Installing new speaker in Maverick bass amp. Help!

[Default]

From the looks of the circuit, they were concerned about the bass overloading the outputs. This might be an excuse to replace the 7247 with a 12 ax7, to get a little more punch.

 

[capnjuan]

Yeah; I stumbled with the audio/linear too. Recalling that this is a bass amp, the impression is controlling overload; might be okay for a guitar, a little distortion / fuzz / twangie taste but not sure that's good for bass; farty is one thing , but farty and buzzy is another...

The schemo indicates 370V ahead of the 7247 15K/2W plate resistor. I'm not sure what the drop is but there's at least 300 - 325 volts on the plate. For whatever reasons, Guild Engineering was crazy about the 7247 as a driver appearing in all of their 7591, 8417, 6L6, and 6550 amps. Yes, I'm aware that a 12AX7 can handle the voltage at idle or light duty, but driven hard for sustained periods .... I mean I think it's there for its durability; the 12AX7 might work for toe-tapping in the backroom but ....

If Dreamlander is going to play guitar through it, then the audio taper pots might be okay but not sure what that would do for it as a bass amp? My Tbass presents the same problem; optimizing for guitar at the expense of the amp's bass ability or vice versa; I'd like to reduce the uf in the coupling caps downstream of the driver but doing so may end the amp's career as a bass amp. Anyway, there's the deal.

 

[Default]

Point taken! :lol: :lol:

 

[danerectal]

As always with amps, I'm way out of my element. I wonder if one could substitute a dual/concentric pot to achieve the later design of overload/limiting.

 

[capnjuan]

As always with amps, I'm way out of my element. I wonder if one could substitute a dual/concentric pot to achieve the later design of overload/limiting.

Hi Dane; that's what DL's pot is; a dual / concentric or 'ganged' pot; see my pic or the upper half of default's. The dotted line indicates that both resistive elements are operated by the same shaft. See the pic of DL's pot above; they are 'stacked'.

The chin-scratching here has to do with whether the replacement should have audio a/k/a logrithmic taper or linear taper. It's just a guess but I think the design anticipates feeding signal to the back half of the 7247 in a 'linear' manner; that is, for each partial rotation of the shaft, there is a proportional increase in signal strength (decrease in resistance to ground). This might have been designed to keep the driver and outputs from bogging down.

Audio taper works a little differently where the change in resistance to ground for each partial shaft rotation is non-linear; that is, a little bit of shaft rotation results in a greater change in resistance to ground. Most audio volume controls are 'audio taper'. If the original was both; 1/2 audio taper and the other half linear taper, the market no longer supports a control like that; it's either audio or linear for both parts of the control and the question is, if forced to chose one or the other, which one is 'better' for the amp or screws it up the least.

Right now, I don't think I have an answer but we have several solder-heads lurking and maybe one of them will have something to offer on the subject. cj

 

[Default]

As always with amps, I'm way out of my element. I wonder if one could substitute a dual/concentric pot to achieve the later design of overload/limiting.

That's a pretty intriguing idea, actually. Effectively, you'd have a concentric master volume. Solves the audio taper vs. linear taper problem since you would be operating both volumes separately. Control knobs would have to be replaced. :?

 

[capnjuan]

Oops; my muff Dual concentric will give you independent control of both pots but both they still have to be either audio or linear taper, no? Assuming you can find one with 1 meg pots...cj

 

[danerectal]

I understand the math of the pots. I just am thinking if you could conform to just log taper; you may have more defined variation for both pots, and also that you would then be able to ditch the fetters of only one means of adjustment. I've never seen a 1megohm concentric pot, but I'd think one would easier locate a concentric than a ganged...

 

[capnjuan]

I understand the math of the pots. I just am thinking if you could conform to just log taper; you may have more defined variation for both pots, and also that you would then be able to ditch the fetters of only one means of adjustment. I've never seen a 1megohm concentric pot, but I'd think one would easier locate a concentric than a ganged...

Hi Dane; I'm not sure but I think it's the other way around; there are lots of ganged/1 meg pots, not as sure about dual concentric 1 meg regardless of taper. These URLs are from default's earlier post and all contain 1 meg / ganged pots ... but no concentrics at 2 X 1meg. Not saying they aren't out there; they're just not in plain view:

http://rangeraudio.com/stock21.htm
http://www.evatco.com.au/pots.htm
https://taweber.powweb.com/store/potsord.htm

If Dreamlander wants to play guitar through it, the ganged, audio taper pot would probably be okay but not sure what it would do with a bass guitar. Also, the 'mod' to twin audio taper pots could present headaches if/when he wanted to sell it. cj

 

[capnjuan]

Potz: linear v. audio taper, stashing reference URLs

http://www.geofex.com/Article_Folders/p ... tscret.htm
http://www.vias.org/feee/exp_dc_07.html
http://sound.westhost.com/pots.htm

Question put to the boyz on The Gear Page: may require registration to view. Ask Mr. Wizard


cj

 

[Default]

http://www.has-sound.com/parts.htm

$25 bucks. Ouch!

2 meg audio pots for coastie.

 

[Default]

Located a 1meg/1meg concentric pot in my junk box, of all places. I'm looking for knobs to fit, or I can solder the shaft together to make it a dual-gang. Also located a 2 meg pot, solid shaft.

 

[capnjuan]

Located a 1meg/1meg concentric pot in my junk box, of all places. Also located a 2 meg pot, solid shaft.

Got any idea whether the dual/concentric is audio or linear taper? cj

 

[capnjuan]

located a 2 meg pot, solid shaft.

