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Some technical questions for the experts

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Posted on July 21, 2017 at 07:17:40
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002
Good Morning!

I'm starting the years in the making guitar amplifier rebuild and repair next week and as a novice I have a few questions in no particular order.

Who do you recommend as a source for resisters and capacitors? Do you buy in batches or individually depending on the project needs?

Should each resistor and capacitor be removed and tested independent of the circuit?

Is the starting point to test the entire device using a signal generator and scope? How then do you test sections and components?

Thanks for any advice and recommendations. And I appreciate your patience with these questions.

Sim





 

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What amp is it?, posted on July 21, 2017 at 08:04:01
Vintage, I take it (since you posted in this area). 8^)

If it's a Fender, Vox, or Marshall --- there is a TON of data and info on the Net. You need a schematic, that shows nominal voltages. I'd measure all the B+ points, cathode voltages, idle bias on the output tubes, plate voltage, etc. Compensate for today's outlet voltages.

You should have a speaker cab, you know works. At least a stand-alone speaker that is good. 1/4-inch mono jacks and plugs. Speaker extension cord (do NOT use guitar patch cords for speakers). Many of vintage amp issues are speaker related.

I get most of my basic parts from Antique Electronics Supply. Fancy caps and resistors from Parts Connexion. You will prolly need to replace all PSU caps, bypass caps, and bias supply filter cap(s). Measure the DC passing coupling caps (on grid side).

Replace anyone that is bad. What cap you choose is up to the owner's tone desires. Replace them with what he wants. Not what you want. Guitar amp tone is a very personal thing. What sounds great to you, could irritate the heck out of the owner. Then, you'll need to rip out all those new caps and redo the whole thing. Only do mods that both of you agree to. Such as reverting BALANCE pot back to BIAS pot --- or other "blackface mods."

Do you play guitar or at least have access to a known good one. Many times the amp is fine, but the customer's guitar has issues. So, fixing the amp becomes frustrating and fruitless.

Heavy chord playing usually shows overdrive and speaker problems. Individual notes displays overtones and false notes.

In vintage Fender amps, rarely does the iron (PT and OPT) need replacing. Unless, the owner obvious shorted out the section(s). If you need iron, try to get as close to OEM as possible. Don't go for fancy stuff (like Mercury Mag), unless the owner agrees to it. IRON changes tone. Sometimes, for the worse. And they take forever to break in.

As for tubes... again it's up to owner's taste. Usually, as close to what he has in the amp now, is the best. Keep the tone as close to what he is used to... if that is what he likes.

Have fun! I love R&R'ing these old amps. Back to the original sound they had, in most cases. The amp MUST be reliable. Esp if the owner gigs. So keep this in mind, too!

 

RE: What amp is it?, posted on July 21, 2017 at 08:08:53
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002
The amp is a Baldwin C-1... I have replacement transistors coming from Russia.

Sim

 

And..., posted on July 21, 2017 at 08:15:10
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002
I do have the schematic diagram. And I'm the owner. Primary guitar is my Gibson es 330 from 1964 and a couple of others. So I can not be limited other than by my ears.

Sim

 

RE: What amp is it?, posted on July 21, 2017 at 08:17:22
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002
I also snagged a new echo unit and will add new speakers and likely fully rebuild;ld the cabinet primarily because I'm a woodworker and have some nice wood laying around... may in fact use wormy chestnut.

Sim

 

All solid-state amp?, posted on July 21, 2017 at 08:41:45
Is this amp for you or a customer? I have very little experience with SS amps. Just the usual PSU cap changes. Placing three prong wire. Etc.

Is the amp working? Does it have sound issues?

 

RE: All solid-state amp?, posted on July 21, 2017 at 13:49:21
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002

The amplifier is mine. I first heard this amplifier around 1964 at a dance. This is an eBay acquisition that never worked. At the time was using a Sears Silvertone stacked unit with my Gibson That was a tube amplifier and served me for all of my R&R band days in the '60s. I gave that amp to one of our neighbor boys who had just taken up guitar. I was busy with career and family at that [point in my life. So this is a challenge and hopefully a fun project especially if I can resurrect it.

