Rejuvenating old Alamo tube guitar amp (recapping, etc)

[leigh]

I got an Alamo Embassy guitar amp a few months ago. It works, but has no low end. I've got some ideas for helping it out that I wanted to run by you knowledgable folks.

First, as background, the Alamo Embassy is similar to a Fender Champ. I haven't finished tracing the Alamo's circuit, so for a rough reference, here's a Champ schematic. The Embassy has 4 tubes: 5Y3GT, 12AX7A, 6V6, and a 6AV6 (which is used for a tremolo circuit).

My general plan is to recap the audio path, figuring that the old caps may have lost some of their capacitance, and thereby their ability to pass low freqs. Correct so far? (As a side note, if I pull an old cap out of the circuit and the multimeter shows it at full capacitance, can the cap still be worn out in some other way that the meter doesn't see?)

More specifically, I also want to make a few optimizations to the input section of the amp. Please tell me your thoughts on these steps:

1. Remove the input resistors (75K on the Champ schem, 47K in the Embassy). I've been told these are not necessary if you're only using one input at a time.

2. Remove the input capacitor (.02 on the Champ, .005 on the Embassy). If I'm feeding the amp with only passive sources, there's not going to be any DC bias on the input signal. So it's safe to remove this cap? Unless it also interacts with the grid leak resistor....(see below)

3. Beef up the grid leak resistor (shown as 5 meg on the Champ). Right now, on the Embassy, that resistor is only 220K, which someone had modded from whatever the original value was. Since the cathode of that first tube stage is connected directly to ground, my understanding is that the grid resistor is responsible for setting the bias of that stage. 220K seems low, then. According to what CJ says in this thread, grid leak resistors should be at least 5 megs. Also, according to what PRR says in the same thread, that input coupling cap is necessary to maintain correct bias, so maybe it should stay put.

Thanks in advance for any clarity you good people can add to this.

Leigh

 

[adamasd]

I will reccomend changing any dieing parts before you worry about mods. After that is done see how it sounds and try it with an external speaker cab, alot of those old speakers from the 50s and 60s are really lacking in the bass by todays standards. If i remember correctly the output transformers on those are really undersized, but I have not seen one of these in a very long time. Upgrading the output transformer will really help on the bass if this is so. I like the Hammond 125ESE for this job, or the 125DSE or 125CSE if room is tight, although to me the 125ESE sounds better in these single ended 6V6 amps. When the coupling caps in the audio path get bad they leak some DC through to the next stage throwing off the bias, so you need to check the DC on the grid of the next tube to see if they are any good. Get it all up to spec and hear how it was ment to sound before you worry about mods, you might be surprised.

My first amp was a old Wards Airline 1x8 champ like combo, I did not like the sound so I made my first tube amp with its parts, a few years later I got bored and remade it it, and I was really surprised, once all those old leaky caps were replaced and its tiny tiny output transformer was repalced it was a fantastic amp.

For some output transformer comparison,

http://area23.org/~silas/pics/amp1.jpg
There it is my first amp, you can not even see its tiny tiny output transformer hidding behind the 6V6.

http://area23.org/~silas/pics/9021a_2.jpg
And the Airline after rebuilding with its new output transformer.

You really need a good air gapped transformer on the output or bass will suffer, alot of those old amps had no airgap on the output transformer, my airline being one of them.

adam

 

[leigh]

[quote author="adamasd"]I will reccomend changing any dieing parts before you worry about mods. After that is done see how it sounds and try it with an external speaker cab, alot of those old speakers from the 50s and 60s are really lacking in the bass by todays standards. If i remember correctly the output transformers on those are really undersized[/quote]

Good points, adam, I will certainly try to get the amp back to spec before changing the circuit around. It looks like the output transformer has already been replaced, and while it's not physically huge, it is what's sold as a common Champ replacement (it's a TF-103, I don't know the manufacturer.) If I get the rest of the amp restored, and the bass still sucks, I'll definitely try plugging it into a different cab, and/or swapping in a bigger output trannie.

NOW, about getting the amp back to spec, as I noted before it looks like the grid leak resistor has already been replaced. It's a modern resistor, while other resistors in the amp have those olde schoole, strictly cylindrical, dark brown bodies. I've got no idea who worked on it, but my guess is that the value that's in there now is low. As I noted above, it's 220K, whereas grid leak resistors are usually in the megaohm range. Without a schematic I don't know for sure what "spec" was, but I'm guessing this resistor should be higher than its current value. Seeking educated guesses about this...

Leigh

 

[Gus]

Try removing the trem tube I have a kalamazoo model two that gets louder and sounds better with the trem tube pulled still has the stock caps and tubes.

Also try a new cathode bypass cap at the output tube you can lose bass there. I added one to my model two(only thing I did)

As posted above try a different speaker or even try a 4x12. A small SE into a 4x12 can be very cool.

Here is something to try turn the amp on plug in a cord leave one end open and turn up the vol to hear the hum and let it cook. Let the amp run for hours >3 hours. Only do this when you are there to watch it( I like to do it outside on a cement etc surface. I like to power up amps every 6 months and let them reform the caps. The heat seems to help reform the caps.


The stock caps might still be good after reforming. I bought a silvertone SE 8" amps in pieces at a guitar show years ago for $20.00. I put it back together and let it cook, only part that was bad was a 68k resistor IIRC.

Yes the transformers are a little on the small side in some older se amps but with a good speaker they can often sound good.


a link http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/M2/

 

[adamasd]

NOW, about getting the amp back to spec, as I noted before it looks like the grid leak resistor has already been replaced. It's a modern resistor, while other resistors in the amp have those olde schoole, strictly cylindrical, dark brown bodies. I've got no idea who worked on it, but my guess is that the value that's in there now is low. As I noted above, it's 220K, whereas grid leak resistors are usually in the megaohm range. Without a schematic I don't know for sure what "spec" was, but I'm guessing this resistor should be higher than its current value. Seeking educated guesses about this...

