# Schematic Help

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#### Atomic_Sheep

##### Member
I started building a clone and encountered some questions regarding the schematics:

1.) Why does this capacitor have a rating of 1kV?

2.) What unit is next to the 25? It doesn't look like the V just has cut off tops. If you compare to other V's, the Vs have a rather pointy end, this one looks more like a u but that doesn't make sense, at least to me.

3.) If there are no units, then I presume for capacitors we're talking farads and for resistors we're just talking ohms?

4.) Why is one PF and the other one is uf? Why is there a difference in capitalisation of the farads?

I'm no electronics expert, learning as I go along, but details matter and I don't want to blow anything up.

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#### jpanhalt

##### Well-Known Member
Most of your questions are simply answered as typos or sloppy review. We all make such mistakes.
@#2: The unit appears to be "V" for volts. That is not surprising, as 6800 uF is a fairly large capacitor.
@#3: No units is most likely uF for capacitors. Agree that a resistor without a unit is in ohms. Note that "3K3 " means 3.3 kohms

#### Les Jones

##### Well-Known Member
1 The voltage across the capacitor will be 488 volts. It could be higher than this as the transformer will probably give more than 345 volts when lightly loaded. So you would probably choose one with at least 700 volts rating. Probably 1Kv is the next highest voltage rating after 500 volts.

2 V (For volts.)

3 I think for capacitors it will be uF in the case of C13. When it is in hundreds I think it will be pF. For resistors I think it will be ohms.

4 It is easier to write 47 pF rather than 0.000047 uF and it is easier to write 10 uF rather than 10000000 pF I don't think the person drawing the schematic meant any difference between upper case and lower case for Farads.

Les.

#### Atomic_Sheep

##### Member
Thanks for the prompt response guys!
I guess good and bad news. Makes me worry about how reliable these schematics are . When I talked about blowing up components, I was joking, now I have a feeling that jokes aside, it can happen.
I had a think and at the moment I'm guessing but I think I'm going to assume caps lock was on when 47PF was typed. Because there's a whole bunch of 47pf capacitors in the schematics, but this is the only 47Pf/PF capacitor. That's a very big difference and I have a feeling caps lock was to blame. I'll put this on my to do list i.e. to see whether I can figure out the logic behind the electronics.
Back to it!

#### audioguru

##### Well-Known Member
Why are you trying to make a circuit with parts that are about 80 years old?

#### AnalogKid

##### Well-Known Member
1. 1 kV was a common rating for small capacitors in tube days.
2. 6800 uF, 25 V. Yes, that is a V.
3. No units = microfarads C13 is 0.22 uF.
4. Because picofarads are 1,000,000 times smaller than microfarads, and writing 0.000000047 uF on a part is hard.

4a. Sloppy reference designation practices. There actually is a written set of standards for using the abbreviations of scientific quantities. For example, volts, ohms, henries, and farads are not capitalized when spelled out, but the abbreviations, V, <omega>, H, and F, are capitalized because those quantities are based on proper names. The abbreviation for siemens is S, but for seconds it is s.

But - people take liberties in schematics to make things more legible. It is common to see all capital letters in all abbreviations.

ak

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#### Les Jones

##### Well-Known Member
Hi AK,
I think you have made a typo on answer 4 1000 should be 1000000

Les.

Oops. Edited.

ak

#### Atomic_Sheep

##### Member
Why are you trying to make a circuit with parts that are about 80 years old?

Because the sum of those 80 year old parts (I'm guessing you're referring to the tubes?) come at a modern price of \$5,000 RRP and I don't fancy spending that much for them . But I'm sure it's not a simple case of finding any old parts for this thing to reproduce the tone of the original, but there's a lot of unknowns currently for this project for me.

1. 1 kV was a common rating for small capacitors in tube days.
2. 6800 uF, 25 V. Yes, that is a V.
3. No units = microfarads C13 is 0.22 uF.
4. Because picofarads are 1,000,000 times smaller than microfarads, and writing 0.000000047 uF on a part is hard.

4a. Sloppy reference designation practices. There actually is a written set of standards for using the abbreviations of scientific quantities. For example, volts, ohms, henries, and farads are not capitalized when spelled out, but the abbreviations, V, <omega>, H, and F, are capitalized because those quantities are based on proper names. The abbreviation for siemens is S, but for seconds it is s.

But - people take liberties in schematics to make things more legible. It is common to see all capital letters in all abbreviations.

ak

Great thanks!

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#### schmitt trigger

##### Well-Known Member
The schematic has several issues that would have been caught if it had been properly reviewed.

For instance, there are several JFETs whose gates appear to be not connected anywhere.
And which don't have reference designators, either.

#### AnalogKid

##### Well-Known Member
The FETs are analog switches driven by a part of the schematic not shared. But I agree with the missing ref designators and signal names.

AND, Why are there *three* sets of tone controls plus a multi-band graphic equalizer?

ak

#### Atomic_Sheep

##### Member
If you are interested, the link to the full 8 pages of the schematics are here:

https://www.thetubestore.com/lib/th.../Mesa Boogie/Mesa-Boogie-Mark-V-Schematic.pdf

schmitt trigger, I think I see what you're saying. Complicates things somewhat.

AnalogKid it's a three channel amp. But I think I see what you're saying, having tone controls plus is a graphics equaliser is a bit redundant. I'm not sure why it's done that way to be honest, good question.

#### Nigel Goodwin

##### Super Moderator
AnalogKid it's a three channel amp. But I think I see what you're saying, having tone controls plus is a graphics equaliser is a bit redundant. I'm not sure why it's done that way to be honest, good question.

It's an instrument amplifier, it's good to have as many options as possible, as you're trying to colour the sound to get what you want. It's also quite likely the amp was available with or without the graphic, hence the tone controls were there already

#### AnalogKid

##### Well-Known Member
The graphic eq might be there to reduce feedback.

ak

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