Another Supro 6424 build (at least, when I have all the parts)

[Bash]

Hi everyone,

Some time ago I build an AC18 (like a single channel 18 Watt AC15, without tremolo) and now I want to build a matching Supro DualTone amp.
Like some others here I would like to build a Supro 6424 amplifier, based on Steve Luckey's schematic and layout.
I changed the lalyout a bit, so it will fit all the turretboard and chassis that I have.


So, yes. It will be a vertically mounted amplifier.

Attached you can find the PDF with my updated layout. I thought it might be fun to add the possibility to switch between parallel and series for the preamp.
Will this be a good idea?
Is there any reason why I really should not do this?


BTW I updated the layout and removed the series option. It did not sound nice at all.

[bmccowan]

Hey Bash, I think you accidently put the handle and feet in the wrong place on that amp.
Kidding aside, as it looks like very nice work, do you have a schematic drawing of that switch arrangement?

[sluckey]

Sounds like a fun project. I'll be following this thread with interest. Nice looking amp in your pic.

[SILVERGUN]

I thought it might be fun to add the possibility to switch between parallel and series for the preamp.
Will this be a good idea?
Is there any reason why I really should not do this?

In an attempt to help you stay straight, what do you mean by "parallel and series"?


Sluckeys input switch is a great idea and there is no reason why you shouldn't try it. I'm assuming this is what you were referring to:

[Bash]

Sluckeys input switch is a great idea and there is no reason why you shouldn't try it. I'm assuming this is what you were referring to:

No, I wanted to try something else.

The two preamps in the original Supro design (and in Sluckey's schematic) are used in parallel.
The input signal is split and handled seperately by the two independent preamps.
His switch makes it possible to use either preamp one, preamp two or both in parallel.

My idea is to offer two inputs. One will use only a single preamp.
The other will use both preamps. By default this will be in parallel, but I wanted to add a switching option to use them in series.

Hopefully this would provide more gain, and possibly you can use the first preamp's volume to define the gain and the second one to be used as some kind of master volume.

[Bash]

Kidding aside, as it looks like very nice work, do you have a schematic drawing of that switch arrangement?

Somewhere on a piece of paper, done with pencil.
I translated it quickly to a layout (see attachement in my original post).

[bmccowan]

I have a hard time following layouts - but I think your layout has some mistakes - ex. look at pin 6 on your preamp tubes.

[Bash]

I have a hard time following layouts - but I think your layout has some mistakes - ex. look at pin 6 on your preamp tubes.

Damn, you are right. I connected the OT to pin 6 instead of 9 (because the numbers are shown around the tubes on the schematic, my brain turned the sixes into nines).
Thanks.

I updated the PDF in the original post.

[sluckey]

 

[Bash]

I am pretty sure it would not have sounded like that... 

[Bash]

And Sluckey, I would also like to really thank you for your great and informative site (which I plundered a bit).

But, coming back to my original question: is there any reason not to try out connecting the preamps in series?

[SILVERGUN]

And Sluckey, I would also like to really thank you for your great and informative site (which I plundered a bit).

But, coming back to my original question: is there any reason not to try out connecting the preamps in series?

Most of us would like to see a schematic drawing of how. It can be in pencil on some toilet paper, that would be better than trying to decipher the circuit from the layout.

[sluckey]

Lot of people try to cascade one preamp into another. Might take a little fiddling around. The amp sounds pretty gritty as is but when you cascade the preamps, it's gonna sound hot and nasty. Go for it. Let us know what you think.

[Bieworm]

Lot of people try to cascade one preamp into another. Might take a little fiddling around. The amp sounds pretty gritty as is but when you cascade the preamps, it's gonna sound hot and nasty. Go for it. Let us know what you think.

But be aware that the volume boosts hard in cascade mode... it ain't just more gain. Forget about switching that thinking it's the same volume level. I did that with my 18W and it's boom!! Muuuuch more volume

[Bieworm]

I have a hard time following layouts - but I think your layout has some mistakes - ex. look at pin 6 on your preamp tubes.

Good catch. The brown wire shoukd be at the nr9 lugs instead of nr6. For the preamp tubes that is..

