Guild Maverick Bass amp - modify for guitar?

[alange5]

I purchased a "broken" Guild Maverick Bass amp.  Turns out all it needed was a power cord (which was replaced with help from posting on this forum - thank you!).  With very little money invested in it, I'm debating whether I should modify it to make it more guitar-friendly, or flip it.


Here's the schematic:
http://www.prowessamplifiers.com/schematics/misc/Guild_Maverick_Bass-7591s_SS_rectifier_7247_driver.html


There is a guitar version of this amp (Maverick Lead), but I can't find the schematic. 


Any suggestions?  I'm guessing I should look at bypass cap values and caps in the tone section?  I'm a little confused by the 3-position tone switch - not sure how to interpret the schematic.  Also, the dual-gang volume control puzzles me.  As far as I know, the guitar version only has one volume control between V1A and V1B'




thanks in advance

[drgonzonm]

At least one boutique amp manufacturer, Dave Zimmerman, author of "The Guitar  Amplifier Player Guitar" states that bass amps require a broader frequency range than guitar amps. 
There are several guitar players who preferred Fender Bassman amps for their guitar playing.   

[John]

Play through it for a while and see what you think. You might find that you want to lower the bypass caps in the preamp to something like 4.7 uf, and then increase your coupling caps to .022. Or, you might like it just fine the way it is. Just flog it, dude! :)

[HotBluePlates]

+1 to the above.

Play the amp, see if you like it. Unless you can articulate what the amp does which doesn't work for your playing, there's no way to really go about modding it to be "better for guitar".

[alange5]

+1 to the above.

Play the amp, see if you like it. Unless you can articulate what the amp does which doesn't work for your playing, there's no way to really go about modding it to be "better for guitar".

It sounds decent for guitar with the treble knob maxed, the bass knob way down, and the 3-way tone switch in the top (most treble) position.  It also doesn't "kick in" until the volume is about halfway up.  Anything lower is not very articulate, and a little "boxy" for lack of a better word.


The other two tone switch positions introduce a TON of bass, and sounds incredibly boomy.


Here are a couple Guild schematics from other guitar amps with a similar circuit:
http://www.prowessamplifiers.com/schematics/images/Guild%20Thunderstar%20Lead%207591%20split%20chassis%206GW8.pdf
http://www.prowessamplifiers.com/schematics/images/Guild_Thunderbird_Superstar_FMIC.pdf


The bypass caps are the same, the preamp coupling caps are smaller, and the coupling caps before the power tubes are bigger.  Aside from that, I'm assuming the cap values on the tone switch and tone control are different, but I can't really wrap my head around that part of the schematic.

[John]

Easiest thing, and pretty cheap too, is to lower your bypass cap values. With those 33uf, there's no bass frequency reduction at all. To start, just clip or unsolder one leg of each one, play it and see, lift the leg of the next one, etc. If those caps are old there's no real reason to save them, so I would just remove them completely and see what I thought.

[PRR]

The tone knobs are James stack, very popular in hi-fi, much less common in instrument amplifiers. I'd keep that for now.

The Normal channel has that tone switch. It is NFB hackery. Cut it right the heck out. Let V1b be a straight simple amp stage.

The Trem/Rev channels has that bridge-Tee filter. Snip one end of the 220K so it don't do much.

This will give a flat (not top-shy) amplifier. Whether it has enough "flavor" for brilliant guitar, you need to play it more with the bass-stuff cut out to see.

[Steve_P]

The tone knobs are James stack, very popular in hi-fi, much less common in instrument amplifiers. I'd keep that for now.

The Normal channel has that tone switch. It is NFB hackery. Cut it right the heck out. Let V1b be a straight simple amp stage.

The Trem/Rev channels has that bridge-Tee filter. Snip one end of the 220K so it don't do much.

This will give a flat (not top-shy) amplifier. Whether it has enough "flavor" for brilliant guitar, you need to play it more with the bass-stuff cut out to see.

You are looking at the wrong schematic. He has the Maverick bass amp and not the Maverick Lead or T-Star. There is no reverb or term on that amp. It's a pretty decent bass practice amp, but you can only do coffee house gigs with it.

 I didn't have the time to get to you at LTG, but try it through a different speaker or cabinet. I think the Oxfords that came in the Mavs were less than impressive. I'd see how it sounds through a 2x10 or 4x10 cabinet. I have a Mav Lead and it's a screamer.

