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Bias help, please.

RickV

Member
Messages
565
I picked up a '77 Princeton Reverb this past weekend. After playing it a little, I thought I'd pull the chassis and have a look. Everything appears original, except the bias circuit cap. Looks like someone decided to put an 80uf/450v in there. It's the size of a filter cap one would find in a fender dog house.

I plugged in the bias rite and powered it up, only to watch the bias climb up over 70 mV on the meter. I got out the probes and checked plate voltages. They are really low, like 280-300.

I changed the bias cap to a 33/50v I happen to have. The 1w bias resistor read 110k, so I changed to a 100k (what it calls for). The 1/2w 22k only registered 18k, so I replaced it with a 22k. All of this made little to no difference to the plate current and bias. I think the amp was recently retubed, so I popped an old pair of 6v6's in, while I messed around with it. One starts to red plate fairly quickly. I even went with a higher value bias resistor, but it changed very little. The AC on the rectifier looks good at 330v.

It's been many moons, since I worked on any amps, so I've forgotten much of what I knew. Not sure where to go from here. I haven't checked values of other resistors. The bypass caps look original, so I'll change them soon. Could the hot bias & low plate voltage have anything to do with the diode in the bias circuit? I have a Fluke meter with the diode setting on the dial, but have never used it.

Any help would be greatly appreciated.

Rick
 

phsyconoodler

Member
Messages
4,315
Well to start with,put your bias rite aside for now and concentrate on your negative bias voltage.Remove the tubes and rebuild the bias supply until it delivers about -35v DC.When it is,then you can pop the tubes back in and test the current.
Go to www.schematicheaven.com and locate the schematic and layout for your amp and follow it to rebuild your bias supply.It's really a very simple supply so it should be straightforward.
Keep in mind the capacitor orientation and replace the diode as well.You MUST have negative voltage when you are done.
 

VaughnC

Silver Supporting Member
Messages
17,746
I agree...get your bias supply working with about -35vdc before plugging in the power tubes.
 

donnyjaguar

Member
Messages
4,201
I had a look and some of the bias supplies are clearly designed by one without a clue.
You need an electrolytic capacitor from where the two bias feed resistors (220k-470k typ) join to ground. If you think about it, without it signal feeding one half of the output section will show up on the other half out of phase. This will reduce the drive level and affect the impedance relationship.
 

RickV

Member
Messages
565
Ok. Pulled all tubes, but the rectifier. It has 380 VAC coming off the rectifier, but only -.335 VDC on the other side of the diode. The resistor measure correct at 100k & 22K. Changed the bias cap (80/450V) to a 47/100V. That's about double the original spec. All the heater wires are running 3.4 VAC.

The power tube wires marked 410 VDC on the layout are running 512 VDC. All the wires on the preamps marked as 160 VDC are reading close to 500 VDC.

Seems like the problem should be obvious, but I'm not sure what to check next. Bias is basically non-existent at -.335 (want -35 VDC).
 

drbob1

Gold Supporting Member
Messages
27,154
The heaters are 3.4v? Something is seriously wrong with the power supply-is that the right transformer, or is it a 220v one?
 

RickV

Member
Messages
565
Don't know. Just picked this up the other day. It came with the schematic and owners manual, which is dated 1977. The schematic is labeled for domestic and export princeton reverb amp. It definitely looks original.
 

kinmike

Silver Supporting Member
Messages
1,800
Check the AC voltage by the pilot light. If it's 240 you could get an outboard step down transformer.

Mike
 

kinmike

Silver Supporting Member
Messages
1,800
I don't think I worded that correctly. If you find it is a 240 transformer for whatever reason, you could use the step down tranformer.
 

WaltC

Member
Messages
2,129
if you measure the filament voltage from one side to ground it will measure 3 volts or so, if you measure between the two sides, it will come out 6.3 (+/-).
 

RussB

low rent hobbyist
Messages
11,171
What rectifier tube are you running?

Note that with all tubes removed the voltages will read high
 




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