My new amp build - not singing the blues!?

[funkgang49]

Finished version 2 of a SE build that I was hoping some of the many learned scholars and fellow tube tinkerers here on TAG might lend me their extra eyes and experience to help solve an issue with.

Everything lights up - jewel light, all tubes, and I get reasonable voltages throughout the amp but, no sound.

The preamp is straight from a 5E5-A with the exception that I have 150K on V1a plate, 1K cath., 220K V1b plate, 1.5K cath. AND V1 & V2 are 6SL7's.

Controls are: Vol.1, Vol.2, Bass, Treble, & Presence. A Master Vol. pot is installed but as of yet not connected (I'm also thinking of intalling a VVR with a control on the back panel).

The Phase Inverter (12AX7) is where I begin to really veer from the 5E5-A circuit. Inspired by the Gulsekt 88 post: https://tubeamparchive.com/viewtopic.ph ... ht=gulsekt which most importantly led me to the much appreciated help of Matt h. Thanks Matt! I implemented a LTPPI which among other things allows me the possibility in the future to use the "unused" plate output to drive a reverb.

I do not have a Schemo as of yet. My enthusiasm to make this build a reality has outpaced my fumbling ability to update my previous versions schem.

A couple of things that might help those who might help me:
1. When the amp power is turned on I can hear a very quiet hum through the hooked up speaker.
2. The only other sound that comes through the speaker is when I touch a probe to the #3 pin of the output tube - I can hear some scratchiness. No other pops, scratches, whurrs, or spits when touching the probe to any other pins on any other tube.

Here are my voltage readings with JJ 6L6:
Power Supply String 30/22/22/10uf
A - 380v 30/22uf
B - 278v 22uf
C - 272v 10uf

V1 - 6SL7
Pin#1 Grid - 0v
Pin#2 Plate - 126v
Pin# 3 Cath. - 1v
Pin# 4 - Grid - 0v
Pin# 5 - Plate - 122V
Pin# 6 - Cath. - 1.1v

V2 - 6SL7
Pin#1 Grid - 0v
Pin#2 Plate - 170v
Pin#3 Cath. - 1.55v
Pin#4 Grid - 170v
Pin# 5 Plate - 274v
Pin# 6 Cath. - 171v

V3 - 12AX7 (PI)
Pin#1 Plate - 276v
Pin#2 Grid - .1v
Pin#3 Cath. - 4.8v
Pin#6 Plate - 277v
Pin#7 Grid - .1v
Pin#8 Cath. - 4.8v

V4 - 6L6 Power tube
Pin#3 Anode - 392v (out to OT) *scratchiness heard when touched by probe.
Pin#4 Screen - 379V
Pin#5 Grid - .1v
Pin#6 Anode - 380v
Pin#8 Cath. - 61v (pin#8 tied to pin#1)

Here are some gut shots:


[img:640:480]http://s25.postimg.org/lh2e0s51r/110.jpg[/img]

[img:640:480]http://s25.postimg.org/xwz3uiydr/111.jpg[/img]

[img:640:480]http://s25.postimg.org/a79o5u00f/112.jpg[/img]

[img:640:480]http://s25.postimg.org/gz038oozz/113.jpg[/img]

[img:640:480]http://s25.postimg.org/72z091j7z/114.jpg[/img]

Here are some shots of the top side of the chassis with view of the control layout and Transformers:

[img:640:360]http://s25.postimg.org/di2mjgfbj/107.jpg[/img]

[img:640:360]http://s25.postimg.org/ovp5unpu7/108.jpg[/img]

[img:640:360]http://s25.postimg.org/64n8khv9r/109.jpg[/img]

[matt h]

(deleted)

[JazzGuitarGimp]

I don't understand why you've put a LTPPI into a single-ended design? Obviously, I am missing something.

[matt h]

(deleted)

[Jana]

61 volts on the cathode of the 6L6 is way off. The tube is probably shut off. Something is wrong with the 6L6 tube circuit or the tube itself.

[funkgang49]

All that being said, if I'm reading what I believe to be the cathode resistor properly at 500ohms, you're biasing that tube *awfully* hot. Like, over dissipating by quite a large margin. As a 500ohm resistor does seem like an appropriate value, this is making me think that there's a wiring error near the grid of the power tube.

Right you are Matt! I forgot to put in a 220K resistor from power tube grid to ground in place of the non-connected MV - DOOOH!

AAAaaah-aaaah, the angels sing, the clouds part and the sun shines - there is SOUND!

Lou, for the same reason it was suggested, used, and successful in the GULSEKT88. It's my pet theory, and I've yet to be proven wrong, that the biggest (and perhaps most overlooked) part of the reason most SE designs sound like toys and don't have the feel the big boys do is that most people forego the PI. If the big amp sound you're chasing was an amp with an LTPI, having a differential amplifier in the amp tends to be a big part of that. Want to include a presence circuit like a big amp? It's a handy place to do it, as well.

Exact-o-mundo! One of my main goals with this amp was to get big amp sound from a small SE amp. Also to have enough umph to drive 6L6 and bigger output tubes to their full potential namely KT-66, KT-77, KT-88, etc.

61 volts on the cathode of the 6L6 is way off. The tube is probably shut off. Something is wrong with the 6L6 tube circuit or the tube itself.

