Single Channel AB763 W/no trem

[dunner84]

Hello,

Last year I started building a single channel AB763 without a tremolo circuit. I started working from my own layout which I designed based on the hoffman one. I was already pretty far into the project before I discovered Sluckey built almost the same amp. His layout happens to be very similar to mine, so I used it as a reference for my build.

I don't have a 5AR4 on hand so I rigged up a SS rectifier to do y initial testing before my tube arrives. I realize that the voltages are all going to be higher, but I am thinking they are too high.

The amp starts with no issues - all the tubes glow - no hum.

With a guitar plugged in, I get no output; however, I can hear the signal in V2.

When I tap the pins of the 6l6s in V5 and V6 I do hear output.

Hear are my voltages:

B+ 490

V1:
2 - 2.3
4 - 299
9 - 299

V2:
2 - 8.9
4 - 485
6 - 8.9
9 - 486

V3:
2 - 2.5
4 - 302
9 - 308

V4:
2 - 450
3 - 407
4 - 456
7 - 450
8 - 390
9 - 450

V5:
1 - -27
3 - 491
4 - 467
5 - .2

V6:
1 - -27
3 - 491
4 - 467
5 - .2

V7:
2 - 490

Here is a link to Sluckey's schematic and layout.

I have not had a chance to really troubleshoot anything, but I did go through the amp, and I cross referenced everything with the layout.

I did not write down my voltage drops at each node, but I will do that later when I can get back to it.

[sluckey]

You have problems with all tube voltages except V7. They make little sense. Please recheck.

[dunner84]

I had a chance to scour over the amp and verified things against the schematic and the layout, and I found a few issues. I had missed a jumper on the board, and my reverb dwell pot was wired incorrectly. I fixed that up and the tube voltages are making more sense; however, there is still no output.

Here are the new voltages.

B+ 480

V1:
1 - 278
2 - 0
3 - 2.1
6 - 278
7 - 0
8 - 2.1

V2:
1 - 466
2 - 0
3 - 8.5
6 - 466
7 - 0
8 - 8.2

V3:
1 - 276
2 - 0
3 - 2.1
6 - 273
7 - 0
8 - 0

V4:
1 - 236
2 - 57
3 - 100
6 - 244
7 - 65
8 - 95

V5:
1 - -27
3 - 475
4 - 450
5 - -27
6 - 475
8 - 70

V6:
1 - -27
3 - 475
4 - 450
5 - -27
6 - 475
8 - 70

V7:
2 - 480

[sluckey]

You say V3 pin 3 is 2.1V but pin 8 is 0V. Did you forget the jumper between those pins on the socket? That would certainly kill all sound.

Otherwise, voltages look much better. Except... What are the real voltages for pin 8 of the output tubes? I know 70V aint right. I suspect that's really 70mV? If so, you are cooking those 6L6s! Adjust the bias pot until you have about -45 to -50v on pin 5 of each 6L6. -27v is way too low for a 6L6.

PS... We need to see a schematic of your amp.

[dunner84]

Thank for the reply. the jumper is in place. I am using the schematic you have posted for your TDR. I have built the same amp with the addition of NFB switch, dwell control, and I don't have a 50v bias tap. On the bias though, I can't get it any more negative than -27v.

[sluckey]

the jumper is in place

Crap! I was hoping that was it. So, how do you account for 2.1v on one pin and 0v on the other? Mistake writing it down maybe?

On the bias though, I can't get it any more negative than -27v.

That's a bigger issue. Fix that first then come back to the no signal problem. Pull the 6L6s until this is resolved.

I assume you are getting AC for the bias circuit from one end of the HT winding, probably about 330VAC? There should be a big resistor just before the cathode of the diode. Maybe 220K or 100K. Make that value smaller to increase the -27V. I would set the bias pot to the mid position then start experimenting with resistors until you get one that gives about -45V at pin 5 of the 6L6s. Hopefully this will allow you to swing between approx. -40v to -55v. You can use 1/2W resistors to experiment but I'd use a 2 or 3W for the permanent fix.

