Critique My Schematic

[lowell]

Hello all,
This is the first design/schematic I've done and would like some advice. I have yet to design power supply, but you can catch the drift. My main concerns are R1 and if leaving both tone stages connected to grid of second tube is a bad idea. I'm also curious of the OD stage should be where it is in the chain. anyway check it out if interested.

[daz]

I'm not someone who can really critique it, but before someone else does you might wanna say what V1's plate resistor values are.

[tubeswell]

Hello all,
This is the first design/schematic I've done and would like some advice. I have yet to design power supply, but you can catch the drift. My main concerns are R1 and if leaving both tone stages connected to grid of second tube is a bad idea. I'm also curious of the OD stage should be where it is in the chain. anyway check it out if interested.

Hi Lowell

JM2CW on the pre-amp

1) What is the difference between channels 1 and 2? (if it is only the extra cap on the treble pot - then you could make that switchable and do away with the extra channel/ and save that extra triode for some place else (like a CF into the tone stack)

2) I am also curious about the taper on the pot going to the grid of V2. (Is there any reason why you have the 33k in parallel with the pot input and pot ground? On the face of it, that will make the whole value of that component 32.9k, which will shunt a lot of signal to ground at that point - on top of which the local feedback - R26, will reduce the gain of that stage even further)

3) The parallel triodes for the OD bypass stages might be biased too negatively for a decent OD sound (equivalent to putting 3k on one 12AX7 triode - For 2 triodes try 820R instead).

4) Also I am sure you are aware that the 100k plate resistors will give you a steep load line (equal to 50k on a single 12AX7 triode stage). This will give a more asymmetrical transfer characteristic, but at the expense of gain (if that's what you want to achieve).

5) I think two sets of parallel stages there might be a bit over the top (given you have a couple of stages in front of them anyway). You could achieve really good OD with just adding two triode stages to the clean amp (if you set it up something like an SLO100 preamp before the FX loop). But each to their own.

6) Also, having the insertion point between the driver stage and the PI might be the wrong place for the OD stage(s) - why not put the OD in front of the tone stack(s)?

JM2C

[Daver]

It looks like you're going for something Dumble-esque and having a separate EQ for the overdrive. As drawn, V1B won't work because it doesn't have a cathode resistor. I assume you meant to have the cathode resistor and bypass cap shared between V1B and A. Separate Rk and Ck for each stage would allow you to tune each a bit for clean and OD modes. You'll need to add a blocking cap before the phase inverter (V2B) to allow it to bias correctly. I agree with tubeswell's point #2 regarding the very low gain for V2A. I think you may want to switch the "channels" after the EQ's rather than at the grids. You'll probably get a popping noise you can't get rid of. That's all I can think of now.
Dave

[Don Moose]

I don't see separate volume pots for the two preamp channels - you might want to add those and a second layer of selector switch/relay after the tone stacks. Without that switch, the two tone stacks are going to interact a bit.

The doubled triodes in the OD section are kind of a waste, in my stupid opinion. That's a whole tube you could use to .. buffer an FX loop or convert the PI to an LTP.

Hope this helps!

[lowell]

Thanks so much guys for taking a look and spending the time to offer advice on this. Don do you not think that the 2 parallel tubes in the OD will make it very quiet? Or is this unnecessarily quiet in your opinion.

Daz, Don, and Tubeswell thanks you caught some dummy mistakes on my part, I'll fix those... oh and I think you're saying that 2 tubes in parallel in the OD stage sharing cathode/plate resistor/capacitors divides what the tubes see on their cathodes/plates into 2? So I should put 200k plate load resistors and 3k cathode resistors?

Daver you're right on w/ the Dumble thing in the OD. There is however a coupling cap before the PI - C19 on clean channel and C22 on OD. So leaving the input connected to both grids and switching the output of the tone stacks would be better?

[tubeswell]

Daz, Don, and Tubeswell thanks you caught some dummy mistakes on my part, I'll fix those... oh and I think you're saying that 2 tubes in parallel in the OD stage sharing cathode/plate resistor/capacitors divides what the tubes see on their cathodes/plates into 2? So I should put 200k plate load resistors and 3k cathode resistors?

Almost. If you want the each parallel set of triodes to be set up to be equivalent to a 12AX7 single typical fender triode gain stage, you should try a 200k common plate resistor and an 820R common cathode resistor. (The plates are in parallel, so for the load to be the same, the value of a common plate resistor has to be double in order to get the equivalent current flowing across each triode. However in order for the cathodes to each carry the same amount of current as a cathode for single stage, the cathode resistor has to be halved, because you have twice the current (now flowing through one resistor) as you would have had for one stage, if you are aiming for each side to be biased the same (about -1.5V grid to cathode) as you would for a normal grid. The B+ needs to be about 280V.

However if you want a higher-gain parallel stage then make the common plate resistor (say) ~390k and the common cathode resistor 270R (but be prepared for really grungy distortion). Keep each grid load resistor at 1M. The cathode bypass cap probably needs to be quite small value (.1uF or .01uF or even smaller) and the coupling cap after the parallel stage should probably be smallish too (390pF-820pF), otherwise I'd say you'll get too much flab in the bass. In fact, you might find that you end up having to tweak the coupling caps and bypass caps throughout the whole circuit to get a decent tone with the OD engaged - not to mention inter-stage attentuation/padding. And then it might not sound how you like it on any clean setting (Its really difficult to get a good clean AND a good OD tone out of the same amp without having lots of cap switching etc).

If it was me, I'd just be either making a nice clean sounding amp, OR a nice OD sounding amp, but not trying to mix the two together. JM2CW

[lowell]

Ok yeah makes sense w/ the cathode R and Plate R... thanks!

[guitician]

I would use a small coupling cap from R2 too. That would keep "flab" down.

[lowell]

Why only on the side that R2 is on? Why not C3 and/or C4?

[6267]

The V1 cathodes should be tied together as drawn, I believe...one side will only have a resistor and one side will only have a cap as drawn.

[guitician]

Why only on the side that R2 is on? Why not C3 and/or C4?

Lowering C3/C4 would give less bass on the clean side as well as the OD side.

[lowell]

guitician not sure i follow. R2 is the grid stopper for one side of the power tubes grids. adding a cap there would change the tone for one side of the OT, affecting both the OD and clean channels.

[guitician]

I meant R2 as in that relay DPDT switch.

[lowell]

there is C19 going into the switch and for the OD there is C22. Do you think those shall be changed?