Challenger CH-18 Questions

[duce67us]

I'm working on a CH-18 conversion to guitar.
I Believe there is another schematic floating around, but the tone circuit is completely different than stock, including values and circuit placement.

I'm using the Fender/Marshall Hi-Lo input circuit, and moved the phono volume pot to just before the PI. (using it to replace the 470k to ground in the stock layout)
I have swapped out all of the filter caps, and rebuilt the power supply circuit completely.  All new resisters and coupling caps.  Everything is sounding very well.

Now I would like to work on the tone circuit, but don't understand enough about the Neg. Feedback tone circuit.

Any suggestions of similar 4 position switched tone schematics that I could look over?

What is the purpose of the .0005uf cap that goes between the plate of one of the output tubes and the 100k in position 2 of the tone switch?


Thanks in advance

[jjasilli]

Feedback signal can be sourced from a power tube plate.  If so, the cap does 2 things: 1. blocks HT DC; 2. tunes the frequency band of the feedback sigal. 


Your amp sources feedback sigal from both the power tube plate and from on OT secondary tap.  Both lines of feedback sigal are run to a 4-postion SW. In Position2 the two NFB signals are combined. In all other SW positions the OT's feedaback signal alone is run through various filters, selected by the SW, to alter tone.


The OT is definitely providing negative feedback. I can't tell the polarity of the power tube feedback.  If negative, it's adding to the OT's negative feedback signal.  If positive, it's subtracting from the OT's negative feedback signal.  This could probably be figured-out a priori, or empircally with a DMM or scope.

[duce67us]

Thanks for the insight,
I do have both a DMM and a scope.  I can inject a signal, and probe around.  So, If I see an out of phase signal between those points, then we have a case of positive and negative feedback interacting to subtract some of the feedback signal.
Position 2 is named "mellow", by the way.   

The OT is definitely providing negative feedback. I can't tell the polarity of the power tube feedback.  If negative, it's adding to the OT's negative feedback signal.  If positive, it's subtracting from the OT's negative feedback signal.  This could probably be figured-out a priori, or empircally with a DMM or scope.

[2deaf]

Both sources of feedback have the same polarity.

[jjasilli]

Both sources of feedback have the same polarity.

Cool.  How did you determine that?

[2deaf]

The NFB connects to the cathode of the first half of the 6SL7, so it must be the same polarity as the grid of that stage in order for it to be negative feedback.  That first half of the 6SL7 inverts the signal at the plate and then the 6V6 inverts it again, so it is back to the same polarity as the grid on that 6SL7 stage and as the NFB from the transformer.

[2deaf]

Position 1:  The NFB circuit is a bandpass filter, but the signal is taken after the first capacitor which makes it a high-pass filter at that point.  Since this is negative feedback, the frequency response at the speaker will be the opposite of the response at the point used in the NFB circuit.  So a high-pass NFB will cause a high cut at the speaker and they named that one "Deep Bass".

Position 2:  The NFB circuit has all frequencies from the transformer plus some highs from the 6V6 plate.  This will be flat with some highs cut at the speaker and apparently sounds "Mellow".

Position 3:  The NFB circuit has a flat response just like the typical ones you see all the time.

Position 4:  This is the bandpass filter again, but this time the signal is taken from the point that it really is a bandpass filter.  It is not quite like the standard bandpass that is just a low-pass and high pass-connected together, but I am guessing a center frequency around 800Hz.  This means that the mid's around 800Hz are going to be scooped out at the speakers leaving the bass and the treble.  Brilliant.

[duce67us]

Thanks for the explanation on the tone controls. 
No doubt this circuit works fairly good as a PA amp, but for guitar, it really doesn't do the trick. Don't get me wrong, the amp has a good clean sound, and approaches overdrive but doesn't get ANY distortion.  It just gets freaking loud. 

So, the NFB Tone control must go.   When I started on this project, I thought it might end up like a 5c3 circuit, with two volumes and a tone control.

I guess I could switch between a range of fixed resisters 470k, 220k, 100k, 47k ?
Any suggestions on schematics I can study/research that might use this approach to tone control?

[PRR]

> approaches overdrive but doesn't get ANY distortion.  It just gets freaking loud.

Play louder. It HAS to distort sometime.

