Looking for Supro Thunderbolt 6420 Build info

[Helmholtz]

Interesting, thanks Helmholtz. The schematic Im working from, other guys mentioned from Weatherford, http://www.laurensbuijs.nl/wp-conten...1864129669.jpg have two caps at the screen node, comment says the extra cap could be 20 or 30uf, in addition to the 10uf in the "power supply" strip. In the photo I found, the second cap was mounted right on the tube socket pins. The way those amps were laid out, it looked like an afterthought, or maybe a repair, as I didn't think caps in those days were sturdy enough to take the heat of having the lead soldered right to the socket pin like that. Does this cap's intended use require mounting that close to the screen pin?

It is generally advisable to mount screen supply components close to the socket pins, thus avoiding long wires, which introduce parasitic inductance and increase the risk of HF oscillation.
As indicated above, I favour the use of separate (470 Ohm) screen resistors, which additionally damp HF effects just like grid stoppers. With screen resistors there is no need to connect the cap closer to the socket than in other designs. The screen resistors should be directly connected to the socket as usual.

[soundmasterg]

Interesting, thanks Helmholtz. The schematic Im working from, other guys mentioned from Weatherford, http://www.laurensbuijs.nl/wp-conten...1864129669.jpg have two caps at the screen node, comment says the extra cap could be 20 or 30uf, in addition to the 10uf in the "power supply" strip. In the photo I found, the second cap was mounted right on the tube socket pins. The way those amps were laid out, it looked like an afterthought, or maybe a repair, as I didn't think caps in those days were sturdy enough to take the heat of having the lead soldered right to the socket pin like that. Does this cap's intended use require mounting that close to the screen pin?

Some of the earlier schematics listed at Hoffman Amp's schematic section list the power transformer in addition to the output transformer. Here is an earlier tube rectifier schematic that lists both, from Classictone.

https://el34world.com/charts/Schemat...20tr_tbolt.pdf

And here is an earlier solid state schematic that lists several options included the Heyboer power transformer model that I have that was originally cloned from an original of one of the ones I worked on.

https://el34world.com/charts/Schemat...pro_6420_2.pdf

Some of the amps seen had different value electrolytics for the final filtering stage in the amp, as noted in those schematics and the one you have too. I didn't get around to posting those pics yet, but I still plan to do so later.

Greg

[Leo_Gnardo]

Ooo, paper in oil, Im in! Ive been looking for *genuine* unicorn hair dielectric, but could only find faux unicorn hair, that rumor has it, is from rodents in Camden NJ. No way Im putting that in my amp.

Well those rodents might have been inhabiting the old RCA tube factory. Lotta mojo magic there, don't pass it up! You could be on to something. Or maybe come down with Hanta virus.

[soundmasterg]

Here are some chassis and cabinet pics of an early tube rectified Supro Thunderbolt. You'll notice the use of a lot of ceramic coupling caps. This amp also had a mod to utilize the unused triode that the owner did.

Greg

[mikepukmel]

Thanks everyone, for tech info, links, photos, this is awesome. So, for this amp, Id like to be able to run it dimed (as close to 11 as possible ), since that's supposed to be where its character is revealed. To that end, as seems to be with new tubes, new tubes aren't as durable running higher screen (voltage or current or both?). Im most interested in reproducing as close to the original tone as possible. That said, if all other components are close to original values (except a few that we can't get any longer), if I add 470R screen resistors, but also leave the power supply chain as is (1k followed by 27k followed by 100k) will this change the amp character much? Helmholtz mentioned that it will have less power, and the breakup will be different. Of course, this isn't 1965 and I can't get 1965 made RCA tubes, but all things considered...

[nosaj]

The screen supply is already 1K below the B+.

Yes, individual screen resistors is always good practice, especially with modern tubes.

Thanks everyone, for tech info, links, photos, this is awesome. So, for this amp, Id like to be able to run it dimed (as close to 11 as possible ), since that's supposed to be where its character is revealed. To that end, as seems to be with new tubes, new tubes aren't as durable running higher screen (voltage or current or both?). Im most interested in reproducing as close to the original tone as possible. That said, if all other components are close to original values (except a few that we can't get any longer), if I add 470R screen resistors, but also leave the power supply chain as is (1k followed by 27k followed by 100k) will this change the amp character much? Helmholtz mentioned that it will have less power, and the breakup will be different. Of course, this isn't 1965 and I can't get 1965 made RCA tubes, but all things considered...

Well Hell son....save your self some money and ditch that Volume knob.

In all seriousness you have proven yourself a tinkerer...try it both ways because as we all know the only thing that really matters is what YOUR brain processes and sounds good to YOU......

Have fun and keep it up.

nosaj

[mikepukmel]

Thanks Nosaj, will do! This is way fun stuff to tinker with!

[soundmasterg]

Thanks everyone, for tech info, links, photos, this is awesome. So, for this amp, Id like to be able to run it dimed (as close to 11 as possible ), since that's supposed to be where its character is revealed. To that end, as seems to be with new tubes, new tubes aren't as durable running higher screen (voltage or current or both?). Im most interested in reproducing as close to the original tone as possible. That said, if all other components are close to original values (except a few that we can't get any longer), if I add 470R screen resistors, but also leave the power supply chain as is (1k followed by 27k followed by 100k) will this change the amp character much? Helmholtz mentioned that it will have less power, and the breakup will be different. Of course, this isn't 1965 and I can't get 1965 made RCA tubes, but all things considered...

