18watt stout in AO35 chassis

[dmp]

I wanted to Build a higher gain amp and so started a build in this small hammond chassis. Using the stout turret layout with the hammond ao35 transformers.
Everything is according to the stout schematic except I subbed a 1K resistor for the choke. I have a few other parts to order and upgrade too for power handling.
It's working but I have some crackly distortion that I don't like when the level is transitioning from clean to distorted. And it distorts very early in the volume knob, it's basically maxed at the halfway point. Maybe that's normal? Once it is heavily distorted the crackly noise is gone or covered.

Voltages on the B+ rail are strange too - lower than expected (measured voltages in blue on the attached schematic).
I tried a few different tubes in all positions. The sound is similar through different speakers. More or less noticeable but present in all. 

[dmp]

Pictures

[dmp]

I have a short recording of the noise but it's not allowed to upload. So this is a mp3, rename from pdf to mp3

[Willabe]

If the EL84's are biased too hot they will distort early and will draw too much current that will drop the B+ dcv.

And EL84's don't need much signal to drive them into distortion.

Is your volume pot an audio taper? A linear taper pot will get loud very quickly as you rotate the pot.

[Latole]

I have a short recording of the noise but it's not allowed to upload. So this is a mp3, rename from pdf to mp3

 The schematic with voltages

Lower voltage = more distortion.
How many AC volts on tubes heaters ?
Under 6,1- 6 ; Power transformer is not powerful enough.


 

[dmp]

If my math is right the EL84s are biased at 10.2 W, almost the same as what the stout build doc shows. I could bias them colder as a test.Both pots are audio taper, 500KAI'm not too concerned with excess gain - it is the grainy crackly distortion that I don't like right now. And the low B+ voltages are making me worry the output transformer has a short.

[dmp]

Latole, the schematic with voltages is in the first post stout_voltages.pdf
You can't open it?

[Latole]

Latole, the schematic with voltages is in the first post stout_voltages.pdf
You can't open it?

See my answer, I have the schematic open

I read 6,08 Vac ; pt is under power

[dmp]

I also thought the low heater voltage was strange, especially since the B+ and 5v windings were correct. These AO35 transformers have been used in a ton of 18watt guitar amp builds though and the PT is oversized, not undersized. I don't think -4% should make the amp sound bad, usually +/-10% is the 'OK' range. 
The carmen ghia was built in these same AO35 and I saw a schematic in another thread here with similar B+ voltages to mine. Not sure why the stout schematic has higher voltages.

[dmp]

I probed the amp with a 'listening amp' and the signal is good up to the output of the PI, C8 / C9.  No crackle.
I lowered the bias with R25=150 and the crackle was still present (amp maybe sounded better though)The DCR of the OT are primary 83k & 71k to CT with no resistance to chassis, and 1.2ohm secondary with 86k to chassis [edit: disconnected FB resistor and much higher to chassis, 7 Mohms].  I think I might try to get a different OT unfortunately

[shooter]

the crackle

this is typical of bad soldering joints, sloppy socket pins, possibly a speaker dragging the voice coil

[dmp]

the crackle

this is typical of bad soldering joints, sloppy socket pins, possibly a speaker dragging the voice coil

Thanks for the ideas. I went over all the connections and checked / cleaned / tightened the sockets. I have several speakers and amps and I took a break yesterday to play other amps to give my brain a reset. I don't hear this through the speakers with different amps.
At this point I am pretty convinced it is a characteristic of the AO35 output transformer.  It is highlighting high frequencies I don't like.
I swapped a stancor 6v6 (5k) PP output transformer I had into the amp and the crackle was gone. Amp sounded fantastic. With this hyperfocus though, I could still hear a tiny bit of what I was hearing before though, it was just really subtle. I then put the AO35 transformer back in, confirmed the noise was there again (it was) and installed the RC filter on the primaries that is in the AO35 (and Carmen Ghia 'conjunctive filter'). This really tamed the crackle noise. So I don't this transformer is for me, going to order the Heyboer.I still don't understand why I am not matching the Stout B+ voltages.

[bmccowan]

Nice job dealing with that tiny chassis. I tried - found it too crowded and robbed the iron and a few parts for another chassis.

I still don't understand why I am not matching the Stout B+ voltages.

It's a different PT, eh? The B+ voltage I see people getting from the AO-35 PT with a 5Y3 are about 300, not the 350v in the Stout.

[dmp]

Nice job dealing with that tiny chassis. I tried - found it too crowded and robbed the iron and a few parts for another chassis.

Yes, I know what you mean. I've worked on tight PTP amps that were very difficult. I made the turret board to make it easy to mod & experiment once it was done.

It's a different PT, eh? The B+ voltage I see people getting from the AO-35 PT with a 5Y3 are about 300, not the 350v in the Stout.

I guess it must be?  The Stout layout shows 300VAC but it uses the Heyboer 18 watt power transformer, which is spec'd at 290vac at 125 mA.  Tubes are drawing 88mA. I still don't understand why the stout has voltages 20% higher 
But I now think my voltages are OK and am going to move on. Ordered a Heyboer OT this morning.

[DummyLoad]

Did you try another rectifier type? 5V4? 5AR4? 5R4?

--Pete

[dmp]

I can try that once I get the new OT in.  A different rectifier could give higher B+ (I'm not sure the amp needs higher B+, just strange the stout has higher B+)

[dmp]

With the Heyboer 18w output transformer the amp sounds great

[dmp]

Made a case for it.
I'm going to want to add switches for different features (feedback, fat, gain stage, etc...)I already tried bypassing the gain stage and really like that option. And turning off the negative feedback is really nice. Basically reverting it to the 1974 Marshall schematic.

[dmp]

With a 5V4 rectifier the B+ is 337v, good suggestion