5R4 has a 2 ampere filament, and the 5U4 runs around 3 amperes. What is the average current draw from the B+? A 5R4 might do it too, and run the transformer a little cooler (by 5 watts).
-Chris
-Chris
The loud hum problem is on turn-on during the first 10 seconds or so. Then it goes quiet.
It's a new fully assembled Chinese amp with a few mods to make the heater hum quieter, and changed the input impedance to better match my tubed preamp. The rectifier is old stock. I have three 5U4G's and two 5U4GB's. The EL34's are Suguang, as are the two 6SL7's.
Dave
I see a bluish glow when I turn the amp on from cold. I don't see it after that.
Dave
Update:
I'm hoping I figured it out. However, first I wanted to mention that I installed a surge protector and 1/2 amp fuse in the power transformer's center tap to ground wire. Makes me feel a bit easier while this is ongoing.
Now, as I mentioned earlier, because of a stroke back in December, everything is somewhat fuzzy when I try to focus on it. I also mentioned I was going to double check my soldering, as I'm sure it was sloppy compared to what I've been doing most of my adult life. This time, besides my workbench light, I used an LED flashlight up close and examined everything. A couple weeks back I'd bypassed the volume control and changed the parallel grids to ground resistors to 100k resistors, Well, the right channel resistor wasn't soldered, but was just resting lightly on top of the solder. I soldered it up and so far, over the last couple of hours, it is behaving perfectly. Time will tell.
I don't know if that could cause this problem or not, but it certainly wasn't right. I'll wait and see (fuzzily, of course) if thew problem occurs again.
Thanks for all the help, guys!
Dave😀
I'm hoping I figured it out. However, first I wanted to mention that I installed a surge protector and 1/2 amp fuse in the power transformer's center tap to ground wire. Makes me feel a bit easier while this is ongoing.
Now, as I mentioned earlier, because of a stroke back in December, everything is somewhat fuzzy when I try to focus on it. I also mentioned I was going to double check my soldering, as I'm sure it was sloppy compared to what I've been doing most of my adult life. This time, besides my workbench light, I used an LED flashlight up close and examined everything. A couple weeks back I'd bypassed the volume control and changed the parallel grids to ground resistors to 100k resistors, Well, the right channel resistor wasn't soldered, but was just resting lightly on top of the solder. I soldered it up and so far, over the last couple of hours, it is behaving perfectly. Time will tell.
I don't know if that could cause this problem or not, but it certainly wasn't right. I'll wait and see (fuzzily, of course) if thew problem occurs again.
Thanks for all the help, guys!
Dave😀
OK, I'm calling that hum starts at time zero. Normally, there should not be any type of hum or hiss or anything coming through your speakers until the tube heaters are half-way warm, and some B+ voltage is actually flowing through the rectifier. The fact that you have hum from your speakers immediately means you have 60hz or 120hz powersupply / heater voltage getting to your output transformers.
To summarised from above:
The added SS diodes in the feed to each tube recifier anode handles (blocks) the Peak Inverse Voltage (PIV) and takes the PIV stress off the tube rectifier. It is the PIV stress which causes arcing in the tube rectifier.
Some have expressed concerns about Reverse Recovery spikes from the added SS Diodes. The series tube rectifier will not allow reverse recovery current spikes (it blocks them) so those added SS diodes can be any "garden variety" diodes (of sufficient voltage rating and current rating), there is absolutely no need to use ultrafast soft recovery SS diodes or any other "fancy" options. The added SS diodes have no audible effect whatever.
That addresses the component issues.
Then as also stated above, there is the design issue.
There needs to be a minimum value of wiring series resistance in the feed to each tube rectifier anode to control the peak current (particularly at power up). If there is not enough "native" resistance from winding secondary resistance plus reflected primary wiring resistance, then you need to add some series resistance to "top up" the winding resistance to the minimum required value. This is a step a lot of designers skip, they just assume that the winding resistance will be enough when in fact it often is not.
That minimum required resistance value will increase as the size of the first cap increases. The datasheet value of minimum resistance applies to the datasheet value of maximum capacitor. If you exceed the max. cap rating then you must add additional series resistance.
Method:
Measure the secondary winding resistance from CT to one end.
Measure the primary winding resistance
Calculate the effective series resistance (limiting resistance) = secondary resistance from above + primary resistance x [(Volts Secondary)/(Volts Primary)] squared.
A picture is worth a 1,000 words - so see what the Wiz says and look at his picture about limiting resistance for valve (tube) rectifiers here:
The Valve Wizard
CAUTION: For those dual primary trannies where you wire 2 x 115V in series for 230V operation or wire in parallel for 115V operation you get different results for the limiting resistance calc.- design for the 115V option and it will be safe for the 230V option.