The Sovtek 5881 that everyone loves to diss on because of its lacking sound in comparison to many other 6L6 types actually sounds great in these Tbolt amps and they last a long time too. They can take the punishment, and you can run them without screen grid resistors too like the originals if you want. The guy I fixed those three Tbolts for used those Sovteks in one, and the other two had NOS Sylvanias, but when the Sylvanias died he put the Sovteks in both the other amps and they sound just as good in that circuit and last a long time. Whenever I get around to building my clone (I have the transformers and chassis already) then I plan to use those Sovteks myself.

Greg

[Helmholtz]

The Sovtek 5881 that everyone loves to diss on because of its lacking sound in comparison to many other 6L6 types actually sounds great in these Tbolt amps and they last a long time too. They can take the punishment, and you can run them without screen grid resistors too like the originals if you want. The guy I fixed those three Tbolts for used those Sovteks in one, and the other two had NOS Sylvanias, but when the Sylvanias died he put the Sovteks in both the other amps and they sound just as good in that circuit and last a long time. Whenever I get around to building my clone (I have the transformers and chassis already) then I plan to use those Sovteks myself.

Greg

Tubes can't have a sound on their own. Tube parameters are independent of frequency up to the 10MHz range. Sound is the result of the interaction of the tube and its circuit. In other words, different circuit - different "tube sound".

[nosaj]

The screen supply is already 1K below the B+.

Yes, individual screen resistors is always good practice, especially with modern tubes.

Tubes can't have a sound on their own. Tube parameters are independent of frequency up to the 10MHz range. Sound is the result of the interaction of the tube and its circuit. In other words, different circuit - different "tube sound".

I beg to differ have you ever dropped one?

nosaj

[Helmholtz]

I beg to differ have you ever dropped one?

Such kind of tube sound testing I prefer to leave to those, who consider it significant.

[soundmasterg]

Tubes can't have a sound on their own. Tube parameters are independent of frequency up to the 10MHz range. Sound is the result of the interaction of the tube and its circuit. In other words, different circuit - different "tube sound".

I know some believe that, including Stan who often posts here. If you are swapping tubes in and out and using the same circuit, the amp sounds different when you swap tubes in and out. It would make sense that different tubes would have different internal resistances and capacitances, since they are a product that is produced from many individual parts that all can't be exactly the same. So if you want to call it interaction between the tube and circuit, thats fine, but since the sound changes when you swap the tubes, then it is likely a result of those resistances and capacitances interacting with the circuit. Everyone tends to simplify and say that it is the tubes that sound the way they do, and perhaps that is wrong, but if you have ever swapped between different tube types in an amp, you would know that the sound does change when you do so. Since the tubes are one of the variables and the circuit is not changing, then what is it that is changing the sound? The end result is that those Sovtek 5881's work well, last a long time, and sound good in this circuit, and that can't be said about some other circuits with those tubes.

Greg

[Justin Thomas]

I believe what Stan was usually saying is that a certain "brand" of tube cannot have a certain "tone" assigned to it across all zillion examples of that tube. He would say that Pair A would sound different from Pair Bin the same circuit IF all else was left the same. But, if Pair C is the exact same as Pair B, the two pairs may or may not sound the same. He also said that any differences that might be heard can be compensated for by small circuit tweaking.

Or as Juan used to put, these particular tubes on this particular day dounded this particular way at these particular operating parameters, while my ears were in this particular state...

Sure you could notice differences, but these vendor descriptions of "Tube A is creamy in the bass under distortion" or whatever just cannot be relied upon to sound the same in YPUR amp.

All that said, yeah, those Sovtek 5881WXTs are tough as nails and take a TON of punishment. Uf it's any help I ran a pair in a VT-40 for 2 hours straight, dimed (yes, Master Volume too), at 600 plate volts. Not a single falter.

Justin

[Helmholtz]

If you are swapping tubes in and out and using the same circuit, the amp sounds different when you swap tubes in and out.

No doubt.

Since the tubes are one of the variables and the circuit is not changing, then what is it that is changing the sound?

The tube is an essential part of the circuit. "Sound" is the result of the (linear and non-linear) transfer characteristics of the circuit with a given tube. A tube having different parameter values and characteristics changes the circuit behaviour in many respects: frequency response, distortion, dynamics. In a different surrounding/circuit configuration the same tube may cause different sound changes. So the tube can't have an own sound character, but one tube may sound better in a given circuit than an other one.

If you change the value of a single resistor in a circuit, the sound may change noticeably. This doesn't justify to attribute an own sound quality to the different resistors, even though one of them may make the special circuit sound "better".

[soundmasterg]

No doubt.



The tube is an essential part of the circuit. "Sound" is the result of the (linear and non-linear) transfer characteristics of the circuit with a given tube. A tube having different parameter values and characteristics changes the circuit behaviour in many respects: frequency response, distortion, dynamics. In a different surrounding/circuit configuration the same tube may cause different sound changes. So the tube can't have an own sound character, but one tube may sound better in a given circuit than an other one.

If you change the value of a single resistor in a circuit, the sound may change noticeably. This doesn't justify to attribute an own sound quality to the different resistors, even though one of them may make the special circuit sound "better".

So what I am saying is that since each tube is going to have different values (i.e. different internal resistances and capacitances, different gain, input impedance, output impedance, etc.) then swapping one 6L6 tube for example into a circuit vs a different 6L6 tube will change the sound, and the circuit hasn't changed at all, except in how it interacts with the new tube and its different internal parameters. So the tube and all that entails as far as different internal values between tube types and the fact the tube is an active device has changed the sound as far as how it is reacting to the circuit. Simply saying that the tube changed the sound is probably not accurate since there are all these other variables involved, but saying that the sound did change when you swapped the tubes is accurate, and the tube itself had an impact on that sound change is also accurate.

Greg