Thunderstar Bass questions : that insane Tone Switch

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I recently acquired a sweet old Guild Thunderstar Bass amp for real cheap because of some issues - primarily some nasty, constant crackle. The filter caps have been replaced some time in the last few years with new, clean looking JJs, so I didn't touch those. The tubes are old but nice American RCA glass and seem OK.

The bias resistor was way out of spec causing an imbalanced current to the power tubes - like 75mA to one and 50mA to the other! I don't know how the tube wasn't redplating, but after replacing that resistor I got them balanced to a more reasonable level (~70% dissipation). I replaced the super old 30uF 10V cathode bypass caps with 22uF 50V caps that I had on hand - I can't find small 30uF caps anywhere. That killed off most of the crackling and cleaning the switches, pots and jacks did the rest. It sounds real cool! It's kind of between a Tweed Bassman and an Ampeg b-15 to my ears. I like it for both bass and guitar.

1: Even with all new electrolytics, there's still quite a bit of hum even with the volume at 0. Should I recap more of the amp? I'm looking at the coupling caps next, but I thought those didn't need to get changed. Should I buy a new pair of 6L6s?

My main questions are about the 3-way tone switch. The way the schematic is drawn is baffling for one. I can tell that it's providing some negative feedback to V1B but which parts are engaged when is confusing as hell. In the bright position (open circle) the amp sounds very scooped, in the dark position (closed circle) it's VERY dark - like fully-rolled-off-tone-on-a-P-bass dark. In the center position, there's about 10dB more gain in the circuit. It's way louder and gainier than the other positions. Honestly it's kinda cool to get that much tweedy overdrive, but it seems real odd to me.

2: Can anyone explain how the switch actually works? I've scoured the web for a wiring diagram for a 12-pin 3 position switch to no avail. Like, which pins are active when?

3: Is this the way the tone switch is supposed to work? Or are the components drifted way off spec? My next thought is tor replace all the resistors and ceramic caps and see if that balances the positions, or maybe just install a switch to bypass the whole thing.
 

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That's the way that tone switch is supposed to work. This is a bass/organ amp. Have you tried the hum balance pot in the back of the amp? You start out in the middle position and turn it a bit clockwise or counterclockwise until you find the position with the least heater hum.
 
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Thanks! That saves me an order of unnecessary parts and another round of disassembly, soldering, etc.

I have used the hum balance in the back and it definitely helped. There's not a crazy amount of hum, so if I've got it as low as it can go, that's OK with me.
 

mavuser

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the one i had sounded holy grail with a Telecaster, in the *normal channel. it sounded so good and clean, like the amp did not even need reverb (which it does not).

for bass, I also would say the tube bass tone was as good as it gets, in the bass channel. the Guild Thunderbass cabinet I had, however, was not so great for bass (the smaller wheat grilled 2x12). I think for bass the cabinet and speakers makes the biggest difference.

but again, for guitar, the cab and speakers were perfect!
 
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I may install a tone bypass switch, once I put it back on the bench I’ll have a listen without that thing in there. I get having a set of different tones, but it’s super weird that the middle position adds so much gain.
 

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I may install a tone bypass switch, once I put it back on the bench I’ll have a listen without that thing in there. I get having a set of different tones, but it’s super weird that the middle position adds so much gain.
Because that is the bypass mode on the tone switch? That's what the schematic seems to indicate.
 

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IMG_5120.jpg


Hello

Thanks to this thread I finally bothered to figure out what that switch does.

My flimsy drawing shows in upper row the three positions of the 3-position triple switch.
Section A of the switch is passive between tone controls and volume.
Sections B & C are in feedback loop of V1-b

Beneath the three position of the switch is shown what is effective in circuit at that position.

In center row of my drawing you can see that in position 1 there seems to be a mid-cut.
Positions 2 & 3 seem to cut high frequensies.

Bottom row shows the feedback loop of V1-b
Position 1 is straight feedback without any filters.
Position 2 & 3 have filtering.

Obviously one could experiment with different component values on these filters to make the amp more suitable for ones liking.

Hope someone gets some ideas of this.
 
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ahhh of course! That’s why it’s so loud- no negative feedback to V1. The way the switch wiring is drawn on the factory schematics really threw me for a loop.

Thanks for drawing that one out! It seems Guild used this same basic circuit on a number of their amps.
 
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Looking more closely at what @Nuuska so kindly wrote out I think it could be fun to play around a bit.

The mid cut in the bright position is a bit extreme to my ears (could be component drift tbh), but the amp is very clean in that position.

Position 2 does have NFB, but mostly in the highs .

So position 3 (dark) just sends an unattenuated HPF'ed feedback signal to completely cancel out high frequencies. That makes sense based on what I hear from it.

So a tone bypass would indeed sound a bit different. That might be a fun, easily reversible mod after all.

Thanks all!
 
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