Supro 6424 Vibrato channel signal cutout

[Apexelectric]

Just built a Supro 6424 and am very pleased with the overall results. I used a Hammond organ amp as a donor for the iron and used new parts for the rest. Channel 2 works great and sounds wonderful when turned up. Channel 1 is great as well but the signal will drop out when the volume is at max and will come back after a little while. The engaging or disengaging of the tremolo switch will tend to help restore the signal. The tremolo effect does not affect the cause of the signal drop.

Schematic is just like Sluckey's as I found the project when trying to figure out how to use the transformers I pulled from the Hammond AO-43. Once again, channel 2 works flawlessly. All voltages seem in line and output seems to be right around 21W.

Plate voltages are as follows

V1A 124
    B 218
V2A 150
    B 134
V3A 307
    B 110
V4 and V5 366

B+ 370
Preamp node 312

Output tube cathode resistor 500ohms
Voltage drop across resistor 29.2

Does this seem like a capacitor issue or a preamp tube issue?
It sure seems to have to do with a power overload of some sort but I'm not sure what component to look to for the resolution.

Thanks in advance!

[92Volts]

Bad solder joint maybe?

Could it happen as soon as you turn the volume to max, or does it appear to happen after a while suggesting it's a heat problem that builds up over time (and builds up faster at high volume)?

[sluckey]

Pull the trem tube. Still happen?

[PlumMojo]

I belive believe its something to do with thw biasing of it.

If you  bias with a led you can see the light turn on and off when its

I say this bcuz i had the same problem and when intensity is turned up all the way the light will fade out.

Try to use a led for a cathode resistor not only is it gonna be stronger but  you can see the speed of it and its functionality.

If you used the same values as the *lucky scheme u must look around a trouble shoot.

Are you using channel two at the same time?

[Apexelectric]

Signal is gone this morning altogether on Channel 1 and Channel 2 was weak for a little while but came back strong.

Removal of the tremolo tube doesn't change the problem. Still no signal. Problem seems to come and go. Have gone through all my solder joints and probed around with my chopstick with no results.

Have the channel switch wired so that it's one or the other with a standby in the center.

[sluckey]

Totally bypass the select switch, ie, connect the input jack directly to both 2.2K grid resistors. Does it work now?

If not, the problem has to be associated with V1 and circuitry. Make sure your filament connections are good to the V1 socket.

Show us some pics...

[Apexelectric]

Checked filament string . Ok
Unsoldered input wires from switch and jumpered tip of input jack to both grid stoppers and no change other than channel 2 volume control works for both inputs and no signal amplification on V1A. Checked all wiring from plates to volume inputs, from tone controls to grids, from mixing resistors to PI, from plates to coupling caps and nothing is crossed. Solder connection is solid on lug that connects mixing resistors to signal input to pi.

Bad mixing resistor?

[Apexelectric]

Mixing resistors check out

[sluckey]

no change other than channel 2 volume control works for both inputs and no signal amplification on V1A.

The channel 2 volume control may be a clue. I'll go to the shop and make a couple checks and report back soon.

[Apexelectric]

Thanks Sluckey!

[Apexelectric]

Not sure what I did but it seems like I'm back in business. In reattaching the mixing resistors something changed. Hopefully it stays thais way.

[sluckey]

Plate voltages are as follows

V1A 124
    B 218

These voltages indicate that you may have the cathode (pins 3 and 8) wires crossed. Check that closely and compare to the layout. Then recheck voltages for pins 1, 3, 6, and 8 and report back. Don't say V1A plate, or V1A cathode. Just give me the pin number and the voltage you measure. Oh yeah, turn the tremolo switch off so that doesn't interfere with pin 3 and 1 voltage readings.

Also, I'm confused by your statement about the volume 2 controlling both inputs. How could you tell? Please explain that a bit more. Anyhow, the volume controls do not interact on my amp and that's how it should be on your.

I want you to make this simple resistance check with amp turned off. Connect one meter probe to chassis. Connect the other probe to the junction of the two 270K mixing resistors. Turn all pots max counterclockwise (zero). Meter should indicate about 400K ohms. Now turn all pots max clockwise (ten). Meter should still indicate about 400K ohms. Might be a bit different from the first reading but should be in the ballpark. Report those two resistance readings.

I see you're back in business. I'd still like for you to make these voltage and resistance readings. THX...

[Apexelectric]

Pin 1 is 232
Pin 3 is 2.5
Pin 6 is 124
Pin 8 is 1.1

There was an interaction in which the volume control for channel 2 was affecting the volume of channel 1 signal.

I get 200K when the pots are fully counter clockwise and 360k in the opposit direction. Channel 1 tone is the only knob that doesn't change the resistance readings from 200k when turned from the fully counter clockwise positions all the others have an effect on the resistance readings.

