Supro 1690T build

[sluckey]

Is the amp back to your original wiring? If so, I have a couple more things to try...

First, turn on the pot switch and be sure nothing is plugged into the F/S jack. Now measure resistance from V1 pin 8 to V2 pin 8. Should be zero ohms. Next measure resistance from V2 pin 8 to chassis. Should be 3.9K. Are both readings correct?

Now put a temporary jumper across the two INTENSITY pot lugs ( the ones with the wires connected). This takes the INT pot out of the circuit and should result in maximum tremolo intensity. Turn the amp on. Do you have tremolo now?

[dbishopbliss]

Is the amp back to your original wiring? If so, I have a couple more things to try...

Amp is back to original wiring. Shunt and Ground are wired together. When nothing is plugged in, Shut, Ground and Tip are all connected. If I plug in a cord (not a footswitch) then the Tip is no longer connected to Ground.

First, turn on the pot switch and be sure nothing is plugged into the F/S jack. Now measure resistance from V1 pin 8 to V2 pin 8. Should be zero ohms. Next measure resistance from V2 pin 8 to chassis. Should be 3.9K. Are both readings correct?

Pretty much... when I connect the leads of my DMM directly to each other I am getting 0.02 ohms. When I am measuring pin 8 to pin 8 I'm getting 0.05 ohms. This is the same value I'm getting at the switch.

Now put a temporary jumper across the two INTENSITY pot lugs ( the ones with the wires connected). This takes the INT pot out of the circuit and should result in maximum tremolo intensity. Turn the amp on. Do you have tremolo now?

Nope.

[dbishopbliss]

I got a NOS 5V4G today and put it in the amp. Tremolo still doesn't work but I didn't expect that to make a difference. Here are the latest voltages:

C1 = 382V
C2 = 330V
C3 = 302V

V1, 1 = 90V
V1, 3 = 0V

This is where things get interesting...

V1, 6 = 90V initially seems to fluctuate increasing to 140V and keeps climbing.
V1, 8 = 1.8V when the pot switch is off (pushed in).
V1, 8 = 1.9V when the pot switch is on (pulled out), but changed over time to 2.3V. Related to V1,6 behavior???

V2, 1 = 50V
V2, 3 = 0.5V

V2, 6 = 254V
V2, 8 = 3.3V with switch off, intensity pot on 0
V2, 8 = 3.2V with switch off, intensity pot on 10
V2, 8 = same values as V1,8 with switch on.

Power tubes... I don't think there is anything important here.
V4, 3 = 377V
V4, 4 = 329V
V4, 8 = 24.9V

V5, 3 = 378V
V5, 4 = 329V
V5, 8 = 24.9V

[sluckey]

Double check your wiring and component values in the trem circuit.

We gotta get that trem tube oscillating. Here's a test to do that will make it easy to tell when the tube is oscillating. Disconnect the yellow wire from the pot switch. (I'm looking at the pic, not the layout.) Connect the anode of a red LED to the end of that yellow wire and connect the cathode of that LED to the ground buss. Now we have a visual indicator. The goal is to make that LED flash. There will be no tremolo signal sent to V1 while the LED is connected. It's possible that the LED will be flashing without doing anything else.

If the LED is not flashing then change the three caps in the trem circuit.

[dbishopbliss]

Double check your wiring and component values in the trem circuit.

We gotta get that trem tube oscillating. Here's a test to do that will make it easy to tell when the tube is oscillating. Disconnect the yellow wire from the pot switch. (I'm looking at the pic, not the layout.) Connect the anode of a red LED to the end of that yellow wire and connect the cathode of that LED to the ground buss. Now we have a visual indicator. The goal is to make that LED flash. There will be no tremolo signal sent to V1 while the LED is connected. It's possible that the LED will be flashing without doing anything else.

If the LED is not flashing then change the three caps in the trem circuit.

