Replacement of tube amplifier kit filter choke

[luiHS]

Hi.
I have a tube amp kit that I bought many years ago, I think at least 30 years ago, I'm going to see if I can assemble it.

The fact is that I am missing a piece, specifically Z1, which I think is a double choke to filter 50Hz AC. Since it's something so old and I suppose it was a custom made part, I can't find the original replacement. What strikes me is that the two filter chokes are in a single encapsulation.

In the list of materials in the magazine neither the value of the choke nor the current is indicated, only the TA30 reference. I have been looking at other schematics in a book that I have and they usually use 10H chokes but they are usually to supply a lot of current because they also feed the final power tubes. In this kit the choke is quite small and only feeds the low power tubes of the previous stages (ECC82, ECC83), not the final tubes (KT88).

In short, what would be the appropriate value to find a replacement for this filter choke?, value of the inductance and current in mA. And if anyone knows if there are double chokes.

Each channel installs 1 x ECC83 and 3 x ECC82 tubes, and 2 x KT88 for the final power stage. 

[TimFox]

I believe it is actually a double choke:  1/2 of it is a series impedance that drives filter capacitor (C26 in series with C27) to reduce the power supply ripple to the voltage amplifier stages.
The other half connects the HV supply to a similar capacitor on the other channel.
Since the current draw of the voltage amplifiers is much less than that of the output stage, you may want more than 10 H in each of two isolated chokes (if you have space to mount them) with sufficient insulation voltage (> 450V).
Those capacitors are not specified on the drawing:  what is the capacitive reactance of the series combination at 100 Hz?
The choke inductive reactance should be substantially greater than that (to avoid resonance at 100 Hz).

[luiHS]

I believe it is actually a double choke:  1/2 of it is a series impedance that drives filter capacitor (C26 in series with C27) to reduce the power supply ripple to the voltage amplifier stages.

Those capacitors are not specified on the drawing:  what is the capacitive reactance of the series combination at 100 Hz?

C26 and C27 are 100uF 400volt
R49 and R50 are 100K 2W

[TimFox]

50 uF at 100 Hz is -j 32\$\Omega\$
10 H at 100 Hz is +j 6300\$\Omega\$, which should be plenty for this application.

[luiHS]

50 uF at 100 Hz is -j 32\$\Omega\$
10 H at 100 Hz is +j 6300\$\Omega\$, which should be plenty for this application.

The filter chokes do not indicate their value in Henries for the inductance, and in mA for the current they support ??
The choke would then be 10 Henries. But how many mA?

[TimFox]

If you look at the full wiring diagram (only a bit was posted) and add the actual resistor values to it, there are enough voltages indicated on the drawing to calculate the total DC draw of the voltage amplifiers.
For example, (R53 + R54) has (400 - 210 = 190 V) across it for one (of many) branch of the DC draw.

[luiHS]

If you look at the full wiring diagram (only a bit was posted) and add the actual resistor values to it, there are enough voltages indicated on the drawing to calculate the total DC draw of the voltage amplifiers.
For example, (R53 + R54) has (400 - 210 = 190 V) across it for one (of many) branch of the DC draw.

R53 and R54 are 22K.
The calculation of the consumption of each one of the triodes of V4 would be calculated like this?

I = (400-210) / (22000 + 22000) = 0.00431 = 4.31mA

[floobydust]

One choke drops 30V and wild guess say 10-25mA load , it would be great to know the full schematic and part values though.
Hammond small chokes are all several hundred ohms.

[luiHS]

One choke drops 30V and wild guess say 10-25mA load , it would be great to know the full schematic and part values though.
Hammond small chokes are all several hundred ohms.

Attached complete schematics and list of components.

I know about Hammond transformers, I have another tube amp with 6550, from which I took the schematic from Kevin O'Connor's Principles of Power book, it uses Hammond 1627SE output transformers and 273BX power transformer, a Hammond 193J for the choke.

[floobydust]

That helps a lot more.  BoM a bit unusual for "100.000 ohm" means 100kΩ or "220.000 pF" is 0.22uF I think.
Looking at voltage drops I get V4 10.13mA, V3 12.11mA, V2+V1 5.14mA = total 27.4mA and 30V across it gives DCR 1,094Ω.
I would use one choke and feed both channels because C25+C26 filter out crosstalk enough. Economy designs just used two resistors.
For the money, it's a precise design including 1pF cap C28 that is a bit exotic. I would add individual bias and balance trimpots.

[luiHS]

That helps a lot more.  BoM a bit unusual for "100.000 ohm" means 100kΩ or "220.000 pF" is 0.22uF I think.
Looking at voltage drops I get V4 10.13mA, V3 12.11mA, V2+V1 5.14mA = total 27.4mA and 30V across it gives DCR 1,094Ω.
I would use one choke and feed both channels because C25+C26 filter out crosstalk enough. Economy designs just used two resistors.
For the money, it's a precise design including 1pF cap C28 that is a bit exotic. I would add individual bias and balance trimpots.

