Old Amp Hummm

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Hey, I have an old "Doric" brand Italian amp from the '60s I bought in a curiosity shop a while back. It's 6v6 (so probably 15W) and powers a 12" speaker. From what I can glean, Doric may have been some kind of sister brand to VOX, but there is very little Doric information on the web, other than a couple of organs which bear the name.

Anyway, I like this amp a lot and find it has amazing reverb for lead work. It used to cut-out after prolonged playing, but I retubed it and that's no longer an issue. But it still hums more than I'd like, and the reverb hums so much past 12 o'clock it's unusable. The hum makes it poor for recording. I'm going to get it fixed up real soon, but I just wanted some information before I bring it to the shop. Is replacing the caps about what needs to happen to limit by background noise issues? Anything more I should expect?

Thanks in advance,
GB
 
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I could post pics later if anyone is interested. You can see its basic style on my avatar photo. It's the grey-grilled amp on the far right.
 

teleharmonium

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Guild Beginner said:
Hey, I have an old "Doric" brand Italian amp from the '60s I bought in a curiosity shop a while back. It's 6v6 (so probably 15W) and powers a 12" speaker. From what I can glean, Doric may have been some kind of sister brand to VOX, but there is very little Doric information on the web, other than a couple of organs which bear the name.

Anyway, I like this amp a lot and find it has amazing reverb for lead work. It used to cut-out after prolonged playing, but I retubed it and that's no longer an issue. But it still hums more than I'd like, and the reverb hums so much past 12 o'clock it's unusable. The hum makes it poor for recording. I'm going to get it fixed up real soon, but I just wanted some information before I bring it to the shop. Is replacing the caps about what needs to happen to limit by background noise issues? Anything more I should expect?

Thanks in advance,
GB

Usually what you would get done in that case is replacing electrolytic caps and adding a chassis grounded 3 prong power cord. Sometimes to get an amp really quiet it's necessary to do a bit more like twisting the filament supply wires below the tube sockets, making some minor circuit changes to better isolate the ground path from the audio circuit, or even changing around the grounding scheme in the chassis, but it makes sense to do the standard stuff first and then see if you think you need to do more and whether it would be cost effective considering the value of the amp.

Old Italian amps sometimes have questionable build quality but they can sound great. I've heard some cool Farfisa amps. It's possible your reverb tank might need replacing, which might lead to a need to change the reverb drive and recovery circuits to better suit a modern tank.
 

capnjuan

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Hi GB; what TH said.
 
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Thanks to the both of you. I have an amp tech who ows me a favor, so I'm getting it re-capped for free, but wanted to know about anything else I could do for it. I hope the reverb tank lasts.

Do either of you know much about the two lines that go into the tank? I've tried switching how they were plugged (they were unplugged when I bought it), and it sounds the same. I would tend to assume there is a right way to assign what plug goes in and what goes out but they both work.

Old Italian amps sometimes have questionable build quality but they can sound great.

I think that's the issue. The craftmanship looks pretty cheap, but it sounds so good and unique I'm willing to spend a bit of energy on it.
 

capnjuan

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Guild Beginner said:
... reverb tank ... I've tried switching how they were plugged (they were unplugged when I bought it), and it sounds the same.
Hi GB;

Reverb tank: there are two sets of mission-critical parts in the reverb tank; the transducers that join the in/out wiring to the springs and the springs themselves. If the transducers and springs are good and the wiring connections aren't crudded up, the tank will work. In fact, you should hope it does; buying a reverb can is easy - determining which reverb can in terms of input/output impedance is a serious pain.

Cables: if the reverb works irrespective of which cable is plugged into which end, then the tank's input and output impedances are apparently matched and, if as you say, it doesn't make any difference, then I guess it doesn't make any difference. Are the in/out jacks on the reverb tank marked? The old Gibbs/Hammond tanks are usually stamped next to the jack. Are the cables hard-wired at the chassis end or jacked into the chassis? If jacked, not marked at chassis?

If there's a third transformer on the amp, then the reverb is transformer- and not speaker or capacitor-coupled. The secondary of the reverb transformer is connected by cable to the input of the reverb can. If you trace the transformer wiring, it has two pairs of wires. One wire of pair 'A' is connected to the reverb drive tube. If you find a wire tied to a tube, then you follow pair 'B'; one of those is connected, probably on a wiring strip, to one of the reverb cables. That is the reverb out or 'send' cable which - if it made a difference - would be plugged into the 'input' jack on the tank.

Got pics of the chassis interior? CJ
 

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Hi GB,

I just read the post, "its a clean machine". The owner of a guild amp just clean it up, including cleaning off rust, and it got me to thinking about your problem. Considering the great advice you've already gotten this seems almost trivial, but rust can wreak havoc with grounding (at least it does with old cars), seems like the same might apply to an old amp.
 

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Guild Beginner said:
I could post pics later if anyone is interested. You can see its basic style on my avatar photo. It's the grey-grilled amp on the far right.

Please?
 
