I just purchased this amp. Upon inspection, it appears to be all stock right down to the Gibson branded tubes. I slid out the amp tray and it's all original parts, so I know it will have to be re-capped. I tested the tubes by plugging them into a champ I have. The rectifier and 12AX7 seem good, the 6V6 crackles when the amp is cranked, or if you tap the tube.
I have ordered all the parts to re-cap and a mullard 6V6, but I couldn't find the 20uf 25v bias caps on the first stage and output. All I could find were 25uf caps. Will these be ok?
I also plan to add a grounded cord, replace the power supply resistors, replace the 470 ohm output bias resistor, and remove the cracked suicide cap. Any other suggestions? Will these repairs effect the value of the amp? I don't plan to sell it, but I'm curious. I only paid $50 for the amp so even after the repairs I think I got a heck of a deal.
Thanks, Rick
I have ordered all the parts to re-cap and a mullard 6V6, but I couldn't find the 20uf 25v bias caps on the first stage and output. All I could find were 25uf caps. Will these be ok?
I also plan to add a grounded cord, replace the power supply resistors, replace the 470 ohm output bias resistor, and remove the cracked suicide cap. Any other suggestions? Will these repairs effect the value of the amp? I don't plan to sell it, but I'm curious. I only paid $50 for the amp so even after the repairs I think I got a heck of a deal.
Thanks, Rick
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The 25uF will be fine - in theory it will extend the bass response a little - but in practice it won't make any noticeable difference.
$50 is an excellent deal.
$50 is an excellent deal.
Yeah, when I saw it pop up on a local buy/sell facebook page for that price I hoped I wasn't too late to get it. The seller told me that a friend of his found it under a bed in his recently deceased grandmothers house and had given it to him. He said he had no use for it so that's why he was selling it. I can't wait to get the repairs done and power it up to hear how it sounds.
Here's a few photo's in case anyone would like to see it.
Here's a few photo's in case anyone would like to see it.
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20uf is not a standard value. It was common back when, but not today. 25uf is common today, but the standard value closest is now 22uf. Considering that caps in that era had mile wide tolerance anyway, the 10% shift from 20 to 22 is irrelevant. A brand new 20uf cap back then could have measured 30uf right out of the box, and still be within tolerance.
Just to be anal about old stuff, that 6V6 that crackles is bad, yes, but look closely at its pins. The glass part has wires coming out the bottom, kinda like a 12AX7 does, but those wires then fit into the hollow pins on the base of the tube. They are then soldered into those pins. SOmetimes that solder fails. You can occasionally "save" a good old tube by carefully sweating some fresh solder into the tiny hole at the end of the pin. Hey, what is there to lose? if you can find those old Gibson labeled tubes working in the amp, it strikes me as an appealing touch to have them there. Retro chic, as it were.
Nice looking piece.
Just to be anal about old stuff, that 6V6 that crackles is bad, yes, but look closely at its pins. The glass part has wires coming out the bottom, kinda like a 12AX7 does, but those wires then fit into the hollow pins on the base of the tube. They are then soldered into those pins. SOmetimes that solder fails. You can occasionally "save" a good old tube by carefully sweating some fresh solder into the tiny hole at the end of the pin. Hey, what is there to lose? if you can find those old Gibson labeled tubes working in the amp, it strikes me as an appealing touch to have them there. Retro chic, as it were.
Nice looking piece.
I'll check the pins. It would be cool to have a "Gibson" set of tubes in the amp. The 12AX7 sounds good in my champ so I'm sure it's fine as is the rectifier.
Any other areas I should look at when I'm in the amp?
Any other areas I should look at when I'm in the amp?
It is a very simple amp, you have mentioned caps, there are only so many of them to change. I suppose a resistor could be way off value, but a little drift won't matter much. Clean controls and jacks. Clean and tighten sockets. Speaker is old, and may sound great. otherwise you MIGHT consider different speakers. Not much else to consider.
I did do a quick power up without tubes just to make sure the transformer was doing it's thing. It had power on all outputs and didn't overheat or smoke. I have so far done preparations for a 3 prong cord. I opted to add a 3 position terminal strip. The center position is grounded and held to one of the transformer bolts with a nut and star washer. The outer two will be the inputs to the transformer. One side will tie directly to the AC Nuetral (White) wire the other tied to the AC Line (Black) wire through the fuse then switch. I haven't got the cord yet, so I've just wired the fuse/switch and terminal strip.
I've also replaced the multi-cap with Sprague Atoms. I haven't fastened them down yet, but the wiring is done on them.
Before:
After:
I used some cloth covering from extra wire I had to cover and color code the leads of the caps. I used the original cap wires to connect to the amp.
I'll post more as I go.
I've also replaced the multi-cap with Sprague Atoms. I haven't fastened them down yet, but the wiring is done on them.
Before:
After:
I used some cloth covering from extra wire I had to cover and color code the leads of the caps. I used the original cap wires to connect to the amp.
I'll post more as I go.
You can buy new power cords, but I have gone the cheap route the last few decades. The ubiquitous "computer" power cords, the ones that connect to the IEC male on your chassis, are usually sitting around the shop, and can certainly be found dirt cheap, make excellent power cords. I just cut off the female IEC from the one end, then strip and bare the wires that result.
