18Watt.com

Hi all,
What began years ago was converting a filmosound specialist EL84 version, to an 18Watt Superlite.
I gutted most of the amp leaving the heater and output TX wiring.

I'm using the lite 2 / Superlite schematic.
I am wiring this point to point. The amp chassis is very tight, there really isn't room for a board, and, these things are supposed to be a rat's nest anyway.

My voltages are a little high... I'm concerned about going further without some advice.

So far:
Starting with the PT, and power supply / B+.
These amps have a 315-0-315VAC, two pairs of filament windings, V1, 2, 3, 4 is 6.3VAC with center tap. for the EZ81 it's 6.3 or 5V winding. Similar to the PT in the superlite schematic.

It has two three-section cap cans First is a 400VDC 40-40-20uF second is a 400VDC 15-15-15uf. I wasn't planning on using the 15-15-15
I've tested the caps, all read close to what they are supposed to, 42uF, 21uF.
For starting purposes, I've used the 220K 1watt, For the dropping resistors; 2.2K 2W Node B and 8.2K 2W Node C and 1K 2W to pins 9 on V3 & 4.
I've just put these in to test voltages before finalizing the sizes for the B+.

This is my concern;
From my Power transformer, I'm getting 315VAC to ground it's 5% higher than the 290VAC on the drawing. No biggie, the wall voltage is higher at 126VAC.

When I put the Rect tube (EHX 6CA4 / EZ81)in I get 435VDC Pin 3, which seems really really high. That's 128%

Node A 435VDC
Node B 435VDC
Node C 437 VDC ? Yes I re checked that.

Because of the above the dropping resistors do not seem to drop any Voltage and I'm concerned about moving forward, or, will adding tubes and load begin to drop the voltage?

Is the 40-40-20uF cap pushing up the voltage as well?

For reference, I took voltages with only the rectifier tube in:
Rectifier pin 1 315VAC, 3 435VDC, 7 315VAC
V1 pin 1 422VDC, 3 .1VDC, 6 422VDC, 8 .1VDC
V2 pin 1 424VDC, 2 .1VDC, 3 .1VDC, 6 422VDC, 7 .1VDC 8 .1VDC
V3 pin 2 .1VDC, 3 .1VDC, 7 430VDC, 9 430VDC
V4 pin 2 .1VDC, 3 .1VDC, 7 430VDC, 9 430VDC

Thanks in advance to any advice,

Leif

you need all the valves in for a true B+ reading,the current draw of the tubes will bring the 315V closer down to 290V.
the multi electrolytic can is also an issue,400V rating is not enough you want at least 475-500V,even with the valves in when you switch on you get a surge of well over 400 volts.also you need to test it for leakage as well as capacitance,it might test good for capacitance but still be leaking voltage especially if its old.
good luck

That's odd that node C is higher than B. Are you sure you have the correct dropping resistor values? And.. maybe drop the B+ with a series of zeners.
AFAIK caps don't raise the voltage, they keep it steady...

First thing first, yes I had my nodes reversed... looking at it upside down. Sorry. My mistake.

With all tubes in voltages are better, the PT is still putting out 315VAC, on the filmosound drawing, it's supposed to be 284VAC based on 117VAC input. My wall voltage is 126VAC.
Node A pin 3 EZ81 is 375VDC
Node B 353VDC
Node C 315VDC
I'm getting some hum, I suspect the caps are leaky, as was mentioned I'm going to grab some this afternoon, see if that assists.

I plugged in a guitar and it makes a sound, I fear with the above voltages it may be a bit stiff or sterile. Rather than live with a variac that I still haven't wired up, I think that putting another dropping resistor could work?

Based on the voltages above, if I place a 8K2 from pin 3 of the Rectifier and node A, will it drop 40VDC as it does between node B&C? Is this calculable, or is it trial and error at this point?

Like I said. You need to put zeners in the B+ line. 6x 5V zeners and you're in the 345V ballpark. You have no idea what a difference the right voltages can make on the sound of the amp

Just remember that Zeners don't provide the AC isolation of dropping resistors. That won't be a problem if they're being used in addition to the proper value resistors.

Jack

What Jack said, and I wouldn't string that many Zeners, especially in a tight space/PTP build. A 5W 20V Zener is probably all you need. I'd use just one, and see how it sounds. Review the thread in my signature for how to do it correctly (B+ Voltage...).

True PTP is exceptionally sensitive to layout/lead dress, so you may be chopsticking a bit to get the amp as clean as it can be.

Thanks,
Josh

JMPGuitars wrote: ↑

Wed 03/03/21 11:14 pm

What Jack said, and I wouldn't string that many Zeners, especially in a tight space/PTP build. A 5W 20V Zener is probably all you need. I'd use just one, and see how it sounds. Review the thread in my signature for how to do it correctly (B+ Voltage...).

True PTP is exceptionally sensitive to layout/lead dress, so you may be chopsticking a bit to get the amp as clean as it can be.

Thanks,
Josh

Josh, and Jack;
Thanks!
I had searched for this topic and surprisingly got lost in the journey (as usual)
That is a simple and easy fix. especially due to the "mess" the compact chassis these amps have.

Cheers,
Leif

Leif,which type of tag strips are you using for the point to point? can you post some pics.

geoff 1965 wrote: ↑

Thu 03/04/21 1:49 pm

Leif,which type of tag strips are you using for the point to point? can you post some pics.