That grid-leak input stage may be throttling you. Have you changed to Fender-type cathode bias yet? (Figure a 6F5/6SF5 is half a 12AX7, old-skool.)

On tone switch, identify wires to wiper and pin 3. Cut the switch out. Connect W and 3 together forever. Flat response, just a whisp of NFB, a good guitar amp power stage.

Any of the older Fender tone networks can be inserted after V2. If you can make another 1 or 2 holes, use Fender TMB or TB. Otherwise steal a one-knob (probably a treble-cut) from an older/simpler Fender.

[jjasilli]

That grid-leak input stage may be throttling you. Have you changed to Fender-type cathode bias yet? (Figure a 6F5/6SF5 is half a 12AX7, old-skool.)
Great idea.  Or swap at least one of those tubes for a 12a_7, for cascading gain stages.  (But check heater current draw; mabe add a filament tranny if needed.)

[duce67us]

That grid-leak input stage may be throttling you. Have you changed to Fender-type cathode bias yet? (Figure a 6F5/6SF5 is half a 12AX7, old-skool.)

On tone switch, identify wires to wiper and pin 3. Cut the switch out. Connect W and 3 together forever. Flat response, just a whisp of NFB, a good guitar amp power stage.

Any of the older Fender tone networks can be inserted after V2. If you can make another 1 or 2 holes, use Fender TMB or TB. Otherwise steal a one-knob (probably a treble-cut) from an older/simpler Fender.

Yes, I have the standard two input jack High / low  from a Fender/ Marshall wired to the coupling cap on the First 6SF5(V1), Delete the 1M to gnd.   The phono input was truncated up to the point of the second 470k to gnd on the second 6SF5(V2).  V1 now has a 1k Rk bypassed by a 25uf, V2 has the 2k2 Rk also bypassed with a 25uf. 

I really didn't want to drill the chassis, but I'm not opposed to swapping out the tone switch for a pot. 

I do have some octal twin triodes, should it be necessary to add a gain stage or two. 

[PRR]

I'm saying to copy the *complete* first stage from later Fender, with 1.5K cathode resistor and bypass cap.

The grid-leak will de-bias itself on strong inputs. OK for mike or early-1950s pickups. Mid-50s most g-amps switched to cathode bias to handle the hotter pickups.

There's plenty of gain there. One 12AX7 simply replaces two 6SF5; since you have the 6SF5 in place, no reason to change that. The 6SL7 stage has good gain; the inverter topology preserves all of the one side's gain, unlike the long-tail which splits gain in half.

The stock NFB around the output stage kept gain down for good PA performance and to leave some reserve for tone boost. I don't think it is at all right for guitar. The mod I propose sets the gain high, lets the power stage run more raw. If still in doubt, just snip the NFB altogether.

Have you made voltage checks to see if parts and tubes are reasonably happy? It's a sturdy beast but must be over 65 years old.

[duce67us]

I'm saying to copy the *complete* first stage from later Fender, with 1.5K cathode resistor and bypass cap.

The stock NFB around the output stage kept gain down for good PA performance and to leave some reserve for tone boost. I don't think it is at all right for guitar. The mod I propose sets the gain high, lets the power stage run more raw. If still in doubt, just snip the NFB altogether.

Have you made voltage checks to see if parts and tubes are reasonably happy? It's a sturdy beast but must be over 65 years old.

I understand that I should copy the input stage,  I was looking at the Fender 5C/D3 era circuits.  I guess I should use the 5E3?
I did make voltage checks and am pretty sure that all the parts are happy. 

Tonight I did remove all neg. feedback, including the tone switch.  I installed a single knob tone circuit like the 5D3 uses with the parts I removed from the switch.
Progress!  a bit more noise ( old caps) but better overall behavior.

[duce67us]

PRR - Thanks for the push!
I have measured voltage ( and current) and have the notes somewhere from back when I rebuilt the power supply section.
I took some measurements again last night after changing both the 6SF5 Rk to 1k5.   The new tone control works good, but does introduce a mild hf hiss.  I suspect it is because I used the original "vintage" caps.

