I wish I had this earlier while testing but this is very helpful. Thank you! I will use this now.
The only thing not working right now is the pilot light. Its connected at the end of the 6.3v line after all the tubes have been wired. I checked for continuity and it works... and also evidenced in the 6V6s and 6SL7s powering up.
I bought it from Partsexpress (Parts Express Red Jewel 6.3V Tube Amp Pilot Light Assembly). I must not be doing something right in how I connected it. I had purchased two and tried both so I am certain an inoperative bulb is not the problem.
I bought it from Partsexpress (Parts Express Red Jewel 6.3V Tube Amp Pilot Light Assembly). I must not be doing something right in how I connected it. I had purchased two and tried both so I am certain an inoperative bulb is not the problem.
The only thing in the schematic (DIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic) that I changed was the addition of a volume pot. I added an ALPS 100KAX2. Its a 100k ohm taper pot at the input. Works fine but I don't possess the technical nous to determine how it affects the rest of the circuit
There is only one customer review at the bottom of that page, and that one review says that two of these bulbs were purchased, and both were defective...and one was a dead short.
That was either a very unusual coincidence, or a symptom of extraordinarily poor quality control.
That said, a bulb is trivially easy to check with your DMM. Test the bulb for continuity on it's own, and measure it's filament resistance to make sure it's not a dead short. Test again for continuity with the bulb mounted in its holder (and the holder disconnected from one of the 6.3V wires.
There is one more test you can do, which is to set the meter to a suitable AC range, and verify that there is in fact 6.3 volts AC across the bulb holder terminals.
One of those three tests should tell you what the problem is!
-Gnobuddy
if buy good opt and nos audio tubes.... 6v6 sound can use as high end amp .in my area we just use nos audio tubes from us and europ brands . picture shows my last 6v6 project .
For those interested in tube design, the dynaco st-35 schematic shows how cool a dual grid power tube is. One can wire the second grid in reverse phase. The first stage grid to the gain stage grid to nullify what was there to make way for what will be there.
Playing with tube rectification is really cool also. Requres power reserves more than the wall can give. Otherwise they are quite slow and beautiful sounding. Slow to rise and quick to fall.
Playing with tube rectification is really cool also. Requres power reserves more than the wall can give. Otherwise they are quite slow and beautiful sounding. Slow to rise and quick to fall.
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So the amp works but something is wrong. The pilot light does not come on and I checked the pilot light and it's not shorted. If I play the amp, it works fine. The music plays without any hum. I've played it for an hour or so a couple of time but when I went to switch it back on again, the fuse was blown. I have two fuses installed. One came integrated into the IEC plug and I installed a separate fuse holder as well. Don't ask me why I did that. It's a stupid reason - I realized after receiving the IEC plug that it came with an integrated fuse. By then I had already drilled a hole in the chassis for the separat fuse holder.
Anyways, the issue is that something is not right. The pilot light does not power up and the fuse (both at the IEC plug and fuse holder) blows arbitrarily sometimes. B+ was stable at 320volts but I haven't checked voltages at the socket or other places in the circuit.
Any suggestions on what I should be doing next? Thanks!
Anyways, the issue is that something is not right. The pilot light does not power up and the fuse (both at the IEC plug and fuse holder) blows arbitrarily sometimes. B+ was stable at 320volts but I haven't checked voltages at the socket or other places in the circuit.
Any suggestions on what I should be doing next? Thanks!
Something is drawing too much current. If the B+ side of things looks okay, then it's likely the excess current draw is on the heater winding - particularly since you're seeing the no-pilot-light symptom.
Excess current draw comes with definite symptoms (other than blowing fuses): abnormally low voltage is one of them. Try measuring the RMS (AC) voltage across the pilot light (bulb holder) terminals. What reading do you see?
Other symptoms include excessive heating, and associated unpleasant smells. There may also be other signs of heat, such as discolouration.
With power off, and all filter and bypass caps fully discharged, try sniffing around the circuit - your nose can reveal the location of a problem.
Keep in mind that excess current draw doesn't happen for no reason - you either have a defective component, or a wiring error. The trick is to track down which, and where.
-Gnobuddy
Excess current draw comes with definite symptoms (other than blowing fuses): abnormally low voltage is one of them. Try measuring the RMS (AC) voltage across the pilot light (bulb holder) terminals. What reading do you see?
Other symptoms include excessive heating, and associated unpleasant smells. There may also be other signs of heat, such as discolouration.
With power off, and all filter and bypass caps fully discharged, try sniffing around the circuit - your nose can reveal the location of a problem.
Keep in mind that excess current draw doesn't happen for no reason - you either have a defective component, or a wiring error. The trick is to track down which, and where.
-Gnobuddy
Thanks, Gnobuddy.
Pilot lamp measures between 6.4 and 6.5 volts when I dial 125v in the Variac. I connected the 125v on the power transformer and not the 115v wire. B+ shows no change. Pilot lamp did not light up. Amp plays music.
The fuses I have installed are 2amp rated at 250volt fast blow fuses. Could that be contributing to the problem?
