DR One-channel - Too much treble response! - SOLVED

[Lectroid]

Greetings,

My build is an adaptation of Hoffman's single-channel DR, with a couple of mods from sluckey's Tweed Deluxe Reverb: I added a master volume, and replaced the .047uF  midrange cap with .022uF.  I'm currently voicing the amp and have the most persistent problem.  The Treble knob is crazy strong.  At any Treble setting above 3 it adds a harsh, buzzing quality to the notes. This happens with single coil or humbucker pickups although not as bad with humbuckers. 

I have spent the last several days researching here.  Mostly the blame fell on oscillation, so I've been trying various tweaks I've read, mostly placing 50-200pf caps in various places.  One across the Master volume leads helped somewhat.  I've tried them parallel with the preamp plate resistors but couldn't hear much of a difference.  The next best one was 120pF across the LTPI plates.

But still, while the sound is improved, there is still a harsh buzzy quality unless the Treble knob is dialed way down.  It also seems like the Mid pot also adds some buzz when I turn it up high.  The tone stack is the classic Fender.

Any tone from bass through bright midrange comes through fine, although definitely brighter with single-coils.  It's just the treble side that's out of control.

Should I replace that .047uF tone cap?  Would replacing the 250pF treble cap help here?  Changing the slope resistor?   

[PRR]

Have you tried reversing the OT primary wires? Tried another speaker?

[pdf64]

Have you tried other black / silver panel type Fender amps and not experienced this problem?
Repeat the above query, but through the speaker model you’re using with this build?

The point being that you may not like the circuit, or you may not like the circuit through that speaker.

If it’s the circuit, I suggest to reduce the tone stack treble cap value from 250pF to 120-150pF.

[Lectroid]

@PRR,

Have you tried reversing the OT primary wires?

l will reverse the leads; it's certainly worth a shot although it doesn't sound like when I've made that mistake before.  I've never built an AB763 before but I've played Deluxe Reverb, Bandmaster, and Bassman amps.  This amp has a very Fender-y clear sound over most of its tonal range--as long as the treble pot is dialed way down.

Tried another speaker?

Now you've got me thinking about impedance mismatch.  My test speaker is a 12" Hammond organ model, as is the OT, from a Hammond AO-43 chassis out of (IIRC) an H-100.  I know that the OT puts out 8 ohms but I don't know the final impedance of all the organ's speakers taken together was 8 ohms.  Could pushing an 8 ohm signal through a 4 ohm or 16 ohm speaker system cause this symptom?

I will try another speaker today, a Jensen C12Q that I know is 8 ohms.

@pdf64,
I have a 120 pf cap; I will try replacing the treble cap in the tone stack.

[pdf64]

I suggest to check the taper of the treble pot.

[Lectroid]

It's a 250K pot.  Across the outer leads I measure 247K.  At roughly 50% rotation I measure 35K, at about 75% rotation I measure 125K.  More later.

[Lectroid]

I tested the 120pf cap as the treble cap in the tone stack.  It made the harsh-tinny-buzzy quality of the notes worse.  Then I replaced the 250pf cap, and put the 120pf cap across the plate resistor of the triode immediately following the tone stack.  That was some improvement, about a 30% drop in the harsh, buzzy quality.

Bear in mind that this effect happens only on the higher notes, say from 600Hz upward, more pronounced on the higher frequency notes.

I then took the Mid pot completely out of the signal chain and that helped somewhat more.  In that configuration I could actually dial in about a third or half of the Treble pot without getting the harsh overtones.

I also lowered the bias current from about 26-27ma to 24-25ma.  These are new JJ 6V6GTs.

I hope this makes sense to someone.  I'm well out of my depth.     Willing to try anything.  I have to go out tonight but I can report back on the other stuff tomorrow.

Thanks!

 

[pdf64]

It's a 250K pot.  Across the outer leads I measure 247K.  At roughly 50% rotation I measure 35K, at about 75% rotation I measure 125K.  More later.

That’s fine, I just wanted to make sure it wasn’t linear.

Those idle currents seem rather high?

