# Thread: 1960's Supro Thunderbolt scratch build copy

1. Remember we found the error on the 470k resistors [should have been 470 ohm] and I put in 1K. All of the last voltages are with the 1K in place and the 270 ohm cathode resistor.

Here's an updated drawing of what is actually in the amp.

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2. Ok I tried a 240 ohm 5W resistor just for a test. Here's what voltages I got.

V4 - 71mv across 1 ohm resistor
p3- 442v
p4- 435v
p5- .111
p8- 35.93

V3 - 68mV across 1 ohm resistor
p3- 442v
p4- 435v
p5- .163
p8- 35.95

I kinda like the tone better with the 270 ohm resistor in, so I'm going to switch back. But I have a question about wattage calculation. Chuck you had mentioned "Measure the cathode voltage and divide by the cathode resistance. This is your current. Multiply that number by your "plate to cathode" voltage to get your "watts at idle" figure"

How do I figure the cathode resistance? I entered in 240 ohms and came up with around 60w per tube. If I use the voltage across the 1 ohm resistor method for calculating current, then I get around 28W per tube. Where was my error?

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3. Your error was that you didn't divide the 60W by two. Both tubes current runs through the 240 ohm resistor.
So 36V divided by 240 ohm = .15A or 150mA. That is for 2 tubes, so current is about 75mA each tube (if they are matched).

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4. Originally Posted by mort
Remember we found the error on the 470k resistors [should have been 470 ohm] and I put in 1K.
That was the screen grid resistors. I'm talking about the G1 (input grid) resistance. It's 470k with a 47k grid stop resistor. That's 517k on a circuit with a max value of 500k in an amp that will probably be cranked as often as not. Bad idea.

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5. Originally Posted by mort
Ok I'm getting back around to working on this and was planning on upping the value of the cathode resistor to decrease plate current a bit and realized that I had actually installed a 270 ohm 10W, instead of a 200 ohm like my schematic showed. I had read a note somewhere about using anywhere from 200-330 and used a 270 but forgot to reflect that on my drawing.

The above comment about losing tone above 240 has me wondering what the amp would sound like if I dropped the value down to 200 or 220(sounds great now but maybe it could sound even better?). I expect lowering the value would increase plate current so now I need another strategy of decreasing current.

I guess one way would be to raise the grid resistor values but as mentioned already that may not be a great idea. ... how should I go about this?

Remember that a stock amp using a 200 ohm cathode resistor to bias the power tubes is running a lower B+ than your amp and that changes the bias calculation. The stock amp is biased hot AB and probably somewhere around 80-90% of the max plate dissipation. It sounds great, and works fine...the only downside is that the power tubes wear out sooner than if it was biased cooler. A proper cathode resistor to get your amp in the same range can be calculated or you can sub in different types and figure out what the dissipation is, and see if you like the tone or not. The comment on the schematic regarding using a 240 ohm if desired was for people who didn't want their tubes to wear out so quickly. Another thing to remember is that since your amp is cathode biased, you need to remember to subtract the cathode voltage from the B+ before you calculate your dissipation.

Greg

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6. Originally Posted by g-one
Your error was that you didn't divide the 60W by two. Both tubes current runs through the 240 ohm resistor.
So 36V divided by 240 ohm = .15A or 150mA. That is for 2 tubes, so current is about 75mA each tube (if they are matched).
ok yeah ... not sure why I didn't see that... sometimes I miss the obvious

OK so with the 270 ohm back in the circuit here are my numbers. And point taken about the higher B+ voltage vs the B+ in the pdf

V4 68mA(444 - 36.62) = 27.7W
p3- 444
p4- 437
p8- 36.62

V3 64mA(443 - 36.62) = 26.0W
p3- 443
p4- 437
p8- 36.62

So it looks like I'm running around 90% at idle... I may try a 300 ohm cathode resistor just to see if it still sounds good but according to what soundmasterg has mentioned it may lose something...

Oh hey one more question... would I raise or lower one input resistor if I wanted to make the inputs high/low ?? Lower one value to make it a high input??

1

7. Originally Posted by mort
would I raise or lower one input resistor if I wanted to make the inputs high/low ?? Lower one value to make it a high input??
That is more effectively done with switching jacks that use the grid resistors placement in the circuit to create different voltage divisions. Look at any Fender or Marshall schematic to learn how it's done.

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8. Hi Amp Gurus, This thread is getting a bit old in internet terms, awesome info on Mort's build. its exactly what I was hoping to do (once I get all the kitchen and bathroom repairs done, and the yard cleaned up and the bad tire replaced ... ).

In case Mort isn't monitoring this thread anymore, anyone have any idea where he got the eyelet board from? Is it a custom made eyelet board, or maybe one of those standard N hole boards with a few eyelets added?

Thanks!
Mike

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9. Originally Posted by soundmasterg
If you look at the original schematic that Sean Weatherford compiled with my help and others help, (S6420.pdf above) the B+ with the solid state rectified version is right around 425V. Your voltage is a bit higher, so the proper cathode resistor value should be a bit higher too. With the stock amp, a 200 ohm resistor puts the bias up around 80% to 90% of the max dissipation for the 6L6GC. Sovtek 5881's actually sound great in these amps because they like to be biased hot. In any case, with a stock amp and a 200 ohm cathode resistor on the power tubes, tube life is shorter than many fixed bias amps such as your typical fender. With a stock amp, changing to a 240 ohm cathode resistor cooled the tube bias down, but made the sound noticeably less sweet. Tutuapp 9apps ShowboxYou will probably have to experiment around to get to a good resistor value.

A couple other things....

