Absolute maximums: new tubes, old tubes

[fossilshark]

Is everyone ready for my monthly n00b question?

So I am about to fully rebuild my LA60BL clone (bandmates are tired of listening to the radio during practice, need to redo all the wiring and lead dress) and I think the plate voltages are too low.

The original schematic got 590VDC right off the rectifier going to the output tube plates. Isn't that WAY too much for modern tubes?

Currently I have 430VDC right off the rectifier. This seems a bit low compared to other amps I have worked on.

My plan is to change out the current diode configuration (as seen in the original schematic here) for a standard full bridge rectifier configuration, this would boost the DC voltage a bit correct? The power transformer I used has 400vac winding that I use for B+, I am thinking this should be enough.

And the title question: What is the max plate voltage I can run modern output tubes at? Or is the bias (dissipation in watts) more important here?

Here is the original schematic and a pic of the power transformer I am using.

[elams1894]

Modern EL34s could possibly handle the plate voltage at high 500v's, however the screen voltage may be a different story. Perhaps go for JJ KT77s, they can handle pretty much anything.

[tubenit]

Why do you think 430v is too low??      Why not leave it that way?  What do you think 590v is gonna get you?


With respect, Tubenit

[pdf64]

... I think the plate voltages are too low.
The original schematic got 590VDC right off the rectifier going to the output tube plates. Isn't that WAY too much for modern tubes?

Currently I have 430VDC right off the rectifier. This seems a bit low compared to other amps I have worked on.

Where has the mains transformer come from?
What issue do you think the 430V is causing?
Yes, A 590V HT supply will probably cause problems, it's above the screen grid voltage limit.

My plan is to change out the current diode configuration (as seen in the original schematic here) for a standard full bridge rectifier configuration, this would boost the DC voltage a bit correct? The power transformer I used has 400vac winding that I use for B+

Your mains transformer doesn't seem to have a centre tap on the 400V HT winding?

FYI changing 2 phase rectification (as per the Laney schematic) to full wave bridge would cause a massive increase in the rectified DC HT voltage, almost double.

So what rectification arrangement are you currently using?
If no CT then half wave or full wave bridge seem to be your only options.
Does the winding actually measure 400V AC with the amp idling?

Why have you chosen the Laney Supergroup when schematically it's little different to Marshall designs if a similar vintage?

[fossilshark]

... I think the plate voltages are too low.
The original schematic got 590VDC right off the rectifier going to the output tube plates. Isn't that WAY too much for modern tubes?

Currently I have 430VDC right off the rectifier. This seems a bit low compared to other amps I have worked on.

Where has the mains transformer come from?
What issue do you think the 430V is causing?
Yes, A 590V HT supply will probably cause problems, it's above the screen grid voltage limit.

My plan is to change out the current diode configuration (as seen in the original schematic here) for a standard full bridge rectifier configuration, this would boost the DC voltage a bit correct? The power transformer I used has 400vac winding that I use for B+

Your mains transformer doesn't seem to have a centre tap on the 400V HT winding?

FYI changing 2 phase rectification (as per the Laney schematic) to full wave bridge would cause a massive increase in the rectified DC HT voltage, almost double.

So what rectification arrangement are you currently using?
If no CT then half wave or full wave bridge seem to be your only options.
Does the winding actually measure 400V AC with the amp idling?

Why have you chosen the Laney Supergroup when schematically it's little different to Marshall designs if a similar vintage?

The PT I got on ebay a very long time ago, no idea what it was pulled off of. It does measure 400vac right off the secondary. I am currently using the half wave per laney schematic.

It has always sounded super muddy with a frequency range that just sounds like crap when jamming alongside anyone using a different amp, but to be fair I think that is more of an issue with the preamp voltages being too low. I will try raising the preamp voltages first by changing the drop resistor values and see if that helps.

I am also wondering if the lower B+ affects power amp distortion as I do quite often run it cranked.