Reflowed a couple connections on TONE 1 and got it to respond. Now getting 4000k fully counter clockwise on all and 360k in the opposite direction.

[sluckey]

Now getting 4000k fully counter clockwise on all and 360k in the opposite direction.

4000K??? That's 4Meg. Maybe you added one extra zero when you wrote that?

Is the amp still behaving correctly? Tremolo sound good?

[Apexelectric]

Yes. Sorry typo.

[Apexelectric]

All seems just fine at this point. Need to give it a work out when the opportunity arises. Is 20-21W dissipation ok for those tubes. Seems like 24W would be max so I wasn't going to change the cathode resistor again. It's at 500 ohms now. Amp seems plenty loud for 20W.  Working on a cabinet now to pair it with a 12" speaker

[Apexelectric]

Thanks as usual for all the help!

[92Volts]

All seems just fine at this point. Need to give it a work out when the opportunity arises. Is 20-21W dissipation ok for those tubes. Seems like 24W would be max so I wasn't going to change the cathode resistor again. It's at 500 ohms now. Amp seems plenty loud for 20W.  Working on a cabinet now to pair it with a 12" speaker

Output tubes are 6973s, with 12W per tube limit? 20-21W (for the pair) is already a bit hot. Push-pull amp tubes can soak up more power when actually playing than idling. 70% of maximum is usually recommended for that reason, which is more like 17W for the pair.

So, you really shouldn't lower that resistor to increase to 24W. If you don't play very loud or very long each session, it's probably fine to leave it as-is, but if you're playing gigs or don't want to spend $ on new tubes any time soon, you might want to decrease that current.

[Apexelectric]

Output tubes are 6973s, with 12W per tube limit? 20-21W (for the pair) is already a bit hot. Push-pull amp tubes can soak up more power when actually playing than idling. 70% of maximum is usually recommended for that reason, which is more like 17W for the pair.

Seems like my plate voltages are a tad high. Would I be better off trying to drop the b+ or just change the OT cathode resistor to a larger value?

[sluckey]

70% is the usual recommendation for fixed bias amps. However, most cathode biased amps typically run at 100%.

A tad high??? What exactly do you measure on pin 9 and pin 7 of both tubes?

[Apexelectric]

The tube data sheet says 17W for max on a pair, cathode biased @ 310 plate volts and 12W max dissipation per tube.

I'm at about 335 and 340 VDC on pins 7 and 9 and 365 VDC to pin 8

[sluckey]

Pin 7 is the cathode. Should be approx. 25 volts. Please recheck. I'm wanting to calculate the idle dissipation and need the cathode voltage to do that.

[Apexelectric]

29 volts across the 500ohm shared resistor

[Apexelectric]

11.7W per tube?

[sluckey]

29 volts across the 500ohm shared resistor results in 58mA flowing through the tubes. Use this number when calculating plate dissipation. I know that a small part of this current flows through the screen grid. I just ignore the screen current. This means that the actual plate current is slightly less than the cathode current so the actual plate dissipation will be slightly less than the calculated dissipation.

Plate voltage of 340v and cathode voltage of 29v means there is 311v actually across the tube. Use this number when calculating plate dissipation.

So, 311V X .058A = 18 watts. This is shared between two tubes. If shared equally, then each tube is dissipating 9 watts.

[Apexelectric]

Perfect! Guess we're good the way it is then. Thanks again for the assistance.

Mike

[sluckey]

Yours looks just like mine. I'm satisfied that the amp is all it can be. Play it a while and see what you think. I will say it's not my most favorite build. I still don't see what all the Supro hoopla is about.

[Apexelectric]

Dimed on Channel 1 with a Les Paul, it has some nice character.

[Apexelectric]

Plate voltage of 340v and cathode voltage of 29v means there is 311v actually across the tube. Use this number when calculating plate dissipation.

So why would I use 311V if I've already read voltage between plate and cathode(pin 7 and 9)? Wouldn't that already take into consideration the drop across the resistor?

[sluckey]

If you put one meter probe on pin 7 and put the other meter probe on pin 9 then you already have the plate to cathode voltage and you should use that number to calculate plate dissipation. It's not clear to me that you did this.

Most people just measure tube voltages in reference to chassis ground because it can be unnerving to hold two probes on a tube socket at the same time. Even though I have steady hands I still don't like to do it! When you measure voltages in reference to chassis ground you will have to subtract the cathode voltage from the plate voltage.

