Thunderbass amps: Convert 8417s to 6L6s

capnjuan

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I recently bought a Thunderbass amp threaded here Works if you shake it More general discussion on Thunderbass amps here: http://www.letstalkguild.com/phpBB2/viewtopic.php?f=7&t=4710

The amp showed up and, after a couple tubes changed, powered up nicely. In anticipation of rebuilding the power supply, I started to go over it and found that, although the amp arrived w/ 6L6s, it was wired for 8417s - like at least John K's and maybe others. Most particularly, the two large light blue cylinders - resistors - in the amp didn't match the 6L6 schematic.

jtbassunderPS.jpg


The pinouts for the 8417 and the 6L6 are identical:

6L6v8417-1.jpg


But, as shown in Guild schematics for the same amp, their electrical characteristics are different. Considering the prices of 8417s:
The Tube Store each 79.95
Toobey Doo each $54.95
Tubes and More each $54.55
Tubeworld pairs $110-$135, individuals $55-$60
The fact that JJ, ElectoHarmonix, Groove Tubes, and the others are not supporting the 8417, and the plentiful supply of 6L6s, I'm going to rewire it for 6L6s. The good news is that for the 6L6s to work correctly in an amp wired for 8417s, only a few changes are necessary.

The power supply and outputs from the 6L6 schematic are shown below. The red below indicates the primary high voltage DC which goes directly to the output transformer and then to the plates; where the amplified AC signal exits the tube. The green below is the screen voltage; the screens control the flow of electrons from the cathode to the plate and it is the flow of electrons that 'imprints' an identical copy of the AC waveform onto the plate but at a much higher amplitude; e.g., amplified. Finally the yellow indicates the bias wiring. The yellow should have been extended to indicate connection to the cathode - opposite the plate (too lazy to do the pic over :oops: )

The colored text boxes indicate the differences in electrical characteristics between 6L6- and 8417-wired amps with the 6L6s requiring higher plate and screen voltages but at only 40volts differential whereas the 8417s require less plate voltage but nearly 100volts differential between the plates and screens.

6l6e.jpg


There are 4 blue circles around resistors with related text boxes. Three of the four resistors are direct swapouts; take out the resistor of 8417 value, replace with the 6L6 value. Simple enough; the remaining resistor to the far visual left will probably have to be adjusted to get 460 volts. Both schematics shown a 10 ohm/10watt resistor in that position. If the transformers are identical then slightly less resistance ought to generate higher B+. If not, it may not matter that much because what is more important is the voltage differential between the plates and screens; the absolute value of the B+ isn't that important; somewhat less voltage would only mean slightly less rated output.

So, other than possibly having to either calculate, or determine by trial and error, a value for the left-most resistor, changing 3 or 4 resistors will convert an 8417 amp to a 6L6 amp. The only other comment is that this power supply has gobs of capacitance; essentially reserve power to support higher volumes and lower frequencies; after all, it is a bass amp but with a guitar channel. No effort will be made to dirty up the guitar channel. The amp has .27uf coupling capacitors; large enough to knock down high frequency hash or grunge so that any effort to optimize the guitar channel will be mitigated by the coupling caps; if the caps are changed, the amp might be somewhat compromised as a bass amp.

Comments welcome; hold the jeering to a minimum.....

cj
 

capnjuan

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:twisted: :twisted: :twisted: :twisted:
but frankly wouldn't object if I could find a design/schematic; I'm not repeat not interested in bench cobbling ......besides, who's to say the 7027 won't go the way of the 8417?
 

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Yeah, I scoped something out on line re: 7027's. The 6L6's that JJ and Sovtek make are capable of handling the voltages that 7027's were designed to handle, so there's no point in buying 7027's at all, even though they're in production.

I was jerkin' yer chain, John, cause I know the aggravation this is putting you through!
 

capnjuan

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Default said:
Yeah, I scoped something out on line re: 7027's. The 6L6's that JJ and Sovtek make are capable of handling the voltages that 7027's were designed to handle, so there's no point in buying 7027's at all, even though they're in production. I was jerkin' yer chain, John, cause I know the aggravation this is putting you through!
I'll tell ya what I tells the Grahamster when he's being pestiferous; I got this guy - yeah - I got people ....

