Supro Thunderbolt Build

[TubeJunkie]

Okay, so I'm on to my next build, which I've decided will be a Supro Thunderbolt clone, with three simple modifications: I'm going to cascade V1-B into V1-A using one input I'm going to call 1 + 2 (input 2 will remain original using V1-A only), I'm adding 100Ω 5W screen resistors, and I'm wiring pins 1 & 8 together to be able to use EL34 tubes.  I got the idea of cascading the triodes from an article by Jeff Bober, a former contributor to PREMIER Guitar.  After reading the article, I contacted him and he helped me (THANKS JEFF!!) with creating the circuit described in his article: https://www.premierguitar.com/articles/26171-ask-amp-man-modding-a-supro-6420-thunderbolt

Below is the original schematic used (from Weatherford Amplifiers), the modified schematic, and the layout.  Before I start any build, I like to get input on the design - both the schematic and the layout.  I would appreciate any feedback regarding mistakes, or even suggestions on the final design.  I am trying to keep it simple by not adding any additional knobs or switches; unless, of course, there is a very compelling reason to.  Thanks in advance for your input.  I'll probably begin the journey within the next week or two.

[sluckey]

Looks like a good project. Schematic looks good. I didn't look at the layout too closely but the cascade mod looks right.

[bmccowan]

That should be fun. Looking at your layout, I likely would simplify the power filtering to be 20 - 20 - 16 - 16 or something near there. No need to try to match exactly what Valco did, IMO. In fact, having worked on a lot of Valco amps, it is clear that they did not always try to match what they did the day before.
And maybe all the filter caps that serve the preamp should be grounded on that bus. Probably doesn't matter much though.

[TubeJunkie]

That should be fun. Looking at your layout, I likely would simplify the power filtering to be 20 - 20 - 16 - 16 or something near there. No need to try to match exactly what Valco did, IMO. In fact, having worked on a lot of Valco amps, it is clear that they did not always try to match what they did the day before.
And maybe all the filter caps that serve the preamp should be grounded on that bus. Probably doesn't matter much though.

I thought I'd start with the original, see how it sounds, and then maybe make changes (like different filter cap configurations).  You're right on the pre-amp filter caps, that 10uf next to the 4uf should be on the pre-amp buss (change made and re-posted).  What I find interesting is how much of what made older amps sound as good as they did/do, was just sheer dumb-luck.  From one day to the next, the parts being used would change, and then the specific person wiring-up and soldering a specific amp using that days parts, the result could be magic, or just ho-hum.

[bmccowan]

I thought I'd start with the original, see how it sounds, and then maybe make changes (like different filter cap configurations).

I think that is smart. However the parallel 10 & 22 uf filter caps at your B+C I would suggest is the same as using one 32uf cap at that location. Maybe others would recognize a difference.  Will save you some space on the board. BTW your layout is much neater than anything Valco ever built! I actually have an original Thunderbolt - but its a different variant - has a tube rectifier. I am off at a cabin, but when I get home I'd be happy to check out any component values for you. Cool amps - they emphasize power tube distortion rather than preamp.

[TubeJunkie]

. . . I am off at a cabin, but when I get home I'd be happy to check out any component values for you. Cool amps - they emphasize power tube distortion rather than preamp.

I'd appreciate that.  After searching the internet, there's dozens of variations with slightly different values on many of the components.

[bmccowan]

At home again. I opened mine up to take a look. I recalled that mine had a tube rectifier - I was wrong, it's SS. I worked on it over 15 years ago; that's my excuse. Anyways, all resistor values are exactly the same as the schematic you posted. The .01 .001 .003 and .005 caps match and are ceramic disc. The .05 signal caps I replaced with .047, so I'm pretty sure they were all .05. Preamp cathode bypass caps I replaced with values pretty close to the ones in the schematic. Original 20-10-10 can cap is disconnected but still in place. I installed 20, 16, 16 single caps inside the chassis (tons of room in there) - the single 4 is still in place, I'm surprised I didn't replace it, but I didn't. No evidence of an additional single 20 cap, but its possible I removed one thinking it was an earlier mod. The only other difference is that I have a 25uf cathode bypass on the power tubes but can't say if one had been there originally and my cathode resistor is a newer 250 rather than 200 - likely I upped the value.
In all, knowing Valco, I was surprised that the amp was virtually identical to that schematic.
Have fun with the project.

