• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

JE Labs 300b round 5. MUSIC!! MUSIC!!

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Hi Everyone,

Thanks Frank, John and everyone else that helped. The amp is working quite well. It plays MUSIC!! and sounding better with every hour. Meter still dose not work right, I'll fiqure that out. Heater volts for 300b is high and will work on that. It sounds wonderfull. Thanks to diyAUDIO.com for a great forum and to all that helped a new guy get started in this hobby. Here is a pic of the amp and myself, the amp is much better looking. Oh, my wife is thrilled.
 

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Well done.:cool: :cool: :cool: :cool:
It looks good complete too!

I'd posted to your original thread before I saw this one.;)

Don't leave the heater volts issue too long, it'll shorten the tube's life. Also they might not work as well when you eventually correct the voltage.
Funny, I only thought about that yesterday. It's probably because you are using Schottkys.
 
diyAudio Senior Member
Joined 2002
300B

Hi,

Wow....man!

Does this amp look good or what!!!

Congratulations Paul,a formidable job you did.

Don't leave the heater volts issue too long, it'll shorten the tube's life. Also they might not work as well when you eventually correct the voltage.

Especially if it's over 10% of the 5V rating.

I'll do a :headbash: session for having overlooked the high efficiency of the Shottky diodes.

Meter still dose not work right, I'll fiqure that out.

Is it just inaccurate?
If so,a trimpot is the easiest way IMO.
When connected in series with the meter adjust it till the readings from DVM and panelmeter correspond.
Measure the resistance of the trimpot at that posistion and replace with a close enough standard value.


Happy listening,;)
 
Thank you

Thanks guys for the compliments.

Both are now up and running. The heater voltage for the 300b filaments are way to high. On one I used a 100ohm resistor and the other a 1.1k resistor. The 1.1k is actually higher at 9.5 volts and the 100ohm is at 7.2 volts. How do I correct this? The resistor is in series with the posistive lead coming off the diodes into the first of two 10v 10k uF capacitors. The caps are coupled on the posistive lead by a choke. Anyway, they sound great and once again I really appreciate all the help and friends I've made here. Should I put a resistor in series with the negative path?
 
Switch off straight away!

As Frank said 10% high is the safe maximum.
Where are these resisitors you're talking about?
They can't be in series with the heaters - not enough current would flow. We should be talking an ohm or 2.

The resistor to alter will be either in series with the rectifier bridge, or, between the 2 positive terminals of the heater supply smoothing caps.

There is some advantage to having a resistor in both top and bottom of the heater supply smoothing, but you must make sure that the grounding is done on the tube side of the bottom resistor.
 
diyAudio Senior Member
Joined 2002
RE:Switch off straight away!

Hi,

Where does this 9.5 V come from?

The 7.2 volts correpond with the rectified 5 VAC and as D'haen suggests should be reduced to 5VDC by inserting the resistors.

Paul,if these voltages are correct you're permenantly damaging the 300Bs.
As it is now even the 10V caps can blow up as well.

:att'n:

If you could make a drawing of current wiring of that part it would surely help.

Cheers,;)
 
Hi,

I think I know what is wrong. When I built the circuit, the trace for the resistor goes straight through, no break. Hence, the electricicty goes around the resistor or straight through, no need to jump up into the resistor, does'nt have too. So, any suggestions on resistor values? I believe the 9.2v is coming from the PWT it was just wound wrong, should be easy to tame, same for the 7.2v. Will let you know and thanks for your help again. Oh, on the post for wiring the amp I posted a diagram of the circuit, if you refer to this, it will show where I made my mistake.
An externally hosted image should be here but it was not working when we last tested it.
 
The basic answer is that since R=V/I, then 2.2 ohms on one, and 4.2 ohms on the other. Though it does puzzle me why they are so different. Did you measure them under the same conditions?
Measure again across the tube pins with the tube plugged in, and then calculate the value required:
R=(V - 5)/1
In reality, the loading will change, so the final value will differ slightly.
Both at least 5 watt rating. (10watts better). They will get very hot.

Cheers,
 
Hi,

Sorry it took so long to reply, I went back to work, nuff said!

Anyway, the measurement for the 300b heater volts was taken with the tubes not in the socket, hence the high voltage readings.

As I found out, the tube needs to be in the socket to complete the circuit and get a true volt reading.

After some trial and error experiments I now have the voltage to 5.2 and 5.3 volts DC on the heaters using a 1 ohm 10 watt resistor on each rectifier circuit, testing the volts with the tubes in place.

I ordered two 1.2 ohm 12watt resistors to correct the voltage hopefully closer still to 5 volts exactly. I installed two new 16v 10k uF electrolytic caps in the supply, eliminating the 10v caps.

The PWT was built and tested at 117v for a 380-0-380 volt output, but a 120.89 volts a/c wall socket volts it reads much higher at around 400-0-400 volts output, hence the B+ is higher hovering near 500 volts, the PWT barely gets warm and the OPT is cool to the touch after an hour of being on. No smoke, yet!

It does sound, well, magical. A muscian friend, who's ears I trust implicitly called them "demon amps". He was listening to recordings he has listened to numerous times in the past and was absolutlely impressed with the fact that he was hearing information he has never heard before on other amplifiers.

I'll try to post a pic of the wiring, later today.
 
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