Ugly distortion with tube amp when you play

[Ampzone]

So someone brought me a ceratone kit amp build to help figure out a problem with nasty distortion when you play or pick even lightly. It will idle fine and has the typical light amp hiss. It is Dr103 Hiwatt style circuit. The voltages preamp bias plates look normal, a chopstick test reveals no loose joints. No DC that I can find on the coupling caps on the preamp plates or cathode. OT check primary under 20ohms.

My audio probe with mini the amp picks up my 100hz test tone, so far I am up to the input grids on pin 5 output section.

Stumped!

[Ampzone]

There no hum so I'm kinda inclined to rule out bad coupling caps

[Ampzone]

PS caps in circuit read well over 100+ in K

[Ampzone]

in addition to the ugly distortion the amp signal is very low and choked off with a guitar plugged. Typical level noise on the input grid on V1 when you touch with a chopstick etc is very reduced.

[The Dude]

Can you post a schematic so we know what we're working with?

[Ampzone]

Can you post a schematic so we know what we're working with?

I found this layout
http://i.imgur.com/6rLTy56.jpg

[bluto]

FWIW I had the same deal on an amp I built from scratch, tuned out to be a cold solder joint (this was at about the time I got old to need reading glasses ;-)

[Ampzone]

FWIW I had the same deal on an amp I built from scratch, tuned out to be a cold solder joint (this was at about the time I got old to need reading glasses ;-)

I know it reminds me of the similar issues in the past, but where is the loose joint if it is this, running through the amp with my homemade audio test probe with a 1k tone shows the signal going through down through the stages.

Just strange with the power almost cut by 2/3rds and super ratty distortion when you play at a light pick attack.

[drewl]

Did it ever work properly?
Could be many things, bad wiring, wrong part, incorrectly installed part, bad connection, etc.

You need to check all the parts, wiring and connections or look for the proper voltages throughout.

With a voltmeter, scope and signal source the problem should be easy to find.

Could very easily be incorrect output tube bias.

[Enzo]

Bad hum might be caused by bad FILTER caps, but not usually by COUPLING caps. Filter caps smooth the B+ and other power supplies. COupling caps are the ones that connect the signal path stage to stage.

[Ampzone]

Did it ever work properly?
Could be many things, bad wiring, wrong part, incorrectly installed part, bad connection, etc.

You need to check all the parts, wiring and connections or look for the proper voltages throughout.

With a voltmeter, scope and signal source the problem should be easy to find.

Could very easily be incorrect output tube bias.

AFAIK no it did not, voltages look good, bias is good, that much I can see with the meter. I have a tektronix scope which is a new tool that I have yet to master, e.g translating what I seeing in to a diagnosis. The audio probe and tone generator are on hand and show signal 1k getting through that I can confirm.


Thinking out loud what could cut the signal level at V1 input grid by 2/3rds? Normally when you touch this with a non conductive item such a chopstick it get really noisy with hum and the sound of your contact. On this amp it's barley noticeable when you make contact.

[Ampzone]

Bad hum might be caused by bad FILTER caps, but not usually by COUPLING caps. Filter caps smooth the B+ and other power supplies. COupling caps are the ones that connect the signal path stage to stage.

I understand, there is no hum, just slight hiss as you would expect however it is greatly reduced and when you plug in and play normally set to clean volume levels it distorts, almost a clipping cut off sound. So far the coupling caps around V1-V3 show no DC leakage that I can see wit the DMM.

Whatever it is, it gets "upset" when you ask more of it (playing with guitar), in idle mode it sits quietly, albeit to quiet as the gain/level as you would normally want to see in a guitar amp is way to anemic.

Does any of this clue you into an area I should zero in on? I may have to lift this whole board up and re-seat the solder joints and re-trace the leads which I do relish.

[The Dude]

If you have a scope, please use it! That is the best way to figure this thing out. Inject a signal, trace it through the circuit and see where your signal gets lost/lowered/distorted. As I posted earlier, it would be helpful to have a schematic so that you knew where to scope.

[Ampzone]

If you have a scope, please use it! That is the best way to figure this thing out. Inject a signal, trace it through the circuit and see where your signal gets lost/lowered/distorted. As I posted earlier, it would be helpful to have a schematic so that you knew where to scope.

As I mentioned earlier I am new to the scope and translating what I am seeing into a diagnosis. Here is a similar schematic I found online for the preamp section:
http://www.hiwatt.org/Schematics/DR103sn903pre.gif

[Ampzone]

and power section which looks to be very close to the same
http://hiwatt.org/Schematics/DR_100wPS.gif