2 wrong values in an EQ section

[pbman1953]

In the EQ section, I noticed on a Super twin that the are 2-10uf's/ 500v instead of 2-4uf's.


What would be the difference if anything?


Thanks

[sluckey]

   I know this one... Is it 6?   

Or maybe it's 12. Crap. Is this a trick question?

[Dave]

Okay, I have to admit, that was funny.


Dave

[pbman1953]

It may be funny but doesn't answer my question unless I don't get the joke

[PRR]

Yeah, about 6.

He seems to mean these parts are 4u on plan and 10u in amp.

The real goal is to be (near equal! and) much-much less reactance than 22k Ohms at lowest working frequency. Actually, less than 1/Gm of V2, which you may note is doubled-up. 1Kohm at 50Hz is 4uFd. 10uFd at 50Hz is 250 Ohms, maybe wee bit better.

Another way to see it: bigger than the 1.0uFd in the 100Hz band filter. 4 is bigger than 1. So is 10.

4 should be cheaper but it is a very odd value and 10 may be more available.

[pbman1953]

Yeah, about 6.

He seems to mean these parts are 4u on plan and 10u in amp.

The real goal is to be (near equal! and) much-much less reactance than 22k Ohms at lowest working frequency. Actually, less than 1/Gm of V2, which you may note is doubled-up. 1Kohm at 50Hz is 4uFd. 10uFd at 50Hz is 250 Ohms, maybe wee bit better.

Another way to see it: bigger than the 1.0uFd in the 100Hz band filter. 4 is bigger than 1. So is 10.

4 should be cheaper but it is a very odd value and 10 may be more available.

Appreciated!




Also, both amps at the front end, have a 750 uf. My Studio Bass has a 470uf instead of the 750uf. This amp has a 680uf,but the Studio's gain is sooner. Is that the reason? 

[WimWalther]

"4 should be cheaper but it is a very odd value and 10 may be more available"

May also be the 10uF part is used in several other locations / models and as such it's cheaper to use here due to economy of scale (they are bought in large quantities).

[PRR]

> 750 uf. .... 470uf instead ...This amp has a 680uf,

This (anything over 25uFd) is about hum from a leaky cathode. If the cathode is clean, there is no need for a super-size cap. If the cathode leaks, the more cap the better. But never spend more than you need to to sell the product.

[pbman1953]

If it was yours, would you put in the 25 or leave it?






in that area, besides the tube, what part has the most effect on gain?

[PRR]

> in that area, besides the tube, what part has the most effect on gain?

[pbman1953]

> in that area, besides the tube, what part has the most effect on gain?

Thanks, I suppose that those should left alone ?


Also, did you see my question about leaving the 680uf in favor of the 25uf on pin 8 of 7025V1A. Or would you install a 25uf?


Thanks

[PRR]

680uFd is a 44 cent part. Leo thought it was worth the cost. Buy it and try it.

[pbman1953]

680uFd is a 44 cent part. Leo thought it was worth the cost. Buy it and try it.

Looks like I have to clear up something. The schematic calls for a 750uf. On someone's suggestion I was told to get one close because the 750's are hard to find. I found a 680 so I installed that. 

[PRR]

I'm sure the exact number is immaterial.

Anyway at +100%/-50% tolerance, 750 and 680 are the same part?

[pbman1953]

You're asking or telling me, I'm confused

[HotBluePlates]

You're asking or telling me, I'm confused

He's telling you. "680 = 750" near enough with the tolerance these parts have.  Which was the same point made about 10µF rather than 4µF in your initial question.

[pbman1953]

You're asking or telling me, I'm confused

He's telling you. "680 = 750" near enough with the tolerance these parts have.  Which was the same point made about 10µF rather than 4µF in your initial question.

I understand and thanks but the original suggestion, even months back when asked, was to install a 25uf.


How does the 25uf stand now? Forget it and leave the 680? Wil the 25uf make any difference or none?

[HotBluePlates]

... the original suggestion, even months back when asked, was to install a 25uf. ...

That was another misunderstanding.

This (anything over 25uFd) is about hum from a leaky cathode. If the cathode is clean, there is no need for a super-size cap. If the cathode leaks, the more cap the better.

25µF is an effective bypass for a 1.5kΩ cathode resistor down to:
   Frequency = 1 / (2 x π x R x C) = 1 / (2 x π x 1500Ω x 0.000025 F) = ~ 4Hz

You asked about gain, but PRR's note is that "25µF" gives full-gain down below the audio range.

But then he said, "hum from a leaky cathode," as in heater-to-cathode leakage hum.  That hum would be 60Hz, about a 2nd-fret "B" on your bass.  Since both amps in the question seem to be bass amps, killing 60Hz dead seems a priority, so the used super-big bypass caps on the input stage where leakage hum would be very annoying.

And in this application "750" = "680" because neither are about some kind of precision-target, they're about squashing the possibility of hum.