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Okay, I took some pictures. Getting the whole chassis documented would be a chore as you'll soon understand, so I did the best I could without wanting to unscrew everything. It looks like the tank connects to the reverb knob while the other cable heads into a hole which I'm guessing leads to the big transformer? I'm thinking it must not matter which way they are plugged--nothing is labeled.

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dapmdave

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That's a cool-looking amp. Lots of cabinet with mucho space inside. And white chicken-heads!

Dave
 

capnjuan

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Guild Beginner said:
... It looks like the tank connects to the reverb knob while the other cable heads into a hole which I'm guessing leads to the big transformer? I'm thinking it must not matter which way they are plugged--nothing is labeled.
Hi GB; I guess the 'no label' thing adds to its Italian charm :wink: . The spacious cabinet will contribute to the tone; closed back and grill gives a bass reflex quality; when the cone moves backwards, it sends out sound waves too ... in this case, they'd be reflected off the back wall and out the grill ... the British thingie ... and overcomes the (notorious to some) woolly bass of the semi open-back cabinets of the early 60s ... remembering all the time that one man's lima beans are another man's strawberries ...

The cylinders with the red dots are the can caps; surprised to see two of them but ... also there's 110V power available to the touch in the red box. The green is the incoming line power and one side hits the fuse post and the other the power switch. The power transformer's primaries can be seen scuttling out the blue hole to the fuse and switch respectively. If you have the cord plugged in, don't touch anything in this area without first wetting your finger and putting it in your ear. The can caps and the cord have to be replaced.

Is one of those tubes a rectifier? The absence of a standby switch suggests one of them is but you never know. Don't see a third transformer and hard to believe there's enough space underneath so your reverb is either capacitor-coupled a la Ampeg or maybe tube-coupled but that is the least-common way of doing it. Yes; one reverb cable goes to the switch but the switch can either be on the input or discharge side of the can; not dispositive. The amp has two channels; one for the reverb and the other ... not. Plugging in either jack takes you to the tube with the magenta circle upper right. CJ

GBsDoric02.jpg
 
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Thanks Juan. I'm pretty sure I replaced a rectifier-type tube. In addition to the 6v6s and 12AX7s, I also replaced a 6CA4 and a 6DR7. Are either of those last two rectifiers?

The amp has two channels

I had experimented and came around to thinking the "Reverb" jack was for a footswitch, because it reacted to one. You can plug into that jack though--it just sounds really far away and muddy.

So I'll replace the cord, and the caps. Including the can caps?

Thanks Again.
 

capnjuan

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Guild Beginner said:
Thanks Juan. I'm pretty sure I replaced a rectifier-type tube. In addition to the 6v6s and 12AX7s, I also replaced a 6CA4 and a 6DR7. Are either of those last two rectifiers? I had experimented and came around to thinking the "Reverb" jack was for a footswitch, because it reacted to one. You can plug into that jack though--it just sounds really far away and muddy. So I'll replace the cord, and the caps. Including the can caps?
Hi GB: Ok; yes, the 6CA4 is the rectifier thus no need for standby. The 6DR7 is a twin-triode; having two physically identical halves but each has its own operating characteristics ... like a 7247 / 12DW7. One half tolerates 250VDC on the plate with a moderately high amplification factor of 68; higher than a 12AT7 (60) and about equal to a 5751 (70). The other half just tolerates 150VDC on the plate and has a low amplification factor of 6 suggesting that the higher voltage/amplification side is reverb drive and the lower voltage/amplification side is reverb recovery; don't need a lot of energy to return the wet signal to the line. Ok; 'reverb' jack is for the footswitch. Wasn't sure whether the 'reverb' switch included a set of off/on contacts on the back; switch on for function, turn to raise/low depth.

Just to clarify; the caps that must be replaced are the two can caps and any other electrolytic caps ... the type commonly used as cathode resistor bypass caps. Your tech will know. Alternatively if you disassemble it and take pics, I can spot up any other 'must go' caps. The rest of the coupling caps ... that's elective on your part although it's possible that your hum is a function of a defective coupling cap ... not blocking DC allowing DC to appear and get amplified in the audio circuit. After he replaces the can caps, your tech will know whether any more of the coupling caps have to go ... that is, the amp might be still humming up a storm but the can caps have to go regardless. Good luck with your amp. CJ
 
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Thanks CJ. I like to go into repair ventures with a little bit of knowledge beforehand. You've been very helpful.

Assuming I can get the hum under control, I'm guessing I'll want to put a better speaker in there at some point. I've had good results with Warehouse speakers out of Kentucky. I'll have to pull the speaker out and get some numbers, but I guess it will be something like a 15W 8ohm replacement? I noticed you (CJ) used a Webber speaker on your cool Gibson. Does anyone here think having an alnico-type speaker in their collection is worth it? I've never had the pleasure to try one. I don't know if it would be worthwhile for this grade amp, but for that "British thing" they interest me.

