Picked up a `71 Maverick guitar amp... got questions

RickV

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Got it from a guy that had it in storage (garage) for at least 10-12 years. The baffle had been hacked from a 2x10 to a 1x15. No grill or logo. Original OT was replaced with a 1962 Fisher 800B trannie. Missing a couple tubes. Cabinet is solid and fairly clean for its age. Power cord had been replaced with a 3 prong. I replaced the missing tubes, made a 2x10 baffle loaded with two Weber smooth cone Sig10 ceramics, covered with silver cloth (Fender clone) and fired it up. I could only get a faint signal when in the standby position only. Took it to one of the better techs in Dallas. He said the trannie was the wrong type (ultra-liniar?) and wasn't even wired up correctly. The power tubes didn't look too healthy. He replaced the OT with MM trannie. New EH 7591 tubes. Replaced missing plate resistor on preamp and replaced the cathode resistor and cap on the power amp. Cleaned the pots and checked the bias (cathode biased). The verb is OK, but I think the tube may be tired (microphonic). The trem isn't working, but I may need a footswitch to turn on.

The amp sounds really good, especially with single coils (G&L S-500 and a couple of P90 loaded guitars). The tone controls seem to be somewhat interactive, kind of like the Fender tweed circuits. The Webers actually sound really good with this amp. Lots of punch and crunch at higher volumes. I'm surprised how loud this thing gets. I think I read somewhere that these were rated at either 50 or 60 watts. I wonder if two 7195's are really capable of that output. I'm guessing it puts out more like 25-30 watts.

Got one problem. The tech informed me of this before I picked it up. The amp sounds really good, but when cranked all the way up with humbuckers, it tends to distort in a not so pleasant way. Doesn't seem to do it with single coils. The distortion is hard to explain, but more like a clipping from signal overload. I haven't tried any pedals though it, yet. My LP has Classic 57's and are set on the low side for lower output. I get the same clipping with that that the tech got. Doesn't seem to be quite as bad if I trim the tone knobs back a tad. The tech said he tried several different OT's to see if that had an effect, but didn't make any difference. He thought it might have something to do with the power supply design, as he couldn't find anything wrong with components or wiring. Any ideas? Not that big a deal, as I don't see a need to crank it to 10 (8 will be plenty for me).

This is a very cool amp. Can't wait to gig with it. It's so funky looking, it's kind of cool.

RickV
 

capnjuan

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Hi Rick; welcome to LTG and congratulations on your new amp. Twin 7591 output depends on both plate voltage and bias. The plate voltage in the schematic is 460V but the schematic indicates cathode-, not fixed-bias 7591 Data Sheet indicates maximum output of 28 watts at 460V / cathode bias and a Zout of 9000 ohms. Your guess of 25-30 watts is about right.
Rick said:
... tech in Dallas ... said the trannie was the wrong type ... and wasn't ... wired up correctly .... He replaced the OT with MM trannie. New EH 7591 tubes. Replaced ... the cathode resistor and cap on the power amp ... The trem isn't working, but I may need a footswitch to turn on. ... Got one problem. ... humbuckers, it tends to distort in a not so pleasant way. Doesn't seem to do it with single coils.
If your amp is cathode-biased (must be - tech says he changed the cathode resistor) and the OT has the proper Zout / 9000 ohm rating, then the source of the distortion has to be in the preamp ... and since he checked it, the power supply wouldn't figure into it.

That model has a 390K grid bias resistor, a 220K plate dropping resistor, and a 1.8K cathode resistor bypassed by a 32uf capacitor. You can hot check to see if all of the resistors and the cap are within tolerance but if this were my amp, I'd just replace them all; other than the preamp tube's bias which is set by the interaction of all three of these resistors, there's no other reason to get grunge with one type of pickup and not with another.

Trem/footswitch; usually the footswitch grounds the effect so if you don't have a footswitch then it can't be grounded ... that is; the trem ought to be on now. Get anything by spinning the trem controls? If not and if the circuit is ungrounded, then it's likely that one or more of the two .01uf and one .022 caps in the trem circuit is shot .... the most common cause of trem no workee. Good luck with your amp. CJ (.... the long soon :wink: )
 

RickV

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Excellent! Thanks for all the great info. I have a few questions I'd like to PM you about.

