Old amp reverb repair

[graylensman]

I've got an old old tube amp. The reverb has never worked in the twenty years that I've owned this. Is it advisable to try and fix this myself (assuming it is fixable in the first place)?

[PRR]

You can diagnose.

Turn Reverb up. Bump the amp hard. Does it booooing? Does it hiss?

It might be good to know what brand and model....?

[graylensman]

You can diagnose.

Turn Reverb up. Bump the amp hard. Does it booooing? Does it hiss?

It might be good to know what brand and model....?

The amp has a hum to it no matter what. When I lift one corner of the amp and then thump it on the floor, there sounds like a reverb kind of noise, but it's extremely faint. The amp is an Alamo Jet - 55 watts of total rock power. It was old when I acquired it twenty-five years ago.

Oh, and I know my initial post was quite non-specific. I know that I know nothing about this topic. 

[Paul Marossy]

There's generally only a few things that can go wrong with a tube reverb circuit.

1. bad preamp tube
2. problem with reverb tank
3. faulty reverb transformer
4. RCA cable damage
5. damaged/faulty reverb control pot

It could possibly be other things, but those are the most likely culprits.

[PRR]

Alamo Jet was a cheap hunk of junk. Such "junk" has unique tone and is modestly collectable today. However at its age it will need "work". It is perhaps worth considerable fixing. More than I'd care to walk-though via cyber-space. If you are in south Texas (where most Alamos are), I know a competent tech sensitive to old-amp needs.

But to be clear.... the big bottles are one 5Y3 and one 6V6? This is a 4-watt amplifier. The "55 Watts" is how much it sucks from the wall, not how much it puts to the speaker.

[Mark Hammer]

If it is identical to the Alamo 2564 Jet schematic I'm looking at right now, it is an odd hybrid amp that is about as far away from the traditional  "three tube-er" (12AX7/6V6/5Y3) amp as you can get. The reverb section is solid state, as is the driver stage that feeds the 6V6, and the reverb is fed right from the input triode, before the signal hits any EQ-ing or anything else.

It's a really interesting amp design.  Worth resurrecting. If I am reading the schematic correctly (and it IS a blurry one), the transistors in the reverb stage (2N3708) are still readily available.  There do not appear to be any electrolytics in the reverb path, either within, going to, or coming from, so replacing an old dried out one doesn't seem to be an issue.  You should probably consider feeding your direct guitar signal to the input lug of the reverb-amount pot, just to verify that the obstruction is within the reverb circuitry itself, and not simply a reverb-amount pot that needs replacing.  The signal from the reverb return that passes through that pot also passes through a DC blocking cap on the way to the mixer-driver stage, so you shouldn't worry about causing problems by directly injecting the guitar signal to that pot.

BTW, do you have a schematic?

[graylensman]

Thanks again for everyone's interest.

Mark, it is indeed an Alamo 2564 Jet. There's a tatty info paper glued to the inside. Has a handwritten serial number (81309). It also shows a tube lineup with symbols, as follows:

6AV6, 12AX7A, 2N3708, 2N3708, 6V6GTA, 5Y3GT

I don't have a schematic.

I plugged my guitar straight into the jack for the footswitch and got no sound. How would I directly feed the guitar signal to the input lug? Understand that I'm a noob with this stuff.

And feel free to tell me to leave it all alone before I screw it all beyond repair.   

[Joe Hart]

Understand that I'm a noob with this stuff.
And feel free to tell me to leave it all alone before I screw it all beyond repair.  

You're a noob. I just want to make sure you KNOW that there is stuff inside a tube amp that can KILL you, right?? Just a public safety announcement for your health and wellbeing.
-Joe Hart

[graylensman]

LOL - Joe, thanks for the warning. I've been lurking here for a couple of months, and that's one of the things I picked up early on. That's the reason I decided to post here first before I attacked the amp with a screwdriver.

So, if the consensus is "leave that thing alone", I'm okay with that.

[Joe Hart]

I plugged my guitar straight into the jack for the footswitch and got no sound. How would I directly feed the guitar signal to the input lug?

