Thunder Star/Bass Amps : what's a 6GF7?

This thread identifies the function of the 6GF7(A) tube in John K's and Rwood's copies of this amp. Assuming there are diodes present as indicated below, then the 6GF7 is a voltage regulator controlling voltage to the 'screens' of the output tubes and, except for the 'driver', the other three pre-amp tubes are 12AX7s. I'd guess that for the remaining working copies of this amp, this gadget contributes significantly to how come people think these amps sound so good.

Listed as footnotes here are links to existing LTG threads on the several models of Guild bass amps. While DKL has several schematics, to my knowledge none includes a 5th apparently 'pre-amp' tube, the "... one in the middle..." per Rwood's earlier post. There are 2 versions w/ 4 tubes in the preamp; one has 6L6 outs, the other 8417s; these tubes have the same pinouts, circuits very similar but apparently no schematic for this particular model. Finally, there is yet another model, also without a schematic, w/ 6550 outputs - like matsickma's and possibly megadan's - and probably with 5-12AX7s, 1-12AU7A,and 1-7591A; more on the 7591 below.

These are pics of the '5th' tube amp; 1st not Rwood's, the others are and regenerated from the footnoted posts:



Tube deck view with pesky little pc boards; must be removed to check voltages or other maintenance - 6GF7 in the approximate center:



Under chassis view:



6GF7(A) - The 'X' tube:




This tube was widely used in older TVs to control vertical deflection and is a twin-triode; similar to the common 12AX7 family. Unlike a 12AX7, the two sections have different voltage-handling characteristics but it's function isn't audio amplification.

The blue square box looks like the diodes/rectifier; if they are small caps to filter RF, then the diodes could be mounted on the strip in the rectangular box. The red square is the '5th tube' / 6GF7; the screen voltage regulator.



The 7591 tube, commonly used for output including the Thunderbird combo amp and found in the in 6550 output model is probably also a voltage regulator for the 'screens'. The 'screens' are metal structures inside the tubes that, well, without overcooking this post, control the tube's operation. Regulators are common in hi-fi audio however most modern gear uses transistors for the application.

And Rwood, if you're out there, the 7247 'driver' is likely the one in the yellow circle - look for the wiring leading from the center/'wiper' leg of the master vol control to the 4.7K input resistor.




Existing LTG Posts:
http://letstalkguild.com/phpBB2/viewtopic.php?f=7&t=80
http://letstalkguild.com/phpBB2/viewtop ... 839&hilit=
http://letstalkguild.com/phpBB2/viewtop ... highlight=

Other bits and pieces:
http://www.diyaudio.com/forums/showthread/t-73176.html
http://www.audioasylum.com/scripts/t.pl ... s&m=185665


cj

This is just excellent info, Cap'n. I'm going to decode my Thunderbass this very evening.

capnjuan said:

And Rwood, if you're out there, the 7247 'driver' is likely the one in the yellow circle - look for the wiring leading from the center/'wiper' leg of the master vol control to the 4.7K input resistor.
cj

Click to expand...

Yes, I am actually right here! And agreed, that's the tube I had with worn-off lettering that tested waaaaaay off the scale on one triode, as a 12AX7, but turned out to be decent as a 7247. I put a new 12DW7 in it's place, a tube which is another designation for the 7247. I am still coming to grips with a phase inverter that has mismatched triodes, but it's all coming together slowly.

Thanks for the clarification on the 6GF7, and it is connected to the screen grids of the power tubes so 'voltage regulation' does make sense.

The serial numbers, btw, on the two examples that I have seen that use 6GF7As, are 6126 and 6165. Manufacturing date on the later is Feb 1968. It belongs to Dave C., a lifelong friend for whom I am hoping to get it up and running again.

Great info,


RWood

S/N on my Thunderbass is 7119, after RWoods, but with no 6GF7 tube. If the 6GF7 was in the earllier versions, and was responsible for such excellent tone, whay would it have been dropped later?

Also, I do have 4 x 12 AX7, not 3 x 12AX7 and one 7247 (or equivalent). What difference might I expect when I swap out the 12AX7 for the right tube?

Hey capn,

Your a hard guy to keep up on with your prolific number of posts!

I hope to add a few comments before the Labor Day holiday is over.

