Hello all

I'm planning to clone the circuit of my Kriesler Multisonic 11-104. It looks fairly straightforward, but the schematic doesn't appear to show the voltages at the HT secondary of the power transformer. It does give some post-rectifier voltages though, but I'm nowhere near educated enough to know how these might bear on the PT specs. I'm hoping someone can suggest the voltages I would be looking for if I were to get a new Hammond PT. I'm submitting the relevant schematic section for inclusion here. Thanks in advance.

Edit......if I simply take out the rectifier valve and measure the voltage across the secondary directly, will that be enough info for me to order a Hammond equivalent? That is, if I came up with, say, around 600V AC, then could I be confident that a 300-0-300V Hammond would be a good choice for a clone of this circuit? I realise of course that I have to determine the current requirements, but that shouldn't be a big problem.

Kriesler 11-104 Power Supply Circuit Diagram

Further edit...does anyone know what the "-7.5V" means which is written near the PT centre tap? Cheers.

Yep, that will work just fine. Remember though that the voltage you read will be a little on the high side since the transformer will be unloaded. When the normal operating load comes onto it, the terminal voltage on the transformer will drop, anything up to 10 - 20 percent depending on the quality of the transformer. Generally, the specified ac voltage output of the transformer given on the circuit, and on the replacement transformers, will be the voltage under normal load, they will all read a bit higher when not loaded

So if you read 600, then 300 or so a side would be better interpreted at around 250 - 270 a side. In general, my advice would be to select the next LOWEST voltage transformer available to the voltages you read. Going too high may stress filter electros, and may even push some valves beyond their operating limits depending upon the circuit design

Going slightly lower however will have no discernible effect upon performance, and may even slightly enhance vale lifetimes

Edit......if I simply take out the rectifier valve and measure the voltage across the secondary directly, will that be enough info for me to order a Hammond equivalent?

Yep, should be fine.

Thanks so much for the replies. I really love the sound of the Kriesler for LPs, and it would be nice to be able to make a clone. The process will also give me the confidence to service the original, which is now making some odd noises. New filter caps will be my first order of business. Cheers.

Document uploaded.

does anyone know what the "-7.5V" means which is written near the PT centre tap?

It's a -7.5 volt rail. It supplies the negative voltage used by the g1 of each valve.

That technique is known as "back bias".

Regarding the "back bias", I presume this is what is being done by the circuitry connected to the centre tap, just above the PT in the schematic, and including the reference points "bias 1" and "bias 2". In other words, the HT tap is still just a standard configuration of (say) 250-0-250 and there's nothing else going on here? I'm still very much in the learning phase here, thanks for your very helpful replies. Cheers.

The back bias may be more than one resistor or a tapped one. It was used to save on Cathode resistors and to set a fixed bias on all valves.

The centre tap of the HV winding will remain as the most negative point, the chassis will be positive relative to it and is often the voltage of the grid bias of the OP valve/s.

This configuration can cause issues, as in most cases the first filter cap is isolated from the chassis and the negative of it goes to the centre tap not the chassis. It is also the one that has a higher voltage on it, always above the voltage of the "B" voltage the valves are getting. E.g. there could be 310VDC at the cathode of the rectifier and 250VDC on the other side of the choke (on load).

The other trap is that where there is a polarised cap from CT to chassis, across the "Back bias" the negative goes to CT and positive to chassis.

The back bias is also a guide to what is going on in the set. Everything has to be running correctly for it to be right.

Thanks Marcc

I've measured voltages again after replacing filter caps and a very fried resistor. On the secondary, I get 520VAC unloaded, and around 495VAC loaded. Since Hammond specs are, as I understand, based on a loaded transformer, would a spec of 450VAC be a good choice to use if I want to clone this circuit?