Plugging into the jack for the footswitch would do nothing (except maybe turn the reverb on or off). But it has nothing to do with the signal path. If you can find the lug on the reverb pot then you could hold the guitar cable and touch the tip to the lug. So you have your guitar with the 1/4" cable plugged into it (and the volume is up on the guitar), then touch the other end to the lug on the pot. You should hear stuff. You can test it by touching it to the lug on the input jack (then it would act like you had just plugged the cable into the amp). I like to use a cable with a plastic/rubber end on it just for a little insulation, but if you stay away from the tubes and the giant caps you should be okay (but you never know).
-Joe Hart

[Brymus]

Since it "is" a fairly old and slightly collectible amp.
I would leave it sit until you learn more or take it to a good tech,and pay to have it restored electronically.
You could restore the cosmetics yourself,and if you express an interest in learning the tech might explain the circuit and whats going on to you,the economy is quite slow,and I dont see hoards of people having tube amps restored like a few years ago...

If you want to DIY ,this is from what I gather an SE (single ended) amp.
You could take this up at SEwatt.com there are lots of guys who would be interested in this over there and would likely walk you through it if you post adequate photos. (they specialize in SE designs at SE watt)
Just dont let them convince you to gut it and build one of the commercial projects they have going or a modified Valve Jr in its chassis.(not a bad idea really but kills the collectable vintage charm)
Heck if all else fails write Jack White and offer it for sale to him....

PS : Mark,could I get a copy of that schematic,I am very interested in the odd design of it ?

[Mark Hammer]

If anyone wants a copy of the schematic, drop me a PM with an e-mail address that can accept attachments.  It's not the clearest of PDFs, but it's what I have.

[Paul Marossy]

If it is identical to the Alamo 2564 Jet schematic I'm looking at right now, it is an odd hybrid amp that is about as far away from the traditional  "three tube-er" (12AX7/6V6/5Y3) amp as you can get. The reverb section is solid state, as is the driver stage that feeds the 6V6, and the reverb is fed right from the input triode, before the signal hits any EQ-ing or anything else.

Oh, it's one of those things. There were several Gregory amps (http://www.diyguitarist.com/GuitarAmps/GregoryAmps.htm) that used a solid state reverb send and return, but I haven't heard of a solid state driver for a tube power section before. That's very unusual.  

Anyway, I've had some amp techs write to me about various Gregory amps with a non-functional reverb. So far, the problems with those amps were in the solid state section of the reverb circuit. Maybe that might be the case in OP's amp?

[PRR]

> it is an odd hybrid amp

Yes, I saw that. Rather clever. But would be hard to talk an experienced tube-head through a diagnosis. And it seems that graylensman is not yet "experienced", and wise enough to know it.

> schematic, drop me a PM

Is it this file?
http://www.schematicheaven.com/bargainbin/alamo_jet_2564.pdf

There's so much to go wrong, some of it in strange ways. The reverb driver and recovery are powered by the 6V6 cathode voltage. The mixer is powered by a steep drop off the 290V rail, with a bias scheme which suggests very old transistor hack-design. Also there is a tingle-cap(*); that is if it hasn't failed open (or short!!).

And I just bet it has the cheapest parts available. In 1965(?) this could be left-overs from the failing radio industry, parts literally intended to just-barely survive a 30-day warranty. That it works at all is mildly astonishing.

It wants a highly experienced technician to go-through, fix what's leaked broke or drifted, without "improving", except for the obvious safety issues.

(*) Sometimes called "death cap", though death is unlikely. If he had just got this amp, I'd be real concerned. But he's had it 20 years so I'll overlook the risk. Just stay off dirt and concrete until it can be converted to proper modern 3-pin cord with correct internal grounding.

[graylensman]

 I'm sort of honored that my original post garnered this much interest.

Seems like a diy fix has the potential for some very disastrous results. I've made it through 47 trips around the sun, and I'd like a few more, so... I think I'll just let this one be.

If anyone's really curious, I'd be happy to snap some photos of the amp (the tubes and what-not are visible in the back of the cabinet).

Thanks again! 

