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Post subject: Red Knob Twin doesn't have much gain
Posted: Fri Aug 12, 2011 1:43 pm
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I picked up a cheap used Red Knob "The Twin" amp yesterday. The gain knob gives me barely any overdrive coming out of it. Is that how they are made, or could I have a bad tube - or any other ideas why it doesn't have much, if it should?


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Post subject: Re: Red Knob Twin doesn't have much gain
Posted: Fri Aug 12, 2011 3:03 pm
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This amp should have plenty of gain. In fact so much so that they can sound pretty muddy. Might need some fresh tubes, or the gain pot might have gone bad.

FWIW

T2

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Post subject: Re: Red Knob Twin doesn't have much gain
Posted: Mon Aug 15, 2011 4:47 am
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Prolly tired or DOA tubes:

1.) Check the power tubes.

2.) input stage tubes

4.) phase inverter.

5.) Way outta spec idle bias.

6.) Wrong tubes in certain sockets.

7.) Too low B+ on power tubes, for a variety of reasons.

8.) Too low C+ (or anode voltage) on signal tubes and PI.

9.) Blown or mis-wired speakers.

10.) Bad solder points (esp to ground) or improper connections.

11.) Bad or mis-soldered control potentiomenters.

and most important and NEVER to forget:

12.) Phase of the Moon.

13.) Color of your sox.

[in other words anything and everything]


Try a thoroughly inspection of each stage from the power tubes (in alone, all other tubes pulled), back to the input. Adding each tube one-by-one. No luck finding the area? Are nominal voltage reading off the power supply and bias power supply are within 20% of schematic? Then do you feel comforatble (and you will be CAREFULLY sticking a small metal probe into areas with 200-475 VDC :wink: :wink: :wink: )? You can tap the signal grid of each stage from power tube to the input---with an INSULATED small metal probe. Tap the socket's tang or the signal grid position. Just the tang! Try not to make an accidental connection between the tang and ground (or any other component). If you have ANY heart condition requiring anti-arrythmic drugs or a pacemaker... read no further and take amp to tech.

The pop noise should get louder and louder as you move from the power tubers back to the input (or going from lesser gain stages to the highest gain stage--the input). If not, try new tubes. No better? Start checking voltages around the offending area. Are the volt within tolerance (15-20%)? Then, do the "ancient" wooden Chinese chopsticks test on components in the bad area. Try to isolate the bad component(s).

If this sounds like high voltage rock & roll suicide.... seek a good amp tech. (really does sound crazy, even to those of us that do this all the time. So don't be embarassed).

Good luck with the amp!


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Post subject: Re: Red Knob Twin doesn't have much gain
Posted: Mon Aug 15, 2011 6:16 am
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Rock Star
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Looks like maybe you should have gotten the Concert...Plenty o' Gain in those puppies.

And after 27 years I just recently had my '84 Concert serviced and retubed...that is what a reliable Point to Point wired amp is all about.

T2

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