It looks to me that the Thunderbass amp shown in your first pictures is an earlier model Quantum. There is a little metal label located bottom center on the front of the amp. Typically that label says "Quantum Bass Amp" or similar.
That may be the reason there is a difference between the shown amp and John K's amp. Guild used the same front panels for both the Thunderbass and "Thunderbass" Quantum. Typical Guild efficiency.

M

matsickma said:

Your a hard guy to keep up on with your prolific number of posts! Hi Mike: it's easier for me, I'm self-employed and mostly office-bound, I suspect you're more of a road warrior, no?

I hope to add a few comments before the Labor Day holiday is over. Please do; I've considered taking a shot with one of these but there seems to be an awful lot of copies of them sitting on people's benches ...

It looks to me that the Thunderbass amp shown in your first pictures is an earlier model Quantum. There is a little metal label located bottom center on the front of the amp. Typically that label says "Quantum Bass Amp" or similar. Thank you; saw the little tag but couldn't get the resolution up high enough to read it.

That may be the reason there is a difference between the shown amp and John K's amp. Guild used the same front panels for both the Thunderbass and "Thunderbass" Quantum. Typical Guild efficiency. With John K's post, I had come to much the same conclusion and that's a 10-4 on Guild labels and model names too thank you: Thunder Bass / Star / StarBass / Quantum / Thunder this / Thunder that / Double Extra Thunder ... :evil: ) M

Click to expand...

You do have a 6550 model, don't you? What is it called? Dumb question of the year coming: have a schematic for it? Beauty pics of its insides?

Best wishes, cj

john_kidder said:

S/N on my Thunderbass is 7119, after RWoods, but with no 6GF7 tube. If the 6GF7 was in the earllier versions, and was responsible for such excellent tone, whay would it have been dropped later?

Also, I do have 4 x 12 AX7, not 3 x 12AX7 and one 7247 (or equivalent). What difference might I expect when I swap out the 12AX7 for the right tube?

Click to expand...

Speculating, the differential in tone quality might not have translated into better sales and / or the number of 'come-backers' might not have justified the additional cost. Alternatively, it might not have worked as well as they wanted and no one wanted to screw with de-bugging it. Further, this amp and all tube amps were losing market share to transistor amps. For bass and for continous high volume, the transistor is more rugged, doesn't need an expensive, heavy, impedance-matching transformer, and produces accurate fast notes although, IMO, at the expense of tone quality.

Some theory: the regulator's function is to 'stiffen' the operating characteristics of the output tube and to compensate in part for 'voltage sag' which can be heard in older, 'brown' 6V6 amps like mine. Strike a chord hard at moderate volume and, as the signal tails off, you can hear the 'waaoowaaoowaaoo', wailing, tremolo-like sound: no change in pitch, only amplitude. In older amps it's a trade-off; if you want the 'brown' thing, you get the rest of it for free. In that amplifying bass requires a lot more energy from the power supply, I'd guess that without a regulator, the notes would loss some 'edge' or distinction and the amp would have some the short-comings normally attributed to tube amps; a wooly or tubby sound particularly in the bass.

Do you have tube deck / chassis pics of your amp? Does it look just like Rwood's? Reading Mike's post, you may have yet another version of this amp. :evil: Given Mike's remark, I'm confused (again) about which model you have; the tube count/type doesn't match. Otherwise, the only other explanation and if you have the '...little tube in the middle...' is that someone stuck a 12AX7/12AU7 where the 6GF7 should be; same 9-pin socket, same dual-triode, but w/ different voltage handling capacity and heater supplies - the 6FG7 uses 6 volts on the heaters.

If you have a 'tube in the middle' model and you put a 6GF7 there, my guess is that you'd get the same tone quality but the note would be drier, clearer, with better definition and maybe better sustain which, in a bass amp, would be an especailly good thing. The 4-tube preamp models (both 6L6 and 8417) use a 7247 as the 'driver' feeding the output tubes. Since in Rwood's pic, there is a pre-amp tube proximate to the output tubes; if you have the same model, it's a fairly safe assumption that that is where the 7247 goes.

cj

RWood said:

....for whom I am hoping to get it up and running again. RWood

Click to expand...

Glad it helped and hope you get yours running. I wouldn't mind taking a shot with one of these but it'd be encouraging, given the lack of schematics, if there were a few more running copies around - that is - more evidence that they can be rehabbed and work reliably; I'm prepared take the tone for granted.

cj

capnjuan said:

the only other explanation and if you have the '...little tube in the middle...' is that someone stuck a 12AX7/12AU7 where the 6GF7 should be
cj

Click to expand...

