Quantum Bass

capnjuan

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If I may John, this is a picture of John's amp and it's the subject of this LTG thread.

GuildQuantumhead.jpg



For those of you who could live with 50 watts of screaming Guild power, this is the amp to own; the top-of-the ThunderBass model line including regulated screen power.
 

capnjuan

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Wait a minute ... is that fish I smell? That news source also carried an article about "Al-Qaeda recruiting suicide bombers with promise of halfway decent job in afterlife"
 

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Replaced electrolytic caps and cord.

img0644e.jpg


Extension for highvoltage leads from pt. The go to a Fender double pole, single throw standby switch. Due to the voltage-doubling circuit, I needed a dpst to interrupt the ac before it was rectified. Unfortunately, the switch was rather bulky and I had limited room. If I had used a smaller spst switch after the rectifier, it would have resulted in a high, no-load voltage on the first set of 250 mfd caps. Stacked, They're good for a thousand volts, but this beast had 530 volts on the plates of the 8417s. The black caps are the bias caps. Not pictured is the cap for the screen voltage. On the schemo, it looked like a 20 mfd cap was called for. In this amp, a 2 mfd was actually in place. I replaced that with a 4.7 mfd cap.

img0647y.jpg


"Driving sideways"
Standby switch by the cord. Hum balance is adjusted by the mark 1 ear. Just turn it back and forth until you reach the position of least hum. The big tubes are 8417s - like a 6l6, but on steroids. They were one of the last tubes to be designed, therefore one of the hardest to get. In fact, I was not able to get a matched set. Guild, however, designed this amp with a bias pot for each tube, meaning that you can adjust each tube's bias easily with a voltmeter and a screwdriver. You can see the round test probe jack at the top.
In front of the powertubes, the 7247 phase inverter tube plugs into the pc board. A white wire provides negative feedback voltage from the speaker jack. Negative feedback reduces distortion (and volume) not that volume should be an issue.

In front of that is another pc board holding a 12ax7. The master volume pot is so close that a signal lead was shorting out on the tube shield, creating a nasty buzz until I rotated the pot out of harms way.
img0650m.jpg



According to the USPS, delivery was attempted today, and why would they lie to me? :roll:
 

capnjuan

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The standby switch; this is it below? Does the switch on the lower left front corner of the faceplate do anything now? There was a member passing through here several years ago with an 8417 T-Bass head; he was looking for tubes for it. I suggested he consider converting it to 6L6s and he gave me the "... over my dead body ..." thing. Nice work!

kidder03.jpg
 

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Yes, that's where the standby went. The switch on the front still shorts the signal out, but the polarity switch is just there for good looks.
 

capnjuan

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Default said:
... but the polarity switch is just there for good looks.
... it's nice to have good-looking details on the control panel. :wink:
 

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The Quantum sings.

What a sweet tone - it's so clean and true, very little noise - but without opening it up it's like having Pavarotti at the dinner table humming softly. You just want to get him on stage and let him go for it. I'm deeply in campaign mode right now, so can't take the few hours/days I will to get intimate with this thing. I need to get it outside to crank it up even a little - it will just blow the walls out of my little country home. But there's no doubt it will be my main amp from this moment forward - it's just too good not to be.

More detail as time passes. But for now I'm the happiest guy on this board.

Thanks Steve for all your careful and highly competent work, and thanks John for recommending Steve and for your always-helpful commentary. I'll bet this old girl is happier now than she has been since new.

Cheers all - may the rest of your day(s) be as good as mine.
 
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