Chasing out obscene bass response from a fender MV Twin Reverb

[goldstache]

So I am restoring a friends 70's Fender twin.  It has a MV but no push pull.  It also has the 70uf filter caps.  I did a slight rebuild, caps, screen resistors, checked voltages, tubes, you know all that good stuff.  However, problems still persist.  Namely, Almost a distorted bass motorboat when playing hard.  Speakers are new.  Bias and voltages are well within tolerance.  Can't seem to chase it out. Tried the cathode resistors on pre's and that didn't help.  Wondering why it sounds so distorted in the lows???? 
Swapped tubes, even tried to take bass out with cathode R/C relationship as well as coupling caps at the plates.  To no avail. It still is mushy down there. 
Any suggestions are what to check next. 
I suppose it should be noted that I have the master on 10 and adjust channel volume from there.  Twins usually stay clean as can be in this volume relationship.  It also happens on the dry channel(1).  So I am wondering if I need to rebuild driver section?  Is this a common problem when the driver is not functioning properly? 
A lil' stumped.
Thanks!!!!

[worth]

No offense meant.. but do you know how to bias your amp ?

[goldstache]

Yes it is idling at 32ma per tube.  Which is about 70% of dissipation.  So to my ears yes.

[eleventeen]

Just for fun, if you have any sort of external speaker cab, maybe you could try powering it with the TR and disconnecting the internal speakers. Maybe there is part that is microphonic with some low guitar freqs when the vibrations from the speakers are applied to said part. Just a thought, simple test.

[phsyconoodler]

I never play Fender Super's or Twins with the bass control past 3 on the dial. There is simply too much bass available with the coupling caps they use.
  If you wanted to,you could drop them to marshall values and the preamp bypass caps to at least 1uf or even .68uf.Then you would have way more control over the amount of bass.

[goldstache]

I've tried the other cab idea.  Better, but still a bit of sizzle in the breakup that isn't just air.  I tried the marshall values and it really gleaned too much control from the pots.  I thought it would help too. 

[Willabe]

I tried the marshall values and it really gleaned too much control from the pots.  I thought it would help too.

So find what works for you somewhere in the middle between the Fender and Marshall values.


               Brad     

[goldstache]

I may have to revise.  This does not sound like low end you get from cathode/plate bypass relationship.  It gets extreme when you play harder.  I kinda think something else is up????!!!! 
Thanks for the insight
Have a great Christmas!!!

[HotBluePlates]

Try unhooking the feedback wire that runs from the from the speaker jack hot terminal back to the board.

Any change in the motorboating/sizzle?

[smackoj]

well, it would not be unheard of for a 80's era guitar player to dime a TR so, I'm with eleventeen for a KISS approach first. i.e. test other speakers and I would think definitely swap out the 4 p tubes with a known good set.....what does she do now?

OJ done it fo sho

[HotBluePlates]

Speakers are new.

Forgot to ask... What are the new speakers?

[plexi50]

This is really interesting. Usually power supply caps can cause note decay and nasty bass breakup. But if all else has been replaced and you still have this i would still have to say bias,cab or speakers. The process of elimination is pretty straight forward. Then again i myself are still running accross stange little issues that do cause bass notes to distort. I have to think bias,cab,speakers. Some cabs are comming apart and you just cant see it, creating bass note distortion. When you find the problem let us all know what it was. Sometimes it's the real simple and obvious things that we overlook

[goldstache]

Thanks for the tips.  I have tried it with other cabs.  Still persists.  I changed the stock speakers to Jensen C12n's brand new.  Still there.  I was wondering if the amp is over filtered.  It is the AA270 circuit Im pretty sure.  Which has 70uf filter caps.  I read that using 100uf would be okay.  From my own tinkering I have noticed that over filtering if anything solidifies bass handling.  Equating to headroom in the lows.  So I'm at a loss.  I will try the 820ohm negative feed back resistor.  Don't see it getting better by removing it. But trial and error.  Tubes are stock but test fine.  I already tried a matched quad replacement to see if that was it.  Bias seems to be good.  Though it sure does sound a bit like a bias problem.  It's that kind of distortion/clipping but just in the lows. 

[goldstache]

I removed the 820ohm NFB resistor and it improved.  Went with it.  Still not sure why the introduction of NFB would give it such a strange bass response.  Thanks everyone!  On to the next project!

