Preamping twin question

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Preamping twin question

Postby jeffm725 » Sat Nov 01, 2008 9:06 am

If I Preamp a twin, does that then forgo ever using it normally? Meaning with the pre-amp mod, can I still us the amp as a regular combo with the Power tubes in it when I want to?
I assume that when you do use it as a pre-amp, that you have to pull the power tubes out of it, right?

What I am looking for is that I would like to be able to use the twin pre-amped when I want but also "normally" when I want. (not at the same gig though).

I see that the Pre-amp mod uses the vibrato pedal jack. In the pre-amp mod, Do people use the vibrato pedal jack as a stock rca jack with an a adapter to get it to 1/4 inch, or do you replace the rca jack with a 1/4 inch jack?

What would I have to do to be able to accomplish this?
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Re: Preamping twin question

Postby RiverRat » Sat Nov 01, 2008 9:57 am

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Re: Preamping twin question

Postby tigerstrat » Sat Nov 01, 2008 10:36 am

You can very easily and quickly restore to standard Twin operation by plugging the tubes back in (in their original respective sockets) and plugging in a speaker load. If you bring the quad of tubes with you (in a padded box) and number which went where (to keep the bias the same) then you basically have brought a backup power amp that weighs about a pound... but you lose the "master volume" adjustability that the external amp gave you.

I added only one 1/4" pre out for both channels, in the hole that had housed the Vibrato pedal RCA jack, but you can do separate ones for Normal and Vibrato.
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Re: Preamping twin question

Postby jeffm725 » Sat Nov 01, 2008 11:05 am

Thanks Rat and Tiger (shoot, we have a regular zoo in here, would call you both by your first names but dont wnat to be presumptuous some people prefer not to have their real names on a board, I respect that)............
OK that brings me to the next question, Tiger, 1/4 inch jack fit into rca jack hole without drilling? I really want to not mess with any unchangeable mods.

Also, is this really as easy as it looks on dozin's site? I mean we are talking just adding 1 wire from the tap point to the jack, no? That in in of itself looks very unobtrusive.

I think I remember Jerry using channel 1 as the preamp, is that correct?

Tiger, you tapped both channels? but use the same output jack ? That makes sense because I cant see using both channels at once anyway.
Which channel do you use as a preamp? 1 or 2? Obviously if you were to use channel 1, it would call for the use of an outboard reverb.
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Re: Preamping twin question

Postby tigerstrat » Sat Nov 01, 2008 1:46 pm

jeffm725 wrote:Thanks Rat and Tiger (shoot, we have a regular zoo in here, would call you both by your first names but dont wnat to be presumptuous some people prefer not to have their real names on a board, I respect that)............
OK that brings me to the next question, Tiger, 1/4 inch jack fit into rca jack hole without drilling? I really want to not mess with any unchangeable mods.

Also, is this really as easy as it looks on dozin's site? I mean we are talking just adding 1 wire from the tap point to the jack, no? That in in of itself looks very unobtrusive.

I think I remember Jerry using channel 1 as the preamp, is that correct?

Tiger, you tapped both channels? but use the same output jack ? That makes sense because I cant see using both channels at once anyway.
Which channel do you use as a preamp? 1 or 2? Obviously if you were to use channel 1, it would call for the use of an outboard reverb.


very unobtrusive. yes on all of the above. although some people have stated the RCA jack hole needs to be enlarged to accommodate 1/4", this was not the case on my 68 Showman Reverb (exact same chassis as Dual Showman Reverb or Twin Reverb)... simple single wire lead for the tap.

I use Vibrato Ch1 for guitar, and on rare occaissions for a "Phil" rig, I use Normal Ch1 into the same Yorkville power amp, but switch to two 8ohm 2x10 cabs instead of the single E120.
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Re: Preamping twin question

Postby RiverRat » Sat Nov 01, 2008 3:13 pm

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Re: Preamping twin question

Postby tigerstrat » Sat Nov 01, 2008 6:12 pm

RiverRat wrote:Jerry did use Channel 1 as Waldo indicated in my other thread...