[dunner84]

Thanks again for the quick response. I rechecked that voltage, and it was indeed a writing error.

There is currently 100k resistor off my HV winding. I will see what I have on hand, and try to get the bias voltage straightened out.

[dunner84]

The next smallest resistor I have in that wattage is 22k. I had another 100k, so I threw it in parallel, and I have the bias voltage sitting -52v max, so I still have some playing to get a more ideal range, but I think it will work for now. The pin 8 voltage on the 6l6s is still around 90v. I am positive the meter is in volts not mV.

Still no output; however, I can hear sound from the speaker when I probe pin 1 of V1 now, just not pin 2, so maybe there is a problem with the input jacks? I have gone over them a dozen times...  perhaps I need to take a break and come back to it later.

[sluckey]

so maybe there is a problem with the input jacks?

Sometimes it's confusing to wire the Fender HI/LO jacks. You could always just use a single jack to see if you get sound.

Here's a quick and easy way to verify that your jacks are wired correctly...

Connect your ohm meter to V1 pin2 and chassis.

1. With no plug in either jack, resistance should read about 34KΩ.

2. Insert a plug (may use a guitar cable but don't connect a guitar) into the "HIGH" jack. Resistance should read about 1MΩ.

3. Now insert the plug into the "LOW" jack. Resistance should read about 68K.

If any of these 3 resistance readings are incorrect, the jacks are not wired correctly.

[sluckey]

The pin 8 voltage on the 6l6s is still around 90v

That's impossible if you have a 1Ω resistor between pin 8 and ground. That would mean you have 90 amps of current flowing through those tubes! Your whole workbench would be on fire. 

What make and model meter do you have?

Oh wait a minute! Do you even have anything connected to pin 8? My board layout does not show anything on pin 8. That would explain a very high voltage and also explain no sound.

[dunner84]

The meter is a fluke 115. And I have nothing connected to pin 8..........

[dunner84]

I have the two 1 ohm resistors to ground just sitting there happily all by themselves.

[sluckey]

About the bias voltage... If you copied my circuit you have a 10K bias pot and one end of the pot connects through a 10K resistor to ground. Sound familiar? If so, change that 10K resistor to a 27K resistor like the super reverb uses. That'll bump the negative bias voltage up considerably.

[dunner84]

Thanks for your help. I really appreciate it. The amp has come to life! I suspect the bias is still too hot, so tomorrow I will make the changes to the bias circuit you suggested, and dial it in. Other than a bit of fizz and mild hum it sounds great. I think that correcting the bias will clean that up. I will also give the reverb a wurl tomorrow as well.

[dunner84]

Are there any good ideas where I can find a good DC source to elevate my heaters? Normally I would use  pin 8; however, this amp had a fixed bias, so that doesn't work in this case.

[sluckey]

Sure. Use one of the B+ nodes. Node C will be a good choice. It's well filtered.

Now build a simple 2 resistor voltage divider like this. The values I used in this example will provide 45V. Change resistor values to obtain a different voltage.

I'll be surprised if elevating the heaters will be beneficial in this amp.

[dunner84]

Thanks for the circuit. You may be right about not needing it. It is something I can try once this is all working properly.

I swapped the 10k resistor for a 22k which made me able to adjust it between -65 and -82v, so I bumped the HV tap back to 100k from 50k.

Here are the current symptoms:

I get output but its a bit weak and very distorted. I am working with 4 6l6 test tubes. 1 is a bad tube for sure 1 is a good tube for sure. My pin 1 measurements are as follows 33vdc on V6 and .1Vdc on V5, and they stay like that regardless of which tubes I have in place. My pin 1 to ground resistors are 10r not 1r

I have NFB switch that lifts the NFB, and there is no discernable difference in the tone or output level when I switch it.

With no reverb tank installed, I get a loud squeal when I turn the reverb past 7. With the tank installed this does not happen. I do hear reverb but it is faint.

