Just got a Thunderbass Head, couple questions

[markjazzbassist]

I just bought a Guild THunderbass head and i can't wait to get a cab and use it with my bass. I have a couple questions though. I've read that the polarity switch is not to be messed with if you have a 2 prong cord, i do, mine's the original cord. which way should i hae the polarity switch set? it's currently in the UP position.

also what year is mine, the serial number is 6794. it has 8417 power tubes in it, 2 of them.

here's some pics

 

[capnjuan]

Hi Mark; welcome to LTG and congratulations on your new amp. The polarity switch is left over from the era when commercial and household wiring was pretty much a 2-wire system; a hot and a common. In the pic below, the Power switch interrupts both of the incoming 110V lines; the hot and common. It's followed by the Polarity switch which connects one 110V line to a capacitor ... or the other 110V line to the capacitor. Back in the day, this was to keep from having to unplug the amp and plug it in 'the other way' to abate AC hum.

You can put the switch in either position ... might or might not have any effect on AC hum. If the caps are old and just by listening, you might not be able to distinguish AC hum from DC but that's why you have a tech.

You need to get that cord upgraded to a 3-wire version and have that capacitor disconnected. The capacitor can fail two ways: 'open' ... it doesn't do anything or it can fail 'short' creating a path to the chassis for the incoming power. Since your instruments get their ground from the chassis, the risk is that the AC voltage will travel up your instrument cable to your guitar ... There's a reason they call that the 'death cap'. Anyway, it needs to go.

Because Guild amps didn't enjoy the success of other makes, there doesn't seem to be any way to match serial numbers to dates of manufacture although '66 - '69 is probably a fair guess. Yours is the much rarer 8417 model. Guild adverlit for your model; although the text says 6L6s / 50 watts, quite a number of them were like yours ... 8417s / 100 watts:.

You're going to find that finding backup or replacement 8417s is a spendy proposition; for example, $75/ea :shock: The 8417 has the rare ability of to use less DC voltage to generate 100 watts than a 6L6 uses to generate 50 watts ... that's also on my list of things I don't understand. The good news is that the amp has independent bias pots for each tube so you don't need matched pairs ... which is good ... since you can't buy matched pairs

I had one of these but I converted it to a 6L6 amp; all the ugga-bugga you see in the pics is mostly the effort to get all new power supply capacitors in it ... the 'conversion' involves only a few little pieces and parts LTG thread link.

So .... good luck with your amp, welcome, and I hope somebody can help you out with the speaker cabinet!

 

[markjazzbassist]

thanks a lot man. i will find a tech ASAP and get that power cord converted, the previous owner said the 8417 tubes were fine, so i'll ride them out as long as i can, i'd prefer the higher wattage actually, so $75 a pop isn't too bad, i'm used to retubing old ampeg or orange amps at $200-300 (4 6550 or KT88) so $150 is cheap

thanks again.

 

[capnjuan]

You're welcome Mark. I have the schematic for the amp; send me a PM with your email address, I'll send it to you. Congratulations again on the amp; the model I renovated produced a wall of warm, clean, glassy tone ... it was impressive.

 

[taabru45]

John, I do get an education from you..DC hum....hmmmm....I thought AC was the major problem there...and smart to remind people about the 2 prong plugs...When I was a kid, my friend turned on the small amp while holding a microphone, he was suddenly throwing the microphone from one hand to another, it took me more than a couple of seconds to get to it and turn it off..... I was laughing so hard...while not knowing the actual peril my friend was in....I like how God tolerates fools....Steffan

 

[capnjuan]

... DC hum ....hmmmm.... I thought AC was the major problem there

Hi Steffan; no ... hum comes in a lot of different flavors including DC.

 

[taabru45]

Around here it also comes in the key of eh! Steffan :wink:

 

[capnjuan]

Around here it also comes in the key of eh! Steffan :wink:

Only in Canuckistan :wink:

 

[Default]

Guild did some forward-thinking engineering when it came to their amps. Beside the individual bias adjustments for each power tube, they also fed filtered dc directly from the power supply to the tubes, in addition to the 6.3vac supplied by the transformer. Very cool stuff!