Why this schematic? See:

I have not had time to work on it since gutting it. I will keep you guys updated.

[danhei]

The PT I got on ebay a very long time ago, no idea what it was pulled off of. It does measure 400vac right off the secondary. I am currently using the half wave per laney schematic.

The Laney schematic is not half wave and requires a center tapped transformer winding. See the Two Phase Rectifier section at  https://www.valvewizard.co.uk/bridge.html


Have you done a true half wave rectifier setup?

[pdf64]

... I think the plate voltages are too low.
The original schematic got 590VDC right off the rectifier going to the output tube plates. Isn't that WAY too much for modern tubes?

Currently I have 430VDC right off the rectifier. This seems a bit low compared to other amps I have worked on.

Where has the mains transformer come from?
What issue do you think the 430V is causing?
Yes, A 590V HT supply will probably cause problems, it's above the screen grid voltage limit.

My plan is to change out the current diode configuration (as seen in the original schematic here) for a standard full bridge rectifier configuration, this would boost the DC voltage a bit correct? The power transformer I used has 400vac winding that I use for B+

Your mains transformer doesn't seem to have a centre tap on the 400V HT winding?

FYI changing 2 phase rectification (as per the Laney schematic) to full wave bridge would cause a massive increase in the rectified DC HT voltage, almost double.

So what rectification arrangement are you currently using?
If no CT then half wave or full wave bridge seem to be your only options.
Does the winding actually measure 400V AC with the amp idling?

Why have you chosen the Laney Supergroup when schematically it's little different to Marshall designs if a similar vintage?

The PT I got on ebay a very long time ago, no idea what it was pulled off of. It does measure 400vac right off the secondary. I am currently using the half wave per laney schematic. ...

Does the winding have a centre tap CT , or is it a single winding, 2 wires?
If so, a half wave rectifier implies one wire is connected to 0V common ground, the other wire to a diode; correct?

What winding V AC do you measure with the standby switch (if fitted) open (ie no current flowing)?
And then winding V AC with the standby switch closed, amp idling, drawing current?

A FWB rectifier would increase the rectified V DC to well over 500V, perhaps near 550V, but will greatly increase (eg double) the current drawn from the winding.

And modern EL34 may not cope with their screen grid voltages been pushed so high.

[fossilshark]

I didn't want to have to break into the good cigars but I have an 8023H (hammond plexi replacement) power transformer in the shop that I am going to use instead of this crappy ebay transformer. No point in me sitting on good parts forever haha.

The output transformer is a classictone plexi replacement, remember those? I think they will work well together.

I will probably stick with the laney schematic 2 phase and drop the voltage down to around 500v with drop resistors.

500v is probably the most id want to run modern tubes absolutely cranked correct?

[mresistor]

That PT is 345-0-345. If you wire it up with a two phase rectifier then you should get around 470VDC unloaded. And you can easily get your negative bias voltage by tapping the HV secondary lead.

[fossilshark]

Ah right. 470vac should be plenty.

Did not realize how undersized the old power transformer was... No wonder it got so hot!! 

[fossilshark]

Another comparison shot.

[Merlin]

With that transformer you're looking at 500V no load, 450V idle, 430V cranked.

[mresistor]

Another comparison shot.

Well it's certainly more handsome..   

[elams1894]

Wonderful transformers are those ClassicTones. Such a bummer the company is no longer around.

[fossilshark]

I guess this thread is now going to be my supergroup rebuild log.

I am upgrading all the components I consider "sub par" aiming to mostly use parts that I have laying around. That being said I dropped a hot dollar on alot of new passive components for it as well. I thought about just series/paralleling caps to get the oddball 250pf/220pf/47pf but i decided to just order the correct value. Got myself a nice #47 lamp fixture with the purple jewel too.   

All new potentiometers, the set that was previously installed had all mismatched shaft diameters and didn't have the correct linear/log specs. very excited to see what the knobs look like when they come in.