[Paul Marossy]

After looking at the schematic linked above (which is the same as what Mark Hammer emailed me), I would suggest taking it to a very experienced amp tech to fix it. That's a real strange beast, and as PRR pointed out, it would not at all be an easy one to troubleshoot (probably even for experienced people).

[wavley]

Heck if all else fails write Jack White and offer it for sale to him....

You let the secret of the burst box out of the bag!!!!  It's one part Alamo Jet and one part Scarlett Johansson.  I shouldn't really say any more, but it involves some wiggle, some jiggle, a faulty reverb tank, all in a box with a hot 6v6...

[Brymus]

I'm sort of honored that my original post garnered this much interest.

Seems like a diy fix has the potential for some very disastrous results. I've made it through 47 trips around the sun, and I'd like a few more, so... I think I'll just let this one be.

If anyone's really curious, I'd be happy to snap some photos of the amp (the tubes and what-not are visible in the back of the cabinet).

Thanks again! 

I would like to see some pics of it if its not too much trouble,thanks

Wavely,sorry ,I didnt know,sounds good though 

[PRR]

Just mumbling to myself.... maybe useful to whoever rehabilitates this vintage beast.

First: there is a classic small guitar amp with one 12AX7 and one 6V6. Best known as Fender Champ, or today's Epi Valve Jr, but almost everybody had such a thing as the bottom of their range. Alamo actually had lesser amps (Capri, Fiesta: 1W out), with Challenger as the basic 4W model. They also had Montclair, with a whopping four small bottles and three big bottles (14W output, 2ch, trem), but you can't make such a fancy machine cheap.

Champ and Challenger have preamp, volume/tone/ driver, power tube. It is possible to combine the first two stages, but performance is less, and it isn't much cheaper.

Fender was always pushing the possibilities, for the music but also to stay ahead of cheap copy-cats. By early 1960s he had Reverb and Tremolo. How could Alamo add these things to their 2nd-cheapest chassis, without major cost? Trem needs at least another triode, and good Reverb is very nearly another small amp (small power driver, high-gain recovery).

About this time TI had put transistor production in HIGH gear. Can you tart-up a tube chassis with a few small transistors and get working (sell-able) frills?

With 1965-vintage transistors it is hard to get a guitar-pickup input. Wisely Alamo kept this tube.

Trem oscillator design is also hard (or non-obvious) with an early transistor. Alamo used a conventional tube trem oscillator.

So if you don't wanna punch another tube-socket, you at least have to find a gain over 10 with at least 15V peak output some other way. This is barely possible with a vintage transistor. And of all the stages in a Champ, the driver adds the least "color", a good place to do it different.

For reverb you need a high-gain recovery stage. At typical tube impedances, with low-leak Si parts, this is not too hard. The transistor here gives gain of at least one hundred, adequate.

The reverb driver needs high current. Transistors can do this; though finding low-volt high-current in a Champ chassis is tough. The Alamo's reverb driver (and recovery) is fed from the 6V6 cathode.

The blurry numbers on the available plan do not really add-up: it takes 18V to hold the 6V6 to 42mA at 280V G2, but it looks like a 270 ohm cathode resistor which implies 66mA. I suspect the "270" should be 470 (a typical 6V6 bias) for ~~38mA here, and another few mA to the reverb driver.

> the transistors in the reverb stage (2N3708) are still readily available.

Not proven that the transistors are bad. In fact I would look at caps and resistors. These transistors are probably Texas Instrument's early Si parts, and were made VERY well. OTOH the resistors and caps are likely the cheapest sweepings. And, of course, bad (tarnished) connections.

Yes, 2N3709 is 30 cents at Mouser, but I suspect the actual parts used did not cover the full range of '09 specs.

2N3708 series is rated 30V breakdown, and general-purpose transistors are all 30V-60V today. We know the 200mA rating is not needed because power comes from a 14W 6V6 at 330V so 42mA is about the most it can pass safely. Likewise the 625mW rating is moot because there won't be 100mW in the circuit and any modern Si part can handle that.

hFE for the '08 part is given as 45-660! This is the "catchall" spec part. If you were fussy, you could pay more for the '08-'11 parts sorted by hFE ranges. However the Alamo designer felt he could get useful performance with "any" hFE, so was not paying for hFE sort. Since any general-purpose small Si part you buy today is likely to be in the 45-660 range, it will work as good as new. Oh, any not-dead Si transistor will pass the Vbe spec.