Smelled the insides of the chassis and tried to spot anything abnormal. Used a chopstick To make sure connections are still soldered on. No discoloration. I kept a close eye on this every time I start up half expecting my transformers to go up in smoke.
The voltage certainly sounds normal. If you have voltage at the bulb holder terminals, but the bulb doesn't light, there are only three possibilities: a bad (high-resistance) solder joint, a defective bulb socket (not making contact with the bulb's own contacts), or a defective bulb.
I would first closely inspect the soldered joints to the bulb socket, or maybe just re-do those two joints. De-solder, clean and tin the mating parts, re-solder.
Still no joy (bulb still dark)? Apply a few volts directly to that pilot light bulb (no socket) and verify that it lights up. Those few volts can come from three or four AA cells in series, or a suitable low-voltage power supply. If you don't already have a spare 5V USB power supply lying around, you can probably find one for a couple of bucks at the local thrift store.
If the bulb lights when powered directly by a battery or 5V source, but not when it's in it's screwed into it's socket, you have a bad socket, or a bad solder joint to the holder. Track down which one, and fix it. (If it's not the soldered joint, it has to be the socket, by elimination.)
I looked up your schematic, and I see that the drawing says "2A", but nothing else. The original Dynaco diagram is even less helpful, showing no fuse whatsoever.
Your amp is supposed to be able to put out roughly 20 W RMS (both channels combined). At 2 amps, you can draw 240 watts from the mains. That should certainly be more than enough to power a 20W valve amp; valve amps are very inefficient, but even so, it won't need ten times more power from the wall than it puts out to the speaker.
So: a 2A fuse seems an okay starting point. Fast-blow could be an issue, but I don't recall your saying that the fuse was blowing instantly when power was applied. So my hunch is that the problem is not the fuse, but something else.
Right now, you have two problems (intermittent fuse blowing, dead pilot light), and the cause of neither one has been discovered.
My suggestion is to divide and conquer. Remove all the valves from their sockets, and solve the pilot light problem first. And find out if the fuse is still blowing with all valves removed, and only the pilot light loading the transformer.
Please remember to be careful! There will be dangerous voltages at several points in the circuit when plugged in, even with no valves installed.
Keep us posted here as you solve the pilot light issue - go through the suggestions I made in this post, one by one.
-Gnobuddy
In a transformer at start up, a transient current up to 10 to 15 times larger than the rated transformer current can flow for several cycles, in addition, uncharged electrolytics are current hungry too.
Fast blow fuses are treacherous, sometimes in normal operation they heat and degrades, then, at the next start up they blow up.
I usually use regulated PSUs, with soft start and 2A fuse on that power (220-240V mains), so you can use 4A for 120V mains, you can also add a 4Ω7 NTC series resistor to reduce current at start up.
The fuse is more a fire protection, I have seen many TV sets with the fuse after the power switch, which sometimes has the bad habit of being in short... 🙄
And don't worry, due to Murphy's law, the fuse is protected by the most expensive component in the amplifier. 😛😀
Parts express has done some really poor quality control on these pilot lights. I bought two and both didn't work. I bought some new 6.3v lamps from a surplus store today and now the pilot light on the amp lights up bright and nice at 6.3volts. The only other reviewer, as Gnobuddy pointed out, also bought two lamps and both did not work. I am amazed at how bad a source this must be.
So now on to solving the case of the blown fuses. I've played the amp for 2-3 hours since the last time the fuses blew. They are still intact
Sheesh. That's really ridiculous. (Though I put the blame on the manufacturer, I doubt Parts Express could possibly quality-check all the hundreds and hundreds of items they sell.)
Excellent, one mystery solved!
Once more, excellent! One reason I suggested you solve the pilot light problem first, was that I was wondering if you had a defective pilot light that was drawing far too much current from the transformer, which was causing the fuses to blow.
I guess time will tell whether the fast-blow fuses are a problem or not, but at least the situation is looking more promising now. Congratulations on your successful build!
-Gnobuddy
You said you are running 2A fuses, correct?
Take a look at your heater current requirements. For four 6v6 tubes you will need almost 2A right there. They are 450ma each. Then for the 6sl7 you are looking at 600ma for two tubes. In total 1.8A for the output tubes and 600mA for the drivers.... 2.4A of heater current. Now for plate current...140ma (35ma each) for the 6v6 plates and 9.2ma for the drivers. All told you have to supply around 2.6A of current.
Now, taking into account in-rush current, which is a few cycles as said earlier, starting up a cold filament, etc..... Put in a 3.5A fuse and you will be just fine. Were I not firmly ensconced on the sofa at the moment, I would pull mine and see what the value is. I'm pretty certain it is a 3.5A because I was having some of the same issues with blowing fuses until I took plate current into account.
If you don't smell something burning/over heating in the amp by now, then you probably won't. I think that you are just stressing the crap out of the fuses with your heater and plate current.
Just my 2 cents worth.
Take a look at your heater current requirements. For four 6v6 tubes you will need almost 2A right there. They are 450ma each. Then for the 6sl7 you are looking at 600ma for two tubes. In total 1.8A for the output tubes and 600mA for the drivers.... 2.4A of heater current. Now for plate current...140ma (35ma each) for the 6v6 plates and 9.2ma for the drivers. All told you have to supply around 2.6A of current.