Some meters (such as Flukes) have a relative button. IOW you can make a measurement and push the relative button. The display goes to zero and any subsequent measurements will be relative to the first measurement. For example, you measure the cathode voltage as 29v. Press the relative button while displaying 29v. The display goes to zero. Now measure the plate voltage. The reading will be referenced to that 29v reading you had when you pushed the relative button. In this case you don't need to subtract anything. Just don't forget you're in relative mode later on.   

[Apexelectric]

Ok. That's what I thought. So I'm a little higher on my dissipation. More like 20W. What do you think? Drop it a little more?

[sluckey]

I'd leave it just like that. The tubes are good for 12 watts each. If you want to see something scary, calculate the power using the voltages on the original schematic. 

[Apexelectric]

Yeah I saw that it had a 250ohm cathode resistor. Lower plate voltage? Or was that thing running at 24W?

Thanks again Steve!

[sluckey]

Look at the schematic and do the math. All the needed voltages are on the schematic.

[Apexelectric]

30 watts!?

[sluckey]

That's what I get.

[Apexelectric]

Looks like Supro is still doing a 6973 amp and pushing 25W through it. Got me a bit intrigued. Might try running it closer to the 24W max and see how much it changes the tone. Any idea on the Hammond output transformer capacity? I know it was originally used with a pair of EL84/6BQ5's so unless Hammond typically overbuilt the O/T's maybe my curiosity might bite me in the butt.

[sluckey]

Hammond is tough. Put the original value cathode resistor in it and see/hear what you think. I ran mine like that for a while before I dropped the power. I don't recall any change in tone.

[Joel]

Most people just measure tube voltages in reference to chassis ground because it can be unnerving to hold two probes on a tube socket at the same time. Even though I have steady hands I still don't like to do it! When you measure voltages in reference to chassis ground you will have to subtract the cathode voltage from the plate voltage.

This is why I use one of those grabby probe tools on one of my mulitmeter leads.  I've had a 240V tickle before, and I'd be quite happy to never have another thank you very much.  I use an older version of one of these: http://www.fluke.com/fluke/uken/accessories/Test-Leads-Probes-and-Clips/AC283.htm?PID=55685. Set it up, turn the amp on, then probe with the other one as normal.  Much safer.

[PRR]

> those grabby probe tools

Am I the only one using roach-clips for a legal purpose??

I used to have to buy four 10-packs of clip-leads just to have two meter-leads at my bench, the others "walked off".

I used these ALL the time (I could find mine), even in low-volt work. Slipped leads do damage.

[Apexelectric]

I make my living as an electrician so I'm pretty comfortable and skilled in using test leads in precarious places. Manage to only get zapped every now and then. A little bit of Electrical tape on the leads to where just the tip is exposed is sometimes a good way to prevent damage from a slip of the lead. Some of the test leads have this as a built in option, except a removable plastic sheath.

[shooter]

for a legal purpose

didn't you're state pass medical use, we've had it a couple years now, n I keep loosing my gator clips

[Apexelectric]

  Im in Colorado. 

[bmccowan]

I have a similar amp and a similar problem,
I just finished a Supro 24 Lite build using a Hammond AO-39 chassis and transformers. Punched a hole for an additional 9 pin socket, similar to Sluckey's AO-39 conversion. As I brought it up on a variac, it started a high pitch oscillation at about 90v AC. Chopsticking, I tracked that to the leads from the power tubes to the output transformer. They ran close to the signal path at the PI, so I moved them and corrected the problem. But, now if I run the amp full volume (which I only do to test) the amp cuts out for about 10 seconds and then comes back. Just like the build in this thread. So I am thinking that an oscillation out of hearing range might be going on. or?? Any ideas before I do something silly, like shield the OT wires? Thanks in advance.

BTW, to Sluckey's comments - I find the Supro tone finicky - I have two Valco 6973 amps, and I have to play with the tone controls (amp and guitar) to get a tone I like. But I like the tone better than the AC-15 I built out of another AO-39. Guess I just like big bottle 6V6 and 6L6 tone better. Best sounding amp I've built; a 1947 SE Supro 6SL7 > 6v6 > 10" Weber Alnico.  Sounded so good I built a carolina blue cab for it.

[bnwitt]

Im in Colorado. 

Well I hope your head is on a swivel when you're out on the road.

[Apexelectric]

I traced my problem back to a bad solder joint at the mixing resistors off the tone controls.

[sluckey]

I have a similar amp and a similar problem,
I just finished a Supro 24 Lite build using a Hammond AO-39 chassis and transformers.

Would you post this problem under your other AO-39 thread? Also post a schematic and some pics. Maybe we can help you figure it out. I looked over at Watts website but could not find any info on a "Supro 24 Lite".

[bmccowan]

As requested, this issue is moved to my other AO-39 thread.
Thanks
B Mac