What to build next in the ODS-100 department ???

[David Root]

Man, you are fast, Steve! It would take me a month to do that.

[67plexi]

Thank you David. I'm trying to get finished with all the builds so I can move on.
Start up did not go so smooth 504 volts B+ hot bias keeps climbing, on the PPS started with a 3k 5 watt
need a 4k or in that range. I had a 6k to much B+5 175v.
I feel like removing the PT and go to the ocean and float test it or take it to the gun range.
I'm open for suggestions.
504 volts under load is to high for a ODS-100

[dcribbs1412]

Thank you David. I'm trying to get finished with all the builds so I can move on.
Start up did not go so smooth 504 volts B+ hot bias keeps climbing, on the PPS started with a 3k 5 watt
need a 4k or in that range. I had a 6k to much B+5 175v.
I feel like removing the PT and go to the ocean and float test it or take it to the gun range.
I'm open for suggestions.
504 volts under load is to high for a ODS-100

Sorry Steve...
just so I understand you had a 3k 5watt and got 504V
and 175V with a 6K? or is this a mistype...

Darin

[67plexi]

My B+1 is 504 Volts under load /V-1 is 175 volts with a 6k 5watt on the PPS
the bias something is wrong 67 volts AC. bias 35mA start two power tubes
85mA finish tubes after warm up.10k pot 33k tail.
bias pot no change.
may have to change out the transformers. if I cut the AC input voltage
with a autotransformer the bias is ok.

[lovetone]

Check the negative voltage on the grid of the output tubes, if the current is rising then the -ve volts must be going higher say from -48 drifting up towards zero.

If you remove the output tubes, power up the amp and check the -ve bias voltage it should be constant and be adjustable from the bias pot. You will get a lower than normal reading (lower being more negative) as there is no load from the output tubes.

This will give you a good indication if the bias circuit works correctly, it might be that you have a faulty tube that has a grid leak as it worms up.

Also check the bias supply capacitor is not warming up it could be faulty drawing current an pulling up the bias supply.

Hope that helps.

Geoff

[ic-racer]

When I first fired up my HRM the tubes shot up to 180ma cathode current. That happened because I forgot to attach the ground wire to the bias supply. You might want to double check that.

Also, what value resistor are you using on the bias supply board?

[67plexi]

4k bias board resistor. It may be a leaky bias cap. ROE 47uf/250v
I need to dig out my capacitor tester and tube tester.
my grounds are all good. The bias pot does nothing tubes in or out.

[lovetone]

At this stage I don’t think it’s the capacitor, we need to work out why the pot has no effect.

What –ve volts are you getting?

Which schematic are you working to?

Can you post a few images?

[67plexi]

If I cut back the input voltage so I have B+450 I have 30mA on the second
set of power tubes and only 8mA on the first set or pair.
I had this before and changed to PT and it was all good.
if not it is runaway bias to 85mA at 504 volts.

I put a new bias board on no help. 3k9 bias board resistor.
on the false center tap for the filaments it was burning up a 100r 1/2 watt
I put in a 4 watt 100r ok now.
I put a 56k tail on the 10k bias pot no change.
#183 layout This is a NOS set of transformers from 1986 date code
I have set of lower voltage transformers see what it does.
On the OPT I have never seen such a difference in current flow.
This transformer set could be trash.

[lovetone]

I think there is a wiring problem as the filament ct resistors have over heated. The high HT at 504V is not a problem. You need to do a few other tests before you change the transformers.

What is the –VE bias voltage? Check this at the output tube grid to ground should be around -48v

With output tubes in can you measure the grid voltage on each output tube (pin5)?

For now run the amp with 540v on the plates

Any chance you could post a couple of high resolution images so I can take a look?

Cheers

[67plexi]

Thank you

[67plexi]

Pin 5 voltage is off PT-1 355v, PT-2 355v, PT-3 167v PT-4 170v

[lovetone]

Boy that’s a busy picture!!

Is that burning on the tube base to the right of the bias pot?

It looks like there has been some arcing between pins 6 and 7 this could have been the cause of the filament resistor over heating.

It all looks a bit tight and close. Take your time and make sure all the wiring is correct with no shorts.

Any chance you can get the voltages a asked for?

[lovetone]

Pin 5 voltage should be –ve at say -48V

Check the volts again with out power tubes.

[67plexi]

PT-1 & 2 -48 Volts PT- 3 & 4 76 Volts pin 5

No tubes