Assume 15V supply to the reverb. Pencil 4mA, very variable.

The driver appears to be biased with a 470K resistor to Base. Assume 14V across it, 0.03mA base current. Times hFE=45-660 is 1.3ma-20mA. But 20mA in the emitter resistance (let's pretend the tank is 1K DCR) cancels most of the 14V across 470K. A high-hFE part might suck 10mA, which seems like a large increase of 6V6 current. Maybe the designer "knew" he was not getting high-hFE parts (that someone else was buying all the 2N3711s that TI could sort-out). 100-150 is about as much hFE as this stage can use.

So we expect 10V-18V at driver collector, and one to several volts at the driver emitter.

There is an "issue" here. The reverb driver loads the preamp. This load may be as low as 22K or as high as 400+K. The output of the preamp is about 100K. So change of transistor will cause change of gain. Both the reverb path and the direct path. If that's truly a 2N3709, then Alamo just didn't care about production consistency. Rather, could not afford to care: Fender and Gibson held the high-price field, and outfits like Alamo were crunched to make something enough cheaper to sell enough to stay in business.

The recovery stage will also bias very differently depending on specific hFE. The base network dribbles around 2uA (0.002mA) into the base. Collector current could be 0.1mA or higher. hFE over 150 would saturate the transistor; again I bet the designer "knew" his '09s did not have hFE over 150.

The mixer and 6V6-driver stage has gain of 4 to 14 for hFE from 45 to 150. If the reverb driver is also low-hFE, the low-gain end will be pretty weak. Higher-hFE parts in both places give good gain (for a cheap basic amp).

The driver stage can't run on the cathode voltage because it needs a peak swing of double the cathode bias voltage. It does not need a lot of current to swing the 6V6 grid. Alamo rigged a divider to get apparently 44V. If the collector is nearly centered (20V-26V) it will swing the 6V6 to the full blistering 4 or 5 Watts. However the bias is not robust against hFE, and the plan we have shows a hand-penciled "4.6V" at collector. This will distort badly long before the full 4W output. And as I read the bias system, most hFE values will sit low. This may be a design mistake, or it may be "the Jet sound!".

I think the target hFE for all three transistors is hFE= 80 to 150. If it gets too far outside this narrow range, you can ship it and sell it and hope unhappy customers don't find you. Buyers would typically out-grow this amp (or the whole idea of becoming a guitar-star). Since it still has life and charm, if a transistor is indeed dead (unlikely), you want about any Si NPN but selected for hFe around 100.

For full 4W without extra distortion, Q3 could be selected for extremely low hFE, or the bias could be modified.

Oh, there is a fourth transistor, the FET at Q3 emitter to make the tremolo happen. This is not critical, and does not get any stress; however FETs were very new and I do not recall how their long-term life was. If there's waggle at the gate but no trem action, and the cap has been replaced, put another FET here.

I suspect the Intensity pot is not wired as shown. Take notes before you mess with it.

> a tube lineup ... 6AV6, 12AX7A, 2N3708, 2N3708, 6V6GTA, 5Y3GT

Ah, this is different from the available plan. At a guess, the mixer/driver was found to be unsuitable, and replaced with a tube triode (6AV6 is a half a 12AX7). There's no mention of the FET; it could instead tremolate at one of the tube grids.

[graylensman]

Here are a couple of quick shots, taken with miPhone.

The amp its own self:

You let the secret of the burst box out of the bag!!!!  It's one part Alamo Jet and one part Scarlett Johansson. 

I started thinking about wavley's post. So I opened up the back and sure enough, there was Scarlett.


After I hustled her upstairs to the bedroom, I shot this view of the tubes and stuff.

I tried to give it a bit more definition via Photoshop, but there's not much to work with in this image. As time and interest permit, I'll get a better camera and shoot some more shots.