Uberschall Build - Blocking Distortion!

[craigmillard]

Hi Guys,

I have been building a pedal version of the Bogner Uberschall as seen here:



but without the Mods, stock!

Im getting really bad blocking distortion if the gain is turned above 75% and i cant get rid of it!!
Dropping the coupler caps down to 1.5nF helps but it its still there..
How did this preamp work like this in the real amp?
What am i missing??

Please help:)

Cheers
Craig

[J0K3RX]

So, you are building the actual tube version in a pedal enclosure?

[craigmillard]

Yup, compressed it down to a 1590dd, sort of a preamp stack thing im working on.. Will publish once im happy with it:)

Im not getting any squealing and it sounds great until the gain is above 75%, i cant see how this doesn't happen in the real thing.. unless this schemo is wrong but its the same as all the rest on the interwebs..

I get farty distortion and if i strike the strings real hard i can almost cut all sound!

c4, c8 are now 1.5nF which helped a bit but took away some of the low end.. thinking the 68k/68k network need to be altered to drop more??

[seedlings]

Try increasing R6 to as high as 470k, and try adding another grid resistor from the junction of R11/R13 to the grid of V3.  That one can be up to 470k as well.  One or both of those will make a world of difference, without changing gain.

CHAD

[craigmillard]

Cheers Chad,

I was thinking of upping the grid resistors tonight, i did add 470k after R6 but didn't make much difference while debugging. Will try the next stage too though..

What i don't understand is how this works, without blocking distortion, on the real deal.. lol

[seedlings]

Cheers Chad,

I was thinking of upping the grid resistors tonight, i did add 470k after R6 but didn't make much difference while debugging. Will try the next stage too though..

What i don't understand is how this works, without blocking distortion, on the real deal.. lol

Could be layout... could be that 'someone' allowed a schematic with known errors to be released... you just never know!

CHAD

[craigmillard]

Can layout have that much impact on blocking distortion then? I thought layout was more oscillations and feedback etc.
As i understand blocking distortion is caused by the RC networks and time to recover to centre bias than layouts..

[seedlings]

Can layout have that much impact on blocking distortion then? I thought layout was more oscillations and feedback etc.
As i understand blocking distortion is caused by the RC networks and time to recover to centre bias than layouts..

Yes, I agree - the layout probably doesn't affect blocking distortion.  Too much clipping, at a high voltage can cause the grid to go positive (or otherwise affect bias), so the tube can sound like a mis-biased transitstor (at least that's how my mind understands blocking distortion).

What I did not say clearly was that the layout/circuit that is publicly available might have errors.  This is a complete wild guess.  With 3 gain stages, and only one 50% voltage divider, there will probably be blocking distortion.  Although, it could be why they needed a small 2.2n coupling cap.

CHAD

[wavley]

Do you have the resistors on the tube socket itself?  It's really important to do this, especially on high gain amps.  I can't tell you how many amps I've fixed this on by moving the resistor from the input jack to the tube socket.  There is a very great Aiken article on this and how the Miller Capacitance is nearly as important as the value.  I was really happy when I recently got a Cornford and opened it up to find that they had done just about all of the "good practice" things like that.

Can layout have that much impact on blocking distortion then? I thought layout was more oscillations and feedback etc.
As i understand blocking distortion is caused by the RC networks and time to recover to centre bias than layouts..

Yes, I agree - the layout probably doesn't affect blocking distortion.  Too much clipping, at a high voltage can cause the grid to go positive (or otherwise affect bias), so the tube can sound like a mis-biased transitstor (at least that's how my mind understands blocking distortion).

What I did not say clearly was that the layout/circuit that is publicly available might have errors.  This is a complete wild guess.  With 3 gain stages, and only one 50% voltage divider, there will probably be blocking distortion.  Although, it could be why they needed a small 2.2n coupling cap.

CHAD

Layout has everything to do with oscillation AND blocking distortion.  Controlling bass will also really help, and grid resistors as high as 470k, but it's very important that they be as close to the actual grid with as short of a lead as possible.

