TeleWreck - Liverpool 15 W

[El_Martin]

Hi!

Fired up my Liverpool yesterday evening for the first time.

How it all began...
Started building amps two years ago. After 20 yrs. the solder iron is busy again.
First built was a Champ w. beefiy PT&OT. Gathered some of the parts floating around and did a GA-5 Variant w. diode rectis. Dabbled w. some home made OTs. Currently the GA-5 is my main amp. Vol. bright sw. and switchable feedback. All the amp I'll ever need. This one's the amp for the island.
Did I say EVER???

No! Started a 15 W pp Liverpool in june. I have come across a Telefunken geiger counter in a nice 19" housing. Gutted it, drilled some extra holes.
I kept the tubes of the Telefunken.

It was hard to stay close to the proven layouts, since the little chassis is a bit crammed. Did my one turretboard, made my own tools for staking.
The board of the powersupply is vertically mounted sideways.
Made my own mistakes, too.

It's not as clean a machine as Dana's, Allyn's or the other usual suspects.

I will clean it up after the final touch and there's sure some things to be re-done (PS).

Ciao
Martin

[El_Martin]

some more pics:
populated boards
turret + ps

[El_Martin]

another gut shot.
not the final lead dress...but it's working.

hum is 5 mV into 8 Ohm at vol=0 (idle). When grounding the gate of v3 the hum decreases significantly.

Still there are some old dummy preamp valves in it. Going to post the actual values w. new tubes.

Did anybody care to take measurements of the hum signal of his TW?
Up to now my filters are a bit small: 32 uF 32 uF 22+ 22+ 22

I guess it was Paul Ruby, who recomenndes in another post to keep the distance between C1 of the PS and the rectifiers low."minimize the area of the antenna".
Would some pF capacitors to ground ease the problem?
On the rectifier and paralleled w. C1?

Ciao
Martin




Ciao
Martin

[doctord02]

if there is any way to avoid it, you'd be well advised to not have that input jack down near the PI and the long wiring run across the whole chassis. Even tho it's shielded, thats just asking for trouble.

[geetarpicker]

I'm not sure how accurate my Fluke 112 is at really low voltages but I get 7mv of hum into 8ohms on both my original Express and my two Express clones with the volume off. I get about 50ma of hum and noise when I raise the volume up a little passed half way (other knobs on 1/2) with no instrument plugged in of course. The only shielded cable on any of these amps is from the input jack to V1.

[El_Martin]

TNX, Glen!

I guess the Fluke is accurate enough.

I'll try to relocate the input jack.
About the grounding scheme: Thought to break up the ground buss on the controls. At least the Presence should be grounded single imho.

Let's see (hear), what new tubes and some other measures can do.
I'll keep you informed.

Did I say, I like your playing, Glen?
If not: I LIKE YOUR PLAYING!

Ciao
Martin

[drhulsey]

...hum is 5 mV into 8 Ohm at vol=0 (idle)....

At what points are you measuring the hum

[El_Martin]

Hello Tim!

I simply clamped the DVM set to AC to the output jack connections.
You don't need to open up the chassis, just put your clamps parallel to the solderings at the loudspeaker (if open back).
Volume down. Tonestack/presence set to zero (that's how I did it). If you have installed a master (nononononotinaTrainWreck), crank it full to get comparable readings.

Ciao
Martin

[El_Martin]

Hi!

Calculated the static power dissipation of yesterday's measurements.
295 V @ anode
8 V across 100 Ohms Rk
-> around 12 W for each tube (disregard the screens)
(I'm using 6N14N, russian versions of the El84)

switched to 120 Ohms, took voltage readings: 10 Watt static

What Rk did you use in your 15 TW Liverpools?
150 or even 180 Ohms?

Seems quite hot to me, but I'm no Vox user.

BTW: relocated input jack to the other side w. the preamptubes.
Nada. Same hum as before. Not even harmonically richer

Hopefully tomorrow I'll double the first filter C and try some shielded cable from the vol pot. Maybe vice versa.

Ciao
Martin

[geetarpicker]

Martin,

Thanks, I appreciate it!

GK

[drhulsey]

...I'm using 6N14N, russian versions of the El84...

I bought an octet of the 6N14N's to try out in a Liverpool build. How do they sound

[El_Martin]

Hi!

I'm a bit short in time. Will spend tomorrow in the dilbert zone. Some travelling and kind of a meeting: gathering of equally uninformed people.

Tubes sound fair so far, will report these days. Wasn't able to fully crank the TW. Neighbors --- decided to not force it

Ciao
Martin

[LOUDthud]

I'm new here at this forum and this issue is something I've been interested in for a while. The last three amps I built, two 5E3's and a 150W bass amp, I used the grounding scheme from Kevin O'Connor's TUT books. They were very quiet and output noise is dominated by hiss. I call this scheme the "Chain of stars".

This led me to examine layouts at Hoffman, Brown Note and to this site with TW and 'umble layouts. The common thing I see that deviates from O'Connor is the grounding of the HV center tap directly to the chassis (as Leo Fender did). My first reaction is, "How can this work at all?" The HV center tap is the noisest wire in the entire amp.

Here's my conclusion: The grounding of the center tap creates a buzz on the ground side of the first filter cap (with respect to the chassis). You can probably measure it with a fluke DVM. Something like several millivolts. The output stage and the phase inverter can reject this as common mode supply noise. The problem occurs when the filter caps for the preamp are connected to this noisey ground. The caps conduct this noise right to the B+ for the preamp stages and some of it makes it's way into the signal chain where it gets amplified. Sometimes, it gets right to the input of the first tube depending on the grounding of the input jack. To prevent this, (in most "approved" layouts) a separate chassis connection is made for the ground of the preamp filter caps, usually near the preamp.

On the photo of the power supply board in this thread, I see a common connection to the ground of all the filter caps. This should be cut and a separate wire run to the chassis, some distance from the power transformer, for the 3 preamp filters.

Just my opinion, I'm still learnin.

[El_Martin]

Hello LOUDthud!

Interesting idea...Dunno KOCs books, but he sure knows a thing (or two).
At least his polarizing statements are worth thinking about.

Seperate ac and dc grounds, hi and lo current grounds...
I'm sure the grounding scheme as is can be improved.

Thankyou for the input, man!

Ciao
Martin

[El_Martin]

Hi!

Shielded cable from volume pot helped a bit. 10% less hum.
Connected a scope to the output.
100 Hz noise, sharp peaks - rectifier switching noise?
I use FREDs UF4007.

Changed the ceramic capacitor 1nF. Still microphonic. Maybe it's the parallel cap?

Did not touch the grounding yet. That's for tomorrow. And a closer look at the rectifiers...

Ciao
Martin