Stromberg Carlson project - too much bass

[bmccowan]

I'm converting an old Stromberg Carlson PA for guitar use. Schematic is attached.
Its a 2x6L6 with 2 single triodes in the preamp and a 6SC7 PI.
It was unmolested (also not maintained) as it had all S-C labeled tubes and caps, and all solder joints looked original.
I like to get them working stock first before adjusting the circuits. So I replaced the bad caps and installed input and output jacks, proper power cord. I did change the first stage cap from .05 to .02.
I'm ignoring the phono input for now.
Good news is that it works fine and is louder than I expected given the low B+ voltage for 6L6s (310 on the 6L6 plates.)
There is too much bass and not enough clean tone.
I have a plan, but before I dive in, I thought I would ask some advice from you kind folks.
My thoughts, and I will test after each change:
1. Replace V1 50uf bypass cap with small value - maybe 2uf.
2. Replace the 5Y3 rectifier with either 5U4/5V4/GZ34 to bring up the voltage.
3. Increase the NFB by reducing the NFB resistor. It is now 1 meg!
4. Repurpose the phono volume pot as a bass or midrange control.
Logical?
Other ideas? I am not looking to gut it and clone a Fender or other as for this amp I prefer to work with the existing circuit as much as possible.

[stratomaster]

Replace the cap you changed with a 0.0047μF.  I wouldn't make any other changes until you've tried this.  Excess low end can eat into the clean headroom.  Fix one and the other follows.

Beyond that, it'll take some experimentation. I'd be inclined to mess with rectifiers and voltages as a last resort.  Increasing the NFB seems like a less risky proposition.

[shooter]

maybe start by crayoning in the values on the schematic so you don't have to bounce eyeballs from BOM to schematic every 2 seconds


i'm not very design wisdomee, but it looks like V2 has it's own local NFB thingy going on + the global NFB from the speaker
clip c1 from the split plate R on V2 see if that gives you more treble-less bass
maybe make V2 grid a standard leak, stop instead of a ?"variable bias"? through the VOL pot.
 

[bmccowan]

Stratomaster - that cap change is a nice improvement. Thanks
Shooter - I will likely skip clipping out that small cap for now, but it seems like changing v2 to typical grid leak is a good idea regardless. and it frees up that phono vol pot. I'm not familiar with that feedback setup at v2.
I found another version of the schematic - attached. V2 is drawn differently but I think its the same electronically. And it has labelled components

[shooter]

schematic - attached.

?

[bmccowan]

 Oh yeah, I have that problem with emails too.

[AlNewman]

That's an interesting tone stack.  I'd almost try unhooking it at the signal end and see what that does.
It might squeal.

[shooter]

much better, makes lot more sense.




Agree^^  I would also unhook that plate cap, you can always solder it back, they tend to suck highs pretty bad, leading to a perception of "too much Bass".  They are used for HF oscillation issues, mitigating "noise"


if you have a scope, feed the amp ~~200hz, measure Vin, n Vout of V1 n 2.  change to 1Khz repeat.  Vgain = Vo/Vin  so you can see if Bass has more gain or treble is attenuated. (NOTE: always verify amplitude of input signal after you change frequency to keep things math happy)

[ac427v]

That is an interesting tone control. I hope you tinker with it so I can learn from your experiences If I understand it properly, it cuts treble and mids at one extreme and treble only at the other extreme. In other words, all settings of the tone control remove some treble.

C5 also cuts treble; so removal may balance sound to your liking.

Unrelated idea is to use the phono control for master volume fun.

A more efficient rectifier tube will increase the B+. Will that change the current load on the power transformer and cause over heating? I don't have theoretical training so I don't know the answer. I'm speculating based on instinct.

[bmccowan]

Shooter - thanks for that. I will try removing that small cap. I do have a scope - not much experience with it. Good chance to learn more.
AC427 - C5 is the same cap Shooter mentions (I think) - I will definitely tinker with the tone control. I think you might be right about the intent, but in reality it doesn't do much.
Phono control - Once I monkey with the other stuff I will try repurposing that, thinking:
 - MV as you suggest
 - Using with the existing tone control for a James B&T stack
 - Framus mid control - Once I find where I want the bass and treble response in an amp I rarely change the settings, but playing with an aggressive mid, like the Framus, gives some interesting tone changes. Its not a popular circuit and I do not know why.

[shooter]

have you rolled in speakers yet, I've found 50% of "too much...too little.." can be "fixed" with speakers

[bmccowan]

Good point - not yet, so far into a vintage Jensen P12Q. I have a 12" Greenback, 15" Jensen, and 10" sig Webers I can try.

[HotBluePlates]

I'm converting an old Stromberg Carlson PA for guitar use. ...
I'm ignoring the phono input for now.
...
There is ... not enough clean tone.

IMO, you're starting in the wrong spot.  Begin with the "Phono" input only.

