Blackface CF preamp mods

[SILVERGUN]

Continued from this thread: http://el34world.com/Forum/index.php?topic=15798.new#new

I took a couple of minutes and desecrated one of sluckey's beautiful schematics in an attempt to have a visual for the suggestions that were given in the links on that old thread....
So this is my take on a single channel version of what is being suggested:

[sluckey]

You may want to block that high DC voltage that's headed to the volume control.

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DOH! 
Edited schematic to reflect sluckey's catch, but not sure if that was intended by the "modder" 

[Willabe]

Edited schematic to reflect sluckey's catch, but not sure if that was intended by the "modder" 

He made a mistake when he left it out.


                   Brad   

[SILVERGUN]

Edited schematic to reflect sluckey's catch, but not sure if that was intended by the "modder" 

He made a mistake when he left it out.

I get that....
I meant to imply that when following someone's instructions word for word,,,if they left that out....what else?

I must say that's the first time I've seen the volume control preceed the tone stack.

I'm gonna take a couple minutes later and see how my schematic matches up with those amended layouts in the coolnorm.pdf

[HotBluePlates]

I must say that's the first time I've seen the volume control preceed the tone stack.

Then why not switch their position to what you're familiar with?

It would seem that if the volume is not somewhere near maximum, that the nice low source impedance of the cathode follower is being raised by the volume control resistance. I don't see a reason to have the volume control first (but maybe I'm slow...  ).

[PRR]

> block that high DC voltage that's headed to the volume control.

You could just as well (and 50 cents cheaper) put the Volume *after* the tone-stack, as Fender did in many Blackfaces.

I'm uncomfortable with C12 being shared over two sequential stages. That's an oscillator. The mix network loss may keep it out of trouble. But for another 50 cents you could do separate cathode networks.

And R5 could be 820 Ohms, like Fender did on his direct-coupled volt-amp catchode-follower affairs. 1.5K puts the first plate way high for the cathode follower. This mix-up is often found on modern designs, and may lead to high distortion (which may be desired). Flipside is that R12 could probably be the 1.5K that Fender used everywhere else.

[sluckey]

I'm uncomfortable with C12 being shared over two sequential stages. That's an oscillator. The mix network loss may keep it out of trouble. But for another 50 cents you could do separate cathode networks.

That's straight from Leo's AB763 reverb amps. That 50 cents (probably a nickel back then, went in Leo's pocket. Heck, he even powered six triodes from the same filter cap in those amps.

[SILVERGUN]

Man, did I screw the pooch on this one...
My original intention was to provide Willabe with a schematic that matched the word for word mod instructions in post 1....

But, I realize now that he was asking for a schematic that reflected the hard to decipher layout changes from the coolnorm.pdf

Looking at them now I don't see many similarities that would make the 2 different mods 'relative'.....so I'll just stick to the notion that I was transcripting the spoken word mod 

I must say that's the first time I've seen the volume control preceed the tone stack.

Then why not switch their position to what you're familiar with?

I was trying to follow instructions..
I thought that if it was important enough for him to go back and write a correction, than I should include it in my "screwedupmatic"

Geetar hackers like to imagine that there's gonna be one game-changing component flip that will make the heavens open up and rain excellence down upon us. 

If I get a minute tomorrow I'm still gonna try to provide our friend Brad a schematic from the coolnorm suggestions

[Willabe]

If I get a minute tomorrow I'm still gonna try to provide our friend Brad a schematic from the coolnorm suggestions

 

Like Sluckey has said many times, looking at the electrical schematic (not the layout) helps you to see the circuits flow.

I'm thinking he drew up layouts just for guys to be able to try/wire up the mods quickly and see/hear if they like them?



                            Brad                                    

[SILVERGUN]

Like Sluckey has said many times, looking at the electrical schematic (not the layout) helps you to see the circuits flow.

it's a personal preference thing....

I really enjoy perusing a nice clean schematic....sometimes my wife will say "what are you looking at?"...and i'll say "space shuttle stuff" 
.....I prefer to keep her thinking that this is more difficult than it actually is, then I don't have to explain why it takes up so much of my 'free' time.

I'm thinking he drew up layouts just for guys to be able to try/wire up the mods quickly and see/hear if they like them?

I have to agree....and I also agree that it makes it much harder to see 

[HotBluePlates]

I'm uncomfortable with C12 being shared over two sequential stages. That's an oscillator. The mix network loss may keep it out of trouble. But for another 50 cents you could do separate cathode networks.

