Supro 1690T build

[dbishopbliss]

In the planning stages. Here's a link to the schematic: Supro 1690T.


I'm trying to find a suitable power transformer now that ClassicTone is out of business. Based on the schematic (snippet posted below), I believe I should be looking for a 620VCT transformer. However, I have a number of NOS Mullard 5AR4 rectifier tubes that I would like to use instead of having to buy a 5V4G. From what I understand the 5AR4 has a lower voltage loss than the 5V4G.


Should I look for a transformer with a lower voltage? If so, how much?


If I went with a 5V4G (which I might do because its closer to the original and has more sag) then would the Hammond 290HBZ be suitable? Is 150mA enough for two 6L6s and three 12AX7s?


290HBZGENERIC120V @ 60 Hz620V @ 150 mA50V Bias6.3Vct @ 2.25A5V @ 4A

[dbishopbliss]

Trying to answer my own question... Looking at the 6L6GC datasheet I see the Max Signal Plate current is somewhere between 54 and 78mA per tube. I'm not sure of the exact voltages (schematic doesn't include them). Does this mean that if the plate is seeing 300V I should be ok with the 290HBZ, but if its 250V I should find something else?

[sluckey]

Hammond 290HBZ should be fine, but expect the voltage to be higher than you expect. However, my choice would be Hammond 272FX.

     https://www.hammfg.com/files/parts/pdf/272FX.pdf

Every new PT I've dealt with in the past 20 years put out more voltage than shown on the data sheets. That also includes several Hammonds.

[shooter]

you have some control over plate volts and current with the bias resistor Rk.  You also have some control with type of rectifier, and "dropping" stages in the PS.  so as long as you're ballpark close.  I've only done 1 build where I tried to get REAL close to datasheet, then when I wasn't happy with sound, i tweaked and the amp overall wound up +/- 30% from datasheet

[dbishopbliss]

I just got a reply from Leon C on the gear page. He said he had 362V on the plates of the power tubes.


I see the 272FX is rated at 600 @ 115. Is the reason for the higher voltage due to higher mains closer to 120?

[dbishopbliss]

the Hammond 272FX for the Power Transformer looks like a good choice. Now, on to finding an output transformer. This is where things get weird....


The schematic says the OT measured between 9K and 10K into a 1.6 ohm load. It also says that the transformer was underpowered. On the other hand, the S6420 Thunderbolt transformer was rated 35 watts, 5K into 8 Ohms. Leon C used the Thunderbolt transformer and his clone sounds pretty good to me.


I think I mentioned in another thread that I will probably go with the OT Transformer made by Trinity rated for 22 watts. It has a 5K and an 8K primary. I'm thinking I could wire a switch to go between the two primaries. This way, I could use the 5K which is like the Thunderbolt and the 8K which is closer to the value in the original.


Thoughts? Any other ideas for a good output transformer choice?


P.S., If I decide I really don't like the Trinity transformer then I could always use it to build a AC15 lite. :-D

[dbishopbliss]

I have never seen inputs like this Supro before. I'm not exactly sure how to lay them out.


One of the odd things is that the "treble" inputs are not shorting jacks. Seems like this might be noisy. The signal is going through a small cap - I'm assuming this filters out the bass frequencies which is why its the treble input.


What is happening on the V1A tube? The schematic says there is a 6.8M resistor from grid to ground. I have never seen such a large resistor before in any circuit.







Here the layout I have come up with. Mechanically, the connections are there (I think) but perhaps it could be improved upon. Let me know what you think.

[shooter]

I believe the 6.8M R is called grid leak bias?  Notice there is no cathode R

[sluckey]

One of the odd things is that the "treble" inputs are not shorting jacks. Seems like this might be noisy. The signal is going through a small cap - I'm assuming this filters out the bass frequencies which is why its the treble input.

Correct. Not noisy. Notice the 100K to ground on the normal jack. Sunn did something similar with it's bright inputs. No shorting jacks were used. They just permanently connected a 150K from the grid to ground.

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

What is happening on the V1A tube? The schematic says there is a 6.8M resistor from grid to ground. I have never seen such a large resistor before in any circuit.

