Tube amp power supply choke

BruGalli

New Member
Hello everyone,

I'm building a tube amp based on the 6N2-6P1-6E2 design from a chinese-made pcb.

I want to add an in-line choke in the power regulation section, right after the two AC rectifying diodes. However, I noticed in most other designs, there is always an electrolytic cap (~100 uF, 450 V) before the choke, and there is usually another cap right after.

Is there an absolute need to have the first cap before the choke?

Thank you!
Bruno
 
Two basic types, choke input power supply and capacitor input power supply.

There is lots of info available about the difference between.
 
Swinging chokes are generally rated with a range of inductance dependent upon the current draw through it. The higher the current the lower the inductance. A swinging choke is not necessary for a choke input power supply. I occasionally will use the same power supply for two different output tubes, for instance 2A3 and 300B. The 300B requires about 100Vrms more plate voltage than the 2A3, so for the 300B I add a capacitor before the choke, converting the choke input supply to a standard pi filter. So with the same power supply I can get either 300Vrms or 400Vrms.

To be suitable for choke input, the choke must meet "critical inductance." In other words it must have sufficient inductance to act like an inductor or the power supply is simply capacitor input with the capacitor after the non critical inductance choke the true input to the power supply. There are several formulas for determining critical inductance, but in general 4H - 5H will be sufficient. When selecting a choke for a typical pi filter it is best to select one with modest inductance (2H - 10H) and low resistance. I like to keep the choke less than 60r. Don't forget that resistance in the power supply will act to inhibit current flow through it by dropping more voltage as the current increases.

In the end I always run my power supply design through Power Supply Designer II before building it to ensure the choke meets critical inductance and the rectifier tube is not stressed. It is a valuable tool and worth getting to know.
 
I understand better now. There is a lot more design considerations involved here, and will dig further into the matter.
I had bough a Raphäelite choke, 150 mA, 68 ohms to help rectify even more the DC generated for the B+ from the board.
I had thought of inserting it simply between the rectifying diodes and the next capacitor filter stage to improve ripple rejection.
I hadn't thought that doing this will/could add resistive load and may reduce the resulting 330 Vdc voltage.
But the CHC topology makes more sense to me, yet I didn't want to alter the board in order to also insert the first stage cap.
In the end, I may leave the regulation circuit as it is and simply check if there is any resulting hum when the amp is finished. If so, I'll try adding the choke.

Thanks guys for the clarifications!
 
Forgot to mention that my Raphäelite choke, 150 mA, 68 ohms, is also rated at 5 Henries, so I guess it makes it a swinging choke from what I understood from the previous discussions. So from what Palustris said above, it may not be requied at all anyways...
 
Forgot to mention that my Raphäelite choke, 150 mA, 68 ohms, is also rated at 5 Henries, so I guess it makes it a swinging choke from what I understood from the previous discussions. So from what Palustris said above, it may not be requied at all anyways...

It probably is not a swinging choke.
 
I understand better now. There is a lot more design considerations involved here, and will dig further into the matter.
I had bough a Raphäelite choke, 150 mA, 68 ohms to help rectify even more the DC generated for the B+ from the board.
I had thought of inserting it simply between the rectifying diodes and the next capacitor filter stage to improve ripple rejection.
I hadn't thought that doing this will/could add resistive load and may reduce the resulting 330 Vdc voltage.
But the CHC topology makes more sense to me, yet I didn't want to alter the board in order to also insert the first stage cap.
In the end, I may leave the regulation circuit as it is and simply check if there is any resulting hum when the amp is finished. If so, I'll try adding the choke.

Thanks guys for the clarifications!
It is not necessary to use a swinging choke for a choke input filter but the choke should be specified for choke input duty. In other words it needs to handle the full current load of the amp at full power. Whenever you use a choke input filter it really lowers your B+ so one would want to know what B+ you need and what the high voltage secondary is from your PT. Knowing that you can use PSUD2 to get an idea of how changing the values of a LC or CLC or CRC, etc. type filter will affect your B+. So you really need to know the specs of the PT , its secondary high voltage output, and the DCR of that winding to input in PSUD.
The choke you purchased is probably not for LC type filter. Unless you are making a single ended low wattage amp, but it may be ok in a CLC filter for a PP el84 type amp. Again, without more info on the amp you are building it really is just guessing.
I use chokes all the time in my own amps since they help current regulation, which is a good thing. And since most of the amps i build are single ended current demand is an important consideration.
 
Yes, the amp I'm building is a single ended type (based on 6P1 output tubes), low wattage amp (2x3.5 Watts) that I plan on using with 4 ohms output transformer (Raphäelite OP5K8A) and speakers (EPI A70).

As you said, this amp being low power will make current demand from the power supply much less of an issue. Even though this choke I bought may not be a swinging type, I'll be tempted to try it out based on your comment.

Cheers!
 
Consider picking up an old copy of the ARRL radio amateur's hand book from the
mid 60's. I have the 1964 version which goes into detail about power supply design
choke input, cap input, rectifier tubes, MV tubes. lots of good data for tube audio.

I think I paid $1.00 for this at a hamfest.
 
Thanks for the suggestion. I definitely will try to get my hands on a copy.

They are pricey on eBay, but I'll keep an eye for it when I go to flea markets or the likes.
 
Thanks for the suggestion. I definitely will try to get my hands on a copy.

They are pricey on eBay, but I'll keep an eye for it when I go to flea markets or the likes.

I have copies from 1945 up to 1995..It's funny to see the tec change over the years.

Go to a hamfest to do your shopping, they have tubes too.
 
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