Yet another 124 build

[ijedouglas]

It lives!

It's pretty loud!

Set FET trimmer to min, od and PI balance trimmers to mid.

Using a light bulb limiter I brought it up sans tubes. B+=462, 458, 431, 364, & 358. Bias has -62 available, set it for that.

Tubes in, let heaters do their thing a couple of minutes then out of standby. Dummy load hooked up prior to powering on. B+=462, 459, 436, 294 & 285. Heaters at 7.3??? Will that be ok? Wall voltage is 125. Set bias at 30mA (one tube) for now, 55-60% of 25W at 462v. 4x6L6 tubes.

Got 2x12 4ohm speaker, shut down amp and hooked it up. Nothing plugged in to input, all controls at 0. No 60hz hum! No obnoxious feedback, my lucky day. But definite noise, static like white noise. Manipulated all controls with no effect on noise, just a couple of clicks first time I tried each of Mid and Bright switches but subsequent operation of these two controls resulted in no clicks heard.

Lightly tapped each tube. V1 slightly microphonic all others silent.

Plugged in a guitar and tried it out-wooohoooo it works! I'm the only one here now so cranked it UP. Loud, feedback at will. Too soon to comment on tone but clean is clean OD has crunch. Unplugged guitar, static is gone but slowly comes back.

Our cat just came in meowing at me, PO'd that I woke her from her nap.

I'm STOKED!! Need to calm down a bit before I put my hands back in the amp to check out plates voltages etc...

Congrats!!! What a cool feeling

[GAStan]

It lives!

It's pretty loud!

Set FET trimmer to min, od and PI balance trimmers to mid.

Using a light bulb limiter I brought it up sans tubes. B+=462, 458, 431, 364, & 358. Bias has -62 available, set it for that.

Tubes in, let heaters do their thing a couple of minutes then out of standby. Dummy load hooked up prior to powering on. B+=462, 459, 436, 294 & 285. Heaters at 7.3??? Will that be ok? Wall voltage is 125. Set bias at 30mA (one tube) for now, 55-60% of 25W at 462v. 4x6L6 tubes.

Got 2x12 4ohm speaker, shut down amp and hooked it up. Nothing plugged in to input, all controls at 0. No 60hz hum! No obnoxious feedback, my lucky day. But definite noise, static like white noise. Manipulated all controls with no effect on noise, just a couple of clicks first time I tried each of Mid and Bright switches but subsequent operation of these two controls resulted in no clicks heard.

Lightly tapped each tube. V1 slightly microphonic all others silent.

Plugged in a guitar and tried it out-wooohoooo it works! I'm the only one here now so cranked it UP. Loud, feedback at will. Too soon to comment on tone but clean is clean OD has crunch. Unplugged guitar, static is gone but slowly comes back.

Our cat just came in meowing at me, PO'd that I woke her from her nap.

I'm STOKED!! Need to calm down a bit before I put my hands back in the amp to check out plates voltages etc...

Congrats!!! What a cool feeling

You're absolutely right Ian, thank you.

[GAStan]

Looking into the 7.3vac heater voltage. Checked it again and it was 7.1. Checked with 2 different DMMs just to make sure-same readings on both. Then realized our refrigerated a/c was running, Checked and wall voltage was down a few volts.

Checked current draw of heaters, 5.48 amps.

20230923_203735.jpg

Doing the math 5.5 amps with .091 ohms gives .5 voltage drop and .136 ohms .75 volts.

A vendor I'm making an order from has R.47 and 1R 2w metal oxide resistors available. 5 of the R.47 paralleled gives R.094 and 4 gives R.1175 and so on....

Is this a good idea-using resistors to drop the voltage? The resistors will handle the wattage. If there is a better way I'm open to suggestions.

Another question, can the high heater voltage contribute to the noise I'm experiencing?

About the wiring mess:

20230923_201623.jpg

Looking closely at the power and standby switches and fuse holder shows they protrude into the amp uncomfortably close to the power tubes. They might be ok but I'm not happy with them so they will be replaced. The power switch isn't even hooked up, I'm using the switch on the power strip it's plugged into. I do have the fuse wired in. This will all be properly dressed once the new switches are received.

[ijedouglas]

Looking into the 7.3vac heater voltage. Checked it again and it was 7.1. Checked with 2 different DMMs just to make sure-same readings on both. Then realized our refrigerated a/c was running, Checked and wall voltage was down a few volts.

