#164493  by Pointclear
 Tue Jan 29, 2019 3:40 pm
waldo041 wrote:
Tue Jan 29, 2019 3:09 pm

You should leave that .47 cap in there and work on the deficiencies you now hear in your preamp!

~waldo
.... man, thanks for this thread discussion. I’ve already dropped it down to a .1uf, and it feels a bit more usable (or maybe just a little less of a shock to the system). I have the idea to doing a toggle with several different output cap values.
I can’t believe the amount of tone shaping that this cap position makes. It changes the feel of my guitar in a way that I couldn’t have anticipated.
I miss the open, airy feel of the straight wire though. Maybe I’ll miss it less as I play it this way.
 #164498  by Pointclear
 Tue Jan 29, 2019 5:02 pm
waldo041 wrote:
Tue Jan 29, 2019 3:50 pm
what cap are you changing?


I added an output cap at the preamp tap - .47 orange drop. It was super, super tight and spanky. I felt it was a little much and dropped it to a .1 cap, which is what brad uses in the sms. It’s got a little more breathing room. I’m probably going to try a few more output caps and settle on one in between .1 and .47 then go back into the preamp and fiddle with the gain in the reverb circuit. I feel the output caps drop some of the grit and texture I was getting going straight out with out a cap.
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 #164499  by waldo041
 Tue Jan 29, 2019 5:50 pm
the Twin already has an output cap, one for each channel. They are represented on the Fender Schematic with their X and Y jumper wires between their output caps and their 220k mix node just before the phase inverter cap. You would not just add another to the end of the line, if i am guessing that is what you have done.

~waldo
 #164501  by Pointclear
 Tue Jan 29, 2019 6:34 pm
waldo041 wrote:
Tue Jan 29, 2019 5:50 pm
the Twin already has an output cap, one for each channel. They are represented on the Fender Schematic with their X and Y jumper wires between their output caps and their 220k mix node just before the phase inverter cap. You would not just add another to the end of the line, if i am guessing that is what you have done.

~waldo
Oh geez. That’s exactly what I have done. Hence why the .1uf may sound better than the .47uf.

I have my preamp tapped just before the phase inverter input cap. I’ll go back through and see what’s in the mix before that. I do have the master volume version of the twin (73), so I have just a wire from the master volume pot going to the phase inverter cap,(though I have didsabled the circuit) . I’ve also done the mod that puts effects on both channels and puts them in phase with one another.
 #164504  by waldo041
 Tue Jan 29, 2019 7:37 pm
Pointclear wrote:
Tue Jan 29, 2019 6:34 pm
waldo041 wrote:
Tue Jan 29, 2019 5:50 pm
the Twin already has an output cap, one for each channel. They are represented on the Fender Schematic with their X and Y jumper wires between their output caps and their 220k mix node just before the phase inverter cap. You would not just add another to the end of the line, if i am guessing that is what you have done.

~waldo
Oh geez. That’s exactly what I have done. Hence why the .1uf may sound better than the .47uf.

I have my preamp tapped just before the phase inverter input cap. I’ll go back through and see what’s in the mix before that. I do have the master volume version of the twin (73), so I have just a wire from the master volume pot going to the phase inverter cap,(though I have didsabled the circuit) . I’ve also done the mod that puts effects on both channels and puts them in phase with one another.
Ok, so the novice level. What you first need to do is find or figure out what and where your amps actual output capacitor is. If you are just tapping into the circuit where the 220k mix node resistors meet at the input of the phase inverter capacitor, that is NOT the correct tap point. And without really knowing/seeing the innards of your modified amp, i can't be much more help. I can advise you to use that calculator i posted and not just try any cap to cap unless you want hear what ice pick sounds like when you push the knee to 158hz or 769hz and lose all that low end content when not matched correctly. You have heard the potential.

~waldo
 #164506  by Pointclear
 Tue Jan 29, 2019 8:23 pm
Definitely a novice at electronics. I can hear and I can solder. It’s just enough to make me dangerous... but I’m determined to figure this rig out, and I think I’m on the right track thanks in a big part to this thread and your advice.

I have read that Jerry has each channel tapped separately, and I’m assuming that tapping at the output of each channel is the correct place to tap.
 #164514  by waldo041
 Wed Jan 30, 2019 1:51 pm
Image

Do you see the X and Y boxed? there should be 2 each? These 2 items marked X and Y represent a Jumper wire from one Boxed letter to the same on the other end. These 2 jumper are used to feed the preamp into the power amp. The power amp section actually starts before the phase inverter cap, thats the input to the Power Amp Section. The 2 - 220k resistors leading to it are what are known as mix resistor of a mix node. Their is only 1 power amp and 2 preamps so the mix resistors are used to try and keep one channel from bleeding into the other. but still allow both channels to work at the same time. ANYWAY, the X is the Normal channel and the Y is Vibrato channel. The Y next to the .1 vibrato channel output capacitor, that is your tap point. Same would be for the X in the normal channel next to the .047, thats the normal channel tap. Both are where the actual preamp sections end and the power amp section begins. The latter on the other side of the jumper wires at the 220k's where the other x and y are.

It is -21 degrees here today!

~waldo
Last edited by waldo041 on Wed Jan 30, 2019 2:05 pm, edited 1 time in total.
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 #164515  by waldo041
 Wed Jan 30, 2019 2:01 pm
NOTE: this is Not Jerry's amp but this is an Alembic modified Twin. It even has the Banana Jacks they preferred over the 1/4" speaker outputs.


