GAStan wrote: ↑Sun Sep 24, 2023 8:05 pm
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.
Yup, definitely old enough
If your phone can do voice recordings, that should be sufficient.
So maybe to calibrate,is it louder than speaking volume?
Is the hiss present on clean or only on OD (on my amps, the OD has quite a bit of hiss... so does the Dumble FYI)
Loud whisper/low talking voice. You can carry on a normal conversation near it without raising your voice.
There is NO 60 cycle hum like in the SRV video you posted, unless a guitar is plugged in and volumes turned up (as can be expected).
Personally I think it's in the power section. The hiss is consistent in sound and volume. Changing the controls, volume, TMB, drive etc... has no affect on it. It is present all the time, while playing and with nothing plugged in. I plugged in an empty guitar cable into the effect return with no change.
This is why I asked if the high heater voltage could cause (or at least contribute to) the noise I hear. My thought was that the high voltage could be overheating the cathode causing electrons to "cook off" and cause some noise.
I'm used to seeing heaters glow more of an orange and emit a fairly low amount amount of light. These are bright yellow.
I'm not going to power it up again until I have the resistors installed. There is no need to, and I feel it's just going to shorten the life of the tubes. Once I do power it up I'll poke around with an o'scope probe. It's something I look forward to--good old fashioned component level troubleshooting. Seems like troubleshooting today is just changing assemblies until the unit works. But I'm just rambling on...
martin manning wrote: ↑Sun Sep 24, 2023 3:14 pm
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 ;^)
I do need to recalculate with 4.5 amps. I'm slow sometimes but do get there eventually. Thank you!
Correct, and you will also need to estimate the heater winding voltage at 4.5A. You have one data point at 5.5A, and you can get another by measuring the unloaded voltage. With that you can solve for the resistance to add. I’d put one resistor on each leg to keep it balanced.
martin manning wrote: ↑Mon Sep 25, 2023 1:18 am
Correct, and you will also need to estimate the heater winding voltage at 4.5A. You have one data point at 5.5A, and you can get another by measuring the unloaded voltage. With that you can solve for the resistance to add. I’d put one resistor on each leg to keep it balanced.
Too many times recently a Variac would have been useful, so I just ordered one. Then I can measure actual current at 6.3 volts in addition to the unloaded voltage as suggested. Once resistor specs are determined, proper ones will be ordered from Mouser.
Possibly. I appreciate you don't want to cook your tubes but when you do switch it back on, there's a simple test you might try to see if the noise is coming from the preamp or the power section which is to plug a jack into the passive loop RETURN jack (assuming you are using a Switchcraft 12A for the return) that will lift the shunt off the tip and isolate the power section from the preamp.
As in this version:
#124 Schematic_1988_Skyliner.pdf
Equally possible it might be originating in both preamp and power sections, but you are likely to hear a difference I think.
How the heater circuit could be pulling the extra Amp is something you want to get to the bottom of, but for the purposes of reducing the voltage, you might double the expected power dissipation for the order of resistors and you will be in a safe ballpark area.
Strike That! I just read the rest of your post more carefully. No difference = strongly suggests it's coming from the power section.
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dbharris wrote: ↑Mon Sep 25, 2023 2:16 pm
The hiss is definitely coming through the speaker and not just from the amp itself?
-Dan
Definitely from the speaker. Amp is silent with dummy load connected.
Gotcha. Echoing what others have said.... LED lights and especially LEDs connected to dimmers are often really noisy even rooms apart. Swapping tubes is quick and easy. If you think it is in the power amp, play with the PI balance trimmer. I would think the hiss would get louder as you turn up the master though and you mentioned it is steady. You may also want to ohm out your ground points against chassis ground to see if any show resistance above a couple of ohms. Other than that, the scope is your friend. Good luck!
I was looking at the specs for your power transformer (Showman / Twin Reverb Power Transformer 125P34A - MOJO763EX)
I would be interested to hear what filament voltage you end up with when you dial in the variac to supply 117V/120V. I'm pretty sure it's going to get you a bit closer to the 6.3V you are spec'd for the filaments but even given the non-linear resistance of the heaviest current hungry power tubes (mainly) I struggle to imagine it is going to give you 6.3V. Hammond transformers, which are actually not all that bad, also come out higher around 6.7V with a European 230V supply, and to cure that I tapped it on the primary side to 240V. The odd thing is that your loaded B+ voltage looks close to the expected power factor of 1.414 x 340 (Vsec) and considering power losses from a string of rectifying diodes & etc. The bias voltage though, at -62V is at odds with the published 55V tap.
You should treat the published voltages in the schematic. for plates especially, more as a guide than doctrine - tubes can induce a significant swing away from the ideal - but if you get around 200V on V2 plates and 190V on V1 you are very close.
Something to consider here then is that you appear to live in an area with hot wall voltages (me too) and the transformer is just doing its job. The variac may be the key to cooling the beast. I would be tempted to see what voltages you get over the amp at the voltages the transformer was built for. Also, if that cures the issue of noise.
Edit: I swear by the variac. During the biasing and tuning phase of the build I apply a consistent voltage of 230V. It's the benchmark and I have to readjust the variac at different periods of the day to compensate for varying grid supply. I've seen voltage swings of +/- 7V during a normal day. When the amps are out in the world, it's another matter, and I don't lug the variac around with me. It's a valid complaint for tube amp users and MrD's amps are particularly susceptible to voltage swings and dirty power supplies. The price you pay for perfection!
The challenge to this is what to do with the IEC cutout. I have searched extensively for a power outlet that fits and can only find the IEC type. Once I receive the chassis and take actual measurements I will look more closely at other options I am considering.
As always I welcome any comments, opinions and advice offered.
Glenn
Why not do what MrD did? Only option really if you can't find a receptacle of some other kind to go in the hole... cover it with a plate.
My final solution was to use a blank octal cover. Size was perfect and holes VERY close. 4-40 screws might have worked but I didn't have any. I enlarged the holes in the cover to 3/16" then #6 screws fit just fine.
20230927_104701.jpg
Still waiting for the Variac to arrive so reading on TAG a bunch. Decided to get some 6PS and 448 caps to replace the 715s in there now. I didn't orient to outer foils so would need to pull the anyway to check. I don't think they're the main source of noise (orientation not type) but they may be contributing. What kind of noise do they cause if not oriented correctly? Hum? Hiss? Other?
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GAStan wrote: ↑Wed Sep 27, 2023 9:06 pm
I will do that and report back the results. Thank you for the suggestion.
yes, and then if all ok, put back PI tube and pull first tube V1 , recheck on clean, should also be very quiet, if there is some noise, problem is in PI or between V1->PI elements, lead dress, grounding (bad grounding topology can create hiss yes ) etc
sometimes i get hiss when i pass with Ub+ wire over the elements on board, so Ub+ wires should be supressed to the chassis..
Found a problem. One of the 6L6 tubes is pulling 59uAmps on G1, the other 3 pull less than I can measure on my Fluke 87. What I measure is .3vdc on the 5k1 grid resistor and 13vdc on the 220k from the bias pot.
Edit: problem follows tube.
What this is causing is two tubes (was initially v6 & v7) to be biased at -45v pulling 30mA @ cathode and the other 2 tubes biased -58v pulling <2mA.
Missed the discrepancy before because like a fool made the ASSumption that all tubes were pulling same current as the I monitored while initially setting bias.
Contacting vendor to replace tubes now.
Last edited by GAStan on Thu Sep 28, 2023 7:39 pm, edited 1 time in total.