Not sure I have a Red LED handy. This is when I really miss Radio Shack. I wish places would ship a part like that in an envelope instead of charging $5 to send in a giant box. I can buy one from Mouser and pay $7.99 in shipping or I can get a box of 100 LEDs in 5 colors sent to my home with free shipping for $4.99.

Any thoughts or concerns about the V1, Pin 6 voltage changing like that? Pin 1 voltage doesn't seem to change. I will admit I haven't left the amp on for more than 10 minutes at a time. Could this just be to the tubes warming up?

[sluckey]

You don't really need the LED. Just leave everything connected normally. Set the speed pot to midrange. Connect your DMM to V2-6 to monitor plate voltage. If the voltage is steady, the oscillator is not working. If the voltage is changing erratically, the oscillator is working. If your DMM has a bar graph (like my Fluke 87V) you can see the plate voltage actually changing smoothly at the speed of the oscillator.

Change those 3 caps if the oscillator is not working.

I'm not concerned about the plate voltages on V1 at this time. But tell me, how does the amp sound, both channels?

[PRR]

We used to be able to pull LEDs out of old computers and appliances.

[dbishopbliss]

But tell me, how does the amp sound, both channels?

Power was out here all day so I didn’t get a chance to check voltages. LEDs will here tomorrow so I will try that.


I brought the amp up and plugged it in to a proper cabinet. The tremolo channel is grindy but articulate. Plugging into the treble jack is almost like a treble booster.


The normal channel has a bunch of bass but it’s tight with no tubby or flubbiness. It’s cleaner and bigger but still has some grit. Although, my guitar has some hot P90s that makes every amp have some grit so I hoots try it with my Tele tomorrow.


So how does it sound… It sounds like the gods of rock have decided to sprinkle fairy dust into the amp. it’s really awesome. Unlike anything else I have.

[dbishopbliss]

With the speed mid range and intensity pot all the way clockwise, The voltage on V2, six is steady at 199 V. When I turn the tremolo off the voltage increases to 256 V.


If I change the intensity pot to be all the way counterclockwise, then the voltage increases to 215 V. If I turn off the tremolo than the voltage is 267 V.


The voltage is steady no matter what I do.


Should I replace the capacitor is one at a time or do all of them at once?

[sluckey]

I would do all three. Don't throw them away.

Are you absolutely sure it's wired correctly? I can't tell from looking at the pics because you're feeding the off board wires under the board. Use your ohm meter to verify all wires go to the correct place. The layout agrees with the schematic so just make sure the wires are connected correctly. Do this before changing the caps.

I'm thinking the INT pot may be connected backwards but that's not a biggie. Just set it mid way. And, remember, when the oscillator is working the plate voltage will be fluctuating.

What DMM do you have?

[dbishopbliss]

I would do all three. Don't throw them away.

Hopefully, I have spares in my drawer so I don't have to wait for new ones to arrive.

Are you absolutely sure it's wired correctly? I can't tell from looking at the pics because you're feeding the off board wires under the board. Use your ohm meter to verify all wires go to the correct place. The layout agrees with the schematic so just make sure the wires are connected correctly. Do this before changing the caps.

I verified using my continuity tester before connecting them. I can try to lift up the board and get my phone underneath to take a picture.

I'm thinking the INT pot may be connected backwards but that's not a biggie. Just set it mid way. And, remember, when the oscillator is working the plate voltage will be fluctuating.

The schematic says speed pot works backward. Maybe I wired the intensity pot backward too.

What DMM do you have?

Fluke 77V - no graphing.

[sluckey]

No need to take a pic. I believe you.

Your DMM does have a bar graph and it will 'pulse' at the rate of your tremolo speed.

[Willabe]

I don't know if you've seen this, very good way to double check a builds wiring;

https://el34world.com/Forum/index.php?topic=17701.0

[PRR]

Fluke 77V - no graphing.

