Help me with OLD tube amp!

[oscillating]

Item in question is a rauland model 1916 pa amp. Looks pretty simple. All octals. Took it to a tech and he said he didn't want to mess with it. Told me I was crazy, but didn't really elaborate. I'm new to repairing tube stuff, so I wanted some help. Found schematic inside chassis. Does this look like it would make a decent guitar amp? Found tubes for it, looking for can type cap. It calls for a 40/10/10/10, I found a couple of different ones but they are 40/20/20/20. Is that too much differenc for the circuit? Would I need to replace all of those paper and oil caps? Needs new 3 prong chord. Then I'm hoping the transformers are good. Have some cool(?) Ideas like 2 stages of 6sj7, and changing tone stack, but need to get it running first... Help me.

[52 Bill]

Help me.

I would not spend a dime on any parts until I tested the transformers.

That is a nice little amp, and could be used as a great starting point for a small Princeton sized amp.

Have you turned it on yet?

[oscillating]

kind of...

Testing tubes and transformers was really what I was hoping the tech would do. He said to rig some wires to it and see if it made any noise. Big help. I saw a thing about using a lightbulb to limit power, but sounds a litte crude. But, if it will work I will try. Also have some circuit questions, like is there enough signal to use baxandall tone stack? Is there NFB in this circuit? Like off the 8 ohm speaker tap going to cathode of 6sc7's? Not sure about that.

[oscillating]

Turned it on several years back, but no speakers were hooked up. The light bulb came on, not sure if 6v6's and recto were glowing.
Some of the metal tubes tinkle like broken lights, so they need replaced.

[52 Bill]

Turned it on several years back, but no speakers were hooked up.

Never run a tube amplifier without a speaker or other load connected. It can cause damage to the amp.

A light bulb limiter can be used to prevent damage to things while testing an amp. I use one all of the time.

What sort of skill set do you have? Have you ever worked on a tube amp before? Are you safe with testing a high voltage power supply? Remember that an amplifier can cause you bodily harm if you are not careful.

[oscillating]

I do commercial hvac, so I'm ok with the testing side. Its the component value changes and the math that gets me. I've been learning about tube amps and this one in particular for about a year.

[Enzo]

I don't know your tech, of course, but I suspect he sees something he sizes up as a $5 yard sale item, and then he knows it will need a complete rebuild - all those caps replaced - at his labor rate. My shop charges $60 an hour, for example. He also knows from experience a certain percentage of customers will approve a repair budget, but when it comes time to pick it up and pay the $200-250 is cost to repair, they never return. So he declines to do the work.


It looks like a cool amp to me, and likely a good basis for a project, but I am not mystified by the tech.


I am no HVAC guy, but maybe like someone looking to spend $2500 to rebuild an ancient home furnace, when a new high efficiency unit could be installed for half the amount.


As to your questions, a 40/10/10/10 can be replaced here with a 40/20/20/20 no problem. But you have tons of room in there, and many of us these days will replace the can cap with four individual caps. Caps these days are MUCH smaller than they were when that was made 50 years ago or more. So they can be installed where neded around the insides. The can cap was in its day efficient, but now, I just leave them there for appearance but unwired.

yes, all those wax covered caps are going to be bad, I'll bet my lunch money.


Yes, there is negative feedback through the 5.6k resistor from ther 8 ohm tap.

The light bulb may be simple, but hardly crude. Look up "light bulb limiter" there are a zillion hits on google. They discuss making it - simple - and using it. it is not for reducing power of the amp, it is for limiting the amount of power the circuit can draw from the wall under fault conditions. Like a low tech GFI or something.

[oscillating]

Alright. That's what I was hoping for. I was thinking about that with the can cap. Take out the transformer sockets for the mic pre's and maybe mount the caps to a board and secure it there.
And is the nfb something I would want or not for guitar? Is it there to reduce noise or increase bass response? (See? I only know enough to ask questions.)
I plan on taking out the phono circuit and that leaves me an empty socket. I was thinking dual 6sj7's for one channel. Or using that socket for something else. I would like to have more tone control, so I was thinking baxandall, and using the phono and tone knobs as bass and treble controls. If I don't have enough signal (thought experiment time), could I use the socket (with appropriate tube choice and supporting circuitry) (6sc7?) As a cathode follower from the mixer 6sc7?
How about a 6sc7 as send/return for fx loop? Also not sure if that's possible.
Anyway, I thank you all deeply for taking the time to help me sort this out and wrap my mind around it. I enjoy learning stuff, so this is a fun project for me, stretches the grey matter a bit. And I'm hoping this amp will "sound old." So I'm hoping that will be inherent to the circuit, and not the sum of it's ancient parts. I know the output transformer will be part of the sound equation (and an old speaker.)But I'm hoping the cap and resitors that get changed won't be.