[craigmillard]

Currently i have R2, R6 and R17 on the Socket with no lead. These are the only two dedicated grid stoppers in this circuit are they not?

I just find it strange that i have to modify this schematic to get it to work

[wavley]

Currently i have R2, R6 and R17 on the Socket with no lead. These are the only two dedicated grid stoppers in this circuit are they not?

I just find it strange that i have to modify this schematic to get it to work

Yeah, but it wouldn't be the first time... go look at the Ludwig thread.

When I was in pro audio repair most of the warranty work I did was bringing devices up to the current revision of the schematic and service manual work arounds for things that weren't properly tested at the factory.  The Mackie 1604 was on rev H of ribbon connectors when I left 8 years ago, there is a whole lot of after the fact repairing that goes on.

Try increasing R6 to as high as 470k, and try adding another grid resistor from the junction of R11/R13 to the grid of V3.  That one can be up to 470k as well.  One or both of those will make a world of difference, without changing gain.

CHAD

Plus one on this, I would try something right after the divider.

You might try an audio probe and see which stage is blocking for a better idea of where you should be concentrating your efforts.

Also, sometimes components make a difference, even when they're the same value, I run into this a lot in the GHz work I do, especially at cryogenic temperatures.  I know that's an extreme example, but I remember a particular amp (I think it was a Fender Tonemaster) at the shop that it's design was so on the hairy edge of working that there was a very specific set of parameters for power tubes with which it would work, anything else would send it into wild oscillation when using the as designed stock circuit... even with the same brand of 6L6's. 

[craigmillard]

Well i have had a bit more of a play and added in 2 grid stoppers one at the second stage tube grid and one at the 3rd stage, started both at 470k and still had the distortion, up'd to 1M on both and it is still there but a little better!!

Would altering r6 and R7 or R12 and R11 to drop some more voltage make any difference? im stumped!


 

[seedlings]

Might not be blocking distortion.  Are R46, R7, R13, R18 (grid resistors) grounded well... maybe a cold solder joint?

CHAD

[craigmillard]

yer they all look good and I re-flowed them for good measure:)

Pretty sure it is blocking distortion as it sounds like a gated transistor very farty and fades back in as the cap comes back etc...

Would dropping more voltage help? im happy to deviate a bit from the schem if necessary now

[seedlings]

yer they all look good and I re-flowed them for good measure:)

Pretty sure it is blocking distortion as it sounds like a gated transistor very farty and fades back in as the cap comes back etc...

Would dropping more voltage help? im happy to deviate a bit from the schem if necessary now

Good.  Could it be something that comes after this effect in the signal chain?

CHAD

[craigmillard]

This is purely guitar to preamp to mini SS amp on bench, so nothing else to contend with..

I have noticed if i switch to single coils it improves a bit too, i have quite hot pickups Semour Duncan JB's..

[wavley]

yer they all look good and I re-flowed them for good measure:)

Pretty sure it is blocking distortion as it sounds like a gated transistor very farty and fades back in as the cap comes back etc...

Would dropping more voltage help? im happy to deviate a bit from the schem if necessary now

Good.  Could it be something that comes after this effect in the signal chain?

CHAD

This might be a great place to look.  For example: if you're pushing the front end of a tube amp with this, with it's high gain/high output, it might be your amp that is suffering from blocking distortion now.  

Also, I didn't scroll over to see that V6, does it have a grid stopper on it's socket?  

[wavley]

This is purely guitar to preamp to mini SS amp on bench, so nothing else to contend with..

I have noticed if i switch to single coils it improves a bit too, i have quite hot pickups Semour Duncan JB's..

Oops, I was posting while you were posting this.

[craigmillard]

the fartyness stays even with the volume turned right down on the preamp.. V6 doesnt exist in mine as that is a suggested mod to the factory default schemo, mine ends at the tonestack and vol.

[wavley]

Maybe try 4.7nF for C8, it's what's in my Cornford and it's the lowest setting of a FAC control in an Orange.