'68 Thunderbass "Quantum Bass" amp

RWood

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Hello,

I'm working on a '68 Quantum bass amp, ser.no. 6165. This one has 8417 power tubes.

There's a tube in the dead center of the chassis that the lettering has all but worn off of. It's a nine-pin tube, an RCA 6GF7A . According to the data sheets, it's a dissimilar triode.

Other T-bass chassis that I've seen pictures of don't have this tube at all. It has a dedicated filament winding from the power transformer.

Needless to say, a schem of this amp would be a huge help, but if anyone knows what this tube is for, please enlighten me!

Thanks,

RWood
 

RWood

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Thanks for the link, John.

Matsickma, you out there? Any ideas on this tube?

Here are some reference pics:


Serial number 6165:
DCsThunderbassAmp014.jpg


The tube in question is right in the center:
DCsThunderbassAmp021.jpg


DCsThunderbassAmp017.jpg
 

RWood

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The tube in question is probably not a rectifier...... it apprears to be a 6GF7A. GE's spec sheet lists this as a Dissimilar Twin Triode (a hi mu and a low mu in one bottle).

Is this the voltage-doubler circuit I've heard about with 8417 output tubes?

Any other amp-fixers out there?

RWood
 

megadan

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I believe the amp you have is in fact a Thunderbass, not a Quantum.

My Quantum looks much different than that, but yours looks similar to this Thunderbass on ebay at the moment:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 0028322454


I do know that Guild put some tubes in odd places on my amp, there sometimes doesn't seem to be much rhyme or reason to it, but I guess it makes sense, since they sound so darn good ;)
 

RWood

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Thanks, the amp I'm working on does have the 'Quantum' badge on the front panel, and it also says Thunderbass on the chassis.

This one is very, very similar to the one you pointed out on eBay but that one doesn't appear to have the 6GF7A tube, which would be located right between the output transformer and the aluminum cap can in his picture.

Other than that, and mine using 8417 power tubes instead of 6L6's, they appear to be identical.

I have re-tubed the one I'm working on and I've replaced all the coupling caps and cathode bypass caps. I'm not getting any signal yet, so more troubleshooting is in it's future!

RWood
 

RWood

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Here's one on eBay that's actually the first one I've seen that appears identical to the one I am working on. It's Ser # 6126, being sold by a nice guy in CA named Mike who replied to my questions about the amp. Unfortunately he doesn't have a schem or a tube chart.

For future reference here are a few pictures of his amp:

Pic 1

Pic 2

Pic 3
 

RWood

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Darryl Hattenhauer said:
What guitars and styles are best for a Thunderbass?

Why, a Guild Starfire bass, of course! 8)

Just look how excited these girls are!

1966starfireBass.jpg


and

guild-Quantum-1967.jpg
 

Darryl Hattenhauer

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Boss, bitchin', and groovy. A vision of what will soon reach New Zealand.

How would these be with a big hollow acoust/electric like a B30 or B50?
 

matsickma

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Hi All,

The Thunderbass picture posted by RWood has an unusual feature in the bass speaker cabinet. This particular cabinet was advertised as one that had two different speakers and a crossover in it. One of the speakers is a bass speaker and the other is a guitar speaker. Guilds idea was to have bothe speakers and a crossover to route the lower frequencies through the bass driver. This allowe the cabinet to be have a frequency response that worked well with either instrument.

I have never actuall seen one of these in person but Guilds literature describes this feature. The only external cabinet feature that indicates this cab is different from others is the orientation of the Guild logo. On standard cabinets the Guild logo is orientated horizontally when the cabinet is arranged in the tall position. On the "dual mode" cabinet the Guild logo is horizontal when the cab is in the low position.

matsickma
 

RWood

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Well, two-and-a-half years in my basement proved to be just the right amount of time to let this amp age, to get me up to speed on several design quirks I had never seen before, to locate the correct schematic, and to get help at various points from the kind folks here at LTG. Thanks all.

The amp did indeed come back to life, the owner was ecstatic about it when I surprised him with it, and most importantly, it sounds great. Worth the learning curve and the time it took.

The major breakthrough occurred a month ago when I rebuilt the bias circuit. Yes, that major cluster in the northwest territory took a shot of courage to dive into, but it's my belief that the domino effect of many problems that this amp had all began with the bias.

It's not much less of a cluster now but here it is with correct values and dialed into the correct .300 VDC on the cathodes. Upper left >

Finalchassiswiring029-1.jpg


Notice also the cigar-shaped resistor mounted to the right of the bias pots. After repairing the bias, my B+ voltage went from an alarming 626 VDC down to a near-manageable 595. Still 6% over the 560 volts that's required to energize the output transformer and the 6GF7 tube. This 10W, 500 ohm resistor brings down the B+ that last 6%. It works independently of the other B+ nodes, all of which were very close to spec.

