Advice needed...

[TriodeLuvr]

So, I had a new Rectron RS60 FWBR on hand and though it wasn't the form factor I was considering it's peak reverse voltage is 400VDC, 6A.
I mounted it on a bracket and hooked it up. Now I've got:
C3-A: +125V / C3-B: +121V / C3-C: + 116V
V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.
V2(12AX7) +63 p1, +65 p2, 557mV K's, 10mV g's.
The B+ went up for sure and the DC on the scope looks better as well.
Output tube plates went up by +6V, screens up by +5V. 12AX7 plates up by +4V. Surge at startup is +135Vrms, ..but that's a peak voltage of +190, ..yes? I'm wondering if multicap voltages as written on the can are rated at peak or rms values Well, I'll check tomorow...RS604.jpg

It's alive! Those numbers all sound correct. I was wrong earlier about the 12AX7 - the 220K anode resistors really limit the current. I'm used to seeing these tubes draw a little more current than that. Anyway, the voltage drop across the 10K power supply resistor indicates total current of about 0.5 mA being drawn by the 12AX7, and the voltage at the two anodes equates to 0.25 mA each. So, that adds up correctly.

The filter caps are fine, in terms of the voltage being applied. Terms like "RMS" and "peak" only apply to AC voltages. The caps are rated for DC, and that's what you're measuring. +135V on the first cap during a cold start is within its rating.

When you add the 6AV6, voltage at the last cap will probably drop about 5 or 10 volts, depending on how the 6AV6 is biased. That should still be enough for everything to work OK, but you can tweak it later if necessary.

Jack

[JMPGuitars]

V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.

Your screen voltage should be lower than your plate voltage. Do you have screen resistors in there now? If so, what value?

Thanks,
Josh

[TriodeLuvr]

V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.

Your screen voltage should be lower than your plate voltage. Do you have screen resistors in there now? If so, what value?

Thanks,
Josh

According to the schematic, there are no screen resistors. The screens are connected to the same B+ node as the CT of the output transformer, so anodes and screens will always be "upside down." The charts for the 50C5/6CU5 indicate that this shouldn't be a problem. At these voltages and grid bias, the anodes are operating much closer to the dissipation rating than the screens. Don't know if this is optimal in terms of output power or distortion, but it's a safe operating point for the tubes.

Jack

[The4thWatcher13]

OK, it was late and my brain was stuck in AC land. I believe that, pursuant to my previous question, that since the multi-cap is now getting fully rectified DC from the bridge rec that an rms value is no longer a consideration regarding the +135 surge value on the cap.

[The4thWatcher13]

[The4thWatcher13]

Sorry, I wrote the post above before my page updated and I saw your responses. I'm parsing them now..
Thanks!

[JMPGuitars]

According to the schematic, there are no screen resistors. The screens are connected to the same B+ node as the CT of the output transformer, so anodes and screens will always be "upside down." The charts for the 50C5/6CU5 indicate that this shouldn't be a problem. At these voltages and grid bias, the anodes are operating much closer to the dissipation rating than the screens. Don't know if this is optimal in terms of output power or distortion, but it's a safe operating point for the tubes.

Jack

The screens should be lower than the plates. Screen resistors add protection, and IME improves the sound.

That's also supported by the docs. If you look at the datasheet for the 6CU5 you'll see that both the maximum values and typical operation points have the screen lower than the plate. https://frank.pocnet.net/sheets/049/6/6CU5.pdf

Thanks,
Josh

[The4thWatcher13]

...At these voltages and grid bias, the anodes are operating much closer to the dissipation rating than the screens. Don't know if this is optimal in terms of output power or distortion, but it's a safe operating point for the tubes. Jack

The screens should be lower than the plates. Screen resistors add protection, and IME improves the sound. That's also supported by the docs. If you look at the datasheet for the 6CU5 you'll see that both the maximum values and typical operation points have the screen lower than the plate. https://frank.pocnet.net/sheets/049/6/6CU5.pdf Thanks, Josh

I noticed that as well, Josh, but you two guys are the ones with the real life experience here so I figured I'd defer to your cumulative experience. A lot may have to change before this puppy is rockin' so I'm not averse to trying it one way and then changing it again. Unless you guys come to a consensus. Thank you, Sir.

