Ugly distortion - Solved!

[johnnyreece]

Hey, all! (Schematic and Layout updated 4-22-14 with fixes implemented)

I've got my cake pan amp up and running, but I'm getting an ugly distortion...Here's a clip of the sound:

https://soundcloud.com/john-lamb-25/amp-distortion

In short, it's basically a Lite IIB preamp/PI with a 6SN7 in P-P (cathode biased) for a power section. I've received some advice from folks on another forum, but I figured I'd expand my search. Here's what I've tried:

-Different speaker (thought was it sounded like voice coil rub)
-Bias adjustment
-Different OT taps
-Separating cathode on V1
-Different guitar (thought it might be a fretting-out issue)
-Chopstick for cold solder joints
-Tube rolling

Nothing really changed the distortion dramatically (although, it sure sounds ugly using different OT secondaries...yuck).

Schematic:

[IMG773]http://i218.photobucket.com/albums/cc165/moto_xxx199/Amp%20designs/Octalpus30_zps20326cec.png[/img]

Layout:

[IMG740]http://i218.photobucket.com/albums/cc165/moto_xxx199/Amp%20designs/OctalpusLayout3_zps93838977.png[/img]

Pics (although, I DO have knobs now. Fancy ):


[IMG768]http://i218.photobucket.com/albums/cc165/moto_xxx199/Amp%20designs/49323D02-8F22-47CE-9F57-582365748E3F_zpsqgifrmzn.jpg[/img]

[IMG768]http://i218.photobucket.com/albums/cc165/moto_xxx199/Amp%20designs/F541F4ED-DF87-4453-9DCB-29E23EBD0B19_zpsn4aci5rm.jpg[/img]

[IMG768]http://i218.photobucket.com/albums/cc165/moto_xxx199/Amp%20designs/6F38E103-B520-42CF-A84F-9E1DB0937DF3_zpsh1y5dnit.jpg[/img]

[IMG:768:1024]http://i218.photobucket.com/albums/cc165/moto_xxx199/Amp%20designs/F6BF83CA-0B3A-47FE-B242-409913CC3017_zpsynhse4ak.jpg[/img]

I don't have access to a scope, or I'd try to track this down myself. Unfortunately (or fortunately), I don't have much experience with bad sounding distortion to really venture a guess at what is going on. I appreciate any ideas you guys might have!

[xtian]

Can you measure voltages and report?

[Phil_S]

Here's a WAG or 3 for you. One thing at a time, of course, or you won't know what results in what.

You may be driving that power triode too hard.

Here are some things I'd try. I'd do it in this order:

1) Change the cathode bias resistor from 360 to 1K.

2) Reduce the grid leaks from 220K to something less, as low as 100K. For starters, tack in parallel 220K resistors, to give you 110K. Then, readjust coupling caps to the same R/C constant (.002u/220K is the same as .001u/110K).

3) Step up the grid stoppers from 2.2K to 5K.

My somewhat limited experience with the 6SN7 as a power tube lead me to choose a concertina over the LTP and paraphase inverters. I tried all three. The LTP and paraphase sounded bad. Of course, your design is very different from what I tried.

Good luck.

[johnnyreece]

Can you measure voltages and report?

From memory...I know that the Plate-Cathode measurement on the power tube is 340. Plates on the pre/PI I think are around 130/170-ish. Around 13 on the power tube cathode resistor. More solid measurements when I get home and can smack it around a little. Any voltages in particular?

[johnnyreece]

Here's a WAG or 3 for you. One thing at a time, of course, or you won't know what results in what.

You may be driving that power triode too hard.

Here are some things I'd try. I'd do it in this order:

1) Change the cathode bias resistor from 360 to 1K.

2) Reduce the grid leaks from 220K to something less, as low as 100K. For starters, tack in parallel 220K resistors, to give you 110K. Then, readjust coupling caps to the same R/C constant (.002u/220K is the same as .001u/110K).

3) Step up the grid stoppers from 2.2K to 5K.

My somewhat limited experience with the 6SN7 as a power tube lead me to choose a concertina over the LTP and paraphase inverters. I tried all three. The LTP and paraphase sounded bad. Of course, your design is very different from what I tried.

Good luck.

Sorry if I've not updated...the cathode bias resistor actually is 1k. I'll definitely try the other two, though! It's too bad...it really sounds pretty good other than that bit of nasty...I was pleasantly surprised when I switched it on the first time and it actually worked...This last little bit is just confounding...

[Phil_S]

You may benefit from adjusting the plate load resistor and cathode resistor to increase gain (and maybe plate voltage, too.) It looks to me like you've dressed both V1 and V2 as if they are 12AX7's. The 6SL7 has about half the internal plate resistance (44K vs 82.5K) and 70% of the gain.

