Filter choke in 6G15

[tubeswell]

I have one of these 90mA chokes

http://www.tubesandmore.com/scripts/...item=P-CF22699

1) Will that do the trick for the choke in:

a) supplying 1 x 6V6GT and 2 x pre-amp tubes in a 6G15 setup? (I suspect it would be fine); or

a) supplying 1 x 6V6GT and 4 x pre-amp tubes in a Hoffman/Weber/Torres style stand-alone 'revibe' (combination of 6G15 reverb with a 5G13 tremolo setup)?

2) Would it be okay for a 1/2-wave rectifier choke filter in the same circuits (i.e. one where the diode-rectified power supply goes straight into a choke before it hits any reservoir caps)?

[vaia10]

I have one of these 90mA chokes

http://www.tubesandmore.com/scripts/...item=P-CF22699

1) Will that do the trick for the choke in:

a) supplying 1 x 6V6GT and 2 x pre-amp tubes in a 6G15 setup? (I suspect it would be fine); or

a) supplying 1 x 6V6GT and 4 x pre-amp tubes in a Hoffman/Weber/Torres style stand-alone 'revibe' (combination of 6G15 reverb with a 5G13 tremolo setup)?

If you place the choke after the power tube's plate it will work for sure. (And it's not worth smoothing before the screen anyways...)

2) Would it be okay for a 1/2-wave rectifier choke filter in the same circuits (i.e. one where the diode-rectified power supply goes straight into a choke before it hits any reservoir caps)?

Are you planning on using the exact same transformer as with the Pi filter? If yes, the choke filter will drop a lot of voltage - much more than you probably want... Randall Aiken has a very nice page on filter chokes:
http://www.aikenamps.com/Chokes.html

Cosmin

[tubeswell]

If you place the choke after the power tube's plate it will work for sure. (And it's not worth smoothing before the screen anyways...)



Are you planning on using the exact same transformer as with the Pi filter? If yes, the choke filter will drop a lot of voltage - much more than you probably want... Randall Aiken has a very nice page on filter chokes:
http://www.aikenamps.com/Chokes.html

Cosmin

I got the 90mA (125C1A-equivalent) choke with the concept in mind of having a choke input filter before the OT Plate supply.

I know that if I go with a bridge rectifier, I'll only need a 220-240VAC secondary, which I can then chuck a Pii (capacitor input) filter after, and get it set up like the Hoffman revibe unit.

But I have a 2nd-hand PT already in my stash that is 376VAC on the secondary (with no CT), and if I use a 1/2-wave rectifier followed by a choke input filter (i.e.; without a reservoir cap in front of it), I maybe can get ~340VDC B+ (I think this is right? - check my math. [(2 x 1.4142)/(22/7)]x 376 = 338). If I then put a series resistor after the choke input filter, followed by another shunt filter cap, I could maybe drop the B+ a bit more and smooth the ripple out a bit more.

So I was wanting confirmation that this 90mA Choke would be up to the job of a choke input filter in this application (1 x 12AT7, 1 x 12AX7, 1 x 6V6GT = <90mA at maximum draw right?), before I put in it and blow it up please :-)

The original choke in the 6G15 is spec'd at 50mA and is used in a capacitor input filter http://www.unclespot.com/FenderXFMRchart.html, and the Hoffman revibe unit also specifies a 125C3A in a capacitor input filter. I also have a 125C3A-equivalent, in which case if I went that way, I'd need to order a smaller PT with (say) a 240V secondary.

However if I want to consider utilising the spare 376VAC PT with a choke input filter, Aiken's site (from the link quoted above) says that:

"the choke must be capable of handling the entire current of the output tubes as well as the preamp section. Note that this doesn't mean just the bias current of the output tubes, but the peak current at full output. This usually requires a choke about the size of a standard 30W-50W output transformer, since the choke must have an air gap (just like a single-ended OT) to avoid core saturation due to the offset DC current flowing through it,... "

I doubt that a 50mA choke can handle the plate and screen supply of a 6V6 as well as a 12AT7 and a 12AX7 at max current draw. Is it likely that a 90mA choke could tho?

[tubeswell]

I wonder if my answer to this question about the max current draw of the whole circuit might lie in a large part on whether the 6V6 is biased to run in Class A or not? (because if it was running in Class A at say 36mA, ~ 340V on the plate, then I probably wouldn't get any more than about 50mA max through the choke would I?)

[vaia10]

How big is this choke? I mean, what are its physical dimensions? Also, how big is its inductance? In a choke input filter you need to draw a minimum current of approx. V/L (RMS/H), where V is the transformer's secondary voltage. As far as the current rating goes, the choke ought to be able to handle the bias (DC) current + the AC peak current (which is approx V/(1155xL), RMS/L - cf. Morgan, "Valve Amplifiers".

