Another Simms Watts bias problem

[Newbietube]

I have a problem with a simms watt. Its a 100 watt, but has been fitted with KT77's. There are also a few differences to the schematic, there is no bias tap on the transformer but instead is taken from one of the HT's via a resistor then just one one pot with reversed diode then split via two resistors then to the power valves via resistors. The amp arrived to me with the pot maxed out giving -80v, although the schematics suggest -50v. Is this normal or has this been done for the kt77's? AT the moment I'm getting a very low thin sounding output, is this a result of the -ve bias being too high? What is the effect of the -ve bias being set wrongly?
AS you've probably guessed I'm new to all this though have worked on loads of different ss gear so please be patient.
Cheers Just

[TomMcNally]

hi -

I don't know that particular amp - but speaking in general ... the bias VOLTAGE is not all that important, except in initial setup and troubleshooting. What IS important, is the voltage you measure across the resistor(s) that go from the cathodes to ground. Typical amplifier have 10 ohm resistors, and the voltage is easily measured and mentally translated from millivolts to milliamps. In other words, if you measure 500 mv ( 1/2 a volt ) across the cathode resistor, the tube is pulling 50 ma of current, which is probably a good starting point for the KT-77s. Adjust the bias pot while looking at the current and see what it does. If it's too low, like 10 ma, the amp will sounds nasty, if it's too high, the tubes will be bright red. Also - you may run into a shared cathode resistor for both tubes, that's fine, you just need to do some math (knowing the value of the cathode resistor)

Here is a cool online calculator that may help ...

http://www.webervst.com/tubes/calcbias.htm

[mesherm]

From the Simms-Watts schematics I was able to find online, it appears the output tube(s) cathodes are connected directly to ground so there is no real way to check the idle current directly. From my experience with KT77s they usually bias similar to EL34s and -80 volts is way too high. That would put a KT77 into complete cutoff and result is a very stunted sound for sure as well as possibly raising the B+. I would reset the bias back to -50 volts as recommended and let it run for a while checking for any KT77 plates glowing red or the power transformer getting excessively hot. If you are keeping the amp I would recommend wiring in a 10 ohm 1 watt resistor from each KT77 cathode pin to ground so you could check for tube matching and proper bias setting. A normal value for a KT77 would be around 50ma each tube which correlates to .5 volts across a 10 ohm resistor as Tom has stated.

[dhuebert]

Do you have a scope? I set my bias so the crossover notch just disappears at full power (where the top and bottom of the wave are just starting to flatten). Randall Aiken and Randy Jamz disagree on this method but I like it.

Don

[Newbietube]

Thanks all for your input. I tried taking the bias back to -50v but one of the tubes started to glow, so I 've backed it off to -60v. The amp now seems to be working properly.
Its not actually mine, though is perfect for a makeover if I can convince the owner to let it go. I'm thinking of pulling out all the pre amp section and recreating a Mesa or Ampeg preamp section to make this into a Bass amp. Does anyone have any idea what the value of the inductors should be in the SVT midrange controls as the schematics I've found only quote an Ampeg part number?
Cheers Just