Resurrecting a 61 Reverberocket

[Mongo]

Hi all,

I just found this forum and it looks like a wealth of information. I have a 61 (first Issue) Reverberocket (6V6, 6SL7, 6SN7, 5Y3). It sat in the basement at my mom's house for 20+ years. I recently saved it and would like to get it back in player shape. At this point only a few tubes heat up.

I figure first off I can expect it needs a complete cap job. Then I can approach any other failed components. What do you all think of the cap kits available? Fliptops has one here http://tinyurl.com/2yhp7v for $38.50. Is this a good idea or should I just go to the local electronics shop and pick up the caps for much less?

They also have a tube set here http://tinyurl.com/2sm5k6 for $89

Should I just expect to drop $140 from the start to get the foundation of repairs done hoping it springs to life with little left to do?

I expect with the considerable change in component values/tolerances I should expect to have to dial in the bias. At that time I would definitely appreciate the 'best practices' on what the experienced among you considers the way to approach it. Ive researched cathode bias techniques and tube socket adapters, etc until I'm blue in the face.

Oh to have a good tech in the DFW Texas area who I could sit down with to apprentice for a week. I'm more than handy with an iron and scope, I just don't have the experience inside a tube amp.

Any help would be most appreciated.

Mongo

[Enzo]

I for one hate throwing parts at something without finding out what it needs. It might need caps, it might not. It might need new electrolytics, but not the coupling caps. FInd out what it needs first, that us what I believe.

Same thing with the "tube set." I wouldn't assume ALL the tubes were worn out. Sitting in the closet doesn't wear them out, in fact people all over spend premium money for new old stock tubes that have done exactly that. The 6V6s might be worn, might not. make the amp work and give it a listen. Unless they are flat out shorted, weak tubes won't hurt the amp while testing it.

Without me looking it up, aren't the Rockets all cathode biased? That would mean there is no bias to adjust. Just make sure the cathode resistor and bypass cap are OK. The only adjusting would be a new resistor value. Your high voltage will likely be higher than what schematics call for, since mains voltages are usually higher these days. But I don't think these amps ran all that hot back when, so I don't think that represents a problem.

Having said all that, if the tubes ARE bad, well sure, replace them. But chances are that some are fine. In my shop I would not bother with a tube kit, I'd just order the tubes I needed individually. Those are all types I would likely have in stock already, but then I am a commercial shop. But if you don't feel like shopping, then buy a kit.

Tubes that don't light up are either bad themselves or they are not getting heater current. I can see a bad tube, but if several don't light, then I suspect the wiring, or perhaps oxidized sockets or loose sockets not making contact with the tube pins.

Some others here will not agree with these views, so listen to everyone. It is not always right versus wrong.

Welcome to the forum.

[Mongo]

Enzo,

Thanks for jumping into this thread. From the research I've been doing, it seems I keep finding that caps will die over time, especially if they have not been under power for over 10 years. With this at 20+, shouldn't they all be dead/dry/leaking?

I'll pull the tubes and clean all of the sockets and power it up to verify the heater voltage first. What is a practical way to verify if a cap is good?

I used to work a bench and rebuild electronic voltage regulators, etc. but never needed to test caps. All the scope work was for transistors, etc and was all pre-defined procedures.

A good 'how to' on verifying caps would be awesome. I'm fine with a meter and can pull a leg and do all the resistors no problem.

Thanks.

[Enzo]

You have electrolytics that can dry out over time. Not all do. If the unit powers up, the main test to me for e-caps is their function. A filter should smooth the DC, if there is a ton of ripple down the B+ string, then obviously the caps are not doing their job. The amp will be hummy or not respond well to peaks. Dried out caps tend to more or less disappear, so clipping a fresh cap in parallel will tell you instantly if the cap was weak.

That is a totally separate issue from leaky caps. You cannot measure leakage on your ohm meter. WHat the cap does at 1v from the meter is not a good measure if what it will do when several hundred volts are applied. Leaky caps will drag down voltages or even blow fuses.

After the e-caps, ther are also the coupling caps - those 0.01uf or whatever that pass the signal along. In real old amps they were "paper" caps. More modern caps are "film" caps. They pass signal and block DC. You find a leaky one by finding DC voltage coming through where it shouldn't. If there is 200v on the plate of a tube, and a cap to the grid of the next tube, and you find 23v at that grid, then I suspect that cap.

Sometimes these caps don't leak the DC so much, but they don't pass the signal much either. A famous example is the caps in the tremolo oscillator circuits. Usually three caps in series with resistors to ground between them feeding back from a plate to a grid in a triode. WEak caps make weak oscillation or oscillation that won't sustain itself.

[Mongo]

Well it sounds like there's not much I can check other than DC leakage on the coupling caps. If the couplers are not passing signal,will the show as open on a meter?

Ripple, I'm assuming, should need a scope (don't have one at the house) and leakage on an e-cap, as you stated, can't be done with a meter.

With so few conclusive tests to do, would I be better off just replacing them all?

[Satamax]

http://www.geofex.com/ampdbug/ampdebug.htm

[Mongo]

Thats a good page, I've read it in the past and will go back through it in detail.
Thanks.

