330V 25mA

unless i am mistaken you are looking to get around 330Vdc. if you use full wave rectification on the 640Vac secondary, you should get approximately 452Vdc, you can make a voltage divider with four 25 ohm resistors and get approximately 336Vdc across three of them.

It's very little current. Why not get a light duty Variac or similar auto-transformer to step down the secondary potential to just what you need?

emc_c

You can end up with about 330 VDC from 640 VAC centre tapped by using a choke input filter. Similar to the typical pie filter used on 300 V (odd) power supplies but without the first capacitor to ground.

"The choke input supply will have a choke immediately following the rectifier. The main advantage of a choke input supply is better voltage regulation, but at the expense of much lower output voltage. The output voltage approaches (2*sqrt(2)/Pi)*Vrms of the AC voltage. The choke input filter must have a certain minimum current drawn through it to maintain regulation"

Google Choke input filter power supply.

Center tap to ground. Half-wave rectify both ends of transformer, tie them together and filter as desired to get the little bit of 330 DC you need. I'd design the filter, but I'm old and tired, and there must be some EE student that needs the exercise.

You probably mean the transformer output is:

<<I have a trasformer that has a secondary 640v CT and 6v.>>

Just use a fullwave rectiier [2 diodes or combination of 2 diodes connected as anode or cathode common with 3 legs] & filter rated for 400VDc.

Half-bridge used as Full-wave-rectier [2 discrete diodes or one 1/2 Bridge Pos Output]

Half-bridge used as Full-wave-rectier [2 discrete diodes or one 1/2 Bridge Neg Output]

Both above make a Full-wave bridge rectifier [If Source is without centre-Tap] output will be equall to Source voltage DC approx .

If the source has a CT & outputs are connectected between com & one at (+) & the other to (_) the output will be 320(+) - 0V - 320V (-) V aprox.

that is Dual supply.

Hi kokkoplus,

You don't mention your power amp. if it is also a 'tube' model then they usually possess a power take-off, auxiliary socket, which is supplied with DC from some way down the power bus and this is specifically intended for pre-amps or even tuners. You will find that the 300 ish line is easily obtained within the power amp amyway.

One point to remember about solid state rectifiers is that they produce full output at the instant you switch on whereas the 'load, ie the tubes does not appear until the heaters have warmed up. During the intevening time you will over-load any capacitors with the rating as provided by Quad. This situation will be avoided if you delay the energising of the solid state rectifier until after the tubes are warmed up.

You can derive the voltage you need without a transformer by full wave rectification smoothing with choke and reservoir capacitor if no other source is available.

Do remember though that preamps are extremely sensitive to 'ripple' on the HT line and also to hum induction on the heater supply.

You may well find that the S/N ratio is somewhat below the makers spec unless the HT and heater supplies are derived from the power amp.

Having messed about with hybrid amps for many years I can only advise you not to be over confident in the results. Be patient and if at first you don't succeed.......keep your soldering iron hot!

Happy New Year.

Massey.

Dear Massey 726,

you are right about the plate voltage coming from the power amp. In fact the Quad22 is called "control unit" and has two umbelical cords that connects it with two Quad II, the tube power amps that I wish I had but I don't.

Instead I am in the process of building my first tube amp which is a clone of the Dynaco st-35. I even got the original output transformers! I am much more familiar with solid state so I might say something stupid...

From the schematics it looks like the pre Quad22 works with 330 volts and the Dynaco st-35 with 320 volts. Is it a problem? I also wondered if scavenging the HT from the power amp would make it run out of juice (I've read something similar would happen on some Quad22 + Quad II).

Thanks about your consideration on rectifiers. I will see if I can find a way to use solid state diodes and have the voltage slowly get to the nominal value. Or just delay it for 30 seconds.

Thanks and Happy New Year.

Hi kokkoplus,

The actual volts are not that critical as if you look inside the pre-amp there are some series dropper resistors and storage electrolytics en-route to the anodes. Its all rather conservatively rated and your extra volts won't hurt anything as it would in a solid state line-up. If you were at all troubled you could always increase the first resistor in the power feed chain ;but I wouldn't bother. The EF86 and ECC83s are very tolerant beasties!

As regards the delay-on time you could go as far as measuring the time taken by the filaments to bring the tubes into action and set the interval accordingly. As I was using a set of diodes arranged to rectify the whole DC supply to the power amps as well there was a damned big load and I put a 1500 ohm resistor shunted by a set of relay contacts in series with the DC supply line. The resistor gradually raised the voltage in the network and a 555 based timer circuit then activated the shorting contacts. This saved there being a big 'thump' when the HT was activated.(I value my speakers and ear drums!).

You could always try building a pair of good quality 50 Watt power amps if you felt so inclined.

Good luck and best wishes,

Massey.

Hi there Massey.. I was hoping you might help me with you opinion on an amp design issue. I just noticed your posting and I too have a similar issue to Kokkoplus in that I have uncovered a simple 40Watt guitar stereo power amp design (one 20Watt side shown in link below) using a single EL34 for each side. It looks like it might be configured as class A?. It shows a supply voltage of 300Volts to the tubes immediately after a 10K 10 watt drop resister but it doesn't indicate what the supply voltage form the transformer is to get this. I actually have a power Transformer lying about with 320v-320v secondary and I was wondering could I use the one transformer to power the two sides (or do I ideally need two separate power transformers).. In any case I am not sure how to derive the required 300v... any thoughts.? Thanks a million. http://www.kbapps.com/audio/tubeamps/40p.html

Hi fayser1,

I would reckon the single power supply is OK voltage wise with full wave rec and smoothing via choke and 32 mF caps the only problem might be in terms of the current requirement and whether your tranny is upto it.

The EL34s are easy beasts to live with even if the harmonic distortion is a bit raunchy. Plenty of NFB usually helps.

Tell me more about the tranny and I see what comes to mind.

Best wishes,

Massey

Hi.. thanks for the prompt reply... very kind... The traffo I have is the TP40D found at www.allenamps.com... It talks of 320v-0-320v at 180mA ?? not sure if that gets me where I need to be... In particular if each side is working in Class A will that mean more current draw? If one traffo suffices then does one just build one power regulator circuit and feed both amps halves from that ? Cheers Tony

Hi fayser,

I don,t have my old valve book to hand at the moment, its not been consulted for a few years! So from memory I think you are OK with the supply capacity.

I would put the retifier and main smoothing as a single supply. Then split and drop via say 100 ohm w/w resistors into each of two reservoir capacitors of at least 32mF. These should be rated to 450V, you may have to settle for a pair of 16s in series,if you do then strap a 100k 0.5 Watt resistor across each to stabilise the network.

I have over run transformers in the past and got away with it, just give it plenty of ventilation, I would even suggest putting one of those handy little cooling fans found in the back of PCs if necessary..

Best wishes,

Massey.

Hi Massey.. not sure I understand what you are getting at here... do you mind running it by me again?..am I looking to rectify each half (320 -0) (centertap)and the (0-320) separately or some such? Thanks again Tony

Hi fayser,

No, the bridge rectifiier or thermionic valve GZ34 or equivalent are connected across the ends of the 320-0-320 winding, the 0 tapping becomes the ground or return path for the pair of amps and all associated components. You then 'smooth' the full wave output via the first capacitor and inductance before splitting the DC into two discrete supplies one to each power amp network. I suggest addind the decoupling resistor at the point of separation to assist in reducing interaction between the two amps.

You can dream up all sorts of tweeks to make things sound sweeter, it all comes down to how much spare time you feel like devoting to this fascinating 'hobby'.

Good luck and keep in touch,

Massey.