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Little tech questions from your day

All your geeky questions about electronics, microphones, tape machines, etc.

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Re: Little tech questions from your day

Postby Nate Dort on Tue Mar 13, 2018 6:31 am

kazoozak wrote:Probably gonna pick up a can of DeoxIT from Guitar Center on my way home from work, is there anything I should be wary of or should I consider a different brand of contact cleaner?


Ideally, you'd use DeoxIT Fader F5 FaderLube (or something like MG Chemicals 801B), as it lubricates the carbon contacts inside the pot. Regular DexoIT will strip away any lubricant and wear out the pots faster, but they'll stop being crackly for a while.
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Re: Little tech questions from your day

Postby TomWanderer on Tue Mar 13, 2018 8:36 am

I've got a Teisco Checkmate 50 head that I hadn't played in years because the original power cord was not grounded and intermittent. I replaced the cord with a three pronger on Saturday morning and played it for a few minutes...it sounded great, I was feeling pretty good about myself. This was at a relatively low volume, amp on the workbench hooked up to a 1x12 on the floor about 5 feet away.

Yesterday evening I put it in another room (the practice room) hooked it up to a larger speaker cabinet and gave it a spin with the mics on just to be sure it was grounded properly and not going to give a shock. It didn't give a shock, but it has quite the proximity buzz especially on the "effect" channel which is also the "dirty" channel. If I stand right in front of the amp head I get a nasty buzz that lessens if I get farther from the amp/don't stand directly in front. I tried different guitars and cables. Now I'm not feeling so good about myself!

Is this just something I didn't notice before because I hadn't used this combination of amp and cab before? (the old cab I used it with was shorter and it was closer to the floor). It might be time to take it in. I had it gone through when I bought it about 8 years ago, lots of leaky caps replaced, etc., I don't know why he didn't do the cord then!
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Re: Little tech questions from your day

Postby Dr Tony Balls on Thu Mar 15, 2018 11:52 am

I managed to fry a Vintage 30. Anyone ever done a recone with one of those preassembled recone kits? Seems like its not much more complicated than gluing in the spider and basket. eh?
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Re: Little tech questions from your day

Postby garthplinko on Thu Mar 15, 2018 12:26 pm

Not for a Vintage 30 but had good success years ago w/ using one of those kits on an EVM 15B. Pretty sure I openly discussed here UTSF
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Re: Little tech questions from your day

Postby TylerSavage on Thu Mar 15, 2018 12:48 pm

TomWanderer wrote:I've got a Teisco Checkmate 50 head that I hadn't played in years because the original power cord was not grounded and intermittent. I replaced the cord with a three pronger on Saturday morning and played it for a few minutes...it sounded great, I was feeling pretty good about myself. This was at a relatively low volume, amp on the workbench hooked up to a 1x12 on the floor about 5 feet away.

Yesterday evening I put it in another room (the practice room) hooked it up to a larger speaker cabinet and gave it a spin with the mics on just to be sure it was grounded properly and not going to give a shock. It didn't give a shock, but it has quite the proximity buzz especially on the "effect" channel which is also the "dirty" channel. If I stand right in front of the amp head I get a nasty buzz that lessens if I get farther from the amp/don't stand directly in front. I tried different guitars and cables. Now I'm not feeling so good about myself!

Is this just something I didn't notice before because I hadn't used this combination of amp and cab before? (the old cab I used it with was shorter and it was closer to the floor). It might be time to take it in. I had it gone through when I bought it about 8 years ago, lots of leaky caps replaced, etc., I don't know why he didn't do the cord then!


anything else on the circuit in the practice room? You could have better power on your workbench ... same speaker impedance? Try changing some preamp tube positions and see if it changes (ie: first two in second slot, etc.)
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Re: Little tech questions from your day

Postby llllllllllllllllllllllll on Fri Mar 16, 2018 7:37 am

I have a head and I need to figure out how many ohms it is rated for. I prefer to match the cab for more bandwidth. It is a rebuilt Showman and the maker doesn’t remember what it is. I know these amps were either 4 or 8 ohms. I’m using kind of an under powered 8 ohm cab with it right now, and I’m not sure if the color from the speakers are being pushed or from a slight impedance mismatch.

What’s the best option here?