ALSO big news for me: I ordered 10ft of 10 different colors of M22759/11-20-x "mil spec" wire. I want to switch over to using super high quality wire instead of that crappy cloth pushback wire I've been using. I also ordered 10ft of M27500-22RC1S06 shielded cable. Very very excited for that.

I just happened to have some extra 32uF+32uF cans as well as x4 cap holder. I think an extra one of those on top of the x2 50uF+50uF caps should do this amp well.


Overall extremely excited to see what an extra 7 years of knowledge will do for this build. Sure the chassis is swiss cheese but its MY swiss cheese. Might even mess around and have some faceplates made up for it.

Desoldered every component and cleaned up all the excess solder. Got the components laid out but not soldered in yet, still waiting for a handful of parts. I thought about making a turret board to make the layout a little more compact but meh it's not like I can ever sell this amp anyways because of the chassis being so mangled. 

[elams1894]

Looking very neat and tidy so far, nice work!

[fossilshark]

Looking very neat and tidy so far, nice work!

Thanks!

Had some more time to work on it. This M22759 wire is so nice. Super easy to strip and sucks up solder really well. Its a bit hard to maneuver but I've worked with worse.

Got the x3 JJ caps mounted and heater wiring done. We are going with small knobs for the tone + presence and big knobs for gain + volume. Not happy with the knobs but the reproductions of the knobs on the original supergroups wouldn't fit the pot spacing so these chinesium knobs will have to do.

I wont have time to work on it until the week after next. Chomping at the bit 

[elams1894]

super good stuff so far. The preamp tube tube sockets look nice, however just an observation on the power tube filament cables. It looks like the filament cables (yellow, black) have been untwisted and then routed on opposite sides of the power tube socket so you have the yellow cable going to one side of the socket, and the black to the other forming a loop?

If so, and you never will know until you finish the build so take with a grain of salt, however you might find that you have a large magnetic field around the socket (due to the configuration of the cables) rather than magnetic field cancellation. In other words, you need to maintain the black/yellow twisted pair throughout the whole routing process to preserve the cancellation aspect of the twists. Splitting the cables and twisting them upon themselves offers no cancellation, and when routed in a loop around the socket, the loop forms the magnetic field.

Hope all that made sense. And as I said earlier, the loops around the socket may or may not induce hum, however I thought I'd put it out there for consideration. Thanks for posting the images, looks great!!

[fossilshark]

Interesting ok. I followed valvewizard for the 12ax7 heaters but I couldn't find any examples of the correct way to do power tubes. If someone could post a picture of that i would really appreciate it.

[HotBluePlates]

... I couldn't find any examples of the correct way to do power tubes. ...

I'm gonna go out on a limb and say attach one wire to Pin 2 and the other wire to Pin 7.

It doesn't need to be more complicated than that.

... power tube filament cables. It looks like the filament cables (yellow, black) have been untwisted and then routed on opposite sides of the power tube socket so you have the yellow cable going to one side of the socket, and the black to the other forming a loop?

... you might find that you have a large magnetic field around the socket (due to the configuration of the cables) rather than magnetic field cancellation. ...

In the 1990s, I tried out "heater phasing" based on some cautionary words I'd read.  I could not hear a difference in hum either way.

Since then, I've seen numerous production amplifiers that do not twist heater wiring, and yet do not suffer hum issues.  I would recommend being neat, but not stressing it much beyond that.

[elams1894]

Indeed, like I mentioned, you may, or you may not have issues. Who knows? However sometimes it pays to eliminate variables where you can, before you need to trouble shoot. If one is going to the trouble of building a nice amp with all the expense that goes with it, and doing a great job with the preamp sockets, perhaps it may pay to extent that ethos to the power tubes.

I doubt you'll run into too much issue in this instance (being the power tubes), however just food for thought for a future build perhaps.

[fossilshark]

Oh that's a slick way of doing it. Definitely going to try that out on future builds.

Even if it has nothing to do with hum, aesthetic is very important