Presence control in 5E8A - Tweed Twin

[Diablo]

I just ordered a low powered tweed twin kit and have been looking over the schematic before the kit arrives. I notice the presence control circuit is different than the Tweed Bassman - 5F6A. I built a 5F6A and modified the presence control circuit to duplicate the Fender Tweed Bassman Reissue, and I really like the presence control. The RI presence circuit is a 25K pot in series with a .1 mfd cap, and all in parallel with a 4.7K resistor to ground. The Bassman uses a 27K NFB resistor, and the Tweed Twin uses a 56K NFB resistor. So, is the presence control circuit on the tweed twin a "good one", or could it stand for circuit mods to improve it?

http://www.schematicheaven.com/fende...5e8a_schem.pdf

[tubeswell]

So, is the presence control circuit on the tweed twin a "good one", or could it stand for circuit mods to improve it?

Dunno - I reckon the only way to find out is try building it and see (but then again, I have used that excuse myself before ;-)

[loudthud]

The Bassman re-issue circuit is a minor variation of the original 5F6A circuit intended to eliminate the tendancy of the control to be scratchy because of the DC across the pot. The disadvantage is that the capacitor is never completely out of the circuit. The 5E8A circuit shares this same disadvantage. Because the two amps have different phase inverters and use different impedance loads, the gain structure and thus the feedback circuits differ. I think you'll find that to get the presence on the Twin to act like your Bassman, you will need to increase the .1uF to .22uF or maybe .33uF. Leave the 56K as is. If the presence acts squirrelly when the control is all the way up, install a 100 ohm resistor in series with the cap.

[Diablo]

Thanks Loudthud. That's some easy mods to try. I was getting a big headache calculating resistance ratio while comparing the two NFB circuits. The twin has a 56K/1.5K ratio for the NFB and uses a 4 ohm tap. The Bassman has a 27K/5K ratio and uses the 2 ohm tap. So it looks like the twin uses less NFB than the Bassman. But, you're right about the different phase inverters.

[marcfrom]

Hi Guys,

I just finish a 5E8A - Tweed Twin based on original schematic.

The presence control working, but very, very subtle. Is it a normal thing on this amp?
I already build a Bassman and the presence control is much more effective.

Thanks

Marc

[tubeswell]

Hi marcfrom

Apart from double checking your pot and resistor values in the NFB loop and whether or not the OT you are using has the 'correct' impedance ratio for the 5E8A NFP loop;

The Global NFB insertion is not the same in both types of amp. The 5F6A uses a LTP inverter, which is a differential amplifier whereby the presence control is not only affecting the roll-off point of the NFB through the LTP tail resistor to its cathode, but also affects the roll-off point through the 'passive' grid of the LTP. With a LTP, the output signals on both plates are a 'sum' of the input signals on either grid. That's my take on it anyhow

[marcfrom]

The OT come from Mojo, it's a 2000R /4R. I think it's the good one.

I will check resistors value.

I read in another thread to invert primary or secondary wires of OT. What do you think about it?

[Jazz P Bass]

.
I read in another thread to invert primary or secondary wires of OT. What do you think about it?

If the primaries are inverted the output signal will change (with a negative feedback loop in circuit)
Insert a signal into the amplifier & monitor the output voltage.
Make a note of the voltage.
Turn off the amp, drain the power supply & invert the output transformer primary leads.
Turn on the amplifier.
If the signal voltage now increases, then the primaries are now incorrect & you have positive feedback.
If the signal amplitude decreases then now the polarity is correct.
Negative Feedback will always lower the signal.

[tubeswell]

Hi marcfrom

A 2000R/4R OT has a 500:1 impedance ratio (which is 22.36:1 VAC ratio), whereas methinks a more-standard Tweed Twin OT is 4200R/4R (which is 1050:1 impedance ratio - or 32.4:1 VAC ratio).

If indeed this is the case (and I stand to be corrected about the 5E8A OT impedance ratio), then the source signal for the NFB would be lower with a 'stock' 5E8A OT, and you would need to adjust the NFB voltage divider in your amp in order to get the same amount of NFB as a 'stock' 5E8A. I say this because the presence control in a 5E8A is an odd beast. At the CW end of the presence pot rotation, the presence pot is not just a straightforward low-pass filter that dumps the high frequencies that are being fed back in the NFB loop (and thus has the effect of 'enhancing' the brightness of the signal overall), but it is also effectively making the .1uF cap (that is in the presence control) into a bypass cap for the 1k5 cathode resistor of the driver stage in front of the cathodyne splitter that the NFB is being inserted into (which has the effect of providing a treble-boost to the driver stage)

Now it occurs to me, that if you have a 'higher than usual amount' of NFB being fed back to the cathodyne's driver stage, then the low-pass filter in the presence control won't have as-great-an-impact on the HF roll-off in the NFB circuit, unless you increase the size of the size of the NFB resistor. So if it were me, I might try replacing the 56k NFB resistor with 100k to begin with and see if that helps. No guarantees expressed or implied.

[sgelectric]

Marcfrom,
It appears you may have gotten the OT for the 5F8 Tweed Twin.
The 5F8 is the "high power" Tweed Twin that uses 4- 5881s in the output section and 1- 5AR4 (or originally an 83) rectifier.
The 5E8a "low power" uses 2-6L6Gs, hence the higher OT impedance spec. It uses 2-5U4GA rectifiers.

http://www.webphix.com/schematic%20h...5e8a_schem.pdf
http://www.webphix.com/schematic%20h..._5f8_schem.pdf

SG

[marcfrom]

I have inverted the primary, the voltage decrease about 3V on the plate, so i suppose the polarity are correct.
The presence control is not so hard than my Bassman but it's working. So i suppose everything is ok.

I tried the amp with 4x12AX7 instead 12AY7 and i really like the sound like this, but a friend told me because the cathode follower the 12AX7 in V3 could no take it and burn...

What do you think about it?

Thanks

Marc

[Jazz P Bass]

I have inverted the primary, the voltage decrease about 3V on the plate, so i suppose the polarity are correct.

I would not expect a plate voltage change when inverting the OT primary leads.
Ideally, what you would want to measure is the output voltage at the speaker.

[marcfrom]

May be it's because i mesure with cold amp and before with hot amp.

Anyway, i think everything are ok with the presence.

Now i'm on the the idea of the 12AX7 x4, have you a suggestion ?

Thanks

Marc

[loudthud]

Certain brand tubes fail when used as cathode followers. Search for "spiral filament". I think it's JJ and EH brand that have the problem.