Graham.jpg


No problem default; the Sovtek 6L6s w/ the buttons are fine with the 'Fender' style retainers; amp formerly had a handle on the bottom that doubled as a support to hold the amp and faceplate up in the front - handle probably on sale at White Van Music - anyway, it's gonna get a top-mounted handle to make the upside down thing a thing of the past. I think also larger amp feet in the front, smaller in the back; duplicate part of the up-facing chassis orientation.

Just about done getting rid of the toene cheese ....ugh....but it really sounded good when it was fired up; forward, bright, clean, strong ... if I only knew how to play the bass... :(
 

capnjuan

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Default said:
My first guitar teacher told me, "If you can't be good, at least be loud."
My best friend in jr HS and I played pool endlessly; our motto? 'If you can't shoot good, shoot hard' :lol: The Thunderbass is a big pushy amp; not hard to understand why they were popular w/ bass players. j
 

capnjuan

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fronobulax said:
capnjuan said:
if I only knew how to play the bass... :(
Never stopped me, although it probably should have.
:wink:
Now you're funnin' me .... OTOH; if you didn't get past where I got on the clarinet ... uh ... that would be different. My jHS band had 14 clarinets seats; I was #13; #14 was a guy named Phil who chewed gum while he played ... :shock:
 

capnjuan

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These are some progress pics from the power supply rebuild on my Guild Thunderbass 8417/6L6 amp. Spent some time considering putting it back as 8417 v. converting it to 6L6. Our BBer losethisskin made a persuasive argument for 8417s but, at the end of the day, finding some of the high wattage resistors needed for 8417 configuration was too difficult and it'll be a 6L6 amp.

The first pic is a partial schematic; the dotted lines are color-coded to the text to show which caps in the schematic match the caps in the as-bought amp:

schem.jpg



This pic shows the new caps from the tube deck side; orientation and closer up. The 2-section bias cap shown in the tube deck view is no longer in production. Its sections will be replicated with individual electrolytics.


newcaps.jpg



Bottom view: as-bought on the left, in-progress on the right - the bottoms of the caps are the brown, oval 'discs' or circle for the bias cap. As with the 2-section bias cap, the 4-section 40uf/450V cap is also no longer in production. The work-around is a 3-section 40uf/450V can cap and a loose Sprague Atom 40uf/500V that you see at the bottom of the right-hand section. So far, the two rectifier diodes, primary B+ and bias supply voltage dropping resistors (seen mounted on the rail on the left), and the caps have been changed.

oldnew.jpg


In the pic on the right, the screen / guitar-bass / driver supply wiring is tied at the cap end and laid back for now pending getting the rest of the power resistors some of which mount on the rail on the right. The two, big blue resistors seen in the pic on the left are no longer in production. Going to 6L6s gets around finding a replacement for one of them; the 20K/25W replacement used to kill off some power to the screens, will be a 'Dale'-style; aluminum housing with fins to dissipate heat.

These 'dynamite' style resistors were clamped into place; the 'Dale' will be mounted directly to the chassis for both safety and heat dissipation reasons. In the upper right center of both pics, you can see that the blue resistor on the right is in the process of self-destructing; the housing is separating from the resistive element.

When the resistors arrive, all the passives and terminations on the center rail will be done over which is/are the dropping resistors to adjust the B+ to the values needed by the guitar and bass channels, and the driver board. Out of view to the right is the bias network which will also be completely redone. There is nearly no other way to ensure that the amp will work as well as it did when it left the showroom other than taking in apart and putting it back together. In so doing, all manner of little things show up; for example, slightly burned B+ plate connections at the output tubes and so on.

I have a pair of way-too-expensive, new, 'Gold' Groove Tube 6L6s (#3) backed up by a pair of Sovteks that I got from default. I also have a Tungsram 7247 driver and am in the market for some sweet-sounding 12AX7s to round out the tubes. cj
 

capnjuan

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Thank you John. cj
 

matsickma

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Hey capn,

Where do I pick up a 1500 Ohm, 10W 10% resistor? I need it for the power amp section for a Superstar amp. The type I'm looking for is the ones that are 1 7/8 inch long with a 3/8 inch x 3/8 inch cross section.

