Rickresource Rickenbacker Forum

Hi, my 70's Fender Twin almost exploded last week and now just makes the worst noise ever. At what point is a Twin beyond repair?
Please help

cheers

It depends on the shape it is in cosmetically, how it sounded to you before the big bang, and most importantly how attached are you to it? 70's Twin's are not worth as much as the 60's models and don't sound as good IMHO, but if you really like it fix it. I would think spending $100-200 on it if it is in good shape is not unreasonable.

Simon....I have a Silver Face Twin Reverb....the hated 'linear' version.
I bumped into a guy I did not know at a music store and he said it didn't work and didn't want to spend the money to fix it so I bought it from him for $125.
I took it home and in about 1/2 hour had it fixed with a new power switch for $3.00.
Get an estimate on how much $$ it will take to repair it. Find a local amateur radio guy and he will probably do it for free....I would do it for you but you wouldn't want to pay the shipping!!!

haha

Dave

Literally speaking, a Twin is never beyond repair... any qualified amp shop can bring a long gone Twin back "from the dead".

The question really is: At what point does it become financially silly (re: fiscally irresponsible) to perform such a
"resurrection"?

My guitarist buddy had a '69/'70 non-master volume Silverface Twin that he paid $200 for. It was subsequently "born again" courtesy of the wizards over at legendary Kendrick Amplification. It was given the old "Blackface" mod, new EVM 12L speakers and a cosmetic "once over" all for about $600.

All I can say is "WHAT AN AMP THAT WAS!!!"

His amps have become smaller and lighter as he's grown older (and wiser, I guess...), but he's yet to play through a rig that sounded as sweet as that Twin.

Your Twin will need all it's electrolytic capacitors replaced, besides what else broke, due to it's age. Probably some resistors as well.

The age is the problem with your amp. If it has been gigged really hard, then maybe moving on to another one would be better. If lightly used, then maybe a rebuild/repair is in order.

I have fixed several 70's Fenders here in Lima with original electrolytics, they really only need to be replaced when they start to hum. They are not a service at a "certain period of time" thing like some people will have you believe. Although it is recommended that they be replaced in 10 years, but in reality I have seen them last much longer. I have seen them last for over 30 years sometimes, depends on the climate, how often you use the amp, etc. Actually a lesser used amp is going to need new caps sooner than a well worn amp. A prime candidate for new caps is an older amp that has been sitting unused in a damp place like a cellar for several years.
I have a 17 year old SVT with perfect caps in it. (well they were last August when I left the states, these I am wondering about because it has not been on for a long time)
And that is how I understood his post, when is it time to give up the ghost on an amp? A good twin is worth sinking some money in.
Well... I suppose you could go get a modeling amp and put it on "twin", they sound exactly the same, haha!

I've tested unused recent production electrolytic caps in my shop that failed, sometimes due to excess leakage, but usually due to dielectric absorption. The Sencore LC-75 will perform this test easily, but you can do it with a few components and a DMM. I doubt that very few repairmen test new caps for leakage at full rated voltage, and even fewer for dielectric absorption. The dielectric absorption can become quite severe before the part fails in the circuit. Anything new over 5% DA should be rejected. Dielectric absorption is measured by bringing the cap up to full voltage, quickly discharging it, and testing for residual voltage. For a 150 volt cap, bring it up to rated voltage, discharge it through a low ohm large power resistor, then immediately measure the voltage. If it is over 7.5 volts, toss the cap. DA forms an "internal battery" that will not discharge, and reduces the capacitance of the cap. I would do DA tests first, as a much higher percentage of caps have failed this test for me. Some of the older caps you find laying around may not pass this test, but recent technology caps should pass it with flying colors, or they are definitely bad and will get worse. Conversely, caps with higher than normal leakage often reform and do fine, so don't toss them before leaving them charged for a while and retesting.

I bought some Nichicon computer grade caps that I am using in my Heath W5M rebuild, and they all tested fine. Most of the gray colored IC brand axial caps sold in many guitar repair parts sources registered very high DA, usually around 30%. While bad for high end audio unless heavily bypassed by film caps, their reduced capacity and sluggish response might sound good in a distortion circuit. You could use 50% more capacitance and make up for the DA loss. Older electrolytric caps had much higher DA than new caps, and it could be a reason for vintage amp sound. When using modern caps, you could try using about a third less capacitance to make up for the lack of DA.

I think the worst thing is to let an amp sit around. Every time I've left an old amp around for any extended time it has hummed when I turned it back on. I've got to recap my other 70's head now.

Dielectric absorption greatly reduced the capacitance and caused the hum. Old caps will reform slightly after being on for a while, but it will get worse with time. I have a vintage Marantz 125 tuner from the 70's that would quit humming after being on for 10 minutes, but now it's permanent. In sound quality it blows away these cheap digital tuners people buy today, so I'm going to recap it. It has polished aluminum faceplate and lots of cool colored lights when it's turned on. I call it the Ginza Strip look. It's one of the last USA designed Marantz tuners, and one of the better performers. It's like a Model 150 without the tuning oscilloscope, otherwise identical performance. Since vintage Marantz has turned really collectable, I'm going to fix it up. The electrolytic caps lasted 20 years. It has great construction with the separate circuit boards shielded in their own separate boxes.

A tube amp should be recapped about every 10 years if it's used much, because the heat cooks the caps. I think carbon comp resistors are **** as well, no matter what the tube amp gurus say. You can use metal oxide resistors or several other types that don't have the ringing of early metal film resistors. I built a preamp with metal film resistors and don't hear the sonic colorations that other people say they have. I also used cheap E.C.I. metalized polyester film caps that are said to have a warm sound for signal paths, so maybe the two cancel each other out. The cheap gray plastic covered IC brand electrolytic caps are ****** enough to sub for original electrolytic caps in antique tube amps. I would prefer Sprague Atoms in any modern amp, and Sprague Orange Drop polyester or polypropylene caps sound good as signal caps. Polystyrene caps are the ultimate for clean tone, but $$$$.

A lot of audiophiles have replaced their electrolytics in the B+ supply with high voltage SCR polypropylene caps. The lower 150V versions are widely used in audiophile crossover applications as well. Madisound Speaker Components had them for good prices. Conrad-Johnson quit using electrolytics in their tube amps, except in low voltage power supplies for digital control applications in their preamps. I was looking at a Carr guitar amp chassis the other day, and there were those SCR polypropylene caps were in the power supply. You could undersize by 20% safely with poly caps since they have a +-5% rating and electro caps were -25%+50% in the old days. Most of them are way on the minus side after several years of use and buildup of dielectric absorption. Film caps never develop dielectric absorption. Polypropylene caps are also self healing.