That's straight from Leo's AB763 reverb amps.

Would the oscillator circuit be 1st & 3rd sequential triodes sharing a cathode connection (in a hypothetical amp)?

Steve called it; straight from the blackface reverb circuit.

[SILVERGUN]

I must say that's the first time I've seen the volume control preceed the tone stack.

Then why not switch their position to what you're familiar with?

I was trying to follow instructions..
I thought that if it was important enough for him to go back and write a correction, than I should include it in my "screwedupmatic"

Ed, can you tell from the book if it was Dave Funk's intention to have the volume control preceed the tonestack?......and if he comments on that at all?

[Mike_J]

Just another thought concerning the CF in blackface amps.  CF needs another triode.  Tubenit solved that with the use of a mosfet on his D'Mars amp.  The mosfet circuitry acts as the second triode. 

If I am recalling correctly the tone stack on the AB763 is tied to the plate of V1A and the volume controls follow on triode 1B.  The cathode follower I used in the 5f6a amp starts with the first tube having a normal volume V1A and bright volume V1B and then both triodes of V2 are used for the cathode follower tone stack. 

I do not have a lot of experience in these matters but it seems that the best way to place a cathode follower tone stack in a blackface would be to place the volume control before the tone stack and use Tubenit's mosfet circuitry as your second triode for the cathode follower tone stack.

Just my two cents.

Thanks
Mike

[Ed_Chambley]

Some have used a Mosfet.

http://www.blueguitar.org/new/text/threads/from_adam/fender-blackface-mods.pdfHere is the whole thread.  The book is on loan to a friend, but I will get it back in a week or so.  I think Willabe said he had it.

I have done a lot of this stuff to my ole 73 SR, but I reversed all the mods recently to sell it.  Send Steve Aloha a message on facebook.

http://www.facebook.com/steve.ahola

R.G. Keen at geofex has this to say: http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

[Ed_Chambley]

Start With V2 on a Fender Reverb Amp. Disconnect The wire on pin 8 V2 that runs to pin 8 V1. Give V1 pin 8 it's own 1.5k/25uf. Make V2 a Cathode Follower with a 100k across the socket. B+ in on pin 6. Add a 100k to ground on pin 8 of V2 and the tone caps should be 250pf Silver Mica, 100k, .022, .002. This connect to 100k resistor you ran to ground off of pin 8 V2. V3 which is a 12AT7, remove jumpers off of pin 1,2,3. V3 pin 3 make 820ohm/25ufV3 pin 1 add 100k plate load resistor and connect the .02 that go's to the 500pf reverb input capcontrol Place a 100pf SM on your bright switch.Place the cap on the legs of the switch, Not one leg of the cap to the switch and one to the volume pot. Leg one of the volume control runs to the bottom of the bright switch. The top of the bright switch connects to pin 2 of the volume control and also runs to pin 2 V3. If you bright switch is wired wrong it will act more like a volume boost than a bright switch.

[Ed_Chambley]

I will get you more later, but this should help. 

[SILVERGUN]

Just another thought concerning the CF in blackface amps.  CF needs another triode.  Tubenit solved that with the use of a mosfet on his D'Mars amp.  The mosfet circuitry acts as the second triode. 

Thanks Mike!
In this specific application we are 'gaining' a triode by splitting the reverb send stage of a paralleled 12AT7, and just using one half of that tube to push the reverb xfmr, to free up the extra triode needed for the CF

I believe that it was done this way so that guys who were just modding their old blackface amps wouldn't need to add anything..

For me, it works because I have found that I like to reduce the reverb send signal by removing the shared cathode bypass cap anyway...so I won't mind losing that other triode,,,but I will put a cap back on that stage if it will just be a single triode

[SILVERGUN]

Start With V2 on a Fender Reverb Amp. Disconnect The wire on pin 8 V2 that runs to pin 8 V1. Give V1 pin 8 it's own 1.5k/25uf. Make V2 a Cathode Follower with a 100k across the socket. B+ in on pin 6. Add a 100k to ground on pin 8 of V2 and the tone caps should be 250pf Silver Mica, 100k, .022, .002. This connect to 100k resistor you ran to ground off of pin 8 V2. V3 which is a 12AT7, remove jumpers off of pin 1,2,3. V3 pin 3 make 820ohm/25ufV3 pin 1 add 100k plate load resistor and connect the .02 that go's to the 500pf reverb input capcontrol Place a 100pf SM on your bright switch.Place the cap on the legs of the switch, Not one leg of the cap to the switch and one to the volume pot. Leg one of the volume control runs to the bottom of the bright switch. The top of the bright switch connects to pin 2 of the volume control and also runs to pin 2 V3. If you bright switch is wired wrong it will act more like a volume boost than a bright switch.