[SoundmasterG]

At home again. I opened mine up to take a look. I recalled that mine had a tube rectifier - I was wrong, it's SS. I worked on it over 15 years ago; that's my excuse. Anyways, all resistor values are exactly the same as the schematic you posted. The .01 .001 .003 and .005 caps match and are ceramic disc. The .05 signal caps I replaced with .047, so I'm pretty sure they were all .05. Preamp cathode bypass caps I replaced with values pretty close to the ones in the schematic. Original 20-10-10 can cap is disconnected but still in place. I installed 20, 16, 16 single caps inside the chassis (tons of room in there) - the single 4 is still in place, I'm surprised I didn't replace it, but I didn't. No evidence of an additional single 20 cap, but its possible I removed one thinking it was an earlier mod. The only other difference is that I have a 25uf cathode bypass on the power tubes but can't say if one had been there originally and my cathode resistor is a newer 250 rather than 200 - likely I upped the value.
In all, knowing Valco, I was surprised that the amp was virtually identical to that schematic.
Have fun with the project.

That Weatherford schematic was based on three T-bolts I overhauled, plus several other amps from other contributors. Sean Weatherford put it all together from all of us (I think there were 5 others besides me). So with all of the examples the schematic should be mostly correct. The previous schematics out there for T-bolts were completely unreadable.


I have some of the correct Heyboer clone transformers and a chassis all ready to go and still haven't built mine after all this time. Maybe one day. :)

Greg

[TubeJunkie]

At home again . . .

Thanks for the info bmc.  It interesting the changes made to the circuit over the few years it was originally manufactured and sold.  I wonder if it was an attempt to make it more bass guitar friendly, or if they saw the writing on the wall and it was to make it more guitar friendly.

That Weatherford schematic was based on three T-bolts I overhauled, plus several other amps from other contributors . . .

Greg, I'm glad you did that.  As you said, all the others are either unreadable, or they're readable but heavily modified, some to the point of being almost unrecognizable.  I've started my build, with the turret board populated and waiting for the initial soldering. Hope you're able to get your's started one day soon.

[bmccowan]

As so often happens - I thought, as long as I have it open I might as well do a couple of things. In doing so I did find a difference from the schematic and also spotted what I believe is an error in that schematic.
Difference - In a previous email I wrote that I was surprised I had not changed the cap that is marked 4uf on the schematic. It did not look like a 4uf to me (label faces down and was out of view.) I removed it and found that it is a 0.1 cap.
I notice that on all the older hand notated Thunderbolt schematics there is a ? at the 4uf capacitor. In my amp, the 0.1 cap is clearly original with no evidence of any work having been done on the associated joints. My limited understanding of the reason for a small film cap in that location is that it filters out some rectifier noise that larger electrolytic caps do not necessarily tackle. But I studied political science in school; SWTFDIK? - PRR, Sluckey, and others on here will know better than I do.
The schematic mistake - the voltage at A (downstream of the 100K resister) on that schematic should be in the 200s not the 400s. Likely just a typo. 

[TubeJunkie]

. . . I removed it and found that it is a 0.1 cap.

. . . the voltage at A (downstream of the 100K resister) on that schematic should be in the 200s not the 400s. Likely just a typo.

CAP - I've seen schematics with the "4uf ?" notation, seen some with an 8uf, some with a 10uf, and I even found one that had only 3 filter caps.  Your's is the first 0.1uf I've heard of.  Who knows, maybe the builder ran out of 4uf, picked one that was close enough to the size/shape, and wired it in upside down to hide the value so it would pass inspection?  Or maybe as you suggested, and attempt to tame some rectifier noise.

VOLTAGE - I noticed the typo as well on the Weatherford schematic and found one with a more realistic 275Vdc and put that on my modified schematic.  But, as it says on the schematic, those aren't measured voltages, just educated estimates.  I'll take measurements when I'm done with my build and post them.

[bmccowan]

I think it's intentional. I searched for Harmony 420 schematics. That is a Valco built amp. Identical circuit with the exception of some tone stack changes. There are blurry versions of that schematic that have a ? next to that cap; or a 4uf ? next to that cap. The same schematic is posted that clearly shows a .1 cap in that position. And I have a H 420 in my cellar waiting for a new PT. I just looked - it has the .1 cap.
The H420 schematic is attached. The .1 is between the two 100K resistors - right where the ? is in all those schematics.
When I finish the cleanup on mine, I will record the voltages and produce a final schematic.

