I had just rebuilt a 222C and was facing the problem of the too high B+ voltage. I decided to install a choke (6H, 150 ohm) in a CLC , power supply. I also completely rebuilt the power supply and replaced all the resistors in the voltage divider network. Since they were all carbon comp almost all were out of spec on the high side which meant that there was more resistance and higher overall voltages. I believe i also added a single cl80 which put the anode voltage and G2 voltage at only 10vdc over specs.Thanks for doing that Blast. It would be great if thefragger is able to restore his sometime.
I returned to this thread wondering about individually biasing the 7189s and dropping the line voltage a bit. Kegger mentions using CL80 thermistors. I wonder if you or others have used a bucking transformer instead?
I have it's brother the LK-48. Same mondo big output trannies for little EL84s. It's got balls! try the Russian 6P14P-EV for outputs. They warm up a bit slowly but work hard.I just bought this and its on its way!
It needs work, to say the least! I'm pretty excited!! I just have to sell a couple pieces before this one comes home and I'm golden, to keep the audio equilibrium happy in my house.
I'd like to start researching what needs to be done to this amp. Does anyone here have any good H.H. Scott resources that they'd like to share?
Can't use the EL84 because it won't handle the B+ voltage or screen voltage as they are too high, that's why Scott spec'd. in the 7189. Unless you are going to knock down the high voltage somehow and create a lower power amp. What mod. are you considering?I was thinking of modding it to run EL84 and putting in some 6P14P-EV tubes. I haven't heard of the ECF80 tubes before tonight, but I've got my studies to tend to!
You don't need FOUR bias pots, only two. The amp. comes with DC Balance controls for each pair of output tubes. Set the Bias, Balance, check the bias, maybe reset it a bit, check the Balance until they are all the same. You will need to add four low value (I used 10 ohms) resistors to measure the cathode current of each tube. Plus, don't forget to subtract about 5% for the screen current to calculate the plate dissipation.Wow Blast, your amp looks awesome underneath! I should have mine by Monday.
Aside from the links that Tinkerbelle posted earlier I haven't looked around that site much-- I'll be sure to join up there when I really get stumped, though!
I suggested the four bias pots as an alternative to having a matched quad of tubes, so that I could balance them out myself. (unless I'm completely wrong in my understanding how bias pots work, I still have some reading to do!)
I've got some MUR860 (8A, 600V ultrafast rectifiers from ONSemi) kicking around here that I could build a bridge out of, maybe overkill? I was thinking all-new metal film resistors as well. Advantages/disadvantages of metal film vs. carbon comp?
My EL84s handle the voltages okay, because the tubes are biased conservatively. Dave G explained this a while back when I first restored my 222C.Can't use the EL84 because it won't handle the B+ voltage or screen voltage as they are too high, that's why Scott spec'd. in the 7189. Unless you are going to knock down the high voltage somehow and create a lower power amp. What mod. are you considering?
What brand are they?My EL84s handle the voltages okay, because the tubes are biased conservatively. Dave G explained this a while back when I first restored my 222C.
My EL84s handle the voltages okay, because the tubes are biased conservatively. Dave G explained this a while back when I first restored my 222C.