[Willabe]

Sluckey, do you remember Phsyconoodler couldn't get the LFO in a 18w Marshall he built?

I suggested he try a 470K for the LFO plate R just to see if it would oscillate, and the 470K kicked the LFO in gear. He ended up going down to ~300K. Stock was a 220K LFO plate R. He tried many 12AX7's, nothing worked until he upped the LFO plate R's value. 

dbishopbliss, try a 470K R for the LFO plate and see if it kicks it into oscillating. If it does, then keep lowering that plat R's value until it stops. Then up the value back up a little till it oscillates again. Just use gator clip cables. 

Here's the post of what he finished with;   

https://el34world.com/Forum/index.php?topic=16282.msg160098#msg160098

[Willabe]

Ok, Phsyconoodler's 18w Marshall, stock the LFO plate R is 100K. He tried a 220K with a red cathode LED, that combo didn't work. He then went up to a 470K LFO plate R and a red LED for the LFO cathode. That combo kicked the LFO in gear.

But then it was thumping too much so he ended up taking out the LED, going back to an R with a bypass cap for the cathode and lowering the value of the LFO plate R down to 330K. That combo worked well for him.

Looking at the schematic, your Supro's LFO has either a 220K or a 270K?

Try going up to 470K for the LFO plate R. (You might need to also use an red LED with that 470K, just to get it going.) That might kick it in. Any LFO wont oscillate until there's enough gain.   

[Willabe]

I built a 5G9 and couldn't get the LFO to oscillate.          

Went over the wiring and parts values many times, all was good.        

Finally changed the LFO 9 pin socket, ceramic with cinch pins. That fixed it. Must have been a bad socket.

[sluckey]

If you look at the schematic you'll see the plate is connected to a 270K in series with a 150K that has a 500K pot across it. That means the plate resistance can be varied from 270K up to 385K.

But yes, if the oscillator is on the edge of oscillating, increasing the gain by increasing the plate load is a possibility. Worth a try. There's no need for a LED in this circuit. I suggested using a LED only for a visual indicator while troubleshooting. But his meter has the capability to show the plate voltage pulsing in time with the trem speed.

I also suggest increasing the B+ by moving from node C to node B.

But all this is moving toward modifying the circuit. I'd really like to see it operate with the provided schematic, but who knows if it is even correct?
 

[dbishopbliss]

Your DMM does have a bar graph and it will 'pulse' at the rate of your tremolo speed.

So that is what that is... I was thinking something like this...

[dbishopbliss]

If you look at the schematic you'll see the plate is connected to a 270K in series with a 150K that has a 500K pot across it. That means the plate resistance can be varied from 270K up to 385K.

I'm going to lift one end of the 150K resistor. That will allow me to go go up to a 770K load and will be easy to adjust without soldering... plus I don't have a box of resistors to swap in.

But yes, if the oscillator is on the edge of oscillating, increasing the gain by increasing the plate load is a possibility. Worth a try. There's no need for a LED in this circuit. I suggested using a LED only for a visual indicator while troubleshooting. But his meter has the capability to show the plate voltage pulsing in time with the trem speed.

Now I know what to look at.

I also suggest increasing the B+ by moving from node C to node B.

I will try that after removing the 150K resistor.

But all this is moving toward modifying the circuit. I'd really like to see it operate with the provided schematic, but who knows if it is even correct?

And that is a good question. The schematic was redrawn by "Dirty Girl Amps" in 2013. That doesn't mean anyone has actually built an amp using it. I will try reaching out to Leon C on another forum. I know he built one, but I don't know what schematic he used.

[sluckey]

I will try reaching out to Leon C on another forum.

Good idea. Meanwhile try your good idea to remove the 150K.

[dbishopbliss]

Well this is annoying... BEFORE CHANGING ANYTHING,  I decided to check out the graphing feature that I didn't know I had. I connected the meter to ground and V2, Pin 6. This time the voltage was varying quite a bit. I didn't even need a graph to see it.