It's called grid leak bias. Look it up. Whenever you connect the cathode to ground you must apply a negative voltage to the grid for proper bias. You can do this using a fixed bias negative voltage applied to the grid or you can use grid leak bias. The easy way to identify a grid leak bias circuit is the cathode will be grounded and you will see a big resistor on the grid, usually accompanied with a coupling cap to prevent the negative grid voltage from leaking back to the previous stage.

[dbishopbliss]

It's called grid leak bias. Look it up... The easy way to identify a grid leak bias circuit is the cathode will be grounded and you will see a big resistor on the grid, usually accompanied with a coupling cap to prevent the negative grid voltage from leaking back to the previous stage.

Got it! Any thoughts on the layout.

[dbishopbliss]

the Hammond 272FX for the Power Transformer looks like a good choice.

I think I'm going to put the amp into a JTM 45 style chassis. It has the right number of cutouts for the tubes, inputs and controls. I just discovered that Mojo has a "Low Power" transformer for the JTM45 that is also 600VCT and is designed to fit in the chassis. Its back-ordered right now but I can wait.

The schematic says the OT measured between 9K and 10K into a 1.6 ohm load. It also says that the transformer was underpowered. On the other hand, the S6420 Thunderbolt transformer was rated 35 watts, 5K into 8 Ohms. Leon C used the Thunderbolt transformer and his clone sounds pretty good to me.

I think I mentioned in another thread that I will probably go with the OT Transformer made by Trinity rated for 22 watts. It has a 5K and an 8K primary. I'm thinking I could wire a switch to go between the two primaries. This way, I could use the 5K which is like the Thunderbolt and the 8K which is closer to the value in the original.

Trinity is not selling their OT by itself right now (supplies are limited because of covid and they don't want to break up sets). So I can either buy a set and save the PT for another project or start looking for alternatives. Hammond has a couple of options (of course) but the are fairly expensive.


Mojo has a couple of OT designed for other amps that could work:

• Tweed Super with 6K primary (same tubes as the 1690T)
• Trainwreck with 6.6K primary (30 watts)
• Supro 6424 with 7.5K primary (I think this is rated at 10 watts and is probably too low).
• Tweed Deluxe with 8.K primary

Thoughts?

[sluckey]

Layout looks fine. I thought you had already decided on transformers.

[dbishopbliss]

Layout looks fine. I thought you had already decided on transformers.

I thought I had too. :-)


Because Trinity won’t sell their output transformer I am looking at other options. While I was looking I saw the low power version of the power transformer at mojo. Because it would drop right into a jtm45 chassis Without having to cut any additional holes I am reconsidering.

[dbishopbliss]

I've been doing some reading about output transformers and I think I have a better understanding which can help me make a decision. Please let me know if I got this right.


The turn ratio of an output transformer doesn't change. If the schematic is correct, the original 1690T transformer has a turns ratio of about 75. This was calculated by taking the square root of 9000/1.6. So if the Page amp was running an 8 ohm speaker instead of the 1.6 ohm speaker this would mean the load would look like 45K = 75 * 75 * 8.


I plugged the numbers into an online calculator and the load line is very flat. Not sure if I'm reading the values correctly, but it says Output Power (W) at Max g1=5.32. Does that mean 5.32 watts? Maybe x2 and it will be 10.6 watts? I think I heard the the Page amp was only around 11 watts so maybe that makes sense.


I don't think I will be able to emulate that load without having something very custom made. But I did find the following transformers with a 8K primary and 4, 8 and 16 ohm secondaries. If I'm correct, then I could plug in a 16 ohm speaker to the 4 ohm secondary and get in the ball park (~32K load) and would be about 15 watts max.


https://www.mojotone.com/amp-parts/Output-Transformers/Tweed-Deluxe-Output-Transformer-w-4-8-16-OHM-Secondary
https://www.mojotone.com/amp-parts/Transformers_1/British-Style-18-Watt-Output-Transformer
https://www.edcorusa.com/cxpp25-ms-8k
https://www.hammfg.com/part/1750PA?referer=1025 (8.4K primary)


What do you think? Or am I over thinking this and I should just get a transformer designed for two 6L6s?


I guess the good news tweed deluxes and 18 watt marshalls are pretty popular so I could always try both and then sell the one I don't like.

