2 small issues - Any ideas

[shaunf]

Guys, after spending a couple of days playing my new amp, there are 2 relatively minor issues I want to try to resolve, and was wondering whether any of you had any ideas.

Issue 1: Scratchy guitar pots and scratchy amp volume pot. I'm thinking this is relating to DC voltage. From what I am led to believe, this can occur particularly with a VVR installed, when the VVR is at very low levels. For this reason, Dana recommends the installation of the extra blocking cap and 1 meg resistor to ground.

Now the strange thing is: 1.) I have the blocking caps installed. 2.) This still occurs even with the VVR at full power. Any idea what could be causing it, and how it gets past the blocking caps? I have them installed between the input and V1 pin 7, and the Volume pot and pin 2.

Issue 2: When playing with the VVR at full power, and amp volume quite low, say at around 9 o'clock, if I strum chords, I get this beautiful clear, crisp sound that rings out, but as it blooms, there is a slight unpleasant sort of resonant, harsh sound that follows. Hard to describe, but hopefully you get the picture. I've been rolling tubes all day, and this is constant across all of them, so I don't think it is tube related. Any ideas?

[MCK]

Sorry about the issues. No idea about #1 but I think the #2 sounds like the issue discussed in the following thread :

https://tubeamparchive.com/viewtopic.php?t=18692

Hope this helps. All the best

[shaunf]

Thanks for the link. I did read that topic before I started building my amp, and it was for that reason that I didn't install the diode strings in the first place.

[shaunf]

*bump*

Please guys,any suggestions on where to start looking, what to try would be appreciated.

[Tillydog]

My 0.02p, then....

Issue 1: Scratchy guitar pots and scratchy amp volume pot. ...

1.) I have the blocking caps installed. 2.) This still occurs even with the VVR at full power. Any idea what could be causing it, and how it gets past the blocking caps? I have them installed between the input and V1 pin 7, and the Volume pot and pin 2. ...

Where do you have the 1meg resistors installed? They need to be on the grid side of the DC blocking cap (i.e. the pin 2/7 side). What type / rating of cap are they?

Issue 2: When playing with the VVR at full power, and amp volume quite low, say at around 9 o'clock, if I strum chords, I get this beautiful clear, crisp sound that rings out, but as it blooms, there is a slight unpleasant sort of resonant, harsh sound that follows.....Any ideas?

Bad solder joint? Dirty tube socket?

Can you make a recording?

Andy

[shaunf]

My 0.02p, then....

Issue 1: Scratchy guitar pots and scratchy amp volume pot. ...

1.) I have the blocking caps installed. 2.) This still occurs even with the VVR at full power. Any idea what could be causing it, and how it gets past the blocking caps? I have them installed between the input and V1 pin 7, and the Volume pot and pin 2. ...

Where do you have the 1meg resistors installed? They need to be on the grid side of the DC blocking cap (i.e. the pin 2/7 side). What type / rating of cap are they?

Issue 2: When playing with the VVR at full power, and amp volume quite low, say at around 9 o'clock, if I strum chords, I get this beautiful clear, crisp sound that rings out, but as it blooms, there is a slight unpleasant sort of resonant, harsh sound that follows.....Any ideas?

Bad solder joint? Dirty tube socket?

Can you make a recording?

Andy

The 1meg resistors are 1watt carbon film ones, and are installed on the grid side, and grounded to the cathode ground. The caps are 0.1uf, 100v.

You can see them in this photo.
[IMG768]http://img.photobucket.com/albums/v206/ ... G_1645.jpg[/img]

Re the second issue, I can do a recording on the weekend. Will post then.

Thanks for the response.

[shaunf]

Bloody hell, I'm rather embarrassed. This second issue that I was experiencing, with the wierd sort of ringing aftertone. Well, I fixed it. Nothing to do with the amp at all.

I was just playing my Les Paul through one of my other amps, and noticed the same thing. Unplugged my guitar and noticed the top E string, although in tune, had this wierd accoustic ring to it. Check the nut side, all fine, checked the bridge, looked fine at first until I looked closer and realised that the string wasn't properly seated in the saddle slot. Not like obviously way off, but just not in the slot properly. Reseated string and problem solved.


Please go right ahead and have a laugh at my expense!

Now just to sort my DC voltage issue.

[funkmeblue]

did you remove the original 1 meg resistor from the input jack?

[Tillydog]

Please go right ahead and have a laugh at my expense!

Nah, you're OK - Been there, done that (or similar, anyway) - I think most here have. Just glad it's sorted.

What you've done with the caps / resistors looks OK to me (assuming that the wiring is correct - I can't tell that the white wire from pin 2 and the screened cable from pin 7 actually *go* to the caps, but I'm assuming that's what you've done.

If there is still a problem with the VVR on full, then I don't think it's the VVR (I'm familiar with the problem from my own TW style amp), so time to look for other causes...

Did the amp work OK without the VVR? Or did you build it in from the start?

