Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ALBATROS1234 on April 07, 2018, 08:47:46 am
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i want to try my first clone. gonna build a supro spectator. the transformer i have came from a revere t-100 , its spec as measure with no load are 362-0-362 and 6.7v. the r-t-r used a 6x5 rectifier but the spectator used a 5y3. i want to do this amp but using the 6x5 but is it ok to use this single 6volt winding to power the rectifier and my heaters on 2 tubes?i assume its ok because the r-t-r did it but the schematic is kinda confusing (see below) what adaptaions must i make to the spectator schematic (also see below)
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i guess what i am asking is if i wire it up as per the spectator schematic where do i branch off to supply my heaters is it as simple as just paralleling the 2 heater pins of the 6x5 with the other 2 or do i have to put a cap or something to block high voltage from getting to the heaters? i do also have a 5y3 should i use that even though the voltage is to high for the heater(6v vs 5v)? or should i hook up a second filament xfrmr to supply my heaters and let the big guy just run the 6x6 and plate voltages?
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also upon closer examination i noticed the supro comet is the same circuit with a couple of changes(see below) on the comet the tone is after the second triode stage as well as when the second triode gets its hv feed there is a branch going to ground with a 500pf cap . what is the purpose of this cap and ground path? i get that the comet type post tone will not be as interactive as the spectator because its not directly connected and the fact that its post triode should make it have a greater overall effect but i dont get the cap to ground at tiode plate 2.
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If you use the 6X5 just connect it's filaments parallel to the other tubes 6.3v filaments. If you use a 5Y3 you cannot connect it to the 6.3v filament string because the 5Y3 filament and cathode are the same element and there will be B+ voltage present on the heater. You must use a separate 5V filament winding with a 5Y3. No exceptions!
The 500pF cap is a high frequency snubber. It sends high freq signals such as hiss to ground. Some people call it a smoothing cap.
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i want to do this amp but using the 6x5 but is it ok to use this single 6volt winding to power the rectifier and my heaters on 2 tubes?i assume its ok because the r-t-r did it
The criteria is the total current draw of all the filaments. To spec out the filament supply portion of the PT, go to the tube charts and find the current draw of the filaments of the tubes it's feeding in the stock schematic. Add that up = current handling capacity of the PT's filament supply. Then do the same to calculate the current draw of the tubes you plan to use.
If the new current draw is lower, the filament supply voltage may be higher -- due to Ohm's Law & the poor voltage regulation of trannies. Use resistors or diodes to drop the voltage, if needed.
If the new current draw is higher, the filament supply voltage may drop. That's OK down to about 5.3V. But the tranny may run hotter. In the unlikely event that this poses a problem you can add a supplemental filament tranny for all or part of the heater supply.
500pF cap: I think sometimes we see them bypassing the plate resistor to similar effect.
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Thanks guys for the answers. The power transformer was running that very 6v6 and 6x5 in addition it also had a 6k6, 6sj7 and 2 6j7 types. I will be using only 1 6sl7 with the 6v6 and 6x5 so I am sure there is plenty of headroom for the transformer.
So opinions on the superior schematic of the 2 ?
1. Spectator volume and tone between the 2 triode stages.
2. Comet volume after 1st triode tone after 2nd triode plus snubber/smoothing cap on plate 2 of the 6sl7.
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The one with the most components is clearly superior.
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lol ....so the comet has 1 extra (smoothing)cap so its superior. i just meant did you guys have an opinion on whether the volume and tone in the spectator sandwiched btw the 2 triode stages would be better or the post triode tone with the smoothing cap was a better engineered solution. my uneducated guess would be that the 1st one would be better because i have read tone stacks rob signal although one simple tone knob might not matter so much also i have never used the smoothing cap so maybe regardless i should try it with and without to see if it made much difference.
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I've never heard of either of those amps. Since you want to build one, just do it. It will be pretty easy to move the tone control around to see which you prefer. Let us know what you think.
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you got it man, not sure the exact date on the comet but the spectator seems to have come around about 1950. its a seems to be valco/supros version of a champ(chicago champ?) valco called it a model 510. i saw some you tube videos and liked the tone. they sound a bit different that a champ which is ok sounding but the old valco/supro/national stuff from the late 40s thru the mid 60s are the tone i like alot, more grainy/raw throaty than a fender type imho. i am going to try and do a push pull supro not sure which model thought i might throw together a spectator too first because its smpler and will allow me to get my feet wet with tube rectification.
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Probably a good idea to keep your feet dry with tube rectifiers.
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i will try and remember that buddy.