Any progress on eliminating the speaker as the cause? eg it may even have a whizzer cone.

[mwelch55]

You said you replaced the .047uF  midrange cap with .022uF.  Wouldn't that end up making the tonestack treble control stronger?  What was the purpose in that change? 

[Lectroid]

You said you replaced the .047uF  midrange cap with .022uF.  Wouldn't that end up making the tonestack treble control stronger?  What was the purpose in that change?

It was from sluckey's Tweed Deluxe Reverb and I believe it is a common mod done to tighten up the bottom end.  But you're right, the .047 is the stock AB763 value, and I never actually tried the amp with it.  I will sub in a .047 cap today.

@pdf,

Those idle currents seem rather high?  Any progress on eliminating the speaker as the cause? eg it may even have a whizzer cone.

At 410 plate voltes, the bias currents have the amp at 70% according to RobRob's calculator.  I'm okay lowering them further if it will fix this issue?

Will do some speaker swapping today as well, also switch the OT leads just to be sure that's not the issue.

[acheld]

switch the OT leads just to be sure that's not the issue

I had nearly identical symptoms recently with a rebuild of my Princeton Trem-O-Nator due to the OT leads being switched.  Took me a few days to realize that was the problem -- didn't think the symptoms could be from that 'cause the amp did not squeal like others I have heard. 

In a new build, that is probably the first place to look when faced with weird distortion.

[Lectroid]

Thanks to everyone for their suggestions so far.  The amp is not there yet but it's getting closer to what a DR should sound like.  Sorry this is long.

Status:
Replaced the .022uf cap in the tone stack with the stock .047uf.  It was a definite improvement.
Removed the Mid pot out of the circuit. Also improvement and that helps some. 
I'm going to pull out the Master volume out and see if that helps.

I did swap OT leads, and the amp produced that loud, unmistakable squeal.  I then switched them back and the squeal stopped.  They are now: brown=sleeve and green=tip. The sleeve is grounded to the PT ground lug. 

Biggest problems:
#1) When I took the voltage measurement on the PI 'b' triode's grid, the amp hummed loudly when the DVM probe touched the grid pin.  No such hum on the 'a' triode's grid, or at any other test point.  Any thoughts on what could cause that?

#2) The NFB switch suddenly stopped working. When I touch the NFB tap to the Tip contact now, the amp squeals the familiar "mismatched OT leads" squeal.    Attached is a drawing of my set-up, also voltages (on cold amp) and a new schematic.  Areas in red are still present but not wired in right now.

Any wisdom on speaker jack wiring with regards to NFB? 

#3) (related to #1?)  This amp has too much distortion above low volumes.  The guitar and amp Volume knobs will both increase distortion.  I wanted an amp that would distort with increasing volume but this is too much.   The distortion starts at around 1-2 on the volume knob.  Don't think it's all even-order harmonics either.  Any other suggestions of where to look?  Likely culprits I can chase down? 

#4) I have a 10K mid pot, would 25K be better?  I see that value on some circuits with Mid.  I'm happy leaving the Mid control off, but wouldn't mind getting it to work if I could.

#5) Also in the reverb driver I had no 2.2K cathode resistor so I used a 1.8K.  Reverb seems strong, but could that be a problem? 


I know this is a lot to process, but any help solving any of these would be appreciated.

Thanks again!

[acheld]

The NFB switch suddenly stopped working. When I touch the NFB tap to the Tip contact now, the amp squeals the familiar "mismatched OT leads" squeal.    Attached is a drawing of my set-up, also voltages (on cold amp) and a new schematic.  Areas in red are still present but not wired in right now.    Any wisdom on speaker jack wiring with regards to NFB? 

Your schematic guides speaker jack wiring for your NFB signal -- so yeah, NFB comes from the tip.  If you think about it, if you tried taking NFB from the black lead, you're grounding that part of the circuit.

Now why you get a squeal when you connect your NFB tap to the tip suggests that something is miswired.  I would recommend looking closely at your wiring in relation to your NFB and PI.   Your schematic is correct --

Maybe some hi-res pics would help us.   