Thanks,
Rick
 

capnjuan

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In a PM said:
... I don't see a 390K on the schem (10-8-70). Where would I find it (what tube, pin, etc.)? Same with the plate dropping resistors ... The only 1.8K resistors I see are coming off pin 3 & 8 of V1 (12AX7). They both run in parallel with 32uf caps. Are those both the cathode resistor(s) you are referring to?
Hi Rick;

V1 Cathode Resistors:

Yes; the 1.8K resistors bypassed with the caps ... those are the two cathode resistors ... replace both and the caps too ... you won't find 32uf on the market, 25uf/25 volt will be fine ... the difference isn't worth talking about. If you can't find 1.8K, 1.7K okay see below ... not less than 1/2 watt.

V1 Plate Resistors:

One end of each is connected to V1 pins 1 and 6; replace them value for value 220K (old color scheme; red/red/yellow), not less than 1/2 watt ... 1 watt ok ... a little belts and suspenders but, all things being equal, they will last longer and resist drift better than 1/2 watt.

V1 Bias Resistor:

The grid bias resistor may be mounted on the input jack. There is a 680ohm grid leak resistor ... one end is tied to V1/pin 7 the other end ... where ever it is ... is connected to the 390K grid bias resistor ... the other end of the grid bias resistor is connected to ground.

Give my your email address; I'll send you the schematic I have ... probably the same ... but we'll be one the same page that way. Finally, if you can take a few pics and put them in the thread ... will help the others and will help me spot stuff.
John
 

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Hey Rick, nice to see you over here! In reference to the tremolo, you don't need a footswitch - it works without one.
I have JJ 7591s in my amp.
Tone controls are a Baxandal circuit, not the Fender, "we'll ground everything out until the player's happy" circuit. :wink: The three position tone switch actually does something noticeable. :D
Don't know about the unpleasant distortion. I didn't notice that with mine.
My amp is not terrifically loud, but it still has the stock CTS speakers. They might be tired, but I'm not going on the road anytime soon. :cry:
I'm not unhappy with them, but Webers are a future possibility.
I don't think you could get 50 watts out of it, 25-35 is more like it. My Vibrolux is louder.

Obligatory amp porn- not sure why pics aren't loading.

Gallery
http://img144.imageshack.us/g/p9260044o.jpg/
 

RickV

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Thanks guys! I will pull the chassis sometime this weekend and attempt to post pix. I was cranking it a little while ago, using my LP Jr (P90). I get what I refer to as unpleasant distortion with that guitar, too, but only with the volume wide open and the tone controls turned mostly all the way up. This sound is best described as like an over compression of the signal. If I hit a hard note or cord, the signal seems to get extremely compressed, with a momentary drop in volume. If I back of fthe tone controls, it will go away. Another thing that really affects this is the three position tone switch. It's best in the bass position and progressively worse in the middle, then treble positions. The switch works really good, with a big difference in the different positions. The good news is the amp has no hum, crackle, pop, etc.

Email is: hamer-guy@tx.rr.com
 

capnjuan

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Default said:
...In reference to the tremolo, you don't need a footswitch - it works without one.
?? How do you turn it off and on .... eh :? ?

As ever; no guarantee that the schematic matches anything but not seeing a way to switch the trem off/on other than the FS?

mav02-1.jpg
 

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Went down and plugged into it just to make sure, but, yes, works without the switch. The switch grounds out the signal to kill the trem, I guess. Didn't get into it too much, the blower motor went bad on the furnace and until the repairman shows up tomorrow, the rabbits' cage is right in the firezone.




You don't want to know. Trust me.
 

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RickV said:
Thanks guys! I will pull the chassis sometime this weekend and attempt to post pix. I was cranking it a little while ago, using my LP Jr (P90). I get what I refer to as unpleasant distortion with that guitar, too, but only with the volume wide open and the tone controls turned mostly all the way up. This sound is best described as like an over compression of the signal. If I hit a hard note or cord, the signal seems to get extremely compressed, with a momentary drop in volume. If I back of fthe tone controls, it will go away. Another thing that really affects this is the three position tone switch. It's best in the bass position and progressively worse in the middle, then treble positions. The switch works really good, with a big difference in the different positions. The good news is the amp has no hum, crackle, pop, etc.

Email is: hamer-guy@tx.rr.com

That's odd. Admittedly, it's cathode bias, but it has diode rectifiers and some pretty manly filtering. Maybe after the caps get changed. Do you have a 7247 in the phase inverter position?

Hopefully, once the furnace is fixed, I can mess around with it tomorrow night.
 

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mav02-1.jpg


If you roll down the depth, it shuts the trem off. The footswitch does that, but less subtly.
 

