Supro inspired amp to build soon just finished 1st schematic

[swingarm]

Greetings,

I'm preparing to build this amp and just finished the schematic. The original schematic used a different PI that I do not have a lot of experience with and thought that I may like to add a master volume or other mods down the road so I used the long tail pair.
So anyway, If you fellas could have a look and share your knowledge and general feedback positive negative etc I would sure appreciate it. The bias circuit is a little more complicated than it needs to be but I thought individual tube biasing would be nice so I can use weak or strong tubes. I also omitted the tremolo from the circuit

[Alan0354]

OK, this is JMHO from my very limited experience in designed only 2 amps:

1) I would put one more 12AX7 as the second stage. Usually the schematics of most amps I've seen have two gain stage before driving into the long tail differential stage. I'm afraid you don't have enough gain in the signal chain to make the amp sound good.
1) 470K grid stop resistor to the power tube is too high, should be no more than 4.7K
2) Quite a few amps did the triode switch, my personal and comments from others are the sound is not that good, it's wimpy.
3) I can't see the bias circuit clearly, I assume you adjust the bias according to whether you select cathode or grid bias.
4) I experimented a lot with the bottom channel type of "T" filters, I never get any one remotely I consider acceptable sound......But that might be just me. I would consider putting the tone stack of a typical Fender/Marshall tone stack on the second channel. That way you have the more tweed type tone stack for the top channel and the Fender clean channel for the second. But as I put in 1), the Fender tone stack is high loss and you definitely need the second 12AX7 to get the signal up.

From my experience, if you don't have the second 12AX7 stage, you are not going to have enough gain to get any OD, so putting in a master volume won't do you any good. Even with two stages gain, a master volume won't likely give you any meaningful distortion. To back up my assertion, try on any of the 70s SF fender amps with master volume. Try the "normal" channel with the volume to 10 and master volume down, you are not going to get any distortion unless you have a supper hot guitar. The normal channel is a two stage preamp. The bright channel has 3 stages to get distortion.

Hope that help.

[swingarm]

Thank you for your reply.

It appears i made an error on the grid stops for sure thanks. I want to post the original supro schematic for some comparison and maybe link a clip. Maybe someone can identify the Phase inverter in thsupro les paul full drive.mp3at amp. If so maybe they could compare and contrast the revision to to the long tailed pair? If removing that part of the amp is where the magic is I will keep it there.

s6424_schematic.pdf

[EFK]

All of the valco made amps used a paraphase inverter and trying to run it as a ltpi is going to completely change the character of the amp. The paraphase design ca. mid 1950s into @ 1961/62 is particularly important as it is somewhat unbalanced and responsible for much of the 'trashy' or raunchy character of these amps; it adds considerably to the distortion, as by the time the volumes are up to around noon or greater, the single preceding stages are supplying enough drive to begin to distort the input the paraphase. Furthermore most of the truly classic Supro/valco tones are attributable to grid leak inputs which add yet more to the lo-fi nature of these amps. It appears you have based much of your schematic pre-inverter to the 6424 amps, which were the final version of the earlier 1624T type circuits. These are the brightest and thinnest of the type.

You can run a master volume post paraphase, obviously the cross line type which I think sounds like ass but you can also run the popular fisher/larmar or whatever type if you are willing to add an extra caps/resistor - you basically need an extra cap and power tube grid resistor to "insulate" the voltage divider necessary to a paraphase from the symmetrical variable grid leak resistors inherent to the fisher type master volume.

None of the real Supro/valco amps of the 1950s/early 1960s used grid stops on the power tubes. The later 60s Thunderbolt did, but it was a bass amp and despite it's ridiculous ebay pricing, is merely 'meh' for guitar imho.

They also never used individual screen resistors, and I've not found any need for them either with nos 6973, 6cz5 or with the newer EH 6973 which don't sound the same but seem to hold up in a pretty tough way. You can also use 6CM6 if you match up pairs within 2-4 mA. No need to change the cathode R sub-350VDC.

I guess what I'm saying is that with some fairly dramatic changes as shown, you no longer have a Supro/valco amp. It may sound great, but it's not a valco.

[Chuck H]

Swingarm, EFK has the right of it above. So much of how this sort of amp sounds, especially clipped is in the PI, power amp and power supply specifics and NOT in the preamp. With that being the case, a PPIMV is tonal suicide for this design. Believe it or not, any mojo the preamp is contributing to the tone could be simulated with a much simpler circuit. Using that preamp with a different power amp is precisely the reverse of how "I" would begin simulating that amp. The key players in that amps tone/feel are going to be in the plate voltages, drive level/balance on the power tube grids, bias and bias shifting under pressure for the power tubes, load matching, power supply sag envelope, etc. Pretty much everything BUT the preamp. It's a common error on the part of novice designers to approach it from an angle that the preamp is the tone and the power amp just makes it louder. This is dead wrong. So, as mentioned, it's probable that the amp design you present would be a perfectly good amp, but it wouldn't sound like that Supro.

[swingarm]

Thanks for the great info fellas really appreciate it! I think I will just emulate the amp the best I can and play around later. It seems this schematic is the best one, No? The 1624t floating around the net seems incomplete? Do you fellas know where to find "the best one"

supro_s6424.pdf

It's a common error on the part of novice designers to approach it from an angle that the preamp is the tone and the power amp just makes it louder. This is dead wrong. So, as mentioned, it's probable that the amp design you present would be a perfectly good amp, but it wouldn't sound like that Supro.