Now, taking into account in-rush current, which is a few cycles as said earlier, starting up a cold filament, etc..... Put in a 3.5A fuse and you will be just fine. Were I not firmly ensconced on the sofa at the moment, I would pull mine and see what the value is. I'm pretty certain it is a 3.5A because I was having some of the same issues with blowing fuses until I took plate current into account.
If you don't smell something burning/over heating in the amp by now, then you probably won't. I think that you are just stressing the crap out of the fuses with your heater and plate current.
Just my 2 cents worth.
2.4A of heater current at 6.3 volts, yes. On the primary side of the power transformer, at 120 volts, this gets multiplied by the ratio (6.3 volts/120 volts). That means the heater current amounts to only 126 mA, or 0.126 A, of current draw from the 120V mains.
Transformers aren't 100% efficient, so the actual current may be a few percent higher. Let's say 140 mA, or 0.14 A, to leave a big fat safety margin.
35 mA seems high for a 6V6 running at 320 volts (over 11 watts steady-state dissipation), but let's go with those numbers. Call it 150 mA at 320 volts. Stepped down to 120 volts on the primary side, that translates to 0.4 amps.
Using the numbers we have so far, the total steady-state current draw from the mains AC outlet is only (0.4 + 0.14) amps, or 0.54 amps. That's about a quarter of the 2A fuse's rating - the fuse won't blow unless the amp draws four times more current than it's supposed to.
It doesn't work that way, because most of the current draw (heaters) is at only 6.3 volts. You have to calculate the power (not current) draw on the secondary side of the transformer. A perfect transformer would draw the same power from the wall on the primary side, in reality, say 10% or 15% more to allow for transformer losses.
You can then convert that power back to current, this time at 120 volts.
You may be right about that. Fuse ratings are always a bit of a balancing act. If the fuse is too sensitive, it will blow unnecessarily, but if the fuse is too beefy, it will only blow after expensive parts of your amp have already cooked themselves to death!
-Gnobuddy
Thanks for pitching in, guys! I had a nasty fall while biking last week and still recovering. Haven't done much testing apart from changing out the pilot lamp. The amp has seen close to 10 hours of use since the change and neither of the fuses has blown so I am guessing the earlier pilot light may have caused the issue.
Ugh. Sorry to hear about the fall. I hope you didn't break anything. Not that you have to break something to be in plenty of pain for a long time from a fall off a bicycle. 
As for the faulty pilot light having been the issue, that was my hunch - that both the symptoms you were seeing were caused by the same fault. Probably way excessive current draw through a shorted or very low-resistance bulb. (The unexpected part being that you still measured 6.3 volts - you must have a generously over-rated power transformer!)
Anyway, I'm glad you got it sorted out. I hope you recover well from the fall.
I quit cycling some years ago, after one friend was killed by a car, another recovered consciousness lying face-down in the street with a transit bus stopped a few feet from his head, a third somersaulted into the bed of a pickup truck that hit him, landing with a broken arm, and an 18-year-old boy I knew died in a horrific motorcycle accident. It was too much, too soon, and I just couldn't deal.
-Gnobuddy
As for the faulty pilot light having been the issue, that was my hunch - that both the symptoms you were seeing were caused by the same fault. Probably way excessive current draw through a shorted or very low-resistance bulb. (The unexpected part being that you still measured 6.3 volts - you must have a generously over-rated power transformer!)
Anyway, I'm glad you got it sorted out. I hope you recover well from the fall.
I quit cycling some years ago, after one friend was killed by a car, another recovered consciousness lying face-down in the street with a transit bus stopped a few feet from his head, a third somersaulted into the bed of a pickup truck that hit him, landing with a broken arm, and an 18-year-old boy I knew died in a horrific motorcycle accident. It was too much, too soon, and I just couldn't deal.
-Gnobuddy
Ugh. Sorry to hear about the fall. I hope you didn't break anything. Not that you have to break something to be in plenty of pain for a long time from a fall off a bicycle.
As for the faulty pilot light having been the issue, that was my hunch - that both the symptoms you were seeing were caused by the same fault. Probably way excessive current draw through a shorted or very low-resistance bulb. (The unexpected part being that you still measured 6.3 volts - you must have a generously over-rated power transformer!)
Anyway, I'm glad you got it sorted out. I hope you recover well from the fall.
I quit cycling some years ago, after one friend was killed by a car, another recovered consciousness lying face-down in the street with a transit bus stopped a few feet from his head, a third somersaulted into the bed of a pickup truck that hit him, landing with a broken arm, and an 18-year-old boy I knew died in a horrific motorcycle accident. It was too much, too soon, and I just couldn't deal.
-Gnobuddy
I quit cycling, many many years ago, when in the city a taxi ran a stop sign and i T boned him. I flipped over the handle bar right onto the hood with my face looking at his. Fortunately, i was totally unharmed and got a chance to vent my rage at the idiot driver. At least i got a chance to upgrade my bike which was ruined. But, i gave up biking soon after since it it too dangerous. Make that, there are too many idiot drivers who don't know what is going on around them.
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