6AS7 Push Pull

Post by **Nick** » Thu Dec 20, 2007 9:55 pm

With valves of that cost, I would bring the voltage down.

What sort of DC heatring were you using before?

Paul Barker · Post by **Paul Barker** » Thu Dec 20, 2007 9:56 pm

simon wrote: I changed the 300B filaments to AC and this may have helped. Checking the voltage though I'm getting 5.4V AC which I would have thought was a little high. Should I be concerned and put some resistance in there to bring it down?

Yes

simon · Post by **simon** » Thu Dec 20, 2007 10:07 pm

Simple CRC. 5V Hammond filament tx into bridge into 4700uF - 0.1R - 4700uF. When I checked the DC voltage it was down at 4.4V, bit of a shock but I guess the KRs must draw more current than standard 300Bs. I'm a bit surprised it's so much over now though. I can tweak the bridge with schottkys to give me a bit more voltage to play with, but I'd like to stay with AC if I can. Presumably the extra resistance I need to add shouldn't affect sound?

Paul Barker · Post by **Paul Barker** » Thu Dec 20, 2007 10:57 pm

I doubt it.

simon · Post by **simon** » Thu Dec 20, 2007 11:10 pm

Okay, if I want to drop 0.5V, and according to the KR website the 300BXLS draw 1.9A I need a resistor of 0.26R of 3W or so. Am I right in thinking I need to put 0.26R in one wire or 0.13R in both wires from the tx? As I happen to have 4 No. 0.1R resistors this would be a good start perhaps?

Lee S · Post by **Lee S** » Thu Dec 20, 2007 11:14 pm

I would put one in each wire.

simon · Post by **simon** » Thu Dec 20, 2007 11:42 pm

Sorry, I wasn't very clear, I meant put 1 No. 0.1R in each wire. Thanks Lee.

Have to say the amps sounds wonderful now.

Post by **Nick** » Fri Dec 21, 2007 12:20 am

I would put one in each wire.

Now, this is interesting (well to me anyway), like Lee, my instinct would be to put them in both lines. But I can't for the life of me think of a reason why that would make any difference, can anyone else?

Lee S · Post by **Lee S** » Fri Dec 21, 2007 1:24 am

It certainly won't alter current flow through each resistor, nor the power consumption, so it makes no difference there. I can't think of a reason either Nick. It seems to be one of these "best practices" that no one really questions why they are doing it, but they just do it. Maybe it looks neater and gives the builder a sense of "balance" or feng-shui in the amp!!

Would it be better to put the two in series in one side of the circuit? Would it be better to have one of the correct value? Which side would it be best fitted in? Would it be better to have two of half the value in each side? I guess all of the above will work and work in exactly the same manner. To burn off half a volt or so.

"No idea" is my answer.... just instinct too I guess....

richardcooper2k · Post by **richardcooper2k** » Fri Dec 21, 2007 1:32 am

here is my understanding, is it wrong ?

normaly 0v would be earth, which theoretically has a zero resistance drain for any noise, so is theoretically clean.
in this case the two wires from the rectifier are just at voltages relative to each other, there is no zero resistance noise drain to earth from either. so neither is clean, so both benefit from filtering....?

Lee S · Post by **Lee S** » Fri Dec 21, 2007 1:37 am

What rectifier? We are using AC aren't we?

simon wrote:but I'd like to stay with AC if I can

richardcooper2k · Post by **richardcooper2k** » Fri Dec 21, 2007 1:48 am

whoops, sorry, didn't read thread carefully enough, there was talk of it earlier on

in that case i can't "see" what difference it would make other than its easyer to put in one resistor rather than two

but then i need glasses !

Paul Barker · Post by **Paul Barker** » Fri Dec 21, 2007 8:32 am

Lee S wrote: Would it be better to put the two in series in one side of the circuit? Would it be better to have one of the correct value? Which side would it be best fitted in? Would it be better to have two of half the value in each side? I guess all of the above will work and work in exactly the same manner. To burn off half a volt or so.

"No idea" is my answer.... just instinct too I guess....

It's hard as it is to find suitable resistors in the parts box as it is.

The beauty of winding your own secondary on a non descript torroid is that you get the voltage right so don't have to fart about like this.

When I wound power transformers I used to put sufficient primary connections to get the secondry voltages right.

If you calculate your transformer properly you can get voltage bang on. I am fortunate that my mains is always 240 to 245.

Start with that and then work out true power consumption, the regulation (percentage difference from theoretical linear voltage transfer) is usually between 5 and 10% if you have selected the correct primary wire diameter and correct core size. You pic your point along the 5 to 10% based on the actual power consumption.

After a little experience you get cock on. But to allow for slight variations you do +/- 5v and +/- 10v taps on the primary.

But in my present experience the best way to heat filaments is to wind your own secondary on a torroid, and stop when the voltage is right.

Darren · Post by **Darren** » Fri Dec 21, 2007 8:53 am

I normaly select a mains trans from my stock with the best/nearest HT value I'm looking for.

Most have 5 & 6.3V for heaters. So my PX4 amp has dropping resistors for both rectifiers and signal valves. In this case they all use 4V.
I only used one resistor for the 5V tap and two in the 6.3V to help disipate the heat. Both were/are on one side only.

BTW, they do get very hot, hot enough to melt the cable insulation for a cm or so. They could really do with a heat sink each.

steve s · Post by **steve s** » Fri Dec 21, 2007 9:48 am

thats just what i do darren, i have a stock of 10watt 0.5 r and 1r, they run ok..still hot.. doubled up ( 20watts) it

in the last year i have been rewindnig the fillament transformers to get them just right for the valve..

audio-talk

6AS7 Push Pull

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