Looking for Supro Thunderbolt 6420 Build info

[mikepukmel]

I would at least twist the grid wires. As the grid signals are opposite phase/polarity twisting will provide some shielding and produce a little capacitance (maybe 10 to 20pF) between the wires which may help to avoid oscillation.

Thanks, will do. Should these leads also run along the chassis, or is it better to make them shorter 'flying'?

[mikepukmel]

Will hum less if it’s connected directly to the ground lead of the reservoir cap. I suggest following the grounding layout in Merlin Blencowe’s grounding article. (Especially Fig 15.12 and Fig 15.14) http://www.valvewizard.co.uk/Grounding.pdf

Thanks, reading now!
<edit>

OK I have the main ideas, but run into something Im not sure about (and probably get wrong). I got the "local star" concept. A filter cap that feeds a section of the amp, should have its negative lead tied to the 'grounds' in that section. Not sure what to do about what Merlin calls 'daisy chain', though. I think this is what I got wrong in the Fender Deluxe Reverb I hacked together.

So, if I put a few lugs on the chassis, for this amp I think 3, would that work like Merlin's drawing? One for the preamp, one for PI one for power tubes, and one power supply + first reservior cap. There would be no wire between the local stars, the chassis would serve that purpose?

[mikepukmel]

So, if there were 3 lugs, one PS, one PI and one for the 'quiet stuff', with the quiet lug mounted at the far end of the chassis, ...

[mikepukmel]

One last question This amp will go close to 500v with no load, which is close to the filter cap rating. Ive seen bigger amps, like Fender Twin, use two caps in series, and 'balance' resistors. The AB763 used two 70uf, 350v. caps. They put two 220k 1W resistors across each cap.

Question: how did they calculate that they needed 220k 1w resistors, and won't those resistors suck power from the PT that would not be used in an amp that didn't have these resistors? I.e. they had to up the windings a bit to account for these losses (to heat)?

Thanks!

[tubeswell]

Thanks, reading now!
... Not sure what to do about what Merlin calls 'daisy chain', though. I think this is what I got wrong in the Fender Deluxe Reverb I hacked together.

So, if I put a few lugs on the chassis, for this amp I think 3, would that work like Merlin's drawing? One for the preamp, one for PI one for power tubes, and one power supply + first reservior cap. There would be no wire between the local stars, the chassis would serve that purpose?

Not quite. In the 'daisy chain' system Merlin describes, all the grounds are daisy-chained together into one ground buss/wire, but only the input stage end of this daisy chain ground wire is attached to the chassis. The output tube end of the ground wire is floating (because it it weren't, you'd end up with a ground loop. Similarly, if you split the daisy chain up into components, and grounded each section separately, you'd be more likely to get ground loop hum, depending on whereabouts the chassis mains ground was situated w.r.t. each of the other signal stage grounds in the split-up ground).

The idea behind the daisy chain is that the most sensitive pre-amp stages get a ground that is as-close-as-possible to chassis ground potential, and the other higher-current signal ground returns are further away from this, so that the daisy chain acts like one long (albeit low-resistance) voltage divider, and therefore any micro-rises in ground potential along this voltage divider caused by the high-current ground returns, will be smaller nearest to where the chain is connected to the chassis, so that there will be less interference (through filter cap reverse-shunting noise in) to the power supply rail of the most noise-sensitive ground returns in the input end of the pre-amp.

[Dave H]

One last question This amp will go close to 500v with no load, which is close to the filter cap rating. Ive seen bigger amps, like Fender Twin, use two caps in series, and 'balance' resistors. The AB763 used two 70uf, 350v. caps. They put two 220k 1W resistors across each cap.

Question: how did they calculate that they needed 220k 1w resistors, and won't those resistors suck power from the PT that would not be used in an amp that didn't have these resistors? I.e. they had to up the windings a bit to account for these losses (to heat)?

500V across 440k is just over 1mA. The PT won't notice that.

The value of the balance resistors is chosen to make the current through them much greater than the capacitor leakage current. If the cap leakage current is 10uA then it's only 1% of the resistor current and won't affect the voltage division and 1mA won't waste too much power so there's no need to go below 220k.

[Helmholtz]

Problem with leakage current is that there is wide variation and often little information available.
Leakage current depends on capacitance, rated voltage, working voltage, operating temperature and the cap's operating history.

For a general purpose 100µ/500V E-cap the formulas in the Siemens/Epcos literature give a typical leakage current of 53µA at 20°C. Operation at 50°C increases the value by a factor 4.
The same cap is considered acceptable as long as its leakage current is < 590µA @ 20°C or < 1470µA @ 35°C measured at its rated voltage. Operation below rated voltage strongly decreases leakage, e.g. at 50% of rated voltage leakage is lower by a factor 7.

