Thoughts on the 835 Bassamp?

[jjmt]

[Note: I've edited this post the morning after to remove some confusion related to posting prematurely.]

Hi! Not quite 20 years ago I made an ebay purchase of an Ampeg bass amp. I spent very little time with it then, and it has been in storage and moving from house to house. This evening I rediscovered it under the basement stairs and decided that maybe it was time to take on this project. And now I admit to being a little bit stumped.

This is a 1x15 combo with blue sparkle covering. Except for the back panel, which is an uncovered replacement. Front and back, it looks like this and this.

There is no schematic (however, there is a schematic for the 835N posted on this site, I have discovered), but the tube chart says it's an 835. It's got a pair of EL37s, and there seems to be a bias adjustment pot, so I guess it's fixed bias. Upper and lower chasis are here and here. The "faceplace" is on the back, angled, and can be seen here.

It's got some dings in the cabinet and a missing back panel, so I doubt it is of interest to any collectors. I think I'll recap it and press it into service. Since this is a pre-portaflex model, it seems to be pretty neglected by researchers, and there isn't much I can find on the internet. It does not seem to be mentioned in Hopkins and Moore's book. Any thoughts or insights would be welcome.

[vibrating string]

jjmt,

I have been posting a recent recapping and general cleanup job on my Ampeg GuitarAmp. The amp is very close to the amp you have in its lineage. I have a serial number that puts it at a May 1956 manufacturing date. I have posted my recent changes and some pics in the "workshop" section of this forum. Until very recently, this "50's Ampeg amplifiers" forum was overwhelmed with spamm, but that seems to be resolved.

I used the 835 schematics to help diagnose and repair my amp as well. I have drawn up some sections of my amp while replacing components.

I acquired this amp so many years ago that I cannot recall where....but I think it was a yard sale item and I got it for cheap. When I turned it on, it hummed so badly that I turned it off and put it aside for 25-30 years. I had a Gemini VI that I used part of that time. I decided a couple of months ago to see if I can fix the GuitarAmp so I could stop lugging my Gemini to/from a buddy's house.

I love the sound of this amp, especially the tremolo. I opened up my Gemini last week to replace some caps in that, and while I had it opened, I replaced the tremolo speed pot in the Gemini from 500k to 1M to extend the range of speeds on the tremolo down to close to what the older GuitarAmp has.

So check out my posts at http://www.vintageampeg.com/forum/Thread...aramp-1956

(I have yet to figure out how to post active links.)

Larry

[jjmt]

A couple observations since my last post.

I've been scouting around for pics and schematics for this amp. The first thing that I discovered was that the black, uncovered back panel appears in other pics, so maybe it is original. Odd, but, OK.

There seem to be several versions of this amp, but I have not found one that has the same tube compliment as this one. The model number is 835, but other 835 seem to have a different preamp, and there are other designations (N and D variants come to mind), but they are also different. I'm beginning to wonder if any two 835s are the same? LOL. The Jensen alnico C-15-N also seems to be an odd choice. I'll have to check the date code on that....

I've decide to get to work. I figure I'll recap the power amp and install a 3-prong cord, and see where things stand once it is operational (I haven't actually plugged it in - there is no real point to doing so at this stage). The guts of the lower chassis looks like this.

The electrolytic caps for the power section have been replaced with two 40mfd Atoms. That's a reasonable solution, and I could just replace those. But there is this now-unused cap can, and that bugs me. I think I'll re-stuff it, as there seems to be nothing available with the correct voltage and temperature ratings. The can for the preamp I can easily replace with a current production 3x20mfd can at 450v/85C. Maybe I will do that, or maybe I will re-stuff it. Haven't decided.

I'll also replace the cathode resistor by-pass caps. I'll probably just use standard 50mfd/50v caps for those, unless I convince myself that I should try to more closely match the values present. I'll also replace the Astrons with something current (I used mojotone caps in my M15 with good results) and check the various resistor values.

