|
Home | Forums | Rules | Articles | diyAudio Store | Blogs | Gallery | Wiki | Register | Donations | FAQ | Calendar | Search | Today's Posts | Mark Forums Read | Search |
Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum |
|
Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving |
![]() |
|
Thread Tools | Search this Thread |
![]() |
#1 |
diyAudio Member
Join Date: Jul 2011
Location: Denver, Colorado.
|
![]()
I'm building phono amp. First I wanted to use two 12AX7 tubes with heaters rated 6.3V @ 300mA. My transformer is 6.3V @ 600mA, so I'm safe here. What if I change one 12AX7 to 6922, which heater is rated 6.3V @ 365mA. I'm going to be 65mA over limit. How much trouble I'm going to get into? Another transformer I have is 6.3V 1.2Amp, which is overkill. What is my best option? Go back to two 12AX7s or bigger transformer.
|
![]() |
![]() |
#2 | |
diyAudio Member
Join Date: May 2005
Location: USA
|
Quote:
|
|
![]() |
![]() |
#3 |
diyAudio Member
Join Date: Sep 2003
Location: Midland, Michigan
|
Try it. The additional load is only 8.3% above the transformer rating.
Most transformers can deliver more current than their specs indicate. If the transformer gets too warm, replace it with the larger one.
__________________
Frank |
![]() |
![]() |
#4 |
diyAudio Member
Join Date: Sep 2004
Location: Minneapolis, Minnesota, USA
|
Sorry but I must disagree, running any component at the design limit is a bad idea, running it over is a terrible idea. Have you considered what the consequences would be if the AC line voltage was 10% high (which is still within specs as far as the power company is concerned by the way)? At 10% over voltage those tubes wouldn't be drawing 665mA as expected (which is already more than 10% overload remember) but would be drawing more like 720 mA, or 20% overload. Do yourself a favor and be safe, use the larger transformer.
Mike |
![]() |
![]() |
#5 |
diyAudio Member
Join Date: Jun 2002
Location: Macedon NY
|
I'd use the 1.2A transformer in either case - it'll run cooler. 0.6A transformer probably won't overheat at 12% over rating, but output voltage may be low.
|
![]() |
![]() |
#6 | |
diyAudio Member
Join Date: Jan 2008
|
Quote:
What you could do is run it with a resistor as a load for an hour or two and see if you are happy with the temperature. You will need load resistors for each secondary. What I do is first run at 1/2 the load, then keep reducing the resistance by a small amount every 30 minutes or so. At some point you say "this is hot enough". This depends on your level of conservatism. Some people are fine with going 10% over the nominal limit, some want to stay well clear of it. One other thing. You can safely push the heater secondary of the B+ it not running at full load. The total heat load from all the windings (primary and secondaries) is what matters. People here are saying you can do it. But I don't like running transformers near their limit, they get hot. |
|
![]() |
![]() |
#7 |
diyAudio Member
Join Date: Jul 2011
Location: Denver, Colorado.
|
I must agree with everybody. I think I could push transformer extra 65mA and that running it near the limit is not a good idea. Well, it started as two stage SRPP 12AX7 amplifier. That is right on the limits of the transformers. The choke is rated at 8mA, so I may not have enough current to run 6922 tubes, anyway, unless I can run SRPP 6922s at 3mA. Most likely I stay with all 12AX7 tubes and 600mA heater transformers. If someone has some idea here it is:
Power transformer=450V CT @85ma tube=EZ80 - heater powered by main transformer choke=60H, 8mA I know it is good for two stage SRPP 12AX7 two channel phono amp. Can I make it first SRPP 12AX7 and second SRPP 6922? |
![]() |
![]() |
#8 |
diyAudio Member
Join Date: Aug 2008
|
I'd recommend replacing the EZ80 with 2 diodes. More current for your heaters, less heat, and more voltage to play with. You can't lose!
|
![]() |
![]() |
#9 | |
diyAudio Member
Join Date: Feb 2009
Location: Calgary
|
Quote:
I made the mistake once (many moons ago) to rig a 4 A transformer to a string of 12 V, 60 W (= 5 A) exterior lights. I figured, meh... what's a little over-current between friends. It'll be alright. It worked just fine until one day when I forgot to turn off the lights. I came home to a blown fuse in the fusebox inside the house (fuses on the primaries of transformers are for pussies!). The tranny had managed to get a 10 A, 230 V fuse to blow when its windings fused together. The shed where the transformer was mounted stunk of burnt enamel. I took the supply apart the next day. The transformer core was still hot to the touch...!! I could easily have caught the shed - if not the house - on fire. ~Tom |
|
![]() |
![]() |
#10 | |
diyAudio Member
Join Date: Jan 2008
|
Quote:
It does change the sound. If you have a push-pull amp (this does not apply to single ended designs) and you push it by turning the volume control up the drop in the voltage across the rectifier provides a degree of "compression" and reduces the dynamic range. (the common term for this is "sag") Most HiFi enthusiasts do not think of this as a good thing but many guitar players do. If you want to preserve the sound of the tube rectifier and also use solid state diodes then place resistors in series with each diode (between the diode and transformer) such that the B+ voltage remains the same as with the tube. Roughly 50R might be a staring point. (but as I said, "Most HiFi enthusiasts do not think of this as a good thing.") Compression is an interesting effect. It reduces the loudness of the peaks but this has the counter intuitive effect of making the music sound louder. Compression is added intentionally by recording engineers to most modern pop music for this reason. |
|
![]() |
![]() |
Thread Tools | Search this Thread |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
Hagerman Bugle power-supply or for +/-15dc 100ma generic supply. | tweakk | Swap Meet | 3 | 23rd February 2011 06:46 PM |
Overload recovery | lndm | Tubes / Valves | 28 | 12th May 2006 12:11 AM |
Pearl input overload? | Saurav | Pass Labs | 9 | 13th October 2003 05:09 AM |
Subwoofer overload protection? | Axeman | Subwoofers | 24 | 5th September 2003 04:44 PM |
heater supply (xformer specs are 6.3V 2.5A) as supply for a power LED? | jarthel | Tubes / Valves | 10 | 21st July 2003 01:30 PM |
New To Site? | Need Help? |