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]

Originally Posted by Enzo

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]

Originally Posted by 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...

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]

Originally Posted by Mongo

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]

Originally Posted by Alex R

...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.

[Mongo]

Originally Posted by Enzo

Rules like "caps have a 10 year life" are silly to me.

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.

This is all excellent advice. I'm glad I came here. Having little experience in tube amp repair, I had only published standards to rely on.

I was a diesel mech for 12 years, I worked in the auto industry for 5 before that. I never replaced a battery until it died or gave signs of death before a trip. I fully understand your motivations. I just needed to address the baseline expectations for component viability so I didn't overlook the obvious.

Nothing worse than spending weeks trying to figure out a problem only to have a veteran take one look and go, oh those caps are ancient, you should have replaced those right off, first thing. Everyone knows that...

[J Martin]

I still think its best to replace 30 yr. old filter caps. I can see the point of taking it one step at a time for a beginner, though. In my position, having done hundreds of these amps over the years, being pre emptive has worked out for the best. Old filter caps degrade the sound before they hum or short. Everything stands on the power supply.

[Enzo]

Nothing worse than spending weeks trying to figure out a problem

You have to HAVE a problem, or at least be aware of one, before this can happen.

Look, it is not like you will go wrong swapping out the caps, I just urge you to assess the amp FIRST.

If you had a Fender Hot Rod DeVille that distorted bad and you had been tearing out your hair looking for it, and I walked up and said, "That plate resistor in the phase inverter is open." You might also be frustrated. But that is merely the voice of experience, it doesn't say anything bad about you.

A story. I also repair video monitors for people in the arcade industry. A large portion of the trouble in these things as they age is dried out caps. They are on 16-24 hours a day and it gets hot on those chassis. For common models, people sell cap kits. If your monitor craps out, install the cap kit, and most times it solves the problem.

One regular customer came to me often, then discovered cap kits. I had helped him learn to use a meter and enough troubleshooting to narrow hte problem to the monitor or other area in the game. OK, I am all for self-reliance. Usually it worked. When the kit didn't work, he had zero clue as to what to do. Instead of learning how monitors worked, he learned which cap kit to order. Now in his case, this is probably the most expedient approach. But I use it to illustrate the difference between the shotgun approach (change everything) and real troubleshooting. In his case he denied himself the learning opportunity.

If he had learned that the vertical sweep bypass cap caused foldover, he would know that on any model that would be the place to look. Even models without kits available. BY learning how to chase down problems you gain experience and knowledge.

This is my main complaint about Gerald Weber's books. They have a lot of interesting information in them, but he has project after project where he shows a layout picture, and tells you to change this and that resistor and this and that cap, and maybe move a wire here or there. And when done, your amp is now blackfaced or whatever. Never a single mention of the schematic or what the parts actually do, no learning of concepts. Someone who does this winds up with something he wants maybe, but still knows zero about what he did.

It is in this frame of mind that I give the advice I give.

[Mongo]

Originally Posted by J Martin

I still think its best to replace 30 yr. old filter caps. I can see the point of taking it one step at a time for a beginner, though. In my position, having done hundreds of these amps over the years, being pre emptive has worked out for the best. Old filter caps degrade the sound before they hum or short. Everything stands on the power supply.

I'm guessing you all have bins at your disposal to pull the caps you need and replacing them is as simple as a solder swap out. I'm gonna need to pull all of the values from the schematic and figure out what type they need to be.

It would be awesome to have a list like this:

Here's all the caps that are in the amp.
Number, Type, Voltage, Value, Purpose
C9, ?, ?, .02, ?
C11, ?, ?, .01, ?

I'm figuring that studying the schematic and making such a list is the first order of business. It should familiarize me better with the amp anyway.

I'm looking at the schematic, there's no indication of what type they are.
I guess that's part of the mystery for me. How many different types are there and which ones should be used where? I'm cool with the values and voltages, but e-caps, film caps, paper caps, orange drops, etc... It's all very confusing at first.

[Mongo]

Enzo,

I love to learn and desire nothing more than to know how all of this works. I've rebuilt pedal effects with no schematics by just looking at them and probing them until it made sense and isolated the cause of the problem.

Man, I'd pay to be able to sit with an amp tech and just learn what I needed to study, go study, come back, do what I learned and and learn some more. The days of apprentices seem to be gone. I went to some of the shops in the DFW Texas area and asked if I could talk to the amp tech. No way, liabilities, we'll fix it for you...

I recently had a Hughes & Kettner amp that started distorting when hot on a low G note. I figured, simple things first, open it up and do a visual. In the process of tapping the tubes lightly I noticed pins 4 & 9 on one my EL84's were cracked below the ceramic there the legs enter the PCB. When it got hot there was just enough expansion to gap the crack on the vibration frequency of a low G.

It was under warranty so I brought it to the only authorized service shop here. I told them exactly what was wrong. It was clearly a factory installation issue as there was no stress on the legs during use. They honored it as a warranty repair. I could have swapped out the socket in 30 minutes. Bottom line, they had it for a month! I needed that amp over the Holidays.

I'm willing to do the work necessary to learn, to understand what things do so I will KNOW how to diagnose a problem.

