Bogen CHB100 / Ampeg B15... maybe.

a fine line between stupid and clever

Postby soundmasterg » Wed Jul 27, 2011 2:54 pm

ioginy wrote:Excellent advice, thank you. I am still wrapping my head around how the cathode and plate resistors effect them, and what the requirements of each tube is.

None the less, this is the updated schematic of the alterations I made.

Image

Sadly I must have done something wrong because I am suddenly getting absolutely no sound. I'm going to check all my values and maker sure there are no stray shorts.

Sadly my signal generator is my guitar and my scope is my ears at the moment. Some day soon I'd like to get some proper equipment, but for now I get to take shots at the dark.


THe plate resistor sets the gain, along with the voltage on the tube. The voltage will also affect the frequency response of the tube, as will the value of the capacitor used in the power supply for that node. The larger that cap is, the more bass response the stage is capable of producing. The higher the voltage, the more gain the tube will have, and also the more high frequency response the stage will have.

The cathode resistor sets the bias point of the stage, and a larger value will allow the stage to take in a larger signal before it distorts. However, since the preamp stages are Class A, the ideal bias point is in the center of the curve. If you go one way, then you get to saturation or cutoff quicker, and as you to the other direction, then you get to where you get grid current. Each of those effects can have an interesting effect on the sound too, so voicing one stage to go one way and the next to go the other can add harmonic richness to the sound. For a bass amp where you are trying to get clean, center biasing is probably the best choice. Without doing calculations with the plate curves and voltages and/or having access to a scope and signa, generator, that becomes a little bit hard and you pretty much just end up guessing. You can get there...it just takes a lot longer.

The cathode cap makes the gain on that stage go up, but a cap can be sized such that it will affect all the frequencies or only some of the frequencies at a given stage. For a typical Fender input stage, a 25uf will amplify down to slightly below all guitar frequencies, and going with a 220uf will go down for all bass frequencies. For bass sometimes it is helpful to have a tighter response so the bass doesn't sound loose. Many guitar amps these days will use a 2.2uf or 4.7uf cap on that first stage to get a more controlled bottom end. Bass needs a bit more bottom but not a ton.

The coupling cap between stages will also control the frequency response of a given gain stage. Lower values allow more bass through to the next stage.

The grid leak resistor sets the input impedance along with any series resistors. The lower the input impedance, the more the signal gets loaded down. In other words, the more gain the preceding stage needs to have to have the same effect. So if you used a 500k volume pot after the first stage instead of a 1M, you gain would be cut in half out of that stage.

Lower gain tubes like a 12AU7 have low voltage gain, but high current gain. So they are lower impedance tubes than a 12AX7. They work better with lower value plate resistors and higher value cathode resistors. This allows the higher current capability of the tube to work. They will also have a higher bandwidth response, and they generally sound smoother and more harmonically rich. They will also have more noise if you want the same amount of gain in a preamp since you need more stages to get the same level of gain using all low gain tubes as compared to a couple 12AX7's.

Looking at your schematic as currently constructed, you should have a pretty decent starting point for a good overall sound out of the amp right now. Depends on what your voltages are at the plates.

You could try a pop test with the amp hooked up to a speaker to see where your signal is stopping. Touch your meter lead to the grids of the various stages in order and you should hear a pop out of the speaker if that stage is working. If it doesn't pop, then that is where your signal is stopping, or the stage immediately before it. As you go through the amp, the pops will be less loud.

I highly suggest to get a signal generator and scope at some point. They make life so much easier when trying to troubleshoot! I could take a trade for that scope i have plus those 7868 tubes and sockets you have and some cash, but since you're in Canada, shipping would be expensive.

Anyway, good luck and keep us posted!

Greg
soundmasterg
 
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Postby soundmasterg » Mon Apr 09, 2012 12:49 am

Hey there,

Was just wondering whatever became of your project?

Greg
soundmasterg
 
Posts: 76
Joined: Thu Feb 08, 2007 5:35 pm
Location: Oregon

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