1960's Supro Thunderbolt scratch build copy

[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?

[Chuck H]

Looks OK to me. Looking at it I'm not crazy about the design but it seems to be a popular build. Some modern safety and reliability changes I might make would be to include an earth ground (not indicated in your schematic), a DPST mains switch, move the first filter to the other side of the standby switch and double up on the rectifier diodes (the schems I'm seeing on line use a bridge, rather than a full wave rectifier). None of this should alter the tone one bit.

Were these used by some famous player or used on some popular tunes?

[mort]

OK so I finished it out and brought it up slow. All looked ok so I took it off the limiter.

Some notes:

Good overdriven guitar sound but virtually no clean headroom, this is acceptable but wouldn't mind just a little bit of headroom

tone control doesn't affect anything

volume control is wired backwards (0 is max and 10 is quiet)

low to moderate hum. Would like to quiet the amp a little if possible but *some* noise may be inherent in circuit

Supply voltages
D- 454
C- 449
B- 409
A- 313

[mort]

Looks OK to me. Looking at it I'm not crazy about the design but it seems to be a popular build. Some modern safety and reliability changes I might make would be to include an earth ground (not indicated in your schematic), a DPST mains switch, move the first filter to the other side of the standby switch and double up on the rectifier diodes (the schems I'm seeing on line use a bridge, rather than a full wave rectifier). None of this should alter the tone one bit.

Were these used by some famous player or used on some popular tunes?

I'm not really sure. I'm building this as a sort of surprise trade with a friend. He had mentioned loving these amps so this is what I chose to build as my end of the surprise. I'm building him an amp and he doesn't know what he's getting until it shows up at his door and he's building me a guitar that I won't know anything about until it hits mine. Nothing is critical.. this is all for fun.

[Chuck H]

The only noise that should be considered inherent to audio circuits is thermal hiss. Some tubes are hummy, and some hum is almost inescapable but it should be able to be kept low enough that it can be ignored. The hum is almost certainly because of the preamp tubes, the grounding scheme or both.

I see that you have a ground buss bar. If it's connected to the chassis at both ends that can cause ground loops. Try cutting the bar at the nearest point where the preamp ends and the power amp begins. At the very least you should isolate the power supply grounds from the signal path grounds. And nothing should be grounded in more than one place. If you consider it, the buss bar grounds everything in two places.

One thing that can help reduce internal hum coupling within the tubes themselves is elevating the CT of the filament wind on a DC voltage. Since you're already cathode biased you can use the power tube cathodes as a DC source. Since you wouldn't want a transformer or tube failure to couple the wrong things together you can unground the 6.3V wind CT, run it through something like a 22R. 1/2 watt resistor to the top of the cathode bias resistor.

I've had several new tubes come out of the box with generous hum built in. Mostly the rusky 12--7 tubes.

P.S. Very tidy build.

[bob p]

Were these used by some famous player or used on some popular tunes?

That's the lore. Telecaster + various Valco-made amps.

[loudthud]

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?

I usually wire up ground first. It's the most critical node. I recommend spliting the ground as shown in the attached edited schematic. I can't see what you used for filter capacitors, it may not be possible to split the ground if all the filter caps are in a single can. Each ground symbol is a separate connection to the chassis. It looks like the red/yellow wire from the transformer is grounded at a transformer bolt. Ground the first filter caps and the output tube bias resistor there. Add more ground lugs if you need to. The big ground wire for the preamp that grounds at the input side of the chassis is close enough to the input jacks to be considered one ground connection. Ground the A and B filter caps somewhere along that wire.

[Chuck H]

The big ground wire for the preamp that grounds at the input side of the chassis is close enough to the input jacks to be considered one ground connection. Ground the A and B filter caps somewhere along that wire.

But only have the buss wire connected at the input side. Not the PS side AND input side, as it appears to be now.

[loudthud]

But only have the buss wire connected at the input side. Not the PS side AND input side, as it appears to be now.

