FunkyE9th wrote:I bought the amp as a learning platform. So I don't mind having to put a series resistor in there. If it fails, I'll just fix it again. My idea was to buy a cheap amp (got it used for $160) and play around with it and not really worry about frying it. I learned a lot this week from what I've done to this amp. Maybe some day I'll replace the PT.
My philosophy on amp building or modification is to build stuff that won't fail, ever. The show must go on! But I can see where you're comin' from. I'd throw down $160 on a Pignose in a heartbeat, if I thought about it for a second longer it might sink in that I have all the amps I'll ever need plus enough parts to build a hundred or so more.
In the AFAIK category: As I remember your Pignose is a stripped down late '60s / early '70s Marshall circuit. It's very roughly a 50 watt Marshall converted to 6L6 tubes which makes it a screaming bargain.
Bernardduur wrote:I had a small discussion last night with a HiFi maniac (tube HiFi) and he claims that heating tubes on DC is poisoning tubes. They work but don't last as long as AC heated tubes and they sound worse.
Dunno if that is common good or new; he has a high standard when it comes to tubes
No need to do DC heaters on an amp with normal gain but
rubbish.... I have Stereo Tube power amps my father built (tube maniac and engineer from Bell Labs) from 1956 that run on DC heaters and the current set of working tubes are 10 years old. Newer Vox AC30s are DC heaters. There are many manufacturers using DC heaters, I personally have a CAE 3+ that has preamp tubes I changed 5 years ago and they still sound and work just fine. I also hardly think the 1964 RCA tube receiving manual would suggest the use of DC heaters if there were any problems. Same manual also suggests referencing the center tap to 60~70V DC to reduce hum.
The Valve Junior nutcases know every Band-Aid fix in the book to make the wrong voltages into the right voltages, they stick power resistors inline to drop the filament voltage into the correct range.
It's kind of ironic- I heard the Valve Juniors come stock with DC heaters now... (?!?!?)
I'm one of those nutcases and yup it comes with DC heaters now. Actually, I have version 1 of the valve jr, and modified it for DC heaters. That was my 1st mod ever and the start of my addiction. I should convert it back to AC heaters and see if I can bring the noise down without DC on the heaters.
One of the problems with DC heaters is you run a real risk of inducing more noise by sticking another rectifier in the chassis. There's no reason to even consider DC heating in a simple amp like a valve jr unless you have a really horrendously shitty layout.
DC heaters need more amps ....the rectifier draws alot
you cant float the heaters at a DC potential useing DC to heat them
if you plan ahead to use DC its great......
In general I've found it more advantageous to use a hum pot
using either a ground or a voltage source...like the chathodes from
the output tubes... even saw a transistor reverb recovery powerd from there....
and Ive seen the pre amp fils powerd directly from there too
a couple 12ax7s in series.... another approach is to connect a condenser
maybe .02 from the center tap of the fil. source
free and simple solutions.......
Well actually with DC heaters it is easy to use 12.6VDC reducing the current requirement in half. What do you mean the rectifier draws current?
What would the purpose be of floating the heater at a DC potential if you are using DC heaters?
Again there should be no reason to do DC heaters on this small amp
But with higher gain amps DC heaters on the preamp tubes sure does make them quiet. And.... for $20 you can buy a turn key solution from Mouser that will run all the preamp tube heaters, no other parts needed it seems like a no brainer to me.
a full wave bridge rectifier with a capacitor input load
gives you amps of ..... I d.c. = 0.62 x Sec. I a.c.
an average voltage of ... V d.c. = 0.90 x Sec. V a.c.
to many times DC heaters are adapted to an amp with out
looking fully at it demands
every now and again youll find a circuit or try to "push" the
voltages in a project to effect, to were you might exceed
the heater-to-cathode max limit and have to float a rather
large positive d.c. V to keep within design maximums for the
tube type..... which you can not do with d.c. heaters
its a reason why some amps might eat tubes....
Well I'm certainly with you there.. Also the issue of if regulated you need to make sure you have enough voltage to maintain regulation.
I dont think DC heaters should be "adapted" to any mortal transformer that exists in most amps since we also dont usually know the current ratings but a transformer would usually need to be added which is why I think instead of all that go with the $20 ....V-Infinity VOF-65-12 You get AC in of 100-240V and (adjustable) output of 12.6 VDC at 5.4A all in a footprint of 2"x4" no transformer. You could run the power tubes in series if you have a 50W.
What would stop you from floating a typical DC heater though? You can still float DC to a 60VDC potential.
The Heater to Cathode limitation is also interesting since I have always wondered how Marshall gets away with it on their cathode follower. They are usually pushing over 180V on the cathode, I think you need to add signal as well which could easily push it out of spec. I have also seen some designs that have over 300V AT THE PLATE of a typical Gnd Cathode circuit. Some people get away with murder !
Andy Le Blanc wrote:appologies......
a full wave bridge rectifier with a capacitor input load
gives you amps of ..... I d.c. = 0.62 x Sec. I a.c.
an average voltage of ... V d.c. = 0.90 x Sec. V a.c.
to many times DC heaters are adapted to an amp with out
looking fully at it demands
every now and again youll find a circuit or try to "push" the
voltages in a project to effect, to were you might exceed
the heater-to-cathode max limit and have to float a rather
large positive d.c. V to keep within design maximums for the
tube type..... which you can not do with d.c. heaters
its a reason why some amps might eat tubes....
Last edited by drz400 on Fri Mar 21, 2008 10:18 pm, edited 3 times in total.
I believe the rateing is around 180 to 200 max for a 12ax7
and thats design center max..... havent found a good sheet to quote
absolute max... you can really run into issues with SRPP... mu followers ...etc
when you push supply volts.... older fender models were pretty conservative
and marshalls do push it ..... it come to life expectancy of the tube
I do like the results that dc fils. can give .... hum used to be accepted
now the expectations are differnt... high gain in a tube pre can be a challenge....
with out really clean fil. supply
I should convert it back to AC heaters and see if I can bring the noise down without DC on the heaters.