NEWBIE with an odd Thunderbass/Quantam

[Sunn Man]

Hi all,I'm a newbie on this forum and need some help with a Guild amp.I have a Quantam amp in a Thunderbass frame/cabinet,s/n 01170,have schematics for a 7 tube and an 8 tube version,but mine is a 9 tube version with some varations.It arrived with 6550's for outputs,was expecting 8417's,has an extra tube on the power amp chassis,12AX7A or 12DW7 (7247) and the 300v regulated screen supply doesn't have a 9 pin socket and a 6GF7A,(dual dissimilar triode),it has an octal socket and a 7591,(output pentode).I've exhausted all my sources for info and can't find anything.Desperately
looking for a schematic or any kind of info about this version.In exchange,I'm willing to share my 40 plus years of bass playing and audio electronics knowledge,everything from tube and ss amps to live sound and spkr. reconing with anyone on this forum,willing to help anyone,anytime with anything I can.Currently playing in 3 bands and running a repair shop out of my home.I played a Guild Starfire in 75',wish I had it now.Looking forward to hearing from someone,thanks in advance,Mel.

 

[capnjuan]

Hi Sunn Man and welcome. As a courtesy, FMIC gave LTG all the schematics it had ... but that group doesn't include the very rare 6550 model. If you'll click the PM button under my post and send me your email address, I'll send you what I have; not sure they'll help but they're yours for the asking.

 

[Sunn Man]

Thanks for such a rapid response capnjuan !! I'll get a pm off to you,thanks again,Mel.

 

[capnjuan]

Hi Mel; this is the only 6550 model I know of:

Our member matsickma is pretty good with these later, higher-powered amps; he might have some of the advertising literature on your model. I don't think the one in the pic has a 7591 regulator though ... In any event, I hope the schematics help.

 

[Sunn Man]

Hi John,yes that's the beast !! Thanks for the schematics,I'll go over them later today,I've gotten more info on this amp in the last 10 minutes than the last 6 mos.,thanks again,Mel.

 

[capnjuan]

With the start of the Thunderbass line, Guild fastened on two things; a common power supply design and the use of a 7247 / 12DW7 for double duty as the final gain stage / phase inverter. As you know, it's half of a 12AX7 for the gain stage and half 12AU7 for the phase inverter side.

Because the socket will accept it, a lot of people put 12AX7s there and maybe for casual use, that would work but most of the big-boy Guild designs dump something like 300+ volts on the PI / 12AU7 plate trading gain for rugged performance. Anyway, if Guild had wanted a 12AX7 there, they would have put one there.

Good luck with your amp; pics appreciated when you get the time.

 

[matsickma]

I beleive the amp posted by Capn is an early model 6550 Quantum. The early models have a Preamp out located on the left side of the chassis as shown in Capn't picture. If you were able to read the label I think it would say "preamp output" or something to that effect. The later 6550 models has and AUX output which is the output from the transformer.

Also the 6550 model does use a 7591A tube. I assumed it was used in some kind of pre-driver mode to push the 6550's. If you look very carefully at the large 6550 tube in the middle of the amp in Capn's picture , and look slightly to the right of it you will see a 12AX7 and slightly to the right of the 12AX7 it is the 7591A. It is farther back in the amp so it doesn't look as tall as you might expect.

This is a really loud amp. A pair 6L6's isn't anywhere near its league. It is 100W RMS kind of power.

Capn, How high of a voltage would you have to run a pair of 6550 to get 100W RMS? I suspect these are biased at pretty high levels.

M

 

[Default]

SVT's are 200 watts for six 8550s at upwards of 600 volts.

 

[capnjuan]

... Also the 6550 model does use a 7591A tube ... and slightly to the right of the 12AX7 it is the 7591A. It is farther back in the amp so it doesn't look as tall as you might expect....

This is a really loud amp. A pair 6L6's isn't anywhere near its league. It is 100W RMS kind of power.

Capn, How high of a voltage would you have to run a pair of 6550 to get 100W RMS? I suspect these are biased at pretty high levels.