Open it up and start googling numbers on the transformers
Take it to a tech, let him measure and do maths
Try a 4 ohm and an 8 ohm cab and see which sounds best
Some kind of other detective work

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Re: Little tech questions from your day

Postby twelvepoint on Fri Mar 16, 2018 8:25 am

Personally, I'd just try the 4 and 8 ohm cabs and see what sounds better, but if you're feeling like erring on the side of caution, it's only four screws on the back panel and four hanger bolts to open it up and look at the output xformer. You may even be able to see the product code on the xformer bottom just by taking off the back panel and using a little mirror.
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Re: Little tech questions from your day

Postby Tommy on Fri Mar 16, 2018 10:19 am

llllllllllllllllllllllll wrote:I have a head and I need to figure out how many ohms it is rated for. I prefer to match the cab for more bandwidth. It is a rebuilt Showman and the maker doesn’t remember what it is. I know these amps were either 4 or 8 ohms. I’m using kind of an under powered 8 ohm cab with it right now, and I’m not sure if the color from the speakers are being pushed or from a slight impedance mismatch.

What’s the best option here?

Open it up and start googling numbers on the transformers
Take it to a tech, let him measure and do maths
Try a 4 ohm and an 8 ohm cab and see which sounds best
Some kind of other detective work

Image


Showman = 8 ohms. Dual Showman = 4 ohms. BUT transformer code will tell you. It's the number on it that doesn't start with 606. Google it and the internet will tell you. The output transformer is in the middle of the chassis (the bigger of the two in the middle).

Also, using an 8 ohm cabinet with a Dual Showman, or a 4 ohm cabinet with a Showman WILL NOT HURT the amp. Provided it has the original Fender Schumacher transformer. These were designed to handle a 100% mismatch. I wouldn't go further though.

Also, with Fender amps always use the main speaker output jack when using only one cabinet. Only use EXT when you have 2 plugged in.
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Re: Little tech questions from your day

Postby llllllllllllllllllllllll on Sun Mar 18, 2018 5:10 pm

8 ohm Showman 022897 or something. Plugged it into a 4 ohm cab for a literal minute quietly before I got that code and it did not sound right at all.
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Re: Little tech questions from your day

Postby Tommy on Mon Mar 19, 2018 9:50 am

llllllllllllllllllllllll wrote:8 ohm Showman 022897 or something. Plugged it into a 4 ohm cab for a literal minute quietly before I got that code and it did not sound right at all.


022897 is definitely the 8 ohm guy, but 4 ohms should have sounded fine (maybe just slightly harsher) unless you were using a single cab into the EXT jack.
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Re: Little tech questions from your day

Postby llllllllllllllllllllllll on Tue Mar 20, 2018 8:28 am

Tommy wrote:
llllllllllllllllllllllll wrote:8 ohm Showman 022897 or something. Plugged it into a 4 ohm cab for a literal minute quietly before I got that code and it did not sound right at all.


022897 is definitely the 8 ohm guy, but 4 ohms should have sounded fine (maybe just slightly harsher) unless you were using a single cab into the EXT jack.


Yeah, I’ve had Ampegs and Fenders for years, it wasn’t that. Turned out to be a shit power cable, now trashed!
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Re: Little tech questions from your day

Postby TomWanderer on Tue Mar 20, 2018 11:33 am

TylerSavage wrote:
TomWanderer wrote:I've got a Teisco Checkmate 50 head that I hadn't played in years because the original power cord was not grounded and intermittent. I replaced the cord with a three pronger on Saturday morning and played it for a few minutes...it sounded great, I was feeling pretty good about myself. This was at a relatively low volume, amp on the workbench hooked up to a 1x12 on the floor about 5 feet away.

Yesterday evening I put it in another room (the practice room) hooked it up to a larger speaker cabinet and gave it a spin with the mics on just to be sure it was grounded properly and not going to give a shock. It didn't give a shock, but it has quite the proximity buzz especially on the "effect" channel which is also the "dirty" channel. If I stand right in front of the amp head I get a nasty buzz that lessens if I get farther from the amp/don't stand directly in front. I tried different guitars and cables. Now I'm not feeling so good about myself!

Is this just something I didn't notice before because I hadn't used this combination of amp and cab before? (the old cab I used it with was shorter and it was closer to the floor). It might be time to take it in. I had it gone through when I bought it about 8 years ago, lots of leaky caps replaced, etc., I don't know why he didn't do the cord then!


anything else on the circuit in the practice room? You could have better power on your workbench ... same speaker impedance? Try changing some preamp tube positions and see if it changes (ie: first two in second slot, etc.)


Thanks for weighing in. I brought it to my repair guy's house and he fired it up and explained that the initial hum is just being amplified more by the gain channel. The initial hum isn't really that bad, at least not for a 50 year old amp. When he plugged a guitar into the gain channel the hum really became apparent when he moved the guitar around the room, and that's just the single coil pickups doing what they do, acting like little antennas. He basically said the hum is a touch higher than it should be, but the high gain pickups are making it really stand out. He pulled the phase inverter and, with a bit of hum still present, explained that we are now just listening to the power section and that he could mess around for a few hours on this thing only to have the same amount of noise.