Not having NFB in a DR normally does decrease headroom for clean -- whether that is the whole story, I'm not sure, but it sounds like that portion of your build is likely your priority now.

[Lectroid]

@acheld,

Thanks for the comment.  I'll check the PI wiring carefully.  Any thoughts on #1, the loud hum when I touch the PI non-inverting triode's grid with the voltmeter lead?  I get a true reading but why would one grid of the PI do that? 

Anyway, I have to work a day on other projects, so I'll have to report back in a day or two.

[PRR]

> I did swap OT leads

We usually swap the primary leads. Because one of the secondary leads is usually designated "ground".

[Lectroid]

@PRR,

Yes!  Thank you for catching that.  I actually knew that at one point and then forgot it when it really mattered.  Thanks for not over-estimating my experience.  Again.     That would explain my funny results with the NFB pretty neatly.

I'll be able to swap the primary leads at the power tubes later today and report back. I'm assuming the secondary 'signal' lead is the green one?  NRN if it is.

[Lectroid]

Update:

I've swapped the OT leads at the power tubes and reworked the speaker jack including the NFB tap.  The amp works with no squeal, even with the NFB tap attacked.   So I definitely had the OT primary leads reversed.  Everyone that suggested I do that first, you were right.   

However I still have a very treble-y sound, nothing like what a AB763 Deluxe should sound like.  The buzzy harsh distortion is gone but the amp is just voiced very, very, bright.

I am working on acheld's suggestion of validating the PI circuit very closely but nothing else to report.

[sluckey]

Try using a different type of 250pF on the treble pot. IE, if using a silver mica, switch to a ceramic.

[Lectroid]

@sluckey,

I will definitely try that.

Today I swapped the OT primary leads, and focused on the LTPI: checking values, continuity, match to the schematic, etc. See pictures.  The layout is ugly, I know, but electrically, it works.  Mostly.  The tone is very treble.  The Bass pot works; I can hear it add bass.  I believe the Treble pot is working too.  The preamp appears to be working.  Whatever is wrong seems to be in the LTPI, but I can't see any obvious problem there.   

A couple of things:

1.  When I read the voltage to the non-inverting LTPI input, I don't hear any hum.  When I read voltage on the inverting side (.001uf cap), I read an almost identical voltage (~47V) but there's a noticeable 60 Hz hum.  If I attached the scope to that LTPI input, that is, at the PI end of the .001 cap, I can see the waveform on the scope but it kills the output sound.  I guess I'm grounding the signal somehow but I don't understand how/why it's happening.  I unsoldered the cap and measured it at 94nf.

2.  The first LTPI tube (Zenith 12AT7) was strongly microphonic.  I replaced it with another Zenith 12AT7, but it's still  microphonic.  I can poke the tube or the socket, same result.  None of the other tubes are microphonic. Also the same grounding (?) behavior described in #1 happened with the second tube.  I'll try another replacement tube but I"ll have to order some in.

3.  The LTPI's 82K plate resistor is actually closer to 88K, if that's a problem.

Thinking about the LTPI just now, I see that I have wired the .001uf cap on the triode with the 100K plate resistor, not the 82K resistor.  Is that a possible culprit here?  I'll try swapping that around.

[sluckey]

Probing the LTP PI grids with a meter or a scope will cause funny things to happen because the bias voltage will be upset when you connect directly to the grids. Don't do that.

If you want to monitor the signal going into the LTP PI, look on the preamp side of the .001 input cap. If you want to measure the grid dcv then connect your meter to the junction of the grid resistors and cathode resistors (4 resistors at this junction).

[Lectroid]

Probing the LTP PI grids with a meter or a scope will cause funny things to happen ...  Don't do that.

Okay.  I'm down to trying random stuff just to gather any sort of data that might trigger someone's thinking.  It seems odd to me that one grid would hum and the other not hum so I put it out there.

If there's any sort of test you think I should make, I'll do it.

[Lectroid]

Try using a different type of 250pF on the treble pot. IE, if using a silver mica, switch to a ceramic.

