1948 Supro..? My project....
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Skallywampus
- Posts: 2
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1948 Supro..? My project....
:BangHead:150 hours in to my first amp project. I’m told amp has lower volume than designed..well..how to tell? No published dB info..just experienced ear gauging? Also the 6sl7 second triode is overdriven by the first stage at 1/2 volume causing distortion which the parallel 6v6’s faithfully amplify. How much distortion is ok?....is it all subjective? Here’s my drawn schematic...do you see any obvious problems? Thanks from a noob!
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Stevem
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Re: 1948 Supro..? My project....
Something's not right here!
Your schematic shows 278 volts on the OT input/ primary, yet you show 297 on the plates of the output tubes and that's impossible if the amp where at idle, so please double check your readings so we can help you better.
If you are taking voltage readings at idle and at max output then that might explain the difference you have posted here.
In short all of these small underbuilt ( inexpensive) amps need to have a 270 ohm 2 watt resistor added in series to feed each pin 4 on the output tubes to help to bias them more stable.
This amp will output no more then 7 to 10 watts with two parallel single ended 6V6s , voltage wise with a meter set to AC volts across the 4 ohm speaker you would read anywhere from like 5.5 vac to 6.4 vac in terms of the 7 to 10 watt range I posted up here.
Output wattage is volts X Volts divided by your speakers resistance which is likely 3.2 to 3.4 ohms.
Also preamp wise on your first gain stage you need a 4 to 5.6 meg resistor that your missing to ground off of the input grid pin 1 so in effect the tube is running with no ground reference, not good for sound, but it will not harm the tube.
You can find a good sounding preamp schematic that uses a 6SL7 over at the schematic Heaven site under the Ampeg listings, the R12RB 1966 model.
Your schematic shows 278 volts on the OT input/ primary, yet you show 297 on the plates of the output tubes and that's impossible if the amp where at idle, so please double check your readings so we can help you better.
If you are taking voltage readings at idle and at max output then that might explain the difference you have posted here.
In short all of these small underbuilt ( inexpensive) amps need to have a 270 ohm 2 watt resistor added in series to feed each pin 4 on the output tubes to help to bias them more stable.
This amp will output no more then 7 to 10 watts with two parallel single ended 6V6s , voltage wise with a meter set to AC volts across the 4 ohm speaker you would read anywhere from like 5.5 vac to 6.4 vac in terms of the 7 to 10 watt range I posted up here.
Output wattage is volts X Volts divided by your speakers resistance which is likely 3.2 to 3.4 ohms.
Also preamp wise on your first gain stage you need a 4 to 5.6 meg resistor that your missing to ground off of the input grid pin 1 so in effect the tube is running with no ground reference, not good for sound, but it will not harm the tube.
You can find a good sounding preamp schematic that uses a 6SL7 over at the schematic Heaven site under the Ampeg listings, the R12RB 1966 model.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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statorvane
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Re: 1948 Supro..? My project....
The way the power amp is drawn, you know the 100K grid leak is shared for both 6V6s right?
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Skallywampus
- Posts: 2
- Joined: Wed Jul 29, 2020 8:56 pm
Re: 1948 Supro..? My project....
Here’s the latest..voltage measurements yesterday...and a table of expected normal operation of 6v6 got tubes to compare...
Here’s the thing..this schematic was the one Supro manufactured and sold. I shouldn’t need to do bypass caps....more grid leaks..etc to make it sound good right?.....let’s go original. I’m ok with that. So..here’s my measurements for perusal....starts to distort at 1/4 volume and overall volume is reduced....what do you think is wrong? I measure 2 volts AC throughout the amp....wouldn’t that distort my guitar signal and contribute to distortion in my signal?....shouldn’t the filter cap take that out?.....still looking for solutions for a novice “ampaholic”!
Here’s the thing..this schematic was the one Supro manufactured and sold. I shouldn’t need to do bypass caps....more grid leaks..etc to make it sound good right?.....let’s go original. I’m ok with that. So..here’s my measurements for perusal....starts to distort at 1/4 volume and overall volume is reduced....what do you think is wrong? I measure 2 volts AC throughout the amp....wouldn’t that distort my guitar signal and contribute to distortion in my signal?....shouldn’t the filter cap take that out?.....still looking for solutions for a novice “ampaholic”!
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Re: 1948 Supro..? My project....
"Sound good" is very subjective. Did you listen to/play the original? My experience with most amps of that era (pre 1950) is that they were "cool" but most didn't sound very good.Skallywampus wrote: ↑Fri Aug 07, 2020 11:10 pm Here’s the thing..this schematic was the one Supro manufactured and sold. I shouldn’t need to do bypass caps....more grid leaks..etc to make it sound good right?
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