Some [but not all] ? on Fen-Twin mods

[Runewalker]

1972 Fender Twin Brain, different cab though.

Another thread discussed some power reduction options, and of course that got a litte extreme, and should we expect anything less from this crowd?

A couple of other ideas to explore:

1. A line in - Line out mod, direct into the power amp side? Or where does that feature usually enter and exit the circuitry. Is it worth pursuing in this amp? Is it hard or expensive or both? ---Knowing at this point I will have to purchase the mod service from a tech.

2. Can the normal and Trem channels be separated to produce stereo outputs? Probably not since I assume that there is just one power amp side.

More questions to follow I suspect.

[sumgai]

Rune,

1. A line in - Line out mod, direct into the power amp side? Or where does that feature usually enter and exit the circuitry.

Such a mod is properly installed in front of the phase-inverter stage, which puts it just after all of the pre-amp stages. (This would be the point on the black fiber board where the two 220K ohm resistors meet, about 3/4 of the way over from the input side of the amp.)

.....
Is it worth pursuing in this amp?

That's up to you. You're the current owner, and only you can determine what the overall value is, to you. If you feel that resale value is not your top priority, then by all means, do what you want with your property, no one has a right to tell you otherwise. I can tell you that I'd do it to my own amp, but only if I had a need that I could justify to myself, all other opinions be d@mned.

.....
Is it hard or expensive or both?

It's extremely easy to do. IMHO, it should not be expensive, but that's up to your tech, and then up to you to accept his estimate, or else attempt to negotiate the price more to your liking.

2. Can the normal and Trem channels be separated to produce stereo outputs? Probably not since I assume that there is just one power amp side.

You're correct, one power amp = mono sound out.

You could, in theory, split the channels just before the mixer, and run them out separately as two different "Line Out" connections. (This would be right before those 220K ohm resistors, where they each have a yellow cloth-covered wire soldered to them.)

HTH

I'm curious, what've you got in mind here?


sumgai

[Runewalker]

Most of the modern or at least have the line in/out functions.

All of this relates to my current modeler and to an upgrade at some point.

The 2 channel output of the amp is to take advantage of the split signal (left and right) of the modeler, used principally in the chorusing, detune, panning, leslie emulations, etc. Also would like to get a little sound stage separation with spread speakers.

the question worth is not about market value. The amp is a silverface so doesn't have the mystique of the overvalued blackfaces. But it does nicely what I want it to do.

The question on worth is more of an evaluation of the benefits of a line in/out on this particular amp. Using the modeler as the preamp for example. I've done it with the cheap SS I have and have been unimpressed. So I am not sure there is a aural benefit to modding the TwinBrain in this fashion.

This is an extension of the questions raised in your biasing thread, since I have to put the thing in the shop anyway to get it biased because I am a dummy about that, even with your throrough explanation. I can't readily visualize your instructions.

So bite the bullet and put her in, and while I'm there since I have to pay for at least 1 shop hour, I am making a list of other things I want done. Tops is the power cut switch we discussed.

RW

[sumgai]

Rune,

OK, that was simple enough, let's see what I can do here.

I use a modeler too, the VG-88, ver. 2. (I think I saw that ChrisK has one too, or was that a GR-33.... Chris??) What I did with my Twin was to bypass the entire tone stack in the Normal channel. (I think we had this discussion just a few weeks ago, in another thread that I can't seem to locate right now.) That lets me input a low-level signal from the modeler, and I can implement any amp simulation I want, within reason. Some things still don't sound as good as if I were using a closed-back cab, but most everything I need is very acceptable.

But like you, I also wanted stage separation. Solution: another Twin, of course! But that's too much beef to hustle here, there and everywhere else. Enter the "little" Fender Vibrolux, 40 watts of 2x10" Jensen power that has bigger balls than most anything else in it's class. 38 lbs. also doesn't strain the back nearly as much. ;D Stage separation galore, and I still have beaucoup true Fender tone in the second channel when I need it.

Costs a helluva lot more, but the sound,.... oooh mama, what a sound! Realistically, you should either invest in a small-to-medium PA setup, or else another amp which can be modified as I explained above. In small venues, maybe four players but usually only three, I just sacrifice all the extra spaciousness and lug in the Vibrolux for full-on duty that night. Anything bigger gets the deluxe treatment. ;D

I guess in the very long run, you could split the 4 power tubes into a stereo setup. I did that once for a guy, it didn't turn out too bad. Here's what I did, as I recall the steps:

I jerked the big OT out, and put in an old Bassman OT. I then mounted another Bassman OT on the far side of the amp (running the speaker leads outside the chassis so as to avoid feedback and parasitic oscillations).