John,

Just a quick note about the 6GF7........ while is is a nine-pin socket, it is not a miniature like a 12AX7. It is the Novar (9 pin compactron) size. It's a T9 bulb.

So there would be no way to plug a 12AX7 (or similar size) into it's socket.

According to Tube Asylum, the 6GF7 is a nine-pin version of a 6EM7 which is also a vertical deflection oscillator but in an octal format.

Ain't this fun?


RWood

RWood said:

...is a nine-pin socket, it is not a miniature ... no way to plug an AX7 into it's socket. Dang I was hoping there was an explanation for John K's tube type / count other than yet another model.... :evil:

According to Tube Asylum, the 6GF7 is a nine-pin version of a 6EM7 which is also a vertical deflection oscillator but in an octal format. Ain't this fun? Yes but only in a limited sense of the word :lol: RWood

Click to expand...

Thank you Richard - we may yet find out how these things work!

Regards cj

john_kidder said:

I do have 4 x 12 AX7, not 3 x 12AX7 and one 7247 (or equivalent). What difference might I expect when I swap out the 12AX7 for the right tube?

Click to expand...

The 7247 is a dissimilar pair of triodes, one of which is identical to a 12AX7 and other of which is a 12AU7. The 12AU7 has a lower amplification factor, so the gain characteristics will be different. This might be a good thing, at least as the Phase Inverter in the Guild amp. (and as Cap'n pointed out, this is the socket closest to the power tubes).

Here is a discussion of the two tubes (keep in mind that 12DW7 is a direct replacement for 7247, much the same way 12AX7 replaces 7025. There may have been quality / microphonic differences back in the day, but they are interchangeable now).

RWood

I may have misled with my earlier post.

My Thunderbass has 2 x 6L6 outputs, and (now) 4 x 12AX7 pre-amp tubes. There is no socket for the 6GF7 shown in Capnjuan's Quantum.

The original configuration had, I now believe, a 7247 (or equivalent) in place of the last 12AX7. If these tubes are functionally different, how is it that my amp is working at all? Or am I getting just a fraction of the performance I should expect? And, if so, does the 12AX7 warrant replacement with a 7247?

john_kidder said:

My Thunderbass has 2 x 6L6 outputs, and (now) 4 x 12AX7 pre-amp tubes. There is no socket for the 6GF7 shown in Capnjuan's Quantum. Okay; thank you, now I know which amp you have and have a schematic for it!
The original configuration had, I now believe, a 7247 (or equivalent) in place of the last 12AX7. If these tubes are functionally different, how is it that my amp is working at all? Because they are more alike than they are different with the 12AX7 'faking it' as a 7247. Both twin triodes and can do the same thing - increase signal strength and act as phase inverter. The distinction is that each 1/2 of the 7247 has different voltage handling characteristics; the 'front' 1/2 is the equivalent of a 12AX7 and the 'back', facing the outputs, runs at a higher voltage.
Or am I getting just a fraction of the performance I should expect? And, if so, does the 12AX7 warrant replacement with a 7247? See below.

Click to expand...

This is the 7247 'driver' from the 4-tube preamp amp: same as yours - schematic courtesy of Rwood. Notice that the 1/2 receiving the input (visual left) is being fed by a 100K - 1/2 watt resistor; nearly every 12AX7 doing signal-handling work has a 50K to 150K resistor hanging on the plate (anode: where the B+ appears). Notice that the other half - visual right - has a (low in relation to typical 12AX7) 15K resistor / 2 watt high heat dissipation resistor tied to it's plate.



Because that 15K resistor is so low, more B+ is appearing on the plate (anode), it's running at a higher voltage than the other half of the tube, and the dropping resistor is rated at 2W to dissipate the heat. The driver is being supplied 330V; that voltage is 'dropped' through the two resistors into each tube half; the 1/2 offering more resistance operates at a lower voltage and vice versa.

If you don't switch out the 12AX7 for the 7247 with it's more rugged voltage handling, you'll un-necessarily shorten the life of the 12AX7 and risk burning it out. Worst case, something uglier. Not sure about tone but I'd think you'd get an uptick in gain, brighter, sharper/crisper notes.


Chatter from an older post here re/ 12AX7 and similar re/ gain factor.
http://www.letstalkguild.com/phpBB2/viewtopic.php?f=7&t=1285

cj

Great help from both of you. Lucky I'm a reasonably quick reader - you do fill in the blanks very thoroughly, and with great attention to necessary detail.