[shooter]

get a transformer designed for two 6L6s

transformers aren't made for tubes, they are made to couple AC from primary to secondary efficiently
tube at a specific "point" want a specific load to operate efficiently
none of that matters much in guitar amps 


if you're really geeked, match the transformer to the tube datasheet at operating point you want (ie 300vdc plate, 53mA PP)
get multiple outut taps.  once it's working and you've played for a week through 7 different speakers and 3 guitars, then mess with speaker loads and repeat

[PRR]

....square root of 9000/1.6. So if the Page amp was running an 8 ohm speaker instead of the 1.6 ohm speaker this would mean the load would look like 45K = 75 * 75 * 8.....

What is a "Page amp"?? First time that came up.

It is VERY likely that two 6L6 at reasonable voltage will be loaded in 5k to 10k, NOT 45k. As you noted, that's too high to suck any power.

[dbishopbliss]

What is a "Page amp"?? First time that came up.


It is VERY likely that two 6L6 at reasonable voltage will be loaded in 5k to 10k, NOT 45k. As you noted, that's too high to suck any power.

Sorry, internal conversation. Supposedly Jimmy Page had a 1690T that got damaged so the two 10-inch, 3.2 ohm speakers were replaced with a 12-inch 8 ohm speaker. If my math is right (which I'm terrible at so it might not be), then the load in his amp would be 45K.

[shooter]

two 10-inch, 3.2 ohm speakers

are you confident they weren't wired series for 6.4 ohms?  6.4 ohms and 8 ohms will "sound" nearly identical

[dbishopbliss]

are you confident they weren't wired series for 6.4 ohms?  6.4 ohms and 8 ohms will "sound" nearly identical

I'm going off of what the schematic says.

[dbishopbliss]

Questions about the tremolo circuit:

• I cannot find any 500K Linear pots. I assume an Audio pot will be fine. Unless someone has a source.
• The SPST 500K pots I can find are labeled "for guitar". Is there a reason I should not use a "guitar" pot?
• I'm not sure what is happening with the switch on the speed pot. Is it just an alternative to the foot switch? Could I just use a regular pot and only use the foot switch to turn things on/off?

[acheld]

Here you go:

https://www.mouser.com/ProductDetail/Bourns/PDA241-SRT01-504B0?qs=%2Fha2pyFaduhqlB3AUoaTVovzBtZT46J1paV7L%252BeUkfnAgKkVA0fWx%2FNOmraAQYXX

Hundreds in stock.

[dbishopbliss]

Here you go:

https://www.mouser.com/ProductDetail/Bourns/PDA241-SRT01-504B0?qs=%2Fha2pyFaduhqlB3AUoaTVovzBtZT46J1paV7L%252BeUkfnAgKkVA0fWx%2FNOmraAQYXX

Hundreds in stock.

Thanks for that. I meant I cannot find a SPST 500K Linear pot.

[acheld]

If that's the volume pot on the schematic, I don't see high voltage on it, and I bet a guitar pot would be fine.

[PRR]

"Guitar pot" often means a different knob/shaft.

If you do not need an exact clone, get a pot and a separate switch. (This is better because you do not lose your setting when you turn off, but if you make a million, switch-on-pot can be a penny cheaper.)

[dbishopbliss]

"Guitar pot" often means a different knob/shaft.

If you do not need an exact clone, get a pot and a separate switch. (This is better because you do not lose your setting when you turn off, but if you make a million, switch-on-pot can be a penny cheaper.)

Hmmm, I was thinking about using a JTM 45 chassis that a power switch and a standby switch. But the standby switch isn't really necessary so maybe I will use that for the tremolo.


On the other hand, maybe I won't bother with the switch and just use the foot switch.

[dbishopbliss]

• I'm not sure what is happening with the switch on the speed pot. Is it just an alternative to the foot switch? Could I just use a regular pot and only use the foot switch to turn things on/off?

Why are there two switches on the Tremolo? Is the one on the pot the "master" if you will. That is, if it is open, it doesn't matter what the foot switch does. If it is closed then you can turn the tremolo on/off with the foot switch. I'm not getting it.

