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Post subject: Question about a 1974 Champ
Posted: Fri Feb 21, 2014 5:29 pm
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Just noticed the Champ is operating at about 1/4 normal volume with a barely audible strange distortion underi it all. I connected it to a different speaker with the same results. Both of the tubes light up with the RCA 7517 far dimmer than the 6V6GT. Do I have a tube problem or something else?

I last changed the power tube in the late 80's I believe.
Thanks.


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Post subject: Re: Question about a 1974 Champ
Posted: Fri Feb 21, 2014 6:39 pm
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rwdavis2 wrote:
Both of the tubes light up with the RCA 7517 far dimmer than the 6V6GT.


The Champ chassis features three tubes: A 5Y3 rectifier, a 6V6GT power pentode, and a 12AX7A dual triode as the gain stage and phase inverter.

:?: :?: :?:

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Post subject: Re: Question about a 1974 Champ
Posted: Fri Feb 21, 2014 6:56 pm
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Both of the tubes light up with the RCA 7517 far dimmer than the 6V6GT.

Never heard of a 7517. Are you sure this isn't the manufacturer's code? Or is it a 7408?

:?: :?: :?:


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Post subject: Re: Question about a 1974 Champ
Posted: Fri Feb 21, 2014 8:10 pm
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The side of the tube, I believe says 7517, or I could have transposed from 5717.

But, since I'm not familiar with tube electronics does anyone have an idea other than to correct my original post?


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Post subject: Re: Question about a 1974 Champ
Posted: Fri Feb 21, 2014 9:08 pm
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This is a blackface/silverface Champ chassis......

Image

The small tube beneath the metal shield on the right is a 12AX7. This tube provides the gain stage and phase-inverter duties. The large tube next to it is a 6V6GT. It's the power tube. On the extreme left is a 5Y3GT rectifier. This tube converts incoming AC voltage to usable DC for the other tubes. Loss of any of these tubes renders the amp non-operable.

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Post subject: Re: Question about a 1974 Champ
Posted: Fri Feb 21, 2014 11:26 pm
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rwdavis2 wrote:
The side of the tube, I believe says 7517, or I could have transposed from 5717.

But, since I'm not familiar with tube electronics does anyone have an idea other than to correct my original post?


No such beast as the 5717. The only other octal tube in this era Champ is the 5Y3GT rectifier. Mil spec versions maybe labeled 6106 or 6087.

The "7517" prolly a date of manufacturing code. 17th week of 1975. Maybe GE or Sylvania in origin.


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Post subject: Re: Question about a 1974 Champ
Posted: Sat Feb 22, 2014 7:15 pm
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Does your "7517" look like the "7238" in a vertical line on the right side of the very right Sylvania 5Y3WGT? Maybe the rest of the stamping rubbed off?

In any event, you cannot judge tube strength by looking at how dim or bright it is, during use. Even two of the exact same type (same manufacturer and date of manufacture) --- may not glow the same. Even if they are a near perfect matches, via tube tester results.


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Post subject: Re: Question about a 1974 Champ
Posted: Sat Feb 22, 2014 7:29 pm
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It is a 5y3wgt. I appreciate all the interest but my lack of knowledge about the working of tube amps still leaves me with the problem I originally posted about.


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Post subject: Re: Question about a 1974 Champ
Posted: Sat Feb 22, 2014 8:12 pm
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rwdavis2 wrote:
Just noticed the Champ is operating at about 1/4 normal volume with a barely audible strange distortion under it all. I connected it to a different speaker with the same results. Both of the tubes light up with the RCA 7517 far dimmer than the 6V6GT. Do I have a tube problem or something else?

I last changed the power tube in the late 80's I believe.
Thanks.



OK... into the wayback machine and back to the original problem. :lol:

I suppose you have no access to a Tube Tester? And I know it's not like every other person in the neighborhood happens to have one. :D

First, replace the 12AX7 with a known good one. Sound better?

If not... Do you have a spare 6V6GT and 5Y3GT? Sub one into the amp and try it out. If you find that either tube solves the issue --- you are prolly good to go. As this amp is Class "A" single-ended, so as long as the 6V6GT is not running too hot (red plating), you are prolly ok.

If the tubes don't help, most likely a dried out electrolytic cap problem. Has the main power supply cap can been replaced? How about the bypass caps? If it's been over 15-20 years --- you need to inspect them (or have a good tech inspect and replace them, if needed).

Start with the tubes. Post your results.

Good luck! :D :mrgreen: :D


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Post subject: Re: Question about a 1974 Champ
Posted: Sat Feb 22, 2014 9:20 pm
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I suggest you take it to a local amp tech.

I posted more but changed my mind as it seemed pointless to say.

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Last edited by Powdered Toast Man on Sat Feb 22, 2014 9:55 pm, edited 1 time in total.

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Post subject: Re: Question about a 1974 Champ
Posted: Sat Feb 22, 2014 9:27 pm
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My thoughts, exactly.

Sorry, the Devil made me do it. :D


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Post subject: Re: Question about a 1974 Champ
Posted: Sat Feb 22, 2014 11:27 pm
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Powdered Toast Man wrote:
I suggest you take it to a local amp tech.

I posted more but changed my mind as it seemed pointless to say.


+1

It won't take long for any competent tech to resolve the issue.


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Post subject: Re: Question about a 1974 Champ
Posted: Sun Feb 23, 2014 12:03 am
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Toronado wrote:
Powdered Toast Man wrote:
I suggest you take it to a local amp tech.

I posted more but changed my mind as it seemed pointless to say.


+1

It won't take long for any competent tech to resolve the issue.


+1

A single gain stage, a phase inverter, and a power tube, driven by the most rudimentary of power supplies. Easy to troubleshoot and service.

My guess would be lack of routine maintenance rather than some serious malfunction. Filter can fried, toasty resistor in the power rail, leaky coupling cap, something along those lines.

Arjay

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Post subject: Re: Question about a 1974 Champ
Posted: Sun Feb 23, 2014 4:52 am
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A tech it is then. Thanks for the tips and education. Just hope it is not too expensive. I do not use it much. It's big brother get most of the work.


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Post subject: Re: Question about a 1974 Champ
Posted: Sun Feb 23, 2014 11:04 am
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Once that Champ is running well, you may end up using it more than you think. One of the best little practice & recording amps ever made.

BTW... this amp benefits from a nice replacement speaker.

Good luck with it! :D


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