I'll definitely swap out the 12AX7, and report.

Cheers

Glad to help; also interested in seeing whether anyone can get any one of the three amps I know of running well ... that's code for 'whether they are worth the trouble'. You play guitar / not bass through yours, no?

cj

New forum looks nice!



I believe, but I could be wrong, that I replaced the tube we are talking about with a regular 12AX7 when I retubed my Thunderbass/Quantum.

I haven't had any problems with it.

Hi Megadan, welcome back! Interesting; not that I looked closely but we'd been assuming, without a schematic, that the 6GF7 was a screen voltage regulator - and not directly a part of the audio chain. Also didn't do alot of research into the 6GF7/Quantum because they are rare - I did dig out the old pics of your amp for purposes of the thread.

Anyway, sort of surprised that the 12AX7 works - if that's where it went in. John K, who has a 8417 version of the Thunderbass (so do I now except it will be modded to 6L6 operation) said he'd put a 12AX7 in the place of the 7247/12DW7 driver and amp worked; any chance you did the same; that is, replaced the tube nearest the outputs with the 12AX7?

Has your amp been recapped? Did you do it? Anymore pics of it? Whattya think of it? Play it often?

cj

capnjuan said:

Hi Megadan, welcome back! Interesting; not that I looked closely but we'd been assuming, without a schematic, that the 6GF7 was a screen voltage regulator - and not directly a part of the audio chain. Also didn't do alot of research into the 6GF7/Quantum because they are rare - I did dig out the old pics of your amp for purposes of the thread.

Anyway, sort of surprised that the 12AX7 works - if that's where it went in. John K, who has a 8417 version of the Thunderbass (so do I now except it will be modded to 6L6 operation) said he'd put a 12AX7 in the place of the 7247/12DW7 driver and amp worked; any chance you did the same; that is, replaced the tube nearest the outputs with the 12AX7?

Click to expand...

Thanks!

You might be correct about the tube - I didn't even realize there was a 7247 until I had all ready ordered the tubes, assuming it was an 12AX7. So that is the tube I replaced I believe. But let see.. where is the tube order....

Ah!

Here it is.

I ordered and used:

2 x SV-6550C
1 x TE-7591
1 x 12AU7A
5 x 12AX7



So, that is what I replaced the tubes in the amp with it...
Let me see if I can get the old pics and make a little diagram...

capnjuan said:

Has your amp been recapped? Did you do it? Anymore pics of it? Whattya think of it? Play it often?
cj

Click to expand...

No recap - it needs to be done, but I adjusted the hum balance, and it was basically dead quiet for a 37 year old tube amp! It's at the jam space now but next time it's home I will try and get more pics.

I play it all the time, it's my main amp. I play bass in a pretty loud band. I run it into an Avatar 2x12 and an Avatar 2x10 cab, with either a MM Stringray or a '74 Ric 4001 (my new baby!)

megadan said:

I ordered and used: 2 x SV-6550C, 1 x TE-7591, 1 x 12AU7A, 1 x 12AX7

Click to expand...

I don't think this bunch has a schematic for the Quantum; maybe matsickma would recognize but the 7591 might be the screen regulator in your model and all I can tell you is that the earlier models of this all used 7247s as drivers; doesn't mean your does too - FYI John K put a 12AX7 where it looks like he should have a 7247 and no trouble .... yet.. ha ha ....love ya John ... it's just that the 2nd half of the 7247 has a very high voltage on the plate; higher than most 12AX7s.

megadan said:

No recap - it needs to be done, but I adjusted the hum balance, and it was basically dead quiet for a 37 year old tube amp! It's at the jam space now but next time it's home I will try and get more pics. I play it all the time, it's my main amp. I play bass in a pretty loud band. I run it into an Avatar 2x12 and an Avatar 2x10 cab, with either a MM Stringray or a '74 Ric 4001 (my new baby!)

Click to expand...

Another BBer or two are messing with power supplies in this vintage although not the Quantum model. Yes, loud - looks like alot of fun!

Ah, I was wrong again.

When I realized the tube in there was NOT a 12AX7, I left the old tube in.

I identified it at one point, but now I've totally forgetten! It was all numbers and it was very rare, there weren't any avaliable directly anymore.

Here's a diagram: