Where to add a choke to tube amp

[MHBuur]

Hi there,

I have a 6H choke that I'd like to try out in an amp that was not originally designed with one. However I am not sure where the ideal place is to put it or if it replaces some other part (one of the power resistors I assume, R86 near x23?).

I've attached the PSU section of the amp schematics (complete schematics here http://forum.highlyliquid.com/attachment.php?attachmentid=195&d=1319883369)

Any pointers?

 

[gemini86]

Ideally, I would want it before R86. If possible, add it between your diode bridge and C43/44. If you add it after R86, you'll only be filtering the screen supply and everything after it, which is fine, but you're not filtering the plate supply for the power amp.

 

[MagnetoSound]

I would rather place it between C43 and R86. Keep the smoothing on the high side of the choke.

Another cap at the plate node wouldn't hurt, either. It seems to me that if you are going to the trouble of adding a choke, you might as well go the whole hog.

 

[MHBuur]

Thanks for your replies

So the difference between your suggestions is basically cap -> choke (solution A) vs choke -> cap (solution B)?

I guess the "effect" of the choke inductance will be most significant after the filter cap (B), however the supply will be more stable if I place it before the cap (A)?

What about a possible voltage drop, will that be more significant with one of the solutions? (I'm thinking B is worse here)


And yep, I was thinking about looking at the "cap situation" after I try this out. I'm currently looking for a good deal on suitable caps at ebay. Any other suggestions on improvements are most welcome

 

[ruffrecords]

Just one word of caution. This choke will be carrying a significant dc current. You don't say what dc current your 6H choke is rated for but if it is underrated it will saturate and have little benefit.

Cheers

Ian

 

[MHBuur]

Just one word of caution. This choke will be carrying a significant dc current. You don't say what dc current your 6H choke is rated for but if it is underrated it will saturate and have little benefit.

Cheers

Ian

Yes, that is a problem: it doesn't say. The wire is pretty thick though, but I guess the voltage rating is about the insulation and not the thickness?

 

[ruffrecords]

Just one word of caution. This choke will be carrying a significant dc current. You don't say what dc current your 6H choke is rated for but if it is underrated it will saturate and have little benefit.

Cheers

Ian

Yes, that is a problem: it doesn't say. The wire is pretty thick though, but I guess the voltage rating is about the insulation and not the thickness?

It is the current rating you need to be concerned with. As this choke is intended to pass dc it will have a gap. Just how much dc it can pass before the core sturates and it ceases to act like an inductor will depend on the core material and the size of the gap.

Cheers

Ian

 

[tubegeek]

Do you know the original purpose of your choke? Or the age, or topology of the original circuit it came from?

It's very common to see a choke used only for screens (lower current capability required, unfortunately) in many guitar amps, especially a push-pull type where a little power supply ripple is basically rejected in the plate circuit.

However, it was also very common in a high quality single-ended amp to use a choke for improved power supply filtering (better current capability) and if it were this type it'd be more likely to serve well in your project. This would usually be an older style of amplifier, pre-WW2 as a general ballpark. This was an era when 8 uF was a big (and expensive) electrolytic cap value and so a choke was a cost-effective way to smooth ripple.

If the original amp was single-ended and used triode outputs, then the choke would be most likely to have been used as you are hoping, in the "pi" power supply filters as the top arm of the "pi."

 

[MHBuur]

I got the choke many years ago (used, from a teacher), it was for a general PSU project (lab. psu), however I never finished that project as I got my hands on another PSU soon after.

There is very little info on it and in fact I'm not even 100% certain its 6H (that was something the teacher told me back then). Let me get some photos for you when I get back to the workshop.

 

[ruffrecords]

CJ is the best man to guess the inductors parameters from some pics.

Cheers

Ian

 

[abbey road d enfer]

I got the choke many years ago (used, from a teacher), it was for a general PSU project (lab. psu),

That would tend to indicate that it was intended to withstand a somewhat significant current.

There is very little info on it and in fact I'm not even 100% certain its 6H (that was something the teacher told me back then). Let me get some photos for you when I get back to the workshop.

Measuring the DC resistance may give a hint as to the max current (very ballpark figure though).