Notice the topology from the Phono input:
   1. Input
   2. Volume control (to turn down an over-hot phono preamp or cartridge)
   3. Gain Stage (Mu of 20)
   4. Paraphase Inverter - 6SC7
   5. Power Tubes

Now look at the topology of a 5C5 Pro:
   1. Input
   2. Gain Stage (Mu of 40 or 70)
   3. Volume control (to turn down an over-hot pickup multiplied by the Gain Stage)
   4. Paraphase Inverter - 6SC7
   5. Power Tubes

The Phono side is like a 5C5 Pro, but with the Volume control in the wrong place.  V2 to the Speaker is like a 5C5 Pro with its Volume control full-up; The Phono Volume control is behaving like your guitar's Volume control.  Do you ever play your amps with the Volume full-up, and use only your guitar's Volume control as the sole level/clean/dirt control mechanism?

To that we add another Mu of 100 Gain Stage in V1.  Yeah, I bet you're feeling like "there's no clean tone" since this amp is like the world's first "high-gain modified tweed amp.". I would look to getting the Phono input settled as this amp's basic voice before trying to sort out what to do with the extra gain of the Mic input's stage.

... There is too much bass and ...
...
3. Increase the NFB by reducing the NFB resistor. It is now 1 meg!
...

Negative voltage feedback usually uses a voltage divider to sample a signal, and that divider is made of at least 2 components.  Here, R2 (1MΩ) is one component and comes from the 15Ω tap via C6 (0.003µF).  But V3's grid reference resistor R14 (also 1MΩ) is a path to AC Ground, in parallel with V2's plate load R9 (100kΩ).  This parallel pair is "the other resistor" of this voltage divider, represented as Z2 here.

Working through everything, the feedback appears to apply 1.77v Peak to driver stage V3 Pin 3, while the Voltage Chart notes the bias (Pin 6) of the driver-inverter is 1.1v.  Does it really need "more negative feedback" when the existing amount reduces power section sensitivity by more than half?


Negative Feedback basically turns the driver/phase-inverter/power-tubes/output-transformer into an Opamp.  The gain of an opamp with feedback is basically "Z2"/"Z1" using the terminology from the link above.

The "gain" we should shoot for depends a lot on how much voltage we need at the speaker, vs how much the input to the power section receives. 

The 5F4 Super has a feedback loop configured for 4Ω and a gain of 56kΩ/1.5kΩ = 37.3.  It takes 7.75v RMS to have 15w across 4Ω.  7.75v RMS / 37.3 = 0.208v RMS to get 15w RMS output.

The AU-42 has a feedback loop configured for 15Ω and a gain of 1MΩ/90.9kΩ = 11.  It takes 15v RMS to have 15w across 15Ω.  15v RMS / 11 = 1.36v RMS to get 15w RMS output.

Do we really need "more feedback" in this output section, which will make it even harder to drive?


The Treble tone control is basically what we would otherwise call "Presence" since it reduces the high frequency content of the feedback.  Because of where the feedback is injected, it also directly acts on removing high frequency content of the signal before that signal gets to the speaker & becomes part of the feedback loop.

C6 (0.003µF) working against 1MΩ should be -3dB at about 50Hz.  But I wonder about an old cap or a leaky cap's impact on the frequency range of the feedback (I saw "bad caps were replaced" but don't know whether this one got caught).


To know what we're working with, I would disconnect the feedback loop & the Treble control by unsoldering C3 (0.01µF) and R2 (1MΩ) from C4 (0.05µF), R14 (1MΩ) and V3 Pin 3.
    If C6 was a very small effective value, there is no feedback for low frequency, and a lot of feedback at high frequency.
    If R2 was a very small effective value, there is no feedback for low frequency, and a lot of feedback at high frequency.
    V3 Pin 3 should be "0v" but the 1MΩ there & chance of grid current means C6 was likely meant for voltage-blocking to keep DC off the speaker voice-coil.

[bmccowan]

Thanks HBP for your detailed suggestions. I understand most of it
I need to leave my projects for a bit, but will get back to this and work through your suggestions. I especially appreciate your words on NFB which is always a bit of mystery for me.
Shooter I clipped out that cap - better.

[jjasilli]

I agree with Hotblue's approach.  But I would also add tone control(s), even if it meant deleting the presence control.  In my experience stock Stromberg Carlson PA amps are dark sounding for electric guitar. Furthering Hotblue's approach, you might plagiarize Fender: 5C5 treble control; maybe also add a Tweed-style bass control; or go FMV.  James / Baxandall would probably keep this amp too dark.


It might be possible to brighten the phono input stage by increasing its treble, and/or decreasing its bass response.  You'd have to do the research for that tube (or convert to a more familiar tube).

[AlNewman]

Everything being said, I wonder if you've tried the amp with the phono control dimed.  Or if it's been turned down.