I got in one of these due to an issue of cutting out.
That part will be an easy fix, just two new preamp tube sockets.
It’s got another problem though, it only outputs 11 watts rms and 12.5 or so peak into a 8 .5 ohm load.
The OT I think looks original and the voltages check good I think for this early model with a recto tube.
Here’s a bunch of photos and tomorrow I will post up voltages for those of you who might want to have for reference.
The original can cap has been replaced, but that’s it.
Interestingly the last filter node for the preamp section only has a 1 uf filter, which I don’t think is the case with the later SS recto version.
Under the hood of a 1965 Supro Thunderbolt.
Moderators: pompeiisneaks, Colossal
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Stevem
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Under the hood of a 1965 Supro Thunderbolt.
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
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!
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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Littlewyan
- Posts: 1944
- Joined: Thu Sep 12, 2013 6:50 pm
- Location: UK
Re: Under the hood of a 1965 Supro Thunderbolt.
Are both 6L6s drawing current?
Re: Under the hood of a 1965 Supro Thunderbolt.
And what are the voltages? Cathode or fixed bias?
I build and repair tube amps. http://amps.monkeymatic.com
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Stevem
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Re: Under the hood of a 1965 Supro Thunderbolt.
Here's some voltages
All the tubes test very strong.
Ac in 117.3
Unloaded ac out 730, loaded at idle 702 vac.
Heater to recto unloaded 5.24, Loaded 4.97
Main heater 6.67 unloaded. Loaded 6.3
The following voltages are loaded.
B+ first node 380
2nd node 374
3rd node. 324
4th node 258
Cathode voltage on outputs at idle 28.48
V1 plate 255
V1 cathode 2.42
V2A plate 193.8
V2A cathode 1.98
V2B plate 153
V2B cathode 1.38
The output wave form looks good.
I did a quick check of the OT by pumping ac in and I am seeing a impedence of about 5200 ohms.
I need to do this test again because I could not find my normal test rig that I made to pump in a stable .5 vac on the OTs primary so take that impedence number with a grain of salt for now.
I have to check again but resistance test wise on the OT primary I saw 75 ohms and 87 ohms.
Once I get this working normal again I will test it out with a 5AR4 for the recto and I will report back with those voltages, but at that time I will add another 50 ohm to current 200 ohm output tube cathode resistor.
One thing a did notice last night was that whoever put in the new JJ multi section power supply filter can crushed it pretty well near bottom from over tightening the band clamp.
All the tubes test very strong.
Ac in 117.3
Unloaded ac out 730, loaded at idle 702 vac.
Heater to recto unloaded 5.24, Loaded 4.97
Main heater 6.67 unloaded. Loaded 6.3
The following voltages are loaded.
B+ first node 380
2nd node 374
3rd node. 324
4th node 258
Cathode voltage on outputs at idle 28.48
V1 plate 255
V1 cathode 2.42
V2A plate 193.8
V2A cathode 1.98
V2B plate 153
V2B cathode 1.38
The output wave form looks good.
I did a quick check of the OT by pumping ac in and I am seeing a impedence of about 5200 ohms.
I need to do this test again because I could not find my normal test rig that I made to pump in a stable .5 vac on the OTs primary so take that impedence number with a grain of salt for now.
I have to check again but resistance test wise on the OT primary I saw 75 ohms and 87 ohms.
Once I get this working normal again I will test it out with a 5AR4 for the recto and I will report back with those voltages, but at that time I will add another 50 ohm to current 200 ohm output tube cathode resistor.
One thing a did notice last night was that whoever put in the new JJ multi section power supply filter can crushed it pretty well near bottom from over tightening the band clamp.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
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!
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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Littlewyan
- Posts: 1944
- Joined: Thu Sep 12, 2013 6:50 pm
- Location: UK
Re: Under the hood of a 1965 Supro Thunderbolt.
What is the bias of the output valves? And what is their plate and screen voltages?
When you tested for output did you increase the input signal until just before clipping? And have you tried sending the signal into the grid of V2a to bypass the preamp stage and controls? And what if you do push the signal into clipping have you seen what stage clips first?
When you tested for output did you increase the input signal until just before clipping? And have you tried sending the signal into the grid of V2a to bypass the preamp stage and controls? And what if you do push the signal into clipping have you seen what stage clips first?
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soundmasterg
- Posts: 30
- Joined: Sun May 08, 2022 4:48 am
Re: Under the hood of a 1965 Supro Thunderbolt.
The B+ at the nodes that you reported are quite low. Take a look at the schematic for the tube rectifier version. These voltage came from a local original tube rectified TBolt amp that I repaired for a studio owner and were confirmed from several other amps that Sean Weatherford's contacts provided. (See the credits on the schematic for who they were). I wonder if the can cap is shorting excessive voltages to ground due to damage? You might try subbing in some caps temporarily in place of the nodes on the can to see if the voltage goes up?Stevem wrote: ↑Thu Jun 19, 2025 9:25 am
The following voltages are loaded.
B+ first node 380
2nd node 374
3rd node. 324
4th node 258
Cathode voltage on outputs at idle 28.48
V1 plate 255
V1 cathode 2.42
V2A plate 193.8
V2A cathode 1.98
V2B plate 153
V2B cathode 1.38
The output wave form looks good.
I did a quick check of the OT by pumping ac in and I am seeing a impedence of about 5200 ohms.
Greg
https://el34world.com/charts/Schematics ... _tbolt.pdf
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soundmasterg
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- Joined: Sun May 08, 2022 4:48 am
Re: Under the hood of a 1965 Supro Thunderbolt.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Under the hood of a 1965 Supro Thunderbolt.
Thanks for the reply’s folks.
I likely will not have any more news to post up until after the weekend.
I likely will not have any more news to post up until after the weekend.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
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!
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!
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
2 others liked this
Re: Under the hood of a 1965 Supro Thunderbolt.
Quick up date.
After some high wattage testing of a PA amp I found that a switch in my resistor power output test rig was heading south.
I replaced it and when tested the amp was found to now output 27 watts rms and 34 peak, so all is well in that regard.
Oh, and the OT test out to 5200 impedance.
On Monday night I will post up the voltages now and also when I swap in a 5AR4 and a 250 ohm cathode resistor.
After some high wattage testing of a PA amp I found that a switch in my resistor power output test rig was heading south.
I replaced it and when tested the amp was found to now output 27 watts rms and 34 peak, so all is well in that regard.
Oh, and the OT test out to 5200 impedance.
On Monday night I will post up the voltages now and also when I swap in a 5AR4 and a 250 ohm cathode resistor.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
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!
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!