Just double-check that you have all the ground connections to the turret board made (again, I can't tell from the photo, but there should be 5 connections to the busbar if you've followed the normal layout).

Other than that, post your voltages on V1 & V2, and the associated B+ voltages.

0.02p

Andy

[shaunf]

did you remove the original 1 meg resistor from the input jack?

Hi,

Yes, there is no resisitor on the input jack, only on the board.

[shaunf]

What you've done with the caps / resistors looks OK to me (assuming that the wiring is correct - I can't tell that the white wire from pin 2 and the screened cable from pin 7 actually *go* to the caps, but I'm assuming that's what you've done.

The wires definitely go to the caps, but I'll see if I can get some better photos up tonight.

If there is still a problem with the VVR on full, then I don't think it's the VVR (I'm familiar with the problem from my own TW style amp), so time to look for other causes...

Did the amp work OK without the VVR? Or did you build it in from the start?

I did build it inat the start, but then had some mosfet issues and bypassed it. Unfortunately, I didn't play it much at this stage, and can't remember if the issue was present without the VVR. Easy enough to bypass the VVR if you think this is worth doing, however as you say, the issue is present with the VVR on full power too, so I don't think its that.

Just double-check that you have all the ground connections to the turret board made (again, I can't tell from the photo, but there should be 5 connections to the busbar if you've followed the normal layout).

Thanks, will do.

Other than that, post your voltages on V1 & V2, and the associated B+ voltages.

I've attached my voltage chart.

Thanks for the assistance thus far!

[Tillydog]

I've attached my voltage chart.

Hmm...

Nothing screams at me from that, but V1a voltages look odd:

Unless I'm having a futz, 2.6V on the cathode & 164 on the anode (plate) implies almost zero current through the valve, but the drop across the anode resistor implies ~0.9mA (256V B+).

Is it a 12AX7? Is this one of the valves that you've tried replacing? Is your anode resistor really 100k?

It could still be a missing ground, or maybe a poor connection somewhere in that area. Is the full B+3 getting to the turret that the 100K anode resistor is connected to?

Andy

[shaunf]

I've attached my voltage chart.

Hmm...

Nothing screams at me from that, but V1a voltages look odd:

Unless I'm having a futz, 2.6V on the cathode & 164 on the anode (plate) implies almost zero current through the valve, but the drop across the anode resistor implies ~0.9mA (256V B+).

Is it a 12AX7? Is this one of the valves that you've tried replacing? Is your anode resistor really 100k?

It could still be a missing ground, or maybe a poor connection somewhere in that area. Is the full B+3 getting to the turret that the 100K anode resistor is connected to?

Andy

Thanks Andy,

I'm going to open her up and recheck voltages, swap some tubes and do a bit of poking around tomorrow and I will report back.

[shaunf]

Hi Guys,

An update on the DC voltage issue.

Just to refresh the memory, I had DC voltage on the guitar input (2.03v) as well as on the amp volume pot. With the VVR, when it is set at very low levels, it is to be expected that some DC voltage would be present, but at higher levels it dissappears. Mine was contant regardless of how the VVR was set, and regardless of the fact that I had a blocking cap and 1meg resistor to ground, that was supposed to prevent this.

Did some troubleshooting last night. This is what I found:

1. ) Took a voltage reading directly on pin 7 of V1 - 0.3v.
2.) Took a reading between the blocking cap and the input - 2.03v.

Ok, so from that, it looked to be that the voltage I was seeing on the input was not coming from V1, pin 7, but at the same time, there was no voltage present at any of the other points between the tube and the blocking cap side of the input connection, only between the blocking cap and the input jack itself.

Made up another shielded lead and directly coupled the input to V1 pin 7. DC Voltage gone with VVR set on full. Slight DC voltage present when I dial down the VVR, which is as expected, but it's better than it was with the blocking cap/resistor installed, so I've removed this cap and resistor from my board, and am happy to live with the slight scratchiness of my guitar volume, when the VVR is set below 1/3 power.

Amp Volume pot

1.) Took a voltage reading directly on pin 2 of V1. Got a reading of about 2v.
2.) bypassed the blocking cap/resistor and connected the vol pot wiper directly to V1 pin 2. - No difference - DC voltage still present, regardless of VVR setting.

Can anyone help me resolve this final issue with DC Voltage on the amp volume pot?

Also any ideas as to why the DC blocking cap that was inline with the coupling between the input jack and V1 pin 7 would actually CAUSE a DC voltage issue, as opposed to preventing it, as it was supposed to do?

[shaunf]

So, I finally fixed my DC voltage on the amp pots issue. Suspected after trying numerous tubes in V1, that the issue was a leaky cap. Read up on how to test for leaky caps. Tested the C3 and C4 coupling caps and they were behaving properly. Suspected that the issue then had to be the 500pf mica cap, C2. Tested it and was getting a consistent 0.03v on the pot side. Tested another cap in this position - this one was blocking properly so I swapped it out. Problem solved, and I now have a perfectly behaved Express.