Hi Paul,
Which amp are you discussing? Shoot me an email if you feel so inclined.. I'm editing out your email addy so you don't get spammed..
Kevin
Wavebourn, I reckon a MOV-R across each OT primary half-winding better clamps the energy that could lead to over-voltage induced failure of a P-P OT. About as simple as diodes clamps, especially if you have a few small 240VAC style movs around.
For P-P, I also prefer the cathode fuse (or 'poofable' bias sense resistor), but with an extra parallel cold-bias resistor to keep the cathode voltage from exceeding cathode-heater voltage rating (to save the tube if it is not the problem).
Ciao, Tim
For P-P, I also prefer the cathode fuse (or 'poofable' bias sense resistor), but with an extra parallel cold-bias resistor to keep the cathode voltage from exceeding cathode-heater voltage rating (to save the tube if it is not the problem).
Ciao, Tim
Hi Kevin
Thank Yu for contacting back again
I will resend this message direct to to you, but this is to share with the folks here :
It is NOT the DHT26 , this little jewel is safe and sound , !!
It is the new SEI DHT2A3 I am finishing for my eldest in Brisbane.
She is a user - no EE background at all and I wanted to have a "safe amp" for her to play with.
My thought:
Tube used DHT 2A3 as output , Output Lundalh LL1664/100mA max , cathode bias , B+ 337Vdc , Va 319 Vdc , Vk 43Vdc , Rk 830 Ohm / 20watts ,
Line tube 6SN7, Ra 62K, Va 175Vdc , Vk 7.6Vdc , Rk510 Ohm
Driver tube 6SN7 , Ra 20K , Va 195Vdc, Vk 7.4Vdc , Rk 2K2 Ohm
For good sound , normally use NOS (Sylvania/Tungsol/RCA etc..) type direct heated tubes 2A3 but as yu are aware they are old and prone to breakdown anytime.
Had one case with my old DHT2A3 long time ago , but fortunately that LL1664 withstanded the runaway tube.
Culd not affort that in , my eldest daughter wuld be unaware at all.
I thinking as how I culd further protect the LL1664 from such accident ? By fuse ?
Yrs sincerely
Paul
Thank Yu for contacting back again
I will resend this message direct to to you, but this is to share with the folks here :
It is NOT the DHT26 , this little jewel is safe and sound , !!
It is the new SEI DHT2A3 I am finishing for my eldest in Brisbane.
She is a user - no EE background at all and I wanted to have a "safe amp" for her to play with.
My thought:
Tube used DHT 2A3 as output , Output Lundalh LL1664/100mA max , cathode bias , B+ 337Vdc , Va 319 Vdc , Vk 43Vdc , Rk 830 Ohm / 20watts ,
Line tube 6SN7, Ra 62K, Va 175Vdc , Vk 7.6Vdc , Rk510 Ohm
Driver tube 6SN7 , Ra 20K , Va 195Vdc, Vk 7.4Vdc , Rk 2K2 Ohm
For good sound , normally use NOS (Sylvania/Tungsol/RCA etc..) type direct heated tubes 2A3 but as yu are aware they are old and prone to breakdown anytime.
Had one case with my old DHT2A3 long time ago , but fortunately that LL1664 withstanded the runaway tube.
Culd not affort that in , my eldest daughter wuld be unaware at all.
I thinking as how I culd further protect the LL1664 from such accident ? By fuse ?
Yrs sincerely
Paul
To protect my OPT from valves shorting I added an HT fuse. Then to protect this arrangement from problems when the fuse blows I added a reverse polarity diode and capacitor from the valve end of the fuse to ground. The cap slows and reduces the peak voltage when the fuse blows, the diode then dissipates the energy in the transformer winding resistance and hopefully helps prevent an arc from forming.
As Wavebourn says, a fuse is no help for 'off-load' running. For this you need a snubber (looks like Zobel network) across the transformer.
As Wavebourn says, a fuse is no help for 'off-load' running. For this you need a snubber (looks like Zobel network) across the transformer.
B+fuses
Hi, can you explain what you mean as between the filter caps?
do you mean between the first and the second filter cap?
I have a dyncao MKIV and I've installed the fuses on the secondary AC HT. one on each.
i do have a single ended parallel integrated i'd like to add fuses to B+ so i'm asking because of your warning lol
Thx.
Hi, can you explain what you mean as between the filter caps?
do you mean between the first and the second filter cap?
I have a dyncao MKIV and I've installed the fuses on the secondary AC HT. one on each.
i do have a single ended parallel integrated i'd like to add fuses to B+ so i'm asking because of your warning lol
Thx.
