Tube Rectifier resistor/choke question

[flintrock]

Bumping this thread up, need a little more help, thanks in advance. I have been trying to eliminate what seems to be blocking distortion, that fuzzy distortion thats cutting out the louder you turn the volume knob. If I juice the input with a dirt pedal it behaves better but maybe the distortion pedal is just covering up the sound. I replaced the signal in wire to V1, reduced the input resistor from 68k to 33k, made no difference. I also upped the grid stoppers on the power tubes to 15K from 5.6K also made no difference other than cutting out a some of the highs. Swapped out the .1uf in the phase inverter for a .022, made it worse so I put the .1 back in. Feedback loop was positive so I flipped the OT primaries around and that is fixed with a 47K in the loop. Rechecked the bias, plate voltage is at 350V, screens 323V, running 22.5 and 21ma on the 2 6V6''s which is right in line. This amp is driving me nuts!!

[pdf64]

That design seems to have mitigated for most of the normal causes of blocking distortion; are you sure that's what it is, could you upload a recording?
The Vol 1 channel should be more resistant to it than Vol 2.
It may be the B+ sag that you don't like, if so consider linking across the 5k resistor feeding the screen B+ node.
Pete

[flintrock]

Yes its not as apparent on the normal channel, but still there. I'll try to get a recording up tonight as well as try jumping that resistor.

[flintrock]

Jumping the 5K resistor did not fix the problem. I am using a tube rectifier in place of the SS diodes as drawn on the Huss Plexi schematic. Here is a sound clip I captured tonight.

https://soundcloud.com/politikillpunk/amp

[flintrock]

I think I just might reflow the entire board. There are some wires on the bottom side I want to check too. Maybe rerout some wires while I'm at it. There is quite a few wires near the OT primaries I'd like to clean up. I put the choke on the inside, I wish I didn't as it made the power section a little cramped. It wouldn't be too hard to move topside if its a problem.

[pdf64]

Oh dear, that's not nice!
It sounds like the power tubes are cutting off horribly.
On examination, the bias supply seems to have large resistances and large caps; suggest that you try 10uF caps in place of the 47uFs.
First though, try measuring what happens to the bias voltage when moving from static to cranked conditions.
Pete

[flintrock]

At idle 22.5ma, at halfway it drifts from 24-32ma, fully cranked 32-42ma and fluctuates. Pin 5 on V4 I have -22. I changed the 220k to a 150k previously, I think the bias was off.

[pdf64]

Sorry for not being clear, I meant the bias supply voltage, ie at the node of the grid leak resistors.

[flintrock]

It hovers at -22-23 on pin 5 and doesn't change with volume.

[Jana]

Does it do this if you put the speaker cabinet in another room to isolate the chassis from any vibrations?

What do you see on your scope if you hook the amp up to a dummy load and test with a signal generator? Where in the signal chain does the distortion problem start?

[flintrock]

I've tested it on the bench with a cab 5 feet away on a concrete floor so I don't think vibration is the issue. When I hooked it up to the scope it had a nasty waveform with a signal generator hooked up. I audio probed it with a headphone signal with music hooked up and it seemed to be fine up until the phase inverter. I'm going to do all of this again tomorrow and I'll report back.

[flintrock]

Spent some time on it this morning, what I found is the wave is nice and clean up to the input of V2. Thr peaks are chopped off coming out pin 8.
Pins 1 & 7 - 168V
Pin 6 - 204V

[Jana]

looking at the pictures in your other post, it looks to me like you have an 82K resistor for the cathode resistor for V2 (in parallel with a big yellow cap that I can't see the value). It should be 820 ohms.

The wiring around V2 looks really honky. I would check it over.

What's your voltage at pin 3? It should be about 1 volt, maybe a little less. If that is an 82K resistor you have there, I am guessing your voltage is about 80 to 100 volts at pin 3. Not good. It would sure explain that horrible distortion.

[flintrock]

Yes! Jana very good eyes, I cant believe I did that I missed it as well. I just put in the appropriate 820 ohm (.68uf cap in parallel) and it is ripping now! Sounds fantastic, dead quiet, and loud. Amazing with pedals in front. Thank you all who chipped in on my project, lots of knowledgable people here.

[pdf64]

Ha, that explains the horrid 'cutting off' tone.
The ~90V on the cathode should have rung alarm bells, apologies for missing it.
Pete