[bmccowan]

Might take a little fiddling around. The amp sounds pretty gritty as is but when you cascade the preamps, it's gonna sound hot and nasty. Go for it. Let us know what you think.

The 18W TMB will maybe provide some ideas for the fiddling. I built a TMB and the cascaded channel is indeed hot and nasty. I'm too old for hot and nasty, so I fiddled with it a lot to tame that channel. I plan to convert the power amp to 6973s (just because I like to fiddle) and that will not be a lot different from what you are suggesting.

[Bash]

Most of us would like to see a schematic drawing of how. It can be in pencil on some toilet paper, that would be better than trying to decipher the circuit from the layout.

Well, by popular request: here it is.

[Bash]

Might take a little fiddling around. The amp sounds pretty gritty as is but when you cascade the preamps, it's gonna sound hot and nasty. Go for it. Let us know what you think.

The 18W TMB will maybe provide some ideas for the fiddling. I built a TMB and the cascaded channel is indeed hot and nasty. I'm too old for hot and nasty, so I fiddled with it a lot to tame that channel. I plan to convert the power amp to 6973s (just because I like to fiddle) and that will not be a lot different from what you are suggesting.

Well, I am hoping I will get a bit of nasty.
If I don't like it, it is easily removed to "normal" configuration.

BTW I happen to own a real Supro DualTone (one of the new ones) so I am curious how it will compare.

[SILVERGUN]

Most of us would like to see a schematic drawing of how. It can be in pencil on some toilet paper, that would be better than trying to decipher the circuit from the layout.

Well, by popular request: here it is.

From a thisgoesintothat standpoint your switching idea looks good, with a standard DPDT on-on switch

[SILVERGUN]

If you don't have an OT yet I would suggest snatching up one of these before they are gone. The manufacturer is going out of business.
https://tubedepot.com/products/classictone-output-transformer-40-18063

[Bash]

@Silvergun Thank you for the tip.

Unfortunately it is nowadays a bit of a hassle to get stuff from the USA shipped to Europe (and shipping has become very expensive).

So, I'll first try the same output transformer I used in the AC18.

[Bash]

On another forum (that is mostly in German, which I can read reasonably) I got the following tips:

1. 6973 has Grid #1 connected to Pin 3 and 6, I would bridge these pins to avoid having one side "floating"
2. same for Grid #2 , 6973 has Grid #2 connected to Pin 1 and 8, I would bridge these pins to avoid having one side "floating"

If I look at the schematic I see this as well.
Just not in the layout nor photographs.

I made the adjustment to the layout and added them to the first post.

[sluckey]

1. 6973 has Grid #1 connected to Pin 3 and 6, I would bridge these pins to avoid having one side "floating"
2. same for Grid #2 , 6973 has Grid #2 connected to Pin 1 and 8, I would bridge these pins to avoid having one side "floating"

I made the adjustment to the layout and added them to the first post.

Say what??? No, no, no! The pins are bridged together ***INSIDE*** the tube. Take those external jumpers off the sockets.

[Bash]

1. 6973 has Grid #1 connected to Pin 3 and 6, I would bridge these pins to avoid having one side "floating"
2. same for Grid #2 , 6973 has Grid #2 connected to Pin 1 and 8, I would bridge these pins to avoid having one side "floating"

I made the adjustment to the layout and added them to the first post.

Say what??? No, no, no! The pins are bridged together ***INSIDE*** the tube. Take those external jumpers off the sockets.

I will immediately...
Thanks.

[sluckey]

Haha. Just kidding. I just wanted to make the point that the jumpers already exist inside the tube. External jumpers are redundant but they wont hurt anything. I found a photo that may interest you. Pins 1 and 8 are jumpered on the socket. That's redundant but perhaps there is some reason that I don't see. But pins 3 and 6 are interesting. Notice that the grid resistor is on pin 3, but the coupling cap is on pin 6. I suspect this is just a convenience for the point to point wiring. Anyhow, lot's of ways to skin a cat.

BTW, I just now verified with my eyes and my ohm meter that the jumpers are really inside the tube.

[Bash]

External jumpers are redundant but they wont hurt anything

I figured as much. Just shorting a short 

I found that information as well (should have done that beforehand)

The only reason I can find to jumper 1 and 8 is for using a 6CZ5 instead of a 6973.
For now, I have no intention to do so.

