1st mod = learning the hard way

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studiodunn
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1st mod = learning the hard way

Post by studiodunn »

Hello All.

I recently was given an old Silverface bassman 100 and figured it to be the perfect starting point for me to start modding an amp. I build guitars and pedals regularly and had been fighting the urge to git into an amp as long as I could, but the fender was too much to resist.

So I started with a simple cap job and it sounded better....not good.

I then redid the bias layout and further tweaked the preamp(s).
It sounded even better......still not good

After doing everything I (read: my basic knowledge)I realized this circuit would never give me something I would be happy with so I ripped it out and started over.

I added some tubes, stuck a turret board in and just started working from the PI to the input. I managed to squeeze in a Levi style Jcm and an express circuit. The Express circuit sounded great out of the gate. The Jcm needed much trial and error to get to a nice place.

My friend Alex has been helping me through this process, but I'm sure he tired of all the questions so any advise is greatly appreciated.

Now my problem is getting both preamps into the PI.
Do I just dump NFB, take the 10k LTP to ground and connect Point A to B?

I would like to be able and use an A/B switch to blend both channels.

[IMG:720:540]http://i7.photobucket.com/albums/y269/studiodunn/guitar%20gear/bassman100/frankenamp001_zps3u0kytd1.jpg[/img]
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Phil_S
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Re: 1st mod = learning the hard way

Post by Phil_S »

Hi, and welcome to TAG. You are a brave soul to look past the malware warning. AFAIK there are no malware problems here. I visit almost daily.

If you are using a LTPI, you can insert preamp A into the upper half and preamp B into the lower half. Give up the FB loop.
studiodunn
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Re: 1st mod = learning the hard way

Post by studiodunn »

Thanks Phil.

Yeah, I'm trusting my software more than chrome.

So my question then is - do I just remove the 100r to ground and replace the the 10k with a 100k and go to my 4ohm speaker? Or do I just take the 10k to ground?
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sluckey
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Re: 1st mod = learning the hard way

Post by sluckey »

I would leave the NFB alone. Connect the two preamp outputs together thru two 220K mixing resistors and feed that into one PI input (just like Fender does).
studiodunn
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Re: 1st mod = learning the hard way

Post by studiodunn »

sluckey wrote:I would leave the NFB alone. Connect the two preamp outputs together thru two 220K mixing resistors and feed that into one PI input (just like Fender does).
So would those just need to be in series with each preamps coupling cap?
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Phil_S
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Re: 1st mod = learning the hard way

Post by Phil_S »

Look at the AB763 schematic. It shows a pair of 220K mix resistors, taking the feed from the vibrato and normal channels, and going into a coupling cap that feeds the upper half of the PI. You'll see.
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Post by Stevem »

Take note! If one preamp circuit has more gain stages then the other then they can not be blended/ used at the same time as they are out of phase with each other, that is unless you want the out of phase sound of a Strat with its 3 blade switch in the middle position all the time!
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studiodunn
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Re: M

Post by studiodunn »

Stevem wrote:Take note! If one preamp circuit has more gain stages then the other then they can not be blended/ used at the same time as they are out of phase with each other, that is unless you want the out of phase sound of a Strat with its 3 blade switch in the middle position all the time!
This is one of the problems I have already run into, and the reason I was looking into losing the NFB and splitting the PI. Are you saying regardless of sharing an input or splitting the PI I will have a phase issue?

The right side/JCM circuit is going into the PI from a MV, so it also effected the TW channel. And yes, when I tied them both into the same PI input, things sounded bad with both channels at once. Tying the circuits together also uglied up the JCM channel.

I notice on JCM schems, that is a 100k to the 4ohm tap. On non-NFB 2 channel designs I've seen this be anything from a 10k to 47k to ground.
studiodunn
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Re: 1st mod = learning the hard way

Post by studiodunn »

Phil_S wrote:Look at the AB763 schematic. It shows a pair of 220K mix resistors, taking the feed from the vibrato and normal channels, and going into a coupling cap that feeds the upper half of the PI. You'll see.
I understand this, but my problem is the JCM channel is feeding the Coupling cap off of a MV. It effected the TW input and more importantly sounded bad once the TW circuit hit the same input.
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Phil_S
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Re: 1st mod = learning the hard way

Post by Phil_S »

Well, then, go back to my first suggestion. Insert one channel at the top and the other at the bottom. Lose the feedback. You have options here!
studiodunn
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Re: 1st mod = learning the hard way

Post by studiodunn »

Phil_S wrote:Well, then, go back to my first suggestion. Insert one channel at the top and the other at the bottom. Lose the feedback. You have options here!
I'll try feeding the top and bottom and see where that gets me. Thanks for all the help.
Smokebreak
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Re: 1st mod = learning the hard way

Post by Smokebreak »

Do you have a rough schematic of what you have? Mixers will work to blend, but you will be cutting each amp's signal in half, essentially. I would put them on a switch before the cap feeding the PI, and then you don't have to compromise each channel as much, of you course then you have the compromise that you cant mix both amps;)
One must build both ;)
studiodunn
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Re: 1st mod = learning the hard way

Post by studiodunn »

Smokebreak wrote:Do you have a rough schematic of what you have? Mixers will work to blend, but you will be cutting each amp's signal in half, essentially. I would put them on a switch before the cap feeding the PI, and then you don't have to compromise each channel as much, of you course then you have the compromise that you cant mix both amps;)
One must build both ;)
I'm sure this wont be the last of both these circuits Ill be building 8)

Here are the schematics I used. The PI and Bias end are Blackface Fender specs.
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Smokebreak
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Re: 1st mod = learning the hard way

Post by Smokebreak »

Check out Steve's comment. You have 4 gain stages with the 36 and 3 with the Xpress, as the signals will be out of phase with one another when blended, and will sound like ass.

Audio signals are flipped 180 when they go through a tube's amplification process(Cathode followers do not).

If you absolutely must blend these 2 beasts, you're gonna have to add a unity gain stage to the Xpress, or make the stage before the tone stack in the 36 a plate fed one instead of cathode follower, and you will have in-phase signals.
studiodunn
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Re: 1st mod = learning the hard way

Post by studiodunn »

Smokebreak wrote:Check out Steve's comment. You have 4 gain stages with the 36 and 3 with the Xpress, as the signals will be out of phase with one another when blended, and will sound like ass.

Audio signals are flipped 180 when they go through a tube's amplification process(Cathode followers do not).

If you absolutely must blend these 2 beasts, you're gonna have to add a unity gain stage to the Xpress, or make the stage before the tone stack in the 36 a plate fed one instead of cathode follower, and you will have in-phase signals.
Got it.

Well, I have another empty stage on the express side, but I'm out of real estate on my board
:roll:
In reality I don't think blending is as important as just switching between the two channels.
And ideally via a footswitch. This will be easier to do if I lose the NFB and just use both ends of the PI. I can only assume that will come with some downsides though.

I will just focus on getting both signals into the PI for now. I think I can figure out the switch scenario, and can actually use the bright switch as it isn't hooked up on the JCM side. The Deep switch is already wired as a Fswitchable stack lift.

So, losing the NFB I just remove the 820r and 100r and take the 10k LTP to ground?
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