Ampeg Reverberocket II Restoration

[RobertS_031]

I am a first time poster to this forum and have some questions regarding a restoration of my 1966 Reverberocket II amp (GS-12-R). Have read most of the other posts concerning this amp and particularly the post which explains how to change the old power cord for a new 3 wire cord set. Was very helpful advice which I followed in changing my cord set yesterday. This amp sat in a basement for 40 years untouched-when put away it was not working. I replaced the Main filter can electrolytic with 4 separate caps and brought the voltage up slowly to 120 Volts using a variac. It sounded great and all functions appear to be working well, although I did have to replace both the echo and tremolo footswitches. My questions have to do with installing a standby switch, discarding the .047 @ 1000v bypass cap (as advised in the post explaining how to change the power cord set) and possibly installing an inrush current limiter to protect the power supply components and tubes. 1. Standby switch-some electronics writers seem in favor, some not. Anyone here have opinion on whether this is useful? 2. Bypass cap (.047@1000VDC, "Black Cat")-I replaced mine with a "Safety Cap" which is designed to fail without shorting to ground. I thought that this bypass cap is useful in eliminating emi and other types of interference from finding their way into and out of the amplifier-is this incorrect? Why the advice to discard this portion of the circuit? 3. Old radio guys (which I also like to repair) install inrush current limiters (ICL) in series with the AC line (in the primary power transformer circuit, in line with the hot wire of the power cord and the on-off switch) to give the radio a "soft start"-the ICL is a temperature sensitive resistor which has a high resistance when cold (at start -up) and when warmed up has a very low resistance. This protects both the tubes and the filter caps from high voltage and current surges on start up when the tubes are cold and the caps have yet to be charged up-at least that is how I understand the theory. Was wondering if amplifier restorers use these devices to protect amplifier circuits. It seems like they are a good idea. Anyone on this forum who can speak to these three items?

[RobertS_031]

Stand-by Switch

Okay. Maybe I asked too many questions. While I have the amp apart, if its a good idea (and it seems like it is) to install a separate standby switch, I would like to do it, but I don't really want to drill a new hole in the chrome face. Has anyone elso retro-fitted a stand-by switch to an Ampeg Reverberocket and if so, how did you do it? I could remove the fuse holder from the face and make the fuse internal to the amp and put the stand-by switch in its place, but it seems like that would make it more difficult to change a fuse. Or maybe drill out one of the vent holes in the rear of the chassis and mount it (fuse or switch?) there? Whatever the solution, I would like to make it easily reversible.

Robert

[Enzo]

OK, WHY do you want a standby switch? The amp worked fine all its life without one. SO many small amps do not have them, why do you want to add one? The amp doesn't need one. We have indeed discussed this elsewhere, and I maintain the only purpose for the standby switch is to silence the amp while leaving the tubes hot, all that so you can play without waiting for the tubes to warm after your break.

COncerns like "cathode stripping" are not really relevant to this amp. They are concern to high power high voltage systems - like radiio transmitters in broadcast settings. Analogy: race cars put spoilers and fins on the cars so they remain stable at speeds over 150 mph. Would they then be needed on your mom's car she drives to the store at 35?

The cap from power cord to chassis served to reference the chassis to the power line ground. Once we move to a three wire cord, the chassis is now hard-wired to ground, so the cap becomes irrelevant. The cap was not meant to reduce interference, it was there to reduce hum, hum from lack of grounding. It was am attempt to provide grounding in a non-grounded world.

I am interested, what kind of cap can you put from power line to ground that won't short to ground if it shorts?


ANd I don't see the need for an inrush limiter either. The guitar amps got along without them for decades, I see no evidence of tube failures from lack of them. The tubes we use are quite common and easily replaced when worn, unlike some old radio tubes. I imagine old radio restorers want to maintain the life of their ancient tubes as long as possible. They don't concern themselves with tone. Guitar amp tubes will lose their tone after a while, but will continue to function for a long time. We tend to replace tubes that radio users would use for years.

