The Amp Garage

Oops. Here is the aforementioned picture.

Ok, so I replaced the wood board with vulcanized fiberboard and and wired the EF86 screen to the right side of the EL84 cathode. First the good news: The voltages look good! Just over 20 on the EF86 plate (plan calls for 20) and 30 on the cathode (2v over spec). I'll ultimately have to tweak a coupl...

Martin thanks for highlighting that cathode to screen feedback loop! I looked again and realized that I ran that joining resistor from the *ground* side of the cathode resistor. Will redo that first, remeasure, and go from there.

Joe

Will ditch the wood. I should have researched that! In any case, I had only modeled up to the plate resistor, not across it. Having played around with adding it to my model, and looking at some of the measurements I took across it at full power/overload, I think I do need to increase resistance in f...

Makes sense! Will re-work my power supply resistors. I assume I would NOT want to change the plate load resistor, but perhaps the one in front of it?

Joe

Yep, grid voltage is an issue. It's about 110v with only 40 on the variac. Thing is, this is a directly coupled circuit, which the claim they get away with due to the "starvation" conditions that the EF86 is running at. I'm going to re-measure/solder my power resistors but I wonder if I need to add ...

The tube did not seem to be shorted and I did not get a reading across the speaker terminals. Aware of the stress on the components, I variac'd the amp up. The behavior is a bit unpredictable, and may change slightly when I pull the tube and then put it back in. Today, I was able to push the mains t...

I powered up my Mullard 3-3 build for the first time tonight on the variac. After frying/replacing a couple of power resistors (much higher wattage), I realized that my single EL84 power tube was pulling no less than 120 ma of current with 276 volts on the plate (I was aiming for the 310 stated in t...

Does the red indicate that those numbers are added together and the sum multiplied by (...)^2?

Thanks as always Martin!

Joe

I'm nearing first power-up of this build and have been reviewing calculations, etc, and I'm just trying to clear up limiting resistance for tube rectifiers. The way I've understood this from the discussion up to now is that the internal resistance of the PT is the end-to-center tap secondary winding...

I got the 50ohms at the link below, but should have scrolled down a page as I see this is an earlier edition of the datasheet, with a later version containing the 75 you mention. http://www.r-type.org/pdfs/gz34.pdf As for the winding, I measured the 290-0-290 (unloaded) tap to have about 78ohms from...

Ok, I've run some stated voltages in the 3-3 plan against component values to come up with better current estimates and things are looking much better in PSUD. The Deluxe Reverb PT I want to use has a lower tap of 290v (as measured with no load) and, of course, the 5 volt winding so I modeled the ci...

Never mind. I see that this circuit is running the ef86 at much lower than normal currents so will play around with that, study the data sheets, etc.

Joe

So I downloaded PSUD2 and decided to plug in the stock values from the 3-3 to the best of my ability. The transformer secondary resistance and offload voltage are estimates for a hypothetical 300-0-300 obtained by splitting the difference between values on the 270-0-270 and 340-0-340 taps on my tran...

I was wondering about that 330ohm resistor, as I found Merlin's page about this and he does mention a single resistor in series with the cathode as an alternative to the 2 resistors on the plates, but I appreciate all of the info on internal resistance, etc. I'm going to download PSUD2 and mull all ...