Amp building - making a 18w schematic work with 6V6s and a PT supplying 325v

DannyP

Theoretically (just a vague plan as yet), could you follow a schematic for an 18w amp build that used EL84s and a PT supplying 275v - but swap out the EL84s for 6V6s and use a PT that supplies 325v?

All those resistors and filter caps that produce the nice flat positive DC for the valve plates - would their values have to change?

Also, would the pre-amp valves end up with plate voltages that are too high?

Or is there a schematic out there for a nice amp with 6V6s and a PT that supplies 325v, 5v & 6.3v? I have these things knocking about, and fancy making something out of them. I've built an amp before an I know what bits do what job etc, but I'm no expert on calculating values etc.

ICBM

The only really important difference is that 6V6s need more bias voltage, so a larger value cathode resistor - typically 250-270 ohms instead of 120-150. (And a higher-voltage-rated cathode cap, since the voltage with 6V6s will be over 25V.)

The screen resistors should be 470 ohms instead of the usual 100 with EL84s. Everything else should be fine even with much higher voltages.

DannyP

Cool, that's reassuring. Is there a particular Output Transformer recommended for this application? PP with a pair of 6V6s kicking out 18Watts.

I have an enormous Hammond 1650p OT spare, recommended for 6L6s I believe. But it's too big & heavy. Input impedance is the thing to look out for, no?

DannyP

I just watched this really useful tutorial:



So my spare 1650p has an input impedance of 6600 ohms when used with the correct speaker, but I need one with 8000 ohms for 2x 6V6s.

Any OT recommendations?

DannyP

Would this cheapo far-eastern effort do the job?

http://www.ebay.com/itm/191837757392

Or do I need to shell out for...

http://www.mouser.co.uk/ProductDetail/Hammond-Manufacturing/1650E/?qs=Q/5vwRdkjWLuYLEAT37uWw==

normula1

How about this one: http://www.ampmaker.com/store/18W-push-pull-output-transformer.html

martinw

DannyP said:

Cool, that's reassuring. Is there a particular Output Transformer recommended for this application? PP with a pair of 6V6s kicking out 18Watts.

I have an enormous Hammond 1650p OT spare, recommended for 6L6s I believe. But it's too big & heavy. Input impedance is the thing to look out for, no?

Any output Tx suitable for a pair of EL84s will be good with 6V6s too.  The Ampmaker one mentioned above is the one I use regularly. It sounds great and is easily available.

Input impedance needs to be in the right area (but not necessarily exact) and the overall size needs to be right. As you imply, too big and it won't sound right.

DannyP

Ah cool, Danbury is just down the road from me, so I may see if I can buy directly from the manufacturer.