Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #104359  by claytushaywood
 Sat Oct 15, 2011 6:46 pm
I'm gonna go ahead and get started on some of the Alembic mods on my silverface pro reverb because I like the idea of the preamp tap and the ability to still use the power amp. First I have a question. Both pre alembic and post alembic mods on waldo's site says to remove the 68k input resistor's... then he has the input layout diagram that shows the 68k input resistors in the pre alembic section and the diagram showing the plate resistors and taps in post alembic section says to refer to this diagram for the inputs... what's the deal with the 68k input resistors?

Also, I have some skills. I modded my amp from silverface to blackface, know how to discharge caps and what not to touch when, but i'm not a pro. i was wondering if anyone happened to know some of the differences between the twin reverb layout and the pro reverb... they look very similar. mine is the non master volume early (1973) pro reverb, i can only seem to find a schematic of the amp though, no layout.

does anyone out there use the a.) b.) and c.) bullets of the post alembicazation preamp tap. where a stereo plug inserted into one preamp tap allows the other channel to be used as a normal combo amp with vibrato? seems pretty rad... anyone got the special cable or box?

What type of filter caps are recommended? what about power and output transformer? id like to get a new one of those as i know one of mine is not original.
one more... what does hand selected mean when referring to components? should i measure those for exact tolerance? is $30 multi meter accurate enough?

Anyone got any sound demos of their modded fenders using the amps power section?

Thanks a bunch guys! as always your help is appreciated
 #107510  by claytushaywood
 Fri Jan 06, 2012 12:40 pm
anyone got something on this? bout to order parts and realized I hadnt figured this out yet!

any help would be great! sorry this is kinda in the wrong area
 #107532  by Smolder
 Fri Jan 06, 2012 9:32 pm
I have not done anything in regards to the pre out, so no advice from me there.

I did recap my 72 twin with the milspec cap, replace the tone stack with double .022 225P spragues and swapped out the 250pf with a silver mica. I can hear the dif. I'm running mine with two 6L6's through a k120 (fwiw).

yes, hand selected means they tested the components to match spec. Without being sure what tolerance was used, the hand spec'd means very little.

As for transformers... the output matters a lot more than the power when finding a source. I'm partial to the tone and the value of midwest magnetic components transformers. They are available from triode electronics.

Across the fender line, the major difference in the great years (ab763 circuit) will be transformers, speakers and power tubes. They will either be 6L6 or 6V6. Speaker differences are pretty easy to figure out and null if you just go with the standard jbl prescription. That leave transformers... and though your pro is pretty similar sounding to a twin, having owned multiples of both, they are not exactly the same... nor is my princeton reverb, but is come damn close to that tone with a E110. The showman is probably closer to the twin in that wattage range... and probably the sf bandmaster reverb is the closest. The vibrolux is probably furthest away, but I think this differention is splitting hairs.

Hope that helps.
 #107591  by claytushaywood
 Sun Jan 08, 2012 2:22 pm
Thanks for the response.

For me, I'd say the biggest difference between a twin and a pro is the rectifier. I hear sag more than most people probably, i wish i could have a switchable rectifier--- please dont tell me about a mesa! Getting a twin style output xformer would probably be a good mod as well.
I think hand selected would mean measuring to find the exact number, not just saying oh we got these 5% caps are they all within 5%... It's pretty easy today to match stuff within .1% imo.

I'm most interested in the input jack mod... not the preamp out. it has a diagram at waldo's site that shows some different switching type jacks and some 1M and 68k resistors. however, at the same time right above the diagram waldo's site says the mod removes the 68k resistors. I'm surprised he hasnt jumped to clear up the contradiction on his site. or to correct my interpretation of a contradiction.

I think I'm pretty clear on everything else. I've read a few places that the tanatulum caps are a pretty key part of the jerry mod FWIW
 #107595  by TI4-1009
 Sun Jan 08, 2012 5:16 pm
I'll tell you anyway. I have Mesa Lonestar, The closest Boogie to Blackface specs- and a switchable rectifier. Plus the "tweed" switch that lowers the voltage for even more sag!

There, I said it.
 #107604  by claytushaywood
 Sun Jan 08, 2012 8:02 pm
well i feel dumb- my freshman year of college in the recording engineering dorm was spent with metal heads glorifying double and triple rectifiers. generally despising everything about those amps, i've stayed away from mesas.

I'm sure the lonestar is closer to a blackface than a rec. but are those really supposed to be similar to a blackface? my experience was that it was a good amp... i thought even unique. Blackface definitely didnt come up. Ill have to try again I suppose!

Do they have the blackface tone stack?
 #107606  by TI4-1009
 Sun Jan 08, 2012 8:38 pm ... nestar.htm

Channel 1 is pretty straight, clean, blackface.

But I know what you mean. My son-in-law used to have a triple rectifier monster with the 4x12 cab. That's what I always thought of as "Mesa Boogie". The Lonestar is mainly about clean headroom based upon blackface specs. The 10/50/100 watt selector is a real bonus for me.