Looking for Supro Thunderbolt 6420 Build info

[mikepukmel]

[ATTACH=CONFIG]53878[/ATTACH]

I've built a few tbolt 6420 amp circuits. I like 'em with a B+ around 400 and 5881s running off a shared 270R cathode resistor that is fully bypassed. The spare triode is useful for goosing the front end up, like a switchable single-parallel input stage. (see schematic). The cathode-bias vibe is cool, but they are not the loudest of amps (you can get more oomph from the same number of tubes if you build a 1-channel AB763 circuit sans reverb and tremolo).

The James tonestack in the attached schematic works well. The treble and bass controls have a wide range and a decent mid scoop with the controls at full and relatively flat with both controls at about mid rotation.

Or build it how you want. YMMV

Thanks!

[mikepukmel]

What effect on the tone does adding the bypass cap do?

[tubeswell]

What effect on the tone does adding the bypass cap do?

It makes it a bit louder and tighter sounding when cranked

[mikepukmel]

[ATTACH=CONFIG]53878[/ATTACH]

I've built a few tbolt 6420 amp circuits. I like 'em with a B+ around 400 and 5881s running off a shared 270R cathode resistor that is fully bypassed. The spare triode is useful for goosing the front end up, like a switchable single-parallel input stage. (see schematic). The cathode-bias vibe is cool, but they are not the loudest of amps (you can get more oomph from the same number of tubes if you build a 1-channel AB763 circuit sans reverb and tremolo).

The James tonestack in the attached schematic works well. The treble and bass controls have a wide range and a decent mid scoop with the controls at full and relatively flat with both controls at about mid rotation.

Or build it how you want. YMMV

So, 400v is fairly close to the other schematics folks here pointed to, e.g. one from Weatherford. It has a 0 - 325vac rms secondary and a full wave rectifier (did I use the corect term, 4 diodes in a bridge configuration). Which gives about 460 vdc at the first filter cap The transformer I found on Classictone, is 350 - 0 - 350 vac rms, so through solid state rectifiers, about 495, too high I think. Their transformer, they say is actually for the tube rectified version, so it has th 5v taps.

Working backwards, if I wanted to build a solid state rectified version, to get 410 at the first filter cap, Id need a 0 - 290 or a 290 - 0 - 290 secondary power transformer? The second one seems a little mere common for guitar amps.

Would this transformer work?

*EDIT* Sory copy and paste error
Hammond
290UX 112 117 60 308.5V @ 250 mA 39V @ 250 mA 6.6V @ 3.75A

its got a bias winding, but would not need that.


As aside: I sorta get the idea how a 4 diode bridge rectifier works, with a transformer that has no center tap, vs 2 diode (like fender) with a center tap, but which is better? Not sure what better means, or why the schematics Ive seen for this amp use a 4 diode bridge.

In the bridge, two diodes are always conducting, and the whole winding is used on every half cycle. In the other config, only two diodes are conducting, and only half (from center tap out) is used on each half cycle.

Would one configuration heat up more, e.g. non center tap, since the while winding is used in each half cycle?

[mikepukmel]

Fixed :-)

Studying your schematic, all kinds of cool stuff in there, thanks! Do the elevated heaters help a lot with hum? Bass + treble is a nice add. How does the raw boost work?

[tubeswell]

... Do the elevated heaters help a lot with hum? Bass + treble is a nice add. How does the raw boost work?

Heater elevation and humdinger is particularly useful with S:N ratio in the first stage.

The James tone stack setup in this schematic gives a fairly flat response with the pots at mid rotation, and a decent mid-scoop with both pots at max rotation.

The raw ('boost') switch is a TS-lift switch. Inserting a 1M resistor between the TS and the ground removes the effect of the TS from the circuit (and makes the amp even louder)

[tubeswell]

...Working backwards, if I wanted to build a solid state rectified version, to get 410 at the first filter cap, Id need a 0 - 290 or a 290 - 0 - 290 secondary power transformer?...

Depends on what sort of rectification you use. 0-290 with FW bridge rectifier using SS diodes. 290-0-290 with 2 phase rectifier using SS diodes

... Would this transformer work?
...
Hammond
290UX 112 117 60 308.5V @ 250 mA 39V @ 250 mA 6.6V @ 3.75A...

435 B+ or so, using a FW bridge

... As aside: I sorta get the idea how a 4 diode bridge rectifier works, with a transformer that has no center tap, vs 2 diode (like fender) with a center tap, but which is better? ...

"Better" in this instance, is strictly a function of the winding configuration, which will determine which type of rectifier you need.

[mikepukmel]

Depends on what sort of rectification you use. 0-290 with FW bridge rectifier using SS diodes. 290-0-290 with 2 phase rectifier using SS diodes



435 B+ or so, using a FW bridge




"Better" in this instance, is strictly a function of the winding configuration, which will determine which type of rectifier you need.

Ah got it thanks.

