50W 12xxx spec preamp, not enough gain

[fhn_lopes]

Guys, I've been tweaking my 50w JMP a bit (it has 71 metro board) to a 12000 series preamp specs. Done it last night, the tone is fatter, a bit looser (in a good way) and bit more scooped (just a bit) but I don't get the gain you guys are getiing ala VHI even with a 15kohm pup in front.

At this moment the bright channel is his like this:
68k inputs
100k plates (12ax7)
2n2 couplers
470k mix with 470p bypass cap (didn't have a 560 in hand)
split cathode 820ohm/.68u rk/ck for both stages

the rest is stock like the 71 board, runnig EL34's , NFB is 39k, mid pot is 50k.

I didn't check any voltages yet, do you think maybe I'm getting too much plate voltage at the 12ax7s?

[motrock]

Get rid of the 50k mid pot. Thats not in the original spec. Also, 100k on 4ohm tap on NFB.

Dave Friedman posted Eddie's spec on here a long time ago. Follow that to a T! Thats what I did and didn't stray from it. Plenty of gain. All these little things like 50k mid pot... its bullcrap. Just follow the standard 12000 series spec.

Also... dispite what anyone says... you need the bright cap on the input.

Also... EH 6CA7s!

Last little trick... set amp to 8ohms and plug in to 16ohm cab. That adds extra.

[Mr.Farva]

I say leave the 50k mid pot. Dave has said that is what ed has in his amp. Are you diming the volume and mids? What 12ax7's are you running?

[Carbia]

check voltage in each pin first.
It could be something wrong and you'll waste time trying other things

[motrock]

I say leave the 50k mid pot. Dave has said that is what ed has in his amp. Are you diming the volume and mids? What 12ax7's are you running?

Get rid of it... Eddie never had a 50k pot in the mids. Thats part of the mythology that has built up over the years. Highly doubt Dave said that. And if he did he was probably joking or blowing smoke. NOT TRUE! Take that to the bank. Dave posted the specs some years ago on here. I have no idea where to find it. Its standard 12000 series spec with nothing overly special. I know that there is a .68uf in there, not sure what the bright cap is but you can play with it... but the negative feedback has to be 100k on 4ohm tap!

Preamp tubes are a crap shoot. I just got some vintage preamp tubes. Brimar, Sylvania, and Telefunken. Power tubes the EH-6CA7s are the best. Don't get the older ones because those get worn out. My original Sylvania 6CA7s are loosing something. The EH are just as good as the originals and better... because they are new.

Another thing.... Mallory Caps... they are in your face and have that aggressive thing going on. I recommend them. Just had them put in my 2203 clone and it woke the amp up. Pulled the Sozo's out. They were way to dark and smoothed out way to much.

[fhn_lopes]

Get rid of the 50k mid pot. Thats not in the original spec. Also, 100k on 4ohm tap on NFB.

Dave Friedman posted Eddie's spec on here a long time ago. Follow that to a T! Thats what I did and didn't stray from it. Plenty of gain. All these little things like 50k mid pot... its bullcrap. Just follow the standard 12000 series spec.

Also... dispite what anyone says... you need the bright cap on the input.

thanks motrock! I got a bit confused when you say those little things are BS and for me to follow the std 12xxx specs... do I follow what DF wrote or the 12xxx??

Bright cap at the input? what value to try first? never saw this before

I say leave the 50k mid pot. Dave has said that is what ed has in his amp. Are you diming the volume and mids? What 12ax7's are you running?

I'm running like this: v1 = siemens green label, v2 = GE blue label (long plate), v3 = siemens green label, all NOS. EL34 also Siemens / RFT NOS. All controls dimed, including presence.

check voltage in each pin first.
It could be something wrong and you'll waste time trying other things

Can't see anything "wrong" bc the other channel (that I left stock) has a bit more gain.. I made it two bright channles BTW, the original one I left as is (2k7/0.68, 560pf mix bypass, 22n coupler, etc) and the other is like discribed in the first topic. I mean, with the GE10 boosting the front end in the stock channel I get plenty of gain but the tone falls into that 80's hair metal, tighter and more agressive than EVH.

Anyways I'll check the voltages bc I haven't done that yet.

Thanks for the answers guys, I'll let you know as soon as I get the measures and the results if I manage to tweak a little further.

Cheers

[fhn_lopes]

Another thing.... Mallory Caps... they are in your face and have that aggressive thing going on. I recommend them. Just had them put in my 2203 clone and it woke the amp up. Pulled the Sozo's out. They were way to dark and smoothed out way to much.

My amps has sozo all over (early ones, yellow) and one or two mustards. I find it quite agressive for a plexi circuit, don't feel it that "smooth" ... it is more on the bright side than the opposite too..... the GE 12ax7 plays a huge role here, it is really gainy and bright.. at v1 it made the amp even brighter. I don't feel I need more sizzle actually, even with les pauls the tone isn't dark at all (bright channel of course).
When playing live (and loud) with my strat w/ BKP VHII in the bridge sometimes I roll back the presence or the treble pot a little bit bc it can get harsh.... but not overly bright like a 2204 can be for example.

[Mr.Farva]

Get rid of it... Eddie never had a 50k pot in the mids. Thats part of the mythology that has built up over the years. Highly doubt Dave said that. And if he did he was probably joking or blowing smoke. NOT TRUE! Take that to the bank.

https://flic.kr/p/t3oLaL

[Carbia]

Anyway, with a linear 50k pot you have all the sounds of the 25k plus added mids.

[TWANGGG]

Get rid of it... Eddie never had a 50k pot in the mids. Thats part of the mythology that has built up over the years. Highly doubt Dave said that. And if he did he was probably joking or blowing smoke. NOT TRUE! Take that to the bank.

https://flic.kr/p/t3oLaL

There's also this:

Did evh amp have a 50k mid pot??

Yep.

george

From here:http://forum.metroamp.com/viewtopic.php ... 70#p443884

[motrock]

Ok, this is just weird. First thing I think of with a 50k pot is a nasal Splawn amp.

I am curious as to if anyone else here has tried this and can give me an idea of what having a 50k pot in the mids will do for this tone? What has everyone heard with the 50k pot?

[guitar007]

No mention of speakers? What are you using?

[fhn_lopes]

speakers aren't the problem here... I have a 4x12 with two 1974 Rola creambacks 25w and two scumbacks m75 25w.

I will have time to tweak the amp again only in the weekend... still looking for a 100k at my old parts desk drawer .. I know I have one 100k 1w lying arround, so it goes to NFB. BTW my amp has 39k at 8ohm tap.

[motrock]

speakers aren't the problem here... I have a 4x12 with two 1974 Rola creambacks 25w and two scumbacks m75 25w.

I will have time to tweak the amp again only in the weekend... still looking for a 100k at my old parts desk drawer .. I know I have one 100k 1w lying arround, so it goes to NFB. BTW my amp has 39k at 8ohm tap.

Yeah, the 39k is waaaay to tight. I have 47k on 8ohm on my 2203 to make it more JCM800 spec to tighten it up. 100k on 4ohm will loosen it up. I like the 47k on the 2203 for the tightness. Mainly to play Paul Gilbert, Judas Priest, and other 80s shreddy type of things.

[vanhalen5150]

The 50K pot is an old mod used for slight gain purposes back in the 70's by some techs as well as messing with the input resistors(lower) and ground resistors(higher) on the input jacks. I have a have a amp mod book that mentions it back in the 90's as an old gain mod he had seen in a lot of Marshalls and Fenders modded from the 70's.