I just scrounged up a 220pF ceramic cap and clipped it in and ...  yes, it sounds better.     The buzzy, ringing sound is gone.  I also tried a 2.2nF ceramic and it sounded okay too, with a tighter bass response and punch.  The -3dB point for the 220pF cap is ~2890Hz, for the 2.2nf cap it's ~289 Hz.

Why would a ceramic 220pF be such an improvement over the 250pF silver mica?  What went into the BFDR's during the 60s?

[sluckey]

Don't know why ceramics sound better. The original blackface amps used ceramics. I don't remember seeing any silver mica caps in anything back in the '60s. They may not have even been invented at the time. Here's some high resolution pics of a '64 DR. All ceramic.

     http://sluckeyamps.com/64DR/index.htm

[Lectroid]

@sluckey

Thanks again for getting down into the weeds on this one, and adding to all the help you've already given me on this amp.

[dude]

I’ve been following this thread as my “ear” has been hearing an overly treble or very bright issue, in a 6v6 plexi l built.
In the plexi l’ve just been mixing the normal channel more to compensate. I’be been using mica caps for a good while, l’ll try some ceramic.
Glad to hear you solved the overly treble problem.

[brewdude]

I have also been replacing silver mica caps with ceramic discs the last year or two.
I had bought some of those huge Sozo ($$) silver mica’s and chased many other possible bandaids before swapping out the cap I spent big money on hoping for more mojo.

[Lectroid]

Glad this helped some others, too.  I guess that's why there are so many different types of caps--the value is not the only determinant.  All caps are not created equal.

If you get good results, please post them back here.

[marshallguy]

Try using a different type of 250pF on the treble pot. IE, if using a silver mica, switch to a ceramic.

I just scrounged up a 220pF ceramic cap and clipped it in and ...  yes, it sounds better.     The buzzy, ringing sound is gone.  I also tried a 2.2nF ceramic and it sounded okay too, with a tighter bass response and punch.  The -3dB point for the 220pF cap is ~2890Hz, for the 2.2nf cap it's ~289 Hz.

Why would a ceramic 220pF be such an improvement over the 250pF silver mica?  What went into the BFDR's during the 60s?

Well I’m chasing the same problem and using a 250p polystyrene treble cap. So you’re saying the problem all went away with just changing your 250p silver mica treble cap? I have a different amp, modded up JCM800 Marshall. I installed the same style Fender tone stack for my clean channel off the plate of the first input preamp tube V1.

What I saw with a 1k audio tone injected was a jagged looking sign wave on the tone stack output that got worse the more treble passed to it. I installed a 331p to ground after it and that smoothed out the signal. Now, I lowered it to 400hz input and lo and behold the jagged signal returned! Still chasing the higher frequency note ringing. Can’t see it on a scope at the output but you sure can hear it. I could add a bigger cap to smooth that out but there goes most of my tone and treble. I’m going to try a preamp NFB some where along the line using a small cap and resistor to see if that cures it. I tried slugging a pole with from 120 to 1000p caps on the power tube grids per Merlin. All that did was cause an oscillating squeal making matters worse. Installed a 330p across the PI 47k NFB hoping that was it. It did remove the hump on the 10k square wave input at the amps FX Loop Return as Merlin wrote about that’s about it. Frustrating  problem for sure.

[ac427v]

I would match the Hoffman schematic more closely.

Replace the 100k Master Volume with a 50k pot. This more closely matches the 47k resistor to ground that Hoffman has in this location.
Remove the bright cap on the master volume. It makes your amp sound bright!

[marshallguy]

I would match the Hoffman schematic more closely.

Replace the 100k Master Volume with a 50k pot. This more closely matches the 47k resistor to ground that Hoffman has in this location.
Remove the bright cap on the master volume. It makes your amp sound bright!

After some research it appears it might be intermodulation distortion and phase shifting with the TMB adjustments. I paralleled a 120p across V1 plate resistor which in my case is 220k. That helped a lot. I’ll upload a schematic once I make the changes and reduce the size. The Fender “shimmer” explained on another forum is the treble and bass full up and the mid pot lower. Do the opposite and you just have a flat response for the most part. Duncan’s Tone Calculator gives you some examples.