Image
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 #164518  by Pointclear
 Wed Jan 30, 2019 4:10 pm
Mike! You are the man. Great explanation. I found out that vibrato channel tap point earlier today with a little sleuthing. Dozin has that page about tapping a twins preamp and the layout schematic at the bottom of the page shows tapping right before the phase inverter. HOWEVER, the photo right below that schematic shows a twin tapped right off v4b after that .1 cap. So that is the cap I need to change? From .1 to a .3-.4uf? To get my 40hz knee? That’s the idea right?
I’m curious if there is any real advantage to tapping there as opposed to after the mix resistors besides being able to tap each channel individually. I’m guessing the signal may be a tad bit stronger being tapped right after v4 by not having to go through any more wire, resistors, pots, etc.
 #164520  by waldo041
 Wed Jan 30, 2019 5:43 pm
Pointclear wrote:
Wed Jan 30, 2019 4:10 pm
Mike! You are the man. Great explanation. I found out that vibrato channel tap point earlier today with a little sleuthing. Dozin has that page about tapping a twins preamp and the layout schematic at the bottom of the page shows tapping right before the phase inverter. HOWEVER, the photo right below that schematic shows a twin tapped right off v4b after that .1 cap. So that is the cap I need to change? From .1 to a .3-.4uf? To get my 40hz knee? That’s the idea right?
I’m curious if there is any real advantage to tapping there as opposed to after the mix resistors besides being able to tap each channel individually. I’m guessing the signal may be a tad bit stronger being tapped right after v4 by not having to go through any more wire, resistors, pots, etc.
Each channel already has a coupling cap coupling each stage together. You're just creating another one down the line adding a 220k resistor in series as well. Not wrong, you can tap it wherever you want! I am only letting you know where the experts tap it. Dozin's picture and layout show 2 different things, i don't trust it, but ymmv.

Use the calculator i posted earlier to enter and get the value you need. 40hz is the knee i shoot for.

~waldo
 #164521  by Gypsykid136
 Wed Jan 30, 2019 5:58 pm
I haven’t physically done any of these mods yet as I am still doing a lot of research first before spending the money doing them, but I have found this layout diagram very helpful. This, the Tone stack calculator, and the R/C calculator Waldo posted above have been great resources in understand the “ins and outs” so to speak of amp modifications for me.
Image
 #164550  by waldo041
 Sat Feb 02, 2019 9:06 am
This photo is of a Twin Reverb amp that was pulled from the GD Pool and given to Simon Babbs and was subsequently sold on eBay many years ago. This amp shows Alembic's version of tapping a twin, meaning this is in an early version and one that Dan Healy actually worked on, he replaced those plate resistors and stated all the other caps are now soggy garbage and should be updated if I owned that amp. I cannot verify and purely speculating this but this could be Jerry's Blackface WoS Twin given to a Prankster, who knows? That said, IMO, it was most likely wired like this one for the WoS. The Dual Twins or any Twin for that matter take a hiatus with Jerry and the GD in 75 and don't return until the day on the green in 76.

1= Normal Channel X
2= Vibrato Channel Y
3= Ground Shunt location when a mono plug is inserted into the tap jack.

When a MONO plug is plugged into either one of them, 1 or 2, that section is tapped and #3 shorts/shunts the input into the power tube section to ground.

Image

~waldo
 #164566  by Pointclear
 Sat Feb 02, 2019 9:18 pm
I was THIS close to googling “babbs twin reverb” today and I got distracted. Tonight you post this photo! Mind blown.

What an interesting glimpse! I want to know how/if the grounding shunt effects tone/feel.

I tapped my twin where the Alembic vibrato channel is tapped a couple of days ago, after our last exchange on here. It makes a huge difference in quickness and response. I’ve actually had to change the tone caps in my guitar to compensate for the added brightness.
Interestingly, and I know this is not orthodox, I added a .1 cap to my output jack in series with the existing .1 cap on the board and I like what it does. It may not be the knee at 40hz, but the bass is fast and tight, and I’ve retained more of the texture and sweet bloom on my high treble. It’s got me thinking.
 #164567  by waldo041
 Sat Feb 02, 2019 10:09 pm
Pointclear wrote:
Sat Feb 02, 2019 9:18 pm
I was THIS close to googling “babbs twin reverb” today and I got distracted. Tonight you post this photo! Mind blown.

What an interesting glimpse! I want to know how/if the grounding shunt effects tone/feel.

I tapped my twin where the Alembic vibrato channel is tapped a couple of days ago, after our last exchange on here. It makes a huge difference in quickness and response. I’ve actually had to change the tone caps in my guitar to compensate for the added brightness.
Interestingly, and I know this is not orthodox, I added a .1 cap to my output jack in series with the existing .1 cap on the board and I like what it does. It may not be the knee at 40hz, but the bass is fast and tight, and I’ve retained more of the texture and sweet bloom on my high treble. It’s got me thinking.

You need to either replace or parallel another cap to the Output Capacitor. Adding another .1 in series is not doubling the capacitance. In fact it should be the same knee with or without it since they are both .1uf's. Capacitors when used together act the opposite of Resistors when used together in regards to parallel or series.

That shunt is used to ground the input of the Tube power section so nothing gets in there or out of there in the way of noise. The speaker cable should be pulled as well which also shunts the output to ground so that the tube power section is grounded at both the input and the output rendering it dead with no tubes and sitting idle with them. The 2 ways producing different voltages throughout the amp.

~waldo