I don't have a handle on what 80 volts difference B+ would do to this amp. The 350 - 0 - 350 Classictone, they say is original design, would produce something like 490v dc at the first node. Way higher than your design, or the Weatherford, you both have around 410vdc Ive read some long discussions about B+ etc, was able to pick up some of it, even though I don't have the detailed background. Some part I got (I think) said its 1 biasing that is important, and 2 difference between the plate and screen.

Is lower voltages better "thing" with new build tubes?

[tubeswell]

... Is lower voltages better "thing" with new build tubes?

Depends what you mean by lower voltages I guess. I don't consider 400V particularly 'low'.

As to new build tubes, manufacturing tolerances tend to be a bit variable (not so many items going into the reject pile). Screen over-dissipation is a big factor in tube failure. One way to reduce that is keep screen voltages lower. But there are also other ways, like cooler biasing, running an optimum load resistance (and not allowing the PI to drive the output stage 'too unbalanced' - which can be a problem with Long-Tail-Pairs).

Bear in mind that the output tubes in a stock t-bolt are biased to maximum plate dissipation at idle, and while these are auto-(cathode)-biased, you are still running the tubes hard, even at 410V.

Also, the cathode-biasing of the 6L6s tends to bring the B+ voltage down quite a bit from what you'd expect if you otherwise ran them with fixed bias. If you got a B+ of 470 with fixed bias, it could well end up being anything from 400 to 420 or so with cathode biased 6L6s, depending on the load regulation of the power transformer.

[mikepukmel]

Depends what you mean by lower voltages I guess. I don't consider 400V particularly 'low'.

As to new build tubes, manufacturing tolerances tend to be a bit variable (not so many items going into the reject pile). Screen over-dissipation is a big factor in tube failure. One way to reduce that is keep screen voltages lower. But there are also other ways, like cooler biasing, running an optimum load resistance (and not allowing the PI to drive the output stage 'too unbalanced' - which can be a problem with Long-Tail-Pairs).

Bear in mind that the output tubes in a stock t-bolt are biased to maximum plate dissipation at idle, and while these are auto-(cathode)-biased, you are still running the tubes hard, even at 410V.

Also, the cathode-biasing of the 6L6s tends to bring the B+ voltage down quite a bit from what you'd expect if you otherwise ran them with fixed bias. If you got a B+ of 470 with fixed bias, it could well end up being anything from 400 to 420 or so with cathode biased 6L6s, depending on the load regulation of the power transformer.

Very interesting, more stuff I don't understand yet! Re
"Also, the cathode-biasing of the 6L6s tends to bring the B+ voltage down quite a bit from what you'd expect if you otherwise ran them with fixed bias. If you got a B+ of 470 with fixed bias, it could well end up being anything from 400 to 420 or so with cathode biased 6L6s, depending on the load regulation of the power transformer."

OK so this is why they could have run that 700vac transformer with 495 (roughly) at the first power node, since cathode biased it would end up much lower actual V at the plate? Could you explain ' depending on the load regulation of the power transformer" does this mean if it doesn't sag?

*** sorry, so you mean the actual voltage between cathode and anode?

[tubeswell]

Could you explain ' depending on the load regulation of the power transformer" does this mean if it doesn't sag?

About PT regulation https://www.ecmweb.com/basics/basics...age-regulation A PT’s load regulation is a ‘measure’ of the PT’s ability to cope with a load.

Besides, When you run the tubes in a stock t-bolt, they’re idling at Pmax, so the B+ load is more than what it is if you’re idling them at 0.7Pmax using fixed bias. (More tube current in the stock t-bolt = more load to pull the B+ down further)

[soundmasterg]

Another thing to keep in mind is that if you have a 350V power transformer like you suggested above, with a solid state bridge rectifier that will be 490V with no load. When you put a load on it then that voltage will drop quite a bit. Take a look at Fender transformers and do the simple math to see what a no load voltage is vs what those amps actually had for B+ voltage. For the multiplier, SS is 1.4, GZ34 is 1.3, 5U4 is 1.2, and 5Y3 is 0.9. So with a 350V power transformer as you noted above, SS = 490V no load, GZ34 = 455V no load, 5U4 = 420V no load, and 5Y3 = 315V no load. Then if you look at the voltages in a Fender and compare to what they would be no load, you can get an idea of how much it drops with different kinds of amps. Then you can get an idea of what voltage the power transformer should put out to get to the B+ voltage you are looking for. I say to use Fender because they are the most documented vintage amps.

Greg

Greg

[mikepukmel]

Thanks!

[Helmholtz]

So with a 350V power transformer as you noted above, SS = 490V no load, GZ34 = 455V no load, 5U4 = 420V no load, and 5Y3 = 315V no load.

There won't be much difference in no-load DC voltages between the different rectifiers. It requires load current to produce the different voltage drops.

[mikepukmel]

so the final question: 350-0-350v secondary power transformer from Classictone, which will result in higher B voltages, or a different brand that has about 300 -0 - 300v?