[The4thWatcher13]

Current voltages:
V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.

Jack previously wrote:
"....The charts for the 50C5/6CU5 indicate that this shouldn't be a problem. At these voltages and grid bias, the anodes are operating much closer to the dissipation rating than the screens. Don't know if this is optimal in terms of output power or distortion, but it's a safe operating point for the tubes."

As we move along I'm having to pay more attention to things previously noticed but not used, i.e. the screen and plate & screen dissipation ratings of the 6CU5's and plate dissipation of the 12AX7 you mentioned above. And I was thinking about how you, Jack, understood "the plates being closer to the dissipation ratings than the screens". Now, instead of showing you my mental hen-scratchings I figure I'll just ask you if you arrived at that knowledge using nothing but Ohms laws? i.e. one can figure wattage ExI or I squared x R, ..etc. I get 0.085mA current from the cap but there is 252 ohm in the output primary winding. (I squared x R?) Or is just using the voltage on the plate while considering the winding a resistance correct (E x I = 2.14W?) If there's a whole lot more than what I'm throwing out here to consider I realize you're not being paid for electronics lessons (...though I'd consider it) I'm just trying to keep up with you guys the best I can. So when you say something is this way or that way I have to try to figure out why or I'm just plain ol' slacking. And that ain't me!

[JMPGuitars]

I would put 1K screen resistors, check your voltages, and then play it and see how it sounds.

[The4thWatcher13]

I would put 1K screen resistors, check your voltages, and then play it and see how it sounds.

Sounds like a good idea. Will do.

[TriodeLuvr]

...I'll just ask you if you arrived at that knowledge using nothing but Ohms laws?

I had to use the tube charts. Even if we could use Ohm's Law to calculate cathode current (I couldn't, because I don't know the resistance of the transformer's cathode windings), that wouldn't tell us how much of the current is from the anode and how much is the screen. Both flow through the cathode. What we do know is the anode voltage, screen voltage, and the grid bias that you measured (cathode voltage). This can get us pretty close to determining screen and anode currents using the tube charts. And of course, the dissipation ratings for those two elements are listed in the datasheet.

Incidentally, I used the 50C5 data for all this, not only because that's how the circuit was originally designed, but also because it was more extensive than what I initially found for the 6CU5. In any event, the two tubes are nearly identical, so in the grand scheme of things, either can be used for general analysis.

Jack

[TriodeLuvr]

I just want to add that there's no magic rule requiring the screen to be lower than the anode. Assuming we stay within dissipation ratings, screen voltage merely serves as one of the parameters that determines the operating point. There's nothing wrong with adding screen resistors as Josh is suggesting, but it's just one thing you could put on a rather lengthy list of mods and tweaks to make this sound more like a guitar amp. BTW, it's convenient that this uses a Paraphase inverter. R7 can be easily adjusted up or down to change the balance. That should allow a considerable variety of tones (distortion) to be created.

Jack

[The4thWatcher13]

...I'll just ask you if you arrived at that knowledge using nothing but Ohms laws?

I had to use the tube charts. Even if we could use Ohm's Law to calculate cathode current (I couldn't, because I don't know the resistance of the transformer's cathode windings), that wouldn't tell us how much of the current is from the anode and how much is the screen. Both flow through the cathode. What we do know is the anode voltage, screen voltage, and the grid bias that you measured (cathode voltage). This can get us pretty close to determining screen and anode currents using the tube charts. And of course, the dissipation ratings for those two elements are listed in the datasheet.

Incidentally, I used the 50C5 data for all this, not only because that's how the circuit was originally designed, but also because it was more extensive than what I initially found for the 6CU5. In any event, the two tubes are nearly identical, so in the grand scheme of things, either can be used for general analysis. Jack

Thanks Jack, that's what I was afraid of! I do appreciate the explanation greatly tho. By the way the impedance of the cathode and plate windings are on the original schematic, but we've been using a truncated, pre-amp only version lately. Therefore I've included an updated schematic of the entire amp with this post. This includes the bridge rectifier and the 1K screen resistors just added. I'll definitely be referring to your explanation in the future. Thanks again.

