News:

Smart Search:  If you're...
-not in a board, it will search the entire forum.
-in a board, it will search just that board.
-viewing a topic, it searches within that topic.


Likes 19 Likes

Spencerleehorton

Re: LA-2A point to point build Sowter TXF
« Reply #80 on: January 05, 2015, 01:25:32 PM »
Hi Dave,

I have an old oscilloscope so that's no problem., just so I'm sure with this.
Am I aiming for the voltages on the schematic or have they all changed now I'm using the ecc81?
And your saying my voltages look right with changing 220k to 100k.
Now I should change R17 to 22k pot, but what should I be looking for exactly?
Should I put scope on output and trim 22k till I see wave reduce in distortion?

Regards

Spence


DaveP

Re: LA-2A point to point build Sowter TXF
« Reply #81 on: January 05, 2015, 03:33:30 PM »
Your voltages are all in the right ballpark so you have the same headroom as the original.

Turn R2 down to zero, then put a 1kHz sine wave through the amp and a 680 ohm or similar on the output,  sub your 22k pot for the 10k res and set to 10k.  Then note the height of the sine wave on the scope as you wind up the input.  When it starts to flatten, try adjusting the pot back and forth to see if the sine wave sharpens up again, adjust for best shape and maximum height.  Then carefully remove the pot and measure the resistance, find the nearest value and solder that in.

Now start to wind up R2 and see if the waveform changes.  Then do the same with the next amp and compare outputs.  This exercise will  show up any problems in the output stages.  This is how you track down problems with a careful process of elimination.  Keep a note and post any differences between the amps.
best
DaveP
Soundcloud: Delayed Action.

Spencerleehorton

Re: LA-2A point to point build Sowter TXF
« Reply #82 on: January 05, 2015, 04:00:37 PM »
Hi Dave,

That's fab thanks, I will try this all out tomorrow and report back with the results.

Regards

Spence.

Re: LA-2A point to point build Sowter TXF
« Reply #83 on: January 19, 2015, 04:18:00 PM »
I am not getting any response from the "everything LA2a thread" so I thought I would post here. I have a problem with my P2P LA2a. All tubes work and the voltages below. I am having oscillation issues with the unit. The meter just bounces when in +10 mode. I do not have any grid stoppers installed and using a Sowter in place of T1 (HA100X). In +10 mode the meter just jumps. Gain reduction position and the meter does not go to zero. Cannot get sound to pass thru unit. Also the gain reduction does not function on the meter.

C7C - 191V
C7D - 115V
C7B -262V
R29 - 350V
R34 - 262V
Across NE - 52V

12ax7#1
Pin 1 - 98V
3- 1.0V
6 - 100V
8 - 1.03V

12BH7A
Pin 1 - 109V
3 - 3.5V
6 - 209V
7 - 31V (*) not normal according to what I read. Before R15 voltage is 109
8 - 113

12ax7 #2
Pin 1 - 100V
3 - 0.7V
6 - 100V
8 - 0.8V

6AQ5A
Pin 2 6.8V
5 - 118V
6 - 107V

Any thoughts?

Spencerleehorton

Re: LA-2A point to point build Sowter TXF
« Reply #84 on: January 20, 2015, 05:55:39 AM »
Hi,

I will investigate.

Spence.

DaveP

Re: LA-2A point to point build Sowter TXF
« Reply #85 on: January 20, 2015, 08:09:16 AM »
Bonsai,

The 12 BH7A is biased to cut-off with 31V on it!  It should be within a few volts of the cathode voltage, say 107V.  This is why you are not passing audio.  You have a mis-wire somewhere so you need to follow it through very carefully on the schematic.  Best way to do this is, to print off a clean copy and highlight the lines after you have checked each one.  That way you can see which ones you have done and what's left to do.  R15 should only contact the grid and C2, nothing else at that end.

You may have attached it to R11 or R22 making a nasty feedback loop, hence bouncing.  You have to cure that before we can help further.

The cathodes of V5 are supposed to be tied together and the same voltage.  The neon is about 10 volts too low which is probably why it won't zero.  Get the right neon or put 2x 33V zeners in series instead.  Otherwise all the voltages are correct.
best
DaveP
Soundcloud: Delayed Action.

Re: LA-2A point to point build Sowter TXF
« Reply #86 on: January 20, 2015, 11:29:18 AM »
I am using Caycosta's Layout. Just to be ultra stupid I have V1 (6) --> C2 ---> R15 and V2 (7). Then R15 --> R18--> V2(1). Hope that makes sense.  Thank you. Will try to review wiring tonight and get back to you with results.

Daniel
« Last Edit: January 20, 2015, 12:05:48 PM by Bonsaimaster »

Re: LA-2A point to point build Sowter TXF
« Reply #87 on: January 20, 2015, 06:39:51 PM »
I did not have R26 connected to GND. That seemed to fix the meter problems however it is still oscillating. Still checking all the other connections but cannot find a problem. I originally used a canister capacitor. I may replace that for more traditional ones.
« Last Edit: January 20, 2015, 08:16:53 PM by Bonsaimaster »

Spencerleehorton

Re: LA-2A point to point build Sowter TXF
« Reply #88 on: April 09, 2015, 01:24:59 AM »
Finally got hold of some Sowter 4257 transformers, going to try them on the input.
Anyone have any thoughts on theses?

regards

Spence.

Spencerleehorton

Re: LA-2A point to point build Sowter TXF
« Reply #89 on: April 09, 2015, 02:22:38 PM »
All i can say is wow!!!! what a transformation, loads of bottom end now and sounds really tight.
very happy with the sound, still got the OEP on the output at the moment.

regards

Spence.


 

Related Topics

Subject / Started by Last post
Last post October 31, 2008, 07:52:57 PM
by Kingston
Last post December 28, 2008, 11:14:13 PM
by kb
Last post December 17, 2014, 02:23:44 AM
by barbaroja
Last post December 16, 2014, 10:40:57 PM
by OPR
Last post June 06, 2014, 01:43:04 PM
by duantro