Thanks Doode.
I'll check that out later and see how far off my schemo is....

I think I'm gonna try this with the TDR on my bench,,,that's why I'm pushing......not just for conversation

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And for anyone else who might be reading along here....
I am working with a single channel preamp, so even though the instructions that Ed posted say to start on V2, my schematic starts with V1

I like to try things that I have never tried,,,that's why the first stage CF is interesting to me......we'll see 

Ed,,,those instructions are telling me to put the volume control post tone stack, so that's where i'll try it first.

[Ed_Chambley]

Start With V2 on a Fender Reverb Amp. Disconnect The wire on pin 8 V2 that runs to pin 8 V1. Give V1 pin 8 it's own 1.5k/25uf. Make V2 a Cathode Follower with a 100k across the socket. B+ in on pin 6. Add a 100k to ground on pin 8 of V2 and the tone caps should be 250pf Silver Mica, 100k, .022, .002. This connect to 100k resistor you ran to ground off of pin 8 V2. V3 which is a 12AT7, remove jumpers off of pin 1,2,3. V3 pin 3 make 820ohm/25ufV3 pin 1 add 100k plate load resistor and connect the .02 that go's to the 500pf reverb input capcontrol Place a 100pf SM on your bright switch.Place the cap on the legs of the switch, Not one leg of the cap to the switch and one to the volume pot. Leg one of the volume control runs to the bottom of the bright switch. The top of the bright switch connects to pin 2 of the volume control and also runs to pin 2 V3. If you bright switch is wired wrong it will act more like a volume boost than a bright switch.

Thanks Doode.
I'll check that out later and see how far off my schemo is....

I think I'm gonna try this with the TDR on my bench,,,that's why I'm pushing......not just for conversation

Yes, I know.  I am trying my best to remember, but I cannot remember.  Old age.  Considering your question on the volume control, I do not remember it swapping locations, however I just got finished reversing and removing all the added mods.

This is all I got for now.  I will get more on this.  I was a lot more up on it when I needed more overdrive.  Now I use clean amps and never really thought how distorted guitar amps are, or at least the ones we know.  When I get home I will look through my notes.  I know I made some as I was doing mods before I knew what they actually did.  Painting by numbers.    I used to be afraid I would not be able to get it back, now I don't care because I know I can always put it back stock but where is the fun in that?

[Ed_Chambley]

And for anyone else who might be reading along here....
I am working with a single channel preamp, so even though the instructions that Ed posted say to start on V2, my schematic starts with V1

I like to try things that I have never tried,,,that's why the first stage CF is interesting to me......we'll see 

Ed,,,those instructions are telling me to put the volume control post tone stack, so that's where i'll try it first.

It will be a LOT easier if you do.  Go ahead and add a couple of fun pots in the Cathode for resistors.

You Know you will love it. 

[Willabe]

I think Willabe said he had it.

I do, but I don't see any mod with a volume control before the TS in it.

(We're talking about 'Dave Funk's Tube Amp Workbook, complete guide to vintage tube amplifiers volume 1 - Fender, 1st edition', right?) 


                        Brad   

[SILVERGUN]

This is all I got for now.  I will get more on this.  I was a lot more up on it when I needed more overdrive.  Now I use clean amps and never really thought how distorted guitar amps are, or at least the ones we know.  When I get home I will look through my notes.  I know I made some as I was doing mods before I knew what they actually did.  Painting by numbers.    I used to be afraid I would not be able to get it back, now I don't care because I know I can always put it back stock but where is the fun in that?

OH....I thought that this mod was to get a cleaner tone and reduce some blocking distortion issues at higher vol. settings on original blackface circuits...?

But, because we have been reading a few different posts, pdfs, etc.,,,I'll repost this so that we will be on the same page:
(this was the link that was in the first post of the amp garage post that Ed pointed to on the old thread)
http://blueguitar.org/new/text/threads/from_adam/fender-blackface-mods.pdf

And just to save you from having to look back for it,,,here's a snip of what the MOD is supposed to accomplish:

[SILVERGUN]

It will be a LOT easier if you do.  Go ahead and add a couple of fun pots in the Cathode for resistors.
You Know you will love it.

Write this down somewhere that I actually said this:
"not every amp is gonna be a breadboard"...


....even though they all turn out that way