Power transformer question

[Bernardduur]

Hey all!

I am quite new to the whole amp building stuff so my technical background on high voltage is limited.

The question:
I just finished my build of a mix between the Trainwreck Liverpool preamp with a Express poweramp. I used the chassis of an old ELK Twin amp 50 and also that transformers.

The PT has a 330 - 0 - 330V wiring; I have wired the amp up and now I have around 470V on the powertube plates. And I must say, it is nice but LOUD!!!! I want to bring the voltage down

Can I use only half of the PT secondary wiring to power the amp (without negative side effects). I was thinking of two ways:
- Remove one of the 330V sides (so only you have a 330V to the rectifier and a 0 to ground)
- Replace the other 330V side to the 0V side (so you have both the 330V as the 0 side going to the rectifier).

Thoughts?

Oh, and I'll post some pics later!

[Phil_S]

Lowering voltage won't make a big difference in loudness. It will lower your clean headroom.

With solid state rectification, it is possible to use an HT primary without a center tap. but I suspect what you want to do won't work very well, partly because you will cut the HT voltage in half. I guess I'd say, it's not recommended and you'll have to deal with the lack of a center tap appropriately.

[Bernardduur]

Lowering voltage won't make a big difference in loudness. It will lower your clean headroom.

With solid state rectification, it is possible to use an HT primary without a center tap. but I suspect what you want to do won't work very well, partly because you will cut the HT voltage in half. I guess I'd say, it's not recommended and you'll have to deal with the lack of a center tap appropriately.

I don't mind the loudness, I want some more grit

How can I lower the voltage best to reach Express / Liverpool voltages

Edit: I found this way to lower the B+ voltage. I think I'll try this!

[Ears]

Don't do your second choice if it means connecting one 330V leg to the 0V.

[Bernardduur]

Don't do your second choice if it means connecting one 330V leg to the 0V.

I told you I never do this stuff

Above named worked like a charm; I had to replace the 39k resistor with a 1.5k resistor, but now the voltage is around 330V on the plates.

[Bernardduur]

Hmmmmmm

If I do the above named trick I can get the voltage down only the powertubes won't bias anymore with this low a voltage.........

Is this a known problem?

[Ears]

It is difficult to follow descriptions such as "above named trick"
Are you familiar with the relationships in the attachment?

Don't short-cct secondary windings.

[Bernardduur]

Thanks!

I meant the trick with the voltage divider on the ground connection, the Rocktall Fatness method I posted above.

Great read btw! It is great to see that the top named thoughts of me give me the exact same voltage readings..... ah, well, almost...... , which is kinda annoying as I need lower

[Ears]

Actually I prefer the rectifier relationships as as presented in this attachment but the scan wasn't so good.
Both attachments assume near ideal solid state diodes.

[Bernardduur]

Thanks! We learn new stuff every day!

Can't I drop the voltage with a resistor in series with the B+ placed before the standby switch?

[drz400]

Thanks! We learn new stuff every day!

Can't I drop the voltage with a resistor in series with the B+ placed before the standby switch?

To a point but you will be limiting the current and you might need a very large Wattage to drop anything worth mentioning. This will not give the amp more "grit", to get more lively and edgy you want to increase voltage. If you want more overdrive or distortion you can play with many other aspects of the circuit. If you want to play with B+ voltage you should install power scaling, then you can play with the voltage all you want and select what part of the circuit you want to adjust.

[Bernardduur]

Thanks! I'll check into power scaling.....

I want more OD from this circuit; with the current voltage the unit is quite clean...... I can get only slight OD from it..... any more thoughts on getting more OD from this unit?

[Phil_S]

I want more OD from this circuit; with the current voltage the unit is quite clean...... I can get only slight OD from it..... any more thoughts on getting more OD from this unit?

This wasn't clear to me from your earlier post. Leave the PT at 330-0-330. There are lots of reasons not to just use 330-0.

If you want more grit, try raising the value of the 220K grid leak resistors on the PI. Try 470K. This will move it towards Marshall territory. Note: you are fooling with the R/C constant and at some point it will be appropriate to reduce the .022 coupling caps, but they should be OK at 470K.

You might try to increase preamp gain by lowering the unbypassed 10K cathode resistor on V2 to maybe 8K2 or 9K1. Increased gain will probably increase noise. For testing purposes, you can just tack the R in parallel to the existing one.

Or try some different values of the grid leak resistor on V2. The schematic indicates 33K to 68K. You might try a 33K + 50K pot to dial it in, take a reading, then install a fixed resistor. Also the coupling cap between V1B and V2 can be varied over a fairly wide range (.001 to .01)

Whatever you do, do one thing at a time and test the amp. If you do lots of things at once and the amp sounds bad, you'll have no idea what to undo.

[ampdoc1]

I used a trick a while back that worked very well. Take your power tubes 6V heater wire off the string, and install a 2-3 ohm variable resistor in series with them. This acts kinda like a variac, but only on the power tubes.
Less electrons boiled off the heaters = less output.
I like this method because the entire rest of the amp is working as designed. Also, this is a very easy thing to try out.

ampdoc

[Bernardduur]

Thanks guys

That sound really promising; I will try it tomorrow!

Ampdoc; on both sides of the heater or on only one side?