Checked current draw of heaters, 5.48 amps.

20230923_203735.jpg

Doing the math 5.5 amps with .091 ohms gives .5 voltage drop and .136 ohms .75 volts.

A vendor I'm making an order from has R.47 and 1R 2w metal oxide resistors available. 5 of the R.47 paralleled gives R.094 and 4 gives R.1175 and so on....

Is this a good idea-using resistors to drop the voltage? The resistors will handle the wattage. If there is a better way I'm open to suggestions.

Not sure what you are trying to do here... drop the heater voltage?

Another question, can the high heater voltage contribute to the noise I'm experiencing?

The amp may be noisier while out the cab. Have you mounted it in the cab yet?

About the wiring mess:
20230923_201623.jpg
Looking closely at the power and standby switches and fuse holder shows they protrude into the amp uncomfortably close to the power tubes. They might be ok but I'm not happy with them so they will be replaced. The power switch isn't even hooked up, I'm using the switch on the power strip it's plugged into. I do have the fuse wired in. This will all be properly dressed once the new switches are received.

HAD had heaters, power switches and tubes all within 2-3mm from each other. As long as the wiring is solid and wont move, you should be good. HAD used solid core 18AWG for heaters and they are pretty sturdy and hard to move.

Screenshot 2023-09-23 211500.png

[Stephen1966]

{could] high heater voltage contribute to the noise I'm experiencing?

First, congratulations. It's no easy thing to put one of these together, even with all the background info online. Great to hear you don't have any major problems.

Now the fun really begins

Tell us more about the noise you are hearing, some is considered normal and could be due to the resistors in the signal path - CC and CF. Granted, most of the time these don't pose a problem (some might even consider it part of the "mojo") but give everything a good old chopstick just to be on the safe side. Is it right you don't hear any 60Hz hum? In that case, the heater circuit sounds okay even if the voltage is a little elevated. Also, the noise is in the form of a hiss - does adjusting the pots (controls on the front) alter that in any way? How bad is it? Is it still audible when you crank it and while you're playing? Is the noise constant or intermittent? It's a bit like a game of whack-a-mole at the moment but if you can say where the noise is not coming from, it's getting you closer to the source. It's easy to get lost in the circuit when you start this troubleshooting so start at the input jack and it's grid leak resistor and work from there. If the noise is coming from a resistor, it is more likely to be in that part of the circuit - the early preamp - which is the most susceptible to noise and where the very small input signal is amplified greatly, everything including noise is amplified greatly. Caps on the other hand, can be leaky, allowing DC to sneak past and you might hear this with the pots.

Check you aren't near to any LED light sources. Those things are very noisy and might be heard from an adjoining room. Shut down any potential background sources of noise. Putting the amp in the cab as Ian suggested or just putting a metal dinner tray over the open chassis can help to block the background noise. If nothing you do with the background sources makes any difference, you can assume the noise is coming from within the amp. Mains voltages are also going to swing through the day - maybe try testing at different times of the day. This is where a variac is useful.

Did you get your plate voltages?

5.48 amps - remind me, what tubes are you using. 7.3V or even 7.1V is a tad too high. Dropping resistors are a brute force solution and in the heater supply i.e. not in the signal supply, it sounds like the easiest to apply. I would be careful about where I place them though - heat dissipation might be problem. Other solutions you might consider though is a VVR, zener diodes, or a simple voltage regulator as found in the relay circuit - efficient heat dissipation is still going to have to be thought about though.

In the meantime, if you have a variac, you might try dialling down the mains voltage into the amp until you get the correct heater voltage. It would take the whole system's voltages down with it, but it could give you some ballpark B+ voltages to consider and some indication of where the noise is coming from. My own feeling is that the noise is coming in somewhere in the early preamp stages.

[martin manning]

Checked current draw of heaters, 5.48 amps.

Seems like you are about an amp higher than the expected heater current: 4 x 0.9A (6L6) + 3 x 0.3A (12AX7) = 4.5A

[GAStan]

Right now my biggest concern is the heater voltage. Yes, I want to drop it to within spec for the tubes.

I'm still waiting for the cabinet, will be another week or two.

Thank you Ian for pointing out the close proximity of the switches HAD used. I'll reconsider. It has 18awg stranded for the heaters to the 6L6s and 20awg overcoat to the 12ax7s.

