Your opinions on a few mods

[pine]

I have a Bandmaster Reverb head I converted to a Super Reverb, blackfaced, and was reading in a book about some possible mods to the circuit. Have any of you tried any of these, or do you think they are worth doing?

A) Disconnecting negative feedback circuit at the speaker jack for gain and harmonics increase. Does this sound good?

B) Increase value of first gain stage bypass caps to improve low end. Pin3 of first preamp tube, change 25uF 25 volt cap to 250 uF 6 volt cap.

C) Eliminate series resistance on input circuit by disconnecting 68K resistor on input jacks to improve fidelity (does that mean to reduce gain or does it mean something else)

D) Eliminate series resistance on phase inverter input circuit by eliminating 220K resistor located at point "Y" on the AA1069 layout and connect the wire directly to the .001 uF cap coming off of pin 2 of the phase inverter tube, supposed to increase gain and fidelity (there's that word again, if you increase gain, do you lose fidelity?)

Thanks in advance for your time and input.
Pine

[Enzo]

A. The negative feedback is a wire. You could disconnect it in seconds and then listen to the difference. That would tell you exactly what the sound difference is far faster than us trying to explain what we think it will sound like. In brief it will be a bit louder and a bit more raw.

C.D. Fidelity? Guuitar amps are not about fidelity. Play your guitar into your home stereo. THAT is what fidelity sounds like. Hifi amps are for REproducing sounds. Guitar amps are PART of producing the original sound - they are part of the instrument. That is why some guys play MArshalls and some guys play Fenders. If they were hifi they would all sound the same. Guitar amps are not flat, and not clean, they are not supposed to be, they are designed to add their character to the sound.

D. That 220k resistor is next to another - one from each preamp chanel. They are mixing resistors. They allow signals from both channels to come together into the power amp. I don't know what improvement removing it would achieve.

B. Again, just experiement. If you want to know what 250uf sounds like, just tack it in parallel with the existing 25uf cap. SOlder it lightly or use clip wires. That will wind up 275 instead of 250, but really you'll not notice the difference between those. No need to remove the other one first. Now you can hear instantly any difference.

Do you already have some 250uf 6v caps? 6v is an odd one these days. I have drawers fill of 220uf caps at 16v 25v 35v and higher. No 6v. ordering one at that low voltage special would be a waste of money in my view if you already have common voltages. A 6v cap will sound no different from a 25v cap. Only the capacitance matters.

And does the amp lack bottom end now?

[loudthud]

The 68K input resistor is an interesting mod. Removing them will increase high end slightly but make your amp vulnerable to picking up radio stations. TrainWreck amps do not have this resistor. They (and their clones) are praised by their owners for having extreme touch sensitivity.

The vulnerability of you amp to picking up radio stations depends on many factors such as your guitar, your cable, proximity to the station, frequency and power of the station, and zodiac sign . Not much has been done on this subject but an RF filter might perform as well as the 68K and still provide the benefits of no resistor.

There are actually two 68K resistors and they also serve as mixing resistors if you are using both inputs at the same time, and a 6dB attenuator if you use just the #2 input. If you use the #1 input by itself, the two 68Ks are connected in parallel effectively making 34K. So you will need to come up with a re-arrangement that will do what you want it to with no surprises.

[cbarrow7625]

Most of the mods you list are rally relatively subtle. The one that is easiest and you will notice the most difference from is disconnecting the feedback resistor.

THe 250uF cap ususally makes these types of circuits feel "slow & bloated" to me. Some others may find them "warm". It really depends what your amp needs.

I almost always eliminate the 68K input resistors because I like the "feel" o fthe amp better witout them. just make sure you have a well shielded guitar & a good cable to reduce the possibilities of RF pickup & injsection into the system.

You probably shouldn't eliminate the 220k resistor (unless you are getting rid of the other channel. Those are mixing resistors. You can try making it smaller or even putting a cap around the 220k. that will still give you DC isolation between the two stages but not AC isolation. You could remove the resistor without doing any damage, however. You're not going to break anything. Tube amps are forgiving.

I would just get in there and try a few of these and see if you like any. I used to start with simple suggetions like these & listen to the amp. While I was in there I would make sure to have the schematic next to me so I could see exactly what I was changing & then look for additional things to change in the same parts of the circuit that might make adifference to the sound.

Chris

[Rick Erickson]

They (and their clones) are praised by their owners for having extreme touch sensitivity.

Is there any proven correlation between the lack of the 68K resistors and this touch-sensitivity, or are people blindly making the connection somehow? I remember a Ritchie Blackmore interview in GP Magazine where he said the Marshall amps with a Ground switch sounded better (to him) than the ones without one. That's just one difference in what is in reality two very different designs.

RE

[loudthud]

They (and their clones) are praised by their owners for having extreme touch sensitivity.

I probably overstated that point. It's probably fair to say that it's one factor that people say contributes to the touch sensitivity of TrainWrecks. Apparently, being on the verge of oscillation is another factor. I may try to build one from my own layout and it may end up looking like an RF amplifier with compartments around each stage. The first tube will be close enough to the input jack to have a grid stopper connect the two... no coax.

Sometimes no-techies come up with things that aren't too far off. Marshalls with ground switches may have been made for US markets and have different power transformers or in the case of the Mk-II's have 6550's instead of EL34's. There are people who say they can hear the difference between different batteries in wah-wahs and pedals.

[cbarrow7625]

Yeah, the overall short signal path and hi gain have nothing to do with that "touch sensitivity".

I have just found that in A/B tests, I just prefer no input resistor. To me amps just sound clearer. However, I would almost never remove that resistor form a Fender twin Reverb, or similar, amp. Sometimes it adds a "softness" to the tone that I like.

All of these suggestions are really subjefctive and differ on a case by case basis. Also, if I decided to build one of my amps in a production environment for sales to the masses I would always add an input grid stopper resistor to control RF. I can just control everyhthing from the guitar all the way through the amp so i can get away without one. I would never blindly recommend a mod like that without knowing the rest of the system. Everything does work together as a system after all, the amp doesn't do everything.

Chris

[pine]

Will separating the cathodes in V1 and V2 add to that touch sensitivity (of my Bandmaster/Super hybrid) by disconnecting the wire connecting their respective pins 8 and grounding those pins with 25uF 25V caps across 1500 ohm resistors?

[loudthud]

Separating the cathodes won't make much difference, but being able change the bypass cap will give you options to alter bass response.

The diagram below will give you the ability to run without a grid stopper and if you are getting interference, switch to the II input to eliminate it. Experiment with the value of the 10K resistor between 4.7K and 68K.

[TD_Madden]

These options sound like they came out of Gerald Weber's first book (or his Vintage Guitar column...:-) )

[Bruce / Mission Amps]

Very nice, good idea.

[edge59]

If you like the tone of removing the 68k resistor, and you get RF noise, you can put a ferrite bead on a jumper or the input lead. Mesa Boogie's Lonestar does this and they rarely use the 68k input resistor, thats one of the reasons they sound like a Boogie (and other reasons...) What kind of sound are you looking for? You could spend years trying different mods and things to find what you like as I have You learn a lot this way, but if there's a particular tone you are seeking like a favorite player gets, I would start from there and study their setup or rig. Pick what you like and don't like, see what they are using, try different pedals, amps guitars whatever till you find whats closer to your liking, then keeping mods to a minimum you will have some time to play too
Ed