Voltages are as follows:
Line = 120vac  Current Draw: .5a
Output measurements                                  Power Supply
Vct  = 450v                                                     Va = 450v
Vplate1 = 446v                                               Vb = 300v
Vplate2 = 448v                                               Vc = 235v
Vscreen = 370v                                              Vd = 208v
Vbias = 27.82v                                               Vfil = 6.58vac                                           
Rbias = 611 Ohms

[sluckey]

You should remove that .01µF cap from the grid of V1. That cap prevents V1 grid from having a dc return path to ground which will cause V1 to not work properly. Maybe this is just an error on the schematic?

[duce67us]

You should remove that .01µF cap from the grid of V1. That cap prevents V1 grid from having a dc return path to ground which will cause V1 to not work properly. Maybe this is just an error on the schematic?

Sluckey,

The original schematic above had a 1M to gnd AFTER the cap,  but it is not there now.  Should I Add it back and keep the cap or just delete the cap?

[sluckey]

Either way will work. I'd prefer to just remove the cap. Then the 1M resistor on the input jack becomes the grid return resistor (just like the Fender amps). I'm surprised that the amp works with no grid return resistor.

EDIT... Just looked at your original schematic. V1 was set up for grid leak bias operation. The cap is a very important part of that circuit. But the cap is no longer needed since you converted to cathode bias. But, a resistance path from grid to ground is necessary for either bias circuit.

[duce67us]

* Removed Cap from circuit and uploaded corrected schematic.

I did have one last question that deals with Static Dissipation.

Looking at the schematic at the top,
Vp is 420v , Vk is 21v, Rk is 270 Ohms

(Vp-Vk) * (Vk / Rk) = ?
399 * .0778 = 31.04w
or 15.52w per Tube

Isn't that Biased HOT?

By my measured Voltages a few posts up, I believe I have it dialed down to around 9.5w per tube. 
Am I doing that correctly?

[PRR]

> Isn't that Biased HOT?

Pretty hot for a PA amp.

However IF the primary use was truck PA (6V DC power), this may be a fine design. The number of hours you would drive the streets can't be huge. The vibrator costs really add-up in steady hi-power battery conversion. 6V6 has the lowest heater-power of anything in the 18-Watt class, and also probably the lowest wholesale price of anything near (the PA guy probably also ran the radio repair and tube store).

> Am I doing that correctly?

You computed cathode current but plates only carry plate current. Deduct G2 current. For speech/music the idle will be in the ratio of 70mA Plate, 4mA G2, 74mA nominal total, plate takes 95% of cathode current. So it's 14.68 Watts/plate. Above the 12W "design center" spec but in sight of the 14W "design max" spec.

Also assuming the "6V" will tend to run low more than high (you don't try to PA when cruising 50MPH, probably just-rolling at 500RPM and the generator basically cut-out), the actual stress will be less.

And if 14.68W reduces 6V6 life from 2,000 hours to 200 hours (it won't be that much!), how many hours a year can you get paid to go around the streets blaring "Vote For Smith!" or "Duck And Cover!" ?

[duce67us]

So I added back a feedback loop, and decided to try and make it adjustable, and add a presence control at the same time.
I chose the same insertion points as the original schematic posted, using a 100K pot + 33K fixed for the adjustable feedback, and a 10K pot and 0.1uf for presence.  ( I know a 5k would be best )
It works OK, but when both controls are maxed, (set to 0 ohms) the amp develops a 14k oscillation. 


I'm attaching a schematic of the circuit I added, as well as a pic of the waveform on the o'scope. 

No Input required, dialing back either the Feedback, or presence pot makes it disappear. 

What is the best way to prevent that?

Thanks !!

[Paul1453]

You could try swapping the OT leads on the output tube sockets.

Apparently, the NFB can actually become a Positive feedback loop if the OT leads are reversed.

The 6V6 Plexi I just finished had a horrible squeal.
I swapped the OT leads and the squeal disappeared.

Of course the leads reached the sockets better the other way around.
But I was very happy to eliminate my squeal by just swapping those leads.

[PRR]

> when both controls are maxed, (set to 0 ohms)

Why would you ever do that?

Does it improve the sound as you get close to that point?

[duce67us]

Paul1453 - It wasn't squealing all the time, just as the presence control was maxed, and NFB control rolled to max too.

PRR - Does it improve the sound as you get close to that point?   Just "discovered" the squeal when I added the circuit.

Happy to report that I added another resister, and stabilized the circuit.