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Old 19th July 2014, 03:23 PM   #1
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Default My introduction to tube amps. The good, the bad, and the ugly.

I have posted here a few times about these amps, and would like to say thanks to those who have tried to help me. Here is my story........

I have no experience with tube amps, and I shouldn't touch them. over the past nine months I have ruined a lot of good gear, and am about to try one last ditch effort to make something work.

I found an old Conn organ with a 2 channel amp in it, not working. I have some mechanical skills and am pretty good at figuring things out so I figured I would recap it and see how it goes. well it went great. without any knowledge of the circuit I replaced a few caps and bought some NIB Sylvania 7868's. this amp rocks, though limited in the high frequency range (over 12k).

I bought the tubes from a local guy, and while there he showed me an old Fisher Ventura that he wished he could sell to a good home. well..... I could not believe how awesome the Conn sounded so I figured for 250 bucks the working amp in the fisher should be well worth it.

Lets not forget I have very limited knowledge of tube circuitry.

brought the fisher home and put new caps in it because I should. the owner had replaced the 7591a's with 7591xyz's and changed a resistor to make this work. He gave me the original resistor to put back as I told him I was going to buy some EH 7591a's. so I did all of this and hooked it up. Sound from both channels, though it distorts pretty early and a resistor gets burning hot. As I really have no idea on how to proceed, and really don't want to ruin a vintage fisher tube amp, I shelved the amp, remainder of the console still resides in my basement. this made me feel stupid and a little bummed out as I have about 400 bucks in this and it still doesn't work.

Junk.

thinking I'd better get a little more experience with tube amps, before I got into the fisher, I bought a Magnavox console with a SE 6bq5 amp in it for 30 bucks. Once again I changed the caps, and this amp works good although not very loud before it distorts. I am running Martin Logan speakers and I believe they are just too demanding for the 3 watts I get from this amp. the sound at low levels is so sweet I am at this point completely certain that I need a higher watt, full range amp to complete my system.

Sooooooo............

I bought another Conn amp that is a twin to the first. recapped it and it worked good. I bought a set of Eico cloth lead 6.6k primary OPT's from an ST-70 to put on it thinking this would be awesome. they didn't fit. I had to move the output tubes to make room on the chassis for them. having a new set of 7591a's I figured I might as well change from the 7868 to 7591a's so I replaced the 9 pin sockets with 8 pin, and when all was said and done, one channel does not work and the other sounds terrible.

More junk.

Last ditch effort:
Today I am going to (in anger mind you), rip the damn OPT's off the Fisher, and put them on the first F*****g Conn amp that I got for free (the one that still works). I realize now that although I am mechanically inclined and can usually figure things out, I have no patience and have no business working on tube amplifiers. hopefully this works as at midnight tonight I am throwing all non working gear in the trash, including the fisher console and the Garrard type A turntable in it.

so I now have turned to ****:

1. A good Conn 2 channel amp.

2. An excellent fisher console that worked good when I got it.

3. a pair of Eico Opts

4. numerous capacitors, resistors and tubes.

Why did I post all of this? I don't know. I really wish someone would just come get this crap in exchange for getting one of these to work for me, but as I am so damn impatient, I am going to put the OPT's from the fisher on a conn amp, throw away the rest, and call it a bad year for tube amps. I hope this works.....

In conclusion, I may have some skills, but without patience, I make junk.
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Old 19th July 2014, 03:32 PM   #2
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If someone in the Cleveland Ohio area would like whatever is left after todays follies, PM me.
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Old 19th July 2014, 03:55 PM   #3
kevinkr is offline kevinkr  United States
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Don't be so quick to give up. Start by posting diagrams and pictures of what you have done so far on any one of these projects. Even a cell phone camera and hand made sketches are fine.

I've been doing this for 40yrs now, and still make mistakes - the difference is I have the knowledge and experience to figure them out given a little time and thought.

You just started, and probably have been a bit over ambitious and have undoubtedly made wiring mistakes - we all do. (Or at least I do)

It's the learn to walk before you run thing.

Get Morgan Jones Valve Amplifiers 4th Edition - it will explain a lot.
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Old 19th July 2014, 05:01 PM   #4
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Hi again kevinkr,

I couldn't make a diagram of the mess I have made if my life depended on it. pictures are out of the question as I am a hack and am too embarrassed to post the burnt up messes I have made of all of this.

I am done trying to figure this out, and asking for help. I lack patience, so this is not a hobby for me. I have accepted that. I just want one working amp as I truly love the sound of tubes, and was hoping I would find someone willing to take this fisher console and Magnavox console with all of their parts in exchange for getting this conn with eico trafos working for me. but again my patience has led me to wanting this done now. so I am about to see if the fisher trafos will fit on the remaining/working conn amp without moving sockets and such. if they do, I will try it and probably ruin both amps. someone please pm me and come get this stuff out of my house.
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Old 19th July 2014, 05:52 PM   #5
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Location: Monroe Township, NJ
Saginaw, MI is not next door. However, it's not horribly far from Cleveland, OH.

Mike Samra (merccougar89 @t gmail d0t com), who is a very good candidate to help you get the mess squared away, lives in Saginaw, MI. Drop Mike a line and tell him Eli sent you. (Yes, I'm stealing Groucho Marx's line. )
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Old 19th July 2014, 11:23 PM   #6
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well, I did nothing stupid. I have not further molested the fisher, nor the working conn amp. I got one channel on the conn/eico working. there is no room in this chassis to work, so I am buying the book recommended by kevinkr, and a nice chassis to build the Conn/Eico on. this started out as fun. Now it seems to be a money eating suck pit.
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Old 20th July 2014, 12:00 AM   #7
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I usually start by looking at DC voltages around the amplifier to se if they make sense.
If they don't then that stage is at fault.
Check the components on that stage.

To understand how tube amps work start with something simple like a simple single triode amplification stage. If it is an 12au7 then look for cathode voltage of 1v5 approx.
Look for half B+ at the anode.
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Old 20th July 2014, 12:18 AM   #8
kevinkr is offline kevinkr  United States
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I'm not terribly patient either, and have had to learn to overcome that particular personality flaw.. In my case it resulted in things that worked fine, but looked like they belonged on a lab bench not my listening room.

The book will help a lot, and that new chassis will give you the space needed to build a good amp.
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Old 20th July 2014, 12:51 AM   #9
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My advice is just keep at it, Tiger Woods didn't hit a hole in one on his first stroke. I feel that the rewards no matter how much frustration come from it will greatly out weigh the negative feeling of giving up. And trust me I tore my hair out for a couple of months on a build one time because of an oscillation issue. I was ready to throw the damn things in the lake but I stuck with it and it ended up being a manufacturing error of a brand new output transformer, simple fix. If I gave up it would have eaten at me my whole life and I would have never slept well again.

And don't worry what others think about the work you did in pictures, just post them, there are lots of people on here (myself included) that will help you get it figured out. Whats a couple of burnt/blown components? It's a small price to pay for knowledge, the greatest treasure in the world. I am not scared to crack a few eggs to bake a cake so neither should you

So crack a beer, take a deep breath (not necessarily in that order) and lets get to it.
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Old 20th July 2014, 01:02 AM   #10
kevinkr is offline kevinkr  United States
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^^ Good advice
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