[sluckey]

Don't worry. Be happy. Just build it as drawn. If there is no footswitch plugged in you still have a way to turn off the tremolo. If you want to use a footswitch just plug it in and the front panel switch must be on for the footswitch to operate. BTW, we can't see the full size pic on imgbb.com. Maybe you need to share the image or make it public or something. Much easier to just link to the schematic in Hoffman's library...

     https://el34world.com/charts/Schematics/files/Supro/Supro_1690t.pdf

Or if you want an easier to understand switch arrangement, change to the S-6424 schematic...

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

[dbishopbliss]

Just build it as drawn. If there is no footswitch plugged in you still have a way to turn off the tremolo.

I'm thinking about omitting the foot switch altogether and just have a Tremolo switch. I don't use tremolo very often and it wouldn't be a problem for me to just flip a switch on the amp.


If I am understanding things properly, with the switch closed and no foot switch connected, the cathode of V1B is connected to the cathode of V2A (I know the pin labels are off, it should really be labeled V2B). In this case, the tubes share cathode resistor R12 and the cathode bypass cap.


When the switch is open, then the cathode of V2A goes to ground through C34, but also through R35 before getting to the R12 cathode. 

BTW, we can't see the full size pic on imgbb.com. Maybe you need to share the image or make it public or something.

I'm seeing the full size pic inline. That is why I have been using imgbb instead of trying to scale down and add as an attachment. But, when I click on the image it takes me to the imbb.com site. I thought it would show the same image but it doesn't. Hmm.

[dbishopbliss]

Component layout question...


I am pretty sure I have all the components laid out according to the schematic. I have not included all the connections for the OT, heaters, indicator lights, grounds, etc. Before I finish all of that up I would appreciate some general feedback on the layout of the components.


If you look at the board from Left to Right I have the following:

• Shared cathode resistor and bypass caps (for A and B)
• Filter caps and voltage dropping resistors
• Phase inverter components
• Tremolo components
• Filter cap for C
• Tremolo channel preamp components
• Normal channel preamp components

The biggest question I have is regarding the placement of the Filter Cap for C location. I think I have seen Sluckey layouts where it is closer to the preamp section so that is what inspired the current layout but honestly I'm not sure what are the pros/cons of doing it this way.


Also, the diagram is to scale using a pre-punched JTM45 chassis. There are two light blue circles under the cathode resistors/filter caps area of the board where the OT wires will be routed. Any issues with this?


Your feedback is appreciated.

[sluckey]

Layout looks fine to me. I didn't check for accuracy.

Filter cap C location is fine. That 10µF cap is filtering all 6 of the little triodes. Bad idea IMO. Hope that works out well.

OT wire routing looks fine.

[dbishopbliss]

That 10µF cap is filtering all 6 of the little triodes. Bad idea IMO. Hope that works out well.

I am following the original schematic. Why is it a bad idea? What would you do to improve it?

[AnalogDok]

I know I'm a little late on this but did you ever get the amp completed?
Was your layout good or do you wish you changed it around a bit?

I'm interested in doing a 1690T as well. I've done a 1624T and a S6606 successfully.

Thanks in advance

[dbishopbliss]

I'm on hold for a bit... I bought a JTM45 chassis and I want to use the Low Voltage JTM45 Transformer (300V) from Mojo. They are built by Heyboer but they are way behind in manufacturing and don't expect to be caught up until July or August. I need to go over the schematic again to be extra sure that the layout matches.

[dbishopbliss]

Filter cap C location is fine. That 10µF cap is filtering all 6 of the little triodes. Bad idea IMO. Hope that works out well.

I asked before but you probably missed it... What would you do differently? Have additional filter stages for those triodes?

[sluckey]

Don't know what I was thinking. I just noticed my S-6424 only has 3 filter nodes too and feeds 6 triodes from node C just like the 1690T. I never had any problems with my amp. And the Fender AB763 reverb amps feed 6 triodes from the same filter node. You'll be fine just staying with the schematic. Just ignore my previous comment.   

[PRR]

...That 10µF cap is filtering all 6 of the little triodes....

It's situational. Six stages straight cascade could never be stable. But Supro has two chains, One is 3 stages but the last (the inverter) has major attenuation at grid. The other is a 4-cascade but attenuation and bass-cut between, plus mix-loss where the two channels comes together.

