Safe to put V1a coupling capacitor on a switch for variety?

[Dave H]

You could use tube models without heaters. I think the Duncan modals say 'NH'

[Mr. Bill]

Thanks Dave. Supplying direct voltages for each plate seems to quicken the response time as well. Does this look correct for V1a and V1b plates plus tone stack output? That seems like a lot of signal loss to me, but I've never put an amp on a real scope before. Again, pardon my greenness.

edit: Caught my first mistake. Still tone stack question.

[Dave H]

That looks about right to me for three gain stages (Marshall 2204?) The tone stack can have 20dB loss at 1kHz depending how the controls are set. 20dB is for a 100k slope resistor. It will be less than that if it's 33k.

[Mr. Bill]

I hope it's okay to continue to derail my own thread, but since Chuck got me on this spice thing, I have another question. Does this seem like a reasonable way to simulate a 1M log gain pot set around noon?

[Dave H]

Looks OK to me. It's what I do.

[Chuck H]

That's what I do too. But since I use the cheap Alpha pots I go 150k/850k (they're 15% @ half rotation). The only 10% log pots I know of right now are the Bourns "guitar" pots. If you buy them you have to be sure and check the torque specs. They offer an appropriate torque for amps, but you need to choose it. Otherwise your volume knob will be loosey goosey. I like the 10% pots for the bass control on BF type topography amps. It feels and "looks" more intuitive when adjusted than the typical CTS 25% pots.

[Mr. Bill]

I didn't know that about Alpha pots, thanks for the info. Is there any accuracy problem in supplying direct voltage to each plate so you don't have to get the entire rail correct when messing with preamp values? That's what I've been doing so far.

[Mr. Bill]

So earlier in the thread I mentioned the possibility of making the plate resistor on V1a switchable. Is this not advisable? Again, this is just a prototype that will never leave the room, so it would only be switched while off or on standby. If it's safe to do so, what rating switch would I need? I'm just trying to save myself from having to solder and unsolder parts in while I experiment a little.

[Chuck H]

A switchable plate resistor is something I wouldn't even do on a personal amp. It is unavoidable to put very high voltage on a switch for that. Any switch failure could result in an HV short to the chassis (resulting in analogous part failures) or worse, if the switch has a metal insulated actuator you could have HV on your HAND while holding your guitar (ground). ZAPPPP!!! Nevermind that the amp "would be off or on standby" whenever the switch is flipped. Danger lurking at a misstep! Not going to build it. Find something else to covet

[Mr. Bill]

Loud and clear! I'll just tack those in after discharging while experimenting. I have liked messing with this value in the past, so I'm sure I will do it. I'm working on my layout right now and getting that nice and thorough. I'll run it by you guys before I actually begin the build and see if you catch any other mistakes.

I have ran into yet another question that I can't seem to find the answer to, so I'll just keep bugging you guys. If I add a gain stage and use the 2 jacks, one going to the first gain stage and passing back through the second jack which will give me the stock input....Will I cause some problem or get some type of bleed from the "out of circuit" first stage when using the second jack?

[Enzo]

There are already commercial amps that do exactly that.

[Mr. Bill]

This is a work in progress and is missing all values at this point. Will I be okay with the cathode value switches on the rear panel where the run is super short? Does anyone feel the need to slap me yet?

[Enzo]

Sure, I will slap you now... I for one would REALLY like a schematic rather than a layout drawing. The schematic shows the circuit and electrical relations, while the layout shows where things sit in the chassis.

Just my opinion, but I think you will find things like cathode resistor values to be a circuit tweak, rather than a feature. In other words a little time spent experimenting will find you the value you like best, and just install that. You can wire up switches, sure, but in most cases, you will find one setting you like and never change it again. With shielding and lead dress, you can probably get away with a lot of this stuff, but every switch and wire run you add is just another opportunity for the amp to act weird.

A friend once had me modify his amp to be switchable between fixed bias and cathode bias. I did. For a week or so he tried switching back and forth, then decided he liked one way best, and never switched it again.

[Chuck H]

Well in this case the cathode switch changes the amp from a cold clipper ala standard 800 to a hot rod type preamp mod. So it WOULD be an instant gratification switch and not such a subtle tweak.

And now I have to own up to something I said that, in this case, was misleading. That is the cathode circuit impedance is low enough that you actually CAN get away with longer leads there. So you actually could put that switch on the front of the amp if you like. You still want to practice good lead dress and just sort of keep the leads in their own lane. My experience with back panel switches for preamp voicing is that it's a PITA and a bad idea. One design I make has the "boost" switch on the back for the purpose of lead dress. This is also the model I own and use. One of my customers is always forgetting the switch is there. He's called me to ask why the amp lost gain (when he leaves it unboosted) or why he can't get a clean tone (when he leaves it boosted). I set myself up in a chair to play for awhile, with amp controls just in reach, and now I have to stand up whenever I want to flick the boost switch. In fact I'm redesigning the circuit to eliminate these inconveniences. My Mouser order should be here in a few days.

[Dave H]

And now I have to own up to something I said that, in this case, was misleading. That is the cathode circuit impedance is low enough that you actually CAN get away with longer leads there. So you actually could put that switch on the front of the amp if you like.

I've actually put the switch on the floor with a low gain amp (18W type) and it worked fine. There's a jack socket on the rear panel and I used a 10ft guitar lead to a metal box footswitch. Inside the footswitch there was also a LED and 9V battery connected to the other pole of the switch so you could see when it was in boost mode.

The amp also has a "Contour" switch on the rear panel because I didn't want to drill a hole in the front. You're right, what a PITA it is.