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Messages - Mprall00

Know this is old but just wanted to follow up here to say i did change to 7812 regulator.  Lowered the noise floor of the preamp and sounds a lot better.  But when i put in the 1k resistor for r46 it dropped the voltage to 5v.  220r put it right at 10v.  Sounds fine.  In case anyone else stumbles on this like I did I wanted to share.

I have changed a ton in this amp now.  When i am 'done' i'll do a different post to try to cover everything.

What I am trying to do now - NFB loop so I can add a presence control.  Keep getting screeching positive feedback even when i flipped the OT primary...  Might give up on that one.
Any advice on adding NFB so I can add a presence control?  Tried a couple things and actually deleted a post from yesterday that didn't work.  I tried deluxe reverb style later and got a loud screeching noise.  No damage done but definitely didnt work.

For that I tried lifting the grounded side of R15 and C16 and connected them. Connected 820r resistor from the speaker output to the lifted sides of R15 and C16. Put one end of a 47r resistor to ground and the other end to the connection of the other 3.

The end goal was to add more like a bassman style with a presence control.  Just tried without to start and didnt work at all.

Edit:  Flipped primary because that sounded like positive feedback - same problem.  Hmm....  Definitely seems like I am getting positive feedback either way...  It did firm up bass and treble seems to sustain better.  It is also in phase with my blues junior when run in stereo now.  So all wins, but still NFB does work.

Edit 2: Tried the NFB of an AB165 bassman (47k in series with .1uf fed back into the phase inverter after C10).  It looks like weird compared to most NFB set ups that i tried originally.  It worked this time since it is actually negative feedback, but it reduced signal way to much.  Not sure why the much more common place NFB from other fender models squealed like positive feedback.  Research continues...
Yeah the comment about altering the bias was correct.  The negative feedback in this amp does seem to apply some negative feedback with low resistor values but it has to go so low that it alters the bias of the second triode as well.

So i removed it entirely and put a switch in for bias resistors instead for cool, hot, or normal bias.  Much more interesting.  Then i drilled another hole and made the bypass cap switchable as well.  Not a real noticeable difference between 5uf and 22uf but the .68uf setting really tightens up the bass for drive.  That with the bias switch makes for some cool sounds.
Haha. Late night dumb question. I've even added a cathode follower to a blues jr. Think I was thrown off because I've never seen one on v1 as I was looking up schematics... and it was late. Thanks for the response to the dumb question!
Coming back with another question - can anyone tell me how the 2 triodes of v1 and v2 are working?  Staring at it realizing they aren't parallel.  And they aren't in series... are they?  is the signal coming from the cathode of both v1 and v2?
Another question while I am at it - V1 and V2 don't have a cathode bypass cap.  Worth adding one to pin 3 of both?  Or at least V1?  would raise gain... and depending on value, on specific frequencies?
I'll have to look tonight but i feel like C13 and C5 being so close to pin 5 means i could almost cut the trace and then just solder the resistor to the existing hole that the leg of C5 is already in then directly to pin 5 without modifying the board at all.  Any problem not using the board at all here?  More noise?
Good advice!  Didn't notice that.  I bet it was never a problem with the original design because it was never getting loud enough to get the power amp cooking anyways.  it was so quiet stock...

I doubt I will get it cranked that loud anyways but in case I do...

Would that mean essentially cutting the trace between R14 and pin 5 of v4 and R10 and pin 5 of v5 then adding something between 1.5k and 10k where i cut the trace?  Is there any downside?  Nothing mentioned in what I am reading... did ibanez just design it without to save $.05?
I agree... But if i toss it then i have 5 open holes in the front.  And if I keep it and break the connection as you outlined then i still have to drill a hole for the new gain pot for v2.  And if I am drilling anyways I am going to just solve it with a second input.  So my new input will bypass all of it - buffers, ts, SS, everything - and go straight to the CN3a.  And the stock input will still be there and use the stock daughter board as is.

Truth is, once we changed all that stuff (i changed a lot with your help but the biggest impact was the 12v regulator, R4, R5, R13, R72, R17 as a pot, and the tone stack caps), the SS front end actually doesnt have a bad gain tone.  So therefore I think I will drill one extra hole and have both options, even if I don't really ever use the stock option... Just turn the SS gain all the way down, plug in to the new input, and it will be like it isn't there.