I would not care to bet one way or the other. Especially if somebody "mods" the losses. Me, for one-off work, I might plan to have one more B+ mode (space, lugs) just in case. Which is a cost-item if you build thousands, so Supro may have tinkered it to work.

[dbishopbliss]

I just got my power transformer in so it's time to get back to the build. As a reminder, I'm using a pre-drilled JTM45 chassis. I realized the board I designed is 1 inch longer than a traditional JTM45 board. I can probably make it fit but it will be tight.


As an alternative, maybe I could install a multi-section capacitor similar to what a JTM45 uses. There is a 20/10/10/10 cap available that would allow me to have a separate cap for the two chains PRR mentions. Is there a downside to running multiple wires from the cap to the board?


On another build Sluckey recommended I put the last filter cap closer to the socket instead of running a long wire from the cap to the socket. I could use the multi-cap for the first two stages and put the caps for the last two stages on the board.


What do you think? Your input is appreciated.

[sluckey]

I could use the multi-cap for the first two stages and put the caps for the last two stages on the board.

I like that plan...

     http://sluckeyamps.com/dual_lite/dual_lite.pdf

[dbishopbliss]

The original schematic calls for the first stages to have 20uF and 10uF. I don't think a multi-cap exists with those values. I found the following:

• 30uF/20uF 475V
• 32uF/32uF 500V
• 16uf/16uF 500V
• 20uf/10uf/10uF/10uF 450V (I could leave two sections unused)
• 10uf/10uF/10uF/10uF 450V (I could connect two of the sections in parallel and leave one unused)

What difference would I expect/detect using one of the first two options?

[shooter]

• 30uF/20uF 475V
• 32uF/32uF 500V

I would consider either an upgrade/improvement

[sluckey]

I would use the 32/32.

[dbishopbliss]

The original circuit has three stages of filtering with 10K resistors between each stage. The last stage, "C", is providing voltage to the 2 preamp gain stages, the tremolo stage and the phase inverter (not sure if my terminology is correct). Basically, V1, V2 and V3.


If I introduce an additional stage of filtering, should I just add another 1K resistor and 10uF capacitor? Something like the attachment. I realize that will drop the voltage to V1, V2 a little but probably not by much.

V1 and V2 are used for the preamp and tremolo. V3 is the phase inverter. Would it be better to group V2 and V3 together?

[sluckey]

I suggest you just stick to the original schematic.

[dbishopbliss]

I reworked my layout to use a multi-cap. I need to go back through for a final check to make sure all values, connections, etc are correct. But aside from that, please provide feedback on the layout. I have questions about the following specifically:

• Does it matter if I go from the second filter cap B to the output tube sockets pin 4 and then to the 10K resistor feeding C. Or, is it better to go from the filter cap to the 10K resistor and then to the sockets?
• As previously noted, Cap C is doing a lot of filtering. I moved it further to the right of the board. I could probably move it over even further to positions C' or even C''. Is there any advantage to doing that?
• I attempted to make most capacitors run vertically - mostly because this is what I see on most layouts. Sometimes this requires I use a jumper running at an angle. I had a number of capacitors running horizontally on one iteration of the layout. Is there a reason to avoid horizontal components.
• The board is 10.25" inches which is the same size found in the typical JTM45 layout. I have some extra room after going to the multi-cap. Should I spread things out more? Make the board smaller? Leave room for a D filter cap to experiment with?

If anyone has any other suggestion please chime in. This is my first attempt at laying out a circuit from scratch - Sluckey style.

[sluckey]

There are a few wiring errors in the tremolo circuit. I'll let you untangle them before I point them out.   

[dbishopbliss]

There are a few wiring errors in the tremolo circuit. I'll let you untangle them before I point them out.   

Because I have a Mac now I can no longer use Visio (unless I pay for the professional version) and have been using a tool called OmniGraffle. OG is ok, but a little awkward. I decided to try DIYLC again - I don't remember what my issue was, but it wasn't great the last time I used it. However, I'm finding it much better and easier to use now. Plus, its easy to scale the components to the exact size so I'm re-doing the layout again. Hopefully, I will find the errors.