Tag strips? who needs tag strips?
I didn't bother with any, so far.
That will change, I have to swap out the cap can's, so there will be a board for filter caps, possibly one for the cathode resistors and bypass caps.

I'm still in the "in the process" of tweaking this, im replacing the filter caps, its a tight squeeze. Before that, I inserted the zener, and it dropped my voltage to about 355VDC at node A, and I will add another see if I can get it closer to 340VDC

I'm hoping the new caps will reduce hum, if not its gonna be chopsticking time.

Back to the beginning,
Replaced the caps, added the second zener, voltages are 340, 319, 280VDC.

The chopsticking began around 9pm, a little after midnight i realized I have to make some changes.
I've removed the preamp and phase inverter and I'm starting over from the nodes, to the plates. I'll try and better separate the preamp and watch the signal path.
I'm having a hard time getting rid of 60hz hum.

Am I right to keep the V1cathode resistor and bypass cap on the tube socket, lay them flat with the ground against the chassis?

Do I keep the Nodes and signal path elevated?

Will the nodes place noise into the preamp?

I placed the bypass caps and cathode resistors against the chassis. Decided to put back in a couple of tag-strips to neaten up things. In doing so I noticed that both the HT centre tap and output transformer grounds were connected to the chassis in two separate places, both tucked beneath wires.
I disconnected and extended them to the main ground lug with the hope that it was a ground loop contributing to the hum.

It's running much quieter now, the hum is negligible now.

It was making a static, crackling noise. Sadly it turns out one of the vintage GE 12AX7 pre-amp tubes that came with the filmosound was noisy.

I've got a proper cap-can arriving tomorrow I can finalize the power section and get to fitting the chassis in a combo cabinet.

I can't wait until the weekend so I can crank it up and play the hell out of it!

Well, it popped and then fizzed out.
I shut it down, let it cool off and pulled the chassis from the cabinet.

I powered it up with the light bulb limiter.
Bulb is normal, not bright.
No tubes detonated, flared or looked odd when powered.
Removed the limiter. Powered up again.
No sound unless cranked wide open, and then its fizzy. I tried each preamptube, no change.

Shut it down and one EL84 was super hot to touch. Again no tubes looked like they were red plating. They are JJs, new, but were bought ten years ago. They only have 2 hours on them at most.

Open to suggestions, is it just a tube? I know they go bad, but I've seen more good than bad reviews on the JJs.

Could it be a resistor?

As I said, im open to suggestions.
Thanks in advance.

Leif
have to get a pair of EL84s.

LeifH wrote: ↑

Fri 03/26/21 5:47 pm

They are JJs

There's your first problem. JJs in general have become mediocre, but their EL84 tubes are just trash.

That may not be the source of the problem, but very well could be. This is an example of bad EL84 JJs I posted 7 years ago: https://www.youtube.com/watch?v=21G6TkukwY8

I typically bought tubes in bulk, and returned all of the JJs at the time. There wasn't a single pair that didn't have issues out of the lot.

While you're waiting for new tubes, you might want to unplug the amp, remove all the tubes and check for shorts between all the tube socket pins and such. Inspect the amp visually too, with a magnifying glass.

Thanks,
Josh

JMPGuitars wrote: ↑

Fri 03/26/21 5:54 pm

LeifH wrote: ↑

Fri 03/26/21 5:47 pm

They are JJs

There's your first problem. JJs in general have become mediocre, but their EL84 tubes are just trash.

That may not be the source of the problem, but very well could be. This is an example of bad EL84 JJs I posted 7 years ago: https://www.youtube.com/watch?v=21G6TkukwY8

I typically bought tubes in bulk, and returned all of the JJs at the time. There wasn't a single pair that didn't have issues out of the lot.

While you're waiting for new tubes, you might want to unplug the amp, remove all the tubes and check for shorts between all the tube socket pins and such. Inspect the amp visually too, with a magnifying glass.

Thanks,
Josh

Josh, thanks!

Will triple check for shorts, and with a magnifying glass.

Any suggestions for EL84s?

LeifH wrote: ↑

Fri 03/26/21 6:02 pm

Any suggestions for EL84s?

New production Mullard or TungSol are both good and reasonably priced.

TAD is good if you want to spend extra money for probably no reason.

If you really like spending extra money, I've been curious about Psvane tubes.

I've heard good things about the EL84M, available directly from eastern Europe. Not as cheap as they used to be though.

Jack

TriodeLuvr wrote: ↑

Fri 03/26/21 11:39 pm

I've heard good things about the EL84M, available directly from eastern Europe. Not as cheap as they used to be though.

Jack

The Sovteks? Sovteks are better than JJ, but I haven't personally tried their EL84 varieties. I was pleasantly surprised by their 7025 series 12AX7WA and WB tubes. Their regular 12AX7s aren't exciting, but still better than JJ. I haven't A/B'd them with anything else though.

JMPGuitars wrote: ↑

Sat 03/27/21 7:06 am

TriodeLuvr wrote: ↑

Fri 03/26/21 11:39 pm

I've heard good things about the EL84M, available directly from eastern Europe. Not as cheap as they used to be though.

Jack

The Sovteks? Sovteks are better than JJ, but I haven't personally tried their EL84 varieties. I was pleasantly surprised by their 7025 series 12AX7WA and WB tubes. Their regular 12AX7s aren't exciting, but still better than JJ. I haven't A/B'd them with anything else though.

I don't use them for V1 or V2. But they rule as reverb tube. The EL84 M is a really rugged tube. It's a real difference in glass thickness too..