Tnx,

M
 

capnjuan

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Hi Mike: try these people, Antique Electronic Supply: http://tubesandmore.com/ On the left, click Resistors, then Wirewound, then 10W, then drop-down window for specific value; they do have 1.5K/10W and ID the dimensions as 1.93" x .39" - possibly a little longer than what you're looking for.

I buy from these people regularly and think they do a good job. There are lots of other sites but these people typically have 90% of the stuff I'm looking for although they don't have any resistors above 10 watts.

cj
 

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capn,

Thanks! That will work.

By the way, I ordered the octal cables you referenced for me. I hope they arive this week.

M
 

capnjuan

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matsickma said:
capn, Thanks! That will work. By the way, I ordered the octal cables you referenced for me. I hope they arive this week. M
Hi Mike: I too would take a shot at a factory-made cable; agree that conductor type/size is a risk but, considering the hassle-factor of home-brewing a replacement ... one worth taking; hope it works. cj
 

capnjuan

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This is the preamp/driver supply section of the Thunderbass amp; the new can caps in on the left and right; pic shows the substitution for the large blue/gray resistors on the left with the whiteish 'bar' and large dark cylinder in the upper center on the right on the right:

Slide3.jpg


This is a close up of the two old resistors out of the amp both showing the damage done by the heat to the insulation of nearby wiring - can be seen in the first pic too.

Slide4.jpg


For whatever reasons, the original resistors were installed first and the old red supplies line run over top and the nearby wiring put too close to the other resistor. When the amp was turned over, the weight of these oversized resistors was enough te deflect their leads and allow them to lay up against insulated wiring....this and other stuff is how come these fail. In any event, to deal with the heat from the new resistors; the 'white box' was mounted on its own wiring strip, with shortened leads, and wire leading away to its connections. This way, when the amp is turned over, the resistor won't droop :evil: The other original was replaced by the large, dark 20K/25W humper at the top right; it needed a piece of custom hardware to make it stand off the deck, stay in place, and not pose further risk.

This is the bias network; group of resistors and and two potentiometers whose functions are to ensure that the tube is properly biased. Because this is a 'fixed bias' and not 'cathode bias' amp, these devices control the cut-off of the output tubes; more on the subject somewhere else some other time but when the bias network is done, then the amp will be powered up, checked, and will look at what's what in the signal stuff on the top of the chassis.
Slide5.jpg


cj
 

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Last week I received capnjuan's 6L6 modded Thunderbass. It sounds amazing. Anyone looking to switch out the 8417s should follow his model as it is one of the best sounding amps I've ever heard. On top of being one hell of a guy, capnjuan is ridiculously knowledgeable. However, if you're reading this thread, I'm sure you already know of his almost freakish ability.
 

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Last week I received capnjuan's 6L6 modded Thunderbass. It sounds amazing. Anyone looking to switch out the 8417s should follow his model as it is one of the best sounding amps I've ever heard. On top of being one hell of a guy, capnjuan is ridiculously knowledgeable. However, if you're reading this thread, I'm sure you already know of his almost freakish ability.
 

capnjuan

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As President Reagan once said "There you go again" :oops: :oops: :oops: The fact is that in the land of the blind, the one-eyed man is king; this applies equally to amps and choosing a good laundary detergent. I switched out the bias resistors shown in the last pic and pretty much thought I had the amp straightened out but never got around to firing it up. When you expressed interest, I turned it on and got a puff of white smoke and the plates in the 6L6s - large vertical metal pieces in the tube - started to 'red-plate', glow red hot.

Needless to say this was a source of some stress :evil: At the time, I was so busy, I made contingent arrangements to ship it to default in Philadelphia and see if he could straighten it out. By the time I got to manning up over it, I found I'd connected the bias caps in backwards. In the image below, the green box shows the filter caps for the bias supply; the red box, some of the primary DC filter caps; notice the difference in the symbols. As a practical matter, all bias voltages are negative so the (-) end of the cap is connected to the supply and the (+) end to ground; the opposite is true for the primary DC which is always a positive voltage. Anyway, I had the bias caps in backwards thereby smoking the 1st filter cap (both bias caps and the dropping resistor have since been replaced) and letting the 6L6 run wild thus the red-plating.

guildbias.jpg


Despite your complimentary remarks, capnjuan (President and Chief Engineer of Juanzamps), stumbles and falls too .... :mrgreen: CJ
 
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