Then I add the PIV blocking SS diodes. I have not had a tube rectifier failure in the past 10 years (except the one that got smashed during transport of an amp).
Cheers,
Ian
The added SS diodes in the feed to each tube recifier anode handles (blocks) the Peak Inverse Voltage (PIV) and takes the PIV stress off the tube rectifier. It is the PIV stress which causes arcing in the tube rectifier.
Some have expressed concerns about Reverse Recovery spikes from the added SS Diodes. The series tube rectifier will not allow reverse recovery current spikes (it blocks them) so those added SS diodes can be any "garden variety" diodes (of sufficient voltage rating and current rating), there is absolutely no need to use ultrafast soft recovery SS diodes or any other "fancy" options. The added SS diodes have no audible effect whatever.
That addresses the component issues.
Then as also stated above, there is the design issue.
There needs to be a minimum value of wiring series resistance in the feed to each tube rectifier anode to control the peak current (particularly at power up). If there is not enough "native" resistance from winding secondary resistance plus reflected primary wiring resistance, then you need to add some series resistance to "top up" the winding resistance to the minimum required value. This is a step a lot of designers skip, they just assume that the winding resistance will be enough when in fact it often is not.
That minimum required resistance value will increase as the size of the first cap increases. The datasheet value of minimum resistance applies to the datasheet value of maximum capacitor. If you exceed the max. cap rating then you must add additional series resistance.
Method:
Measure the secondary winding resistance from CT to one end.
Measure the primary winding resistance
Calculate the effective series resistance (limiting resistance) = secondary resistance from above + primary resistance x [(Volts Secondary)/(Volts Primary)] squared.
A picture is worth a 1,000 words - so see what the Wiz says and look at his picture about limiting resistance for valve (tube) rectifiers here:
The Valve Wizard
CAUTION: For those dual primary trannies where you wire 2 x 115V in series for 230V operation or wire in parallel for 115V operation you get different results for the limiting resistance calc.- design for the 115V option and it will be safe for the 230V option.
Then I add the PIV blocking SS diodes. I have not had a tube rectifier failure in the past 10 years (except the one that got smashed during transport of an amp).
Cheers,
Ian
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Then I suspect the tube is gassy. Does it the same with the filament not powered (Try removing one of the transformers 5V secondary at the socket).
No, I meant it happens within about a 10 second period. There is a pause when first turning it on first. I understand about letting the heaters get hot. I guess I didn't describe it well before. The heaters do turn on and glow good before the few seconds of hum and static. Sorry for any confusion. I had a long day yesterday and was tired when I wrote.
Dave
OK, no prob. Next... From the diagram it looks like the driver grid pin #4 is open. Is there not a jumper from the other side grid? The sections won't be driving in parallel without the other grid getting a grid signal, too.
Yes, the jumper is there. The schematic leaves a bit to be desired. I understand there were three or four variations the manufacturer went through, including one with the twin grids wired in series. Mine is wired parallel.
Dave
I'm assuming this is the classic "artificial heater CT" mod. But those resistors are usually 200-500 ohms, not K ohms. The ones you used are tied to ground, correct?
Hi Dave,
Yes, those resistors would normally be 100R, not 100K.
-Chris
Even better is to bias them +30 ~ +40 VDC positive using a voltage divider and bypass capacitor to common (ground). I usually stick the divider at the lowest potential on the B+ supply.
Yes, those resistors would normally be 100R, not 100K.
-Chris
Eli told me of this some time back. I'm also going to put a 1 meg pot in the circuit for fine tuning. Waiting a while to see if my eyes improve more before tackling it. That, and waiting for funds to improve a bit before making another parts order.
Dave😀
Ok, I really thought this was the case. Now, have you tried pulling the driver tubes, turn it on cold, or wait a few minutes if warm, and then listen for the hum? It would be a way to isolate the origin of the hum. Won't hurt anything to run the amp without the drivers. I really suspect the nature of the hum to involve the paralleling of the driver sections.
Haven't tried that yet, but will today. So far, that hum seems the only issue left. Thanks.
Dave😀
Hi Dave,
And the 1 Meg pot is for fine tuning what? If you want to experiment with heater bias - okay. However, you are going to be safe around 35 VDC, there doesn't seem to be a sweet spot.
-Chris
And the 1 Meg pot is for fine tuning what? If you want to experiment with heater bias - okay. However, you are going to be safe around 35 VDC, there doesn't seem to be a sweet spot.
-Chris
As long as you're positive wrt the cathode, you should be fine. Anywhere around 35 volts is a safe and effective voltage. You'll see that once you experiment. You have a long time constant with a 10 uF bypass capacitor. Give it time to reach your set point after adjustment. Its a neat experiment.
-Chris
-Chris
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