An old thread for sure!
For completeness I'd like to add some links that I've found very help in understanding protection:
https://dalmura.com.au/static/Output transformer protection.pdf
https://dalmura.com.au/static/Valve amp fusing.pdf
https://dalmura.com.au/static/Power supply issues for tube amps.pdf
Cheers, Ralph
For completeness I'd like to add some links that I've found very help in understanding protection:
https://dalmura.com.au/static/Output transformer protection.pdf
https://dalmura.com.au/static/Valve amp fusing.pdf
https://dalmura.com.au/static/Power supply issues for tube amps.pdf
Cheers, Ralph
Yes Wavebourn is referring to a CRC type main B+ power supply filter, as is commonly used to suppress B+ ripple voltage (more so than may be needed for a PP output stage amp).
I would recommend not installing a fuse in the B+ feed at all, but rather install the fuse back at the power transformer secondary region - fusing a DC feed (such as the B+ link to the output transformer) needs a suitably rated DC fuse and holder, and adds the risk of collatoral damage to other parts due to the inductive voltage transient that could occur from the large inductance of the output transformer primary.
And for the Mark IV then as you have done, the fuse needs to be in each HV winding arm, rather than the CT, as the power transformer uses a HV tapping for bias supply. For simpler B+ power supplies, then locating a fuse in the CT link is simpler and just as effective imho.
maddmaxxdrummer, imho no parts are ever "safe in every aspect", unless you leave the amp turned off 
If I had to prioritise, I would add a 1N4007 in series with each plate of the rectifier valve. After that I would add a CT fuse, although the ability of such a fuse to achieve coordinated protection would require some assessment and/or measurement of CT current to be able to choose a suitable fuse.
I also note that there is no cathode current sense resistor for each 6P1 - are you regularly checking that the parallel valves are adequately sharing idle current ?
And how are you anchoring the heater supplies? Through each CT to an elevated DC supply?
If I had to prioritise, I would add a 1N4007 in series with each plate of the rectifier valve. After that I would add a CT fuse, although the ability of such a fuse to achieve coordinated protection would require some assessment and/or measurement of CT current to be able to choose a suitable fuse.
I also note that there is no cathode current sense resistor for each 6P1 - are you regularly checking that the parallel valves are adequately sharing idle current ?
And how are you anchoring the heater supplies? Through each CT to an elevated DC supply?
Why do you think they 'cut corners'?
Fusing the secondary CT is often appropriate for any valve amp - it doesn't have to be SE.
Fusing requires some effort to arrange, and is a bit advanced if starting with an unknown amp. The link below gives some background.
https://www.dalmura.com.au/static/Valve%20amp%20fusing.pdf
Fusing the secondary CT is often appropriate for any valve amp - it doesn't have to be SE.
Fusing requires some effort to arrange, and is a bit advanced if starting with an unknown amp. The link below gives some background.
https://www.dalmura.com.au/static/Valve%20amp%20fusing.pdf
I only once ever put extra parts in to protect my output transformers (unless you consider individual self bias resistors for the output tubes).
But years ago, I decided to protect my power transformers.
But with solid state rectifiers that charge large capacitors, the peak current is the inrush current at turn on (and the cold filaments draw more current then too).
When the tubes are warm, the quiescent current, and additional current in high level output push pull cause more current over longer time than the inrush time.
At the primary circuit, I use 2 fuses in series:
I use a fast blow fuse that is just a large enough value to keep from blowing at turn on.
I put a slow blow fuse in series with that which is just a large enough value to work when the amp is warm and when high level music is applied. Push Pull draw can change at high level music (but depends on the class, class AB, class B). Class A should not change much.
Single ended is class A, and the music level should not change the draw by much.
A typical amp might use a 2A fast blow in series with a 1 Amp slow blow in the power transformer primary circuit.
But years ago, I decided to protect my power transformers.
But with solid state rectifiers that charge large capacitors, the peak current is the inrush current at turn on (and the cold filaments draw more current then too).
When the tubes are warm, the quiescent current, and additional current in high level output push pull cause more current over longer time than the inrush time.
At the primary circuit, I use 2 fuses in series:
I use a fast blow fuse that is just a large enough value to keep from blowing at turn on.
I put a slow blow fuse in series with that which is just a large enough value to work when the amp is warm and when high level music is applied. Push Pull draw can change at high level music (but depends on the class, class AB, class B). Class A should not change much.
Single ended is class A, and the music level should not change the draw by much.
A typical amp might use a 2A fast blow in series with a 1 Amp slow blow in the power transformer primary circuit.
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