Help with an AC50

[tooold]

I bought an AC50 in London a few years back. Ugly recover, semi-functional condition. Well, I finally recovered it, but that was the easy part. Someone had gone to town on the amp itself, lots of wiring changes, cap changes, etc.

I've gone through it and put it back to stock (or as close to stock as I could - I put in a GE thermistor where the Brimistor would have been). However, I have two problems:

1. Low volume and hum. Volume and tone controls have an effect, but really low output and AC-derived hum.

2. Bias trouble. I can get my 2V across R27 (one of the two 47R resistors) with a functional bias pot, but it's 7V across R28, and the bias pot has minimal effect.

I've gone over the wiring a bunch of times, everything looks in order, as do all the component values in the circuit. I've obviously missed something, so I was hoping someone here with some AC50 experience might take a look and see if anything obvious comes to mind.

Here are the circuit layouts and schematic I've been using.

[IMG754]http://i115.photobucket.com/albums/n308 ... ss-4-2.jpg[/img]

[IMG702]http://i115.photobucket.com/albums/n308 ... s-4-MS.jpg[/img]

[IMG1021]http://i115.photobucket.com/albums/n308 ... iagram.jpg[/img]

[Tillydog]

No AC50 experience, but it would be helpful to know what voltages you have on all the pins of the EL34s (and power supply nodes).

Also, are C27 and C28 present? I presume they're on the top of the chassis connected via the feed-throughs.

What voltage do you get either side of the thermistor?

Andy

[tooold]

No AC50 experience, but it would be helpful to know what voltages you have on all the pins of the EL34s (and power supply nodes).

Also, are C27 and C28 present? I presume they're on the top of the chassis connected via the feed-throughs.

What voltage do you get either side of the thermistor?

Andy

Hi, Andy -

Thanks for your reply. Here's what's happening on the EL34s:

[IMG:295:295]http://i115.photobucket.com/albums/n308 ... ltages.jpg[/img]

V4 is the one with the bias problem, and the one closest to the speaker transformer on the layout. Voltages on the thermistor are 403 (transformer side) and 402 (choke side).

C27 and C28 are present and, as you figured, wired through the feed throughs.

One strange thing I just noticed while shutting the amp down - I strummed the strings on the guitar I was testing the amp with before shutting down (to theoretically drain the filter caps), and the sound choked off very quickly, rather than the gradual decay I'm used to on tube amps.

Any thoughts you might have would be mightily appreciated.

Thanks,

Myles

[overtone]

oops, I think there may be a few typing errors in your little voltage table, please double check what you gave us for all pins. The mix up is only on paper I hope!
Best, tony

[tooold]

oops, I think there may be a few typing errors in your little voltage table, please double check what you gave us for all pins. The mix up is only on paper I hope!
Best, tony

Well, let's try getting the pin numbers straight!

EL34 V4

Pin Voltage
1 7.7
2 Ground
3 392
4 368
5 -10
6 -10
7 Ground
8 7.7

EL34 V5

Pin Voltage
1 2.2
2 Ground
3 398
4 370
5 -28
6 -28
7 Ground
8 2.2

[chief mushroom cloud]

2 and 7 are at ground? 0V?

[tooold]

2 and 7 are at ground? 0V?

Yep. Checked it a bunch of times, the only possibility is I still (again) have my pin numbers wrong, but if 1 and 8 are tied together, this should be correct...

The AC-50 bias arrangement is really strange. But maybe you already knew that. This amp was also modded five ways to Sunday, so anything is possible.

[overtone]

My guess with the 0V heaters (pins 2 and 7) is that you are reading DC. Flip your meter to AC and you hopefully get a meaningful heater voltage there too.
I would keep homing in on that bias circuit, it is certainly struggling.
Best, tony

[Tillydog]

Well, let's try getting the pin numbers straight!

EL34 V4

Pin Voltage
1 7.7
2 Ground
3 392
4 368
5 -10
6 -10
7 Ground
8 7.7

EL34 V5

Pin Voltage
1 2.2
2 Ground
3 398
4 370
5 -28
6 -28
7 Ground
8 2.2

That implies that V5 is biased roughly correctly ~ 47mA cathode current, but that V4 is running away: 160+mA.

