Super reverb problems after OT change.

[davesareus]

I am new to the forum. since I registered, twice I have typed in two different threads concerning a couple of problems and they did not go through. I am trying it again with this new thread. I will state my comments about somethings I read here that you guys helped me with and see if this one flies.
I want to thank who ever gave the info about changing rectifier tubes and getting a different B+ voltage reading after that was done. That was spot on. My GZ34 went south and I used a new old stock 5U4, B+ went to 318v, changed that with another one it reads 448v now If this goes through I will post another one with more info and a couple questions. Also, information about biasing.

[davesareus]

Super reverb problems after OT change

OK it flew. I have a 1967-68 not sure, AB763 chassis SFSR converted to BFSR specs. Bought it in 1968. A couple weeks ago I had to chng OT and GZ34 rect tube. after the change, Bias was at -34v. I set it at -52v as print calls for, amp had crossover, tried a few more settings and settled on -48v. perfect. I read off the record it can be -45 to -52v. at -44 mine was still a little dirty.

I think it will be fine but is -48v going to be acceptable?

How low can I run the bias and not eat the tubes?
The sound at -42 and -44v was just about right for a low vol dirty sound.

I gather from this forum, lower the bias settings the hotter, higher the colder the tubes run, is this right?

[davesareus]

I forgot to mention I moded the bias setting circuit about 10yrs ago by adding another pot so separate biasing can be done. Don't know where I got it. I can't find the paper work. As I remember I think they used an AC 30 as an example because I have an AC 30 schematic, don't know of any other reason I have it.

[Enzo]

It isn't a matter of how many volts is OK, it is a matter of how much plate dissipation it causes in the tubes. First, this is not a hifi amp, it was never intended to be free of distortion. I never set the bias on the basis of crossover distortion. That is my opinion, you are free to use whatever standard you prefer.

The common rule of thumb is to run the tubes at about 70% of max dissipation at idle. For the typical 30 watt 6L6 tube that is 21 watts. I don't necessarily follow that standard, but it works for many. I wouldn't run them hotter than that myself. Tubes are all different, that is why bias is adjustable. Dissipation is the product of voltage across the tube (plate to cathode) and current through the tube. If your B+ is 480v, 21 watts means about 43ma through the tube. (Through the plate really. The screen carries a few ma itself, which adds into the cathode current if you measure there) Note I never mentioned -48 volts. To get those 43ma, one tube might need -55v, and another might only want-48.


If the plates of your power tubes are not getting red hot areas on them, then you are probably OK. A lot of guys like the tone the hotter it runs. Of course I have heard guys report that the amp never sounded so good as the time the tubes were running bright cherry red - until they failed a few minutes later.

[pdf64]

'moded the bias setting circuit ..adding another pot so separate biasing can be done...I think they used an AC 30 as an example because I have an AC 30 schematic'
Are you sure it's not an AC50, as some AC50 versions have seperate bias controls for each output tube?
It's beneficial to fit 1 ohm resistors in the ground return of each output tube cathode, then, when setting the bias, the current through each tube can be measured and so the tube dissipation calculated, as per Enzo's sage advice.
Pete

[davesareus]

Thanks guys. The schematic I have for the bias pot mod is an AC 30 schematic. I am not sure or don't remember if that is why I have it. It does show a dual pot for bias settings on it. My mod varies a little in the resistor values and connection configuration. Can't find the info I had for the mod. But it works perfectly. I will purchase a bias tool or use the resistor method to check the current. The amp is still slightly dirty as it was before at about 3 on the volume, thats where I run it most of the time. If you go above 3 it quickly starts to dirty up. Before I set the bias, it was highly compressed and dirty even below 3. I did not mean to imply it was now strickly clean. I play a variety of styles of music so I need a cleaner sound than it was before I re-set the bias, it but it still has the tube compression and dirtiness at the same level as before I changed the OT. Liike I said my concern was for the 6L6GC tubes. I also modified the feedback loop, used the ground switch to turn it on and off. Just ope up the loop and it raises the volume about 25%. I have a three prong ac cord now which freed up the switch.