The pot that I earlier assumed was a bias control looks to be a heater balance circuit. I guess that could have been predicted if I had just done some research.

---

So check out my posts at http://www.vintageampeg.com/forum/Thread...aramp-1956

That is one very cool amp! And it cleaned up so well. I never take the time with the cosmetics - something that my wife would very much appreciate.

Getting the trem to work seems to be a frequent topic of conversation for vintage ampegs. On my M15, I had to change a resistor value just make it audible. And it really seems to care about what tube goes in that hole. Oh, and there is a cap that is missing from the online schematic. Without that it sounds like a helicopter. But It works now.

Oh, on my M15 I actually replaced the "death cap". The old one had leaked all over everything and shorted. Ampeg seems to have used a bunch of different values for that cap, and I think I stuck in a 0.1uf because I had one handy. Not sure what it's for. It doesn't do anything that would protect the power switch... It's not technically a death cap, as it doesn't lift the ground or anything. Doing without seems like a reasonable option.

Happily, there is no trem with the 835 that is on my bench now. But I'm sure there will be other interesting challenges that come up.

[vibrating string]

I removed the "death" caps (0.5uf 400V) that was connecting the hot leads to the chassis when I added the grounded power cord. I found no difference in the background noise when I did this. I may regret it someday, but not yet. Both amps are remarkably quiet since replacing caps in both amps along with some drifted resistors in the GuitarAmp. I just did not like the idea of a 110vac wire shorting to the chassis if the cap shorted one day. But I am learning just now that there are X and Y rated caps that will OPEN rather than short circuit if they go bad. What are your thoughts?

Larry

[jjmt]

Just to continue the narrative of my research. The date code on this amp places it as March 1962. What was Ampeg doing, making 835s in '62? That's 2 years after the launch of the portaflex series. Probably they were just using up their inventory of parts? It does mean that I posted in the wrong area of this forum, so, sorry about that.

The more interesting story is the speaker. As mentioned above, it's a bit weird. It's got a date code of February 1962, so it's probably original. But it's stamped as a C15N (a ceramic magnet model), but it has an Alnico-5 sticker on it. It certainly looks like a ceramic magnet. It turns out that it is not unique. I found a thread that discusses a similar situation. Interestingly, the speaker in that thread was pulled out of a '61 portaflex. I guess Ampeg bought a bunch of these. As they were mislabeled, perhaps they got them at a discount? Maybe Jensen simply ran out of the correct sticker? Maybe there is actually an interesting story?

Anyway, I've ordered parts for the power amp. Will report back in a couple weeks with progress.

[Hangman]

The 835 is a great amp, as are all of the amped amplifiers of this era. You are right in wondering if any two 835s are the same. Ampeg was a pretty small company at the time and substituting parts that were on hand for ones that were not, was not uncommon, also the owner Everett Hull was always having the factory do weird custom modifications for customers. So if a customer wanted an 835 with different features, he would make that happen. The customer got what the customer wanted.

I would approach the schematic as a general reference, perhaps the first rev of the schematic, just realize that many factory made amps did not match that schematic and are still considered “right” or “factory”.

I have a 435-SN, and the schematic and amp are totally dissimilar on the front end. They ended up using different tubes, fewer gain stages, etc...

What is the date code for your 835?

Sorry for all the spam in the 50s amped forum. Spam can be hard to keep up with. Every once in a while they robots get ahead. Hopefully it is good for a while.

[jjmt]

What is the date code for your 835?

The serial number, as seen from thee tube chart linked above, is 203160. As I understand it, the first three digits put it in the 3rd month of 1962.

---

So, the maintenance slog through the amp is officially underway. The "before" photo of the guts of the power section is here. From my perspective, the first thing that has to go are those blue Atoms. While there is nothing really wrong with the way the main power supply filters for the power tubes were installed, the three atoms on the preamp section are simply connected to the existing can, effectively doubling the capacitance. That would be, that is, if that multicap was in any condition to function. Not best practice.

