5F4 Tweed Super Question

[stephenl]

I'm thinking about building this one...
What is the purpose of the highlighted circuit segment - feedback/feedforward?

Tweed Super Preamp.jpg

[pompeiisneaks]

This is local negative feedback yes. Anode signals are out of phase with the input signal, and then cathode signals are in phase with input, so the signal goes through an anode, then cathode then through a coupling cap, and then is fed back through that resistor.

~Phil

[Stevem]

Once you get the build up and running I would play around with lowering that 4.7m resistor to get the amp to respond smoother to how loud you play it on average, and this could be a feature you add with a rotory type switch to 3 to 6 different setting of that value,

This will be in terms of how slides into clipping or jumps into clipping ,

[ViperDoc]

Once you get the build up and running I would play around with lowering that 4.7m resistor to get the amp to respond smoother to how loud you play it on average, and this could be a feature you add with a rotory type switch to 3 to 6 different setting of that value,

This will be in terms of how slides into clipping or jumps into clipping ,

Steve, what range of resistor values do you find useful for this kind of local NFB adjustment? Thanks.

[ViperDoc]

I drew up a 5F4 schematic with a few updates based on the present discussion for anyone interested. This is only a draft and has not been tested.

Questions:

1) Do the "Manning Bias" polar caps need to be of higher value for 6L6G tubes?

2) What values would be useful in the Local NFB rotary switch suggested by Stevem? Perhaps fewer would be needed, but maybe not.

3) Is it possible to put the pre-PI .02/.001 caps on a switch for low-end control in a head version when opting between 10-inch and 12-inch speakers?

4) The 5U4G data sheet specifies a [CORRECTED: 3H MINIMUM] choke input (5U4GB EH = 10H)]; most Tweed Super transformer sets include a 4H choke. My research suggests the 4H was a historical factory decision for simplification, not necessarily for best operation. Is this right?

https://s3.amazonaws.com/tubedepot-com- ... 1400187939
https://frank.pocnet.net/sheets/155/5/5U4G.pdf

5) I understand they may not necessarily be appropriate, but I sketched in the 470K PI grid stopper I put on my PR amps that helps control flab; I also sketched in 470R 3W screen resistors as they tend to come and go in Fender amps.

6) The old schematic shows a .05 uF cap to ground right off the B+ line. What is that for?

7) The multi-tap OT is not stock.

Enjoy.

Old 5F4 SCH:

Fender_super_5f4_schem.pdf

3-9-2021 SCH: [misplaced input resistors, wrong nfb tap]
3-11-2021: (fixing errors)

[pdf64]

1/ I don’t see that the cap values need to be increased.
2/ Why discrete fixed switchable resistors, rather than a continuously variable resistor control? But I would experiment with dumping that NFB; why ‘correct’ the wonderful stuff that a DCCF brings to the signal path? If preamp gain needs user adjustable trimming then put a pot after the tone stack, Type 4 style, or if you want to be different, split the CF’s cathode load resistor.
3/ The cathodyne input impedance will be rather high, so I suspect there may not be a noticeable difference between those values.
4/ The rectifier has a reservoir cap, so it’s a cap input arrangement, unloaded VDC = VACpeak; whereas for a choke input, VDC = VACrms (or VACavg?). Specifically the CLC arrangement gets referred to as a ‘pi’ filter.
5/ I’m a fan of fusible resistors for the cathode current sense and screen grid current limiters, 1/2W and 1W respectively. A power valve being the most likely thing to fail in an amp. Ok, the hot switching standby may cause the 5U4 to give them a run for their money.
6/ My guess is that cap was there to take some stress off the rectifier, caused by the hot switching arrangement. But why have a standby at all, nevermind a hot switching type? Silicon protection diodes in series with the 5U4 anodes would be a good idea.
7/ Good idea but the schematic shows the full winding being 4ohms with 8 and 16 taps

[jabguit]

Is it just me or is the B+ to the U4a (U4a?) screen on the wrong side of the 470R resistor?

cheers,

[pdf64]

Well spotted, that wouldn’t work very well

[ViperDoc]

Thanks for catching that. It appears I inadvertently blended the wires when I moved one of them to see the choke better. Will fix tonight.

[ViperDoc]

CORRECTED FILE ABOVE. Thanks for your help.

[pdf64]

Just to note that the NFB feed has moved from the 4 to 16ohm output; that’s doubling the feedback ratio.

[ViperDoc]

RIGHT. I'll get it...Thanks.

BTW, I noticed on the 5U4G datasheet that the capacitor input value is listed as 40 uF maximum. Any problem changing the reservoir cap to 40 or 32 uF? Both my PR amps have a 40uF (40-20-20-20 can) in the reservoir position with a GZ34 rectifier and that's an "upgrade" according to RobRob. Seems to do no harm, and I've never used a 20 uF there, so I have no comparison.

[martin manning]

Bias circuit mods. The cap values aren't too critical. I have 2x 47u in an AB763 build and the bias stabilizes in under 2 seconds.

[ViperDoc]

Thank you, Martin, for the graphic and input. I’m very interested about how you develop these values. Penny for your thoughts? The fixed bias resistor on the old schematic is 56K. I copied the other components from the old schematic as well. That's about as good as I get at the moment! Thanks.

[martin manning]

As I mentioned in your PR thread, this circuit is adaptable to almost any application by adjusting the first resistor on the left. The 10k in the CRC filter, the 10k pot, and the 15k to ground all remain the same. These were selected to get a reasonable range of adjustment with the commonly available Fender-type 10k bushing-mounted bias pot. This is basically the circuit used by Marshall, adapted to the 10k trim pot. The power ratings are determined when you choose the mid range bias voltage you want, and here I used -40V since that is what is shown on the Fender schematic posted above. If you have -40V there with the pot centered at 5k, the current is 40/(5k + 15k) = 2mA (grid current can be ignored). From that you can get the power dissipated in the 15k, the pot, and the 10k (whose average current is the same as the others): 10k, 40mW; pot, 20mW; 15k, 60mW. As you can see, 1/2W resistors will be more than adequate, and the pot (which is at least 1/4W) will be fine. Still fine even if you want -60V bias. The current and voltage drop through the first resistor is difficult because it handles the very spiky charging pulses. There are a lot of details involved, but since it's just the one resistor you can guess from previous experience (the order of magnitude is 100k if you are connecting it to the HT, and 1k if you have a 60VAC bias tap), or run a simple simulation, and then adjust it by trial and error.