So I plugged in a guitar to see if I could hear anything. Nope... no tremolo. Just for grins I decided to connect the meter to V2, Pin 8. Suddenly, I could hear tremolo!!! It doesn't do the "whump whump whump" or go as slow as the Vox, but there is definite audible tremolo. As expected, the voltage on the cathode is varying too. So I disconnected the meter and the tremolo is still there. I poked around with a chopstick but I get no crackling or sound going in and out. I'm going to reheat the joints on the tube socket pins anyway and test again after the tubes have cooled.

[dbishopbliss]

Update... it seems to come and go. Sometimes I can hear the click click click of the tremolo but there is no tremolo. Other times it comes back in. I did notice some pots here and there as I was playing with the Intensity pot. But those seemed to have gone away.

[sluckey]

Remove the 150K

[dbishopbliss]

I will try removing the 150K resistor later today... in the meantime, here is the reply I got from Leon C asking about the schematic he used.

The tremolo circuit in mine is identical to the one in Eric's schematic. Mine has the 6.8k rather than the 15k resistor between the oscillator cathode and on/off jack ring. The previous owner added a 3.3uF cap in parallel with the .001 that goes to ground. I haven't removed it as it works fine as is.

[dbishopbliss]

Ugh... things are getting worse. I swapped in a different set of power tubes just to see if I could hear a difference. Turned it on, hummmm. Then POP! Then smoke!!!! Tubes look ok. Do fuses make smoke? I haven't opened it up yet.

[sluckey]

What kind of tubes? Fuses don't usually smoke, at least enough to see it.

[shooter]

Just for grins I decided to connect the meter to V2, Pin 8. Suddenly,

that sorta thing, makes smoke.  maybe time for a new socket and associated soldered parts?

[dbishopbliss]

Just for grins I decided to connect the meter to V2, Pin 8. Suddenly,

that sorta thing, makes smoke.  maybe time for a new socket and associated soldered parts?

No smoke when I did that. Things started working.

[dbishopbliss]

What kind of tubes? Fuses don't usually smoke, at least enough to see it.

Initially I was using a pair of Sovtek 5881/6L6WGC. I replaced those with a pair of Svetlana 6L6WGC. I think they were originally in the Bandmaster i gave you. Actually, they were the first pair of tubes I tried when I fired up the amp for the first time. I think there was a big hum then too. Time to go open it up see what I can find.

[dbishopbliss]

Looks like I lost the cathode bypass cap.

[dbishopbliss]

I just cleaned up the guts and replaced the cap. Lucky for me I had a 22uF 50V cap, which is close enough to 25uF. I installed the working tubes and everything but the tremolo seems to be working again.


I lifted a leg of the 150K resistor. Still no tremolo. The voltage is changing but there is no change in the sound (no whump whump whump). I've included a link to a video so you can see and hear what is happening.

[sluckey]

Frequency seems very fast. Put your meter in AC mode. Measure the signal voltage. Switch to freq. mode and check the frequency. Verify that the frequency changes as you turn the speed pot. Refer to manual if you don't know how to check frequency...

     https://dam-assets.fluke.com/s3fs-public/77iv____umeng0100.pdf

Post the voltage and frequency (min and max) numbers.

[dbishopbliss]

Frequency seems very fast. Put your meter in AC mode. Measure the signal voltage. Switch to freq. mode and check the frequency. Verify that the frequency changes as you turn the speed pot. Refer to manual if you don't know how to check frequency...

     https://dam-assets.fluke.com/s3fs-public/77iv____umeng0100.pdf

Post the voltage and frequency (min and max) numbers.

Min: 67.5
Max: 81.2
Frequency changes as I turn the speed pot.


Here's another video for your viewing pleasure.

[sluckey]

I want the frequency min max numbers, ie, turn the speed pot max CCW and note frequency. Then turn the speed pot max CW and measure the frequency. What is the slowest trem freq? What is the fastest trem freq?