If the choke is comparable in size and current rating with the OT (and the min. current condition is met), I'd say it should work. I built a SE amp with a 6BM8 (ECL82) output tube and used Weber's universal choke as the main filter (choke filter). The pentode in the 6BM8 is lower power than your 6V6 and Weber's choke is rated at 120mA/9H so I was in a slightly safer position

Cosmin

[tubeswell]

The 90mA choke I have is a 22699 (125C1A-equivalent)

Choke core measures 2 1/4 inch wide x 1 7/8 tall x 1 3/4 inch deep over the covers. Mounting centers are 2 3/4 inch apart.

Rated 4 henries at 90 ma DC. 110 ohms DC resistance (+-5%), 2500 volt DC factory hipot tested.

Open Circuit Impedance (Ohms): 1,500 (min. at 10 V, 60 Hz)


The blurb on the triode electronics site for this type of choke says

"may be used as a power supply choke for a small single-ended amp project" (which is, I was wondering, maybe the equivalent of a 6G15 project?)

[vaia10]

Judging from the dimensions the choke is big enough for this circuit (air gapped too, probably). It's bigger than the OT that's used in the Weber reverb kits. I'm sure that it will work in the choke filter that you have in mind - besides, what can possibly go wrong? (Don't quote me on that though).

I'm sure you're familiar with Duncan's PSU calculator - maybe you can modify the Champ PSU file and see what kind of voltages and currents you get with the choke filter, 1/2 wave rectifier etc.

http://www.duncanamps.com/psud2/psud_list.php

Cosmin

[tubeswell]

Thanks for your advice and suggestions cosmin. One of the perils of having a mac is that the stuff on the Duncan Amps site is fairly inaccessible to me (Dang!).

If the 6V6 is running in Class A, that means the current is at max throughout the entire cycle right? In which case, in the worst case I could expect no more than (say) 38mA for the 6V6, 10mA for the 12AT7?, 4mA for the 12AX7?

[vaia10]

I run PSU with the specs corresponding to your PT and choke. (376V secondary, 4H/110R choke - 90 mA rating). First of all, are you using the same power supply block as in the standard 6G15 schematic (40u cap -> plate,screen, followed by 10K resistor & 40u cap that feeds both the 12AT7 as well as the 7025/12AX7)? I simply added the choke in front of the first cap and set the PT to 376 with a half wave rectifier. With the currents set to 38mA for the PT and 10mA (12AT7) + 4mA (12AX7), you'd end up with 170V (RMS) on the 6V6 anode. The RMS current through the choke 96mA (outch!), with a max swing of 205mA. That's no good. Also, the ripple would be pretty bad from the half wave rectification, the choke wouldn't be able to smooth it out completely.

If you use a bridge rectifier on the same tranny though, the B+ would be 373V (RMS) and the current through the choke 71mA (RMS) - max swing 122 mA.

Disclaimer: I might have run this completely wrong.

Cosmin

[tubeswell]

I run PSU with the specs corresponding to your PT and choke. (376V secondary, 4H/110R choke - 90 mA rating). First of all, are you using the same power supply block as in the standard 6G15 schematic (40u cap -> plate,screen, followed by 10K resistor & 40u cap that feeds both the 12AT7 as well as the 7025/12AX7)? I simply added the choke in front of the first cap and set the PT to 376 with a half wave rectifier. With the currents set to 38mA for the PT and 10mA (12AT7) + 4mA (12AX7), you'd end up with 170V (RMS) on the 6V6 anode. The RMS current through the choke 96mA (outch!), with a max swing of 205mA. That's no good. Also, the ripple would be pretty bad from the half wave rectification, the choke wouldn't be able to smooth it out completely.

If you use a bridge rectifier on the same tranny though, the B+ would be 373V (RMS) and the current through the choke 71mA (RMS) - max swing 122 mA.

Disclaimer: I might have run this completely wrong.

Cosmin

Thanks for your efforts with the PSU Cosmin. I really appreciate it.

I am not sure if those voltages you got are right though - I calculated 338V B+ with a half-wave rectifier off 376VAC (which followed the formula on the Aiken site). Also, I thought if you used a bridge rectifier off 376VAc secondary winding, you would end up with 531VDC. (376 x 1.4142) (- unless I am missing something?, which I could be )

[tubeswell]

FWIW, as a follow-up to this (if anyone is interested) - in the end I got a little PT made up for about NZ$50 that was 240-0-240 100mA and 6.3V 2A with CT. I run it through a diode rectifier feeding a CLC filter with 47uF input and output caps and a 50mA 4H 150R choke. This delivers about 342VDC B+ loaded up, and about 338VDC to the 6V6 plate at idle .