Hi Steve: If you intend this for Coastie's amp, I have this covered. cj

 

[Default]

No idea whether the pot is audio or linear, I can't decipher the pot code yet.
The link for 2 meg pots is just in case you need any others in the future, cj.

 

[capnjuan]

Thanks Steve and for the benefit of Dreamlander and Danerectal, as Steve has already pointed out, these folks make and sell pots.

http://www.has-sound.com/parts.htm#Pots

They make an audio taper, dual/concentric 1 meg pot but my recommendation would be to contact them and ask if they'd make you one that is consistent with the design; front half audio taper and back half linear taper. I posted this question over on the Gear Page and haven't gotten a useful answer yet. If they don't or won't, the only other way to meet the design is to cobble one together from 2 pots; one audio and the other linear taper.

There is at least a reasonable chance, if you use either a dual-ganged or dual-concentric pot where both sections are audio taper, that the amp will perform poorly for bass guitar. If that turns out to be the deal, if/when you try to sell it, you'll either have to disclose it or - if a bass player buys it - risk an unhappy buyer. In any event, welcome DL to the wacky world of Guild amps. cj

 

[Dreamlander]

Thanks for all the info everyone. :shock: My head is kind of spinning right now, but I think I will be able to work something out. I will get some translating from Dane when I get back home next week. :mrgreen: I really have no problem with turning this amp into a strictly "guitar" amp. I have a bassman 100 which sounds pretty good with a bass. And the distortion the better, I just want to get away from the dead farty sound.

 

[capnjuan]

...I just want to get away from the dead farty sound.

Hi DL; agree, to be avoided. The following amps were in production when yours was and yours is the only one that has the linear taper volume control immediately ahead of the driver stage.

Quantum Bass: 8417s w/ 6GF7 regulator; 7247 driver
Thunderstar; 7591s; 7247 driver
Thunderstar Lead: 7591s; 7247 driver
Maverick Lead: 7591s; 7247 driver

As indicated, it might have been Guild's way of keeping the low frequency bass signal from overwhelming the output. As long as you only want the amp for guitar, then my suggestion would be to buy a dual gang / audio taper 1 meg pot and wire it as indicated on the right below. The left side indicates what you have now. If you had the amp open and were looking down with the faceplate nearest to you, this is the orientation of the lugs as well. Also, for purposes of this suggestion, I'm assuming the preamp / audio taper section of the existing pot is closest to the faceplate.

Schematic view:

If you buy a new dual gang / audio taper pot, it can go in and the preamp section gets wired exactly like what you have. The driver / linear taper connections are as shown on the right; there would be no connection to ground and the inbound and outbound signal wires are soldered together on the signal-in lug effectively jumping the back half of the pot out of the circuit.

The only function of the un-used pot section is to provide a tie point for the in/out wiring. If you wished, you could also just buy a single section 1 meg audio taper pot and splice the in/out wiring together. T'were mine, I'd get the twin 1 meg audio pot because it makes a 'cleaner' job.

Good luck with your amp!

cj

 

[Dreamlander]

Sorry, but I get way lost when I look at the schematic , and I am not going to ask you to try to explain on a keyboard since it is near impossible. What i can do is look at the way something is wired and put a new pot in the same way. I would like to replace the pot with the same one that will come out, just not sure if I want to drop $25 on a pot yet. The cheap amp I bought is turning out to be a little spendy. I would like to try the cheaper alternative. I think I get what you are saying about wiring with the ganged audio pot, basically remove the ground from the pot, and then remove the one wire connected to the middle lug (signal out) and connect that wire to the outside lug (signal in) with the rest of the wires. Seems simple enough. I also want to replace the bass and treble pots. How do I know what their value is?
Thanks again for your effort to help someone who knows very little about amps, although my knowledge has increased 10x in the last couple months here.

 

[capnjuan]

Sorry, but I get way lost when I look at the schematic , and I am not going to ask you to try to explain on a keyboard since it is near impossible. S'why so many find the pictures/schematics helpful.

What i can do is look at the way something is wired and put a new pot in the same way. It's a start.

I would like to replace the pot with the same one that will come out, just not sure if I want to drop $25 on a pot yet. If you / the Dane do it, your labor is zero and the difference between a $25 pot and a $5 pot buys you protection from overloading the output. This is a risk/reward decision; for the $20 you have in your pocket and if the outputs bog down, you get do it over and spend the $25 anyway; you will, however, have a perfectly good $5 pot in your pocket.

The cheap amp I bought is turning out to be a little spendy. I would like to try the cheaper alternative. One of the reasons you got it for what you did is that the former owner didn't want to come out of pocket for the speaker and pot and whatever else it needs. If he had, it wouldn't have gone so cheap. Most 35+ year-old / 'player'grade' tube amps need anywhere from $100 - $250 in 'technical intervention' to be useful. Dumpster diving has it's risks. :evil:

I think I get what you are saying about wiring with the ganged audio pot, basically remove the ground from the pot, and then remove the one wire connected to the middle lug (signal out) and connect that wire to the outside lug (signal in) with the rest of the wires. Seems simple enough. You will have to confirm that the section nearest the faceplate is, in fact, tied to the preamp and not the output but, if so, under the dual-audio taper pot scheme; the section nearest the faceplate is wired just like what you have. On the back half; signal out (middle) just moved over to signal in, and no leg to ground. Agree, seems simple enough.

I also want to replace the bass and treble pots. How do I know what their value is? By looking at the schematic; they are both 1 meg / audio taper.

Thanks again for your effort to help someone who knows very little about amps, although my knowledge has increased 10x in the last couple months here. You're welcome DL; if you get stuck, you can PM me or default anytime. Best of luck.!

cj