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Matt Rowland 6th December 2012 04:51 PM

Quad query
Flat earthers look away now! I've rebuilt my prevoiusly working Quad 11 power amps- rewire, new sockets 60uf smoothing before and after choke, GZ34 rectifier. All works well except I'm having to use an auto transformer to keep the operating voltages as specified (340v o/p stage, 330v input/driver, 6.3v heater).
When I use the 240v/com transformer taps as normal for uk supply and run from 240v mains, the TH rises to about 380v and heater 6.6v and the KT66s start to suffer! Does the combination of GZ34+extra reservoir capacity compared with the original GZ32 16+16uf account for this? The only fly in the ointment is the ac heater voltage is higher too. Any ideas?



indianajo 6th December 2012 05:13 PM

Wall voltages are higher than they used to be. Since the higher voltage cuts transmission losses, they are not going down either, unless your building goes on local wind power inverters or natural gas fuel cell supplied electricity.
If your voltages are over the tube datasheet maximums, the 1950's technology to reduce B+ or A (heater) voltage was 1 watt or 1/2 watt carbon comp resistors. I tend to buy metal film resistors as the premium product these days (vishay or multicomp) but you have to buy the 2 to 3 watt variety to get length long enough to sustain tube voltages over time dust & moisture. Wirewound 3 watt resistors are also long enough. If you don't have a can cap to mount them on, turret board from guitar amp builder supplies is probably the best you can get in europe. I can get solder type terminals strips copies of cinch jones designs from in NY or in IL, but the shipping and customs to Europe would be excessive. I haven't seen these in europe. If you have to drill holes in the metal for a mount screw, magnetize the drill bit and use a magnet near the hole to pick up the metal trash. If you have a steel chassis. Otherwise, clean, clean, clean.
If you don't want to vent the heat, a 240-220 VAC single phase transformer is in order. Or maybe cheaper, a different power transformer designed for modern power.
If your plate voltage are within limits but the plate dissipation wattages are too high, you can turn down the idle current from stock to get the wattages under control. Red plates is good for nasty sounding guitar amps, but not for hifidelity.

DF96 6th December 2012 06:14 PM

The GZ34 may drop a bit less voltage. The extra reservoir capacitance will boost voltage a bit too.

In (EU legal) theory the UK mains should edge down to 230V. In (real life engineering) practice it is more likely to edge upwards for the reasons given by indianajo.

Heaters being 5% high and HT being 10% high suggests both effects are in play.

Matt Rowland 6th December 2012 10:30 PM

Thanks for the replies. DF96- I'm feeding the quad with 240v from a variac into the 240v taps on the mains transformer, so it should out put the correct voltage on the secondary. That's why I'm confused.
indianajo- thanks I think reducing the quiescent current is a good idea. I'm just confused as to why the amp seemed top work reliably before I reworked it.



DF96 7th December 2012 09:50 AM


I'm just confused as to why the amp seemed top work reliably before I reworked it.
I wish I had a fiver for everytime I heard someone say that! (or its equivalent for software development)

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