I'll just need to ask lots of ignorant questions until I get up to speed.

[Alex R]

It's simpler than you think. Any chance you could post the schematic you're looking at? Or a link to it? Someone will happily identify caps for you and explain their purpose, appearance etc, and whatever else you want to know.

The way forward with an old amp is to get the chassis out and find a way of firing it up safely - say, with a light bulb in the live mains lead as a current limiter first to check for shorts - then observe what it does that it shouldn't do & doesn't that it should, and go from there. After that you can decide where you stand in the Great Filter Cap Replacement Debate .

[Mongo]

I just threw a web page together will pictures and all of the information I have compiled for this repair so far.

I hope this helps.
1961 Reverberocket

[Alex R]

That's a handy page, thanks. OK a few tidbits:

On both the photo and the schem attached, typical filter caps are circled in red, typical signal caps in green, and typical bypass caps in blue.

Filter caps are electrolytic, large, and in values from 10 uF upwards, with high voltage ratings to match the amp's B+, often 500v. They have a positive and a negative side and you have to get the polarity right. Main filters go -ve to ground, bias filters (lower voltage rating 50-100v-ish) go +ve to ground. The big silver can cap outside the chassis has several caps inside it. You can see on the schem that these caps are in the power supply and go from the B+ high voltage line to ground, filtering out AC and dumping it to ground (caps pass AC but not DC).

Signal caps are film or mica or ceramic type, smaller in size (tho' quite big in the Ampeg as it's so old), with values point-something of a uF, with the v small ones in pF, picofarads. Usually no polarity on these. These are usually either coupling caps, which filter out the DC from preamp stage plates leaving just the amplified signal, or tone caps which shape the tone. (Often the treble cap, that feeds the tone stack, does both these things.)

Bypass caps, which when fitted (they improve gain) go in parallel with the resistor that often sits on the tube's cathode, can be either type and are usually middle-sized to match their values, which as a rule are from about 0.5uF upwards, but with voltage ratings usually below 50v.

On the schemo I've also done some yellow shading to show the main part of the signal path (a bit convoluted in the middle). Understanding the necessary parts of a typical preamp stage will get you a long way inside an amp. Let us know how much of that you understand and we'll fill in the rest.

All these components could have gone wrong, so could all the others in there. But most of them will be ok. Doesn't make sense to go in and replace everything. So we need to fire the thing up and troubleshoot it, order up a few spares, fit them and see is it ok... etc...

[Mongo]

Alex,

I'm glad my page proved helpful. I'll do what I can to provide the info that's needed to help me.

I'll need to chew on that for a bit. I'm busy as a bee today, but have tomorrow off. That's a bunch of great/new info for me, thanks.

I'm sure I'll have some more questions as I begin to get my head around this.

Thanks again.

[Mongo]

I'm still chewing on the schematic and compiling my questions.
I'll check back in a few days, things are pretty busy here.

I really appreciate everyones help.

[dkostek]

Have a 1960 Rocket that my 18 year old son found and likes the sound with his American Tele. He has finally come around and realized just because things are old doesn't mean they are no good. I am tempted to "improve" the amp with modern components (I did change the power cord to 3 wire and disconnected the ground cap for safety) but I am sure the sound will change and the amp will be lesser original, of course. Who knows how long it will last but enjoy it. If it breaks I will fix it. The old adage: "if it ain't broke, don't fix it", followed by "if it ain't broke don't break it" applies here. In my old TV repair days we would routinely change filter caps as a money maker for the shop. Weeks later we'd get a call back because the now improved power supply would stress other weakened components. Like all things in life, there are trade offs.

[Alex R]

Originally Posted by dkostek

In my old TV repair days we would routinely change filter caps as a money maker for the shop. Weeks later we'd get a call back because the now improved power supply would stress other weakened components. Like all things in life, there are trade offs.

I'm considering adding this to my sig. Wise words indeed!

[J Martin]

I think many people have a misconception of "Changing the sound of an amp" as its often expressed as a negative. Really, that is the goal of amp repair. To change a bad or weak sounding amp back to its original sound. I am old enough to remember the mid '60s Ampegs, for one, as many of my friends had them. So, I can tell when they don't sound right. Some parts may not be "broke", but they sure are deteriorated. Replacing them in not wasted money. My goal has always been to bring an amp back to its original form. I test every amp from the view of a player, not just a tech. And this is not to say you need to strip an amp and replace everything, but replacing marginal parts will have a sonic impact.

[Mongo]

I've experienced the 'added stress' concept before and it is valid. The goal for me has always been to find that balance between what is electrically necessary to bring it to an operational state and what is sonically prudent for good tone.

Sure, I could replace everything to factory values and have a close representation of a new one, but would my transformer die in a year? The wise path seems to lead to bringing it back to functional, then letting my ears decide the benefit of further parts replacement.

It sure would be nice to either have a NOS version or someone like J Martin to lend an ear to that evaluation.

One other thing to consider is the speaker. My original one may be an awesome vintage jewel or it may sound like flubby poop. All of these judgments will be through some speaker. I have the original, a Tone Tubby, and the one in my Mesa MKIV to use for evaluation.

My ears will be getting quite a workout once this baby powers up...