Vox AC4 clone power supply - choke advice please

[Yazoo]

I have just built a clone of the Vox AC4. To save money, I used back to back power transformers. I used a full-wave diode bridge rather than a tube rectifier. I am getting good voltages for the heater supply to an EF86, 12AX7 and EL84.

The problem I have hit is that the plate voltages are too low. On the schematic, the voltage is given as 270V and I am getting 260V at the first 33uf power capacitor and only 221V at the second 33uf capacitor. The 1K power resistor between the 2 caps is dropping 39V. To get the voltage up, I want to replace the 1K resistor with a filter choke. I have modelled this in the Duncan Amps power supply designer and I should get around 256V. I am new to this so I would appreciate some advice. Looking at the Hammond filter choke guide, I see the following which might be OK:

156M 3H, 100ma max current 86 ohms resistance, 400V max voltage

158Q 5H, 150 ma max current, 105 ohms resistance, 400v max voltage

What would you recommend please?

[didit]

Hello -

Why not 156L with 5H, 75 ma.& 135 ohms? An EL84 single-ended pulls sensibly 50mA max.

Best .. Ian

[sluckey]

Your amp is only drawing 39mA as is, so chokes with a 50mA to 100mA would be OK. With that in mind I would get the most Henrys with the least DCR. More Henrys means less hum, lower DCR means more B+. The 194B (4H, 90mA, 108Ω) would be another good candidate.

[didit]

Hello -

Checking our homework off this schematic--https://www.thetubestore.com/lib/thetub ... ematic.pdf--and seeing nominal design bogie >= 55mA. Conservative rule of thumb for chokes applied of 150% above typical current.

Best .. Ian

[Yazoo]

Thanks, I need to re-bias the EL84. I’ll do this when I fit the choke. It works but it is running too cold. I just tack soldered the cathode resistor in. I am going to use a 5 watt potentiometer to adjust the bias.

I used this schematic but it isn’t complete. Fortunately I found a good photo of an original AC4 chassis and could see the additional connections on the tubes.

[sluckey]

Doing the math based on the info given by Yazoo, ie, "The 1K power resistor between the 2 caps is dropping 39V.", I get 39mA for "his" amp.

[didit]

Doing the math based on the info given by Yazoo, ie, "The 1K power resistor between the 2 caps is dropping 39V.", I get 39mA for "his" amp.

I saw that too, but he has set out actively after raising the voltage and generally warming it all up to match nominal amp specification.

[sluckey]

His power supply is considerably different than a stock AC-4. He only has 260V directly off the rectifier. The only way he will ever get to the 270V shown on the schematic is to reduce the 1K AND reduce the current load by increasing the EL84 cathode resistor, IOW, run even cooler than where he is now, which ain't what he wants to do.

He can't increase the B+ from 221 up to 270. Just replacing the 1K with a 100Ω choke will get his B+ up just shy of 260V. But when he biases that EL84 hotter, it's likely to pull the voltages right back down near where they are now. He can easily check this out by replacing the 1K with a 100Ω before buying a choke. Worth the effort to try. The choke is still a good idea even if the voltages don't land where he wants them. It's all good and all the chokes mentioned are good candidates for this little amp.

[ChopSauce]

So you're ready to buy a choke just to spare the cost of a power transformer with the good voltages - knowing that it is unlikely to lead you where you want, anyway?

If you leave in Europe, I can sell you a 50VA toroidal transformer with the good voltages that I don't use, for roughly the price of a choke.

[Yazoo]

Thanks for the offer but I’ve already ordered the choke. It only cost £20, so I’ve spent £44 in total for the back to back transformers and choke. It is also a learning process for me and I’m in Covid lockdown so it keeps me busy. If I can get close to 255V on the plate voltage, I will be happy. That would be about 95% which is close enough.

I discovered Digikey as a supplier. Orders over £33 come with free postage and no import duties from America which is some consolation for the loss of European suppliers.

[sluckey]

If I can get close to 255V on the plate voltage, I will be happy.

I think that's doable. Just remember, as you bias the EL84 hotter, that voltage is likely to drop simply due to the increased load on the transformers. Can you post a schematic that shows exactly how your PTs, rectifier, and filter caps are wired?

In the end I'd let my ears decide if this is a success. Numbers are fun to play with but they all sound a bit flat to me.

[Yazoo]

I am waiting for the choke to arrive. In the meantime I’ve removed the 1k resistor. Looking at the original low pass filter of 1k with the 33uf capacitor, I get a ceiling of 4.82 Hz (using an online calculator). With the choke, this would increase to 12.39 Hz, so I have paralleled a 47uf capacitor to bring this down to 7.96 Hz.

Is it ok to follow the 33uf reservoir capacitor with 80uf after the choke or should I also add a 47uf capacitor to the reservoir capacitor to make it up to 80uf as well? Advice appreciated!