[RobertS_031]

I assume that you repair amps for a living and I have only repaired some radios and 1 amplifier (so far) as a hobby so you are the expert here. That said, I have learned about electronics late in life and only by reading, asking questions and then doing. So here, I am only up to stage 2-asking questions. I suppose that the reason to want a standby switch is to make the tubes last longer. Turning them on and off frequently, from what I understand, eventually fatigues the metal and leads to tube failure. Back when tubes were cheap and plentiful, this was not a concern. Today, the NOS output tubes (7591) for this amp cost $48.00 each at TubesandMore (AES) and $65.00 each at Radio Daze-and these are not matched pairs. So, if a stand-by switch would make the tubes last longer, I thought that this is a good thing. Your answer seems to say that even if they last longer, the tone will not be good so it is not worth it. That would mean that I would replace the existing original tubes with modern Soviet or Czech manufactured tubes and not worry about the tube life or the stand by switch or Inrush Current Limiters-correct?
As far as the cap to ground is concerned, there is a write up on "safety caps" at this web site (he also sells these caps):
http://www.justradios.com/safetytips.html. There is also a discussion about using them and the various types of safety caps at this thread topic in the antique radio forum:
http://antiqueradios.com/forums/viewtopic.php?t=56110
I re-read both very carefully and I can not find where it says that these types of caps will not short to ground. Both say that the cap is designed not to explode or catch fire and to fail "safely". Perhaps I assumed that this meant it would not short to ground. Regardless, if you are correct and its placement in the circuit was meant only to provide a sort of ground so as to eliminate hum, then it may be eliminated without consequence when switching over to a three prong cord set where the ground prong is firmly tied to the chassis. One other thought on this cap: after my original posting, I had a discussion with a co-worker who is more knowledgeable about electronics than I am. When he looked at the schematic and the placement of this cap, he thought that it may have been meant as a sort of "cushion", in the same manner that the condenser served in a pre-electronic ignition automobile distributor which contained a set of breaker points. In the Amplifier, when the power to the transformer is disconnected by the switch at turn off, doesn't the same inductive voltage spike occur from the power transformer as in an automobile high voltage ignition coil? If so, then wouldn't this cap provide a "place" for this voltage to go and protect the amplifier switch in the same way that the old distributor condenser protected the breaker points? Thanks for humoring me.

Regards,
Robert

[telemoxy]

My understanding is that you use a standby switch to limit thermal cycling of the tubes.

If you don't want to drill a new hole use a three position off-on-on toggle switch or use the hole for the mic jack if you aren't using it and are willing to disable it.

[Enzo]

Thermal cycling?


Welcome to the forum RObert, and telemoxy too, I see. I didn't mean to sound flippant, I just want to see well defined goals. And though yes I do repair amps for a living, I also make mistakes, and if I say something wrong, I hope someone sets me straight.

When I play the amplifier for an evening, I leave it running, I don't turn the power off every time I take a break. Sitting idle, the tubes are not really going to wear anymore with B+ aplied than they will without. The cathodes will be hot either way. SO unless one plans to turn the amp off at each set break, there will be no thermal cycling. The major concern of folks who demand standby switches is that they fear having the B+ on tube plates the moment they turn on - and are thus still cold - might cause "cathode stripping." But in the low power applications such as most guitar amps, this phenomenon really is not going to occur.

So turning the amp on once or twice a day or something is no stress to the tubes. And I would not be turning the amp off every time I needed a fresh drink or answer the phone. So really tube life is not going to be affected. Certainly the standby won't hurt anything, though.

My point about the tube aging was that they will lose their tone BEFORE any of these other things would come to bear. A tube might be worn out after a year, but would continue to function for ten more years. WOuld sound dull, but it would work. Just exactly like strings. Strings go dead, but they will still play if you leave them on the guitar. Ten year old strings don't sound like much, but they play. SO with that in mind, I am not too concerned if something I do to the amp lengthens the life of the tube from 9 years to 10. I would have replaced the tube long before it got to that stage.

I think NOS 7591s are an amazing thing, the money they get for them. I think the current crop sound OK, and I personally would not be able to justify those prices for NOS. Especially since they are not matched, and who knows what the tubes have been through in the decades since they were made.

I am not trying to talk you out of a standby switch so much as I am trying to let you know that if you don't add one, you really are not going to suffer for it. No need to agonize over drilling a hole - I hate drilling new holes in amps - if you'd rather not. Inrush limiter? WHy not? It mounts inside easily, won't affect the amp operation. I just think the needs of the old radio restoration industry are different from ours. Such limiters are nicer on the power switch, but not on a small amp like this. Really large amps often use the power switch to control a triac or relay to actually complete the mains circuit. I suppose one thing a inrush limiter would do would be allow a little closer fuse rating. The relatively large surge at turn on requires a slow blow fuse of some size. Not sure how much of an advantage that is, but it's something.