[sluckey]

My pin 1 measurements are as follows 33vdc on V6 and .1Vdc on V5, and they stay like that regardless of which tubes I have in place. My pin 1 to ground resistors are 10r not 1r

Pin 1 is not used in a 6V6 or 6L6. Pin 8 is the cathode and that is the pin you should have your 10Ω resistors connected.

If you followed my layout you have a 1500Ω resistor connected between pin 1 and pin 5. If that's your case there should be about -45v on pin 1 when you get the bias supply sorted out.

[dunner84]

Sorry, I really meant pin 8... The 1500r resistor is on pin 1. 10r resistors to pin 8

[dunner84]

I hope everyone has had a great Christmas!

I've made a bit more headway, but I am still a little stuck.

I traced some of the issues I was having to bad power tubes. I also replaced the 10r resistors with 1r to make bias reading easier. I threw in a good set of jj6V6s, and all of the noise issues went away. with 465v on the plate, the hottest I can bias them is to 7w, so about 50% which is rather cold. My initial plan for this amp was to be able to switch between 6v6 and 6l6 tubes, but so far, I can't get the bias range right for either. My local electronics shop is open tomorrow, and I will pick up a few resistors to replace the 100k HV Tap to sort that out. This isn't the issue though.
Here are my current symptoms:

- guitar is distorted even at the lowest volume setting. The preamp pots all work as they should, but the overall signal is a little thin, and distorted/fizzy.
- reverb is very faint. I can hear the reverb tank loudly if the springs get jostled, but barely anything otherwise.
- amp freaks out a bit when the reverb is turned all the way up.
- no discernible change when I flip the NFB lift switch.



I have checked, rechecked, re re re re checked the layout against the schematic, and everything is looking ok. I am not too sure where to go, possible pre-amp tubes?

[pompeiisneaks]

I would think the cold bias is what's making hte tone a bit off.  If you can get that up a bit higher near 70% you'll do better.  If you want to swap out between 6V6 and 6L6 you may need two separate bias resistors that are switchable, or a larger sweep bias pot, but not sure. 

~Phil

[dunner84]

I was thinking that, but I'm not sure that is a symptom of a cold bias. The other thing is that the same issue was present with the 6l6's, and they were biased appropriately... The presence of the other symptoms leads me to believe there is another issue.

[whoops]

Have you checked the values of all the resistors installed?
Measure them or use the colour code to identify the value, write the value in the schematic one by one

Have you measured continuity in the circuit of all the parts , all the wires, all the Ground bards, all the wires from tube to circuit?
Take the schematic and measure continuity of everything, beginning to end

Are you sure all the tubes you are using are Good?

[dunner84]

I have gone through and checked continuity and resistor values vs the schematic, and also done the math on the parralelled resistors.  I have diodes in the rectifier position, and known good power tubes. The 12ax7s are good as they were verified in another amp. I dont have a means of checking the 12at7s, but I have 4 NOS tubes that I have never used, but purchased from a trusted source.
Is there an easy way of bypassing the reverb circuit and eliminating it as part of the problem?

*Edit* If I lift the top leg of the 500p cap, will that remove the reverb portion of the circuit?

[pompeiisneaks]

If you just shut the reverb volume to 0 it should do the same as removing it from the path, unless you've got a bad reverb pot.  You could also just disconnect the output at R21 so it doesn't go up to the next stage I'd think.

~Phil

[dunner84]

I have gone through and verified all of the components values/integrity, and I have now verified all the tubes I am using, and reflowed all of the solder connections again. I even removed the diodes and plugged in a 5Y3. The bias is still quite cold on the 6v6's, but that should not be causing this. The volume does not get over voice level, and it sounds like I have an overdrive pedal plugged in.

[sluckey]

How much voltage on pin 5 of the 6V6s?

[dunner84]

Pin 5 is sitting at -40.6V

[sluckey]

Drop it down to just -36V. Any better?

