122
TheGimp is offline TheGimp  United States
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I tried something similar with "dial-a-distortion" using a pot on the screen but ran into the gain variation problem. So I used a dual pot with the gains reversed to try to overcome it but the tapers didn't work out. A switch is a much better idea.
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Old 12th October 2011, 08:58 PM   #1123
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Quote:
So I went back into the Radio Engineers Handbook and reviewed the material on pentode amps..... Use a pot on the screen during development
I just pot a pot on the screen (1 meg).....and a pot on the plate (1 meg)....and a pot on the cathode (5K)...and a pot in seried with the cathode bypass (5K). I tweaked until I got what I liked and then....(the important part) try a bunch of different tubes, because they are all different. Yes, the screen voltage is the magic knob for plate voltage and the cathode resistor is for plate current. The plate resistor sets the gain.
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Old 12th October 2011, 09:51 PM   #1124
TheGimp is offline TheGimp  United States
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Why not put a current source in the anode, pot in the cathode to adjust the anode voltage and pot in the Screen to adjust gain.
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Old 12th October 2011, 10:32 PM   #1125
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Quote:
I have a problem with the low E string though. And I probably have to dump some gain somewhere.
Hi Printer2, love your work, using your ideas (also Tubekit, the Gimp, Tubelab et al).

I have been addressing this particular aspect by tweaking the bypass cap on the small signal pentode's screen. You have 1u I think. I have been as low as .047u for bass roll off.

Keep up the thread.

Regards to all.

JimG
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Old 12th October 2011, 10:36 PM   #1126
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Quote:
Why not put a current source in the anode, pot in the cathode to adjust the anode voltage and pot in the Screen to adjust gain.
Hi Gimp,
I do that when I'm breadboarding - and for my own amps - but I design for minimum numder of pots on anything that leaves the shed.
Switches are still fair game tho'

Regards
JimG
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Old 13th October 2011, 12:16 AM   #1127
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Quote:
Why not put a current source in the anode...
Most pentodes become unstable when working into a current source. In ideal pentode is a constant current sink. Loading one with a perfect CCS wil make a bistable flip flop. Fortunately there are no perfect devices and a typical pentode can work into a CCS but the gain will be very high and somewhat nonlinear. Thermal drift and aging are big issues. A CCS in parallel with a resistor can make a good high gain stage, but the output impedance will also be very high. CCS loads work very good on triodes though.

There is some design information showing the use of pentodes with CCS's on Gary Pimm's web site.

Quote:
pot in the cathode to adjust the anode voltage and pot in the Screen to adjust gain.......I do that when I'm breadboarding - and for my own amps - but I design for minimum numder of pots on anything that leaves the shed.
I wasn't advocating using pots in the finished product, just explaining one of my design methods. However having too many pots can make it impossible to converge on the right solution without the knowledge of what each one does, and how they interact. Put 2 or 3 pots on each of 4 gain stages and you will never get there. To get 4 stages right, connect up the first stage only and tweak it up with a scope, then add the second......
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Old 13th October 2011, 01:10 AM   #1128
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Quote:
Originally Posted by djgibson51 View Post
Hi Printer2, love your work, using your ideas (also Tubekit, the Gimp, Tubelab et al).

I have been addressing this particular aspect by tweaking the bypass cap on the small signal pentode's screen. You have 1u I think. I have been as low as .047u for bass roll off.

Keep up the thread.

Regards to all.

JimG

The screen bypass is 0.1uF, only so much resolution on the pictures from photobucket I guess. A couple more.

Click the image to open in full size.
Click the image to open in full size.

Click the image to open in full size.

I used the 1.5M on the front for the screen, 5k cathode pot, resistance values pencled in on the front. 500k to the 6V6,, mounted a small 500k on top for volume from the triode, I have a switch on top for going from pentode to triode, a switch on the side to go between 100k to 220k in the pentode plate. Test clips for cap values. Not great but not bad way of finding what sounds right or not. Testing through three different speaker, each with their own sound and the 12" being really loud.

Thanks for the kind words, I am sure the others appreciate interested in what they are doing. Kind of silly though when you can get a Fender Mustang I for around $100. Oh well it keeps us off the streets. Speaking of silly I may have come full circle. My latest variation #15 is pretty close to what I planned for #2. Should have stopped there and called it a day. I'll be rewiring my test amp to see how it sounds and then finish my build which ever way I decide after that. Not much time left. And I have to get some kind of recording setup together also.
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Old 13th October 2011, 01:59 AM   #1129
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Beware of those old carbon comp resistors. Its doubtful that any of them are in spec, and some will change value with temperature and applied voltage. They can create some rather unique sounding distortion that might sound cool in a guitar amp, but can drive you crazy when trying to fix a 85 year old radio!
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Old 13th October 2011, 04:17 AM   #1130
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Originally Posted by tubelab.com View Post
Beware of those old carbon comp resistors. Its doubtful that any of them are in spec, and some will change value with temperature and applied voltage. They can create some rather unique sounding distortion that might sound cool in a guitar amp, but can drive you crazy when trying to fix a 85 year old radio!
It was just convenient to use. I measured the resistors before firing the amp up for the first time, crossed my fingers on the capacitors, brought up the voltage slow with a variac also. Once I finish the amp for you guys I am either going to build a Champ and will be replacing the caps and resistors. I am not a believer in the mojo of old parts, well maybe transformers and speaker. Actually I am at odds with this amp. With the tube rectifier and the more than ample 6v winding it begs to run a Class AB circuit. I measured the current and it is about 45mA on the 6V6 at 300v with the power transformer pretty much cold to the touch. I do not know how much I can pull out of the power transformer but if I could, building a Fender Harvard would suit me just fine.
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