Help Needed:Vintage Amp/NOS Tubes

[TheFullMonty]

Hi all,
So a few weeks back, I picked up a '65 Alamo Electra Twin Ten (Model 2570). It's very Vibrolux-esque, with a similar layout and tremolo circuit. The tube layout is as such from left to right, looking at the back of the amp:
5Y3---6V6---6V6---7199---12ax7---12ax7---12ax7
Their brands, respectively, are:
NOS GE--NOS RCA--NOS RCA--NOS Sylvania--New JJ--NOS RCA--New JJ

My problem started yesterday morning. Everything was perfect at low/medium volumes, but at higher volume breakup, I began to hear intermittent static and odd buzzing noises. I occasionally even heard the amp completely cut out then back in within a split-second. So, with this information, I spent the better part of my afternoon researching these problems, and found every possible recommendation from changing the rectifier tube, to putting silly rubber rings around my preamp tubes. Needless to say, I was frustrated without a definite answer, and decided to sleep on it. (Not literally)

So this morning is when it got really frustrating. I fired her up to try to further pinpoint the problem, and now in addition to the static/noises, I now have a deafening microphonic howl when the amp is at full volume, whether a guitar is plugged in or not. I tried the wooden pencil tap test on all the tubes, and found that the 7199, which I assume to be the PI tube, is the only one that really makes noise through the speakers when tapped. This may be the issue, but I've read that typically, super-microphonic tubes do just fine in the RI position.

So frankly, I have no idea where to start. I'm hoping some of the amp geniuses around here can chime in and guide me in the right direction. Thanks in advance for the help!

PS- Because I know someone will ask, I'll attach a few pictures, although I don't know how helpful they'll be

[Telenut62]

Pull the 12AX7's one at a time, does the issues change? pull the 7199, interestingly that's a pentode/triode combo tube. Looks like the amp as seen some service with the orange drops in there, has the can cap been replaced as well?

[TheFullMonty]

Hey Telenut, thanks for the reply. Will it damage the amp to pull the tubes while it's running? As far as I know, the orange drops have been the only replaced parts in the circuit. Thanks again man.

[Telenut62]

Nope, no damage at all...it may help isolate the prob area, even if it turns out to be the tube itself. If the electrolyte can cap hasn't been replaced, then a fresh one would be a good move. Can you put up a schematic or layout drawing of the amp, I would like to check it out.


7199 tube pricey little bugger....

http://picclick.com/?q=7199+tubes&mi...pCode=zip+code

[TheFullMonty]

Great, I'll be sure to pull the tubes when I get around to it tomorrow. Unfortunately, I haven't been able to track down a schematic for this design, only a few from a later version after they switched from tube to solid state rectifier, and messed with the layout a bit.

[Telenut62]

The only vid up on youtube....

[BobbyZ]

Here's a mod for the 7199 to 6U8a. http://site.tubedepot.com/pdf/7199_6u8.pdf
I have never done it and don't know if it would work on this amp.
It may not be the 7199 anyway. Myself I'd just stick with 7199s.

The pics help because it looks like a new JJ or F&T filter cap. There are more electrolytic caps in there. Have they all been replaced ?

The other thing on a Fender from this time we pretty much know the blue tone or coupling caps are still good. Toss in some electrolytic and maybe a resister or two and it's good to go.
This one could get interesting.

[muchxs]

If you think it's the 7199 swap the 7199.

[TheFullMonty]

Thanks for the replies, guys.

Originally Posted by muchxs

If you think it's the 7199 swap the 7199.

This was my first instinct as well. However, after a quick search on the bay, I found that 7199s are no longer made, which means I'd have to go the NOS route and pay around $50-$60 for a single tube. Call me cheap, but I'd rather not spend that kind of money when it could be something as small as a $10 12ax7. Thanks muchxs!

[TheFullMonty]

Originally Posted by BobbyZ

The pics help because it looks like a new JJ or F&T filter cap. There are more electrolytic caps in there. Have they all been replaced ?

The other thing on a Fender from this time we pretty much know the blue tone or coupling caps are still good. Toss in some electrolytic and maybe a resister or two and it's good to go.
This one could get interesting.

Hey Bobby, thanks for the link. That may be the route I'll have to take if it is indeed the 7199. As far as the caps go, I think the orange drops are the only non-original units. What makes me skeptical of cap problems is, the noise and static started almost overnight. I'm no expert, but don't caps begin to leak, and gradually get worse? Thanks again for your help Bobby, I really appreciate it

[Telenut62]

Originally Posted by TheFullMonty

Thanks for the replies, guys.



This was my first instinct as well. However, after a quick search on the bay, I found that 7199s are no longer made, which means I'd have to go the NOS route and pay around $50-$60 for a single tube. Call me cheap, but I'd rather not spend that kind of money when it could be something as small as a $10 12ax7. Thanks muchxs!