[bmccowan]

The only reason I can find to jumper 1 and 8 is for using a 6CZ5 instead of a 6973.
For now, I have no intention to do so.

Jumping 1&8 allows using 6CZ5 as you state, and also 6CM6 which is closer to the sound of a 6V6. I have 3 6973 amps and added that jumper to all. The reason - the NOS stock RCA 6CZ5s sound better than new EH 6973s. I have bought both RCA and Sylvania 6CZ5s. The Sylvanias do not physically resemble a 6973 and do not sound as good. But as near as I can tell, and I have, out of curiosity, compared the RCA 6973s and 6CZ5s with a magnifying glass, the structure is identical except for the pin connection. Yes they supposedly have a lower voltage rating, and some claim they sound different; but I say they are twins; structure and sound.
RCA 6CZ5s are still available on ebay for $10. Half the price of the EH. Unless the seller knows the above, then they double the price.
So I say connect the pins and try those tubes.

[Bash]

Thanks bmccowan.

That is a good thing to keep in mind.
Unfortunately 6CZ5's are a bit less common in Europe.
Or people know what they are and ask high prices.

[bmccowan]

Ah, you are across the pond. I hope things are going ok in your location. Things are a bit crazed here!

[Bash]

Well, concerning Corona: things are slowly improving over here.
Still, no restaurants, no live music, etc.

Luckily, Dutch politics are mostly boring (which usually is a good thing).

[Bieworm]

Well, concerning Corona: things are slowly improving over here.
Still, no restaurants, no live music, etc.

Luckily, Dutch politics are mostly boring (which usually is a good thing).

Lucky you! Here in belgium it's still prison. No practices.. that's the crap man!!! Brand New amp built and no chance to really try it out...

[Tirol29]

Thanks bmccowan.

That is a good thing to keep in mind.
Unfortunately 6CZ5's are a bit less common in Europe.
Or people know what they are and ask high prices.

try here :
https://electols.com/shopping/fr/audio/68095-6CZ5.html
https://electols.com/shopping/fr/tubes/96774-6973.html

[sluckey]

Y'all peaked my interest in the 6CZ5 so I started looking. Right off the bat I see that the RCA tube manual says pin 8 is internal connection. Do not use. But why? RCA doesn't elaborate. So, why would you want to put a jumper on a 6973 socket to operate a 6CZ5?

Seems to me that if the 6973 socket has pin 1 wired for the screen and nothing on pin 8, the 6CZ5 should drop right in.

     https://www.tubesandmore.com/sites/default/files/associated_files/6cz5.pdf

     https://www.tubesandmore.com/sites/default/files/associated_files/6973.pdf

[Tirol29]

Seems to me that if the 6973 socket has pin 1 wired for the screen and nothing on pin 8

RCA RC-30 1975 receiving tube manual says pin1 and 8 connected to G2 (p 491) 

[Bash]

try here :
https://electols.com/shopping/fr/audio/68095-6CZ5.html
https://electols.com/shopping/fr/tubes/96774-6973.html

Thanks. I will have a look.

[sluckey]

Seems to me that if the 6973 socket has pin 1 wired for the screen and nothing on pin 8

RCA RC-30 1975 receiving tube manual says pin1 and 8 connected to G2 (p 491) 

We know. Not talking about the tube. Talking about the socket.

[bmccowan]

Steve brings up interesting points which made me go surfing. Found this:
http://www.bustedgear.com/repair_Valco_62-90_tube_6973_3.html
When I put the jumpers in my amps - two Valcos and one I built. I blindly followed advice on the Web (always a good idea, right?) and found many amp users who were successfully using the jumpered sockets. Since jumping my amps, I have used 6973s, 6CZ5s, and 6CM6s in them with no negative issues. The 6CM6 lists no connection for pin 8.
But it appears that by using pin 1 on the sockets, that achieves the result? I do not recall which way the PT sockets are wired in the Valcos I own. Guess I'll need to open them up.
Datasheets for 6CZ5 indeed state "internal connection" - to what? is my question. Interesting discussion that is venturing beyond my vacuum tube knowledge level.

