Heater Hum

[Willabe]

3. Or, like pdf64 said, it could be that the verb circuit itself because of grounding is hummy.

3. How do I check this?

You have to understand grounding.

https://www.valvewizard.co.uk/Grounding.html

You have to make a ground star of all the verb circuit grounds with the B+ filter caps ground that feeds the verb circuit.

And if the verb tank has jacks with cables you might need to isolate the jacks from the chassis.

And the tank should only have 1 ground connection, not 2, connected at the jacks to the verb housing. The single cable jack ground goes to the verb ground star.

And you have to figure out how you handle the grounds for the in/out from the main chassis to the verb chassis. That could be the problem itself. This is part of what pdf64 was talking about.

[pdf64]

... after using a spectrum analyzer. Turning up volume on Ch 2 with reverb off gives me 50hz hum. Turning up reverb very high only increases the 50hz slightly, but brings out nasty harmonics at 150 and 250hz. ...

Are both sides of the HT rectifier passing (forward) current?
eg maybe a BY126 or a 5R (?) resistor is bad?

650V diodes seem marginal / underrated here.

[dankinzelman]

I spent a few hours testing with the amp owner yesterday and here are our conclusions regarding the current status:

1. I had the trim pots (intensity volume and intensity reverberation) feeding the reverb circuit at unrealistically (unusably) high levels. After we fiddled around and lowered the gain to usable levels, the hum is now pretty subtle. I will look into the grounding article and your specific suggestions, thank you.

2. The INTENSITY REVERBERATION trim pot makes a big difference in how clean Ch2 is, even with reverb switched off. I presume this is because it raises the input impedance (and thus gain) to the next stage's input grid. Is this correct?

3. I tried clipping bypass caps around the 1st stage cathode of the ECL82, which gave me more wet signal, but this also significantly increased grunge on the reverb. We can hear it also slightly compresses/distorts the dry signal even with reverb switched off (why?). Lower value caps made less difference in reverb level and distortion. A .022uf cap might be a reasonable compromise but we are leaving the cathode resistor unbypassed for now because we don't want the additional saturation.

4. We would like to find a way to have our cake and eat it too, ie get stronger reverb without saturating the clean signal. Suggestions?

5. The really bothersome noise now is just the 150 and 250hz buzz coming from the reverb circuit. Pdf64: I have already changed the B+ diodes to 1N5408 (1000V) diodes. What wattage should the 5R resistors be, and how important is it that they are matched?

6. I have yet to test all three amps back to back (need to re-bias the other two and readjust trim pots based on yesterday's findings), but I think the twisted heater wiring I did on this last one may have indeed made a difference in hum. Based on my memory, this amp is now the quietest of the three. I will report back.

Thanks for all your help.

[Willabe]

6. I have yet to test all three amps back to back (need to re-bias the other two and readjust trim pots based on yesterday's findings), but I think the twisted heater wiring I did on this last one may have indeed made a difference in hum. Based on my memory, this amp is now the quietest of the three. I will report back.

This  ^^^^^^^^  is probably because of this below. Not because of the heater wires being twisted.

1. I had the trim pots (intensity volume and intensity reverberation) feeding the reverb circuit at unrealistically (unusably) high levels. After we fiddled around and lowered the gain to usable levels, the hum is now pretty subtle.

3. I tried clipping bypass caps around the 1st stage cathode of the ECL82, which gave me more wet signal, but this also significantly increased grunge on the reverb. We can hear it also slightly compresses/distorts the dry signal even with reverb switched off (why?). Lower value caps made less difference in reverb level and distortion. A .022uf cap might be a reasonable compromise but we are leaving the cathode resistor unbypassed for now because we don't want the additional saturation.

Is this a typo? Because a .022uF won't do much as a K bypass cap. Only from starting at ~1k up.

Look at this graph from Merlin on K bypass caps showing what frequency and how much boost the value of the cap gives.

https://www.valvewizard.co.uk/OtherStuff.html

[Willabe]

This is why you shouldn't have started 3 different threads on this same amp. You want to keep all the Q/A's together. The thread on balancing the power tubes, that would have been fine for a different thread. (I was going to tell you this and put the threads together, I should have.)

