New express rebuild - blocking distortion?

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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MikeR670
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New express rebuild - blocking distortion?

Post by MikeR670 »

Hi all, I rebuilt my express prototype into a new chassis. Redid everything including boards, it fires up but is very low volume and bad blocking distortion type sound.

Based on the Kelly 90 schematic from this forum.

Hum is ok, it will play but cuts out on notes and this weird blocking distortion that fades the notes away.

I've checked for good voltages, I get pops on all grids when probing, swapped tubes out too. Put a 68 k input grid resistor in, helps a tiny bit.

If I disconnect the NFB loop it stops, but then there's no sound at all. I tried changing the NFB resistor, smaller makes no difference, larger helps a little - but still there.....any ideas?
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xtian
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Re: New express rebuild - blocking distortion?

Post by xtian »

Sounds like what happens when you don't have grid referenced to ground in a preamp stage. Check those carefully.
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MikeR670
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

Could you guys listen to this sound sample? Warning: it's ugly :roll:



Ok checked the grid reference resistors, seems good -[/mp3] although I don't see a grid reference in the schematic for v2. Unless it's the 100k resistor before the tone stack, which I guess eventually goes to ground via the stack.

Is it blocking distortion/ crossover? Seems strangled too...speaker is 8 ohm and I'm connected at 8 ohm tap.

Hope I uploaded the file right, still too dumb to place it on the page lol
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pompeiisneaks
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Re: New express rebuild - blocking distortion?

Post by pompeiisneaks »

It can be confusing, but you need the 'link' to the file and then the mp3 tags like so:



i.e. remove the spaces I'm adding between the[ and the letters (as well as the :// so it doesn't make it a real link).

[ mp3 ] http : / / ampgarage.com/forum/download/file.php?id=47078 [ /mp3 ]

your's had something like

[ mp3 ] [ attachment ] somethinghere [ /attachment ][ /mp3 ]
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pompeiisneaks
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Re: New express rebuild - blocking distortion?

Post by pompeiisneaks »

Sounds almost to me like something's almost at cutoff... biased too cold? Maybe not?

Edit: in fact the idea that removing NFB makes it go silent is reinforced by that. You likely have it on the wrong phase so you have PFB (Positive FeedBack) and with that extra boost it gives it just enough to actually sound okay, otherwise it just dies out? maybe anyway.

Make sure the power tubes aren't almost totally biased way too cold.

Just a wild guess.

~Phil
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xtian
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Re: New express rebuild - blocking distortion?

Post by xtian »

pompeiisneaks wrote: Thu May 11, 2017 10:08 pmin fact the idea that removing NFB makes it go silent is reinforced by that. You likely have it on the wrong phase so you have PFB (Positive FeedBack) and with that extra boost it gives it just enough to actually sound okay, otherwise it just dies out?
Good eyes, Phil. That statement was so bizarre it didn't register with me. Mike, is it true? You lift the gNFB connection from the OT and amp goes silent?
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MikeR670
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

Thanks, here's a gut shot of the new build, I tried to tidy things up and work on lead dress more. Changed the layout to suit keeping heater wires away and power lines under boards with short signal wires over boards. It did mean altering the turret board layouts to orient components the right way around. It's getting messy again because I've just substituted some 6.8 k resistors to power tube grids and 1k on volume lead to pin 2 first preamp tube.

I also added a 6k 10 watt dropping resistor before b+1, and a 68k input grid resistor.
rps20170511_154426_528.jpg
With the bias, I put in a pot and the idiot resistor to adjust. Works great, I have anything between -22 and -64 vdc. At 380 volts on plates, online bias calc shows about -60 vdc at 70%. And lower for cooler running. So I'm confused here: if it's set at that level that's a fairly warm bias right? The tubes are not super hot to touch. But I'm maxed out on bias volts available and if I reduce the vdc via the screw pot, say to -45, the tubes get way hotter and start to look a bit red platish......I used the Rob robinette calc. Lower vdc bias seems to me to be running the tube hotter.....?
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MikeR670
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