My theory is that the amp has always been a little high on the B+ side, and when the bias circuit went out, voltages raised even more. Virtually all of the dropping resistors to the preamp tube plates had blown open. Needless to say, I became adept at removing the three preamp circuit boards.

Before I stumbled on the bias fix, I had replaced every coupling cap and the entire voltage regulator circuit. I rationalized later that while those changes did not fix the amp, they probably made the fixed amp sound better. Another such pair of changes were the massive voltage-doubler capacitors. Look for the pretty blue cans >

Finalchassiswiring026-1.jpg


They fit in the same holes and are the same height as the originals. Nice screw terminals on the underside made for some clean work >

Finalchassiswiring042-1.jpg


A casualty of the repair was one of a matched pair of 8714s, a GE. Here you see it replaced with an RCA - the tube tops are different. Thankfully each gets it's own bias control pot.

Finalchassiswiring021-1.jpg


The amp got buttoned up nice and tight, some Meguiars brought a shine back to the tolex, and the amp should be good for another few decades. I gave the owner detailed instructions on how to set the bias, and also when to set it - anytime a power tube is changed, even if it is one being swapped for the other. I also told him how to adjust the screen regulator circuit, but this is much more stable by comparison. It was built to be that way.

If you are working on a Quantum X, I encourage you to keep the amp running with 8417s. The really do sound good, and man does it blast. It's no accident that these amps originally came with cabs loaded with 15" JBL D130s!

Thanks Capn Juan for your encouragement and also Dr JayBee for rolling up your sleeves and melting solder with me.

RWood
 

capnjuan

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RWood said:
.... two-and-a-half years in my basement proved to be just the right amount of time to let this amp age ... rebuilt the bias circuit. Yes, that major cluster in the northwest territory took a shot of courage to dive into, but it's my belief that the domino effect of many problems that this amp had all began with the bias ... I became adept at removing the three preamp circuit boards ... I rationalized later that while those changes did not fix the amp, they probably made the fixed amp sound better... A casualty of the repair was one of a matched pair of 8714s, a GE. Here you see it replaced with an RCA - the tube tops are different. Thankfully each gets it's own bias control pot ... I encourage you to keep the amp running with 8417s. The really do sound good ...
Hi Richard and congratulations! Strong is the pleasure of solving a long-standing problem and getting fine sound as a reward. And, in the famous words of California vinter; " ... I'll fix no amp before it's time ..." and it's good to have tolerant friends ... My Thunderbass didn't present with problems extending to the circuit boards ... no small blessing in these amps.

I wound up using JJs for those monster filter caps ... an effective, if not necessarily elegant, solution:

newcaps-1.jpg



The only bias issue in the amp was one of the brown, 6.8 ohm/1% resistors had drifted off; I replaced it allowing the range of swing on the two bias pots to more or less match.

bias01.jpg



I considered putting it back as an 8417 amp but I bought it as a two-bagger; a Guild amp and a power supply project and other than me, it had no sponsor. Despite the build-back based on the 6L6 schematic, I never got the 450V or so; I hadn't considered that maybe the transformers were different but I manipulated the screens until the plate/screen voltage ratio in the amp matched the schematic after which it was just adjusting the bias supply dropping resistor to get the bias voltage right .... but not until after blowing the first replacement bias supply caps :oops: :oops: because of polarity ... I also agree with you that so long as you have the hood open, any updating and maintenance that can be done increases reliability and promotes good tone.

At the risk of veering this thread; I'd really love to see more pics of that rig you mount your amps in; great job! CJ
 

RWood

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Default said:
As long as you buy an extra set or two now. NOS is @$75 a single tube. :shock:

I agree they're not cheap. Bargains can be found, though. We got the GE pair for $85 and the RCA single for $55, both NOS. This is actually in line with current production KT88s, which the 8417 is similar to, power wise. But definitely more that 6L6GC, yeah.

capnjuan said:
At the risk of veering this thread; I'd really love to see more pics of that rig you mount your amps in; great job! CJ

Thanks, Cap'n. Once I started working with rotatable chassis cradles, I could never do without. They just remove that obstacle of positioning the thing just right for whatever you are doing. Easy to build, too; the key being the threaded knobs and carriage bolts. I get those knobs at Woodcraft - they are inexpensive.

Here are some examples in small, medium and large. The last one is the Guild:

Chassiswiring021.jpg


Chassis004-1.jpg


Finalchassiswiring039.jpg
 

capnjuan

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Thanks Richard; those look really worthwhile and straight-forward to build; having several helps get around the width variability and the need for adjustable brackets. Cool rigs and congratulations again on the Quantum ... there's a lot to like about those amps! J
 
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