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Old 9th August 2011, 03:04 PM   #1
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Default Muddy 6V6 Power Amp?

I recently finished fixing up a 1940s Tube PA for use as a guitar amp (see Looking for info about a 1940s Amp), and it's almost working perfectly.

The output is very muddy-sounding, with a pronounced treble rolloff. Testing with another amp, there's plenty of treble right up to the power tubes' grids. The output at the speaker has the treble rolloff. The amp uses two push-pull 6V6s with a Hammond 1639 OT (10k:8). All of the parts seem to be fine. I'm currently using two 6P6S's as power tubes. The schematic is here:
http://oi51.tinypic.com/vpkd2c.jpg

And these are the voltages:
A+ - 306V
Plate - 298V
Screen - 302V
Cathode - 21V

I've tested and tested and I still have no clue what's going on. Any ideas on where the treble rolloff is?
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Old 9th August 2011, 04:09 PM   #2
Cassiel is offline Cassiel  Libya
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Quote:
Testing with another amp, there's plenty of treble right up to the power tubes' grids. The output at the speaker has the treble rolloff. The amp uses two push-pull 6V6s with a Hammond 1639 OT (10k:8).
Well, it seems you ready know where the problem is.
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Old 9th August 2011, 08:20 PM   #3
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Yes, I know that the problem is somewhere in the power amp, but I don't know exactly what it is. I'm wondering if anyone can help me pinpoint it or fix it.
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Old 9th August 2011, 09:29 PM   #4
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How old are the tubes? Tubes get muddy sounding when they age.
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Old 10th August 2011, 03:00 AM   #5
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They're almost unused. Just got them NOS, NIB, matched, and tested off of ebay, so I don't think that's the issue.
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Old 10th August 2011, 12:01 PM   #6
Cassiel is offline Cassiel  Libya
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Quote:
Just got them NOS, NIB, matched, and tested off of ebay
Wow, so that makes them great tubes, doesn't it? You may be right but it's either that or the OT. Guessing here you already know your speaker doesn't sound muddy. Guessing also here that you tested the preamp output correctly.
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Old 10th August 2011, 02:13 PM   #7
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Quote:
Originally Posted by Distorque View Post
I recently finished fixing up a 1940s Tube PA for use as a guitar amp (see Looking for info about a 1940s Amp), and it's almost working perfectly.

The output is very muddy-sounding, with a pronounced treble rolloff. Testing with another amp, there's plenty of treble right up to the power tubes' grids. The output at the speaker has the treble rolloff. The amp uses two push-pull 6V6s with a Hammond 1639 OT (10k:8). All of the parts seem to be fine. I'm currently using two 6P6S's as power tubes. The schematic is here:
http://oi51.tinypic.com/vpkd2c.jpg

And these are the voltages:
A+ - 306V
Plate - 298V
Screen - 302V
Cathode - 21V

I've tested and tested and I still have no clue what's going on. Any ideas on where the treble rolloff is?
I dont understand 'testing with another amp, and u got treble to the grids'? Did you drive the output tubes with another amp? Did you mean you compared this amp with another amp?

Anyways, looking at the schem it seems to me the amp is still designed for PA and not guitar. Guitar amps need to be tuned for a brighter response and not like a HiFi amp with flat response. Make it so bright it hurts, and I bet you'll get a better tone for guitar.
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Old 10th August 2011, 07:38 PM   #8
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I took the output from the preamp at the power tubes' grids, and plugged that into the input of a different amp. It sounded normal and there was no treble rolloff. When I took the output from a line out box connected to the speaker, there was a significant treble rolloff. So, I'm losing treble somewhere in the power amp. Also, I'm sure that it isn't a flat response. I could tell if it was. There is a significant treble loss in the power amp, and I'd don't want to start messing around with the tone until I find and fix it.

@Cassiel
Of course I'm not 100% sure that the tubes are okay, but I don't own a tube tester or any other new 6V6s so that's as close as I can come to a guarantee. It could be anything, but I'm more likely to suspect the OT than the tubes. Any ideas about how I can pinpoint or fix the treble rolloff?
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Old 10th August 2011, 07:43 PM   #9
DF96 is offline DF96  England
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Treble loss or bass peak? With a relatively high output impedance you will get a peak at the drive unit bass resonance. This may sound like a treble rolloff.
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Old 10th August 2011, 11:51 PM   #10
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Quote:
Originally Posted by Distorque View Post
Any ideas about how I can pinpoint or fix the treble rolloff?
What test equipment do you have (or have access to..) ?

Sine/square wave generator?
Oscilloscope?
VTVM or good meter?
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