[HotBluePlates]

... channel has that tone switch. It is NFB hackery. Cut it right the heck out. Let V1b be a straight simple amp stage. ...

You are looking at the wrong schematic. He has the Maverick bass amp and not the Maverick Lead or T-Star. ...

You are correct, but if you chop down PRR's comments to what I show in the quote above, it's still relevant to the bass version of the amp.

Part of the tone switch operates in a feedback loop around V1b, the other part monkeys a T-filter between the tone controls and the volume control. It would probably be worthwhile to listen to the tone circuit without the filter (Treble pot wiper connected directly to volume control input), and without the feedback loop around V1b.

[alange5]

I managed to purchase a Maverick Lead schematic.  I bought it from MusicParts - not sure what their policy is concerning posting purchased schematics on forums...  Comparing it to the bass version, there are definitely some differences (other than reverb and trem).


First, both amps have a cap between the input and 680 ohm grid resistor on V1A.  Lead used .01uf, mine uses .1uf.  What's the purpose of this cap? 


Next, the Lead shares the same cap and resistor values between the treble and volume pots, but the Bass has the 1.2M resistor and .001 caps connected to the switch, whereas the Lead does not.  HotBluePlates, I'll take your recommendation and remove the switching from this section.


The Bass has a 330K before the V1B grid, the Lead does not.  I removed the 330K (the only modification I've done so far). 


One thing I definitely need some clarification on.... The Bass volume control is a dual pot which seems to simultaneously adjust resistance before the grids of V1B and V4.  I guess this was a method of getting some bass clarity at lower volumes without overloading the last gain stage?  In any case, it stinks for guitar.  The amp is totally dead (figuratively) until the volume is way up, at which point it really starts to breathe and some of the highs and midrange start to shine.  Unfortunately, at that point, the amp is incredibly loud.  The Lead schematic does not use this setup - it only has the single-gang volume control before V1B.  Looking at the Bass schematic, could I simply remove the second half of the volume pot and replace it with its full value, a 1M resistor? 


As for the speaker, I pulled it out expecting a CTS but it's actually a 1970 15" Eminence (67 speaker code).  When the amp is cranked, it sounds awesome. 

[HotBluePlates]

First, both amps have a cap between the input and 680 ohm grid resistor on V1A.  Lead used .01uf, mine uses .1uf.  What's the purpose of this cap? 

It blocks d.c. which might be leaking from your effects pedals or active pickup circuitry. But most often, this is being used to set a bass roll-off right at the input.

The Bass has a 330K before the V1B grid, the Lead does not.  I removed the 330K (the only modification I've done so far).

Does this mean you also removed the wire from the tone switch to the grid of V1b?

The 330kΩ is needed for the feedback loop tone switch circuit to work properly. If you just remove the 330kΩ and not the other stuff, the results may be unpredictable.

You'll see the same 330kΩ at the grid of V2a, because it also has a flat feedback loop in the form of the 1.2MΩ from plate output (after the blocking cap) to grid.

One thing I definitely need some clarification on.... The Bass volume control is a dual pot which seems to simultaneously adjust resistance before the grids of V1B and V4.  I guess this was a method of getting some bass clarity at lower volumes without overloading the last gain stage?  In any case, it stinks for guitar. 

V1b and V2a are both gain stages with feedback wrapped around them. The dual-volume is likely to prevent the drive to either of the feedback-controlled gain stages from getting large enough to abruptly overload.

You'll probably just want to start with unhooking the feedback loops, and re-evaluating before making a bunch of other changes.

[alange5]

Does this mean you also removed the wire from the tone switch to the grid of V1b?

The 330kΩ is needed for the feedback loop tone switch circuit to work properly. If you just remove the 330kΩ and not the other stuff, the results may be unpredictable.

You'll see the same 330kΩ at the grid of V2a, because it also has a flat feedback loop in the form of the 1.2MΩ from plate output (after the blocking cap) to grid.

I did not.  I took out the 330k and didn't really notice much of a difference.  I didn't even notice it was connected to the switch... I just assumed it was attenuation before the grid.  Then I snipped the leads from the tone switch to the treble and volume pot.  Now the switch is most trebly (and sounds great) in the middle position, whereas it used to have the most treble in the top position.


The switch has me confused.  I'm not quite sure how to interpret that part of the schematic, so I'm not really sure how the feedback loops are introduced via switching.