RickV

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[quote="Default
That's odd. Admittedly, it's cathode bias, but it has diode rectifiers and some pretty manly filtering. Maybe after the caps get changed. Do you have a 7247 in the phase inverter position?

Hopefully, once the furnace is fixed, I can mess around with it tomorrow night.[/quote]

Yup... got all the correct tubes in there, including a new 7247.
 

capnjuan

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Default said:
If you roll down the depth, it shuts the trem off. The footswitch does that, but less subtly.
Ok; roll it down to where it can't be heard ... but that's hard to do with the foot, isn't it? :wink: I thought you stopped keeping rabbits last year?
 

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No, they're still around.


On the other hand, you gave me an idea! What if I whipped up a cable with a 1/4 " plug on one end and a rca jack on the other. Plug one end to the amp and the other to a volume pedal.

Instant subtlety for Steve!
 

capnjuan

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Default said:
... you gave me an idea ... What if I whipped up a cable with a 1/4 " plug on one end and a rca jack on the other. Plug one end to the amp and the other to a volume pedal. Instant subtlety for Steve!
Ok ... fine ... no charge ... but what will the rabbits have to say about that ... them having big ears and all? :wink:
 

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Had no less than three Russian repairmen to fix the furnace. Heat, we have! Also have to make sure they weren't casing the joint. :roll: House dropped into the fifties, so I'm letting everything, pets, amps and guitars acclimate to normal temps.

Back on topic! Just looked through a bunch of threads on google. One old thread insists that EH7591a's are just rebased 6L6's. :?
Here's Dreamlander's thread on his Maverick Bass and EH.
viewtopic.php?f=7&t=5993


7591Atubes002.jpg



Old comparison of 7591's. In this comparison, the Eh's distorted slightly more, while the JJ's didn't put out as much power. It's from an audiophile point of view. and Rick's amp has new speakers - I don't. Interestingly, a couple of threads suggest Weber Blue Dogs for an Ampeg RR2.

http://tronola.com/html/7591a_tubes.html
 

capnjuan

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Default said:
Had no less than three Russian repairmen to fix the furnace.
That sounds like the old joke;
Q: 'how many _____s does it take to screw in a lightbulb?'
A: 'Three ... one to hold the bulb and two to turn the chair'

My 7591/U305R just stares back at me from the bench; I bought what I thought were more/better/gooder 7591s for it ... will go look ... it's been so long ...
 

capnjuan

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Hi Rick; not sure how much of this you want to mess with but the pic below of Steve's Mav IDs the several resistors I mentioned. I'm assuming that the tube you have in V1 is good; you might swap a few in and out to see if that cures your grunge. If you have about 350VDC supplying those two plate resistors (between the end of the resistors away from the tube and chassis ground), then the only way the power supply can be a grunge factor is if one or the other of those 220K resistors is no good.

mav01V1.jpg


The interaction of the grid bias and cathode resistors sets the bias of the input half of V1 ... the first stage to see the signal. Roughly, these two determine the rate at which the tube 'idles' ... how much current flows when the amp's on and no signal is present. If the bias is too hot because one of the resistors has drifted out of tolerance, you can get clipping, notes out of tune, grunge ... all sorts of problems ... same with cold.

A couple of other notes. The resistor marked with the 'X' is part of the reverb drive circuit of V2A ... rated at 5 watts on the schematic ... Guild was pretty good about putting beefy components in critical places. Also; Guild borrowed that silvery rod / ground buss running through the middle of the chassis. That ground bus is a standard feature on a lot of hi-fi gear of the period including Marantz. One end is insulated and the other grounded so that every ground in the preamp is taken to one common location abating ground loop-induced hummmmmmm ... and providing a convenient tie point during fabrication.

Hope this helps; no shame if you don't want to screw with it: "A man's gotta know his limits" Leo Fender.

CJ
 

RickV

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This is excellent info...especially the photo with arrows & lines!!! I've worked on a few amps (mostly BF/SF Fenders) and put together a few kits (including my current gigging amps: Allen Classic 10 and Allen Encore head). I'm not use to working ofF a schematic and a true PTP circuit. It's confusing to me. I definitely plan to check and change a few of these components out. I will try swapping V1 with a couple others, just to rule that out. I will try to find time this weekend to pull the chassis, snap some pix and post. I really like the tone of this amp. Look forward to gigging it (I play in a blues band with three other geezers). The problem only appears when the amp it cranked all the way up. I don't see me gigging at those volumes, but just need to make sure it's reliable.

Thanks again for all the help!

btw... Looks like Mouser has the odd valued resistors and caps.

Rick
 
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