LOL yeah I'm a weakling on theory. I just copy and build things for fun. When I'm building amps its the one (or two) things that make time fly terribly fast!

Thanks again fellas!

[Chuck H]

I mentioned the preamp/power amp relationship because as a novice that's how "I" approached it at first. It didn't take long to realize my error, but if I can save anyone the hassle of a couple of disappointing projects I will

[swingarm]

OK here is a revised schematic. Please have a look and criticize as necessary.

Thanks,
revised supro schem.pdf

[Dave H]

If I'm reading your schematic correctly the grids of the input 12AX7 are floating and TP0 should be grounded.

The PI has changed to Alan's favorite the floating paraphase but it won't work because it is drawn incorrectly. The outputs to both power tube grids are taken from the 'virtual earth' point. They should be from PI tube plates (I was going to say V1 and V2 plates but you have two V1s)

[swingarm]

OK I fixed the test point ground. but when i look at the rest I cant see my error. Not to say its not there

revised supro schem.pdf

I followed this

[Dave H]

when i look at the rest I cant see my error.

Compare the schematics again yours is not the same.

[swingarm]

LOL, ok ,i did thank you for your patients I'm in denial or senile or both !! please confirm that this part is resolved. Also have a look at the switch/input area too if you would .

revised supro schem.pdf

[Dave H]

The PI looks OK now but your TP0 ground symbol has drifted over to the top of the OT B+
I wouldn't use a pot for PI balance. A fixed resistor is good enough.

I'm not sure about the switch you have at the input but the links on the back of it are connecting both inputs together in all three switch positions. There is an 820R resistor across the input. That can't be right.

[Chuck H]

Are we being trolled!?!

I'm having a hard time with the previous discussion culminating in the last schematic for evaluation. If you want a Supro, build a Supro! With nearly all the clipping occurring in the power amp, and all the nuance of how that happens being in the power supply, anything else is, well, anything else!

The OP clearly has some electronics understanding to be improving a bit. Omitting some things that may not matter or adding switchable differences. With that in mind it seems odd then that the bypass cap was omitted from the previous PDF and the potential good of incidental imbalance was ignored. This tripped my troll flag.

A stripped down version of the Supro in question should give good results without the need for any switchable parameters or balancing pots. In fact, those things are almost contrary to the design in some respects.

Just build the Supro design and "Muntz" what you don't need! The internal jumper for channels is a good idea, yes. But it should be eminently obvious to someone who is already implementing circuit differences and omissions that the all continuous switch contact can't work. Why ask? You only need to draw it out and follow the leads!!!

I think we're being trolled.

[swingarm]

Are we being trolled!?!

I'm having a hard time with the previous discussion culminating in the last schematic for evaluation. If you want a Supro, build a Supro! With nearly all the clipping occurring in the power amp, and all the nuance of how that happens being in the power supply, anything else is, well, anything else!

So what?

The OP clearly has some electronics understanding to be improving a bit. Omitting some things that may not matter or adding switchable differences. With that in mind it seems odd then that the bypass cap was omitted from the previous PDF and the potential good of incidental imbalance was ignored. This tripped my troll flag.

Ohhh, it tripped your troll flag did it? you babbled The OP clearly has some electronics understanding to be improving a bit. what kind or word salad is this? is this on topic? are you talking to me or the community at large? What is your motivation?

A stripped down version of the Supro in question should give good results without the need for any switchable parameters or balancing pots. In fact, those things are almost contrary to the design in some respects.

So either i do it as you envision it should be, or I'm a troll for having limited theory? Look here ass, I have built several functioning amps I know some basic stuff, and resent your snobbish "liver on Wednesday" attitude on how things should or should not be.

Just build the Supro design and "Muntz" what you don't need! The internal jumper for channels is a good idea, yes. But it should be eminently obvious to someone who is already implementing circuit differences and omissions that the all continuous switch contact can't work. Why ask? You only need to draw it out and follow the leads!!!

I think we're being trolled.

You think "We're" being trolled? Lol, so desperate to form reality by consensus huh? Is this is the way people are here? WTF?

I guess i need to really prepare and not shoot from the hip huh? Don't want to be publicly humiliated by the self professed elite of the group now do I?

troll
1a. Noun
One who posts a deliberately provocative message to a newsgroup or message board with the intention of causing maximum disruption and argument.

1b. Noun
A person who, on a message forum of some type, attacks and flames other members of the forum for any of a number of reasons such as rank, previous disagreements, sex, status, ect.
A troll usually flames threads without staying on topic, unlike a "Flamer" who flames a thread because he/she disagrees with the content of the thread.

1c. Noun
A member of an internet forum who continually harangues and harasses others. Someone with nothing worthwhile to add to a certain conversation, but rather continually threadjacks or changes the subject, as well as thinks every member of the forum is talking about them and only them. Trolls often go by multiple names to circumvent getting banned.


2b. Noun
An aesthetically repulsive person, often has terrible social skills, usually a woman. See Troglodyte.

You know I see some of these things in you actually, no?

Kiss my ass *tips hat*