Welcome to the world of vintage amps, generally NOS is far superior if you buy carefully....

http://www.ebay.com/itm/ws/eBayISAPI...m=160986585074

http://www.ebay.com/itm/ws/eBayISAPI...m=151006013158

[TheFullMonty]

Well, I ended up doing a little more poking around the amp this afternoon, and have some interesting findings:
1. The microphonic 7199 PI tube no longer squeals at any volume, and is functioning just as it should.
2. I switched out all the 12ax7s with perfect working tubes, and the high volume static and low frequency "fartiness" still persists.
3. The problems only occur in the tremolo channel. The normal channel works fine at all volumes.
4. The amp in general seems to be breaking up at lower volumes than before.

Does any of this info help pinpoint the issue further? Thanks so much for the help guys.

[Telenut62]

So just to be sure, you swap the first two preamp tubes and nothing changes? Meaning the problem doesn't follow the tube. So it's probably a faulty component in trem circuit. Start by checking the input jacks are locked tight, grounded securely or no corrosion issues, you may get lucky and find an easy fix. I'd still be looking at some electrolyte caps/tubes replacement as well though.

[TheFullMonty]

Yeah, all new preamp tubes fixed nothing. I think I'll pull the chassis and have a look around this evening. I've heard a few people with similar problems say that they ended up having a loose solder joint somewhere. I'll report back after I do some more digging. Thanks again guys.

[Telenut62]

Ok, make sure you have one hand behind your back and use only a genuine Chinese chopstick

I could only find this link to a schematic...

http://www.musicparts.com/products.asp?Company=Alamo

[muchxs]

Originally Posted by TheFullMonty

Well, I ended up doing a little more poking around the amp this afternoon, and have some interesting findings:
1. The microphonic 7199 PI tube no longer squeals at any volume, and is functioning just as it should.

I run into this all the time. Amp functions perfectly at home, makes funky noise when I get to a gig. Re-seat the tubes. Sometimes there's a sweet spot in a 50 year old socket. If a tube is slightly loose in the socket it introduces resistance in the circuit that's not supposed to be there. Usually adds a little crackle, too.

I should post my pic of my nice milspec Cinch sockets with silver plated contacts.

Originally Posted by TheFullMonty

2. I switched out all the 12ax7s with perfect working tubes, and the high volume static and low frequency "fartiness" still persists.

DC leaking from coupling caps makes "static". Noisy plate resistors make static.

There's a time to keep the "cherished" original caps and "untouched solder jonts" and a time to do much needed service. If it's an old Fender it's worthwhile tracking down Astrons or blue caps a making the thing look like it hasn't been messed with. Like I said... old capacitors are about as much fun as old milk. You would be astonished what low ESR and no leakage does for an old amp. It's like a whole new amp...

The farting is probably cathode bypass caps.

You need to call BobbyZ if you want untouched solder joints. I hear he has a whole jar full of untouched solder joints he's willing to sell.

Originally Posted by TheFullMonty

3. The problems only occur in the tremolo channel. The normal channel works fine at all volumes.
4. The amp in general seems to be breaking up at lower volumes than before.

So leave the trem channel alone. Treat the normal channel to some fresh caps.

"Breakup" is distortion. There are many ways to get distortion. You can add gain until breakup when everything is working correctly. "Distorted and low" is a common complaint when things aren't working right.

Originally Posted by TheFullMonty

Does any of this info help pinpoint the issue further? Thanks so much for the help guys.

Look at the bright side... you get to keep your 7199. I have exactly one of those that I know of. I probably have two more sleeves full of them somewhere I forgot I had.

[syrynx]

I'm sorry I didn't see this until today. I agree with muchxs's suggestion to re-seat the tubes, but if you'll Re-tension Your Tube Sockets and clean the tubes' pins, I'll bet that most of your troubles will vanish.

Many thanks to our TDPRI brother Billm for the tutorial.

[KyAnne]

Am I not seeing things correctly? Aren't some of your tubes missing the shields?
This certainly will at times produce microphonic situations.

Also corroded input jacks will cause this and drive a Tech crazy. When you pull a plug out of a jack hole, it is supposed to "short". If it is corroded, it will not. It will be resistive somewhat to the input of the stage it is driving. This will cause howl and microphonics like hell. Been down that road with a Super Twin and Fender Reverb amp. If you replace your tubes and tube shields and this hasn't helped any, I would go to an electronic supply and get a "relay burnishing tool". They are a small very finely abrasive thin flat blade that will "get in there" and clean off the crud from years gone past.

I have "thumped" many a tube in my day. And yes, sometimes socket pins need tightening (easy to ruin), and yes even old tube pins (on the tubes themselves)
need to get some cleaning. Hope this helps.

BTW those old Fenders were quiet as a mouse after taking care of those input jacks.