[dunner84]

The sweep range is -40 to -58v. This is because I changed the HV tap from 100k to 68k, and the 10k resistor to 22k to get things setup for 6l6's.

[sluckey]

The sweep range is -40 to -58v. This is because I changed the HV tap from 100k to 68k, and the 10k resistor to 22k to get things setup for 6l6's.

But you ain't using 6L6s any more.

[dunner84]

You are very correct, do you have a suggestion for how to achieve a large enough sweep to accommodate both tube types? Or, what size HV tap resistor will get me in a good range for the 6V6. I just recalculated the dissipation and its only 2.2 watts! way too low, but I don't think it is the reason for this, but maybe it is?

[dunner84]

I replaced the 68k resistor with the 100k resistor, and can dial the - voltage down; however, it has no affect on the current issue. 

[sluckey]

do you have a suggestion for how to achieve a large enough sweep to accommodate both tube types?

Yes. Make the bias pot larger than the resistor connected to ground. I'd try a 25K pot and a 10K resistor, then juggle the resistor as needed.

[sluckey]

The amp has come to life! I suspect the bias is still too hot, so tomorrow I will make the changes to the bias circuit you suggested, and dial it in. Other than a bit of fizz and mild hum it sounds great.

So, what has changed since you said that?

[dunner84]

That was me being a little hot off the line. I had the volume down low because the kids were sleeping. The distortion and volume issue were there, but I could not hear at such a low volume.. It is kind of strange, at the lowest volume setting you can here the clean chime, but there is some light distortion on it. As soon as you pass that initial-on-volume, it dirties right up and that sweet character disappears.
I was reading through your build, and I got excited when you had a similar issue due to the tremolo loading, but then my heart sank when I noticed the 47k to ground...

[dunner84]

I didn't realize that not having the reverb tank plugged in could damage the reverb transformer , so I may have damages it. I disconnected it from the send jack, and measured it. It comes in around 4ohms, which may be OK since it's a small winding, but I can't say for certain. I reattached the reverb transformer to the send jack, pulled V2, and jumpered over the 3.3m resistor, and the the noise floor of the amp shot right up, the issue is still there and is significantly worse. I can hear almost a motorboating type tone in the distortion now.

[whoops]

I didn't realize that not having the reverb tank plugged in could damage the reverb transformer , so I may have damages it.

Disconnect the Reverb transformer leads, measure DC resistance of the Primary and the secondary

Primary should be around 2K
Secondary should around 1ohm

While troubleshooting,I have measured in the past vintage Fender original transformers from the Blackface/Silverface Reverb amps family with 1K on the primary  and other with 30ohms on the secondary, the reverb still worked well and sounded good and the problems the amplifiers had were not related to the reverb transformer.
But as a simple test use the values I showed you and report back

[dunner84]

Thank you for the reverb transformer info, when I get this figured out, I will take a look at it more closely.

With the 3.3m resistor jumped the noise floor went through the roof. I pulled one tube at a time starting at V1, and V3 is the culprit. I rolled a few tubes through, but it seems to be the location, and not the tubes. I took voltage readings again, and I noticed there is 4mV on the grids pin 2 and 7. I am pretty sure that should be 0, but I am not sure what kind of tolerances are ok. I will go over the socket and connections again to see if I missed anything. I have a good fluke meter, but it doesn't measure capacitors very well, and I suspect the voltages on pins 2 and 7 is due to a leaky one somewhere.

[whoops]

I suspect the voltages on pins 2 and 7 is due to a leaky one somewhere.

Why?
Did you use old parts re-used parts when building the amp?

[sluckey]

and I noticed there is 4mV on the grids pin 2 and 7. I am pretty sure that should be 0

4mV is zero in a tube amp.

Don't jumper the 3.3M.

[dunner84]

Ok, so I will ignore that voltage while I troubleshoot this. Is it safe to say my problem is with a component connected to V3?