Later I found a schematic and sent it to him and he pointed out a little 100 Ohm pot on the top of the chassis that is a hum balance. He said this pot often becomes the source of noise. I may replace it, but having the amp turned and a few feet away from the guitar makes a big difference. That was why it sounded different on my work bench, I wasn't dangling a guitar right in front if it while messing with the controls. I did wire the 3 prong cord correctly though!
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Re: Little tech questions from your day

Postby PEPPER! on Fri Mar 23, 2018 2:15 pm

hey, so work gave me an Ampeg Echo Twin that had been a doorstop of unknown origin for a long time, it's the 5Y3-6SL7-6V6 version. weird amp - it's two whole 1x12" 15W amps in the same combo, with options for bridging them or using them separately. it is in rough shape and seems to have been the house amp in a KFC. the board is covered in what seems to be dried Pepsi, and every surface of the amp was really greasy with random spills and blotches everywhere. it could be the poster child for why resting your drink on top of your amp is a terrible idea.

anyway, I cleaned it up, did a little work on it, and got the non-echo channel/amp sounding pretty good. the echo side doesn't work and I've ruled out bad tubes or a bad PT on that side. it seems to have a blown OT, which makes sense as it had no speaker attached for years.

the original ones in there are Ampeg part OT149. I haven't disconnected my working one and measured it but it's the same part number that was in some Reverberockets of that era. between a number of schematics and discussions the good people of the Internet report an input impedance of anywhere from 8.5K to 10K, 15W, 8 Ohm out.

anybody have advice or a recommendation?

here are a few I found that might fit the bill. if I can afford it I'll probably replace both OTs to keep the channels somewhat similar.

Edcor XPP15-10K is 15W, 10K input at 8 Ohm, $27 apiece

Classic Tone 40-18037 -18W, specs say 9.2K input with a 8-Ohm load, $46 apiece

Hammond 1750E - 15W, 8K input with a 8-Ohm load, $40 apiece

there's also the Mercury Magnetics one which doesn't give specs and is $96 apiece.
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Re: Little tech questions from your day

Postby numberthirty on Fri Mar 23, 2018 10:23 pm

PEPPER! wrote:it is in rough shape and seems to have been the house amp in a KFC. the board is covered in what seems to be dried Pepsi, and every surface of the amp was really greasy with random spills and blotches everywhere. it could be the poster child for why resting your drink on top of your amp is a terrible idea.


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Re: Little tech questions from your day

Postby Dr Tony Balls on Sat Mar 24, 2018 8:25 am

I think any of those options look fine to me, functionally. Things to note:

- Edcor is built to order so expect 4-6 week delivery time
- Mercury overcharges like crazy and should never be purchased unless absolutely necessary
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Re: Little tech questions from your day

Postby scrotescape on Sat Mar 24, 2018 10:42 am

Get a used pp transformer off the bay. Or I could find one in the cellar for you.
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Re: Little tech questions from your day

Postby PEPPER! on Sat Mar 24, 2018 12:45 pm

thanks all! will probably go with the Edcor ones. I'd like to replace the tubes as well so the cost of those spread over 4-6 weeks is honestly preferable. unless . . . what do you have in the cellar, scrotescape?
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Re: Little tech questions from your day

Postby biscuitdough on Tue Mar 27, 2018 7:55 am

The cheaper Princeton style from Classictone should work, too:

http://www.classictone.net/40-18045.html

If you don't want to drill a hole, Fliptops has a direct replacement for way too much:

https://www.fliptops.net/catalog/p-1006 ... any-ampegs
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Re: Little tech questions from your day

Postby uglysound on Tue Mar 27, 2018 12:43 pm

Would you spend $300-500 to fix up a guitar that is probably only worth $200? I have a Jay Turser jazzbox (I forget the model number), and I really don't mind how it plays/feels. However, it could use a different bridge, new tuners, and different pickups.

My second question is slightly related. What is the best setup for testing transistor fuzz pedals? I have a bunch of old silicon and germanium transistors, and I'd like to start trying out some different fuzz pedal circuits. I know that fuzz is more fussy about impedance and where it is in the signal chain compared to most other effects.

The main problem I am facing is that I am not in a place where I can be very loud. Is there a small amp (DIY or commercially available) that will allow me to get a good feel for the character of a fuzz pedal? How dependent on volume are they? Is guitar->fuzz->amp->headphones out of the question? Or does it not matter as long as the fuzz is connected directly to the guitar?

I assume single coils vs humbuckers comes down to personal preference?
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Re: Little tech questions from your day

Postby llllllllllllllllllllllll on Thu Mar 29, 2018 10:59 am

Image

This is a schematic for the Rickenbacker M-16 amp. I’ve heard they resemble different tweed Fenders, but what’s the final word on this? Is it really that similar to a Bassman?
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