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Post subject: Re: 76 Deluxe Reverb
Posted: Wed Aug 09, 2017 5:53 pm
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Rock Star
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What you'd be doing is replacing the 820-ohm, 1/2 watt resistor (marked GNFB, in the layout diagram) with another value. The total GNFB network is a 820-ohm AND the 47-ohm resistors. The transitional brown face ran a 56k-ohm and 1500-ohm network (in place of the 820/47 ohm). Tweeds ran no GNFB.

You will lose some output and potentially gain noise and some inherent instability by increasing the resistance or going to no GNFB. You will change the tone and the onset of OD. Empirical testing will determine how far you can go with these changes.

Just remember, the output stage of the tweed is much different than the BF or SF Deluxe. Tweed models having CATHODE bias and BF and SF having FIXED bias. I'd prolly start with the transitional brownface (FIXED bias) GNFB network and see how it effects tone and stability.

Good luck!

:mrgreen:


http://ampwares.com/schematics/deluxe_6g3.pdf


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Post subject: Re: 76 Deluxe Reverb
Posted: Wed Aug 09, 2017 6:08 pm
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Thanks Beemer, I'll let you know how it sounds!


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Post subject: Re: 76 Deluxe Reverb
Posted: Mon Aug 21, 2017 10:03 am
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I didn't like the NFB mod it turns out, so put it back to stock.

I'm biasing the amp and had a question about that. I added 1-ohm resistors to pin 8 of the 6V6's. I'm getting 425 volts on the plates at about 45mA on the resistor at pin 8. This seems a lot higher that on some of the charts I see on line (http://www.tedweber.com/webervst/tubes1/calcbias.htm). More expected values would be like 400V at 21mA.

Can anyone help? I don't want to fry my amp!


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Post subject: Re: 76 Deluxe Reverb
Posted: Mon Aug 21, 2017 1:17 pm
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I like the 820/47 ohm GNFB resistor network the best, too.

45mA per tube? Does each 6V6GT have it's own 1-ohm resistor? Or is the resistor being shared by both tubes?

45mA is way too high, if this is one tube's idle bias current. Can you adjust it back to around 20mA? If not, there maybe not enough negative voltage from the bias supply.

Pull the 6V6GT tubes. Measure the voltage with bias pot and midpoint. Should be around (-35 VDC) at the junction of the two 220k-ohm grid-to-ground resistors. If the voltage greater than 20-25% off, you need to investigate your bias supply.

Good luck! Keep us posted.


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Post subject: Re: 76 Deluxe Reverb
Posted: Tue Aug 22, 2017 8:50 am
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Thanks Beemer:

Its one resistor per tube, and each tube is drawing about 45mV per tube. The current is being measured at the resistor on pin 8. With the pulled the power tubes the voltage at the junction of the two 220k-ohm grid-to-ground resistors is -32 to -59 from one end of the dial to the other. Its about -44 in the middle. I set the mA to 20 for each tube and the plate voltage ends up being 445v.

Other maybe related info: Its an export amp, and the voltage selector on the PT is set at 117. I have a sovtek 5U4G in as the rectifier.


Here is a pic of the bias supply
Image


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Post subject: Re: 76 Deluxe Reverb
Posted: Tue Aug 22, 2017 2:06 pm
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s3mamill,

20mA at 445VDC is much better. Around 9-watts idle dissipation per tube. Or around 65% max rated dissipation.

I wonder if someone had 6L6GC tubes in the amp, when the idle current was 45mA at 425VDC. this would equal about 19-watts per tube. This would be about 65% of 6L6GC max rated dissipation of 30-watts.

Image


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Post subject: Re: 76 Deluxe Reverb
Posted: Wed Aug 23, 2017 8:28 am
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Beemer, maybe there were 6L6's there in the past. Good thought.

Thanks for your help with this!


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