Single Channel 6V6 AB763/Deluxe Lite in Progress

[fiftynine]

I've just built a single channel AB763 pretty much like Steve Luckeys AB763 Lite (not the Hoffman layout though). When I first turned it on it was fine. I set the bias to 19ma with subsequent 445vDC at the tubes (current production Tung-Sol) and heard exactly what I wanted to hear. Very quiet too, no hums....nothing.

Then some event triggers red-plating (like plugging the guitar in) and I get a noticeable hum, a half second ticking noise and it's always in the same tube. I've swapped tubes round and tried another set and get the same thing, same tube after a while. If I turn it off and come back, it's fine again.

I've done the chop stick thing but don't want to leave it on for too long while I'm doing so. If anyone who knows more than I do could suggest where to look, I'd really appreciate it.

Here's a couple of images. The unorthodox stuff that needs tidying up (B+(D) line for example) doesn't seem to cause the problem.

Thanks.

http://i285.photobucket.com/albums/ll70/20sksebay/Picture024.jpg

http://i285.photobucket.com/albums/ll70/20sksebay/Picture026.jpg

[sluckey]

Have you ruled out the NFB loop? Just reverse the OT primary plate leads.

Neat looking build.

[fiftynine]

I thought that if you'd got the brown and blue the wrong way round, you'd get an obvious and very unpleasant squeal???

Thanks. The gap in the board is for an effects loop or reverb circuit if the Neunaber Wet Reverb pedal doesn't work out.

[sluckey]

I thought that if you'd got the brown and blue the wrong way round, you'd get an obvious and very unpleasant squeal???

It's not always so obvious. Since I've been hanging around this forum I've seen maybe a dozen NFB problems that had subtle symptoms. May not be your problem at all, but it needs to be positively ruled out.

[fiftynine]

I'm not going to question advice from someone with nearly 5000 posts so I went to work on it. I disconnected the NFB first and noted it red plating so I wiggled the tube and it stopped. It made a loud bang or two during the wiggle but it's not plated since.

The jury is still out but it's looking good.

[Shrapnel]

I'm not going to question advice from someone with nearly 5000 posts so I went to work on it. I disconnected the NFB first and noted it red plating so I wiggled the tube and it stopped. It made a loud bang or two during the wiggle but it's not plated since.

The jury is still out but it's looking good.

Red plating both tubes?

Sounds to me, IMHO, as if it could be socket related.  Perhaps the socket, as it heats up with the tube, the pin retainer(s) are expanding in such a way that contact was becoming problematic. I'd examine the sockets closely to make sure they are actually good. After all, it's not impossible to get a bad socket or two even in new production.

[HotBluePlates]

Or, you could have a poor solder joint on the affected socket, and good mechanical contact was established when jiggled.

If solder was the issue, remember the rule: You must first form a good mechanical contact with the wire/lug; afterwards, you use solder to permanently hold the existing good connection. Never use solder to make the connection, or to take the place of good mechanical connection.

[topbrent]

Nice looking build!

Is that a Hot Rod Deluxe chassis?

I like the idea of the open section of circuit board space.  Gives you room to experiment later on.
 

[fiftynine]

Shrapnel, it was only ever the first 6V6 that plated. I checked all the joints, contacts, re-soldered and all that carry on. It's still OK.

HBP, but what a mission that would be.......the circuit essentially working before seeing any solder?

Yes, Brent. It's a gutted HRD. I'll stick some more images up later.

I left a gap between the bias and filter caps in case I wanted to add switched cathode biasing. The second speaker out hole is going to be a NFB switch. I've got plenty of space behind the front panel (between the pilot and pots) for a VVR circuit. I'm thinking of a 5th pot, 500k linear to simulate the missing second channel (with the pot at mid point) but I'm not sure if any other positions on the pot will sound right as a Pre-PIMV yet. All that said, I've always felt most at home with the Blackface circuit just how it is.

Which is why I left the big gap. I don't know if I'll be able to live without integrated Spring Reverb. I have a spare pre-amp socket and those 20 terminal strip lugs. I don't need massive reverb like Leos. Here's a simple 1 tube reverb circuit from the Orange AD30R http://users.telenet.be/orangefg/OFG_SCHEM/OrangeAD30_1.pdf

Would something like that be possible with a few component value tweaks? Any other ideas?

[dynaman1]

59, what kind of board material is that??

[topbrent]

Look up the posts on the 1-tube reverb in the files section. 

Tubenit has fitted this reverb circuit into virtually every known electronic device 

[fiftynine]

Here you go, Dynaman. It's run of the mill Tufnol.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=1941132

Oh yeah. 3 threads down in the archives (blush)

[fiftynine]

Dudes, this is getting me down now. The red plating has disappeared but now I've got very little or no output at full volume. I reversed the OT primaries way before this issue and got the squeal so they were fine and still are fine. The NFB is in tact. The power supply is fine, bias fine, OT reads the same resistances as it did new, so that's fine. I've chopsticked, used a magnifying glass to examine everything and re-soldered.