It is probably not a grid leak resistor, it along with the input cap set the frequency response of the input stage, it forms a highpass filter with a cuttoff frequency determined by f = 1/(2*pi*Rg*Ci), Rg is the grid resistor and Ci is the input cap, so f = 1/(2*3.14*220k*0.022uF) so f = 32.9khz. It is letting pretty much everything through it looks like, but it will filter out anything out of the audio range, but it can be lowered for guitar to suit your taste. Once you finish traceing the schematic we can give you more help.

adam

 

[leigh]

Correct me if I'm wrong, but since the cathode is connected directly to ground, doesn't that mean that the grid resistor is doing the biasing? And so that 220K resistor is functioning as a grid leak resistor? I'm fuzzy about these concepts, so I could be totally off the mark.

Leigh

 

[adamasd]

Well then, you are most likely right, 220k is really pretty small for grid leak bias, I think the 12AX7 data sheet reccomends 10Meg, I am still learning grid leak bias, but 220k seems to small to work, or at the very least distort very very easily. I am still figureing out grid leak bias so maybe one of the more knowledgeable folks will chime in here. The 220k resistor does go between the grid and ground right? and not from the grid to a voltage source? That is just grasping for answers though, the finner points of grid leak bias are lost on me still.

adam

 

[Gus]

Leigh

Try a larger value r at the grid leak section. Radio shack had 10meg resistors in the past hanging on the wall they might even have them in the drawers now. Or even solder a string of 1 megs for 5meg. You need the input cap.

First thing I would do is change the resistor and play the amp for a few hours. The older designs were often built with the smallest amount of parts. Sometimes the best sound out of some of the cheaper older amps is to leave them as they are or only change the parts that are bad.

It might be better to just build a new amp or buy a new SE amp (some sell for $100.00) to mod. It might be better to sell the fixed old vintage tube amp

Have you checked the ax84 site?

 

[leigh]

Thanks for the input, people, I've been doing a lot of reading this morning about amp designs. Formerly fuzzy concepts are somewhat shaping up for me.

Just so we're clear moving forward here, I never intended to radically modify this amp. Mostly, I wanted to bring it back to spec, and then maybe take out a couple of unneccesary parts. Since I know now that the first tube stage is using grid leak bias, I know that the input (series) cap needs to stay in place (to block the DC). However, I will try removing the series input resistor from at least one of the three input jacks, to allow for a hotter signal to pass through.

I'm also pretty sure at this point that whoever put that 220K grid leak resistor in there didn't know their shit, and I will swap in a multi-meg value.

That AX84 site is pretty great, thanks for the tip. There's also a lot of detailed guitar amp info at the Tone Lizard site.

As I look at other tube amp schematics, I'm realizing that the Alamo is a bit different from all of them. I was told at one point that the Embassy was almost a Champ clone, which is not the case. They both do use grid leak biasing on the first stage, which I haven't seen yet on any other guitar amps. And it is a small, single-ended amp, but the input section at least is quite different. If I get a full schematic done, I will certainly post it here. But, in short, the Champ 5C1 goes from an 6SJ7 pentode tube, to a volume pot, to a 6V6 output tube. The Embassy, in contrast, goes from half a 12AX7, to the volume pot, back to the other half of the 12AX7, and then to the 6V6.

Cheers,
Leigh

 

[Gus]

Yes that is like a later model champ or princeton but with a grid leak. The fenders changed to cathode bias later on.

Google for fender amp schematics check places like the blue guitar site check ampage.org etc. for other schematics Lots of stuff on the web.

To first test the amp use lower output single coils and no pedals, (guitar to amp) this is more for how it was designed I would guess.

 

[adamasd]

As gus said with low output single coils. grid leak bias breaks up easily, so if you use humbuckers or most single coils you may not even need to remove that input resistor, it may actualy be a bennifit if you use higher output pickups like humbuckers. The only pickups that I have that were not able to get quite abit of distortion out of grid leak are the ones in my old 60s Silvertone, and even those got it to the point of starting to break up. Experiment, its a simple resistor to change around, see what sounds best for your playing.

adam

 

[leigh]

[quote author="adamasd"]It is probably not a grid leak resistor, it along with the input cap set the frequency response of the input stage, it forms a highpass filter with a cuttoff frequency determined by f = 1/(2*pi*Rg*Ci), Rg is the grid resistor and Ci is the input cap, so f = 1/(2*3.14*220k*0.022uF) so f = 32.9khz. It is letting pretty much everything through it looks like, but it will filter out anything out of the audio range, but it can be lowered for guitar to suit your taste. Once you finish traceing the schematic we can give you more help.[/quote]

This turned out to be good advice, although your numbers were off: to examine the input section as an RC network. If you do the math for what the Alamo had when I got it, a 220K resistor to ground, with the input signal running through a .005 uF cap, you get about a 90 Hz rolloff point. Hmm... no bass!

I had replaced the power caps, with no improvement in bass, and so went back and looked at this input RC issue. I had only been concerned with the grid leak biasing function of the input section, and hadn't run the numbers on the frequency response.

So I upped the resistor to ground to 2.4 megs, and golly, the thing actually has bass now! This is definitely not a small difference from before, and plus I recorded before and after snippets to make sure my imagination/wishful thinking wasn't fooling me.

Thanks to everyone for the help in fixing this amp!

Cheers,
Leigh