All the components are new expect for the PT and choke which came out of a 1950s organ. The caps are all new orange drops, and the resistors are all new 1/2 watt metal and carbon film. Pots are the mini alpha pots, and the switches are new carling. It's not a big stretch that new components can be bad, or maybe the heat sink didn't do its job during soldering.

[whoops]

The caps are all new orange drops

So I don't see any reason to think it's a Leaky cap, you had to be really really unlucky

[dunner84]

I have been pouring over these connections, and I am still not much closer. There is no DC on any of the pots, or on the input, and I don't see any AC on the rectifier output.

With the volume cranked right up and no guitar plugged in, I am hearing intermittent swirling.. It is only intermittent though. I also hear a very faint very low frequency pulse on startup, but only at startup. I am not sure that this means anything. With a guitar cable plugged in, the noise floor jumps up a bit, and I am picking up a radio station. That is a first for my house. It happens to my friend all the time, and he is a neighbourhood over. Again, I don't think that is related to my issue. It is just an observation.

I am wondering if there are any key voltages on the board that I could be looking at to help narrow this down.

I still also think it is odd that the NFB switch has no discernible affect on the output/tone.

It is also worth noting that I do not know the exact specs for the choke. It is a good sized choke, and it measured about 175 ohms. It came out of the same amp as the power transformer, and the circuit used the same rectifier tube, and power tubes, so it should be ok in this amp.

[whoops]

With a guitar cable plugged in, the noise floor jumps up a bit, and I am picking up a radio station.

Picking up radio stations is normally solved by increasing the grid stopper resistor value of the V1 tube.
What value are you using?

I read your thread from the start again, and I got really confused, you posted the voltages 2 times and they were wrong or had writing errors, so with misinformation is also hard to help you out.

My advise:
- Do a present voltage chart, without errors or writing mistakes
- Do a video with sound of the actual problems
- post high-res pictures of your circuit and board (you can use photobucket to host the pics)
- Post an exact schematic of your circuit, you can use the schematic you based on and change in the schematic the parts that you changed in your build or the mods you might have done, that schematic has to reflect exactly the circuit you built
- if you used parts from another amp, tell which amp was that, tell the parts specs

Organize all your info, be precise

[sluckey]

I agree with whoops. It's time for you to show us your exact layout (don't just say it's similar to mine) and some hi-rez pics so we can compare your actual build to your layout. It's entirely possible that there is an error on your layout and even if your build completely agrees with the layout, the amp will never perform any better than it does on paper. The pics need enough detail to see values of components and how those components connect. Need to be able to follow a wire from a tube socket to the board. Need to see the wiring for the input jacks.

The fact that you can get a loud squeal by turning the reverb pot up indicates that the power amp is probably working to some degree. If so, it would be logical to look for problems with input jacks, V1, and V3 circuits.

You say the PT and choke came from an organ amp? Which one? Model number?

[dunner84]

You guys are completely right. I read back through everything, and it is all scatter brained. This build in general was a bit rushed, and untidy. I blame kids and holidays

I will be able to post everything properly tomorrow, and I may even have my tubes by then.

[dunner84]

I have not had a lot of time the last few days to work on this, but while was taking photos yesterday, I noticed the V1 and vV3 cathode bypass caps look bulged on the + end. It is really hard to tell because of the angle. The plastic that folds over towards the leg has separated, so I can't tell if it is bulging or if the plastic has come away. Unfortunately, I can't upload pictures right now because I am stuck at work all night, but I thought I would make note of that. I am also not sure what affect that could have on the amp. I know what completely lifting the V1 bypass cap sounds like, and that is it. It could be nothing, but it could be the cause of this issue?

[Willabe]

For what K bypass caps cost just swap them out with new 1's and be done with it.

[dunner84]

That's the plan. But it is an interesting point you bring up. For most of the members on the forum that is the case. My local supplier is terrible, and shipping from mouser is $20 to Canada, Digikey is decent at only $8, but its not exactly fast. For one off parts, it's actually a fair bit more expensive and time consuming. Sorry for the mini-rant. It is a source of frustration for me.