Project ideas please?

[ampgeek]

Indeed. Hundreds of ways to skin that cat mate!! It all depends on how complicated you are willing to get (and how much $ you are willing to invest)...to get there.

Generally speaking, just about any pre-amp plan can be mated to just about any power amp configuration that you see out there in your research. So...the "low" power requirement shouldn't be the sticking point in your overall specification. You just need to decide how many watts you want after all is said and done.

Two completely separate and distinct pre-amps can get really tricky...really quickly...if you are not following a fairly well defined plan right from the git-go.

Switching in/out an additional gain stage (or two) is the most "common" (if there is such a word in this hobby!) approach to start with as you mention.

Take a look at the Hoffman Stout project here.

http://www.el34world.com/schematics.htm#Mods

Scroll halfway down the page and poke around all of the links relating to the Stout. It is based on the venerable Marshall 18W design which is tried-and-true. There are many, many mods to the 18W plan out there that could be considered if you want to spice it up a bit.

This could make a low cost/low risk project for you that is a couple of steps above the Two Stroke plan in complexity.

Also, you might want to go out and tour the music stores in your area and try as many amps as you can to see what size/features appeal to you. With those model numbers in hand, many here could chime in how best to get you what you want.

Cheers,
Dave O.

[livo]

OK. Thanks M and AG. I'll plug away along your suggested lines for now and see what I come up with.

Just looked at Bogner Duende and Bugera BC15. Both 15 Watt 2 Channel (Foot Switchable). That is the sort of thing I think I'm aiming at. In full tube construction although I suppose SS rectified is OK. Can't find schematics for these though which is not surprising.

[livo]

Well thanks again. I've looked at the AX84 and the Stout as suggested and while they are similar in some ways they are quite different in others. Have I got this right?

The Stout looks pretty simple. (please correct me if I'm wrong).
The Relay chops V1-B in or out of the signal path after the Tone and Volume Pots.
V2 is the PI for EL-84 Push- Pull Power amp. via 2 of 0.022uF Coupling Caps.
Need to remote foot switch the relay.

AX84 (with L / R Switch option)
Pre-amp Stages 1 and 2 (V7-A and V7-B) each have own Drive Pot to feed the next stage (ie: Cascading Gain in Lead position).
Switch diverts signal through or past Pre-amp Stage 3 (V6-B) into Cathode Follower (V6-A), followed by Tone Stack. Gain Pot 2 only operates on Lead switch Position.
Need to replace switch with relay and remote foot switch.
Connect to whichever Power Amp desired.

I probably prefer the AX84 with the second Gain Pot as this would give adjustment capability to the Lead switch position which is not available in the Stout.

I think I'm understanding this. Hard to say being self-taught. Give me a slap around the head if I'm completely wrong would you then tell me where.

[ampgeek]

Although I couldn't find an AX-84 with the "L/R" option schematic, what you cite sounds right to me at first blush. Can you share a link to that?

You can add a volume pot to the Stout added gain stage if desired.

Cheers,
Dave O.

[M Fowler]

Hope it ok to post.

[livo]

Although I couldn't find an AX-84 with the "L/R" option schematic, what you cite sounds right to me at first blush. Can you share a link to that?

You can add a volume pot to the Stout added gain stage if desired.

Cheers,
Dave O.

The L/R switch option is just a little addition on the third page of the Schematic link provided above by MF. I redrew the schematic with that circuit included so I could follow the signal path right through.

The link worked for me but just in case, go to AX84 then select Building Blocks, then Preamps, then Lead preamp. The last file in that section is the Schematic.

I like the way the site is set out into building blocks that appear to be able to mix and match. Great resource for a learner like me. I will say that I've gone back and re-read The Guitar Amp Book and also re-visited the Musical instrument chapter of the Audio Valve Amplifier book by "Rainer zur Linde".

The early chapters in this book are a bit heavy going and jump in the deep end. I can do the math and read the Characteristic data but it's understanding the variables in the circuit that are puzzling to me. I know it's not but it all seems a bit arbitrary to me.

For example: Look at the very first preamp stage of the AX84, Stout and the Princeton / 2-Stroke. The Cathode resistor varies from 2K7 to 2K2 to 1K5 Ohms respectively. The bypass cap parallel to these resistors ranges from none, through 0.47, 0.68 and then a jump to 25 uF.

My reading tells me that the resistor value controls the gain of the stage and the cap value controls the voicing. This is great but I'd like to understand the values and how they are derived.

[ampgeek]

Got it! That looks a whole lot like a typical JCM-800 topology (save a few values here and there) that I am laying out right now.

Search for some articles/information on "load lines" to better understand how the cathode resistor is chosen to change the performance of a tube gain stage. Generally speaking, it's about pushing the operating point of the tube either within or outside of it's linear response range.

The tube is biased without a signal on the control grid. With a signal at the grid, AC passes through the cathode (not all of it slips out through the coupling cap after the plate load resistor), through the cathode resistor on it's way to ground. The AC current created with a signal at the control grid increase the cathode voltage (it, too, is "backing-up" against the cathode resistor) which tries to "shut down" the tube thus reducing the gain by decreasing current flow through the tube. The cathode bypass cap shunts the AC component around the cathode resistor effectively retaining full gain. Which frequencies pass are a function of the sizes of the resistor and capacitor. So...you can also shape the voicing of the stage by changing those values.

Cheers,
Dave O.

[livo]

Thanks again. Assembly and following schematics is one thing but I like to try and understand what it is I'm doing.