[pbman1953]

Ok, I think I understand better now. Let me know if I'm on the right track.


The 25uf cap would be enough to reduce hum below the hearing range to 4 hz.
Anything else would be overkill. Also, if larger than 25uf,  hum wouldn't have a chance to happen

[HotBluePlates]

Ok, I think I understand better now. Let me know if I'm on the right track.

The 25uf cap would be enough to reduce hum below the hearing range to 4 hz. ...

No, 25µF is not enough to kill hum.  It is big enough avoid gain-reduction by bypassing the cathode resistor, and is effective at avoiding gain-loss as low as 4Hz.

... if larger than 25uf,  hum wouldn't have a chance to happen

This is the hope of using the massive caps.  IMO, it is why the tweed Bassman has a 250µF bypass cap; the large capacitance is not needed for stage-gain, so it must be there to nix hum.

Experimental results confirm this, but you have to have a bum preamp tube that has heater-to-cathode leakage to know that it works.

[pbman1953]

Ok I get it.


I noticed that my Traynors and Sunn use the 250.


Last question- when the cap is increased, is there any effect on sound or volume?

[HotBluePlates]

... when the cap is increased, is there any effect on sound or volume?

No.  Except that you hope it reduces the volume of hum to zero.

[pbman1953]

Thanks so much!

[pbman1953]

> in that area, besides the tube, what part has the most effect on gain?

First off , thanks for your help on this thread. I'm in the clear as far as the hum cap.


On this diagram you sent. There's a difference between the Studio Bass and the Super Twin in this exact spot. The Super Twin has a 3.9k to ground and the 47k to the EQ. The combination for the Studio is 47k(to EQ)  & 2.7k to ground.


Would that 1k difference effect the volume control/ sound?

[PRR]

> Would that 1k difference effect the volume control/ sound?

Yes, it affects gain. It is reasonable to think Fender was on the same path you are on, tweaking the amplifier.

[pbman1953]

Sorry, I'm back with a peculiar issue. My Super twin is a combo board of the Super twin and the Super twin Reverb. This amp was never a a Reverb version but could of been on the line that the Reverb boards may have been used for the non-Reverb units.


What's puzzling is to decipher the wiring.

I sent the schematics for both and 2 pictures. I have 3 pictures, but the 3rd will be sent after this write up. I'm maxed out for attachments


If you look at the ST -non-Reverb schematic, picture 1- You'll notice that 7025 v1b shows pin 3 connecting to a 1.5k resistor. Mine has a 25uf/50v cap with a 820ohm resistor across it to ground. The 25uf may have been suggested in the past but no one said to change the 820 ohm back to a 1.5k, as on paper.

On the negative side of this connection there's a loop wire connection (picture 1) to a 5uf cap which is connected to a 10k resistor, on it's negative end. On the cap's positive end there's also a 3.9k resistor, Picture 2. The 3.9k is supposed to be on the negative side of the cap and resistor. then grounded, its not. Also connected there , on the cap positive end is 7025V3A's pin 8.

At the other end of the 3.9k resistor, there's a 47k resistor attached.  At that joint there's 2 green wires. One comes from 7025v3A, pin 7, a second leads to a 33k resistor /.1uf cap that goes to the EQ section. Connecting to a black 10uf /500v cap's, negative end. Picture 3 that will be sent to show that. On the non-Reverb, the 47k goes to a .1uf cap with no mention of the 33k resistor before the .1uf cap then leading to the 10UF cap negative end.  Picture 4

On the green wire from pin 7. this wire connects to the 3.9k and 47k resistor. From what I can tell, on paper, the 3.9k resistor should be connected on the negative side of the 5uf cap then go to ground.  The 49k is supposed to continue on to a .1uf cap .  The 49k resistor is actually is connected to a 10k resistor, along with the negative side of the 5 uf cap, It's not connected directly to the .1uf cap. It looks like a triangle with the 5uf cap at the bottom and the 2 angles are the 3.9k and 47k at the top of the triangle.

With the Reverb version I don't see the connections. this seems more like the non-Reverb but the triangle of the 5uf cap and 2 resistors are not clear.


This section is puzzling.

[pbman1953]

picture 3

[pbman1953]

HI Gents,


After some considerable digging I found the mysterious 3rd schematic that was made before the Reverb version of the Super Twin. Many of you may of thought I was annoying but having the schematic makes sense and was very hard to find.


I'll send it here for anyone to examine and keep for their records


Thanks to anyone that helped and was patient with me!

[Brownie]

It may be funny but doesn't answer my question unless I don't get the joke

  Sorry, either do I.  I'm guessing that choice of filters is okay?

[Brownie]

Aah, it's a subtraction equation! The difference

[PRR]

.

[pbman1953]

.

Thank you!!


Can you think of any worth while updates or do you feel it's fine as is?


Thanks!

[pbman1953]

Anyone want to take a stab of the previous question?