- the originals used ceramic coupling caps throughout the amp. Using film caps is generally considered to be a better route to go, but that may get rid of some of the character of the original design.
- You will notice if you look at vintage amps that they either don't use a screen grid resistor, or use a 470 ohm, or a 1k, but almost never much larger than that. Read the attached document, specifically section 8 and they talk about optimum screen grid resistor sizes. The main thing to keep in mind is that you want some protection for the screens, but not so much that you start to lose power.
- You can take a look at a stock Supro Tbolt in the attached pics and the way they made the amp.

Greg
Thanks for this helpful informations

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10. Originally Posted by klimbo
Thanks for this helpful informations
You're welcome!

Greg

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11. Originally Posted by mort
I'm about to finish up construction and will be applying power to it sometime this weekend. Attached are the schematic I started with in PDF format and the other is the schematic that was actually built after adding a stand by switch, grid resistors, and a couple other details or two that were suggested by forumites.

Is there anything that looks like a mistake/oversight ?

All I have left to do is wire the heaters, grounds, and a few other connections. Whattaya think?
Thank you man for taking your time out and posting a helpful information.

Really appreciated your efforts. Have a good day!

Regards

1

12. A proper cathode resistor to get your amp in the same range can be calculated or you can sub in different types and figure out what the dissipation is, and see if you like the tone or not. The comment on the schematic regarding using a 240 ohm if desired was for people who didn't want their tubes to wear out so quickly

1

13. Originally Posted by mort
Ok I tried a 240 ohm 5W resistor just for a test. Here's what voltages I got.

V4 - 71mv across 1 ohm resistor
p3- 442v
p4- 435v
p5- .111
p8- 35.93

V3 - 68mV across 1 ohm resistor
p3- 442v
p4- 435v
p5- .163
p8- 35.95

I kinda like the tone better with the 270 ohm resistor in, so I'm going to switch back. But I have a question about wattage calculation. Chuck you had mentioned "Measure the cathode voltage and divide by the cathode resistance. This is your current. Multiply that number by your "plate to cathode" voltage to get your "watts at idle" figure"

How do I figure the cathode resistance? I entered in 240 ohms and came up with around 60w per tube. If I use the voltage across the 1 ohm resistor method for calculating current, then I get around 28W per tube. Where was my error?
I have several valco amps using 6973 tubes (not 6L6s), and using a slightly bigger Cathode resistor didn't change tone all much, but put the ouput tubes in a safer region. I think in the amp world there are some who perpetuate a misguided fixation on running the plate idle dissipation as high as possible, in the false belief that an amp will sound "Hotter" and more aggressive and maximize power. But as with many things, there are several ingredients in the amp "Stew" that are responsible for a good sound and high output.

Last on my personal list would be running the output tube plate dissipation on the verge of red plating, particularly if it didn't do that back in the 1960's with the lower wall voltages. Remember the wall voltages have increased since that amp was designed, so you may be required (by vintage tube amp law ? ) to adjust your amp, to set things back to right so it runs like it did back in the day.

Only recently I realized how important Screen voltage is to power output, more so than the last few watts of plate dissipation. You saw that first hand when you dropped the size of the too large screen resistors and the amp woke up, so that's a bit more important.

I vote for sticking with the 270ohm or perhaps the 300ohm cathode resistor, as the tone difference should be minimal, and the amp and tubes will run cooler and longer, IMHO.

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14. Haven't been amping in a while so haven't logged in. Had recent notifications on this thread so looked back through it. Sadly, the guy that this was built for, Paul Slagle of Pittman Guitars, died 2 days ago. I built 2 more amps for him after this one and I have 4 of his beautiful guitars. Here's the most recent one he did for me. I'll miss that guy more than most people I've known in my life even though we never actually met in person. He was a real gem. So long old buddy.

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15. Originally Posted by mort
Here's the most recent one he did for me.
That is one beauty of a Telecaster! Is there some kind of polka dot pattern on the control panel, or am I seeing a reflection of some sort of acoustic tile?

I like the little flat spot where the output jack is mounted. Nice practical touch.

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16. Originally Posted by Leo_Gnardo
That is one beauty of a Telecaster! Is there some kind of polka dot pattern on the control panel, or am I seeing a reflection of some sort of acoustic tile?

I like the little flat spot where the output jack is mounted. Nice practical touch.
That's just reflection of the pegboard where he hangs his tools and such. You can see it on the body too.

And as for the amp.. here's a video demo his friend did for me. I've been on memory lane ever since I got the news about his death.

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17. Do you remember what cabinet it is and what speakers it used?
Thanks!

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18. Originally Posted by Pedro Vecino
Do you remember what cabinet it is and what speakers it used?
Thanks!
In the video I have no idea. On the original it was a combo I believe but I don't know what speaker it was.

2

19. By the way, good to see you back around, former 'semi-noob'.

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20. Originally Posted by g1
By the way, good to see you back around, former 'semi-noob'.
Thanks!

I kinda move around between amp building, kayak fishing and football card collecting as my hobbies. The season of amp building again feels like it's coming back around.

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21. Originally Posted by mort
Thanks!

I kinda move around between amp building, kayak fishing and football card collecting as my hobbies.
Kayak fishing is very popular here in the PNW as well. Though the species are probably very different and a neoprene wet suit is a must here. I like to fish but I haven't gotten up the nerve to try kayak fishing in these hypothermic waters yet

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22. How many kayaks can you catch on an average day?

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23. Originally Posted by Enzo
How many kayaks can you catch on an average day?
I told a friend from Cali that I took up mushroom hunting here in the PNW. He asked me if I use a knife or a gun.?.

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