[soundguruman]

It will make an excellent guitar amp. Octal preamp tubes sound old(er) and more classic.
The big hump is to obtain enough good octal preamp tubes, to keep it going. The good ones are scarce and expensive.
As in: lower microphonic, good internal condition.
so when you pick a preamp tube, try to find one which is easily available and cost effective, or pay and hunt.

The phono input may actually be preferable for guitar because phono stages needed more gain and lower noise, but three or four preamp stages is actually what you may want.
also the original RIAA phono eq is kind of interesting in an overdrive...
at least 3 gain stages and a cathode follower, is what I would go for.
In other amps, (similar era) the phono input was chainsaw shred-o-matic. I loved it. I would not discount the phono as a guitar input.

But my thinking is to take a lot of the original construction and stack the stages together, in a rudimentary manor, instead of re-making a lot of the design.
You may find that this simpler method with a few tweaks will work well for guitar, without tearing the entire thing apart.

The original design mic input may be much lower gain than what you would expect from a guitar amp.

[oscillating]

Ok. I like that. But, it takes me back to fix it and try it out as is, or would I be better off to use this to build a new circuit in? I like to rock, but I also like those sweet, mellow cleans. So low gain isn't a bad thing maybe...? Or is it barely audible low gain?

[drewl]

I bet it will sound pretty good as is.
You can always mod or tweak the circuit after you get it working.

[oscillating]

So what are the good options for cap types replacing those wax caps? I'd rather not pay for new, old style caps if modern ones "sound" the same.

[Rhodesplyr]

So what are the good options for cap types replacing those wax caps? I'd rather not pay for new, old style caps if modern ones "sound" the same.

You have to get rid of the paper capacitors, regardless of sound, just like you can't drive on tires with the belts showing through the tread. They will leak DC current from one stage to the next. I like Mojo's Dijon line of polypropylene caps unless you want to get into something like surplus paper-in-oil types. The Dijons are axial lead caps and fit nicely in place of the paper caps. To my ears, Mylar has a different sonic signature than paper or polypropylene. Polypropylene will sound a little brighter than the old paper caps, but even if you found old stock paper capacitors, they'd be leaky, too, at this point.

I get Enzo's point, but I would probably at least have given you an estimate on the restoration or have powered it up in "safe mode" for you to test the iron. Like others have said, this circuit looks like it can be adapted for guitar with few alterations if any, unless you want more tone control options. It looks clean, and the chassis isn't particularly crowded. Using it as a head, you'll have far fewer problems with tube microphonics than you would if the amp were in a combo cabinet, and it has OPT provisions for 4/8/16 Ohms. Very flexible. Good for about 14 Watts or so. (Trying to find non-microphonic 6SC7s to use in combo amps these days can be a losing proposition.)

I usually test amps like this with tubes pulled to avoid damaging good, old-stock tubes. I'd test the power transformer first with the 5Y3 pulled, then, apply B+ voltage slowly via a Variac and a solid-state plug-in rectifier module while watching to see what happens. Leaky paper caps will show up via this test. If there are no capacitor fireworks, you could put all the tubes back in and try it out, briefly. The reason I use a solid-state module is that it's easier to control the DC voltage rise via the Variac than with a rectifier tube.

Rauland made good stuff. I'm currently adapting a 1948 Rauland amp for a guitar customer that Rauland made for the Hammond Organ company.

[Rhodesplyr]

They never sound the same.
The makers of "new old style caps" want you to "think" they do.

But who's around who still accurately remembers what an amp made in the 1950s sounded like when new?

[Rhodesplyr]

The most experienced LEAVE the old coupling caps, and try to use them. This is considered the most "proper" approach. The MOJO dwells in the old parts. That's why we try to preserve the oldness of it....

Complete bullshit. The Mojo is in the circuit and the transformers, not to mention largely in whatever speaker you connect it to and who's playing guitar through it. I get lots of business from another tech in town who employs your approach. After he charges them $$$ just to dust off an old amp, they start to break down.

I had to restore a '58 Fender Harvard, and the original Astrons showed massive leakage. One measured a complete short. I've also seen a Grommes amp where a guy tried your approach. He ended up burning up a pair of Blackburn factory Mullard EL84s in an attempt to keep the original coupling caps. It was a sad day in the shop when I saw that.

I service 1950s Hammond Organs and Leslie speakers all the time, and I know organ dealers who try to dust off an organ and sell it without replacing the mid-50s paper coupling caps. Almost invariably, within a few months of being put back into regular use, these timebombs start to cause problems. I've seen many cases where these paper caps give up before the original electrolytics.

I've also read a book on amp repair written in the 1950s in which the author states that problems in an amp are most frequently caused by failed capacitors. No one was under any illusions back then about their reliability.