Thanks so much for the information! I think I understand now

Also, does anyone know what the "Presence" control does on these amps? Unfortunately the schematic doesn't label what each pot in the drawing corresponds to  on the control panel, but I've narrowed down the "Presence" knob to VR6 down in the power amp section of the schematic. I can't seem to figure out what that pot is trying to do to the signal though. does it simply change the gain of the power amp section?

For reference VR1 is the bright gain control, VR2 is the normal gain control, VR 3-5 obviously comprise the tone stack and VR7 is the master volume control.

One more question... What about the "Bright" channel makes it bright? Is it just a different filter in the feedback loop?

Ok... so I should be able to find out.. Do you now if there were any 4 ohm greenbacks made back in the early 70s? So far I've only found references to 8 ohm and 16 ohm..

How about the amp? It seems to be one of Marshall's earliest SS amps but has almost NO collector value (my friend bought it for the speakers, not so much the amp). He's considering having me replace all the electronics with a hybrid setup (Tube preamp and LM3886 power amp). Would this be tragic for an old SS amp in fine working order?
-Matt

I'm going to be doing some work on an amp for a friend but I wanted to get some info up-front. The amp is an old Marshall SuperLead combo (Solid state) http://www.schematicheaven.com/marshallamps/jmp_masterlead__30w_2199.pdf

The speakers are 12" celestion greenbacks with no markings aside from the date code (at least that's what I'm told, I haven't had a chance to look at them personally).  The schematic seems to show two 4Ohm speakers in parallel however 2Ohms seems to be pretty low for an old SS amp like this. If the impedance isn't marked on the speaker is there an easy way to determine what it is? I know I can measure the DC resistance but is there a range the resistance will fall in to based on the impedance? For example, if I get 10Ohms for the DC resistance is that pretty much guaranteed to be an 8Ohm speaker or is it possible for it to be a 4Ohm?

The amp is dated from the late 60s while the speakers are from the early 70's according to the date code.
-Matt

looks good. I'm digging the retro enclosure

I'm not planning to just use a clipping circuit to control the voltage, as you say, this will clip constantly with such a high-gain preamp. My plan is to use a simple voltage divider after the preamp (similar to the McTube circuit) as the primary method to bring the output voltage "in line" with what the power amp wants to see. Then, I will add a second line of protection against spikes with a series of clipping diodes. This should, in theory, allow the circuit to operate normally under 90% of circumstances, but if the user gets overzealous with the volume, feeds a higher voltage input signal to the preamp (active pickups, pedals, whatever) then the diodes will step in to clip the higher voltages rather than fry my Reverb DSP and/or power amp chip.

Thanks for all the info. I plan to verify the gain with an oscilloscope before I hook it up to my power amp. I l'll probably incorporate some clipping diodes on the output to cap the maximum voltage swing and avoid damaging my power amp. If I do it right I might even get a nice soft clipping sound out of it when cranked. Probably use germanium diodes, I hear they have nice soft clipping characteristics.

do you think that's needed? On the mctube design there's a 10:1 voltage divider just before the master volume pot. These tubes produce so much gain you have to bleed most of it off to prevent clipping the power amp.

I ordered a set of Real McTube PCBs from Tonepad and after studying the schematic for the Alembic F2B I noticed that could build either circuit with just a few component changes. The alembic seems to get rave reviews as a bass preamp (and some good reviews for guitar as well). Since the McTube doesn't have a tone control I'll just build a separate board for the tone stack components and tap in to the signal using the pads for the gain control pot.

So, would this preamp be worth while for "regular" electric guitar? I'm not interested in any kinds of effects in the preamp like overdrive or distortion since there are plenty of pedals for that. I just want a nice, warm, clean tone to work with. I'll be running the output through a digital effects board (reverb, rotary speaker, chrous, blah blah...) and adding a master volume control before running the whole shebang into an LM3886 power amp.

So, any thoughts? Is the schematic found here: http://www.freeinfosociety.com/electronics/schemview.php?id=1191 suficient to recreate the much acclaimed tone of the F2B or is there some other black magic "mojo" involved that I'll be missing?

Regards,
-Matt

Sorry it took so long to reply. Here are the latest pictures of the hurricane.
Also, here's a sound clip a friend recorded for me. Fair warning, it's about 2.5MB.
http://www.mr2-power.com/Hurricane_test_2.mp3

ok, I spent some time studying the schematic you sent and would like to get your feedback on an idea.
I've attached a schematic to show what I'm thinking... Please excuse any gross errors as I'm still learning 

Here's what I did:

• Copied the reverb section from the Fender Frontman 25 schematic from U3-B to U4-A.
• Converted U3-B to a buffer rather than an inverting gain stage. It seems fender is using this (along with U4-A ??) to recover losses from the tone stack. In my case the preamp already has a gain stage to recover those losses so no gain should be required (??)
• Converted U4-A to a buffer as well. For the same reasons stated above. It seems this was used as a buffer to combine the dry and wet channels as well as to provide some additional gain to the combined signal. I don't think I need that additional gain due to the gain provided by the preamp.
• Added a master volume control after the final buffer.

