Troubleshooting a new build amp

[Wardy]

Hi Gang,

I really need some expert help. am reaching the end of my tether with a new build 50W ODS amp.

The situation is that it fires up and all the controls work as expected but the output volume is low. I have traced through the amp with an oscilloscope and narrowed the problem down to V1a. which does amplify but by a factor of 2 when the expected is 45. Its taken me days to get to this stage!

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, checked for no dc on wrong side of caps, correct connections, all earths working, tried 3 valves, inspected every joint with a magnifying glass, reflowed some.

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

has anyone ever had a similar problem?

[Williamblake]

Measure DC Voltages at tube pins.
Doesn't sound sexy, but excel is of big help. No more notes flying around, you can see your progress afterwards.

[Wardy]

Ok Thanks, Sorry I forgot to attach the voltage chart I made previously.

best

Steve

[dogburn]

Could it be the input jacks and/or their wiring? Could the FET board have anything to do with it? Have you tried wiring in an isolated input jack and playing through that?

[acheld]

IF what you're saying is correct, then consider mis-wiring of the input to V1a. 

Pictures of your build would be really helpful, showing wiring of the inputs (both normal and FET), and V1.  High resolution helps greatly.

[kagliostro]

Just to know

didnyou swapped V1 with One other tubes, the problema is there also if you do that ?

Franco

[Wardy]

Thanks People

I’ll try all your suggestions tomorrow. Can’t do anything this evening.
Best

Steve

[Williamblake]

V3 Pin1 is probably just a typo. Are your grids 0 V DC or did you not measure them?

[Wardy]

Many Thanks for all the suggestions and looking at this with me. I really appreciate it.

I have to say I'm 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 have also attached an updated Voltage chart with all the grids. V3 is an added single tube reverb circuit and the pin 1 reading is not a typo. I did wonder about this although 460v was specified here on fender blackface amps. They used 12AT7 in parallel though. not a single 12ax7. The reverb is working ok though. after I get the main problem with volume output sorted I'll revisit the reverb and add some mods to better accommodate the high voltage.

best

Steve
 

[stratomaster]

Something is wrong with your master volume pot. It's supposed to be Vin =Vout at max. You're getting about a 30% drop in amplitude

[Wardy]

Thanks for that info.

I had a play last night and as I had already swapped out the volume pot for another ( Alpha to CTS 1m log) I tried disconnecting the reverb circuit and bypassing it. Also I replaced the shield cable with a new one in case of any issues there. This led to Vin = Vout . I tested the output of the amp and I got max 25w, so better but still half power. I reconnected the reverb circuit and the volume pot issue came back so at least part of the problem is there somewhere.
best
Steve

[Wardy]

Hi Gang,

Many thanks for all your efforts. Richard1001 over at AmpGarage spotted the problem. I had wrong value resistors on the PI grids. I'm partially colourblind and can't read resistor codes so I measure every one as I fit. I must have seen 0.998 show up and as this was expected, carried on. I neglected to note the K on the end instead of M!

Amp now works as it should and I can complete fine tuning and tidying of cables etc.

Thankfully I can then get back to my bass preamp project!

best

Steve

[Wardy]

So I remeasured the power output and although the amp now sound a lot louder and is playable I don't think I'm getting the full power output. I getting a lot of asymmetric clipping with the bottom of the wave clipping a long time befor the top starts to clip and strange shape to the wave. Tomorrow I'll try to take some pics. Could this be the output transformer leads wrong way round? The amp is not squealing though?

clean output power is about 25w after this the bottom of the wavw starts to collapse. the top of the wave doesnt clip till it see 20v rms into an 8ohm load, so 50w as expected. Any ideas?

best

Steve

[Wardy]

So I had another good look at this today.

I checked the PI trim pot and it made no difference on when the wave starts to clip. It was also similar at any of the impedance setting. The bottom of the wave starts to clip at 7.4vrms and the top at 13vrms when on 4ohm load/ so power about 13 clean watts and rising to 42w for the top. Quite a big difference.

I have tried different power valves and currently have in some Tung Sol 7581a that can go to 35w each. Before that I had Nos Philips 6l6wgb's.

I measured the bias and its steady at the grid pins and the -50v supply but rises with increased power at the cathodes/bias check points.

On the NFB circuit I checked all the resistor values and they are right. I removed the wire from the impedance selector and the asymmetric clipping stopped. Can get 13vrms from both sides of the wave. Still a bit under power though. The transformer ratings are 300mv for the power transformer and the paperwork that came with the output transformer says max 75w so not sure why I can’t get more or why the NFB circuit would cause this.

I’d previously swapped the GNFB wire from the 4ohm to the 8 ohm tap as this was recommended by ampgarage as the existing nfb circuit is apparently designed for 100w not 50 on a 102 ODS. It was said that doing this makes it behave the same as the 100w and is correct for 50w. However I swapped it back to the 4 ohm tap and the asymmetric clipping improved a lot . Its still there but now the bottom of the wave starts to clip at 10 vrms rather than 7.4. the top still clips at 13.

I’m not sure whether this is “just the way this amp is” It sounds and plays OK. I tried it with the gnfb disconnected and it had a lot more gain- however the overdrive channel became very difficult to set up as then the volume on V1 had to be kept very low. I had added a mod on a switch as there was a spare hole in the back for some reason. The designer of this amp left out the local NFB on V1b which should be there so I put it in on a switch. I think the switch would serve better if it were changed to a three way resistor setting for GNFB and hardwire the other circuit as it makes little noticeable difference whereas the GNFB seems to make a lot.


Any thoughts on the above? Do you think the amp still has a problem or am I seeing things that aren’t there?

best

Steve