1. I had the trim pots (intensity volume and intensity reverberation) feeding the reverb circuit at unrealistically (unusably) high levels. After we fiddled around and lowered the gain to usable levels, the hum is now pretty subtle. I will look into the grounding article and your specific suggestions, thank you.

I suggested you play with those trim pots back on the 19th, 7 days ago. I thought they could have been set too hot/high.

Would have saved us a lot of fooling around, hunting for the source of the humm, if you would have tried playing with the trim pots back when I brought it up.     

Also, how do you suggest I regulate the trim pots 'INTENSITY VOLUME' and 'INTENSITY REVERBERATION'? My understanding is that they set the input impedance of the following stages and so essentially allow me to trim how hard I am driving respective grids, right? So should I just use my taste in terms of how much gain and distortion I want, or is there a safe/proper way to set them?

Those are there to balance the dry signal with the verb signal and to not over drive the effects channel. Int. verb is how hard you drive the verb tank and int. vol. is how loud the dry signal.

If you think the effects channel is too hot, distorts to early, try turning the int. vol. down. You can probably turn the int. verb up pretty high to drive the tank, it's a dwell control. Then you control the amount of verb by just using the verb pot. 

Just play with them a little until you get a good balance and good sound.

[dankinzelman]

Sorry Willabe I missed these posts. Feel free to combine all the threads if that is your preference.  For what it's worth, I thought about this before creating multiple threads specific to each topic. I've found clear thread titles to be advantageous in my own searches on this forum. I'm sure there's tons of other good info on things like heater hum or tube balance that I haven't read, but I'm more likely to read a thread dedicated to the issue I'm facing than to sift through hundreds of threads with seemingly irrelevant titles.

Regarding the trim pots, my apologies for wasting your time.  I'm grateful that there are many (apparently) knowledgeable people here offering suggestions, but I am a beginner so I'm not always able to pick the best next step among several suggestions offered to me.

For various logistical reasons, it was impossible to do a testing session, so I pursued other solutions in the meantime. I now have the advantage of hindsight, and I plan to compare with the other amps as soon as I get the time to open them up and fiddle with the trim pots to try to get a level playing field. This should hopefully clarify whether the twisted filament wires made a noticeable difference or were a waste of time, which in either case will hopefully be useful information for someone besides myself. In any case, they look kind of cool and gave me some good soldering practice.

Does anyone feel like explaining why the setting of the reverb intensity trim pot affects the dry signal so much (even with the reverb off?) And more specifically, how I can keep the channel's dry signal punchy and clean while at the same time getting a much wider reverb adjustment range? For now, I have to choose between a clean, punchy dry signal and not nearly enough reverb, or a crunchy, compressed dry signal and a good amount of (not super clean) reverb. We've chosen the punchy dry signal, but this means the reverb control on the amp face is essentially useless, any setting below maximum reverb is almost useless, and we still would like much more reverb.

Regarding the k bypass cap sizes, it's possible you are right, the switching between uf, pf, k and nf confuses me. I have printed a cheat sheet and use it, but I tried a lot of things that night and I might have added one too many zeroes in the end. Thanks for the link, that information is golden.

[Merlin]

Does anyone feel like explaining why the setting of the reverb intensity trim pot affects the dry signal so much (even with the reverb off?)

You mean the dwell trimmer? It should have no effect on the dry. Has it been accidentally wired as a variable resistor instead of a pot?

[dankinzelman]

3. I tried clipping bypass caps around the 1st stage cathode of the ECL82, which gave me more wet signal, but this also significantly increased grunge on the reverb. We can hear it also slightly compresses/distorts the dry signal even with reverb switched off (why?). Lower value caps made less difference in reverb level and distortion. A .022uf cap might be a reasonable compromise but we are leaving the cathode resistor unbypassed for now because we don't want the additional saturation.

Is this a typo? Because a .022uF won't do much as a K bypass cap. Only from starting at ~1k up.

Look at this graph from Merlin on K bypass caps showing what frequency and how much boost the value of the cap gives.

https://www.valvewizard.co.uk/OtherStuff.html

I looked at the schematic again, we were testing the k bypass around the first stage cathode of the ECL82 driving the reverb (pin 8).