So, that's about it... does the circuit seem reasonable or am I missing the boat with regard to the purpose of U4-A and U3-B in the fender schematic? One concern I have is that I may need to adjust the gain of the reverb tank driver to avoid saturating the input coil but I'm not really sure if that's needed. The gain currently seems to only be 2x... Can I simply prevent saturating the tank input by choosing the impedance of the input coil carefully?

Edit: One more question.. is there any particular reason fender would choose to use two different types of op amp in the driver circuit? Can I get away with a pair of NE5532 op amps or a single TL074?

-Matt

Thank you for the feedback! Excellent info as always.
I was already planning to add a master volume control in addition to the volume already in the circuit. I'm not sure what the output impedance of that blackface clone preamp circuit is though. The description mentions that it is fairly high, but I'm not sure how to calculate it for an SRPP circuit. Presumably a buffer will be required at the take-off point for the parallel reverb circuit. Here are some follow-up questions I have:

-Should I buffer the signal before the junction where the parallel reverb circuit splits off, or should I just buffer the reverb branch of the circuit? The schematic you linked to seems to buffer before the junction (U3-B).

-Assuming the buffer is placed before the point where the two parallel circuits (dry and reverb) diverge can I simply tie the output of the blackface preamp to the input section of U3-B in the fender schematic? (including C22 and R36 in the feedback loop of U3-B) and then tie the output of U4-A to the master volume pot? Will U4A have a low enough output impedance to maximize signal transfer to the power amp? Or will an additional buffer be required?

-I can't seem to find any reference on the fender schematic regarding what type of reverb tank to use. A quick search online hasn't netted me anything but I'll keep looking. If anyone knows the model (presumably an accutronic?) I would love to know.


Sorry for all the questions. I'm still trying to get off the ground with all this stuff
-Matt

I am in the process of building a combo amp based on a LM3886 chipamp. The power amp section and power supply are built so now I'm focusing on the preamp section. Based on various posts I've read on here I've settled on the JFET based Fender Blackface clone found here:http://www.redcircuits.com/Page120.htm Now I'm investigating the best way to add reverb to the design. I would like something integrated in to the amp rather than an external pedal/box of some kind. The Stage Center reverb circuit seems like a pretty simple design that gets good reviews so I'm leaning that way. I have read the section on spring reverb in teemu's book but,as usual, I still have a couple of questions.

-Where should the circuit be placed in the signal chain? Obviously the stage center was designed as an effect pedal so it was intended to be placed before the preamp stage. However after studying various amp schematics (which include reverb) it seems the reverb circuit is placed between the preamp and the power amp.

-Assuming the reverb should be placed between the preamp and power amp stages, what modifications (if any) should be made to the standard Stage Center schematic? Is it sufficient to simply build the schematic as-is and wire it into the signal chain? Should the input buffer circuitry be removed?

-What is a good way to control reverb level? It seems the stage center unit has effectively two volume pots feeding into a summing amplifier. One pot attenuates the "dry" signal reaching the summing amp while the other attenuates the output of the recovery stage. Is the first pot necessary?

-Matt

Quote from: n9voc on October 15, 2008, 07:19:44 AM
In reply to Armstom:
Built the Stage Center as a stand alone unit (with the spring reverb tank inside it was a "pedal" that is about 35 cm long by 15 cm high and wide).  I use it as both a booster and a reverb unit.  I put it right before the amplifier if using it as a reverb unit for guitar.  I found that it works great with a microphone as well - boosts a dynamic mic output hight enough to effectively drive a spare amplifier, and adds reverb to the vocals!
Good luck to both of you - Josh_777 and Armstrom.  (Armstrom, you might pose the question in the effects forum, this is a "bit" off topic for this thread - just a suggestion!)
Thanks Josh_777 for your kind patience!

Thanks for the information. With regard to this being off topic, I'm not sure I follow. I am not asking about building an effects pedal but rather adapting one of the circuits for use as a reverb channel built in to a combo amp. Perhaps I should start a new topic regarding integrating reverb into an amp.
-Matt

not a problem, my name is Matt Armstrong, hence the ArmstroM (Unix style user name, can't seem to break myself of it).

I can answer your question about wiring in parallel vs. series. Yes, speakers behave the same as resistors. You can really think of them as "dynamic" resistors. The resistance varies as the speaker motor moves at different frequencies so the values you see quoted are really just nominal values. Impedance factors in static resistance of the coil as well as its reactance.

So, wiring two 8Ohm speakers in parallel will "look like" a 4 ohm speaker to your amp while wiring in series will "look like" 16 ohms.

As for the amp I'm building, it's pretty much done. All that's left to do is apply the decal to the control panel. You can hear some clips in the last two posts of this thread: http://www.ssguitar.com/index.php?topic=899.0

Hopefully we'll get some feedback on the process of choosing a speaker impedance though
-Matt