[Enzo]

Your meter will verify a shorted cap, that is about it. Most caps will show some resistance at first and rise to infinity after a period of time that depends upon the size of the cap. So most couplers will show open ona meter.

If there is signal at one end of a cap and not the other, that is suspicious, yes. If you have no scope, set your meter to AC volts and measure points in the circuit. The meter should iignore the DC present and just show signal. In the case of filtered DC, the AC reading will just be the ripple.

I don't consider seeing if the amp works an inconclusive test.

here is the reasoning behind my philosophy. Aside from not wanting to waste parts. If you take an amp and right off the bat, tear into it and replace a large portion of its part, and THEN find the amp not working. Was the problem already there, or was it generated by some part of the rework you did? I belive in finding out what is wrong with an amp before recapping it, unless the need for recap is too severe to continue. Even if I am already convinced I will recap it, I still "fix" the amp first. Once it works, then I will have confidence any new problems are due to my work.

[J Martin]

I think you should at least replace all the electrolytic capacitors. Don't wait for them to fail totally. New filter caps often make the amp sound much better. Over the years this has proved to be the best policy. I own a Reverb-o-Rocket and a Reverb-o-Rocket II myself.

[Mongo]

I don't consider seeing if the amp works an inconclusive test.

here is the reasoning behind my philosophy. Aside from not wanting to waste parts. If you take an amp and right off the bat, tear into it and replace a large portion of its part, and THEN find the amp not working. Was the problem already there, or was it generated by some part of the rework you did? I belive in finding out what is wrong with an amp before recapping it, unless the need for recap is too severe to continue. Even if I am already convinced I will recap it, I still "fix" the amp first. Once it works, then I will have confidence any new problems are due to my work.

This is all excellent advice.

I agree with you on the get it working first approach. My analytical nature/diagnostic approach is to change as little as possible so I know the effect the change made. If it has a response different than I expected, I know the last thing I did and can easily take a step backwards to a known good point.

My questions about the caps stems from a lack of experience and concern that I'd be chasing a problem that I couldn't isolate. If they effectively die over time, I wondered if the first thing to do was to replace them so it would work at all.

[Alex R]

Because filter caps can't be definitively tested without spending a lot more time and money than it would take to replace them, they are an area of uncertainty and thus debate. Generally speaking old caps will exhibit clear symptoms if they are going to be a big problem. Hum, fuse blowing, massive voltage sag, visible bulges or leakage of gunk.

Some repairers change them out anyway if they're past their shelf life; some change them if they seem to be causing a problem. For what it's worth I'm in the latter camp. I say get your amp working, see how you like it, then think about filter caps. My experience is that if they are bad there are usually marked symptoms. Over to you...

[Mongo]

Because filter caps can't be definitively tested without spending a lot more time and money than it would take to replace them, they are an area of uncertainty and thus debate. Generally speaking old caps will exhibit clear symptoms if they are going to be a big problem. Hum, fuse blowing, massive voltage sag, visible bulges or leakage of gunk.

Some repairers change them out anyway if they're past their shelf life; some change them if they seem to be causing a problem. For what it's worth I'm in the latter camp. I say get your amp working, see how you like it, then think about filter caps. My experience is that if they are bad there are usually marked symptoms. Over to you...

Ok, If I'm understanding you correctly, filter caps may show visible signs (bloating, leakage). Since I don't have a matrix of expected voltages at certain points I'll have to go with visible signs or a dead function. If I can get it to power up, then I can approach changing caps if necessary.

I'm still stuck on cap shelf life. I've read in multiple places that shelf life is 10 years. After that they might be able to be re-grown. Since I've more than doubled that (that I know of) shouldn't chaging them be a given? Honestly, when I look at the eyelet board, it looks factory, so I'm potentially looking at 45 year old caps.

Does anyone here really think a 45 year old cap is possibly a keeper?
I'm asking because I don't know.

[Alex R]

Does anyone here really think a 45 year old cap is possibly a keeper?
I'm asking because I don't know.

...just saying that unless the symptoms are there you're best off firing up the amp anyhow and seeing whether or not you need them. Yes a 45-yr-old cap can work ok, I've seen plenty.

[Mongo]

...just saying that unless the symptoms are there you're best off firing up the amp anyhow and seeing whether or not you need them. Yes a 45-yr-old cap can work ok, I've seen plenty.

Thats good to know.

[Enzo]

Rules like "caps have a 10 year life" are silly to me. Car batteries have a 6 year life - or 4 or 10 or whatever. DO you believe that? There are folks who will tell you so. If you buy a Die Hard for your car, and it calls itself a 5-year battery, would you just replace it on its fifth birthday?

Caps they build today are so much better than the ones they made 40 years ago, yet people would apply the same "rules" to them. But even then, we are talking statistically, not individual cases. So what if many caps don't last 20 years? SOme don't last 5 years and some run for decades.

Yes, caps can have visible signs of failure like crap oozing out the vent or bloating. But they also have easily detectable electrical signs, as in the first part of Alex' post.

Hum, fuse blowing, massive voltage sag, visible bulges or leakage of gunk.