• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

should a newbie restore this relic or cannibalize it

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I recently aquired a tube amp relic for $50 off ebay . the relevant parts of the description reads

RCA single ended tube amp removed from a console. IT has had resistors changed to modern types and the cathode bypass on the outputs raised to give better low frequecny pass. All valves are strong. This soulds fantastic. Great warm single ended sound sought after by audiofiles. The inputs are the older RCA type, three pin. I did not convert the inputs to any jacks, the buyer can install the type of inputs he desires. There is no hum. The line-up is: 12AX7, (2) 6BQ5 and 5Y3GT

I find that one channel doesn't work, though the input (1) / output transformers (2) appear okey. the other channel sounds very nice

It's from a console and therefore not much of a looker though i could re-paint the chassis and get it to a relatively respectable state.

beyond that, i have 2 options:
1. try and troubleshoot the faulty channel
(any hints here as how to proceed)
2. rip out all the electronics and re-build it. any suggestions on schematics based on the tubes i have? i wouldwant it as simple as possible (no balance / treble / bass / input selector)

this is my first foray into tube electronics and i am still reading up info.

thanks in advance :)

An externally hosted image should be here but it was not working when we last tested it.
If I were you Iwould have a go at troubleshooting it. It's a good way to learn. Being single ended it's very simple with low comp count so it should be easy enough to trace the circuit through. Check around on the net for similar schematics to give you an idea of the layout. From what you say, one channel works fine and without hum because you did not mention it, so the power supply can be eliminated. From there it's really just a process of elimination...
Looks to me like a good fixer-upper. There are probably going to be a total of 20 components passive and active to it. It' s probably going to need new caps, pots, and tubes. If there is anything wrong with any of the transformers you might as well just toss it overboard.
Measure everything out. If they measure ok, leave the paper and oil caps alone and the carbon resistors, too. They are suppose to give you than 2nd order harmonic distortion that makes you want to cuddle up in front of your stereo with a blanket and a cup of hot cocoa.
Repiar it

I would suggest repairing it. Since you already have one working channel to should be fairly simple to make voltage reading comparisons between the working and none working channel to find the problem.

Get the tube pin outs and draw the diagram of the circuit. Once you have that troubleshooting should be fairly straightforward. Use a high impedance VTVM or other such meter to make the measurements.

Use caution when working with the high voltage levels that you are likely to encounter. They can be deadly under the right conditions.

John Fassotte
Alaskan Audio
Info sources

www.triodeelectronics.com has a page each for both valve data and tube amp schematics, both of which are quite extensive. If the schematic is not there, try a few Google searches and you might turn one up. I've found some very obscure things out there. There are probably other designs out there which are similar and would give rough ideas on operating voltages etc. You're luchy too: it's much easier to find a fault in something thats not working as opposed to something thats partially working or intermittent.

Also at www.duncanamps.com you will find two freeware programs that could be a lot of help. TDSL is a tube data locator, and PSU designer 2 is a power supply simulator. PSU is usually used for design, but could be used to simulate your circuits' PSU to help diagnose difficulties. It's very easy to use.

That said,
-replace the PSU filter cap, it will quite likely be very dodgy by now (safety precaution).
-swap tubes from channel to channel one at a time and see if the problem changes/goes away.
-replace any cooked reistors whether new or not. Many modern resistors are not designed to work at the voltages inside tube gear and can fail because of them.
-depending on what type of caps are in circuit, they might have failed too, even though they look OK.
-Look at the voltages in the good channel (no input signal) and compare them with the other good channel.
-then if you have a CD with test tones on it you can use it as a signal generator to trace the signal through the circuit. If not there are several freeware ones out on the web so you can use your PC and soundcard.

Tube gear has enough voltage and current capability to kill you stone dead if you are not careful even when you are young, fit and healthy. I have lots of years of experience working on HV gear and still get zapped on a rare occaision. It hurts.

-Get some insulated aligator clips for your meter so you can connect the negative, and then move the positive around, whilst keeping your other hand in your pocket. Move one lead at a time. The one thing you must avoid is a second point of contact across your chest. If you accidentally touch the high voltage with one hand, and the other is in your pocket, there is no current path across your chest able to stop your heart. A current path through your hand-arm-body-leg-floor for instance can still hurt like hell, and certainly ain't good for you, but is less likely too kill you.

-if you are unfamiliar with working on HV gear I would prefer you had someone else in the room/house in case of accident.

-wear insulated shoes.

-work slowly. If you get flustered/annoyed go get a cup of coffee, and come back to it with a clearer mind. Enjoy the learning experience.

-wear some safety goggles. A cap blowing up in your face would not be a pleasant experience and could damage your sight.

-work on it somewhere comfortable and uncluttered, with good light.

I don't mean to sound like a nanny, but I have seen the damage HV can cause IF YOU ARE NOT CAREFUL. If you are careful, it's a lot of fun working on tube gear, and the stuff can be quite addictive. When those lil' bottles start to glow and the music comes out, it's great. Whatever benefits SS gear has, tubes have an animus SS never will.

Feel free to ask any questions.

HTH, enjoy.
I would prefer to cannibalize it to get rid of the tone controls and associated components (it might turn out quite expensive to replace all those pots) and build a new chassis. Chassis for tube amp could usually be built more easily comparing to building chassis for solid state amplifier. Four pieces of wood and piece aluminum sheet is the usual approach for building simple enclosure.
You probably need to replace the elkos and volume pot. I would keep the old film caps at first, to just try them out, high wattage wirewound resistors could be useful to keep also.
Yeah run the search engine for 12AX7/ECC83/6N2P 6bq5/EL84/6P14P SE as Brett suggested .
Here is one schematic I have bookmarked so far http://diyaudio.co.kr/power/6bq5s_ecc83.gif

Have fun


have to second what the other members posted, particularly John.
The gadget is so simple and you have one working channel to compare.
You should get the other channel alive, too!

If you want to learn via cannibalizing, get an old Tektronics tube scope no longer working and carress it apart with your soldering iron (instead of using that rude side cutter :) ) and let the Tek folks' technical wisdom slowly penetrate your mind .... they were the masters and if you cannot share space and spirit with the masters, share it with their work as their spirit is i it.

So did i, 5 times, enjoyed every hour. Learned a lot how electronics should be constructed mechanically. Salvaged most of those components, too, alone the ceramic terminal strips the whole thing is built with are worth the job.

And if you are lucky to get an old Tek service manual with your Tek, muse about the scope circuitry and the beautiful schematics spiced with little cartoons, scope signal patterns, parameters wherever you need it.
Such stuff is obsolete today and so is the attitude emanating from it.
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