[PRR]

> removed the 820ohm NFB resistor and it improved

Swap OT primary wires and put the NFB back.

Is that good? (If so, you had the OT phase wrong, but not enuff "N"FB to outright howl.)

You might indeed prefer the no-NFB connection. Depends on speakers and style.

[HotBluePlates]

> removed the 820ohm NFB resistor and it improved

Swap OT primary wires and put the NFB back.

Is that good? (If so, you had the OT phase wrong, but not enuff "N"FB to outright howl.)

 

This is why I asked for this step to be taken. It's a simple 3-minute test, and 2.5 minutes of that is waiting for the iron to heat up.

By the way, I didn't intend for the 820Ω resistor to be removed, but simply the wire that attaches to that resistor to have one end unsoldered. That would break the NFB connection and tell if the loop is the wrong polarity.

[goldstache]

Yeah I just lifted the leg of the resistor out of the tagboard.  Standard break in connection.  No parts were removed.  So did the negative feedback just give me to much low end response?  Is that the understanding here?

[Willabe]

So did the negative feedback just give me to much low end response?  Is that the understanding here?

No.

Swap OT primary wires and put the NFB back.

Is that good? (If so, you had the OT phase wrong, but not enuff "N"FB to outright howl.)

 This is why I asked for this step to be taken. It's a simple 3-minute test, and 2.5 minutes of that is waiting for the iron to heat up.

That would break the NFB connection and tell if the loop is the wrong polarity.

Try what PRR and HBP said.

If the OT primary leads where reversed it will give positive feedback, which will cause problems. It's a very common problem on new builds or when changing an OT. A lot of guys leave the primary OT leads a little long for the 1'st power up just in case they need to be swapped. Then once they know they're correct they go back and trim them nice and neat.

Read this link on the same topic. It's not long.

http://www.el34world.com/Forum/index.php?topic=14822.msg141754#msg141754



                    Brad      

[HotBluePlates]

Yeah I just lifted the leg of the resistor out of the tagboard.  Standard break in connection.  No parts were removed.  So did the negative feedback just give me to much low end response?  Is that the understanding here?

> removed the 820ohm NFB resistor and it improved

Swap OT primary wires and put the NFB back.

Is that good? (If so, you had the OT phase wrong, but not enuff "N"FB to outright howl.)

Did you do what PRR said?

Just trying to get you to make a short test and report your findings. Since breaking the NFB loop helped, 99.9% chance your "negative feedback" is really positive feedback due to wrong orientation of the OT plate wires. So, swap the plate wires and hook the feedback circuit back up.

Is everything fine now?

I used to explain all the little theories I might be considering when suggesting a simple test, but it's much faster and less confusing to just give a simple step to try. If it works, I can then go back and tell you why it worked.

I might go through a similar process when troubleshooting my own amp. Think of the various things that could cause the symptom, then figure what test to try to rule out the most likely causes first. No point in trying to fix something that isn't broken by "fixing" something that's only 5% likely to be wrong.

[goldstache]

Sorry for the delay.  And thanks for your help.  The transformers are wired stock.  They have not been changed.  So Im under the assumption that they are not the culprit.  Seeing as the amp worked for 30+ years the way its wired, Im thinking it lies in other areas.  Just not sure where?

[goldstache]

I should note that the NFB lift help but did not solve it.  Still a weird oscillation/fuzz on only E string notes with bass control at 3 or above.  Its very similar to when you have a bad cap on a cathode of the preamp.  Fuzzy mis biased sound.  Still combing in between other repairs.  Thanks for the help. 

[vatch23]

Hi there, since I have exactly the same problem with my no-MV Twin I am very interested in hearing of you got it solved by now.
Or if someone else can spin further. My amp is an 74 SiverfaceTwin no-MV converted to El34,s. It has JBL e-120,s in it. and I did try different cabs, removing NFB,tubes and all powersupply filtercaps and resistors.
It still has an ugly distortion on low notes. I have another amp same year identical except for 6L6GC,s and it sounds great and dont have this problem.

[goldstache]

Vatch23:  I'm sorry but I'm not sure how I fixed it.  It was a while ago and I think I probably resorted to replacing preamp passives.