xtian wrote: Thu May 11, 2017 10:51 pm
pompeiisneaks wrote: Thu May 11, 2017 10:08 pmin fact the idea that removing NFB makes it go silent is reinforced by that. You likely have it on the wrong phase so you have PFB (Positive FeedBack) and with that extra boost it gives it just enough to actually sound okay, otherwise it just dies out?
Good eyes, Phil. That statement was so bizarre it didn't register with me. Mike, is it true? You lift the gNFB connection from the OT and amp goes silent?
Yes that's right - when I first finished it I forgot the NFB loop. And the amp had no sound, no hum, nothing. So I soldered a wire to the 8 ohm ot tap on the speaker jack and to the 100 k resistor on the board. It then produced sound and some hum. I don't have a Presence pot in at all, wanted to keep less things going on to start with.
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Re: New express rebuild - blocking distortion?

Post by pompeiisneaks »

Do you have a scope? Or a listening probe? I'd recommend trying to find out where the signal is dropping to a weak level. You should see it at the input jack, then post the first stage, etc, and that should help you isolate where the signal is going so weak. As for the bias voltage, you need to calculate the actual current to be sure but those ranges do sound acceptable.

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Re: New express rebuild - blocking distortion?

Post by Colossal »

Mike,

It's hard to tell from your photo (not quite hi-res enough), but check that you didn't inadvertently solder in a 470k resistor for the bias resistor of your PI, instead of 470R. This happens sometimes and it usually makes for a very quiet amp. Just thought I'd throw out that idea before you go tearing into it, only to find nothing else wrong.
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Re: New express rebuild - blocking distortion?

Post by xtian »

MikeR670 wrote: Thu May 11, 2017 5:51 pmI've checked for good voltages
Can we start from the top? What are your plate, screen, and grid voltages?
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

pompeiisneaks wrote: Thu May 11, 2017 11:34 pm Do you have a scope? Or a listening probe? I'd recommend trying to find out where the signal is dropping to a weak level. You should see it at the input jack, then post the first stage, etc, and that should help you isolate where the signal is going so weak. As for the bias voltage, you need to calculate the actual current to be sure but those ranges do sound acceptable.

~Phil
I only have a good multimeter. I swapped the OT leads to the tubes, it's a little better. Still has some blocking and ghost sounds alongside. As a test after swapping leads I disconnected the NFB loop. The amp got quieter, but I could still hear it. It still has low volume overall, but louder with NFB on. It's getting more playable......thanks for the ideas!

The plus side of this game is that as it improves, I can hear it has great sustain, note bloom and some possible tone under all the noise and artifacts.......
MikeR670
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

Colossal wrote: Fri May 12, 2017 12:00 am Mike,

It's hard to tell from your photo (not quite hi-res enough), but check that you didn't inadvertently solder in a 470k resistor for the bias resistor of your PI, instead of 470R. This happens sometimes and it usually makes for a very quiet amp. Just thought I'd throw out that idea before you go tearing into it, only to find nothing else wrong.
Thanks, just checked - it's the right one!
MikeR670
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

xtian wrote: Fri May 12, 2017 1:50 am
MikeR670 wrote: Thu May 11, 2017 5:51 pmI've checked for good voltages
Can we start from the top? What are your plate, screen, and grid voltages?
Ok I removed the b+ resistor I added, thought it might be choking things. Nice big sound now, clean and dirty from guitar volume - which was my big goal :D

Voltages with amp on, no standby, volume low:

B+1 510
B+2 506
B+3 460
B+4 430
B+5 403

V2 pin 1 298
V1 pin 6 265
V3 pin 1 388
V3 pin 6 388

6550C tubes:
Pin 3 ot. 505
Pin 4 b+2 screen 499
Pin 5 grid. -62

It is a whole lot better, thank you all, still some ghosting here and there, but it's usable now. I have a practice night tomorrow, I think I'll drag it to the studio and see how it runs. Might change back the screen resistors etc first. Hope these voltages make sense....I tried dropping them because I wanted to use 6v6s on it as I prefer that sound. But it seems like that caused a lot of issues, so I guess I'm back to the big ol 6550s...for now :lol:
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Re: New express rebuild - blocking distortion?

Post by MikeR670 »

:oops: I meant to write

V1 pin 1 298
V2 pin 1 388
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