Hiwatt Lead 50 bias reistor

[tonezoneonline]

I've got a Hiwatt Lead 50 model SG-50 that the bias resistor is toasted in.
It reads 91k out of circuit on the DMM although the color code is red,red,white,black,white which read either way would suggest several thousand mega ohms.I know that the heat can sometimes change the colors but this is pretty clear.
Anyone know what the bias resistor in this amp should be?Can't find a schematic anywhere for this model .I guess I could check some of the ones that are out there and see if the bias circuit is close.
TIA

[stokes]

If that is the bias resistor,the colors are not right.I dont see a "white" listed as a third color in any of the color code charts I have.And a resistor that large,"several thousand mega ohms",is way too big for a bias supply.All the 50 watt Hiwatts I see have a 27k,try wiring in a 50k pot and dial in the correct bias.Better yet,just get a 100k cermet pot and add an adjustable bias pot to your amp.

[Rob Mercure]

Hiwatts, and some other British amps, are notorius for having marginal bias supplies and many won't produce sufficient negative voltage to limit current flow in some of the new production tubes. You might want to consider adding a voltage doubler as long as you're in there - you, or the amp's owner, will probably find it useful at some point in the future, or even now.

Rob

[tonezoneonline]

Thanks guys .I found a Hiwatt power supply schematic online for a 50 watter and it showed a 100@160 cap with a 110k resistor.I'll try that and see what I get.

[stokes]

That 110k resistor is not the resistor you would alter to change the bias voltage,it would be the 27k resistor after the reverse oriented diode.But if the 110k is the one that burned up,of course you should replace it with a 110k.The schems I have for 50watt Hiwatts show a 100uf/63v cap,but a 100uf/160v cap will give you a little extra voltage protection.There is very little current in a bias supply,if that resistor burnt,you should find out why,rather than just replacing it.That bias cap would be a likely suspect,but I think the diode would likely fry as well if the cap shorted.

[Amp Kat]

There is very little current in a bias supply,if that resistor burnt,you should find out why,rather than just replacing it..

No doubt Stokes and that was my first thought was how could that fry with so little current there. I guess any supply if shorted could pull enough current to increse the wattage but I agree if it's burnt that something is wrong that caused it to fry. You will watch it fry again if just replacing it so maybe tracing back or forward will give you an idea what's frying that resistor.

[tonezoneonline]

Yes ,the cap was bad too.What had me confused is that where the 110K resistor was supposed to be there was a resistor labeled red,red,white,black,white which is a huge value and it was toast.The diode tests fine but I'll change it anyhow.
The 27K bias range resistor has been changed before and is fine.

[Amp Kat]

there was a resistor labeled red,red,white,black,white which is a huge value and it was toast..

No matter how you read it most likely a precision maybe backwards a 91.9k 2% tolorance. The 27k being changed supports the above posters as the adjust bias resistor.