Monster 5E5 Build...

[yankeerob]

Hi there - I'm new here and hoping there's someone who can help...

I've built a 5E5 clone, which I've done quite a few times already, only this time I thought I'd experiment with some bigger iron to see what would happen.

Normally I use a 150mA 340-0-340 PT with all the usual whistles and bells - sufficient 6.3V and 5V windings etc and a 5K6 OT and they produce a fairly consistent 28W output.

This time I thought I'd try building the same amp with a bigger set of iron - I had a 200mA 360-0-360 lying around with plenty of 6.3 and 5V current and a 5K OT so that's what I've used. It's weird but the amp produces around 25W as is (not what I was hoping for!!)...

In it's current state it's got 445V B+ straight off a 5U4 rectifier, 405V plates, 430V screens and around 32.5V on the cathodes (common cathode follower, 250Ω/22uF). I thought this a bit odd and am wondering if I need to drop the cathoder resistor value to make this beast go? I'm not that great at working these things out and haven't found any relevant info on this sort of situation... Anyone got any Ideas?

Like I said it's a straight 5E5 build but with bigger iron. I've attached the schematic of the build.

Many thanks...
Rob

[Alexo]

Hey Rob, I bet the 10% reduction in your OT load, plus the higher plate voltage basically means that your output section is drawing a fair bit more current when it's pushed. More current in cathode bias with a tube rectifier = more sag = self-limiting compression (not necessarily a bad thing).

I wonder if the extra current your bigger PT can provide is actually contributing to the voltage drop across the rectifier...

[yankeerob]

Hi Al, thanks for the reply...

Yer well - here's me hoping that a bit more juice = bit more power...

Worse case is I pull the tube rectifier... stick in a straight silicon one, stuff in a bigger 1st stage filter (and maybe a choke...) and be done with it. That'll kick it's *ss ... but I'd like to see if I can't get it to do say - 40-45W...

I regularly build these as mentioned in my first post - either with a 5Y3 and 6V6's or 5AR4 and 5881's - have even gotten as much as 18W outta the little guy and like I said - regularly get 28W out of the bigger one... There's some suggestion that I try a 5V4 in the little one as there are still quite a few of those around and new production rectifiers are pretty dire!!

But I would like to get my head around the business of what's going on here as I've never had to mess around with these - the single input's a neat trick we came up with by trial and error but it really works sweet...

Thanks again...

[VacuumVoodoo]

In it's current state it's got 445V B+ straight off a 5U4 rectifier, 405V plates, 430V screens and around 32.5V on the cathodes (common cathode follower, 250Ω/22uF).

First: it's not a cathode follower in any traditional sense.

If you have 445V on the first supply cap off standby i.e. 445V on OT center tap than your measurements look strange to me.

1. 32V on the 250R cathode resistro means each tube is pulling ca 65mA.
2. 405V (??) on plate is 405-32=373V between plate and cathode, this is ca 24W dissipation per tube.
3. 430V on screens says there's 10mA flowing in the 1500 Ohm filter resistor, take ca 3ma total for the preamp tubes, you have 7mA left for two screens, that's 3-4mA per screen. Seems roughly ok but not really all the way.
4. 405V on the plates vs 445V on the center tap: 40V drop on the OT primary !? 62mA and 40V drop makes OT primary DC resistance ca 645 ohm! That's not right at all, I would be ok with 10-12x less.

Assume you made a mistake and your plates are at 430V and screens at 405V.
We now see 40V drop on the 1500 resistor: 26mA running through it. preamp leaves 23mA for the screens, roughly 11mA each.
Bit high with no signal but lets go with it.

Say you in fact have 430V on plates and 65mA current in each cathode, this means plate current is 65-11=54mA.
Plate dissipation 54mAx430V=23W --- still high for 5881/6L6

54mA plate current and we now see 15V drop on one half of OT primary. This gives primary DC resistance 15V/54mA= ca 280 Ohm
IMO still high by at least factor 4-5.

Now we also see 430-32= ca 400V between plate and cathode. Plate to cathode saturation voltage for 6L6GC/5881 is ca 70V. This means plate can swing 400-70=330V down from center tap potential.
So we can have at best 370V peak voltage swing on each half of the primary. 370V peak is 262Vrms.
Is your OT a a 5k or 6k6 plate-to-plate? I guess the 5k6 number is a typo and the OT is a 6k6 so each plate sees 1/4th of this = 1650 Ohm. Power that OT should be passing through to the secondary is 262x262/1650 = 41W.
You are not getting it.
Why?
OT? primary resistance seems very strange. Measure it.
Idle current too high for 5881/6L6. Real 6L6GC rated 30W plate dissipation could live with it.
Increase cathode resistor, you'll probably end up somewhere around 400-500 Ohm.
Repeat your measurements and report back.

[Cygnus X1]

Welcome to AmpGarage Rob!

Whut took ye so long?



-Cyg

[yankeerob]

In it's current state it's got 445V B+ straight off a 5U4 rectifier, 405V plates, 430V screens and around 32.5V on the cathodes...

First: it's not a cathode follower in any traditional sense.

OK...

If you have 445V on the first supply cap off standby i.e. 445V on OT center tap than your measurements look strange to me.

I'm glad you say that - they looked weird to me too...

1. 32V on the 250R cathode resistor means each tube is pulling ca 65mA.

That's what I made it so we're in agreement there... a bit high I thought...

2. 405V (??) on plate is 405-32=373V between plate and cathode, this is ca 24W dissipation per tube.

I measured between the plate on each tube and the cathodes to get that figure - is this the correct way of measuring plate voltage on this type of amp?

3. 430V on screens says there's 10mA flowing in the 1500 Ohm filter resistor, take ca 3ma total for the preamp tubes, you have 7mA left for two screens, that's 3-4mA per screen. Seems roughly ok but not really all the way.

