YBA-1 bridged T circuit

[jmcd]

I picked up a lovely Traynor YBA 1 that has a bridged T circuit between the second and third gain stages along with a plate-driven tone stack. I've read through the TUT3 chapter on the Bass Master, but I'd like to know more about the bridged T circuit (what it does, how it does it, and what sorts of component value changes are possible).

[K O'Connor]

Ho jmcd (not to be confused with jcvd)

In general the birdged-T is just a notch circuit with very low-q. This means the notch is not likely to be too deep. In this application, peter used it to add a mid dip when the pot resistance is zero and it does nothing when the pot is at full resistance. The pot is in the ground connection of the T so it decouples it or engages it. being a passive circuit there can only be amplitude cut and more cut.

[jmcd]

Thanks Kevin,

The interesting thing is that there is no pot connected to the bridged-T - at least in the schematic. Here is a snapshot of the schematic for people who don't have their TUT3 open...

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[jmcd]

I also note that the third gain stage has a 68k plate resistor and 9k total Rk (I think TUT3 may have specified Rk = 68k)

[K O'Connor]

Hi jmcd

Correct on both counts. This version is just a straight mid-dip with no control. The V2B stage has a bootstrapped grid-leak for slightly increased input impedance, thus reducing loading on the T. The gain of the stage is also low, mostly to make up for loss through the

Pete did have a ground pot in another model.

[jmcd]

There is a lot of potential for turning this into a guitar amp rather than a bass amp. adding bypass caps on the input stages gets you most of the way there.

[K O'Connor]

Hi jmcd

Even as it is you can play guitar through it and get a good sound depending on what your goal is. For example, it will be pedal friendly since the amp's character is rather neutral, especially if you fit it with real 6CA7s or 6L6s, which sound the same but smell different when hot

As you optimise the amp for more modern guitar tones, you make it more your own. The chassis size and eyelet board makes it very easy to work on for mods or repairs.

Grounding is random and for the original circuit the noise level will be acceptable. As you increase gain you may find grounding needs to be adjusted. One good initial mod is to redo the bias circuit so each tube has its own bias pot. This allows the output stage to be hum-balanced and you can use any tube type or mix of types you desire. Add current-sense resistors and meter jacks to make bias checking/setting safe.

[jmcd]

Thanks KOC,

I started by replacing the original filter caps and adding one extra stage of filtering for the preamp, and then I re-worked the random grounding scheme into your Galactic scheme. I'll modify the bias circuit when I have some free time (next week?).

[jmcd]

Update, with question:

I installed LTSpice on my Mac to simulate the circuit. My results indicate that the 'dip' is in the bass frequencies with the 10n caps used in the YBA-1.





It becomes a mid-frequency dip by reducing the values of C2, C3, and R2:



Am I simulating this correctly?

[K O'Connor]

Hi jmcd

Yes, the simulations look good. The passband around a bridged-T is usually asymmetric.

The first tuning looks like it is supposed to reduce the response hump at speaker resonance, and/or to get rid of some of the 'woofiness" with the typical thumpy country bass tones of the day.

The second tuning would be good for guitar use.

You can make the bridge-T tunable or switchable, as you might suspect.