I think you might mean Input 1 of the Vibrato channel. "Channel 1" could be misinterpreted as "Normal channel"...
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Re: Preamping twin question

Postby jonarobb » Sun Nov 02, 2008 7:12 am

I've done the full preamp and input jack mods as per Waldo and Buck over at Gearheads and it's tasty. I've since used the Twin Head I made for both a preamp out to my power amp and as a stand alone tube head. When I'm preamping out to the power amp I obviously pull the power tubes. When I don't feel like dragging the power amp along I just put the quad of power tubes back in. It's been pretty cool and sounds great. One thing that happened though was I blew one of my K120's. I have a few of them but I realize I just wasn't being careful. I have a 600w power amp bridged and one day with nothing to do I kind of turned it up...., and well, the cone of the K120 blew right out of the magnet assembly!

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Re: Preamping twin question

Postby waldo041 » Mon Nov 03, 2008 7:49 am

on jerry's both channels were preamped, although one channel can be done and an rca hole probably would need to be enlarged. the "preamping a twin" over at dozin.com works, but is not how jerry's was. here is the layout for both channels. simply eliminate the other if you choose to tap only one, normal or vibrato.

Image

this isolates each channel and shunts the power section out. isolating each channel and shunting the power section drops unused portions of the circuit thus reducing the noise. with the proper jack(switchcraft 13) and wired correctly.the normal channel essentially becomes an alembic f2b, while the vibroto channel is the the jerry version with reverb on it(and vibrato if the opto's leg is not snipped).


as for the input mod.
input 1 eliminates the input resistor and eliminates the noise associated with them when used exclusively. i have noticed a slight audible gain also, but others have not. input 2 stays stock wired(with the exception of the jack used) so that its jack can still be used. the mixing resistors both come back into play when both inputs are used simultaneously. so there is 3 scenarios coming from them.

Image

most will see the 68k input resistor of input 1 still in circuit, and it is, but the path of least resistance is used thru the always closed switched jack of input 2. that switch opens when a plug is inserted. actually the preamp tap switched jack is the opposite of this. those boy's were mighty clever.

jeff, dozin's preamping a twin shows the vibrato jack being used, and it can and does work. but that is not how it was done. jerry didn't use the vibrato so using it's rca jack could work if you only preamp one of the channels. by rca jack i actually mean it's hole. with the 13 jack, you can use your amp as a combo with nothing plugged into the 13 jack and the power tubes inserted. and to preamp the same amp, with the proper wiring and 13 jack, simply plug your power amp into the tap and remove the tubes. the removal of the power tubes is to save the output transformer and WAS done on jerry's.

that all said, if one had a seperate power amp, speaker cab AND a combo. i would think that the power tubes could stay in when preamped as long as the combo's speaker stays also plugged into the combo. this would provide the load needed when not used and since the power amp side is shunted it would get no signal. this would make for a sweet backup if the external power amp and cab went down. and would be as quick of a change as removing the preamp plug. NOTE: i have never tried this, but would think it could be done.


peace,
waldo
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Re: Preamping twin question

Postby RiverRat » Mon Nov 03, 2008 2:17 pm

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Re: Preamping twin question

Postby waldo041 » Tue Nov 04, 2008 8:09 am

RiverRat wrote:The connection from Input 2 to grid is 68K (not 136K like the stock Fender Input 2)


Stock Fender Inputs;

Input 1 34k (68k and 68k in parallel = 34k)
Input 2 68k
when both inputs are used input 1 is 68k and input 2 is 68k mixed together.

so on the input mod, input 2 stays Stock. 136k is not the input stock for input 2. input 1's 68k is disengaged from the signal. input 1's 68k goes in series with the 1meg to ground for a 1068k value when a plug is inserted in input 2 versus input 1's 1000k(1meg)value.

peace,
waldo
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