Bypass cap on V2A

rgalpin · Post by **rgalpin** » Wed Jul 13, 2005 1:52 pm

I'm trying to identify the location of a mod I read about where you put a ".68uF across V2A's cathode." I think the cap is put in parallel with an existing resistor but I'm not sure which one.

Anyone heard of this mod? Supposedly it was the standard pre-1974 PCB.

The blue arrow is pointing to the resistor that I think you put the cap across. Any pearls of wisdom regarding this are quite welcome indeed!

Billy Batz · Post by **Billy Batz** » Wed Jul 13, 2005 2:10 pm

Thats should be it. At 820 ohms.

rgalpin · Post by **rgalpin** » Wed Jul 13, 2005 3:01 pm

Thanks. Evidently they stopped putting this cap in when they increased the NFB resistor because the combo was too mushy. I decreased my NFB to 50K and moved the FB wire to the 16 ohm tap. I like the tightness but I miss a little of the gain - I'm thinking this mod may work well to give me some of that gain back - and if it's the way they "used" to do it on the coveted older designs then that makes sense to me.

What do you think? Any cons? Anything to look out for?

Billy Batz · Post by **Billy Batz** » Wed Jul 13, 2005 6:44 pm

Use a push-pull pot to turn it on and off. As far as the stock differences in marshalls I think this one is by far the most different and switchworthy. It adds a lot of gain. As far as i know there are amps that came with the 100k NFB and the bypass but Ive heard people say the bypass got nixed when the NFB got lowered. People here whove built the 69 if they went by the Metro layout then they have the bypass and 100k. It would be a bit hairy for me though. I know that.

The only thing I can suggest is try a few different values. 330u is the 'old' bypass value there and the supposed value in the EVH grail amp. Ive used that for a while (so does Ralle evidently who made that awesom VH1 tone clips) but myself and most find it to be a bit too loos on the lower registers for most other tones. It does give more of a full range boost then the mid centered .68. I use a 5u now but I think experimenting with 1 4 and 5 are good to go for a more full range boost.

joshwilson3 · Post by **joshwilson3** » Wed Jul 13, 2005 9:41 pm

I think this is what you are wanting to do:

Are you basically wanting to convert your 1974 board to a 1969 Superlead board?

What stock value NFB resistor is supposed to be on your 1974 board?

thanks

joshwilson3 · Post by **joshwilson3** » Thu Jul 14, 2005 1:25 am

Here is a MV circuit:

On a MV circuit, what do you guys think about adding a .68uF across V2A's cathode and upping the NFB resistor to 100K like is on the Superleads?

thanks

guest · Post by **guest** » Thu Jul 14, 2005 5:06 am

You'll get more balls and gain doing that. I think it's a great Idea.

Flames1950 · Post by **Flames1950** » Thu Jul 14, 2005 8:21 am

JCM800's will already have a 100K feedback resistor anyway -- in fact, by the time MV's came along in '75 100K was pretty standard, and I don't recall seeing an MV with a 47K before.

joshwilson3 · Post by **joshwilson3** » Thu Jul 14, 2005 9:28 am

Flames1950 wrote:JCM800's will already have a 100K feedback resistor anyway -- in fact, by the time MV's came along in '75 100K was pretty standard, and I don't recall seeing an MV with a 47K before.

Yeah, you're right, and by looking at the schematics, it looks like it goes to the 4 ohm tap.

I think this is a more accurate pic:

rgalpin · Post by **rgalpin** » Thu Jul 14, 2005 10:25 am

330u is the 'old' bypass value there and the supposed value in the EVH grail amp

and now a moment of silence in honor of the EVH grail amp...

i was listening to Fair Warning on the way into work today.

Are you basically wanting to convert your 1974 board to a 1969 Superlead board?

i'm not thinking in those terms really - although that may be what i am doing coincedently. once i learned what the NFB resistor was doing from Mr. Batz and Flames, i found that i like the way the amp sounds with increased NFB. i use the amp live. i don't want to need a pedal to give me the juice necessary to get my solo sound. i have found that the tightness provided by increasing the NFB doesn't leave enough juice for a solo. so, i'm looking for reasonable ways to increase the gain in other parts of the amp. not insane hairy gain... just something to give it a little kick when i open the guitar up for a solo.

What stock value NFB resistor is supposed to be on your 1974 board?

i think stock is 100K off the 8 ohm tap. can anyone confirm that? i am running 50K off the 16 ohm tap. i increased the cap value on the presence in order to lower the cutoff frequency of the control. i increased it to 1.4uF and it sounds to me like the cutoff frequency is letting all the upper mids through not just the sizzle that i usually hear from the presence control. but the amp sounds best for rhythms with the presence on 0 which throws all that NFB back into the signal and tightens it up bigtime.

On a MV circuit, what do you guys think about adding a .68uF across V2A's cathode and upping the NFB resistor to 100K like is on the Superleads?

i have heard that that is too much on the superleads - gets mushy - what i hear people saying is that it is best to choose one or the other - either increase your NFB resistor or add the bypass cap - but not both. but i have not tried adding the bypass cap yet - but i will be trying it soon... especially since... "you know who" had one!

rgalpin · Post by **rgalpin** » Mon Jul 18, 2005 2:01 pm

following up with the results i got from adding the .68uF across the resistor going to V2a on my '74 super lead 1959:

i played out on friday night with it. there's more life in the feel. when you turn the down the guitar to clean up for rhythms the touch response remains even when clean. great improvement IMO. thanks for the help on this.