[sluckey]

That link backs up my point perfectly. Valco had the screen wires on socket pin 8, which works perfectly well for 6873s, since pin 1 and 8 are jumpered together INTERNALLY. But in order to use the 6CZ5, the guy had to move the wires from pin 8 to pin 1, since the 6CZ5 screen is internally connected to only pin 1. Now you can use either tube in the socket.

Still don't know what 6CZ5 pin 8 is internally connected to. Since you have some could you take a close look? And use your ohm meter to see if there's continuity from pin 8 to pin 1. In fact, see is there is continuity between pin 8 and any other pins.

I ran into a similar situation with some old 6BQ5s (EL84s). Tube manual shows pin 1 as internal connection Do not use. But in fact my old 6BQ5s had pin 1 jumpered to pin 2. Modern 6BQ5s or EL84s don't connect pin 1 to anything so it can be used as a convenient tie point. In fact Hoffman uses it to mount a grid stopper resistor in his Stout amp. Works fine with most new tubes, but the jumper in my ols 6BQ5s wouls simply short out the grid stopper in the Stout.

[bmccowan]

I will look and meter.
6BQ5s - I have several old Baldwin and Hammond labeled 6BQ5s. I'm wondering about those?

[sluckey]

Here's my 6BQ5. You can easily see that pins 1 and 2 are jumpered together.

[bmccowan]

I looked under a lighted magnifying glass at Sylvania, Tung-sol, and RCA 6cz5s. Pin 8 has no connection. It just stubs up through the support disc and dead-ends. To be sure, I metered - no continuity with any other pins.
Looking very closely, the Sylvania and Tung-sol clearly came off the same production line - not too unusual. The RCA has some distinct differences. A pic of the RCA is attached.
I have not looked at the old Baldwin and Hammond 6bq5s yet - first I need to find them

[Tirol29]

....
We know. Not talking about the tube. Talking about the socket.

sorry, should have read more and keep silent 

[Bash]

I am still waiting for all the parts to arrive.

Still no chassis or turret board, which makes it a bit hard to start.

Also found out that it has become impossible to get Supro-like tolex. So instead of making it blue, the amp will become exactly the same as the AC18 combo.

I am still on the lookout for a good speaker to put in the DualTone.
For now I think I will go with a WGS G12 C/S.

Any other suggestions?

[sluckey]

Put some long legs on the cab so us old farts don't have to think about bending over.   

[Bash]

Put some long legs on the cab so us old farts don't have to think about bending over.   

I am nearly 51 myself, so maybe I should start taking these things into account.
It also makes it look even more like an ancient television set.

[shooter]

just toss a rope over the stage lights and adjust height as needed 

[SILVERGUN]

I am still on the lookout for a good speaker to put in the DualTone.
For now I think I will go with a WGS G12 C/S.

Any other suggestions?

If you're trying to nail the Jimmy Page tone I would suggest the Jensen P12Q based on the fact that the Sundragon amp wound up using that speaker. See 2:00 in this video.
https://www.youtube.com/watch?v=HNCIAZYS8yc&feature=emb_logo


And, this fancy little pontificator of tone came up with the Jensen C12R:
https://www.youtube.com/watch?v=9nwBSqSREfg

[Bash]

Yay! The parts arrived today.
Finally, I can start.


The Jensens are on my list as well. I will first build the amp and just see how it sounds through the speakers I have at hand (Celestion Gold, Supro DT12 and an Eminence Red Fang).
BTW I am not necessarily looking for sounding like Page.
NO matter what I play, I always end up sounding like myself anyway.

[Bash]

Finally had time to use the soldering iron (a fulltime job and 3 kids tend to get in the way).
I've finished all connection to the pots and the power connection to the preamp tubes and power amp tubes.

Also found a mistake in my layout drawing (connected the 270k resistor to the wrong lug of volume 2), which I have updated in the first post.

Also started hammering away to get the turrets on the board.

[Bash]

Very little time to make any progress, due to preparations for the St. Nicholas festivities upcoming weekend.
So, mostly loose wires still.

Luckily I was able to find a worthy replacement for the blue Tolex (Pewter Trout Slub Bronco, that really is the name), which still has to be imported from the states.
So I will end up with a blond/charcoal pair of amps.