 

[MHBuur]

Alright, here are some "specs"

Part no.: C104994

The wire is 1.5 mm in diameter.
The DC resistance (probably inaccurate) is 0.06 Ohm.
Physical size of the iron block is 6x5x2 cm
55 core layers (sheets)
Core is 2x2 cm
Visible winding layers: 2
No. of windings pr layer: ~10
Estimated length 2-2.5 m

 

[abbey road d enfer]

Alright, here are some "specs"

Part no.: C104994

The wire is 1.5 mm in diameter.
The DC resistance (probably inaccurate) is 0.06 Ohm.
Physical size of the iron block is 6x5x2 cm
55 core layers (sheets)
Core is 2x2 cm
Visible winding layers: 2
No. of windings pr layer: ~10
Estimated length 2-2.5 m

I have a feeling it's closer to 6mH than 6H. Considering the wire gauge, it is probably capable of passing several amps. IMO it's not adequate for the intended application.

 

[PRR]

> The DC resistance (probably inaccurate) is 0.06 Ohm.

Not a tube-amp choke.

In R-C filtering you pick a resistor to drop 5%-20% of the available voltage. For power plate supply, typically 100-2K.

When you want better filtering with less drop, you pay for a choke. It is liable to be 10 times better than a resistor. The DCR is likely to be 10 ohms to 200 ohms.

You have <1/100th of the expected DCR. It would be "possible" to have a tube-like value of inductance with a DCR that low, but it would be BIG. Like a guitar-AMP, not a guitar-plug.

Much more likely this has 100X the current rating and 1/100th the inductance you need for a tube amp. Indeed, 6mH is more likely than 6H.

Personally I am thinking a car/CB-radio power supply. 12V 10A. A 0.06r*10A= 0.6V drop is reasonable for a "car battery replacement", which probably starts from 14V and has a couple part-volt sags down to 12V at full load.

Uh, go to the Hammondmfg.com site, find open-frame chokes, find a similar physical size, then a similar DCR. See the current and inductance numbers. Compare to Fender Replacement Choke numbers. Probably way-different.

 

[MHBuur]

Okay I've been wasting your time, sorry about that - it was not a choke  ;D

Actually the original PSU project was partly for CB-radio use and general purpose "lab" PSU, so it makes sense now. And yes it is very likely that it was 6mH (it was about 15 years ago after all). Anyway, thanks for your response - it has been most helpful, and now I have some idea on how these things look vs their rating.

I have a choke coming in from Mercury Magnets (for another project). I'll try it out instead, and if its doing anything "good" I'll consider another from them or from hammondmfg.com.

 

[tubegeek]

Okay I've been wasting your time, sorry about that - it was not a choke  ;D

What was it? And *I'll* be the judge of whether my time was wasted - not you! Better men than you have tried to waste my time and found it impossible!

 

[MHBuur]

Okay I've been wasting your time, sorry about that - it was not a choke  ;D

What was it? And *I'll* be the judge of whether my time was wasted - not you! Better men than you have tried to waste my time and found it impossible!

Okay, that was not my intention - sorry, I have a bad sense of humor: "it was not a joke" -> "it was not a choke" (as that was kind of the conclusion here.. at least not a tube-choke) 

I'll get back to you when I have the MM choke (right now stuck in customs.. Grrr)

 

[tubegeek]

I was joking too - I was saying - it's impossible to waste my time because it's already a complete waste!

Sorry for the confusion.... English is my 1st language but I still don't know how to speak it!

 

[MHBuur]

Alright, I'm back on track!

I got my hands on a Mercury Magnets choke (APS), the DC resistance is 125 Ohm.
As far as I understood the suggestions above, the optimal place to add it would be between R85 and R86 (after filter caps).

I tried that (see red circle on attached image)... That did not work out well: the amp started making a very unpleasant squealing sound. I turned it off immediately. What could be the cause here?

The sound did not turn off immediately after the power and stand by was turned off (standby has always been dodgy on this amp.. its not shutting off immediately). I suspect the introduction of the choke interferes with the discharge resistors (are those R85 and R101 btw?) somehow..

I did not remove R86, but from what I understand that may be necessary.

 

[abbey road d enfer]

Alright, I'm back on track!

I got my hands on a Mercury Magnets choke (APS), the DC resistance is 125 Ohm.
As far as I understood the suggestions above, the optimal place to add it would be between R85 and R86 (after filter caps).

 
These chokes are not designed to withstand the total current of the amplifier, so they are inserted between the B+ (positive of 1st cap) and the screen feed (positive of 2nd cap). That means you just have to wire it in series with the standby switch. Alternatively you may choose to wire it as a replacement to R86.

  I tried that (see red circle on attached image)... That did not work out well: the amp started making a very unpleasant squealing sound. I turned it off immediately. What could be the cause here?

  The large DCR of the choke makes the power supply not stiff enough, which may generate feedback.