Thanks for your information.
So I could use one or two 10H 65mA chokes like Hammond's 157J?

https://www.mouser.es/ProductDetail/Hammond-Manufacturing/157J?qs=o%2FsL%252BddUClXlxvM12hhtjA%3D%3D

One more question, out of curiosity; What is the use of connecting the capacitors as seen in the attached image?.

[DavidKo]

Probably some old device. HV electrolytic was not common like today. Resistors can be to ensure "same" voltage on both capacitors (tolerances -0% +100% was quite common).

[trobbins]

The main amp is a variant of the Williamson circuit (of which there is bucket loads of on-line info about). 

I can't see the output transformer TU/S in the part list - that part will be the most important aspect of the amp if it is to perform ok.  Sadly the schematic shows C23 as coupling the input stage to the cathodyne phase splitter - that coupling cap adds one more low frequency network than is normally used and is highly likely to cause low frequency instability (even without that coupling cap, the use of an inferior output transformer still causes instability).

I'd suggest using two chokes in lieu of Z1 - the original Williamson used a 30H 20mA choke, but it didn't also feed a preamp section from that feed.

The circuit shows it uses a R39/C20 step filter so hopefully there is sufficient high frequency stability.  I can only suggest that caution is needed to confirm stability (LF and HF), which is not a simple and easy excercise, but could avoid damage and disappointment.

[luiHS]

I'd suggest using two chokes in lieu of Z1 - the original Williamson used a 30H 20mA choke, but it didn't also feed a preamp section from that feed.

What I don't understand yet is how to calculate the inductance value of the choke.

In the book "Principles of Power" by Kevin O'Connor they give some information, but they start from the assumption of always using a 10H choke, they give some explanations with formulas for calculating the PI filter with coil and capacitor.

I put here screenshots from the book, with the information they give, and some of the schematics that are used for tube amplifier power supplies, almost all of them with 10H chokes, but there are also some with 5H chokes, and some with a double filter with two chokes.

Maybe I could use two 15H 40mA chokes.
https://www.mouser.es/ProductDetail/Hammond-Manufacturing/155J?qs=MDqvSWII5j8KLIsvqvWoMg%3D%3D

[JohanH]

It's just an LC filter. In the past capacitors were expensive. So it was easier to use large inductances. At the same time you get a low inrush current that didn't destroy the tube rectifier diode. If you just replaced it with large filter capacitors, you would blow fuses as well. The choke also causes a voltage drop. It could be replaced with an eqivalent resistor, but you would get more ripple.

Use Duncan's PSUD2 software to calculate and see the impact of an LC filter versus other filters: https://www.duncanamps.com/psud2/

As for 5H vs 10H, depends on current demand, available chokes, price, space considerations and acceptable ripple. The general principle is that the larger the choke (and larger capacitor), the less ripple you get.

[TimFox]

Besides the cost of capacitors (and the required high-voltage rating), if a 10 H inductor has a reactance of 3 k\$\Omega\$, but a DC resistance of only 100\$\Omega\$, then it wastes far less DC voltage than a 3 k\$\Omega\$ series resistor for similar ripple reduction.  Note that a push-pull output stage is relatively insensitive to ripple on V_bb, but the input voltage amplifiers are more sensitive to it.

[floobydust]

Haven't seen the 430VDC power supply schematic, but assuming it's using solid-state rectifiers - so B+ surges to a high voltage when the tubes are cold, there is no load on the power supply until the filaments warm up. The voltage rise is much higher for choke-input LC filtering on B+, rarely used though. I would expect B+ to go over 500V and this is the reason the designer uses two 450V caps in series with balancing resistors.

For sizing the choke, Radiotron Handbook advises to avoid resonance at the amplifier's lowest output frequency. That pipe organ music at 25Hz... not only mains ripple coming in to it.
The choke should be as large as possible, what you can afford and fit for best filtering.
The choke should be as small as possible, to save room and money, and have low losses.

[TimFox]

10 H and 50 uF resonate at 7 Hz for this example.

[floobydust]

I also found these chokes used in guitar amps:
EDCOR USA choke offerings are 4H 50mA 170Ω 0.4lbs used as Fender 125C3A, 15H 75mA 347Ω 1.4lbs.
Hammond 194 is another series, shielded.
Sowter UK also has excellent offerings.

[luiHS]

Haven't seen the 430VDC power supply schematic, but assuming it's using solid-state rectifiers

This is the schematic of the power supply, yes it is solid state. On the power supply there are four trimmers to adjust the BIAS of the KT88.

[trobbins]

luiHS, if you haven't made such a valve amp before, and have minimal measurement and test gear, then imho this is an advanced amp to start your learning curve with, even if it is a kit with pcb's and construction details.  I say this as KT88's are quite expensive, and rail voltages are high, and unless the construction details have detailed steps for checking everything out in minute details with low risk, and you have at least a variac for initial testing, then it may be quite easy to overlook something and not realise until too late.

Do you really need the choke(s) to plan your chassis layout in advance, or can you simply use a resistor for each choke for starters? A resistor is not going to significantly affect initial start-up capability/performance, and may well avoid damaging any choke if there was an accidental fault somewhere.

My view is that the design is lacking some basic forms of protection for a modern valve amp, including fusing the 340Vac secondary and making the bias trim pots fail-safe.