Otherwise, I'll find a footswitch. I used a two button footswitch from another amp when doing testing, but I think a plain old one buttom Fender amp footswitch should work for a permanent fixture?

It's exciting to bring new life into something thats been hanging around in my living room looking (and sounding) Midieval for a while. :) I also love talking about amps.
 

capnjuan

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Guild Beginner said:
... but I guess it will be something like a 15W 8ohm replacement? I noticed you (CJ) used a Weber speaker on your cool Gibson. Does anyone here think having an alnico-type speaker in their collection is worth it? ... but I think a plain old one buttom Fender amp footswitch should work ...
Everybody here who messes with amps has their favorite speaker mfr; same with strings, tubes, on and on ... if it's something people can have opinions about, there will be lots of variety. As our BBer JP pointed out some time ago, there are 246 different types of cheese registered with the French government ... there is a lot of room for individual tastes. If you want a true vintage sound, then you'd need a speaker that matched what was delivered in commercial amps back in the day. Most '50s/early '60s amps were shipped with alnico speakers; if you want yours to sound like it just came off the showroom floor, then it's an alnico speaker you'd be after. The '50s generic commercial standard was the Jensen Alnico 5 and later the P12R. The 'budget' Jensens alnicos were the 10" and 12" 'S' models with smaller magnets and voice coils ... commonly with silver frames.

Weber's dupe of the P12R is the Vintage 12A100 and that would be the alnico speaker I'd suggest for your amp ... you can read about it here at https://taweber.powweb.com/weber/ click Speakers on the left and open each section. If the money is an issue, consider the Weber Signature 12, also an alnico speaker; smaller magnet and voice coil ... great tone but with early breakup. If you think you might be interested, I have black-frame re-coned '64 Jensen P12Q / 12A125 Weber equivalent you could have for reasonable money. That would get you more of a 'roots/vintage R&R' sound but if that isn't what you're after, you might want to rethink the speaker selection. I also don't know how readily your amp breaks up and whether you like what it does. Low power, 20-30W alnico speakers, especially P12R types like the 12A100 will tend to break up more quickly than a ceramic speaker with the same weight magnet and voice coil diameter. I personally don't need a lot of power and like 6V6s and prefer amps and speakers that breakup early ... but then I like Gouda where others prefer Swiss.

Yes; if you want alnico, you want a Weber or a vintage Jensen and any single-button FS will work. CJ
 

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From looking at it, I would have to think that it's an organ amp. Closed back, only one input and check the size of that output transformer!

Not that I have anything to add other than the equivalent of jumping up and down and yelling "Me too, me too!"
Very cool amp and I'm not surprised that it sounds good.
Something else to keep an eye out for is "Acetone". It's a keyboard company, but the amps were made by Premier/Multivox...
 
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I personally don't need a lot of power and like 6V6s and prefer amps and speakers that breakup early ... but then I like Gouda where others prefer Swiss.

I think we're on the same page, CJ. Even with the cheese. :lol: I typically fingerpick and often play with acoustic instruments, so I don't find myself needing much headroom. I also live in a basement apartment, so early breakup is a bit more friendly to my landlords volume-wise. I'll do some alnico research and perhaps PM you about your available speaker later, thanks very much.

From looking at it, I would have to think that it's an organ amp. Closed back, only one input and check the size of that output transformer!

That's funny Default--I didn't even consider it could have been an organ amp, though all of the stuff I pulled up about Doric was organs. I was thinking inside the proverbial cabinet. The guy I bought it from had been using it as a guitar amp, and I never questioned it. This might be a silly question (because I have been using it for this purpose), but can you "use" an organ amp for guitar? Is some of the hum having to do with inputting a guitar instead of a keyboard (assuming it really is an organ amp)? It does sound really cool. It has a real Ventures-esque surf tone.
 

capnjuan

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Guild Beginner said:
... and perhaps PM you about your available speaker later, thanks very much.
Anytime GB; good luck. CJ
 

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Guild Beginner said:
That's funny Default--I didn't even consider it could have been an organ amp, though all of the stuff I pulled up about Doric was organs. I was thinking inside the proverbial cabinet. The guy I bought it from had been using it as a guitar amp, and I never questioned it. This might be a silly question (because I have been using it for this purpose), but can you "use" an organ amp for guitar? Is some of the hum having to do with inputting a guitar instead of a keyboard (assuming it really is an organ amp)? It does sound really cool. It has a real Ventures-esque surf tone.


I don't see why not, there might be a difference in voicing (more bass maybe?) I would think that the hum would be a service thing at this point, rather than using a guitar instead of an organ. The single input is what makes me think organ - almost all other amps have at least two, maybe more inputs.
This one doesn't have any input for an accordion :x :cry: !
 

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During the recording of Abbey Road, George Harrison was almost exclusively playing through a Leslie speaker cabinet which was designed for use with an organ. And a lot of rockabilly guitarists play through a Fender Bassman amp (or similar gear). I don't think playing through a non-guitar amp should in and of itself create a problem.
 
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