Once the heater voltage is at a comfortable level the noise issue will be addressed. I'm resisting the urge to power it up until heater voltage is resolved.

A hiss is a good description of the noise I have. No 60hz hum. Operating the controls, even maxing all of them, does not have any affect with nothing plugged into the input. It has been present any time the amp is on and out of standby. There are no CC resistors, only CF and MF.

What type of noise will an LED light source impart? 60hz? My work lamp is LED, within 2-3 feet of the amp.

My thought is to remove the diode and 330R resistor used for the LED from their present location. Replace the current LED with one that has an integral resistor for 12v and power it with the 12v that is already in that corner. Then place the voltage dropping resistors on the terminal strip that the diode and resistor was on.

[GAStan]

Checked current draw of heaters, 5.48 amps.

Seems like you are about an amp higher than the expected heater current: 4 x 0.9A (6L6) + 3 x 0.3A (12AX7) = 4.5A

Great point. However those calcs are at 6.3 volts. Using 7.3 volts gives 5.2 amps.

[martin manning]

Checked current draw of heaters, 5.48 amps.

Seems like you are about an amp higher than the expected heater current: 4 x 0.9A (6L6) + 3 x 0.3A (12AX7) = 4.5A

Great point. However those calcs are at 6.3 volts. Using 7.3 volts gives 5.2 amps.

That is the likely explanation, although the resistance isn't linear ;^)

[GAStan]

Seems like you are about an amp higher than the expected heater current: 4 x 0.9A (6L6) + 3 x 0.3A (12AX7) = 4.5A

Great point. However those calcs are at 6.3 volts. Using 7.3 volts gives 5.2 amps.

That is the likely explanation, although the resistance isn't linear ;^)

True, another great point. Inrush was 10+ volts. Glad it didn't blow the fuse in my meter.

[ijedouglas]

My thought is to remove the diode and 330R resistor used for the LED from their present location. Replace the current LED with one that has an integral resistor for 12v and power it with the 12v that is already in that corner. Then place the voltage dropping resistors on the terminal strip that the diode and resistor was on.

I would try and shield the amp and see if that helps before making any major changes. See if you can cover it or put it in a cab. The other thing to consider is these amps are tube amps and are noisy. Listen to SRV in the studio with his blackface SSS .


Is your LED powered off the heaters or a separate transformer? If off the heaters, I'd try move it to a separate supply and see if that reduces noise.

[GAStan]

LED is powered off heaters as shown on #124 schematic. I'm going to change it to be powered off the 12 volt DC relay power.

Thanks for the SRV video

[ijedouglas]

LED is powered off heaters as shown on #124 schematic. I'm going to change it to be powered off the 12 volt DC relay power.

I apologize, not sure what I was thinking. I meant to say to follow the schematic. HAD (and many others) have used this design with great success. You could also test this by lifting one of the LED supply leads and see if it reduces the noise (without having to redo much). I really think your noise may be coming from external and a cab or temporary shielding may help.

Is it possible to post a clip of the noise?

[GAStan]

Ok listened to SRV video. His seems to be mostly 60hz hum from the single coils of his Strat.

What I have is a constant "hiss" that does not change. After I lower the heater voltage I'll put a scope on it to determine the source.

This morning my wife ran some errands. As soon as she was gone I powered up the amp, plugged in my guitar, cranked it and hit some power chords. About 2 seconds later my teenage son, who had been sound asleep, came walking in eyes wide and grinning ear to ear. I let him play it loud about half an hour until his mom got home. While he was playing I was adjusting controls. This amp is capable of a VERY wide range of tones! The hiss was always present, 60hz hum varied as I'm accustomed to.

[GAStan]

LED is powered off heaters as shown on #124 schematic. I'm going to change it to be powered off the 12 volt DC relay power.

I apologize, not sure what I was thinking. I meant to say to follow the schematic. HAD (and many others) have used this design with great success. You could also test this by lifting one of the LED supply leads and see if it reduces the noise (without having to redo much). I really think your noise may be coming from external and a cab or temporary shielding may help.

Is it possible to post a clip of the noise?

No worries. I appreciate the help.

I need to work on my recording skills before I can post anything...that is learn how to record something.

I have no idea how old you are, but if you have ever heard an old CRT TV that was tuned to a channel with no station and only had static, that's the sound it makes. Similar to White or Pink noise that is used these days.