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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Mon Aug 01, 2011 6:30 am
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sstevep wrote:
...Shimmilou, you have found that about 5mA is reasonable for the control- and screen-grid currents at a typical given DC bias? (cathode I - grid(s) I = plate I, yes) I am happy to bias according to my ear within the aforementioned acceptable range...


Yes, exactly. I like to check the actual Plate current instead of, or as well as, the Cathode current to be sure, and have found that for a 6L6 there is very close to 5 mA difference. And most bias probes are a little more accurate than the amp test point(s). So, 40 mA at Cathode (35 mA Plate), and approx 433 volts on Plate, is only about 15 watts of idle Plate dissipation. So, for a 30 watt tube at this Plate voltage, 40 mA Cathode current would be an absolute minimum requirement. :idea:

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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Mon Aug 01, 2011 8:12 am
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You have a very good point Shimmilou.

If you calculate your bias/ power disipation with grid current your amp is biased "average" And for the same amp if you forget the grid current , your bias could be "hot"


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Wed Aug 03, 2011 4:05 am
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So the most current I am able to get flowing as measured at the cathodes is 35.2/40.8mA. Taking screen grid current into account, and assuming 5mA, which seems reasonable, that becomes 30.2/35.8. This is barely at the minimum as far as the spec for the tubes, and I have the pot maxed.

I still need to verify parts values. But in the meantime, for those I haven't TLDR'ed out of this thread, I have uploaded some actual sound files!

http://ssri-sound-samples.weebly.com/

I'll be curious to hear what folks think this sounds like, in addition to all the technical details...

Thanks,
Steve


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Tue Aug 09, 2011 11:44 pm
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Update... Received new power tubes. This was a matched pair from TubeDepot. They bias to within 1 mA. 40mA is still about as high as I can get the dc bias current, as measured at the cathode.

So... same problem persists, unfortunately.

I broke down and finally brought the amp in to work where I have much fancier toys and test equipment. I removed the chassis and went over the circuit as described on the schematic that is available on Fender's website. I focused on the Vibrato channel. I set up a 1k sine wave at the input to this channel, and adjusted the ac input voltage so that I had 20mVAC at the grid of V2A, as described in the schematic. I then collected the following data:

TP
12: 20.1mVac (spec: 20mVac)
13: 1.83Vdc (spec: 2.06Vdc, 11%Δ)
14: 866mVac (spec: illegible)
15: 6.8mVac (spec: 12mVac, 43%Δ)
16: 424mVac (spec: 690mVac, 39%Δ)

24: 19.2mVac (spec: 31mVac, 38%Δ)
25: 630mVac (spec: illegible)

31: 900mVpk-pk (spec: 2.4Vpk-pk, 63%Δ) (meas w/ o-scope)
32: 300mVpk-pk (spec: 1.0Vpk-pk, 70%Δ) (meas w/ o-scope)
33: 92.2Vdc (spec: 93.6Vdc, 1.5%Δ)
34: 3.62Vac (spec: 5.5Vac, 34%Δ)
35: 3.89Vac (spec: 5.9Vac, 34%Δ)
36: -46.8Vdc (spec: -52Vdc, 10%Δ)
37: 34.6mVdc (spec: 33mVdc, 5%Δ)
38: 33.6mVdc (spec: 33mVdc, 1.8%Δ)
39: 444Vdc (spec: 433Vdc, 2.5%Δ) / 27.9Vac (spec: 30Vac, 7%Δ)
40: 445Vdc / 27.8Vac (spec: 30Vac, 7.3%Δ)
41: 1.18Vac into spkr (spec: 1.25Vac, 5.6%Δ)

Even though some of these numbers are quite far off, the AC plate voltages and the speaker voltage are well within the ±20% described on the schematic. I really don't see a smoking gun here. I'm going to try to get a screenshot off my o-scope with the amp driven well into clipping and hope someone has the experience to compare it to something.

Did anyone have time to download those sounds clips?

Thx
Steve


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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Wed Aug 10, 2011 12:10 am
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A couple of questions, Steve......

What amplitude were you using for that 1kHz signal at the input?

Did you happen to repeat this same battery of tests using the "normal" channel?

On the face of it, I'd say that the problem seems to develop between TP13 and TP15, then compounds itself further downstream from there.

JMWAG

Arjay

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Post subject: Re: SRRI Unequal Bias Current, one tube 27mA
Posted: Wed Aug 10, 2011 2:22 am
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sstevep, I 'm surprise your "old" tube bias was good at 5 ma between them. But it good to have some spare tube.

I have some difficult to translate , to understand some words , but should I understand your problem i always the amp is breaking earlier, like vol at 4 ?

If yes, some resistor in the Phase Inverter circuit ,( the 12AT7 , tube before 6L6 ) could have wrong vallue. If you have 12AX7 in place of this tube, try a 12AT7.

OR could it be the speakers ?

IMO Unless the amp have a big problem like does't work, half power, the voltages are always good or not enoug wrong to help.


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