[sluckey]

My S6424 uses .05µF and 100K for the preamp filters! My guess is they figured the B+ already had enough 60/120Hz filtering and all they needed for the preamps was to decouple the actual guitar signal from the B+ line. Sounds a bit cheap to me.

     http://sluckeyamps.com/supro/supro.pdf

And my Magnatone M2 used a .1µF for the final B+ node. That node fed three consecutive triodes! Sounded OK at low volume but had major motorboating at high volume. My fix was to replace the .1µF with a 47µF cap.

     http://sluckeyamps.com/lil_maggie/Magnatone_M2.pdf

[bmccowan]

I have not played through my Tbolt in a dog's age, and really don't recall how it sounded. At the time I played out some, but no longer and its a bit big for a bedroom amp! I'll see how it sounds with a new .1 in that position but also try other values and report back. I also will try it with and without a bypass cap on the power tube cathodes, as that GP article author preferred the amp without one.
Sluckey - have you tried other values in the S6424? I have a Gretsch version of that model and never thought it lived up to the reputation of that amp. Couldn't be the player

[sluckey]

Sluckey - have you tried other values in the S6424?

No. The 47 was the only new e-cap I had at the time. 22 or 33 would have probably worked just as well.

[bmccowan]

Thanks, but now I'm confused, I thought the 47uf was the fix for the Mag M2, not the Supro? I was curious if you tried different values other than the .05uf in the Supro?

[sluckey]

Thanks, but now I'm confused, I thought the 47uf was the fix for the Mag M2, not the Supro? I was curious if you tried different values other than the .05uf in the Supro?

You're not confused. I made a mistake. I did not experiment with the Supro because it had no issues.

[TubeJunkie]

My S6424 uses .05µF and 100K for the preamp filters! My guess is they figured the B+ already had enough 60/120Hz filtering and all they needed for the preamps was to decouple the actual guitar signal from the B+ line. Sounds a bit cheap to me.

The S6424, also called the 1624T, is the Supro DualTone amp, if I'm not mistaken.  I came across a schematic that is purported to be an S6424 field service schematic (below). It shows the .05uf cap after a 100k resistor and before a 270k.

[bmccowan]

Right - that's what Sluckey has in his schematic.
And thanks Steve for clearing up your comment on the Maggie/Supro thing. Apparently a small cap in the B+ before it feeds the V1 plate resistor is a pretty common Valco thing.
I have read that some audio amp designers do the same as an additional final filter for some rectifier noise that doesn't get filtered by the large electrolytic caps. Doubting my battered ears can hear the difference.

[SoundmasterG]

Greg, I'm glad you did that.  As you said, all the others are either unreadable, or they're readable but heavily modified, some to the point of being almost unrecognizable.  I've started my build, with the turret board populated and waiting for the initial soldering. Hope you're able to get your's started one day soon.

It was really Sean that got it started and followed it through. I think he might have made a post on Ampage and I was one of the few that responded and had the opportunity to work on some unmodified vintage amps and provide him the information. One of the very early versions of the schematic did use a 0.1uF that the others mentioned because that was in all three of the amps I worked on, but the other parties who contributed had a 4uF and disagreed, so Sean went with the 4uF. Maybe he felt a 4uF was more of a likely filter cap value? Anyway I can attest that the three amps I worked on used the smaller value and sounded and worked fine, so some experimentation is probably fine. The originals did use ceramic coupling caps btw.


Glad you're getting your amp started! The board looks good! Once I get my house sold and a car put back on the road and sold, then hopefully I can get back to fun projects and build my T-bolt clone. :)


Greg

[bmccowan]

Maybe he felt a 4uF was more of a likely filter cap value?

I suspect this is the case as nearly all the Tbolt schematics online have a "?" next to that cap. The 0.1 that came out of my amp, physically is the size of the old Sprague 4uf 250v caps I've seen - so its a logical conclusion. I fired mine back up last night after replacing the 0.1 cap with a 0.1 Mallory 150 - Sounds fine. I then changed the second filter cap from my earlier installed 16 to 30uf to match the parallel 10&20 found in most of these amps. Slight (could be imagining it) firming of the bass with that change. I changed the cathode bypass cap from 50uf (I think I previously replaced an old 33uf with a 50 cause it's what I had) to 10uf, and removed the plate to ground .001 cap. Those two changes brightened the amp some. Final changes are that I brought the power tube cathode resistor back to the original 200ohm with no bypass cap. The amp sounds good at medium volume. I cranked it for a moment, but it was 10pm and my wife yelled, WTF are you doing?
As promised I will record voltages. I'll also draw a schematic with the original 0.1 cap and one noting the small changes I made.

[bmccowan]

Voltages - Note that my line voltage is 117. I'm fortunate to live in the boonies at the end of the power line. Not so fortunate during ice storms.