TUT3 Custom Special build

[Bassman]

Hi Kevin et al.

I'm laying out the eyelet board for my TUT3 Custom Special build (again).  Great fun.

Checking the suggested layout against the improved schematic in TUT3 I see a few discrepencies.
Some of them I can work out, some of them I can't.

Is this a good place to ask about them?

Regards.

[K O'Connor]

Hi Guys

You are probably referring to bootstrapping the EQ: do not do this.

Just a thought exercise but not without issue in practise. Go with the conventional grounded EQ.

Otherwise, as with most of the projects in TUT3 and TUT5 that are complete amplifiers, it is wise to split the card by function,where the PA+PSU are one card and the preamp is another. This makes it easier later if you wish to change something or add in an effects loop, say.

[Bassman]

Hi Kevin et al.

Thaks for that.

Very interesting about the bootstrapping and the discreet eyelet boards.  I've been meaning to experiment with bootstrapping on one of my preamp breadboards at some point.

But I was actually referring to a couple of small discrepencies between the schematics (page 14-6) and the board layouts (14-8 to 14-9) in TUT3 for the Custom Special amplifier.

These are the ones I'd like advice on:

1. On the bias power supply, the cathode of the zener diode is connected to a different point in the schematic compared to the layout.  I'm not sure which is most correct.

2. The cathode resistor on the first stage of V2 is 820 Ohms on the schematic but 1k5 Ohms in the layout.  
It's also 820 Ohms in the original Traynor layout, but maybe 1k5 was amended in the layout to reduce the gain?

3. In the layout, the mid and bass pots are wired differently than the schematic.  I'm pretty sure the schematic version is more correct, but the mid cap value in both the schematic and the layout (47n) for the Custom Special differs from the earlier Bass Master preamp value (22n).
I can play with these, but would be grateful for a suggested starting value for the mid cap.

Regards.

[K O'Connor]

Hi Guys

Regarding the TUT3 Custom Special build questions of post-1:

1. The zener in the bias supply has it cathode at ground. The schematic only shows this generically, where the layout shows the specific ground node. No difference.

2. V2 Rk value: Once Rk is below a certain value there is not much change to the gain, but there will be a change to the current through the tube and its harmonic distortion. A wide range of values are useful. The presence of Ck assures that gain is maximised.

1k5 is a much more common value, so it was written on the layout that way out of habit

3. EQ wiring for Bass and Mid controls is a bit different between the schematic and layout, again because the layout wiring is much more common. They are functionally the same, especially considering the EQ is cathode-driven, which tends to make it less effective.

The overall circuit is just one of the many forms of the Fender Bassman head, copied by Marshall, Traynor et al. Whether fitted with "guitar" values or "bass" values for coupling caps or EQ caps, there is a massive amount of bass response, so changing the cap values are more of a nuance to tone and feel. Nuances are important. How they change depends on many other factors, including the instrument used, how the player plays, how hard the amp is driven, the choice of speakers, the music style.

Have fun