After longer discharged periods initial leakage currents may be up to 100 times higher than normal for several minutes.

Of course in a series arrangement only leakage mismatch really matters.

Still, 220k may not always be low enough for divider resistors to ensure equal cap voltages.

Marshall amps often use 56k/2W dividers.

[Helmholtz]

Thanks, will do. Should these leads also run along the chassis, or is it better to make them shorter 'flying'?

I consider it more important to run the OT leads close to the chassis and keep the grid wires distant from the OT wires.
Also the OT wires connecting to the plates should be as short as possible and be twisted as long as it makes sense to minimize field radiation.

[mikepukmel]

I consider it more important to run the OT leads close to the chassis and keep the grid wires distant from the OT wires.
Also the OT wires connecting to the plates should be as short as possible and be twisted as long as it makes sense to minimize field radiation.

Thanks!

[mikepukmel]

Not quite. In the 'daisy chain' system Merlin describes, all the grounds are daisy-chained together into one ground buss/wire, but only the input stage end of this daisy chain ground wire is attached to the chassis. The output tube end of the ground wire is floating (because it it weren't, you'd end up with a ground loop. Similarly, if you split the daisy chain up into components, and grounded each section separately, you'd be more likely to get ground loop hum, depending on whereabouts the chassis mains ground was situated w.r.t. each of the other signal stage grounds in the split-up ground).

The idea behind the daisy chain is that the most sensitive pre-amp stages get a ground that is as-close-as-possible to chassis ground potential, and the other higher-current signal ground returns are further away from this, so that the daisy chain acts like one long (albeit low-resistance) voltage divider, and therefore any micro-rises in ground potential along this voltage divider caused by the high-current ground returns, will be smaller nearest to where the chain is connected to the chassis, so that there will be less interference (through filter cap reverse-shunting noise in) to the power supply rail of the most noise-sensitive ground returns in the input end of the pre-amp.

Awesome, thanks, reading and re-reading. I think I got it.

(Sorry, have to get used to a single "thanks everyone" rather than one for each post to avoid so many thank you posts. Read all and re-read, great info.).

[mikepukmel]

I found this transformer, closest to the HV winding voltages recommended. The others were either too high hv voltage, or way undersized re HV secondary current rating. Is there any problem using this transformer and tying off the 5v winding? Would this work ok for the weatherford schematic?

Size: 236 VA
Dual Primary: 115 VAC, 125 VAC, 60 Hz.
A.C. High Voltage Secondary RMS: 600V.C.T. @ 287mA
Filament Winding #1 5 V C.T. @ 4 A
Filament Winding #2 6.3 V C.T. @ 8 A

https://www.tubesandmore.com/product...0-300-v-287-ma

[mikepukmel]

Help with power/wattage calculation

Hi All,

Working on a build of this schematic. Heyboer transformer 325-0v secondaries.

Supro_6420_2.pdf

The schematic doesn't state the wattage of the 1k resistor, or the 200R cathode resistor.

For the 1k, the power tube plates are fed before this point, So, its only "feeding" the screen grids, and plates on 2 other 12AX7's. Data sheets put max 1 watt for each 1/2 of a 12ax7. 1/2 of the first tube isn't used, so that's max 3w. The svetlana 6L6 screen grid dissipation of 5 watts. (really 5 watts each tube???). That total is 13 watts. But that can't be right. No amp schematic Ive ever seen has a 13 watt resistor in that spot.

So I must be wrong about using max screen, which would probably melt the tube. How high, 3 watts? That is still a 9 watt resistor, with no cushion. How do you calculate this?

Thanks!
Mike

[eschertron]

Use the formula P = I squared r
If I assume 40ma for each screen (a highball number), then 40+40+ couple ma per preamp tube results in 6.4W across a 1k resistor. I'd use 10W cement resistor and forget about problems on the screen grid node.

[mikepukmel]

Awesome, thanks!

[tubeswell]

I found this transformer, closest to the HV winding voltages recommended. The others were either too high hv voltage, or way undersized re HV secondary current rating. Is there any problem using this transformer and tying off the 5v winding? Would this work ok for the weatherford schematic?

Size: 236 VA
Dual Primary: 115 VAC, 125 VAC, 60 Hz.
A.C. High Voltage Secondary RMS: 600V.C.T. @ 287mA
Filament Winding #1 5 V C.T. @ 4 A
Filament Winding #2 6.3 V C.T. @ 8 A

https://www.tubesandmore.com/product...0-300-v-287-ma

This would be fine. If you want, you could build it with both SS rectification (2-phase rectifier) and tube rectification, so you can swap between them and get the different characteristics.