Hamilton Electronics Tube Amp Iso Mod - Current 6V_H_6AV6.png

[The4thWatcher13]

I would put 1K screen resistors, check your voltages, and then play it and see how it sounds.

Josh, I've updated the current working version schematic and included it with the previous post. I've added 1K 1/4W resistors to the screens of the 6CU5's. The maximum output is right about where it was when I started all this, albeit maybe a little cleaner.
The supply section C3-B is +123.2
Screen voltages at zero signal are V4: +118 / V3: +120
V4: down 5V and V3 down 3V.
Seems like you and Jack are in a Sc-R / No Sc-R tug of war!
I'm pretty pragmatic; I'll try anything as long as I think I can get away with not blowing anything up. But screen R's did bring the screens down a little. The protection and sound improvement that you mentioned may become more evident once another pre-amp tube is added. Thanks Josh

[TriodeLuvr]

By the way the impedance of the cathode and plate windings are on the original schematic, but we've been using a truncated, pre-amp only version lately. Therefore I've included an updated schematic of the entire amp with this post.

Can I suggest that you modify the document so it refers to the windings as DCR (DC resistance), not impedance? That's what's being shown by those numbers. Good info to have!

[JMPGuitars]

Josh, I've updated the current working version schematic and included it with the previous post. I've added 1K 1/4W resistors to the screens of the 6CU5's. The maximum output is right about where it was when I started all this, albeit maybe a little cleaner.
The supply section C3-B is +123.2
Screen voltages at zero signal are V4: +118 / V3: +120
V4: down 5V and V3 down 3V.
Seems like you and Jack are in a Sc-R / No Sc-R tug of war!
I'm pretty pragmatic; I'll try anything as long as I think I can get away with not blowing anything up. But screen R's did bring the screens down a little. The protection and sound improvement that you mentioned may become more evident once another pre-amp tube is added. Thanks Josh

You should be using 2W or 3W screen resistors. Jack and I having a difference of opinion is irrelevant. Try searching this site, or google, and you'll see it's a lot more than any one opinion. There's some of that science stuff behind it.

And for the record, having properly rated screen resistors will help reduce red plating, and help avoid things like blowing up. Not having a screen resistor does...nothing for you.

Thanks,
Josh

[JMPGuitars]

There's nothing wrong with adding screen resistors as Josh is suggesting, but it's just one thing you could put on a rather lengthy list of mods and tweaks to make this sound more like a guitar amp.

LOL! If he's not trying to make this sound like a guitar amp, he's on the wrong site! Hahahahah

[TriodeLuvr]

You should be using 2W or 3W screen resistors. Jack and I having a difference of opinion is irrelevant. Try searching this site, or google, and you'll see it's a lot more than any one opinion. There's some of that science stuff behind it.

And for the record, having properly rated screen resistors will help reduce red plating, and help avoid things like blowing up. Not having a screen resistor does...nothing for you.

Thanks,
Josh

Josh, it's not a Marshall. It doesn't have +400V on the anodes and screens. In fact, I'll bet nothing in this amp is anything like what you've worked with in the past. A 1K screen resistor w/ 8.5 mA dissipates less than 80mW. Please also note that this amp is more than 50 years old, and it hasn't blown up yet. How long does it take for a lack of screen resistors to become a problem? If you want to apply science, you'll need to start by using use the charts published for these tubes.

Jack

[JMPGuitars]

Josh, it's not a Marshall. It doesn't have +400V on the anodes and screens. In fact, I'll bet nothing in this amp is anything like what you've worked with in the past. A 1K screen resistor w/ 8.5 mA dissipates less than 80mW. Please also note that this amp is more than 50 years old, and it hasn't blown up yet. How long does it take for a lack of screen resistors to become a problem? If you want to apply science, you'll need to start by using use the charts published for these tubes.

I linked the datasheet, and in every instance, it lists the screen voltage lower than the plate. I also mentioned above, this is a guitar amp website, and I've never heard a guitar amp that sounded better with the screen voltage above the plate voltage. We don't have to agree on the subject, it's okay.