Do you have a rule of thumb/best practice about board size and location? That is... it seems the traditional board size for a JTM45 is 10.25 inches. From the pictures I have seen, it looks like the traditional board allows for the OT wiring to be routed without having to be under the board. However, there is room in my chassis for a 11.75 inch board - this would let me put all the caps on the board. But the filter caps would be under the OT. Is there a reason to avoid this?

[sluckey]

I like your layout just as is. It's neat and logical. I don't see any reason to redo it. I untangled the tremolo circuit. In doing so, I swapped positions with the speed pot and the 100K on the schematic and layout. Take a look and tell me what you think...

PS... You forgot to connect the 6L6 grids to the board.

[dbishopbliss]

Thanks for going through the tremolo circuit. The good news is that when I redrew the circuit in DIYLC I came up with the same layout as you except for one... I had a missing 100K resistor. As I mentioned, I redrew the layout with all the filter caps on the board. The main reason I want to do this is because I already have the capacitors. Its not like a dual cap is that expensive, but I kind of like it with all the components on the board. :-) I have not included all the ground wires, and OT wires yet - those are coming.


I have two things I'm confused about. The first is the tremolo switch. The schematic says there is a switch on the Speed Pot and the Footswitch. Do I really need both? I really don't use Tremolo that much so I would be happy to user either a switched pot or a just a switch on the amp. I wouldn't use a footswitch.


The second thing I'm confused about is the Standby switch. On the schematic there is a note that says, "on power switch." What does that that mean? Are they saying that there was a DPDT switch and that when you turn on the amp the standby comes on... what would be the point of that? Or maybe its supposed to be a 3 position switch? Is there such a thing where the first position would turn on power and the second position would turn on (or is it off) the standby? Finally, the location of the standby is unlike anything I have seen before. It looks like it is connecting the grids of the power tubes. How does that work? Is is connecting each side of the phase inverter so signals cancel each other out?


Finally, there are no screen resistors on the power tubes. I'm thinking maybe I should add those.

[sluckey]

I have two things I'm confused about. The first is the tremolo switch. The schematic says there is a switch on the Speed Pot and the Footswitch. Do I really need both? I really don't use Tremolo that much so I would be happy to user either a switched pot or a just a switch on the amp. I wouldn't use a footswitch.

The switch on the FS jack is the third lug. It enables you to turn the tremolo on/off with the switch on the speed pot ***OR*** with the footswitch. I consider it essential, but you can easily just omit the FS jack if you want to.

The second thing I'm confused about is the Standby switch. On the schematic there is a note that says, "on power switch." What does that that mean? Are they saying that there was a DPDT switch and that when you turn on the amp the standby comes on... what would be the point of that? Or maybe its supposed to be a 3 position switch? Is there such a thing where the first position would turn on power and the second position would turn on (or is it off) the standby? Finally, the location of the standby is unlike anything I have seen before. It looks like it is connecting the grids of the power tubes. How does that work? Is is connecting each side of the phase inverter so signals cancel each other out?

The power switch is a three position switch. Could be an ON-OFF-ON DPDT. Or, it was popular with a lot of amps from that era to use a 2 pole 3 position rotary switch.

Finally, there are no screen resistors on the power tubes. I'm thinking maybe I should add those.

Your call. Tubes will probably last a bit longer. Might affect that "Page" voodoo!   

[dbishopbliss]

I was looking at a photo of a vintage 1690T when I noticed that the foot switch for the tremolo was located between the speed and intensity pots. Using that as a guide, the layout for the components became more obvious. The JTM45 chassis 6 holes for potentiometers. I am going to enlarge one of them and install the foot switch there.


I also noticed that the vintage 1690T must have used a DPDT on-off-on switch as suggested. The vintage switch was label standby (down) and on (up). Did they really have people turn the switch down for the heaters and then up, briefly turning everything off, to turn everything on? Seems very strange. I think I'm going to either go with a single switch - everything on - or two different switches.


On more thing, I decided to use a 47uF and 16uF filter caps instead of 20uF and 10uF as shown on the original schematic.


Other than the mains, heaters, and pilot light wiring I think I have the layout complete.