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DIY 5c1 style amp
DIY 5c1 style amp
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Old 14th October 2019, 07:27 PM   #51
Printer2 is offline Printer2  Canada
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So 18.5V across the 470R resistor, 0.0439mA. The GE datasheet says at 315V, -13V bias and 0.036mA, so not too far off of Fender's take. Seems the newer Champ's run the same voltages as the 360V (350V plate) in the Harvard schematic. They did increase the cathode resistor from 1W to 2W though, also added a 470R on the screen. So say 350-20 = 330V, multiplied by 0.040, 13.2W. So if we have 12W on the plate and 1.2W on the screen it would seem that the Champ is operating within its limits with Fender's values.
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Old 14th October 2019, 07:31 PM   #52
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Printer2 View Post
Effects loop can fit on a pef board, no heater current required. Put it after the volume control and before the second stage.


Click the image to open in full size.
Using LND150s instead of tubes makes total sense, but the 3.2 Hz input high-pass filter doesn't, nor does the unreliable biasing method (self-bias) for the recovery MOSFET.

It only takes two more components to turn this into a reliable biasing scheme. Add one resistor to bias the gate of the 2nd MOSFET with a potential divider, like the first, but chosen to put something like 20V DC on the gate. Increase the source resistor accordingly. And add the second component - a small resistor in series with the source bypass cap - to define the AC gain.


-Gnobuddy
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Old 14th October 2019, 07:40 PM   #53
Tubelab_com is offline Tubelab_com  United States
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Quote:
it would seem that the Champ is operating within its limits with Fender's values.
I have seen old Fender Champs that measure near 400 volts of B+ when plugged into the 122 volt outlet in Florida. Others have reported 380+ volts.

I haven't seen one here, but my wall outlet is around 125 to 126 volts on days with minimal electrical load. It drops to around 122 to 124 during the coldest days of winter.

The dissipation may be right at the edge of spec, and the B+ can be at or over spec, but even the old 6V6GT tubes seem OK with it. They live as long or longer than the 6L6GC's in a Bandmaster.
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Old 14th October 2019, 09:03 PM   #54
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by Tubelab_com View Post
I have seen old Fender Champs that measure near 400 volts of B+ when plugged into the 122 volt outlet in Florida.
The official Fender schematic for my '65 Princeton Reverb reissue shows 440 volts on the 6V6 anodes.

Probably not coincidentally, my second-hand PRRI came with big and beefy JJ 6V6S output valves in it.

I don't know what military-era Soviet tube this started out as, before JJ re-labelled it and changed the pin-out, but the JJ 6V6S appears internally identical to the JJ 7591. And IMO, doesn't look or sound much like current production "real" 6V6s that try to be closer to authentic.

-Gnobuddy
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Old 14th October 2019, 09:11 PM   #55
PRR is offline PRR  United States
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Originally Posted by Gnobuddy View Post
Using LND150s...sense... does the unreliable biasing method (self-bias) for the recovery MOSFET. ...
It makes sense. The LND150, unlike almost any other device we can name, is tightly specified for Id vs Rs, see page 5:
http://ww1.microchip.com/downloads/e...-20005454A.pdf

Also the output is a "small" signal (relative to supply voltage) so we don't need precision of operating point.

K.I.S.S.
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Old 14th October 2019, 10:52 PM   #56
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by PRR View Post
The LND150, unlike almost any other device we can name, is tightly specified for Id vs Rs, see page 5:
I think you are misinterpreting that datasheet.

The datasheet shows that Id vs Rs for one specific LND150 shows little variation with temperature (image attached.) There is no variation shown with Rs because the data is for one (bogey) FET.

I know that data is only for one single FE, because elsewhere on the same datasheet we see that Idss and Vgs(off) each vary over a huge 3:1 ratio (image attached.)

Just like ye olde JFETs of ancient times, apparently. Evidently the rumours of tighter tolerances in MOSFET parameters don't apply to the LND150.

300% variations in Idss and Vgs(off) automatically mean there will be large variations in drain current (Id) between different samples of the LND150, all with the same value of external source resistance.

Along with that will come large variations in headroom and voltage gain as well.

Isn't this exactly what good electronics design is supposed to eliminate? Large amounts of random variation between production samples?

As mentioned earlier, it doesn't take much additional complexity to do much better: just two additional resistors.

-Gnobuddy
Attached Images
File Type: gif LND_150_Is_vs_Rs_vs_T.gif (29.8 KB, 25 views)
File Type: gif LND_150_Vgs_Idss_Tolerances.gif (70.6 KB, 27 views)
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Old 15th October 2019, 01:55 AM   #57
Printer2 is offline Printer2  Canada
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I don't understand why I can no longer see the schematic, I copied the link and pasted it in my browser and it opens.



https://music-electronics-forum.com/...5&d=1445867855



I'm not married to the circuit, fix bias the second stage, I don't mind. I just did not have time to go through a lot of pages to find the ideal circuit just to find it is discarded. I just wanted to show that you did not have to use tubes, sticking it after the volume control does reduce the need for another volume control on the send. On the return I would hope the effect jumpered in has a volume control on its output. You could still get some triode clipping of the first stage before sending it out the loop. I am sure it can be tweaked a little more in terms of parts count and simplicity, smart enough people on the forum.
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