Yes, I think safety cap refers to not exploding or igniting. ANy cap can short. We also use "flame proof" resistors in some applpications. They will get hot and even open, but won't ignite.

If you place a cap from the AC mains to chassis, it won't soften the blow to the power switch. If you want to do that, place a cap ACROSS the switch.

[RobertS_031]

Thanks to both of you for your thoughts on this. Enzo, your advice leads me to think that I have been looking at this problem in a very technical way and ignoring the reality of how the tubes perform over their lifespan. The reason to have the tubes in the first place is to get the great tone from them, so if they lose that before the tube expires, there's not much reason to hang on to the tube. I plan to remove all my original tubes, save them, and buy modern replacements. I can compare the new to the old to see if my ear can discern any difference. With new tubes in the amp and a supply of modern replacements still available I won't worry if I have to replace a tube.

Regards,

Robert

[RobertS_031]

Enzo-One thing you said has made me think over the past day. You said that this amplifier will require a slow blow fuse of some size. The schematic shows a 3 amp fuse but does not indicate a slo-blow. I have not used the amp for 40 years but I do remember that when I put it away, I could not get the amp to stay on-it kept blowing the fuse. It is possible that I replaced an original slo-blow fuse with a quick acting fuse since, at the time , I did not understand the difference. Fast forward to 2008, without an inrush current limiter, will I need a slo-blow fuse for the current surge at start up? 3 amps also seems like a pretty large size for this amp. I have only started this amp up and run it through the variac so I do not know, at this point, if it will blow a 3 amp quick blow fuse without the slow start of the variac. If anyone else on this forum is interested, I took a digital picture of my schematic so that I would have a schematic to follow when the original disintegrates (it is well on the way). It is mostly readable and it does seem to differ from the one shown all over the internet which has been re-drawn by Joe Piazza. If it is possible to post such a picture on this forum, I would do so if it would help someone else out.

Robert

[Enzo]

Not all, but many amps ise slow blow fuses for exactly this reason. MAke your fuse a slow blow and see of those power-on blows don't stop. If it is blowing fuses at times other than power-up, then it is not from inrush. By definition, if nothing else.

Even if the original was not slow blowing, it won't kill the amp to make it one. The thing to do os to monitor mains current into the amp under operating conditions to see if the current draw is excessive in general.

[telemoxy]

Thanks to both of you for your thoughts on this. Enzo, your advice leads me to think that I have been looking at this problem in a very technical way and ignoring the reality of how the tubes perform over their lifespan. The reason to have the tubes in the first place is to get the great tone from them, so if they lose that before the tube expires, there's not much reason to hang on to the tube. I plan to remove all my original tubes, save them, and buy modern replacements. I can compare the new to the old to see if my ear can discern any difference. With new tubes in the amp and a supply of modern replacements still available I won't worry if I have to replace a tube.

Regards,

Robert

I think your strategy is sound.

But keep in mind, that if you compare any new tube to the old ones the new ones will sound better. An effective comparison would be new NOS tubes to new Current production tubes but that isn't practical even if it would be interesting.

[jrfrond]

I am interested, what kind of cap can you put from power line to ground that won't short to ground if it shorts?

Enzo, there are safety caps, which are just a cap with a series resistor, available for this purpose. The series resistor prevents the hot from contaacting the chassis directly if the cap shorts, and is really the only component to use for this purpose, especially if you are into silly things like UL compliance.

However, "death caps" are a thing of the past, and all the amp really needs is a grounded AC cable.

If switch protection is a concern, use a DPST or DPDT power switch and wire the two contact sets in parallel to share the load. This is what I ALWAYS do, especially on those nasty, underrated Fender standby switches.

[Enzo]

I think of those as snubbers. Doesn't a series resistance diminish the effect of the capacitance?

[jrfrond]

They ARE snubbers Enzo, and the carry a prominent UL rating for use on AC lines.

I am sure that the series resistance affects the capacitor's effectiveness, but think of the alternative.

[Enzo]

Just thinking academically about the electronics. I am not against safety. I didn't realize they mounted snubbers for mains bypass.

[jrfrond]

It's not a popular practice except in cases where you have excess line noise. In the case of a new build, I would just punch the chassis for an IEC-RFI/EMI AC Input/Filter with the proper current rating and be done with it.