The problem is that the first can is a 30/20/20/20 at 500 volts. Those seem to be unobtainable. I suppose I could custom order one, but I opted for the more cost effective approach of restuffing the existing can. I've never done this before, so it was a bit of a ride. But here are pics of it before, opened, restuffed, and finished.

If you are interested in doing this yourself, I used Nichicon caps, mouser part numbers 647-UCY2H220MHD (22uf) and 647-UCY2H330MHD (33uf). These are great caps! 500 volts, 105C, 10,000 hours life expectancy. All four caps for less that $10. It did take an entire evening, however, mostly due to the fact that I had to solve the problem of getting the guts of the can out of the base. Heat gun was the solution. Smelled really bad, though. Note to self: install some kind of ventilation in my shop.

[jjmt]

I "finished" working on the power amp section. I removed the Atoms, (including the stick-on plastic things that they were mounted to), re-stuffed both cans, replaced all the Astrons, and installed a 3-prong power cord. I also took some metal polish to the chassis and buffed out the pitting to the best of my ability. You can see the interior and exterior.

I tested the tubes. The phase inverter (6SL7) is branded "Ampeg", so it's probably original. It tested good with remarkably well-matched triodes. I left that. The power tubes (Mullard EL37s) both test good, but unfortunately, were very poorly matched in current draw. I tested them at +380v on the plate with -33v bias and got 50ma and 34ma of cathode current. I swapped out the 34ma tube with another from my cache that also pulls 50ma. The OC3 and the rectifier both seem fine.

I reconnected the power chassis (the power switch is in the preamp chassis), pulled the preamp tubes, and powered it on. I get 430v on the plates of the power tubes, and 320v on the screens (I'm used to amps that run the plates much closer to the plate voltage). 120Hz hum was quite low. Everything seemed to check out OK.

I tested the preamp tubes. The 6SL7 seems original, but tested good. The 6SN7 was dead. I replaced that with a random draw from my stash (a Ken Rad, I think) that tested good. The preamp tubes lit up, but I got no signal from the preamp. Time to go to work on that chassis. I've already identified an open resistor connected to the grid of the 6SN7, so I guess that is why there is no signal. But it doesn't look like anyone has ever touched the preamp, so I think I'll shotgun it. The parts count is not very high.

[jjmt]

It's been a while since I posted an update on this project. I expected to finish this last weekend, but alas, no. I have gotten my work on the preamp section. It's a very interesting circuit, and I will post a full description, and hopefully a schematic, when i get the thing working. But let me just fill you in on a few challenges.

First, the amp was wired by "Chico". Chico did a pretty nice job, overall, but really had a thing about that brown sticky stuff that Ampeg loved to put all over the insides of their amps. I first became annoyed when I discovered that Chico had used it to glue the Astrons to the board, meaning that when you pull the cap from the board the paper wrapping remains and is virtually impossible to remove. That, coupled with the fact that there were a few that I am guessing at the value of were glued *value side down*, was really annoying. He also got the brown stuff all over the solder joints, which did not please me at all. I removed what I could, but it still made for a lot of stinky smoke.

Anyway, then I got sidetracked. When the amp did not give me tube hiss when I connected everything, I decided I needed to trace through the circuit. This required dusting off my tone generator and oscilloscope, both of which have not seen the light of day for at least 10 years. Neither worked. So, I've got the oscilloscope working well enough, but the tone generator is still pretty shaky. I may have to just use a phone app for this purpose, but that's just not my style. I'd much rather use the enormous and ancient Heathkit IG-82, so I'm starting to debug that. One way or another, I should be able to complete the 835 this coming weekend. Right now I'm waiting for some resistors that I've found have failed in the power amp chassis.

[Hangman]

That brown gunk can be annoying, especially when its on a solder joint. But I think I prefer it over having the whole eyelet board dipped in wax like in a silverface Fender.

keep us updated. If you're interested in writing a blog post about your restoration process for this site, let me know. I could add you as a contributor. I'm sure a lot of people would be interested in reading it!