I inserted a second bias circuit, tapping off of the same negative supply (about -50 volts).

For the phase inverter, I jerked the "Vibrato" (really the tremolo) function out, and used that tube for the job - less hassle by not having to drill and mount yet another tube socket.

I used the Ext Spkr jack for the second channel output.

After explaining to the guy why I couldn't just jumper the reverb signal over the the other channel (it would put everything right back to mono), he opted not to go the extra cost of installing two more tube sockets, all the parts necessary for those tubes, and of course, the reverb tank itself. Pity, might have been cool. As it was, the thing sounded pretty d@mned good, if I do say so myself. This was in 1999, and I nicked him $300 for the whole thing.

So yeah, it can be done, but the final value is still up to you. But I'll bet I can predict just what The Unkster would say, if he were here to see this: "That's putting all your eggs in one basket. If one channel fails, you've lost the whole thing while waiting for it to be repaired. At least with two amps you'd have something to play through while waiting on that repair job."

Is that enough food for thought there, Sparky?


sumgai

[Runewalker]

sumgai said:

Rune,

But like you, I also wanted stage separation.
I guess in the very long run, you could split the 4 power tubes into a stereo setup. I did that once for a guy, it didn't turn out too bad. Here's what I did, as I recall the steps:

I jerked the big OT out, and put in an old Bassman OT. I then mounted another Bassman OT on the far side of the amp (running the speaker leads outside the chassis so as to avoid feedback and parasitic oscillations).

I inserted a second bias circuit, tapping off of the same negative supply (about -50 volts).

For the phase inverter, I jerked the "Vibrato" (really the tremolo) function out, and used that tube for the job - less hassle by not having to drill and mount yet another tube socket.

I used the Ext Spkr jack for the second channel output.

After explaining to the guy why I couldn't just jumper the reverb signal over the the other channel (it would put everything right back to mono), he opted not to go the extra cost of installing two more tube sockets, all the parts necessary for those tubes, and of course, the reverb tank itself. Pity, might have been cool. As it was, the thing sounded pretty d@mned good, if I do say so myself. This was in 1999, and I nicked him $300 for the whole thing.

Is that enough food for thought there, Sparky?


sumgai

Intriguing. If you 'steroize' the twin, do you loosed the capability for a mono 100W twin option? Is a guitar run through a signal splitter into two 50W amps as loud as a one cord into a 100W amp?

You can't use the original Power transformer? You have to buy and install two Bassman lever trannies, instead of the original plus one additional power tranformer? (I know these are dumb questions, I am going for that "ignorance is bliss" thing)

Another thought I had was a line out interupt for both preamps. Maybe that what you art talking about in your first post to this thread. I have another amp with line in capabiltiies, albeit it is SS (but not terrible).

"You could, in theory, split the channels just before the mixer, and run them out separately as two different "Line Out" connections. (This would be right before those 220K ohm resistors, where they each have a yellow cloth-covered wire soldered to them.)"

I have another amp with line in capabiltiies, albeit it is SS (but not terrible). In that scenario I could use the norm channel preamp to drive the other amp's power amp.

What would be entailed in that idea?

Also is the preamp out signal too hot for inline to the computer audio card for inline recording (Sonar, GuitarTracks, etc)?

[sumgai]

Rune,

Abbreviated answers, sorry, the meds are starting to really take me down.

Two amps, one with Line In, the other with Line Out. Very good way to experiment. What you hear should give you a good indicator of what modding the Fender would do, and also an idea of what that 'expanded stage presence' will be like.

"Stereo-izing" the Twin. Only one power transformer needed, it'll still be ample for the two channels. In effect, they won't draw any more total power than the original single OT and quad setup. You can always imitate mono/100watts by simply plugging your cable into both channels at the same time! (May need a short jumper between the two channels.)

The overall power output issue (100w vs. 2x50w) is good fodder for a full thread. Start that one, and I'll jump in, but right now, that would just make this topic messy.

The computer's ability to take in signal is up to the computer (or it's sound card....), but in general, they're all set to handle Line In easily. Just for reference, Line is usually recognized as 1.25 to 1.5 volts (rms) into 50Kohms or greater. That's a very loose definition, btw, the impedance can go much lower, and there are other ways to show these same characteristics (dBm, dBu, etc.), but that's the gist of it. BTW, if the output in my mod above (splitting the mono signal at the two 220K ohm resistors) proves to hot, you can just move the take-off point back in the chain, no need to add resistors at that point.

I'm pooped, I've gotta turn in, see ya manana.


sumgai