Hi Mike; yes, I see the 7591 ... a bigger boy where a smaller boy would be expected and yes, I would have thought it'd be closer to the output tubes. I wish I'd save those pics of that EL34-driven Guild PA ... about the only other Guild amp in this power class?

tube data source 6550/KT88:

Push Pull Class AB1 Amplifier
Plate Voltage ................................... 600 V
Grid No. 2 Voltage ............................. 300 V
Grid No. 1 Voltage ........................... -16.5 V
Power Output (approx) ....................... 100 W

 

[matsickma]

Good info Capn and Default.

Another amp that was rated at 100W RMS , 200W P was the Superbird II. Both the Superbird I and II use a pair of 8417 finals but they are rated at different power level. I think you have the marketing sheet that indicates the specs and also the schematics. Maybe if you look closly at the poweramp section of the schematics you will find alternate components for voltage selection.

M

 

[capnjuan]

Hi Mike: I think this is most of the pieces:

Superbird I and II output section, both used 8417s:

Superbird I:

Power supply, 410V primary:

Push Pull Class AB1 Amplifier*
Plate Voltage ...................................... 400 V
Grid No. 2 Voltage ................................ 275 V
Grid No. 1 Voltage ................................ -13 V
Power Output (approx) ........................... 65 W


Superbird II:

Power supply: (having a schematic and being able to read it are two different things) ... looks like 560V:

Push Pull Class AB1 Amplifier*
Plate Voltage ...................................... 560 V
Grid No. 2 Voltage ................................ 300 V
Grid No. 1 Voltage .............................. -15.5 V
Power Output (approx) ......................... 100 W


Advertised at 120 watts ... not 120 watts peak ... just 120 watts which makes sense.

I've never found any web material discussing the end of the 8417 but it apparently had an attractive power-to-weight ratio; high output at low/moderate voltages. I wonder if too many of them went south before they could get the bugs worked out. Not seen in many other makes of the period ... before or after.


*8417 Tube Data

 

[Sunn Man]

Hi there,I've come accross 8417's in some Bogen amps,also the SG series of amps by Gibson,tube output and P.I.,solid state front end.I saved the tranny's out of one of the SG's that I scrapped thinking I'll use them in an amp some day.They used a pair of 8417's and rated the amps at 100 wrms,Mel.

 

[capnjuan]

Hi Mel; the 8417s seem to have come and gone. Even used, they're hard to find. The Thunderbass series and the Superbirds both have individual bias pots ... a nice touch that helps.

 

[Default]

When I measure the voltage on John Kidder's Quantum, it was pushing 529v. It was also biased extremely hot, so that voltage was a little low.

No wonder he can't play it in the house. :shock:

 

[capnjuan]

Hi Steve: on the two partial power supply schematics above for the Sbird I and II, just above the symbol for the power transformer, each has its own (smeary) part #. After changing my Tbass head from 8417s to 6L6s, I had same primary B+ because (slapping self on side of head), that transformer was intended for use with the more efficient 8417 that converted 400 volts DC into 50 watts. At some point, Guild started using 6L6s in the Tbass line and needed a different transformer to supply 450V to get the same 50 watts because the 6L6 was less/more efficient; less current but more voltage.

John's amp is a Quantum (Tbass) and Sunn Man's amp is a Superbird I. It would have been reasonable to infer that since the Quantum was the penultimate TBass and it ran 8417s, that John's Quantum would have been arranged like my Tbass but apparently John's Quantum (or at least its power transformer) is the same transformer found in the Superbird I. Of course, this is a theory ... if we had more facts ....

It's also true that we had some ham, we could have ham sandwiches.

The now-and-then 8417: Link to Quicksilver Audio Amplifiers a maker of higher-end stereo amps formerly using the 8417. The discussion makes no mention of 8417 applications prior to their models ... maybe one of them used to work at Guild? (nyuck nyuck) and some Weber BB 8417chatter. Interesting without being informative.

Yours for no more linearity than absolutely necessary, J