Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #112573  by claytushaywood
 Fri Apr 13, 2012 6:08 pm
I just completed the jerry preamp mods as listed on waldo's site- very cool for having all that info available to the public- very true to the spirit of the dead.

Couple observations...but first let me note that I am still using a pair of reissue jensen c12n's in my pro reverb.
I did the full post alembic mods with a couple additions and slight changes. I used PRP metal film resistors in place of the dale rn65's. I used these in the cathode bypass capacitor positions in parallel with the sprague 150d tanatulum caps. I also threw in some PRP's in the power filtering can and across my output tubes.

The amp is a little harsh now- It's too bright for me at this point. I'm wondering if subbing the PRP's for carbon films in the cathode bypass spots could contribute much to this? Maybe the tanatulums are too bright for me? I have some of those old BC big blue axial electrolytics I could sub, or should I put carbon films in first? maybe both?

I dig the bright clean sound, but I definitely cant imagine this amp with a brighter speaker- maybe the jensen's notably harsh upper mids are wearing my ears out?

A couple other changes I was thinking about...
Using a .047 cap for the bass cap
i've read that increasing the .001 coupling cap in front of the phase inverter ( i think) to .0022 or even up to .01uf could really thicken up or round out a thin or harsh sounding fender
Maybe I'm not digging the .022uf vibrato coupling cap?
I had a recommendation to substitute the silver mica 250pf treble caps with capacitors made out of polystyrene (apparently one of the best capacitors made--- ratings and electric properties wise) They're supposed to sound smoother and not as ice picky as silver micas.
Maybe I'd dig carbon comps or some nice carbon films (takmans) in the plate resistor position or as I said earlier taking out the PRP's of the cathode bypass resistors and using carbon films?

Anyone have any ideas or recommendations? Overall the amp sounds much more jerry like. The lows arent really low- they're nearly mids- but they cut and spank very nicely. The noise of the amp is down considerably. There is a definite hi-fi feeling and an amazing feeling of accuracy. I'm just not feeling this much brightness- it's a little sterile at this point- I do dig bright tones, but the amp is very fatiguing to my ears at this point. Maybe I just subconciously dig fatter sounds? haha I think i'm past the beginners stage where you hate treble (anyone ever notice how beginners never turn up the treble on their amps? i've noticed they typically use really bassy sounds- I sold a zen drive to this beginner and he told me it was harsh sounding haha)

One quick question... the mods say to switch the bright switch capacitor to 120pf silver mica... jerry always had the bright switch off... so what's the point? the 120pf silver mica with the rest of these mods makes the bright switch unusable bright... I mean it's just sick how bright it is with it on. I switched it to 56pf and it's perfect for playing humbuckers out of this amp.

By the way- you guys that play deluxe reverbs. They have a permanent bright switch in them, but I think its a smaller value like 68pf or something lower than 120pf. A great mod for you jerry fans would be to remove that capacitor completely. This will allow you to turn up the treble without destroying your ears.

Any input would be much appreciated!
 #112577  by JonnyBoy
 Fri Apr 13, 2012 6:45 pm
sounds like you did it right, I had the same issue way back when I switched into a bright amp. Try turning down the tone on the guitar, especially if you have a buffer you can really amplify the high end. I use my tone knob a lot, almost as much as I use my volume. It really makes a difference between having your tone ice pick or nice and bright. No one can really tell your issue unless they can see and hear the amp though, most talk will be a stab in the dark. Hope it works out for you!
 #112581  by SarnoMusicSolutions
 Fri Apr 13, 2012 10:59 pm
PRP's are sweet and gentle. Don't worry about those. THe bright/harshness is the new tantalum cathode bypass cap's. They need at least 50 hours of real run time to settle down and mellow out. They're brutally bright at first, but when they break-in, they become more neutral and very hi-fi. Other new cap's also need to break in. New parts are always too bright and harsh. Be patient and never judge at first.

But it is definitely time to try JBL's. Jensens are gonna hurt. JBL's are actually darker, mellower, flatter, cleaner, but with sparkle on top. Jensens are hard and crisp and they bite hard in the harshest treble zone, and with the Jerry mods they are virtually unusable. JBL's are strangely warm and Jensens typify "bright".

And I agree wholeheartedly that in a Deluxe or any smaller Fender with a fixed bright cap that the bright cap needs to be removed for sure.

Let those tantalums break in. They change dramatically.

 #112582  by waldo041
 Fri Apr 13, 2012 11:16 pm
i agree with brad, definately JBL time. JBL's have a flat response and show you what your amp is really doing. the jensens frequecy response is designed bright. i feel for you, those jensens have to be brutal with the jerry mods!

also, once you let that amp break in, i'd consider swapping those cathode bypass caps for carbon film. the metal films will make things quieter, but they will just be contributing to the brightness those 150d's are already adding. just a small change, but something you will notice.

 #112587  by SarnoMusicSolutions
 Sat Apr 14, 2012 8:11 am
Waldo, I think you made a typo when you said "swapping those cathode bypass cap's..." Didn't you mean "cathode resistors"? He's got the the right tantalum cathode bypass cap's there as you mentioned.

For killer carbon film types, go to Mouser and check out the KOA SPR types. They're pale, flat green and sound killer. And they're cheap.

 #112590  by claytushaywood
 Sat Apr 14, 2012 8:37 am
Thanks for the responses! I was thinking it was the lytics... Do you guys think adding a seperate bypass cap and resistor for the shared v1 v2 one would be a nice improvement for these fender amps? I could fit one easily with some perfboard or something. and I am modding my normal channel to have reverb and have a different voicing- so I could try something like a .68uf with a 2.7k resistor. worth doing? Also i'm still thinking the .001 coupling cap near the PI could be increased with favorable results (to me)- maybe even just like .0022uf (would that even make a difference in sound)- what do you guys think? I feel like I've read thats a common mod...
also wondering if a mid range control would help me get closer to jerry's mid range response? Or is the stock 6.8k resistor pretty much there?
What about a presence control or having a switchable NFB?

I was also checking out weber's kits- and they have a kit for adding a tube buffered fx loop that has an adjustable send and return (optional)- the send can be used as a preamp out without disrupting the flow of the amp apparently? With that I could have more control over my preamp vs power amp clipping. and I really dig using a reverb pedal (actually looking for a new one- kinda going between alesis nanoverb or another version- and the tc electronics hall of fame PGS edition- which has some amazing cathedral reverbs)

Anyone have any tube recommendations? I just sold 4 tung sol 12ax7's- absolutely hated em- too much gain. I have been digging this old westinghouse 12at7 in v2 and an Ei in v3. smooths everything out- i get more chime, doesnt overdo the pick attack. Get more power amp fire cuz I can turn the amp up louder. along with a 12ax7 in my phase inverter its been a good combo- but the 12at7's have begun to get microphonic (i feel like microphonic tubes haunt me- i've always had major problems with microphonic preamp tubes) I've tried EHX, JJ, mullard (which i loved the sound of, but they went microphonic way too quickly), Ei, westinghouse, sovtek 12ax7's. I've heard the "fender" brand tube is actually a relabeled 5751- maybe thats the ticket? Anyone have recommendations? I'm not set on 12at7 in v1 and 12ax7 phase inverter- I'm thinking 5751's in both spots might be the ticket.

I really need a 6L6 recommendation- I'm leaning towards these TAD (tube amp doctor) tubes that are supposed to be like old RCA black plates- or these tung sol's that supposedly can handle very high plate voltage, which could be good with the SS rectifier.

Speakers- I've read on this forum that steve kimock thinks a single speaker with 2 tubes is the best combination. I was thinking about using one weber california (i got to compare a california with a neomag and a k120) I didnt like the neomag and actually liked the california better- it was warmer and fatter sounding- probably doesnt cut through like the k... But hey instead of spending $450 on a pair of k's I could get a single 4 ohm cali for $105. I'd like to cut down the weight of my amp by using a single speaker, but will I lose a bunch of spread and perceived volume going from 2x12 to a single? Does it really matter that much in an open back cab? I was thinking a weber beam blocker would help me with soread- yeh? Also the cali's are rated at 80 watts... my pro at 40, so I was thinking using one speaker might have another advantage in taht I'll be able to drive the speaker harder and closer to its sweet spot. whatdya think

As always thank you for the help! Always greatly appreciated... Recommendations, opinions, off topic rants, long winded desciptions, short statements of truth and fleeting moments of brilliance are all accepted with gratitude and are highly valued! Thanks!

--Thanks for the carbon film recommendation- However, I've actually considered changing the plate resistors to these Takman brand carbon films... they arent too expensive, but they aint cheap- they sound fantastic, very warm. I'll check those KOA's out too though. Check out those takman's though if you ever need some low noise great sounding carbon films
 #112600  by claytushaywood
 Sat Apr 14, 2012 1:47 pm
I found a sweet deal on tubes that seem to be very highly regarded. I was originally thinking theyd be fine and that they wouldnt work in a cathode biased amp unless you changed the plate resistors. but now i'm thinking I'd have to do that too in my fender pro reverb combo.

maybe I could get one of the smart dudes opinions.

This store is selling "winged c SED" 6L6GC tubes at an amazingly discounted price. Apparently they test fine everywhere else, but these particular tested tubes are drawing higher than normal plate current, and the store "normally rejects them in their testing screening process because they would run too hot in a fixed bias amps."

Would this just be a matter of a simple bias adjust with my blackface bias circuit? Or would I have to change the power tube grid resistors or something?

They're like half off for winged c tubes- which are apparently some of the best modern production tubes. should I mess with it or would I be better off with something else?

Thank you much!