That measurement taken between the screens and earth

4. 405V on the plates vs 445V on the center tap: 40V drop on the OT primary !? 62mA and 40V drop makes OT primary DC resistance ca 645 ohm! That's not right at all, I would be ok with 10-12x less.

Hmmm - it's a brand new Majestic OT and we've never had any problems with them. Further - I've never had a scenario where the screen voltage is higher than the plate voltage either... not that I can remember anyway...

Is your OT a a 5k or 6k6 plate-to-plate?

Not a typo - we use 60W 5K A-A OT's on two of our amps...

primary resistance seems very strange. Measure it. Increase cathode resistor, you'll probably end up somewhere around 400-500 Ohm. Repeat your measurements and report back.

Well that won't be until tomorrow now but first thing I'm gonna do is measure the DC resistance of the OT primary... What about changing the common cathode resistor for two individual ones? Say 250Ω each?

I'll also post some photo's of the build so you can see what I've done. Many thanks for taking the time to scratch your head over it

[Hellhammer]

So you measured 405v plate to cathode and 430 screens to ground? That adds up doesn't it?

[yankeerob]

So you measured 405v plate to cathode and 430 screens to ground? That adds up doesn't it?

It might do - and you can call me an idiot - but it doesn't tell me why I'm only getting 25W out of it. That's what I was hoping to find out - and maybe find out how to get what I was hoping for which was around 40 or 50...

Try as I might - I have yet to find a good source of reading material about cathode biased amps; theory and design etc - instead I'm forced to come here and ask dumb*ss questions

[Alexo]

Hey Rob, you may get some more power with a 5AR4 but to really get into the 40/50 watt range, you might have to go with a fixed bias design. That should definitely do the trick.

Or if you want to stick with what you've got, cathode biased El34's can definitely yield more power and later breakup, believe you me... At these sorts of voltages and loads, they are pretty much interchangeable with the 6L6 family. You'll want 5 watt, 1K screen resistors though, and you'll have to ground pin 8, iirc.

EDIT! Ground pin ONE, not pin 8! ...and if you do use EL34's, def go with a 5AR4.

[VacuumVoodoo]

Always measure and report voltages referenced to same potential. For reasons known only to themselves some like to use ground for this purpose.

Knowing you measure 405V between cathode-plate and OT 5k Ra-a
I'm expecting:
Primary DC resistance ca 120 Ohm. I'd like it lower but your supplier was probably trying to cut costs and used lowest wire diameter that would withstand the current and not exceed max allowed temperature rise. At specified nominal 60W output power this transformer will have ca 12W copper losses in heat alone due to high primary DC resistance. Expect another 5W in iron losses due to eddy currents. 17W total losses in a 60W OT is high by my reckoning.

About 10mA screen grid current per tube and ca 55mA plate current.
That's ca 23W dissipation. 6L6GC should be ok with it but my preference would be to cut it down to 20W. Try 390-430 Ohm range on cathode resistor. Or ca 820 per tube. It should result in about 40V on cathodes and lower current.

With 5k Raa & 400V plate-cathode you have ca 330V avialable peak voltage swing = 230Vrms. 1/4 tha of 5k is 1250 Ohm and max power is thus 230^2/1250=42.6W

Why you measured 25? Look at cathode voltage at full power output with an oscilloscope. You're probably getting cathode rectification of your grid signal so cathode potential rises probably to the neighborhood of 40-50V causing bias shift to lowering the plate current. Less current=less power.

Around 30W with cathode biased 2x6L6GC or KT66 seems about right in general.

Normally I would send you a consultancy bill at ca 50£/hr but in this case, since I volunteered to help, I'll be happy if you take me to the best place for Yorkshire pudding and a barrel of your best Ale when I happen to be in your part of the Kingdom.

[Drumslinger]

Hey Yankeerob! Welcome!

[yankeerob]

Normally I would send you a consultancy bill at ca 50£/hr but in this case, since I volunteered to help, I'll be happy if you take me to the best place for Yorkshire pudding and a barrel of your best Ale when I happen to be in your part of the Kingdom.

Good ale and Yorkshire puds are no problem in this part of the world - many sincere thanks

[VacuumVoodoo]

You're welcome, Rob. Very cool factory location. Me like!

While I'm at the keyboard: IMO common cathode resistor in PP AB1 PA is ok IF you don't expect to ever run your power tubes into overdrive and grid cut-off. Consider what happens when one tube is driven hard and the other at same time goes into cut-off. What happens to potential on cathode resistor and what does it do to the tube that is overdriven? You may want to consider separate cathode resistors with not too big decoupling capacitors. I'd choose these caps to give the PP tube pair same 3db LF limit as the OT. Simple and effective prevention of flatulent low end.

Good Luck!

[yankeerob]

Thanks for that - did get a chance to measure the a-a DC resistance of OT today - 180Ω (a bit high; no?) and have ordered some 7W 1K's, 820Ω's and 680Ω's to wire in parallel to derive the values close to those you've suggested and have plenty of dissipation capability... (maybe even go for individual 820's with low cap values)

Will keep you all posted on the progress of this project - a bit of a hair-brained idea really but... who knows?!

[VacuumVoodoo]

Did you measure 180R between anode ends of the OT or between CT and one of the anodes?

If between anode ends then 180R is pretty good. It may be a bit asymmetrical when measured CT to anode ends. 80R and 100R is acceptable. 70/110 will be indicative of a simple 3-4 section wind: primary-secondary (4R)-primary-secondary (8 & 16R) Difference in DC resistance isn't really that important but identical number of turns on each half of the primary is.
The most you might need for guitar amp is a 5 section wind. 7 is over kill unless you need taps on primary for different Raa like 5k and 6k6.