I would be disinclined to ramp up on the LTP PI to feed a 6SN7. I might look at raising the value of the tail resistor a bit from 15K to see if that makes a difference.

[johnnyreece]

You may benefit from adjusting the plate load resistor and cathode resistor to increase gain (and maybe plate voltage, too.) It looks to me like you've dressed both V1 and V2 as if they are 12AX7's. The 6SL7 has about half the internal plate resistance (44K vs 82.5K) and 70% of the gain.

I would be disinclined to ramp up on the LTP PI to feed a 6SN7. I might look at raising the value of the tail resistor a bit from 15K to see if that makes a difference.

Yes, I used the 12AX7 values, as my brief research said those values would be close enough. If that could be causing the noise, though, I'll hack away.

[Phil_S]

No, I don't think 12AX7 values are causing the noise. I just think it leaves you with an underpowered preamp with less headroom. There is a remote chance you are hearing something from the preamp. I think the noise you here is from the way the PI interacts with the power tube.

[johnnyreece]

Okay, here are some voltages (measured to ground):

After rectification - 353
1st filter node - 352
2nd filter node - 320

V1 - both triodes - Plate - 118; Cathode - 1.5
V2 - 1st triode - Plate - 195; Cathode - 1.2; Grid - 24.2
V2 - 2nd triode - Plate - 187; Cathode - 1.2; Grid - 26.6
V3 - both triodes - Plate - 360; Cathode - 13.95

Anything look out of whack? I'll attempt some of the mods when I get a spare minute. It's story time for my six year old right now.

[Merlinb]

I would start by adding a grid stopper to the LTP, and biasing it a bit cooler. A hot-biased LTP can lead to some pretty ugly blocking and crossover distortion in a power stage, and I would think it would be even worse when the power stage is a very sensitive little valve.

(Grid stoppers are probably your number one way to tweak the 'quality' of distortion. Don't be afraid to make them bigger.)

[johnnyreece]

Thanks so much for chiming in! I'll try a few of these things when I get home and see what happens. I really do like the sound of it, so I'm hoping we can save the good parts and just get rid of the bad. If this one turns out well, I've got a little SS combo that's just dying to be gutted and converted to something that isn't awful.

[johnnyreece]

O-kay...Here's the latest list of things I've changed:


-PI tail increased to 48k
-Grid leaks/coupling caps in 1/2 (paralleled two resistors, had two identical caps series). I did undo this to steal the 220k's for the next step
-220k grid stoppers on PI.
-"input" coupling cap on the PI changed to .0047uF (saw it on Aiken's site; worth a shot, right?)
-disconnected 1 preamp triode grid (ugh...sounded like a blanket thrown on top of the amp...terribad).

Just realizing I didn't change the cathode resistor on the PI yet, nor have I upped the grid stoppers on the power section. Maybe tomorrow...So far, no improvement. I'm gonna keep hammering on this thing until it's right, or until it becomes something else. Either way, that buzz is gonna die!

[teemuk]

Does the circuit work as intented, no oscillations, no excessive blocking distortion, etc...?

If yes, then you just don't like the sound of the circuit. Personally I have never heard any of these low-ish power, flat-ish response tube amps sound too impressive. Any higher amount of distortion always sounds like muddy bag of farts to me.

[katopan]

I did a 6SN7 push-pull amp years ago and tried out various preamps in front of it. Tried out a lot of the 18 Watt preamps including the Lite IIb and Superlite TMB. Also 2204, Wreck and others. At first I thought wow, but as time went on I noticed and then couldn't not notice a buzziness to the distortion. So many people loved the Firefly (although I had a full LTPI) and the 6SN7 was delivering more bottom end than a 12AU7, I couldn't work out what was wrong. I found the part of the distorted tone that I didn't like got much worse the more the power section was overdriven. I decided that I didn't like the tone of triodes for power valves and went on to build a 2W subminiature version of the 18 Watt Superlite TMB. Back to pentodes and that's a low power push-pull amp that just rocks. Teemuk might have a point.

Having said that I don't remember it sounding like your clip so I'm not sure. My B+ was about 300V. I also can't remember what cathode resistor I used but the load lines I've got on file (which I might've changed once built) suggest I had a 1K shared cathode resistor.

[Littlewyan]

It could be blocking distortion caused by the coupling caps charging up. Increase the size of the output stage grid stoppers or try the Paul Ruby Zener Mod. http://www.18watt.com/storage/18-watter ... fo_311.pdf

Also if your generally getting too much bass I'd start by reducing the size of the first coupling cap in the pre amp.