All my voltages are high because of the wall voltage, the MM PT and because I've only 2 pre-amp tubes.....say 10% higher and I wanted that. But, I've got a massive 380v on Pin 1 V1 but I don't know if that's why there's not output. I've checked and double checked everything at the pre-amp now.

It was sounding great for 2-3 hours then nothing

Amp building is one of those things that can bring a lot of satisfaction then destroy your karma in seconds.

[sluckey]

I've got a massive 380v on Pin 1 V1 but I don't know if that's why there's not output.

Could be. Depends on the voltage on pin 2 and 3. Measure all tube pin voltages and post here. Also post a link to your schematic. I have an idea what your circuit looks like but am not sure if the schematic is exactly the same as my lite.

Here's a divide and conquer thing to try. Crank the volume and tone controls max CW. Use a small plastic handled screwdriver as a probe and place the tip on the wiper of the volume pot. Touch the blade of the screwdriver with your finger. Do you hear a fairly loud buzz/hum? If so, the problem is prior to the volume pot. If not, the problem is after the volume pot.

[fiftynine]

Thanks, sluckey. I'm doing a bit of work on it now so can't post the exact voltages but can tell you that where the Fender schematic says 180, 180, 170 and 180 from left to right, mine is a definate 380 and roughly 250, 230, 250. I think all was fine when the 380 was at 250 but I may have 2 problems.

My build is the same as your Deluxe Lite except I used the standard 0.047 instead of the 0.022 in the tone stack.

I will post up some images in an hour or so.

[tubeswell]

Hi fiftynine

Go through and measure all the (DC idle) voltages on the plate, screen and cathode pins and list them V1: Pin 1 =  etc that way everyone can see where something is amiss.

If you have HT voltage on a plate, it usually means the tube isn't conducting (because when current is flowing through the tube, it flows through the plate and cathode resistors as well and a voltage drop results across those resistors according to Ohm's Law)

[fiftynine]

I have the loud hum when I touch the screwdriver. Here's the voltages, pots at noon.

V1.  372,  0, 5.5,  na, na, 251,  0, 1.87
V2.  221, 56, 89,  na, na, 212, 60,   89

V3.  -49,  na,  445,  445, -49,  446, na, 0
V4.  -49,  na,  443,  444, -49,  446, na, 0

B+ nodes I have 447, 445, 389, 375 and the bias reading at the pot lugs are -25, -50 and -61. Images soon.

Currently researching what you've all said and messing with it.

[tubeswell]

Here's the voltages, pots at noon.

V1.  372,  0, 5.5,  na, na, 251,  0, 1.87

How is V1a set up? 5.5 volts on the cathode is quite cool, (Did you use a large value cathode resistor?) - which would explain the higher voltage in that triode (i.e.: low tube current) Or is it a case of the wrong value resistor in there?

V2.  221, 56, 89,  na, na, 212, 60,   89

If this is for a LTP, what are the cathode vs tail resistor voltages?  

[sluckey]

V1.  372,  0, 5.5,  na, na, 251,  0, 1.87

There's a conflict with V1-1 and V1-3 readings. Ideally, they should look just about like V1-6 and V1-8 (which are OK). The high voltage on the plate indicates the tube isn't conducting, but..., the 5.5v on the cathode indicates it's conducting balls to the walls!

Since your volume is very low, I think the plate voltage reading is probably correct and the cathode reading is wrong. Please redo the cathode voltage. A very likely reason for high voltage on the plate (and low volume) is a high resistance in the cathode circuit. That could be an incorrect value resistor, or a bad ground connection. Measure the cathode resistance, one lead directly on the tube pin 3 and the other on the point where the PT center tap is connected to chassis. It should be 1500Ω if you used my schematic.

I have the loud hum when I touch the screwdriver.

That indicates the problem is in the V1A circuit, likely related to the high plate voltage. Check your cathode resistance to ground, recheck your voltages, and recheck that you have the correct resistor values for plate and cathode.

[fiftynine]

I'm over the moon. The cap off pin 3 read 25uf and even I know it shouldn't read it's value in circuit. I fixed the joint and it seems fine but it's to late to give it some welly tonight. As HBP said, if I'd have made each connection right without solder, that could have been avoided. I have a hunch the red-plating was a bad joint as well. I now have 230/1.74, and 241/1.8.

I take LTP means long tailed pair? The circuit is exactly as sluckeys Lite but with Leos component values except the grey shaded area in the Lite schematic. It's been replaced with a 500k pot. When it's mid position it's roughly the Lite/DR. I maxed the volume/tone and barely opened the 500k pot but didn't hear that first stage overdriving. Maybe I need to lower the voltages there.

Obviously buying from Doug, is there any other way to support this forum to show my appreciation for the advice?

Edited to add these. The first image shows the 500k pot and wires that need tidying up. Might be better with 1Meg there. It's interesting to have a pot that shows you how much the first stage is clipping. I now know part of the BFDR magic is a bit off pre-amp overdrive mixed in. But I quite like the voltages how they are. It thumps like a Vibrolux. Maybe switchable dropping resistors to control the voltages?.....VVR to control the 6V6s.