Yes, the big buss wire will be the quiet ground. It should be only connected to the chassis near the input jacks. The A and B node filter caps ground to that buss wire, they are isolated from the hum on B+ by the 27K 2W resistor. The big buss wire has to be disconnected from the noisy ground in the power amp. The power amp grounds to the noisy ground of the power supply at the transformer bolt. This is not an ideal ground, but it's how old Fender amps are grounded.

[mort]

I'll take a look at the grounding and adjust it as you suggested. It's not a ton of noise but if it can be cleaned up a little then great.


Found my mistake on the tone control. Forgot to ground and solder a couple nodes from the tone control circuit (visible in the first two pics). Easy fix.


What would I change if I wanted to increase the bass response by just a little teeny bit. I like how the amp has a nice growl without that woofy bass sound in it but just a smidge more bass response would be nice.

[mort]

And here's a set of operating voltages taken just now. I can't quite remember how to calculate actual wattage (plate dissipation?). I did put in the sense resistors to make it easier, I remembered that much at least.

V4- 29mV across 1 ohm resistor
3- 466v
4- 185.2v
5- .31v
8- 15.6v


V3- 28mV across 1 ohm resistor
3- 465v
4- 182.6v
5- .68v
8- 15.6v

V2
1- 225.9
2- .037
3- 1.99
6- 251.2
7- .097
8- 2.54

V1
1- 249.1
2- .08
3- 2.62

[Chuck H]

Well that input capacitor has a knee of -3dB at about 100Hz. -4dB at 82Hz. Bumping that up to .022uf should do. But since this is the earliest place to affect the LF it also means the LF here will suffer the most clipping, increasing the woofy. The LF has a pretty straight shot through the rest of the circuit so there's not much to be done other than choosing a speaker with strong bottom end. Increasing the first filter could tighten the bass back up a little. With the diode rectifier there's no need for it to be as small as it is and the 20uf value was probably a matter of cost and availability back in the day. At least double it. I'd go 100uf if it were my call. IMHE it won't detriment the tone to increase just the first filter value. You might notice less 120hz noise though

You could also change the taper on the tone pot so that the perceived bass is greater. If you have a linear pot change it to an audio pot. If you have an audio pot change it to an A10 audio pot.

Another thing will tighten the LF would be a zener diode across the power tube cathode resistor. It won't add LF but it will help tighten the LF you've added when amp is clipping. Use a value chosen as follows...

Get the amp to where it starts to show obvious clipping on the wave form (if you have a scope) Not a full square wave. Just a little flat spot. With the amp operating like this, measure the voltage on the power tube cathode. Choose a zV one volt higher than your cathode voltage. If you don't have a scope just measure the voltage at idle and choose a zV three or four volts higher. The rating should probably be about 5W for the diode. This rating IS available in a DO package so mounting is easy.

What this does is allow the amp to operate as cathode biased up to the point where it starts clipping harder and then the bias morphs to fixed. Thin puts a ceiling on any cathode bias squish from continued bias shift and keeps the amp a little tighter.

If you did all four things, change the input cap, change the tone pot, increase the main filter and add the diode to the bias circuit, I'm certain you would notice a marked improvement in bottom end responsiveness. Maybe not much actual increase in bottom end. You'll need a bigger OT and the right speakers for that.

[Chuck H]

I think you have your PS nodes connected cattywhompus. With the power tube screens on node B and the preamp on node C.

EDIT: None of what I said in the previous post applies before the voltages are corrected and the amp is reassessed.

[mort]

I think you have your PS nodes connected cattywhompus. With the power tube screens on node B and the preamp on node C.

EDIT: None of what I said in the previous post applies before the voltages are corrected and the amp is reassessed.

In the initial schematic they lettered the nodes weird. to keep things easier for me to follow from their schematic to mine, I just kept their nomenclature.

The nomenclature not withstanding, do you still think I hooked them up wrong? I was very careful to not make a mistake there but perhaps I did....

[Chuck H]

Your voltages would indicate that it's wrong. You need to check to be certain one way or the other.