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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 11:34 am
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BMW2002Ti wrote:

1.) Changed the grid-to-ground resistors to precise 150K-phm resistors (from the OEM 220K-ohm ones).

2.) Put is new 1.5 k-ohm grid stoppers.

3.) Put in 470-ohm precise screen resistors.



I see nothing wrong with these component now, If the are there , and are good. By the way the 470 ohms don't need to be precise. Almost any resistor from to 100 ohms to 1k will do the job. Higher is safer


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 06, 2011 12:29 pm
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I appreciate your comments. I just chose the most precise components to rule-out the power section as the cause.

I think perhaps dropping the screen voltage (by using, say a 1000-ohm screen resistor instead of the 470-ohm resistor--- or about 10 volt drop versus 5 volt drop) may also help stabilize the TSRI better. Of course, this might change the tube's performance & the tone.


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sat Nov 19, 2011 5:28 am
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Post Script:

Just installed and re-biased a pair of JAN Phillips 6V6GT tubes and am back in 'tone heaven'. :)

Perhaps not a 'loud' as the Fender Groove Tubes I've been using since the Tung Sol crisis emerged, but that's no bad thing with earlier break-up welcomed - my Tele. honks sweetly and my Strat really sings with some Jimi H. settings on my floor-rig.

I am reliably informed these NOS tubes are pretty bomb-proof so hoping for the aural excitement of the Tung Sols, but with increased reliability - they are marked A 7/86.

We have a gig tonight, so that will be a good test of how they perform in a live environment.

Feed.

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sat Nov 19, 2011 11:14 am
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Good luck! Usually these NOS (esp JAN) 6V6GT are plugged in and forget. Provided the bias is in correct range.


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sat Nov 19, 2011 11:17 am
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23mA

Sound OK?

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sat Nov 19, 2011 11:27 am
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23mA sounds ok if the plate voltage in around 390 VDC AND the plates aren't red.


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sat Nov 19, 2011 11:35 am
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Is that Cathode current? If so, the tubes are idling at about 8 watts, which sounds just right to me. 23 mA Cathode is about 20 mA Plate, times approx 400 Plate volts is right at 8 watts per tube idle dissipation (between 55% and 60%). If your reading is actual Plate current, then the tubes are idling just under 70%, which is still in acceptable range.

You should always check your actual measured Plate voltage, times the actual Plate current, to get the idle wattage of a tube. Typically, you want to bias between 50% to 70% of a tubes max Plate dissipation rating in watts. And typically, the Plate current for a 6V6 tube will be about 3 mA less than the Cathode current. Most bias probes measure Cathode current, which is Plate current plus grid currents.

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 20, 2011 11:27 am
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Firstly, thanks for all your comments - much appreciated and I have to say that with 35 years of live gig experience on the clock, last night my DRRI with the JAN Philips tubes gave me the nearest close shave ever to the ultimate 'live on-stage guitar sound' I carry around in my head.

Those that gig regularly will know that with drums, bass and another guitar blasting away on small stages (pubs mainly), its sometimes hard to tell exactly what IS happening. But my DRRI sang its heart out last night and was so easy to coax into controlled feedback when required.

But to follow on from the last post, my approach to using bias probes is shown in the pic below (ducking for cover): we Brits like to do things on a budget and this 'thing' certainly displays this trait (it was all I could find available some years ago - it works fine and I have kept it ever since despite its questionable approach to external insulation). I can't tell where the leads actually go, but am interested to understand exactly what I am measuring e.g. plate current OR cathode current?

Either way, I am interpreting advice so far that a measurement of 23mV (or 23mA measuring the volt drop across a 1 ohm resistor) is about right?

But where is the best place to actually measure plate voltage (pin numbers?).

Anyway, enjoy a smile at my expense when you review this pic.

Feed.

Image

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Last edited by Feedbackboy on Sun Nov 20, 2011 12:49 pm, edited 1 time in total.

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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 20, 2011 11:54 am
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Nice home made bias probe, I do some like this.

Plate voltage is at pin 3


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Post subject: Re: DRRI: Tung Sol 6V6 Red Plate
Posted: Sun Nov 20, 2011 12:54 pm
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Yes, pin 3 is Plate.

Typical bias probes measure Cathode current using a 1 ohm resistor in series with the Cathode (pin 8 ). This is safer than using the same method at the Plate, as the voltage at the Cathode is low or at ground potential. You would have to check with an ohm meter to see how the resistor is actually connected (to what pin) to know what the maker did. It would be unusual, and dangerous to attach the exposed resistor in series with the Plate as you have high voltage there (400 VDC). I would expect that you would read 1 ohm when checking from the female end to the male end on pin 8 of the probe, and closer to zero on all other pins, and if so this is the Cathode. If you have the 1 ohm reading male to female on pin 3, then that would be the Plate. For all we know, the resistor could be across two pins, hard to say just going by the picture. :o

If Cathode current, subtract 3 mA to get the Plate current of a 6V6, and subtract 5 mA to get the Plate current of a 6L6.

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