Once done I will try to remember to take pictures and write up an easier to follow list of changes in a new post.  These can be fun to learn with since they are relatively cheap for 30 watts.  I got my TSA30 combo for just under $200 used...
I just wired it as a variable resistor.  Actually just got a 1meg pot because the 500k pot at R17 was not breaking up much when cranked and even though i almost always play it clean as a pedal platform I thought having the option of more preamp gain would be nice.

I guess you could replace r13 with a pot as well to control v1 too.  Let me know if you try it!

After tweaking a lot of resistor values and replacing the tonestack, the SS front end actually doesn't sound bad either.  I got the solid state section to be switchable with the stock input jack but it doesn't sound good.  When the SS is bypassed it sounds like the impedance is way off - like something at the input needs that buffer from the SS sections.  I even tried adding a 1 meg pull down on the stock input jack...  It just sounds really flubby with no presence when the SS is bypassed but sounds perfect when i removed the whole daughter board and added my own input jack straight to CN3b.

SO... i am about to drill a hole next to the existing input for a second input direct to CN3b and a hole for the gain pot above and between the gain and bass knobs.  That way I can just plug in straight to the tube section with the new input or plug in to the TS>Boost>SS front end if i want to use it.

Side note: replacing the signal path resistors with metal film and replacing the coupling caps and tone stack made this amp a lot less noisy.  Not sure if one of those thing did more for the noise floor than any another - i just know the whole change helped.  i got mine very old and very used, so it could have been noisy due to age, but this really helped as did a proper bias.
Switch was good.  Rewired with a 5k and 820r.  820r was likely too low but good to hear its working.  5k is a good sweet spot.  Off now sounds like there is no negative feedback.  10k was just high enough in this circuit to be barely noticeable so stock sounded almost like no negative feedback at all.

So I may leave the switch there.  Or I may solder 5k in there and experiment with the switch at other places...  like c4.  Or maybe an off switch for the tone stack...  Or a cold clipper switch.  Wonder if this amp is high enough gain to have that be worthwhile...
Thanks!  Yeah it dawned on me that the 1.5k was the cathode resistor.  But when I had the 10k in there shouldnt it have read 1.3?  I guess with it being that close (and that late at night) it could easily be by own user error or mental rounding...

Also I thought higher was going to be more noticeable.  But now realize looking at it that the parallel resistor is just averaging it out to be basically no change.

Thanks - I will try 5k and see if that is noticeable.

With NFB having so little impact on this amp, is it even worth trying a presence control?  that was going to be next for me...
Attached is the schematic for an epiphone galaxie 10.  I have modded it heavily as a learning experience following another board's advice.  First thing I did years ago was cut the negative feedback resistor out (R15) but now thought I would like to add it back in on a switch to try values.

I wired it like the picture attached in place of R15.  I had tested the switch prior to hooking it up.  All good to go.  Attached it and fired it up - no change in sound at all between 33k, 10k, and negative feedback off.  That can't be right right?

So I took it back apart and used my multimeter to measure the switch on board and every setting (even off) measured 1.5k.

So I took the switch out entirely and measured between the pads - 1.5k.

So I wired a 33k in to r15 and measured on board - 1.5k.

Fired it up with 33k and sounded exactly like when I thought I had the negative feedback removed.

I mean it actually sounds really good after all the mods, but anyone know what is happening?  Can i not modify negative feedback loop in this amp?  Can I not even remove it?
Thanks for all the help.

So the tube amp portion now sounds excellent and plays nicely with pedals and is much quieter now.  1meg is in place for r13.  Switched r18 to 470r.  left r25 at 15k since that is between a bassman and deluxe reverb.  May play with it someday but sounds good now.

Still pretty bass heavy.  Could play with c10 (or c54?) coupling cap but don't have any on hand rated at high voltage.  Just running bass control around 30% and it is good clean but as the new gain is turned up it distorts pretty bassy.

Now thinking about how to work the SS portion back in to the circuit on a switch.  Have an spdt on-on I am going to use maybe at the point you mentioned with r62 going to middle lug and one side going straight to cn3a and the other to C30.

I knew i did something wrong!  Thanks.  Totally misread and thought R13 was in series in to v1 and didn't notice it was going to ground!  Stupid... i think I have it now.