It's this amplifier from Willie Nelson's set-up. https://www.gearslutz.com/board/attachments/so-many-guitars-so-little-time/340000d1365778810-baldwin-c1-custom-professional-amplifier-supersound-6877913729_701aa1bc78_z.jpg

Sim

 

Free Shipping on Parts Orders, posted on July 21, 2017 at 13:51:18
FlaCharlie
Audiophile

Posts: 940
Location: Gville, FL
Joined: June 1, 2003
When I started getting involved in this hobby years ago I quickly realized that I couldn't afford to pay a tech to do all the work and that, if I was going to work on stuff myself, it made sense to have a stock of parts on hand. Since I had collected quite a few tube stereo amps it made sense to set up a shop and stock a large quantity of parts - resistors and caps of various values. If you know you'll be working on a lot of amps this makes sense.

If you're just getting started and only have a few projects it doesn't make sense to order 100 resistors in every value or a wide variety of caps, just get what you need for the project at hand. Of course ordering parts - especially if you're only ordering a few parts at a time - gets very expensive because of shipping costs.

Here's a solution I learned about a few years ago. One supplier, Digi-Key, has an unusual policy that is hidden in the fine print on their website. If you order online from the website you pay shipping just like when you use other suppliers. But, if you mail in your order and pay with either a personal check or a money order, they will ship for free. Just print out list of what you want with the part numbers, mention their free shipping policy and drop it in the mail. It only takes a couple more days to get the order. I use this regularly, even with larger orders.

I have no connection to Digi-Key except as an occasional customer.

 

RE: Some technical questions for the experts, posted on July 21, 2017 at 21:35:37
fredtr
Audiophile

Posts: 1973
Location: Phoenix
Joined: January 4, 2005
Generally you get what you pay for, my suggestions are a particularly good example.

Semi-conductors rarely degrade over time. Using a substitute when you don't need to is asking for trouble.

Electrolytics do degrade over time. So replace them. Coupling caps can be measured for leakage with the unit powered up. Or you can just replace them, depends a lot on what types they are and if you are interested in subtly changing the sound.

Resistors can crack, or increase in value over time. In a lot of circuits resistors can be measured in place, in some cases wait for the resistance to increase as the parallel capacitors charge. Most resistors will be ok.

So my suggestion is change out the electrolytics. Measure the resistors particularly the larger (power) resistors in circuit. Worst case lift one leg to measure. Replace the ones that are out of spec. If the output transistors are easy to remove check the resistance. Replace if necessary. Then power it up, and trouble shoot it. If you have a variac, bring it up slowly and check voltages. Since you have owned it for a while, and have powered it up, if it hasn't blown fuses or smoked resistors, skip the variac.

Don't change any semi conductors (transistors and diodes) unless they are bad. Check coupling caps for leakage and if you like, experiment with different types one at a time, for your sound preference.

I agree with digikey as a source, if they don't have it try ebay or for some of the audio caps the sites that specialize in them. Avoid vintage electrolytics, use the new stuff even if you have to make physical changes and it is ok to increase capacitance, as long as it is within reason (50%).


 

RE: Free Shipping on Parts Orders, posted on July 22, 2017 at 06:29:53
Brian McGowan
Audiophile

Posts: 1635
Location: Wisconsin
Joined: December 16, 2004
Will have to keep that in mind myself,about Digi-Key & free shipping!

 

kudos, thanks, and props, posted on July 22, 2017 at 06:34:07
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002
Thank you for the sage advice. This is quite helpful. I'm excited to get started.

Sim

 

One more question, posted on July 22, 2017 at 07:04:31
spindoctor
Audiophile

Posts: 1660
Location: Virginia
Joined: December 31, 2002
What approach do you take to the old "can" caps with multiple capacitance. Bypass in circuit?
Tx.

Sim

 

RE: One more question, posted on July 22, 2017 at 08:35:16
fredtr
Audiophile

Posts: 1973
Location: Phoenix
Joined: January 4, 2005
Electrolytics have gotten smaller. There are companies that stuff the old cans with the smaller caps to maintain the same appearance. I usually do what you stated, bypass them, disconnect and put the replacement caps underneath.

 

I wasn't aware of that. Thanks! (nt), posted on July 25, 2017 at 19:29:25
1973shovel
Audiophile

Posts: 10110
Location: Greenville SC
Joined: February 25, 2007


 

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