Peavey Delta Blues Rebuild/Plexi Preamp

[wtk0315]

30 watt outside the box amps don't drum up much interest on TDPRI, so I figured I'd share this here.
Schematic:
https://ibb.co/8jH5tNv

Amp is built, but I could use a little assistance to get my PS voltages where I want. Pardon my mishmash labeling system, I copied the node labels off the original Delta Blues schematic.

Anywho, I'm trying to get my plates down to 325, screens to 310. I'm using this page to determine my filter cap nodes:
https://www.ampbooks.com/mobile/classic-circuits/JMP50-1987/

Problem I'm having is that the dropping resistor values for B+, B+1, B+2 are based on 450 volts at the reservoir cap. I'm at 352 and want 325, but I want 320 at B+. B+1 and B+2 should fall in line after that.

Through solder and try experimentation, I was able to get my 320 at the plate node by adding a dropping resistor between the two lugs of the reservoir cap. Problem is, that dropped my already low PI/preamp nodes to way too low. So I experimented with the 8.2K running to the B+ node. I changed it to 5k (I think). This caused my voltage at the reservoir cap to drop significantly (195 volts IIRC) and my 3 parallel 10k resistors to get smoking hot (these resistors were located between the two nodes of the reservoir cap, part of the voltage experiment and not noted on the schematic).

I know this is a lot for a first post. I can't wrap my head around the maths and it'll be another week before I can get back to this thing. I'm now armed with a stash of 3 and 5 watt resistors in multiple values, so if I'm forced to experiment I can. I do remember from reading that you have to think of current as going backwards, which is why a relatively small change in the B+ resistor forced the reservoir resistors to drop a lot more voltage.

[wtk0315]

Here's the original Peavey power supply:
https://ibb.co/wW9F928

I guess due to lack of solid state components taxing the PT, plus the way Peavey wires the filaments, my starting voltage is 352 instead of 332.

[sluckey]

I'm at 352 and want 325, but I want 320 at B+

I would be very happy with 352V on the reservoir caps. The reservoir voltage is subject to change depending on the power tube load. It's possible that the bias is set too cold which causes the output tubes to not draw enough current to drop the plate voltage. What voltage do you have on pin 2 of each output tube? Get the bias set right then see where all the voltages land.

So I experimented with the 8.2K running to the B+ node. I changed it to 5k (I think). This caused my voltage at the reservoir cap to drop significantly (195 volts IIRC) and my 3 parallel 10k resistors to get smoking hot (these resistors were located between the two nodes of the reservoir cap, part of the voltage experiment and not noted on the schematic).

This should not have happened. I suspect you did something wrong. If you want a lower voltage at the B+ node, then increase the value of the 8.2K dropping resistor. (Changing the 8.2K will also affect B+1 and B+2 nodes)

I see a couple issues with your schematic. I'm sure it's just drawing errors.

1. There are no coupling caps between the PI plates and the output tube grids.
2. You have the -36V bias supply connected to the +75V heater elevation source.

[wtk0315]

I actually want to increase B+. The preamp is a Plexi/marshall preamp and I need to have around 320 volts @ B+ node. I don't remember what it measures but it was at least 70 volts too low.

I found some calculators and according to the rough estimates I would need 250R between the reservoir terminals and 1.2K to replace the 8.2 K leading to B+. I'm just not sure how to factor in the parallel caps or if it even matters.

Also good catches on 1 and 2. Those were definitely drawing errors. The 75 V supply only shares ground with the bias supply. The .022 coupling caps are installed between PI plates and Power tube grids.

[wtk0315]

Corrected schematic:
https://ibb.co/SmtzHYK

[sluckey]

You said you had 352V at the reservoir caps before you started experimenting. I suggest you remove that resistor you put between the reservoir caps to drop the plate voltage. This will put the reservoir voltage back to 352V which is perfectly normal for a quad of EL84s. Your screen voltage will be a bit higher since you are using a choke rather than a dropping resistor between the plate node and screen node. I would just accept those voltages as they are. Remember, adjusting the bias pot will also change these voltages a bit. You did not tell us what the voltage on pin 2 of all the output tubes is. That's important.

Now, your plate and screen nodes are a bit higher than you desire, but they are perfectly OK. Moving on down the line, it should be a simple matter of changing the 8.2K that feeds the B+ node. Increase the 8.2K to decrease the voltage at node B+, decrease the 8.2K to increase the voltage at node B+.

So, a big question is, how does the amp sound ***BEFORE*** you started experimenting? Another big question is why do you want to drop the voltages?

[wtk0315]

The all important pin 2 lol. That should be a negative voltage? I didn't measure it at the pin. I just set my bias to -14 V. But I have very narrow range on my bias pot. It only goes from -17V to -14V or thereabouts.

I feel quite useless without the amp in front of me.

I have no reason to lower the plate voltage other than AC30s are considered "hot" with 330VDC on the plates (which is also what this Delta Blues PT is supposed to give me). I was just trying to make my tubes happier.

Sound wise I have to split the discussion up a bit. I have some HF oscillation on the Bright channel. Research shows this to be a fairly common thing in Plexi builds. One fix that came on some factory Marshalls was a snubber cap between the plate and grid of V1A/V1B. I didn't have the parts to try it so I went back to tweaking voltages.

On normal channel where the HF oscillation isn't a problem it sounds like classic rock goodness. Maybe a little too much so, because there's no clean sound to speak of really. Pretty much overdrive up the dial. It gets noisy with the pots maxed, but more reading on that suggests leaving the EQ controls at lower settings than you would run other amps at. So like 3-2-3 on T-M-B where most amps I would have 8-7-6 or something similar. Presence control is the same, turn past 5 at your own peril. Works better at 2.

[shooter]

Pretty much overdrive up the dial

84's drive with a whisper instead of a roar, scope comes in real handy here.  try changing V1 to An AY or AU for testing.  you have 820uF on V1a and V2a, that's ALOT of cap.  take them to 10uF for test.

Always amazed when music folk are "forced" to change the knobs to better sound 

[wtk0315]

Always amazed when music folk are "forced" to change the knobs to better sound 

Lmao. Well most amps sound like mud with the EQ set low. Apparently this amp is not one of those.

[sluckey]

You have a fixed bias amp so don't be comparing it with the cathode biased AC-30. Fixed bias amps usually run cooler than cathode biased amps. The AC-30 is known to run very hot, which means a heavy load on the power supply, which will drop the voltages considerably.

I have two fixed bias EL84 amps and they both run with plates at 348V and screens at 346V, and -15V on the control grids. They are both very happy amps. I also have several cathode bias EL84 amps. The cathode voltage is 10V which has these amps running hot. The plates are 317V and the screens are 295V.

I think you are chasing rabbits. I would fix what's not working properly and set the bias for 70% of max plate dissipation. Then recheck voltages. Let us know how this turns out.

[sluckey]

you have 820uF on V1a and V2a, that's ALOT of cap.

He also has a ,68µF RESISTOR on V2A's cathode. I think what you are seeing are pencil errors! 

[wtk0315]

Bah, shooter you caught two more typos even if you weren't trying lol:

https://ibb.co/LRhD05G

[wtk0315]

Sluckey I'll defer to you in regards to voltages. I just didn't want to build a tube eater. I'll let you know how it turns out when I get back to it and post some build pics for the general public.

Mine looks alot different than Opry Audio's conversion, that's for sure. I went with mostly turret board and cap cans, although my el-84s are hooked up via a long turret strip.

[wtk0315]

This is my power rail, but it's an older pic, not everything is hooked up:

https://ibb.co/1mPC5Wb