67VAC trem signal should be plenty. Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.

[dbishopbliss]

I want the frequency min max numbers, ie, turn the speed pot max CCW and note frequency. Then turn the speed pot max CW and measure the frequency. What is the slowest trem freq? What is the fastest trem freq?

67VAC trem signal should be plenty. Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.

With the intensity pot half way and the switch still in the circuit:

Min frequency is 5.07
Max frequency is 8.62


With the switch out of the circuit:
Min is 4.88
Max is 8.12


It seems like I can hear a very very very  faint tremolo.

[dbishopbliss]

It seems like I can hear a very very very  faint tremolo.

I'm thinking that was wishful thinking and really just the wobble of strings out of tune.

[sluckey]

Did you do this?

Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.

[dbishopbliss]

Did you do this?

Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.

Yes. That is what I meant by saying, “with the switch out of the circuit “

[sluckey]

What is the voltage of the trem signal? Meter set to AC and connected to plate (V2-6).

[dbishopbliss]

What is the voltage of the trem signal? Meter set to AC and connected to plate (V2-6).

Voltage oscillates between 67 and 84 VAC.

[sluckey]

OK, there's nothing wrong with the oscillator. Try this... Connect a gator clip test lead jumper directly from V1 pin 8 to V2 pin 8. Any joy?

[dbishopbliss]

Any joy?

No joy. 


I measured the DC voltage on the cathodes and it was between 1.8 and 2.1 V (something like that didn't write it down).

[dbishopbliss]

I'm reading Valve Wizard Phase-Shift Oscillator for Tremolo page again...


It says,

The effect requires a low frequency oscillator (LFO), which produces a wave (usually a sine wave) of around 0.5 to 10Hz- the "trem' signal". This is then mixed with the audio signal in some way, so that its amplitude (volume) increases and decreases in sympathy with the trem' signal.

The diagram that accompanies the description shows a sine wave signal coming from the anode of the tube. If I am understanding things correctly, the Supro is changing the voltage on the cathode of V1 but it doesn't appear that anything is being "mixed with the audio signal."


Is there a missing connection in the circuit or should the changing voltage be audible?

[shooter]

I probably missed it;
when you plug your guitar into the Trem input jacks does the guitar sound get through?  all the Vol/Tone pots work?, just no trem?

[sluckey]

Is there a missing connection in the circuit or should the changing voltage be audible?

That's why I had you directly connect V1 pin 8 to V2 pin 8. You cannot hear the actual tremolo sine wave. It's too low frequency. But you can hear the effect it has on varying the gain of V1B. It's called amplitude modulation and it sounds exactly like someone quickly turning the volume pot up and down while you are playing.

I've looked at every Supro, Valco, and Gretsch schematic in Hoffman's library. There are only two other cheap models that use a one triode tremolo circuit similar to yours. Everything else has a cathode follower on the output of the oscillator. That suggests to me that your one triode oscillator circuit is inferior and not reliable. Probably why they switched to the two triode circuits.

Whatever, I'm out of ideas. Jimmy never used tremolo anyway!   

[dbishopbliss]

I probably missed it;
when you plug your guitar into the Trem input jacks does the guitar sound get through?  all the Vol/Tone pots work?, just no trem?

Yep. Everything else works and sound good.

[shooter]

has the bypass cap on V1A been removed from the circuit?  If not, lift one leg n test

[dbishopbliss]

has the bypass cap on V1A been removed from the circuit?  If not, lift one leg n test

I’m confused… V1A is not in the tremolo channel and doesn’t have a bypass cap… at least on the cathode.


Are you referring to the 35uf cap on V1B cathode? Haven’t tried that.

[shooter]

I'm following with this schematic, so;
Supro_1690t.pdf (el34world.com)


IF the mix point for dry signal and trem is a cathode and has a bypass cap, (like the linked schematic), lift it n test.