Since the bias voltage is from the same supply, and V5 has a sensible value, there are very few places it could be going wrong - check around RV5 & R24 - maybe RV5 has a noisy / intermittent wiper?

There may be other problems, but it's worth sorting this out, because it looks like you're cooking V4 at the moment.

Andy

[tooold]

My guess with the 0V heaters (pins 2 and 7) is that you are reading DC. Flip your meter to AC and you hopefully get a meaningful heater voltage there too.
I would keep homing in on that bias circuit, it is certainly struggling.
Best, tony

Give the man a cigar. Man, I hate learning in public! Real measurements to follow.

That implies that V5 is biased roughly correctly ~ 47mA cathode current, but that V4 is running away: 160+mA.

Since the bias voltage is from the same supply, and V5 has a sensible value, there are very few places it could be going wrong - check around RV5 & R24 - maybe RV5 has a noisy / intermittent wiper?

That's what I was thinking, but it seems to work as it should (voltage divider) across full travel. But I'll check it again, as well as R24.

There may be other problems, but it's worth sorting this out, because it looks like you're cooking V4 at the moment.

Andy

At the moment, I'm not real happy leaving the amp on for more than a couple of minutes!

Thanks to you both.

[Tillydog]

Since the bias voltage is from the same supply, and V5 has a sensible value, there are very few places it could be going wrong - check around RV5 & R24 - maybe RV5 has a noisy / intermittent wiper?

That's what I was thinking, but it seems to work as it should (voltage divider) across full travel. But I'll check it again, as well as R24.

If RV5 is OK, then its likely C11 is leaky - try disconnecting one end of it and see if the bias voltage then behaves.

Good luck

Andy

[tooold]

Since the bias voltage is from the same supply, and V5 has a sensible value, there are very few places it could be going wrong - check around RV5 & R24 - maybe RV5 has a noisy / intermittent wiper?

That's what I was thinking, but it seems to work as it should (voltage divider) across full travel. But I'll check it again, as well as R24.

If RV5 is OK, then its likely C11 is leaky - try disconnecting one end of it and see if the bias voltage then behaves.

Good luck

Andy

That would make sense, it's one of the old WIMA's. Tone, you know... I'll spend some time with it later today. I really appreciate all the help.

[tooold]

My guess with the 0V heaters (pins 2 and 7) is that you are reading DC. Flip your meter to AC and you hopefully get a meaningful heater voltage there too.
I would keep homing in on that bias circuit, it is certainly struggling.
Best, tony

Heater voltages look OK, 3.1V AC on 2 and 7. Amazing what you can do with your meter set correctly.

[tooold]

That's what I was thinking, but it seems to work as it should (voltage divider) across full travel. But I'll check it again, as well as R24.

If RV5 is OK, then its likely C11 is leaky - try disconnecting one end of it and see if the bias voltage then behaves.

Good luck

Andy

That would make sense, it's one of the old WIMA's. Tone, you know... I'll spend some time with it later today. I really appreciate all the help.

You know, I've been making a list of things to do, and not to do, for people (me) learning to work on amps. I think the one that would probably be at the top of the list (OK, drain caps first) would be "CHECK WHAT YOU'VE DONE, STEP BY STEP, THREE TIMES" before you scream for help.

Andy, the C11 tip was really helpful, because, as you suggested, I pulled one leg. It didn't help, and there was a lot more noise, not less. But, while I was looking at C11, I checked the layout and schematic one more time, and couldn't figure out why I had two wires going into the eyelet on one end of C11 (along with the cap lead), when I should only have one. Turns out... (drum roll) I had one of the 16/16uf filter cap's positive leads going there. Misread the layout completely.

So, I put it where it's supposed to be and, son of a b&$*%, bias is adjustable, not much noise. Get everything adjusted where it's supposed to be, plug in... and it sounds great! Really nice. Louder than hell, too.

So thanks for helping out, and thanks for treating me like I knew what I was doing, even though, at the back of your mind, you were probably wondering what I had done wrong.

[Tillydog]

Glad you got it sorted it out. It's always team effort in these things.

Enjoy

Andy