Help with a new Build ODS

[Wardy]

Hi Dan,

I wrote to the designer of this amp and he has confirmed that with a 200mv signal at V1a the voltage on the plate after the blocking capacitor should be 9v so this is defo the problem, just need to find out why. I'm thinking of just start swapping out surrounding conponents starting with the plate resistor.

hopefully I'll get the other reading you were after later on but up against it now with family matters to attend to.

best

Steve

[martin manning]

Did you check the resistor values carefully? Wouldn't be the first time someone put in something that's off by a factor of 10 or 100.

[Wardy]

Hi Dan,
Managed to get 10 mins to do those readings and have attached.

I hope you can glean something from this, its a mystery to me. I was also going to check the valve socket, although the parts supplied all seem good quality.

many thanks for your help.

best

Steve

Reading update.pdf

[Wardy]

Thanks Martin,

I Just rechecked those around V1 and they are all correct. The only one I couldn't measure in situ was the 22m on the treble pot but hav'nt any more time this eve to lift it.

best

Steve

[dbharris]

Thanks for the additional info. Are those measurements you took in AC, DC or a combination?

May be time to try and lift the board to see if anything is amiss underneath.

-Dan

[Wardy]

Hi,

The measurements were the sinewave pk-pk signal so ac. I did check the caps for any unwanted dc with a multimeter and couldn't see anything. I already lifted the board and couldn't see anything wrong. I've had some snags on the amps I've built before but this one is a real mystery. the others turned out to be a bad or missing joint or a bad valve. This one seems amazingly elusive to fix.

I guess the question is now pretty simple as the problem is specific. What would cause one side of an ecc83 triode (V1a) to output 2x gain when it should output a gain of 45x according to a recent test on another amp thats the same.

I've checked for resistor values. cap values, correct connections, all earths working, tried 3 valves, inspected every joint with a magnifying glass.

What else could possibly cause this? where else to look?

has anyone ever had a similar problem?

[sluckey]

With your 200mV test signal connected to the input jack, use your scope to measure the signal directly on V1 pin 2. Measure directly on the socket. What have you? If not 200mV suspect a bad shielded cable or miswired input jack.

[Wardy]

Thanks Steve

I’ll try that tomorrow, can’t do anymore this evening

Best

Steve

[Wardy]

Ok with the equipment I have I had to use 500mv to get a steady signal (I have a better signal gen on order). There is a very small drop at V1 pin 1 about 16mv. if I connect the other side of the 30k resistor that is connected directly to the same pin I get 500mv so no change in the cable.

best

Steve

[martin manning]

Pins 1 and 6 are the plates. You should have 180-200VDC on those with no signal. Pins 2 and 7 are the grids, so look for your AC input signal on pin 2 or pin 7, whichever is connected to the input coax. The diagram below is an inside the chassis view of the socket pins.

[Wardy]

Many Thanks Martin et al,

Sorry I have been wasting some of your time due to a stupid mistake.
I realised that I have been using the sine generator I have wrong. It has a DC offset button that is default on and I got 12v dc on the signal as well as the ac. Its not the best and I have a better one on order. I have attached new readings with the thing without the DC. It now looks like I'm actually not getting a bad signal from V1a. The amp is still quiet though and not achieving anything near full power.

It would be helpful if someone could review the readings I have and let me know if they are in the right ballpark or if there is a problem in this part of the circuit or not.

I also did an updated voltage chart.

New Readings 7-9-25.pdf

many thanks

Steve

ODS Voltage chart.xlsx

[sluckey]

Voltage chart.xlsx

Maybe you could print those Excel files to pdf. Then more people could see them.

[Wardy]

Oh I didn't realise that.

thanks

[sluckey]

For example, look at the number of downloads in the short time since you posted the latest xlsx file. I'm the only one that has downloaded your xlsx file but the pdf has been downloaded 3 times.

[dbharris]

Hi Wardy,

I've continued to follow along, but I am not seeing anything that stands out to me.

Maybe to try to rebuild the v1 socket connections and temporarily leave off the LNFB network for simplicity.

You can attach photos directly to your posts. That may help more people look at them rather than putting them into a PDF.

-Dan