B+ to high, how to bring it down question

[Tillydog]

I do not have the 1ohm resistor at the moment but I have some on order that I plan to add. when I had my compu-bias probe attached it showed that there was no cathode current.

I'm sure that the compu-bias will tell you what you need to know without the 1R resistors, but they'll do no harm.

You're looking for something like -12 to -14V V at the junction of the 2 x 220k grid leaks for EL84s. If it's much more negative than this, it's likely that you've biased them into cut-off. You need *some* cathode current to hear anything. The fact that you hear something as the amp turns off leads me to think that it's basically working but biased off (but there are other explanations!).

Good luck!

Andy

[roberto]

Something like this?
[img:931:401]http://www.geofex.com/Article_Folders/m ... osfol4.gif[/img]

[Cliff Schecht]

Something like this?
[img:931:401]http://www.geofex.com/Article_Folders/m ... osfol4.gif[/img]

I have this circuit in five amps right now, it works like a charm. I found a bunch of BUS14's really cheap and have been using them with a 100 Ohm/1kOhm resistor combo without a problem (yet ).

You can even get clever and switch in different Zener/bias combos for a poor mans VVR style attenuator (something I've never done).

[roberto]

Yeah, for a VVR there is also this simple solution, but you need higher wattage transistors:

[img:732:537]http://www.zl2pd.com/img/smps/smps_var_zener.jpg[/img]

[dju]

IT IS ALIVE!!!!!!!
Hi all, thanks for all of the replies. I checked to see where things were idea wise yesterday even though I had not yet gotten the voltages asked for. So while I was getting the voltages I checked the voltage at the 220K resistors and it was ~-52v which according to one of the posts it should not be no where near that. I was unable to get it lower than -50v so I swapped out the 47K resister/10K trim pot in the bias section for a 5.6K resistor/25K trim pot and I was able to finally get sound. YEAH!
So the voltages I currently am getting are:
V1=pin1 173v, pin3 1.13v, pin6 232v pin8 2.08v
v2=pin1 178.1v, pin3 1.183v, pin6 322.4v, pin8 179.5v
v3=pin1 346v, pin3 5.3v, pin6 345v, pin8 5.33v
v4=pin2 -20.54v, pin7 401.7v, pin9 405v
v5=pin2 -20.58v, pin7 402.4v, pin9 405.7v
I measured -21.07v at the 220k resistor pair.
My bias probe is showing the plate voltage at 402v, cathode current=16mA and estimated wattage 6.4W per tube.

Now that I have this problem on it's way to hopefully being fixed providing that nobody see's any problems with this I am on to the next problem. Now there is sound but it is not very loud. It would make a great low volume amp for bedroom playing but that is not what I intended for it to be. Any ideas for this new problem?

Thanks for everybody's help, I really appreciate it,
dj

[roberto]

Lower bias voltage to around -12V, until you'll read the right bias current value.

[dju]

ok, I lowered the bias voltage amd played with it between -11v and -13v. the amp did get a little louder but still not 18~20w loud. my 5w marshall class 5 is quite a bit louder. I swapped out the 5K reisitors in the B+ path to 10K per some of the schematics that had them as 10K instead of 5K. No improvement.
So, any other ideas or information wanted that might help get this project to the finish line?

thanks again for all help and suggestions,
dj

[Firestorm]

Probably time to post some pictures. Someone will see something.

[dju]

Probably time to post some pictures. Someone will see something.

let there be pictures.....

http://i1224.photobucket.com/albums/ee3 ... CN0193.jpg
http://i1224.photobucket.com/albums/ee3 ... CN0192.jpg
http://i1224.photobucket.com/albums/ee3 ... CN0191.jpg
http://i1224.photobucket.com/albums/ee3 ... CN0190.jpg
http://i1224.photobucket.com/albums/ee3 ... CN0189.jpg
http://i1224.photobucket.com/albums/ee3 ... CN0194.jpg

please take a look and let me know if you see anything wrong.

thanks,
dj

[Firestorm]

Your PI bias resistor is 470K instead of 470R.

[Cliff Schecht]

Your PI bias resistor is 470K instead of 470R.

[dju]

Your PI bias resistor is 470K instead of 470R.

Daaanng, how did you catch that? I don't know how I missed that when assembling it. I thought I was being so careful when assembling it. And then when I was looking for problems I was mainly looking at connections and not verifying values as much as I should have. Lesson learned. Unfortunatley I don't even have the 470ohm reistor to swap out so I now have to order some.

great catch, thanks,
dj

[Jana]

You could jumper the 470K with an 820R or 1K until you get a 470R. It won't be 470R but it will be much closer to value than what you have and will let you play the amp.

[dju]

You could jumper the 470K with an 820R or 1K until you get a 470R. It won't be 470R but it will be much closer to value than what you have and will let you play the amp.

thanks for the suggestion, I might give it a try. I am planning to go by radio shack to see if they have one. they probably won't as these days they have next to nothing. I also have a coworker that occasionally builds amps among other things and I am going to ask if he has one. I already ordered them so at the least they should be here monday.

thanks,
dj

[surfsup]

The resistor on your power tube looks crispy...like a slice of bacon, though not as tasty.

http://i1224.photobucket.com/albums/ee3 ... CN0190.jpg