Mozz's Filmosound 179 Conversion Plus Tweaks

[willard]

Enzo: Not all that much unless higher gain is the only way to get a smoother overdrive tone. The design goals are more to change the tonal character of the overdrive. Hence NFB was added to obtain another way to tweak overdrive. I was interested in Blencowe's DC cathode follower article because of the oscilloscope images showing the control over compression from the circuit through the resistor value change.

If I were to choose without knowing anything more, I would choose whatever gives the least gain (because I'm trying to build this as an apartment amp) which I guess would be the 6SL7. However because I have no real circuit design/analysis skills, I would feel "safer" using a 12ax7. Then I can "cut and paste" from the Blencowe article.

If lower gain precludes or is more distant from a "creamier" overdrive, I would go with the higher gain and try to use an attenuator to achieve apartment usability.

btw, can the compression change that Blencowe gets from the resistor change be implemented using a potentiometer?

[Enzo]

Well I was hinting that a cathode follower has a gain of a bit less than one. In other words it will not add any gain.

A potentiometer IS a resistor, it just happens to be adjustable.

[willard]

Thanks for the clarification Enzo.

So is the following roughly correct?

1) with the MV mod only, there will be no change in total gain, only a second point to control overall loudness.

2) with the DC cathode follower as well as the MV, there will be little to no increase in gain but a change in the wave shape (compression)

This would be a very good result for my intended use.

[Helmholtz]

Does insignificant signal loading indicate that the MV is ok (that it avoids a specific problem or condition) or problematic in some regard?

It means that the 270k (or a 250k) MV should work fine and will not lower gain by loading.

[Helmholtz]

2) with the DC cathode follower as well as the MV, there will be little to no increase in gain but a change in the wave shape (compression)

If you mean a so-called DCCF, that would add considerable gain.
It consists of 2 triodes: A triode gain stage DC coupled to a unity gain cathode follower.
Using just a CF alone wouldn't make much sense.

Recently I learned that some tubes/brands tend to develop heater buzz in a DCCF and it seems that it's actually due to the high cathode voltage of the cathode follower itself.
I would go with an ECC83/12AX7/7025 here because you might need to select the tube for low noise.

[willard]

Helmholtz: (re: 12AX7 and the cathode voltage noise issue) I will keep that in mind. I think I will try a 6SL7 first, because I have an extra one, to avoid hacking the chassis and will rebuild with a 12AX7 in case noise does arise. By coincidence, the person I referred to who has tried a DCCF with the 6SN7/6SL7 has made contact and sent schematics. He has made several DCCF upgraded Filmosound style amps. He did not provide specific guidance on the tube choice but had no warnings either.

I have attached the updated schematic. The additions are
1) NFB through the (modified) tone circuit
2) DCCF with 6SL7.

The DCCF is adapted from Blencowe's preferred version (higher compression with protection features) plus his "bootstrapping" variation which splits the load resistor. I have also added a pot before the cathode resistor on the output side to allow tweaking the compression level.

I am uncertain about the load resistors on the DCCF tubes plates (ie. the output half of the tube doesn't have one). Blencowe's examples don't either and the 12AX7 and 6SL7 tube data shows both tubes with 300VDC max plate voltage. Do I need any here assuming B+ rail voltage is within spec?

I am also uncertain about the DCCF pot value and usage. The pattern from Blencowe's page is that the cathode resistor is equal to or less than the load resistor in the input section. In my case, the sum of pot value and the cathode resistor can exceed the load resistor. So is the load resistor/cathode resistance relationship significant or accidental?

Again, thanks to all in advance for any comment or criticism.

[Helmholtz]

The DCCF is adapted from Blencowe's preferred version (higher compression with protection features) plus his "bootstrapping" variation which splits the load resistor. I have also added a pot before the cathode resistor on the output side to allow tweaking the compression level.

The compression pot will be scratchy when turned, because of DCV across..

I am uncertain about the load resistors on the DCCF tubes plates (ie. the output half of the tube doesn't have one).

The "output" triode is the actual cathode follower (CF) and has its load at the cathode.

I am sceptical about your tone circuit in the NFB path.
Also here DC running through the tone pot making it scratchy.
It looks at least unusual. Is this based on a proven design?
Any tone circuit in the NFB loop will have little effect at low MV settings.

[g1]

I am uncertain about the load resistors on the DCCF tubes plates (ie. the output half of the tube doesn't have one). Blencowe's examples don't either and the 12AX7 and 6SL7 tube data shows both tubes with 300VDC max plate voltage. Do I need any here assuming B+ rail voltage is within spec?

Max plate voltage is referenced to cathode, not to ground. You will have substantial cathode voltage so that is subtracted from the voltage you measure at the plate.
As Helmholtz stated, CF has load at cathode, and plate is connected directly to supply.

[willard]

Ok. Thanks for the clarifications.

Re: tone circuit. This was taken from what I assumed was the original Filmosound circuit, based on the source of the schematic I used for drawing (not Mozz's). Instead of checking the circuit, I went back with the straightforward tone circuit in Mozz's build. I was initially interested by the possibilities of tone shaping via negative feedback but came to the realization that this has been done (Presence pot). I added a switch and used the NFB circuit from a Fender Twin schematic.

I have replaced the compression pot with a 3 way switch.

For Mozz: do you know why your measured voltages are so much higher that the original B&H ones ?

edit: schematic corrected for small errors and accidental reversion (screen resistors incorrectly wired)

[mozz]

Voltages are high probably because the original transformer has 110v primaries? I'd have to look. I may also have some .1 or .2 ohm resistors in the filament line. Also, there are less tubes than the factory wiring so all voltages would go up.

[willard]

The compression pot will be scratchy when turned, because of DCV across..

Can I add a capacitor (say 0.1uf) before the pot to block the DC?

[Helmholtz]

Yes, you could do that. Maybe use a larger cap value as it's a low impedance circuit.
But then you will need a resistor (e.g. 470k) across the switch contacts to avoid popping.

[willard]

Yes, you could do that. Maybe use a larger cap value as it's a low impedance circuit.
But then you will need a resistor (e.g. 470k) across the switch contacts to avoid popping.

I should have been clearer. I would not be using the switch. Is this ok?


But the idea of using a switch is interesting. Is this what you meant?


What is the function of the switch when open? Would it bypass the DCCF?

As an aside, your comments on the NFB into the tone circuit which led to me scraping the idea and realizing that it has been done as a presence control will be extended by also including a resonance control for the bass frequencies.

An updated schematic will be coming soon.

[Helmholtz]

I should have been clearer. I would not be using the switch. Is this ok?


But the idea of using a switch is interesting. Is this what you meant?


What is the function of the switch when open? Would it bypass the DCCF?

I was referring to your schematic of post 24 where the NFB is switchable.

Your new cicuits here don't make sense to me. What's the diode?

[willard]

I was referring to your schematic of post 24 where the NFB is switchable.

Your new cicuits here don't make sense to me. What's the diode?

It's the diode in the DCCF. That's where the compression pot/switch/selector is. My question about adding the capacitor before the pot was referencing the schematic in post #21. The problem as I understood it was that the pot can't be used on the cathode because of DC. I was revisiting a past comment and thought this would allow reimplementation with a pot.

I understand what you mean by the switch now though. It's the NFB switch that needs the 470k resistor.