[HotBluePlates]

The switch has me confused.  I'm not quite sure how to interpret that part of the schematic, so I'm not really sure how the feedback loops are introduced via switching.

V1b plate output  -> 0.15µF coupling cap  -> wire to components attached to switch  -> wire from switch to V1b grid.

That whole path is a feedback loop. The switch just changes which specific parts are in the loop to change frequency response. The 330kΩ resistor was the resistance against which the feedback loop parts work to define the (frequency-dependent) gain of V1b; if you cut that out, the feedback loop operation is changed.

You might like the amp's overall sound better without the feedback-loop frequency shaping. Certainly the amp will feel less stiff.

The feedback on V2a is likely easier to see. There's a 1.2MΩ running from the tube's output back to its input. Again, it has a 330kΩ series resistor. Gain of the stage is set by the resistors to 1 + (R_f/R_s), where R_f is the feedback resistor and R_s is the series resistor between the signal source and the feedback resistor.
1 + (R_f/R_s) = 1 + (1.2MΩ/330kΩ) = 1 + 3.64 = 4.64

That stage's gain would be more like 60 instead of less than 5 without the feedback.

Same goes for V1b, except one setting yields a flat gain of ~31, another (middle) setting boosts highs, and the last setting appears to add a midrange bump. Again, you'll likely get a more traditional "vintage tubey" sound if you just get rid of the feedback tone switching.

[alange5]

That makes sense.  Thank you for explaining.  If I want to remove the switch completely, would I just go V2A plate -> coupling cap -> 330K -> V2B grid? (assuming I don't mess with the feedback on V2B)


In the meantime, I'll do some studying on the mechanics of the switch and how it's represented in the schematic. 

[HotBluePlates]

... If I want to remove the switch completely ...

I guess what we have been calling "V1b" is really "V2a" and the 2nd half of V1 isn't being used.

Anyway, for V2 pin 7, remove the wire which goes to the switch. Remove the 330kΩ resistor (which may be gone already); only the 1MΩ to ground and 0.01µF coupling cap should connect to V2 pin 7. Next, unhook the wire from the output of the coupling cap connected to V2 pin 6 which runs to the 10MΩ resistor (and all the other stuff on the switch). That will remove all the feedback around V2a.

For V2b, just remove the 1.2MΩ resistor; you can also remove the 330kΩ and replace with a piece of wire (or bridge a wire across the 330kΩ to short it out.

The amp's character will completely change after you do those 2 things.

[Steve_P]

I'believe I have the Maverick bass schematic at home. I'll take a look see when I get home.

[HotBluePlates]

I'believe I have the Maverick bass schematic at home. I'll take a look see when I get home.

Or look at the link in the first post... 

[2deaf]

First, both amps have a cap between the input and 680 ohm grid resistor on V1A.  Lead used .01uf, mine uses .1uf.  What's the purpose of this cap? 

I use a capacitor in this position in bass amps and high gain amps to prevent the speaker cone from leaping out of the basket when I first touch the strings. 

[alange5]

I removed the switch and all the feedback from V2A and it's definitely more "alive".  I have yet to remove the feedback on V2B.  Before I do, do I need a bypass cap on V2B cathode?  The Maverick Lead schematic has a 32uf bypass cap in this position.

[HotBluePlates]

... I have yet to remove the feedback on V2B.  Before I do, do I need a bypass cap on V2B cathode?  The Maverick Lead schematic has a 32uf bypass cap in this position.

If you want. Gain will increase at least 5-fold when you remove the feedback, or 10-fold if you remove feedback and add the bypass cap.

... There's a 1.2MΩ running from the tube's output back to its input. Again, it has a 330kΩ series resistor. Gain of the stage is set by the resistors to 1 + (R_f/R_s), where R_f is the feedback resistor and R_s is the series resistor between the signal source and the feedback resistor.
1 + (R_f/R_s) = 1 + (1.2MΩ/330kΩ) = 1 + 3.64 = 4.64

That stage's gain would be more like 60 instead of less than 5 without the feedback. ...

[alange5]

WAY too aggressive with no NFB on V2B.  I put the resistor back, and I'm satisfied!  Thanks for all the help, I really appreciate it.

[Steve_P]

I'believe I have the Maverick bass schematic at home. I'll take a look see when I get home.

Or look at the link in the first post... 

Yeah, that's what I get for having to use the phone.