In an emergency or if I need a heavy cord, I can buy a heavy extension cord at the store and snip the female end off that. Much cheaper than buying a "replacement power cord".
In an emergency or if I need a heavy cord, I can buy a heavy extension cord at the store and snip the female end off that. Much cheaper than buying a "replacement power cord".
I didn't have any computer cords long enough. I wanted at least a 12ft. cord. I didn't think of buying an extension cord... I may do that anyway so I don't have to wait for the cord to ship to me. Thanks for the idea!
Even if you buy one new, inch for inch, computer cords still cost less than "real" new power cords.
This morning I replaced the 10K & 22K power supply resistors as well as the Bias Resistors and bypass Caps on the 12AX7 & 6V6.
My power cord should arrive tomorrow so I'll likely install it Friday morning. Then will come initial power up and test time. Assuming that goes well, I'll just need to secure the power supply caps and put everything back together. I'm getting excited to hear what it sounds like now...
My power cord should arrive tomorrow so I'll likely install it Friday morning. Then will come initial power up and test time. Assuming that goes well, I'll just need to secure the power supply caps and put everything back together. I'm getting excited to hear what it sounds like now...
You have replaced everything else, I am surprised you have not replaced the two black coupling caps. Have you applied power to see if they are leaky?
The coupling caps tested good with a meter, I'll see if there are problems with the sound when I power it up. From what I've read, it is desirable to keep the originals in circuit if possible as they effect the tone much more than the power supply and bias bypass caps.
By test good with a meter, can I assume that means you checked them for value? The problem with old caps like that is they get leaky - DC comes through them. And a hand meter can't tell you that. A 400v cap might work fine at 50v or even 200v, but leak like a screen door at say 300v or more. Unless you use an actual cap tester that can apply full voltage, your hand meter measurement will never know that.
Once you power it up, check the voltage at each end of those caps. Typically one end will will be connected to a tube plate and have 150-200v on it, while the other end will go to a grid of a following tube, and should have essentially zero volts DC there. If you have the zero volts, then those caps have survived the ages. If you see positive DC voltage, even if it is only a volt or two, then they are leaky.
Leaky means electronically, it doesn't mean something oozing out from inside.
You have replaced most of the parts in it, have you not? Those two parts are not going to be the difference between vintage tone or not. That amp has not sounded like it did new for over 50 years now anyway.
Once you power it up, check the voltage at each end of those caps. Typically one end will will be connected to a tube plate and have 150-200v on it, while the other end will go to a grid of a following tube, and should have essentially zero volts DC there. If you have the zero volts, then those caps have survived the ages. If you see positive DC voltage, even if it is only a volt or two, then they are leaky.
Leaky means electronically, it doesn't mean something oozing out from inside.
You have replaced most of the parts in it, have you not? Those two parts are not going to be the difference between vintage tone or not. That amp has not sounded like it did new for over 50 years now anyway.
I'll check the DC on the grid side of the caps after I power it up. I do have a couple of orange drop caps I could replace them with if the need be. My quest for maintaining as much of the original circuit as possible isn't to keep the amp with as many vintage parts, so much as it's to try to rebuild it and achieve the same or as close to the same tone as it had 55 years ago when it was manufactured. I'll let you know the test results when I power up.
Thanks for all your advise and information I really do appreciate it. At 50 yrs old, I have been an electronics hobbyist my whole life, but just within the past 5 years or so started messing with tube amps. I know enough to know some of the terminology of them, but still have a lot to learn. I thank you for the added knowledge you've directed me toward in this project.
Well, I temporarily wired up my short cord and the speaker to the amp just to start initial testing. I did the tests w/o tubes, then with rectifier, then with all tubes. Voltages are good with right at 165 on the plates of the 12AX7 and 370 on the plate of the 6V6. At idle, the cathode of the 6V6 was at 22. I just now realized I forgot to check cathode voltages of the 12AX7, but I know the resistors measured correct earlier on so hopefully that was good as well... I'll check at next power up.
I did check the coupling capacitors for leakage at idle. the preamp coupler was a few millivolts DC and the output coupler was at about .5V DC. This was less than 1 volt, so I thought I'd see what it sounded like. One word -- BAD.
The amplifier is in a bad sounding distortion at all volume levels especially with a guitar plugged into input 1. The Low's are especially distorted. From what I've read, the sounds I'm getting are indicative of leaking coupling capacitors. So, I have now replaced them with the .022uF 600V orange drops I had in my parts box. Unfortunately, I can't power the amp up right now without waking the rest of the house 😀. I'll power it up tonight and see if that makes an improvement.
Here's a couple of photo's of the new orange drops installed.
I did check the coupling capacitors for leakage at idle. the preamp coupler was a few millivolts DC and the output coupler was at about .5V DC. This was less than 1 volt, so I thought I'd see what it sounded like. One word -- BAD.
The amplifier is in a bad sounding distortion at all volume levels especially with a guitar plugged into input 1. The Low's are especially distorted. From what I've read, the sounds I'm getting are indicative of leaking coupling capacitors. So, I have now replaced them with the .022uF 600V orange drops I had in my parts box. Unfortunately, I can't power the amp up right now without waking the rest of the house 😀. I'll power it up tonight and see if that makes an improvement.
Here's a couple of photo's of the new orange drops installed.
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