Power tube Cathode reads 20v - I'm in the right ballpark now, however a new issue has arisen:

My power supply voltages are all low by about 100V.
Readings are as follows:
At standby switch (choke input lead) - 280v
1st Filter cap (30uf) - 292v?
After 3K dropping resistor 2nd filter cap (22uf) - 277v

Power tube 6L6:
Pin#3 - 285v
Pin#6 - 277v
Pin#8 - 20v

The amp sounds fine just not putting out nearly what it should be in overall volume. Wattage works out to only 10.3 watts.
I checked all my connections and re-flowed all the solder joints but readings remain the same.
Clearly I'm missing something...

[JazzGuitarGimp]

I'm the worst person in the world to give you advice from looking at photos for a wiring error. If there's absolutely no sound except for touching the plate tap, that suggests a wiring error. Jacks are the most common culprits.

All that being said, if I'm reading what I believe to be the cathode resistor properly at 500ohms, you're biasing that tube *awfully* hot. Like, over dissipating by quite a large margin. As a 500ohm resistor does seem like an appropriate value, this is making me think that there's a wiring error near the grid of the power tube.

You mentioned having a master volume planned but not implemented. I'd make sure that you have a grid-to-ground reference resistor on the grid of the 6L6GC. I imagine, though perhaps incorrectly assuming here, if so, apologies, that the MV was to be a post-PI type? If this is missing, it will easily create sound problems. (it would be like the volume control set to "off"). It also could create a floating grid condition, which could be causing excess current draw, thus leading to the rather insane (40some watt) tube dissipation the numbers are giving.

But really, your best bet is to draw yourself a schematic and then use the "divide and conquer" technique. I always build backwards (power supply, power amp, pi, preamp) and test as I go. Not build the whole thing and then power on and scratch my head. It cuts down on the number of surprises and head-scratching moments.

(I've also run the pre of an amp i was building into the effects return of a known working amp to confirm).


Also, your pin descriptions (if not voltages) caused a headcratching moment for me. Pin 6 is being used as a tie point for the screen resistor being fed to pin 4, yes? It's not an "anode". (the plate is your tube's anode!)

If so, there's only 1V of drop across your screen resistor. Depending on the value of the resistor (again, no schematic? no clue.) This could be a point of concern.

Got it. Thanks, Matt. So, it sounds like only one of the two PI outputs is used then - in other words, you don't use an interstage transformer with the primary driven by both of the PI outputs, right?

[JazzGuitarGimp]

All that being said, if I'm reading what I believe to be the cathode resistor properly at 500ohms, you're biasing that tube *awfully* hot. Like, over dissipating by quite a large margin. As a 500ohm resistor does seem like an appropriate value, this is making me think that there's a wiring error near the grid of the power tube.

Right you are Matt! I forgot to put in a 220K resistor from power tube grid to ground in place of the non-connected MV - DOOOH!

AAAaaah-aaaah, the angels sing, the clouds part and the sun shines - there is SOUND!

Lou, for the same reason it was suggested, used, and successful in the GULSEKT88. It's my pet theory, and I've yet to be proven wrong, that the biggest (and perhaps most overlooked) part of the reason most SE designs sound like toys and don't have the feel the big boys do is that most people forego the PI. If the big amp sound you're chasing was an amp with an LTPI, having a differential amplifier in the amp tends to be a big part of that. Want to include a presence circuit like a big amp? It's a handy place to do it, as well.

Exact-o-mundo! One of my main goals with this amp was to get big amp sound from a small SE amp. Also to have enough umph to drive 6L6 and bigger output tubes to their full potential namely KT-66, KT-77, KT-88, etc.

61 volts on the cathode of the 6L6 is way off. The tube is probably shut off. Something is wrong with the 6L6 tube circuit or the tube itself.

Power tube Cathode reads 20v - I'm in the right ballpark now, however a new issue has arisen:

My power supply voltages are all low by about 100V.
Readings are as follows:
At standby switch (choke input lead) - 280v
1st Filter cap (30uf) - 292v?
After 3K dropping resistor 2nd filter cap (22uf) - 277v

Power tube 6L6:
Pin#3 - 285v
Pin#6 - 277v
Pin#8 - 20v

The amp sounds fine just not putting out nearly what it should be in overall volume. Wattage works out to only 10.3 watts.
I checked all my connections and re-flowed all the solder joints but readings remain the same.
Clearly I'm missing something...

Doe the PT have enough B= current capacity to drive the amp? Is it getting hot?

[TUBEDUDE]

10 watts out of a 6L6 SE, °what's to complain about?

[TUBEDUDE]

10 watts out of a 6L6 SE, °what's to complain about?

[TUBEDUDE]

10 watts out of a 6L6 SE, what's to complain about?

[TUBEDUDE]

10 watts out of a 6L6 SE, what's to complain about?

[funkgang49]

Doe the PT have enough B= current capacity to drive the amp? Is it getting hot?

It should. In it's previous incarnation (SE version 1)I was getting 385 - 400v at power tube plate depending on output tube. PT: 315-0-315 B+

Nope. Not gettin' hot at all.

[matt h]

(deleted)

[armillary]

If you put the choke before the first filter cap you will get a very large drop in B+. If you put the choke between the first filter cap and the second filter cap you will get a very small drop in B+. It sounds like you have it before the first filter cap.