50W Plexi, Dual Rectifier, PPIMV Budget Build

[Guitarnut]

My numbers were off but the results are headed in the right direction. I went ahead and swapped the resistor as mentioned above. With it now at 68K, the bias range is -49v to -35v. This is 8v hotter than the max I could get in the original setup. I could raise it another 5v and still be comfortable.

Now the question is, if I l had to use the max -35v setting on the bias pot to get the power tubes singing on the tube recto, how would I limit that at the switch to keep the -42v I want for the SS recto? This would allow me to switch between rectos without resetting the bias.

Could it be as easy as adding a R inline with the lead to the switch?

Peace,
Mark

[martin manning]

I would switch in another resistor in parallel with the 47k for the vacuum rectifier mode. A 56K will do it, I think, but a better set-up would be to have two trimmers, where the existing one would be for SS, and the other would be for vacuum. The vacuum rectifier trimmer would be connected to the top of that 47k, and it's wiper would be connected to the bottom (paralleling the second trimmer's resistance with the 47k) when switching into vacuum mode.

If you want to switch the vacuum rectifier's filament off when the SS rectifier is in use, you'll need a 3PDT switch.

[Guitarnut]

Thanks! That's the opposite of how I was thinking. Makes good sense to me.

So, I would bias for the SS and then when switching to the tube, have the 47K cut almost in half by the paralleled resistor?

Something like this...

[martin manning]

That's what I described above, and should get you close. If that 56k is replaced with a trimmer, then you can dial in the bias setting for vacuum and SS independently. Just do the SS first, though.

[Guitarnut]

That's what I described above, and should get you close. If that 56k is replaced with a trimmer, then you can dial in the bias setting for vacuum and SS independently. Just do the SS first, though.

I'll get another pot this week but for now I'll go with a fixed resistor.

Peace,
Mark

[martin manning]

It wouldn't be a bad idea to check that 56k by clipping in a regular pot (100k, say) and use it to find the exact value. Bias for SS using the existing trimmer based on plate dissipation, then switch to vacuum and clip the pot across the 47k. Adjust it to correct the bias, then measure the resistance required.

[Guitarnut]

It wouldn't be a bad idea to check that 56k by clipping in a regular pot (100k, say) and use it to find the exact value. Bias for SS using the existing trimmer based on plate dissipation, then switch to vacuum and clip the pot across the 47k. Adjust it to correct the bias, then measure the resistance required.

Great idea! Only thing I have on hand is a 250K but it should work.

Before seeing your post I tried the 56K soldered in and the bias isn't quite there. I'm biasing for 70% dissipation which on the JJ 6L6 is 21W. Ideal numbers would be:

SS
21/450=.046

GZ34
21/380=.055

I dialed in the SS using the pot and easily hit the 46mA I wanted. After letting the tube recto warm up for 10 mins the power tubes are only drawing 31mA. 24mA shy of what I need.

I'm going to try your idea and see where it lands.

Thanks again!

[martin manning]

Seems like 70V is way too much voltage drop going from SS to GZ34. Re-check your plate voltage as you get each rectifier dialed in. This is often an iterative process.

[Guitarnut]

After setting the SS current, the plates are now up to 392 on the tube. So my new number is 53mA.

I found a new 100K Wah pot in my parts bin and used it to test for the R value. I dialed in 53mA. I checked the two lugs I was clipped to and they read 92K. In parallel the 47K and 92K are 31K.

I'm a little confused. If the 56K and 47K in parallel (25K) didn't get the current I want, how is it that 31K works? Or should I be using the value at the other pot lug? (6K) Which when paralleled with the 47K would be about 5.6K.

Peace,
Mark

[martin manning]

Use the lugs you were clipped to... that has to be the resistance you paralleled in, right? Maybe there was some measurement or calculation error.

Did you have the plate dissipation where you want it in both modes? If so, what's the difference in plate voltage SS-vac?

[Guitarnut]

Use the lugs you were clipped to... that has to be the resistance you paralleled in, right? Maybe there was some measurement or calculation error.

Did you have the plate dissipation where you want it in both modes? If so, what's the difference in plate voltage SS-vac?

Right.

I just powered up again to check and the the SS was right where it was before clipping in the pot. Plate voltage was 447V. I switched over to the tube and before I could get the probe on the test point, the amp made a small sizzle sound and blew the fuse.

Son of a ...

[martin manning]

A 5AR4/GZ34 should drop only about 10-15V at idle current level, so I suspected something wasn't right.

[Guitarnut]

I've been a bit suspicious of the GZ34 all thru the build. It seems to come up very slowly...not nearly as fast as the 5AR4 did in my Bassman build.

Is there a way to test the tube without powering up?

Peace,
Mark

[martin manning]

GZ34 and 5AR4 are supposed to be the same thing, European vs. US designation, analogous to ECC83 and 12AX7. Swap in the 5AR4 from your 5F6-A and see what you get with that.

[Guitarnut]

After taking a breather to clear my head, I changed the fuse and changed to the 5AR4 and things seem to be on track now. The tube is only dropping about 15V.

I couldn't get there with a 100K pot so I went to a 250K. The number ended up at 150K. This setup has the 6L6s at 70% dissipation with either recto.

I don't have a 150K 1/2W. But I do have 470K and 220K which I can put in parallel for just a hair under 150K.

SS
F1 436V
Pin3 435V
48mA

5AR4
F1 420V
Pin3 419V
50.5mA

Peace,
Mark