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Hi all, I'm slowly putting ideas together for a huge bass amp using either six or eight 6550/kt88s. Having looked around there's not a lot of choice for output transformers on this scale so I've found a UK company that can wind them for me at a reasonable price. What I need to know is how do I calculate the required input impedance required for these or any other valves used in large numbers? If I double the number valves do I simply halve the impedance? Any help is greatly appreciated.
Thanks for reading, cheers Just

Easy.

Look at the specs for 2 x KT88 in PP.

If you have 8 x KT88 (4 per side), the required impedance will be that required by 2 divided by four.

A custom transformer capable of 500VDC @ 1A RMS and "reasonable price"? You lucked out!

Cheers!

Thanks for that. I kinda figured that might be the case, but even at reasonable prices it's a big risk if I get it wrong.
Regarding the transformers I did some poking around the net and then phoned a few manufacturers direct. I called them originally to get a replacement made for an amp I'm fixing, so was pretty pleased when a good price came back. I find as soon as you look on the net for guitar amp stuff the price gets silly. Tube amp doctor in Germany do some really nice replicas of all the classic amp stuff, fender marshall ampeg Hiwatt etc but the cost is rather off putting.
Appreciate the help.
Cheers Just

Oh, silly me posting before my morning tea... divide by two, not four (four for 8 per side)

I'm getting about 1.6k for six valves(3per side). Will be looking for a B+ of 650volts.

1.5 K to 2 k would work it just depends on how ragged a tone you want balanced with the longevity of the output tubes .

Have you ordered?
Hammond 1650W off the shelf.
280W RMS but it is HiFi, that is, 280W at 30Hz, will easily do a Bass Amp of 350W to 400W. It is 1K Raa which would suit 8 x KT88.
You will need a 600 Volt or higher B+ Power Supply.
This is based upon my experience as follows:-
For guidance - I ran 4 off KT88 into a Hammond 1650T (120W rated down to 30Hz with Raa of 2K) and a B+ of 506 Volts. In Ultralinear Mode I got 122W at the onset of clipping and 138W in heavy overdrive.

Cheers,
Ian

Yeah I looked at those but they're rather expensive here in the UK, £200+shipping. I'm hoping the manufacturer I've approached will come up with a better price. If not then this looks like the best option as it far outspecs most of the replicas of ampeg and hiwatt transformers.